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Applied Surface Science (v.253, #15)

Preface by Leonid V. Zhigilei; Aaron Peled (pp. 6275-6275).

Dynamics of plume and crater formation after action of femtosecond laser pulse by M.B. Agranat; S.I. Anisimov; S.I. Ashitkov; V.V. Zhakhovskii; N.A. Inogamov; K. Nishihara; Yu.V. Petrov; V.E. Fortov; V.A. Khokhlov (pp. 6276-6282).

Using microinterferometric method, a transition in laser plume from the regime with spallation to the regime without spallation is experimentally studied for the first time. The transition occurs when the fluenceFinc of incident radiation exceeds a threshold of “evaporation”(Finc)ev. It has been shown previously that the spallation layer is formed at fluence above the ablation threshold(Finc)abl. Thus the spallation exists within the limits(Finc)ablFev.

Keywords: PACS; 71.15.Pd; 79.20.DsUltrashort laser ablation; Pump-probe; Molecular dynamics; Spallation

Laser induced ion emission from wide bandgap materials by S.R. John; J.A. Leraas; S.C. Langford; J.T. Dickinson (pp. 6283-6288).

At fluences well below the threshold for plasma formation, we have characterized the direct desorption of atomic ions from fused silica surfaces during 157nm irradiation by time-resolved mass spectroscopy. The principal ions are Si⁺ and O⁺. The emission intensities are dramatically increased by treatments that increase the density of surface defects. Molecular dynamics simulations of the silica surface suggest that silicon ions bound at surface oxygen vacancies (analogous to E′ centers) provide suitable configurations for the emission of Si⁺. We propose that emission is best understood in terms of a hybrid mechanism involving both antibonding chemical forces (Menzel–Gomer–Redhead model) and repulsive electrostatic forces on the adsorbed ion after laser excitation of the underlying defect.

Keywords: Ion emission; F; ₂; Laser; Time-resolved mass spectroscopy; Menzel–Gomer–Redhead model

Substrate influence in electron–phonon coupling measurements in thin Au films by Patrick E. Hopkins; Pamela M. Norris (pp. 6289-6294).

Accurate understanding and measurement of the energy transfer mechanisms during thermal nonequilibrium between electrons and the surrounding material systems is critical for a wide array of applications. With device dimensions decreasing to sizes on the order of the thermal penetration depth, the equilibration of the electrons could be effected by boundary effects in addition to electron–phonon coupling. In this study, the rate of electron equilibration in 20nm thick Au films is measured with the Transient ThermoReflectance (TTR) technique. At very large incident laser fluences which result in very high electron temperatures, the electron–phonon coupling factors determined from TTR measurements deduced using traditional models are almost an order of magnitude greater than predicted from theory. By taking excess electron energy loss via electron-substrate transport into account with a proposed three temperature model, TTR electron–phonon coupling factor measurements are more in line with theory, indicating that in highly nonequilibrium situations, the high temperature electron system looses substantial energy to the substrate in addition to that transferred to the film lattice through coupling.

Keywords: PACS; 79.20.Ds; 73.50.−h; 73.61.−r; 72.10.Di; 66.70.+f; 73.63.Bd; 65.80.+nElectron–phonon coupling; Two temperature model; Thin films; Ultrashort pulsed laser heating; Nonequilibrium

Temperature dependences of the electron–phonon coupling, electron heat capacity and thermal conductivity in Ni under femtosecond laser irradiation by Zhibin Lin; Leonid V. Zhigilei (pp. 6295-6300).

The electron temperature dependences of the electron–phonon coupling factor, electron heat capacity and thermal conductivity are investigated for Ni in a range of temperatures typically realized in femtosecond laser material processing applications, from room temperature up to temperatures of the order of 10⁴K. The analysis is based on the electronic density of states obtained through the electronic structure calculations. Thermal excitation of d band electrons is found to result in a significant decrease in the strength of the electron–phonon coupling, as well as large deviations of the electron heat capacity and the electron thermal conductivity from the commonly used linear temperature dependences on the electron temperature. Results of the simulations performed with the two-temperature model demonstrate that the temperature dependence of the thermophysical parameters accounting for the thermal excitation of d band electrons leads to higher maximum lattice and electron temperatures achieved at the surface of an irradiated Ni target and brings the threshold fluences for surface melting closer to the experimentally measured values as compared to the predictions obtained with commonly used approximations of the thermophysical parameters.

Keywords: PACS; 61.80.Az; 71.20.Be; 63.20.Kr; 79.20.DsElectron density of states; Electron–phonon coupling; Electron heat capacity; Thermal conductivity; Two-temperature model; Laser melting

Coherent phonon excitation in bismuth by Alexander Q. Wu; Xianfan Xu (pp. 6301-6304).

Ultrafast time-resolved reflectivity of a bismuth thin film evaporated on a silicon substrate is measured to investigate coherent phonons in bismuth. The reflectivity result is analyzed by a linear chirp approximation to obtain the time dependent frequencies of coherent phonons. Not only the optical modes are detected, which are generated by a combination of impulsive stimulated Raman scattering and displacive excitation of coherent phonon, acoustic phonon modes are also observed, which are emitted by the A_1g optical phonon.

Keywords: PACS; 63.20.KrFemtosecond laser; Bismuth; Coherent phonon; Raman scattering; Displacive excitation

Probing nonequilibrium dynamics with white-light femtosecond pulses by J. Michael Klopf; Pamela Norris (pp. 6305-6309).

Femtosecond pulsed lasers have become an invaluable tool for examining ultrafast nonequilibrium dynamics. With pulsewidths of a few hundred femtoseconds (fs) to less than 10fs, these lasers can clearly provide unprecedented temporal resolution. By amplifying ultrashort laser pulses to sufficient levels of energy per pulse, it is possible to exploit the nonlinear optical properties of certain materials to generate extremely broadband pulses. These pulses retain the time structure of the incident pulse, but contain a spectral bandwidth extending from the infrared to as far as the ultraviolet. By generating white-light pulses, it becomes possible to probe ultrafast nonlinear processes over a large range of energies. In this paper, the process of generating white-light ultrashort pulses will be presented, along with a discussion of different probing techniques and procedures necessary for modeling the transient optical data. Finally, results from pump–probe measurements using a white-light probe on indium phosphide (InP) films will be presented as a demonstration of this technique.

Keywords: White-light; Femtosecond; Ultrashort lasers; Pump-Probe; Nonequilibrium

Investigation of nanoparticle generation during femtosecond laser ablation of metals by Sylvie Noël; Jörg Hermann; Tatiana Itina (pp. 6310-6315).

The production of nanoparticles via femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume.

Keywords: Laser ablation; Femtosecond laser; Nanoparticle synthesis; Laser plasma

Time resolved laser-induced plasma dynamics by Xianglei Mao; Sy-bor Wen; Richard E. Russo (pp. 6316-6321).

The structure and evolution of the laser-induced vapor plume and shockwave were measured from femtosecond time resolved shadowgraph images. By changing the wavelength of the probe beam (400 and 800nm), differences in the opacity of the vapor plume were measured as a function of delay time from the ablation laser pulse. The evolution of the temperature and electron number density during and after the ablation laser pulse were determined and compared for ablation in argon and helium background gases. A laser supported detonation wave (LSD) observed for ablation in argon, blocks the incoming laser energy and generates a high-pressure region above the vapor plume.

Keywords: Femtosecond; Shockwave; Vapor plume

Strong emission of particulates towards the incident beam direction in pulsed-laser ablation experiments by Luca Cultrera; Michael I. Zeifman; Alessio Perrone (pp. 6322-6325).

Theoretical predictions suggested that particulates (large clusters and droplets) in pulsed-laser ablation deposition (PLD) move towards the surface normal and constitute a small fraction of the total plume mass. Contrary to expectations, here we report that, independently of the laser beam direction, large clusters are ejected towards the laser direction of incidence, which generally differs from the surface normal. Moreover, fragments and droplets constitute the major fraction of the ablated mass. Cross-sectional SEM investigations performed on the Si targets show that the direction of growth of the columns follows the laser beam direction. These observations have been explained by the change of the microscopic ablation mechanism from monomer evaporation at low local laser fluences to phase explosion at higher local fluences.

Keywords: Silcon; Pulsed laser deposition; Particulates; Plume deflection; Mass distribution

Growth of GaAs “nano ice cream cones” by dual wavelength pulsed laser ablation by C.T. Schamp; W.A. Jesser; B.S. Shivaram (pp. 6326-6329).

Harmonic generation crystals inherently offer the possibility of using multiple wavelengths of light in a single laser pulse. In the present experiment, the fundamental (1064nm) and second harmonic (532nm) wavelengths from an Nd:YAG laser are focused together on GaAs and GaSb targets for ablation. Incident energy densities up to about 45J/cm² at 10Hz with substrate temperatures between 25 and 600°C for durations of about 60s have been used in an ambient gas pressure of about 10⁻⁶Torr. The ablated material was collected on electron-transparent amorphous carbon films for TEM analysis. Apart from a high density of isolated nanocrystals, the most common morphology observed consists of a crystalline GaAs cone-like structure in contact with a sphere of liquid Ga, resembling an “ice cream cone”, typically 50–100nm in length. For all of the heterostuctures of this type, the liquid/solid/vacuum triple junction is found to correspond to the widest point on the cone. These heterostructures likely form by preferential evaporation of As from molten GaAs drops ablated from the target. The resulting morphology minimizes the interfacial and surface energies of the liquid Ga and solid GaAs.

Keywords: PACS; 81.05 Bx; 81.05Ea; 81.05Zx; 61.46.−w; 81.10.−h; 81.15.FgDual wavelength PLD; Nano “ice cream cone”

Raman spectroscopy of SWNTs produced by a XeCl excimer laser ablation at high temperatures by Mitsuhiro Kusaba; Yoshiaki Tsunawaki (pp. 6330-6333).

Synthesis of single-wall carbon nanotubes (SWNTs) was carried out by an ablation method using a XeCl excimer laser. It was irradiated onto a graphite target containing Co and Ni at the temperatures of 1073, 1173, 1273, 1373, 1473, 1523 and 1623K under the atmosphere (0.1MPa) of Ar gas with the flow rate of 12ml/min. The measurement by a scanning/transmission electron microscope and Raman spectroscopy found the formation of SWNTs with the diameter of about 1.3nm and the length of about 2μm in ablated carbonaceous soot. The ratio of peak intensity of 1590cm⁻¹ (G band) to that of 1335cm⁻¹ (D band) in the high frequency Raman spectra increased with increasing the ambient temperature. The radial breathing mode (RBM) in the low frequency Raman spectra shows that the mean diameter of SWNTs increased with increasing the ambient temperature.

Keywords: PACS; 61.46.Fg; 42.55.Lc; 79.20.DsSingle-wall carbon nanotube; Excimer laser; Laser ablation

Peculiarity of metal drilling with a commercial femtosecond laser by B.R. Campbell; J.A. Palmer; V.V. Semak (pp. 6334-6338).

A commercial femtosecond pulse laser was used to study the interaction of ultrashort laser pulses with aluminum. Tests were conducted to measure the average drilling rate over a range of laser pulse energies in both air and vacuum at the wavelengths corresponding to the fundamental and second harmonic of the laser. For the fundamental wavelength, it was observed that the drilling rates in vacuum were significantly higher than that for drilling in atmospheric air. For the laser beam that was converted to second harmonic, the drilling rate in vacuum at the same energy was slightly lower than that for drilling in air. The observed results can be explained by the presence of an energetic nanosecond pedestal in the laser pulse produced by the femtosecond laser system. This nanosecond component provides a major contribution into drilling and it is strongly affected by the optical breakdown plasma that reduces the drilling rate in air. Conversion to second harmonic reduces the relative energy content of the nanosecond component resulting in a higher contrast femtosecond pulse that is not affected by the near surface plasma. The presence of air results in self-focusing of the second harmonic laser beam, causing an increased drilling rate as compared to the interaction in vacuum.

Keywords: PACS; 42.62.CfUltrashort pulse; Femtosecond; Drilling; Aluminum; Pedestal

Mechanism of ablation of CdS at laser wavelengths in the visible and in the UV by María Jadraque; Jesús Alvarez; Rebeca de Nalda; Margarita Martin (pp. 6339-6342).

The mechanisms of laser ablation of CdS targets at different laser wavelengths have been investigated. (CdS)_n⁺ cluster formation is only observed upon 532nm ablation. The time and energy distributions of neutral S, S₂, Cd and CdS show significant dependence on laser wavelength. Bimodal distributions are observed at 266 and 308nm. For the former, the average kinetic energy increases significantly with mass, taking values in the range of 0.3–1.7eV. At 308nm the slow component of the time distribution disappears at distances above the target larger than 1cm. At this wavelength, the observed time distribution appears to reflect mainly the dynamics of the expansion. At 532nm the time distribution is monomodal and the average kinetic energies are below 0.2eV. Clear indications of the participation of thermal (at 532nm) and non-thermal mechanisms (at 266nm) have been found. It is tentatively concluded that the cluster formation observed upon ablation at 532nm can be related to the thermal ablation mechanisms in which the low kinetic energy of the species in the plume and their similar velocities favor the aggregation processes.

Keywords: Laser ablation; Clusters; Plume dynamics; CdS; PLD; TOF MS

Multi-material two-temperature model for simulation of ultra-short laser ablation by Mikhail E. Povarnitsyn; Tatiana E. Itina; Konstantin V. Khishchenko; Pavel R. Levashov (pp. 6343-6346).

We investigate the interaction of 100fs laser pulses with metal targets at moderate intensities (10¹² to 5×10¹³W/cm²). To take into account effects of laser energy absorption and relaxation we develop a multi-material two-temperature model based on a combination of different approaches. The backbone of the numerical model is a high-order multi-material Godunov method in a purely Eulerian form. This formulation includes an interface-tracking algorithm and treats spallation at high strain rates and negative pressures. The model consistently describes the hydrodynamic motion of a two-temperature plasma and accounts for laser energy absorption, electron–phonon/ions coupling and electron heat conductivity. In particular, phase transitions are accurately taken into account by means of a wide-range two-temperature multi-phase equation of state in a tabular form. The dynamics of the phase transitions and the evolution of the heat-affected zone are modeled and analyzed. We have found that a careful treatment of the transport coefficients, as well as consideration of phase transitions is of a great importance in obtaining reliable numerical results. Calculation results are furthermore compared for two metals with different electron–phonon coupling parameters (Au and Al). We have found that the main part of ablated material results from fragmentation of melted phase caused by tensile stresses. A homogeneous nucleation mechanism alone does not explain experimentally observed ablation depth.

Keywords: PACS; 61.80.Az; 79.20.Ds; 64.70.Dv; 64.70.FxLaser interactions; Laser ablation; Laser melting; Heat transfer; Laser-induced spallation

Short-pulse ablation rates and the two-temperature model by B.H. Christensen; K. Vestentoft; P. Balling (pp. 6347-6352).

A numerical simulation of the two-temperature model has been performed for Au, Ag, Cu and Al. Based on findings in the numerical simulation, an analytical model for the ablation rates in the high fluence regime is proposed. Furthermore, it is shown that a temperature-dependent electron–lattice coupling must be introduced to have qualitative agreement with the logarithmic growth in the low fluence regime. For aluminum, experimental ablation rates with varying fluence is presented and compared with the numerical simulation.

Keywords: PACS; 61.80.Ba; 78.47.+p; 81.16.Rf; 81.65.CfLaser ablation; Two-temperature model; Electron–lattice coupling

Theoretical determination of the ablation rate of metals in multiple-nanosecond laser pulses irradiation regime by Mihai Stafe; Constantin Negutu; Ion M. Popescu (pp. 6353-6358).

A detailed understanding of the physical determinants of the ablation rate in multiple nanosecond laser pulses regime is of key importance for technological applications such as patterning and pulsed-laser deposition. Here, theoretical modeling is employed to investigate the ablation of thick metallic plates by intense, multiple nanosecond laser pulses. A new photo-thermal model is proposed, in which the complex phenomena associated to the ablation process are accounted for as supplementary terms of the classical heat equation. The pulsed laser ablation in the nanosecond regime is considered as a competition between thermal vapourization and melt ejection under the action of the plasma recoil pressure. Computer simulations using the photo-thermal model presented here and the comparison of the theoretical results with experiment indicate two different mechanisms that contribute to the decrease of the ablation efficiency. First, during the ablation process the vapour/plasma plume expanding above the irradiated target attenuates the laser beam that reaches the sample, leading to a marked decrease of the ablation efficiency. Additional attenuation of the laser beam incident on the sample is produced due to the heating of the plasma by the absorption of the laser beam into the plasma plume. The second mechanism by which the ablation efficiency decreases consists of the reduction of the incident laser intensity with the lateral area, and of the melt ejection velocity with the depth of the hole.

Keywords: PACS; 40Laser ablation; Photo-thermal model; Ablation rate

Modeling of plume dynamics with shielding in laser ablation of carbon by Kedar Pathak; Alex Povitsky (pp. 6359-6365).

The process of laser ablation of carbon in presence of background gas is simulated numerically. The plume dynamics in laser ablation is important to study for many reasons including temperature of plume particles and shielding of target by previously ablated plumes. Shielding leads directly to the change in energy deposition of incident laser pulse at the target surface and in turn influences the ablation dynamics and amount of material removed. Carbon ablation is studied for single and multiple laser hits typical for synthesis of nanotubes. Two models of correction of ablated velocity and pressure resulting from shielding effect are proposed and investigated. Numerical modeling of this plume dynamics and its integral effect of shielding is challenging due to inherent high nonlinearity of the problem. Some of available numerical techniques handles nonlinearity but are dissipative, e.g. Godunov type schemes. Other techniques are less dissipative but fail to account for strong nonlinearity typical for initial stages of ablation, e.g. the ENO-Roe. To effectively model this highly nonlinear plume dynamics a combination of two of above mentioned schemes is developed so as the numerical evaluation of fluxes is close to their physical values and the scheme has minimum dissipation. The non-monotonic behavior of ablated mass as a function of time duration between two laser pulses is studied.

Keywords: Laser ablation; Carbon ablation; CFD; Godunov numerical scheme; ENO-Roe numerical scheme; Multiple plumes; Vorticity dynamics; Shielding effect

Exploration of pulse timing for multiple laser hits within a combined heat transfer, phase change, and gas dynamics model for laser ablation by Nathan Mullenix; Alex Povitsky (pp. 6366-6370).

Laser ablation involves heat transfer, phase changes and/or chemical reactions, and gas dynamics. All three of these processes are tightly coupled with each other. A model has previously been developed to simulate the nanosecond scale laser ablation of carbon. This model has been extended to accommodate longer term simulations and multiple laser pulses. The effects of varying the timing of a second laser pulse by tens of nanoseconds are explored. It is shown that by changing this interval one can control the total mass ablated and the mass transfer rate.

Keywords: PACS; 79.20.Ds; 64.70.Hz; 47.11.−j; 46.15.−xLaser ablation; Carbon ablation; Hertz–Knudsen equation; Multi-dimensional heat transfer

Modeling of vapor-droplet plumes ablated from multiple spots by Igor Zinovik; Alex Povitsky (pp. 6371-6376).

The study aims at modeling of plume shielding aspects of laser ablation processes with multiple laser pulses applied to multiple targets. The main obstacle with the efficient use of multiple laser pulse technologies is an attenuation of the laser irradiation by previously ablated plumes. Dynamics of plumes is described by the axisymmetric Euler equations describing a vapor-droplet ablated mixture rolling-up in the surrounding ideal gas. For multiple laser pulses, the role of absorption of laser beam by previously ablated plumes is evaluated varying a model parameter that defines the fraction of laser energy absorbed by the ablated mixture. Absorption of laser beam by plume may cause its secondary explosion that cleans the target surface and, subsequently, increases the mass ablated by the consequent pulse. Dynamics of plumes ablated from two targets with possible time delay between two laser hits is investigated as a representative case of multiple targets. Shielding of the surface between targets appears to be significant if the second pulse occurs before the first shock wave passes the second target.

Keywords: Laser ablation; Multiple targets; Multi-phase plume; Euler equations; Plume roll-up; Vorticity

Incorporation of chemical reactions into UV photochemical ablation of coarse-grained material by Yaroslava G. Yingling; Barbara J. Garrison (pp. 6377-6381).

A coarse-grained representation of material can significantly speed up molecular dynamics simulations. The difficulty arises when the simulations need to include chemical reactions. We have developed a methodology for including the effects of chemical reactions in coarse-grained molecular dynamics simulations, namely the Coarse-Grained Chemical Reactions Model (CGCRM). The model adopts physically and experimentally based parameters of a specific material, such as photochemical passways, the probabilities, and the exothermicities of chemical reactions. We have applied this approach to elucidate the effects of photochemical reactions on laser ablation of organic and polymeric materials. We find that the model provides a plausible description of the essential processes.

Keywords: Laser ablation; Simulations; Coarse-grained; Photochemistry

Computational investigation into the mechanisms of UV ablation of poly(methyl methacrylate) by Manish Prasad; Patrick F. Conforti; Barbara J. Garrison; Yaroslava G. Yingling (pp. 6382-6385).

Molecular dynamics simulations with an embedded Monte Carlo based reaction scheme were used to study UV ablation of poly(methyl methacrylate) (PMMA) at 157nm. We discuss the onset of ablation, the formation and distribution of products in the plume and stress relaxation of the polymer matrix. Laser induced heating and bond-breaks are considered as ablation pathways. We show here that depending on the nature of energy deposition the evolution of ablation plume and yield composition can be quite different. If all of photon energy is converted to heat it can set off ablation via mechanical failure of the material in the heated region. Alternatively, if the photon energy goes towards breaking bonds first, it initiates chemical reactions, polymer unzipping and formation of gaseous products inside the substrate. The ejection of these molecules has a hollowing out effect on the substrate which can lead to ejection of larger chunks. No excessive pressure buildup due to creation of gaseous molecules or entrainment of larger polymer chunks is observed in this case.

Keywords: Computational investigation; Mechanisms of UV ablation; Poly(methyl methacrylate)

Effects of thermal energy deposition on material ejection in poly(methyl methacrylate) by Patrick F. Conforti; Manish Prasad; Barbara J. Garrison (pp. 6386-6389).

The effects of absorption of 7.9 and 5.0eV photons by the polymer poly(methyl methacrylate) are studied using molecular dynamics simulations. By rapidly depositing a critical amount of thermal energy in the surface region (greater than 0.03eVÅ⁻³), a pressure wave is formed which causes spallation of the substrate. If there is only one photon absorbed per monomer unit of the polymer, the 7.9eV photons can supply sufficient energy density to initiate ejection.

Keywords: Ultraviolet absorption; Molecular dynamics; Thermal energy; PMMA

Simulation of the expansion of a crystal heated by an ultrashort laser pulse by S.I. Anisimov; V.V. Zhakhovskii; N.A. Inogamov; K. Nishihara; Yu.V. Petrov (pp. 6390-6393).

Molecular dynamics (MD) simulation of expansion of quickly heated Lennard–Jones (LJ) crystalline thin film is performed. The heating time is assumed to be much shorter than the characteristic expansion time. Such situation occurs when a femtosecond laser pulse is absorbed by a crystal. We established that plastic rarefaction wave is formed and propagates from the vacuum boundary into solid.

Keywords: Ultrashort laser ablation; Molecular dynamics; Plastic wave; Spallation

Numerical modeling of short pulse laser interaction with Au nanoparticle surrounded by water by Alexey N. Volkov; Carlos Sevilla; Leonid V. Zhigilei (pp. 6394-6399).

Short pulse laser interaction with a metal nanoparticle surrounded by water is investigated with a hydrodynamic computational model that includes a realistic equation of state for water and accounts for thermoelastic behavior and the kinetics of electron–phonon equilibration in the nanoparticle. Computational results suggest that, at laser fluences close to the threshold for vapor bubble formation, the region of biological damage due to the laser-induced thermal spike and the interaction of the pressure wave with internal cell structures can be localized within short distances from the absorbing particle comparable to the particle diameter. This irradiation regime is suitable for targeted generation of thermal and mechanical damage at the sub-cellular level.

Keywords: PACS; 78.20.Bh; 47.11.−j; 61.80.Az; 64.60.HtComputer modeling; Nanoparticles; Cell targeting; Laser damage

Effect of CCC bond bending vibration on the photodissociation of cyclobutane by Yusheng Dou; Yibo Lei; Zhenyi Wen; Yubin Wang; Glenn V. Lo; Roland E. Allen (pp. 6400-6403).

The effect of CCC bond bending on the photodissociation of cyclobutane to form two ethylene molecules was investigated by performing semiclassical electron-radiation-ion dynamics simulations and also by examining the potential energy surfaces of the electronic ground state and lowest excited states. These potential energy surfaces, calculated at the CASSCF/MRPT2 level with 6-31G* basis sets along a reaction path determined by the semiclassical dynamics simulations, show well-defined energy minima and maxima in the intermediate state region. It is found that in addition to rotation of the molecule around the central CC bond, CCC bond bending plays an important role in determining the features of the potential energy surfaces for the intermediate species.

Keywords: PACS; 31.15.Qg; 87.15.HePhotodissociation; Cyclobutane; Potential energy surface; Tetramethylene diradical; Semiclassical dynamics simulation

Ring opening reaction of 1,3-cyclohexadiene studied by semiclassical dynamics simulation by Yusheng Dou; Shuai Yuan; Glenn V. Lo (pp. 6404-6408).

A semiclassical dynamics simulation study is reported for the ring opening reaction of 1,3-cyclohexadiene (CHD) triggered by a femtosecond-scale laser pulse. The results clearly demonstrate that, following the excitation by the laser pulse, the ring opening occurs at ∼110fs and the molecule decays to the ground electronic state at ∼210fs due to non-adiabatic transition of electrons from LUMO to HOMO orbitals. Isomerization of the product of the ring opening reaction, 1,3,5-hexatriene (HT), to various stable isomers are also well demonstrated by the simulations.

Keywords: PACS; 31.15.Qg; 87.15.HeRing opening; 1,3-Cyclohexadiene; Isomerization; Semiclassical dynamics simulation

Polymer ablation: From fundamentals of polymer design to laser plasma thruster by L. Urech; T. Lippert; C.R. Phipps; A. Wokaun (pp. 6409-6415).

UV-Laser ablation of polymers is a well-established method to structure and deposit polymers, but the mechanisms of ablation are still controversial, i.e. photothermal or photochemical processes. An approach to probe the ablation mechanisms and to improve ablation is to incorporate photoactive groups into the polymer structure.The investigation of the ablation behavior of designed triazene polymers showed that the ablation mechanism is always a combination of both photothermal and photochemical processes, but the ratio can be changed by using different polymers and irradiation wavelengths. Also the quality of structures in the triazene polymers is superior at an irradiation wavelength of 308nm compared to commercially available polymers.Polymers can be designed not only for UV irradiation, but also for applications in the IR range, but with different requirements. One application for designed polymers in the near-IR range is as fuel for the laser plasma thruster, which is used as propulsion system for small satellites. With commercially available polymers the necessary thrust could not be achieved. A specially designed polymer-absorber system for this application produce more energy in the form of thrust, than the laser delivered.

Keywords: Laser ablation of polymers; Designed polymers; Triazene polymers; Energetic polymers; Laser plasma thruster; Infrared; Ultraviolet

Realignment process of actin stress fibers in single living cells studied by focused femtosecond laser irradiation by Ryohei Yasukuni; Jean-Alexis Spitz; Rachel Meallet-Renault; Takayuki Negishi; Takuji Tada; Yoichiroh Hosokawa; Tsuyoshi Asahi; Chisa Shukunami; Yuji Hiraki; Hiroshi Masuhara (pp. 6416-6419).

Three-dimensional dissection of a single actin stress fiber in a living cell was performed based on multi-photon absorption of a focused femtosecond laser pulse. The realignment process of an actin stress fiber was investigated after its direct cutting by a single-shot femtosecond laser pulse irradiation by high-speed transmission and fluorescence imaging methods. It was confirmed that mechanical force led by the femtosecond laser cutting propagates to entire cell through the cytockelton in a 100μs time scale. The cut actin stress fiber was realigned in the time scale of a few tens of minutes. The dynamic analysis of the realignment induced by single-shot femtosecond laser gives new information on cell activity.

Keywords: PACS; 87.17.−d; 87.80.Rb; 52.38.Mf; 42.62.BeFemtosecond laser; Laser processing; Fibroblast cell; Actin cytoskeleton; High speed imaging; Fluorescence imaging

Submicron foaming in gelatine by nanosecond and femtosecond pulsed laser irradiation by S. Gaspard; M. Oujja; R. de Nalda; C. Abrusci; F. Catalina; L. Bañares; M. Castillejo (pp. 6420-6424).

We compare the foaming characteristics induced by irradiation with single ns and fs laser pulses of UV, VIS and IR wavelengths on gelatines differing in gel strength (bloom values 75 and 225) and in crosslinking degree. We have observed that while laser irradiation with nanoseconds leads to the formation of a microfoam layer at 266nm, and melting and crater formation at longer wavelengths (532 and 1064nm), fs pulse irradiation leads to submicron foaming at all wavelengths studied (266, 400 and 800nm). These results show the possibility of controlling the submicrometric foam structure in this biomaterial and can shed light into the working mechanisms of fs laser nanoprocessing in biomaterials, where increase of temperature, thermoelastic stress generation, and stress-induced bubble formation are mediated by the generated plasma.

Keywords: PACS; 52.38.Mf laser ablation; 82.53.Ps femtosecond probing of biological moleculesGelatine; Biopolymers; Femtosecond laser processing; Nanostructuring

Femtosecond laser-induced crystallization of protein in gel medium by Kazuhiko Nakamura; Yosuke Sora; Hiroshi Y. Yoshikawa; Yoichiroh Hosokawa; Ryota Murai; Hiroaki Adachi; Yusuke Mori; Takatomo Sasaki; Hiroshi Masuhara (pp. 6425-6429).

We succeeded in generating femtosecond laser-induced crystallization of hen egg-white lysozyme (HEWL) in the irradiated area by suppressing convection of the solution using a highly concentrated gel. When a laser pulse, whose energy was above the threshold energy for bubble formation, was focused along with a linear scanning of the stage, HEWL crystallization was enhanced at the surface of bubbles on the irradiated line. The relation between the bubble formation process and the crystallization is discussed.

Keywords: PACS; 52.38.Mf; 47.55.dd; 81.10.−h; 87.14.Ee; 87.15.NnFemtosecond laser; Laser ablation; Light-induced crystallization; Protein crystal; Hen egg-white lysozyme

Properties of conductive polymer films deposited by infrared laser ablation by S.L. Johnson; H.K. Park; R.F. Haglund Jr. (pp. 6430-6434).

Thin films of the conducting polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) were deposited by resonant infrared laser vapor deposition (RIR-LVD). The PEDOT:PSS was frozen in various matrix solutions and deposited using a tunable, mid-infrared free-electron laser (FEL). The films so produced exhibited morphologies and conductivities that were highly dependent on the solvent matrix and laser irradiation wavelength used. When deposited from a native solution (1.3% by weight in water), as in matrix-assisted pulsed laser evaporation (MAPLE), films were rough and electrically insulating. When the matrix included other organic “co-matrices” that were doped into the solution prior to freezing, however, the resulting films were smooth and exhibited good electrical conductivity (0.2S/cm), but only when irradiated at certain wavelengths. These results highlight the importance of the matrix/solute and matrix/laser interactions in the ablation process.

Keywords: PACS; 81.15.Fg; 73.61.Ph; 52.59.RzInfrared laser; PEDOT:PSS; Thin film deposition; Conducting polymers; Resonant vibrational excitation

Influence of the photoionization process on the fragmentation of laser desorbed polycyclic aromatic hydrocarbons by K. Thomson; M. Ziskind; C. Mihesan; E. Therssen; P. Desgroux; C. Focsa (pp. 6435-6441).

Characterization of polycyclic aromatic hydrocarbons (PAHs) samples has been performed by laser desorption combined with multi-photon ionization technique using two different geometries of the ionization laser beam. This comparative study evidences the strong influence of ionization laser fluence on PAH fragmentation. Through a ∼10³ enlargement of the ionization probe volume and 10⁴ reduction of laser fluence over previous studies, fragment free mass spectra are obtained with higher sensitivity and selectivity. The ability to measure fragment free PAH mass spectra is a very important step in the end goal of measuring complex unknown mixtures of PAH desorbed from solid surface such as soot samples.

Keywords: JEL classification; 68.43.Tj; 82.80.Rt; 33.80.RvPolycyclic aromatic hydrocarbons; Laser methods; Desorption induced by photon stimulation; Photoionization; Mass spectrometry

IR laser ablation of doped poly(methyl methacrylate) by S. Gaspard; M. Oujja; E. Rebollar; M. Walczak; L. Díaz; M. Santos; M. Castillejo (pp. 6442-6446).

We investigate the TEA CO₂ laser ablation of films of poly(methyl methacrylate), PMMA, with average M_W 2.5, 120 and 996kDa doped with photosensitive compounds iodo-naphthalene (NapI) and iodo-phenanthrene (PhenI) by examining the induced morphological and physicochemical modifications. The films casted on CaF₂ substrates were irradiated with a pulsed CO₂ laser (10P(20) line at 10.59μm) in resonance with vibrational modes of PMMA and of the dopants at fluences up to 6J/cm². Laser induced fluorescence probing of photoproducts in a pump and probe configuration is carried out at 266nm. Formation of naphthalene (NapH) and phenanthrene (PhenH) is observed in NapI and PhenI doped PMMA, respectively, with relatively higher yields in high M_W polymer, in similarity with results obtained previously upon irradiation in the UV at 248nm. Above threshold, formation of photoproducts is nearly complete after 200ms. As established via optical microscopy, bubbles are formed in the irradiated areas with sizes that depend on polymer M_W and filaments are observed to be ejected out of the irradiated volume in the samples made with high M_W polymer. The implications of these results for the mechanisms of polymer IR laser ablation are discussed and compared with UV range studies.

Keywords: PACS; 82.50.Bc (processes caused by infrared radiation); 36.20.Cw (molecular weights (macromolecules and polymers)); 61.82.Pv (polymers, organic compounds)Poly(methyl methacrylate); Polymer molecular weight; IR laser ablation; Laser-induced fluorescence; Optical microscopy

Femtosecond laser-induced cleaving of protein crystal in water solution by Masafumi Kashii; Yoichiroh Hosokawa; Hiroshi Kitano; Hiroaki Adachi; Yusuke Mori; Kazufumi Takano; Hiroyoshi Matsumura; Tsuyoshi Inoue; Satoshi Murakami; Kazuomi Sugamoto; Hideki Yoshikawa; Takatomo Sasaki; Hiroshi Masuhara (pp. 6447-6450).

For precise X-ray diffraction (XRD) measurement giving the three-dimensional structure of proteins, it is important to prepare high-quality single crystals with suitable shape. As a new processing technique to obtain such protein crystals, we employed femtosecond laser-induced cleaving of protein crystal in a growth vessel containing water solution. An intact protein crystal was precisely processed without mechanical contact in its sealed growth vessel by focusing femtosecond laser pulses. We confirmed that three-dimensional processing of the crystal in its supersaturated solution was realized using multiphoton absorption and that the processing was efficiently enhanced by the cleaving behavior attributed to a photomechanical mechanism of the femtosecond laser ablation.

Keywords: PACS; 52.38.Mf; 42.62.Be; 87.14.Ee; 81.20.−nFemtosecond laser; Laser processing; Protein crystal; X-ray diffraction; Photothermal ablation; Photomechanical ablation

Characterization of lysozyme films produced by matrix assisted pulsed laser evaporation (MAPLE) by Andreea Purice; Jørgen Schou; Peter Kingshott; Nini Pryds; Maria Dinescu (pp. 6451-6455).

Thin lysozyme films of thickness up to more than 100nm have been produced in a dry environment by MAPLE (matrix assisted pulsed laser evaporation) from a water ice matrix. Analysis of the films demonstrates that a significant part of the lysozyme molecules is transferred to the substrate without decomposition and that the protein activity is preserved. The film deposition rate for 1wt% lysozyme has a maximum at 2J/cm² of about 1ng/cm² per laser shot. During the film production the deposition rate is constant without any sign of depletion or accumulation effects in the water ice target or in the growing film. Scanning electron microscopy (SEM) images demonstrate that the silicon substrate is completely covered by lysozyme films thicker than 100nm. Deposition was also made from a target with pressed (100%) solid lysozyme, but the deposition was difficult to handle and with a much slower rate than that from a water ice matrix.

Keywords: MAPLE; SEM; Lysozyme film

Computational and experimental study of the cluster size distribution in MAPLE by Elodie Leveugle; Leonid V. Zhigilei; Aaron Sellinger; James M. Fitz-Gerald (pp. 6456-6460).

A combined experimental and computational study is performed to investigate the origin and characteristics of the surface features observed in SEM images of thin polymer films deposited in matrix-assisted pulsed laser evaporation (MAPLE). Analysis of high-resolution SEM images of surface morphologies of the films deposited at different fluences reveals that the mass distributions of the surface features can be well described by a power-law, Y( N)∝ N−^t, with exponent − t≈−1.6. Molecular dynamic simulations of the MAPLE process predict a similar size distribution for large clusters observed in the ablation plume. A weak dependence of the cluster size distributions on fluence and target composition suggests that the power-law cluster size distribution may be a general characteristic of the ablation plume generated as a result of an explosive decomposition of a target region overheated above the limit of its thermodynamic stability. Based on the simulation results, we suggest that the ejection of large matrix-polymer clusters, followed by evaporation of the volatile matrix, is responsible for the formation of the surface features observed in the polymer films deposited in MAPLE experiments.

Keywords: PACS; 81.15.Fg; 79.20.Ds; 02.70.Ns; 61.80.AzMatrix-assisted pulsed laser evaporation; Cluster size distribution; Molecular dynamics; Laser ablation

Matrix-assisted pulsed laser evaporation of polyfluorene thin films by T. Tunno; A.P. Caricato; M.E. Caruso; A. Luches; M. Martino; F. Romano; D. Valerini; M. Anni (pp. 6461-6464).

Poly(9,9-dioctylfluorene) (PF8) thin films have been deposited by matrix-assisted pulsed laser evaporation (MAPLE) using a KrF excimer laser. The influence of the laser fluence (50–500mJ/cm²) and the nature of the solvent (chloroform, toluene, tetrahydrofuran) on the films properties have been studied. The chemical composition of the deposited films was investigated by Fourier transform infrared (FTIR) spectroscopy and compared with the one of spin coated films. To investigate the effect of the deposition parameters on the optical properties of the films, photoluminescence (PL) measurements were performed. Poor structural and optical properties were observed for films deposited starting from chloroform solutions. When using toluene as solvent, the spectra characteristics improved with increasing laser fluence, while wide PL spectra were observed. The characteristic emission bands of the PF8 polymer were nicely detected for films deposited starting from a tetrahydrofuran (THF) solution. Moreover, in this last case, the PF8 structure is preserved at high laser fluences, too.

Keywords: Matrix-assisted pulsed laser evaporation; Photoluminescence; Polyfluorene

Assessing the effect of the matrix in resonant infrared MAPLE by D.M. Bubb; A.O. Sezer; J. Gripenburg; B. Collins; E. Brookes (pp. 6465-6470).

The ablation yield of polyethylene glycol is measured as a function of laser pulse energy and polymer molecular weight for both free running and Q-switched Er:YAG lasers. The results show that the deposition rate is dependent on the absorption coefficient in a way that is consistent with the blow-off ablation model but that the linear dependence of the efficiency on laser pulse energy is also consistent with a steady-state ablation model. The results for both lasers are remarkably similar given that the pulse durations are 350μs and 85ns, respectively. Furthermore, polystyrene thin films are deposited as well and it is found that the morphology depends on the THF/MeOH ratios in the matrices. The significance of these results is discussed as well as future directions for this research.

Keywords: PACS; 61.41.+e; 78.30.−j; 81.05.Lg; 81.15.FgMAPLE; Laser processing; Polymers

Uniform thin films of TiO₂ nanoparticles deposited by matrix-assisted pulsed laser evaporation by A.P. Caricato; M.G. Manera; M. Martino; R. Rella; F. Romano; J. Spadavecchia; T. Tunno; D. Valerini (pp. 6471-6475).

We report morphological and optical properties of a colloidal TiO₂ nanoparticle film, deposited on a quartz substrate by using the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. Atomic Force Microscopy demonstrated that a good uniformity of the deposition can be obtained. The presence of agglomerates with dimensions of about 1μm in size was noticed. Form UV–vis transmission spectra, recorded in the 200–800nm range, the optical constants and the energy gap were determined besides the film thickness. The optical constants resulted in agreement with the values reported in literature for TiO₂ nanoparticle thin films.

Keywords: TiO; ₂; MAPLE; Colloidal nanoparticles

Polycaprolactone biopolymer thin films obtained by matrix assisted pulsed laser evaporation by R. Cristescu; A. Doraiswamy; G. Socol; S. Grigorescu; E. Axente; D. Mihaiescu; A. Moldovan; R.J. Narayan; I. Stamatin; I.N. Mihailescu; B.J. Chisholm; D.B. Chrisey (pp. 6476-6479).

We report the successful deposition of polycaprolactone polymer by MAPLE using a KrF* excimer laser ( λ=248nm, τ=7ns). According to FTIR spectra the deposited films have similar chemical structure to the dropcast material. The fluence plays a key role in optimizing the performances of MAPLE-synthesized polycaprolactone structures. We demonstrated that MAPLE allows for controlling the morphology of films to the level required in targeted drug delivery of pharmacologic agents.

Keywords: Controlled drug release; Polycaprolactone; Nanostructures; Pulsed laser deposition; Matrix assisted pulsed laser evaporation

Laser fluence dependence of the elastic properties of diamond-like carbon films prepared by pulsed-laser deposition by Xiao Liu ⁎; T.H. Metcalf; P. Mosaner; A. Miotello (pp. 6480-6486).

We studied the temperature dependence of internal friction of variety of amorphous diamond-like carbon films prepared by pulsed-laser deposition. Like the most of amorphous solids, the internal friction below 10 K exhibits a temperature independent plateau, which is caused by the atomic tunnelling states—a measure of structure disorder. In this work, we have varied the concentration ofsp3 versussp2 carbon atoms by increasing laser fluence from 1.5 to 30 J/cm². Our results show that the internal friction has a nonmonotonic dependence onsp3/sp2 ratio with the values of the internal friction plateaus varying between6×10−5 and1.1×10−4. We explain our findings as a result of a possible competition between the increase of atomic bonding and the increase of internal strain in the films, both of which are important in determining the tunneling states in amorphous solids. The importance of the internal strain in diamond-like carbon films is consistent with our previous study on laser fluence, doping, and annealing, which we will review as well. In contrast, no significant dependence of laser fluence is found in shear moduli of the films, which vary between 220 and 250 GPa.

Keywords: PACS; 62.40.+i; 63.50.+x; 68.35.GyInternal friction; Diamond-like carbon; Double-paddle oscillator; Tunnelling states; Internal strain

In situ Raman spectroscopy of annealed diamondlike carbon–metal composite films by C. Jin; H. Zhou; S. Graham; R.J. Narayan (pp. 6487-6492).

Diamondlike carbon films and diamondlike carbon–metal composite films may provide increased component reliability, decreased fuel consumption, decreased noise/vibration/harshness (NVH), and decreased lubricant use in next generation automotive components. Raman spectra were obtained for diamondlike carbon, diamondlike carbon–platinum composite films, and diamondlike carbon–gold composite films, which were annealed to a temperature of 523°C. The Raman spectra for these films were fitted using a two-Gaussian function. The variation of the G-peak position, the D-peak position, and the I_D/ I_G ratio was examined as a function of temperature. The unalloyed diamondlike carbon film demonstrated greater thermal stability than the diamondlike carbon–noble metal composite films. These results suggest that the operating temperatures of the diamondlike carbon-coated automotive components must be kept under careful consideration.

Keywords: Raman spectroscopy; Diamondlike carbon; Composite materials; Annealing; Nanostructured materials

UV assisted oxidation and nitridation of hafnia based thin films for alternate gate dielectric applications by K. Ramani; C.R. Essary; V. Craciun; R.K. Singh (pp. 6493-6498).

The synergistic effects of NH₃ ambient and ultraviolet illumination on the dielectric properties of hafnia based gate dielectrics are reported in this paper. The films were processed at relatively low temperatures (∼400°C) by pulsed laser ablation and UV oxidation technique. UV illumination and the NH₃ ambient created a thin and a denser interfacial layer (at the film–Si interface) comprised of HfSiON bonding. As a result of the interfacial layer modification, a leakage current density lower than 10⁻⁴A/cm² and a dielectric constant of ∼21.7 were extracted from the best samples processed in NH₃ and under UV illumination. The nitrogen doped HfO₂ also exhibited a thinner interfacial layer (∼12Å) in comparison to the films processed without NH₃ ambient.

Keywords: Hafnia; UV assisted oxidation; Hf; Si; O; N

Structural and optical characterization of undoped, doped, and clustered ZnO thin films obtained by PLD for gas sensing applications by C. Ristoscu; D. Caiteanu; G. Prodan; G. Socol; S. Grigorescu; E. Axente; N. Stefan; V. Ciupina; G. Aldica; I.N. Mihailescu (pp. 6499-6503).

The synthesis by pulsed laser deposition technique of zinc oxide thin films suitable for gas sensing applications is herein reported. The ZnO targets were irradiated by an UV KrF^* ( λ=248nm, τ_FWHM∼7ns) excimer laser source, operated at 2.8J/cm² incident fluence value, whilst the substrates consisted of SiO₂(001) wafers heated at 150°C during the thin films growth process. The experiments were performed in an oxygen dynamic pressure of 10Pa. Structural and optical properties of the thin films were investigated. The obtained results have demonstrated that the films are c-axis oriented. Their average transmission in the visible-infrared spectral region was found to be about 85%. The equivalent refractive indexes and extinction coefficients were very close to those of the tabulated reference values. Doping with 0.5% Au and coating with 100 pulses of Au clusters caused but a very slight decrease (with a few percent) of both transmission and refractive index values. The coatings with the most appropriate optical properties as waveguides have been selected and their behavior was tested for butane sensing.

Keywords: PACS; 81.15.Fg; 78.66.−w; 68.37.LpZnO; PLD; Thin films; Optical properties

Preparation of the La_0.8Sr_0.2MnO₃ films on STO and LAO substrates by excimer laser-assisted metal organic deposition using the KrF laser by T. Tsuchiya; K. Daoudi; T. Manabe; I. Yamaguchi; T. Kumagai (pp. 6504-6507).

La_0.8Sr_0.2MnO₃ films were prepared on SrTiO₃ (STO) and LaAlO₃ (LAO) substrates using excimer laser-assisted metal organic deposition (ELAMOD). For the LAO substrate, no epitaxial La_0.8Sr_0.2MnO₃ film was obtained by laser irradiation in the fluence range from 60 to 110mJ/cm² with heating at 500°C. On the other hand, an epitaxial La_0.8Sr_0.2MnO₃ film on the STO substrate was formed by laser irradiation in the fluence range from 60 to 100mJ/cm² with heating at 500°C. To optimize the electrical properties for an IR sensor, the effects of the laser fluence, the irradiation time and the film thickness on the temperature dependence of the resistance and temperature coefficient of resistance (TCR: defined as 1/ R·(d R/d T)) of the LSMO films were investigated. An LSMO film on the STO substrate that showed the maximum TCR of 3.9% at 265K was obtained by the ELAMOD process using the KrF laser.

Keywords: LSMO; TCR; STO; LAO; KrF laser

Structural properties of single and multilayer ITO and TiO₂ films deposited by reactive pulsed laser ablation deposition technique by F. Fotsa Ngaffo; A.P. Caricato; M. Fernandez; M. Martino; F. Romano (pp. 6508-6511).

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser. The films were deposited on SiO₂ substrates heated at 200 and 400°C. ITO and TiO₂ films with uniform thicknesses of about 400 and 800nm, respectively, over large areas were prepared. X-ray diffraction (XRD) analysis revealed that the ITO films are formed of highly orientated nanocrystals with an average particle size of 10–15nm. Atomic force microscopy (AFM) observations indicate rough ITO films surfaces with average roughness of 26–30nm. Pores were also observed. TiO₂ films deposited on the prepared ITO films result less crystalline. Annealing at 300 and 500°C for three consecutive hours promoted formation of TiO₂ anatase phase, with crystal size of ∼6–7nm. From the scanning transmission electron microscope (STEM) images, it can be seen that the TiO₂ films deposited onto the prepared ITO films present a relatively high pore sizes with an average pore diameter of ∼40nm and excellent uniformity. In addition, STEM cross-sectional analysis of our films showed a columnar structure but no evidence of voids in the structure. Therefore, films exhibited large surface area, well suited for dye-sensitized solar cells (DSSC) applications.

Keywords: RPLAD; ITO; TiO; ₂; Multilayers films; Annealing; Roughness; Pores; AFM; STEM

Laser-assisted synthesis of semiconductor chromium disilicide films by A. Luches; S.A. Mulenko; V.P. Veiko; A.P. Caricato; V.A Chuiko; Y.V. Kudryavtsev; A.V. Lopato; A.A. Petrov; F. Romano; D. Valerini (pp. 6512-6516).

Different photo-assisted techniques were employed for chromium disilicide (CrSi₂) semiconductor film fabrication. Flash evaporation of CrSi₂ powder on the Si substrate heated to ∼740K leads to the formation (according to XRD study) of amorphous films. Post-annealing at 920K leads to the formation of polycrystalline CrSi₂ phase. Crystallization is improved by further annealing with 1500 Q-Switched Nd:YAG laser pulses. Optical properties of the as deposited and annealed CrSi₂ films have been investigated in the 240–1100nm spectral range by using spectroscopic ellipsometry. The formation of CrSi₂ semiconductor phase was additionally confirmed by the temperature dependence of electrical resistance of the films treated by Q-switched Nd:YAG laser. The band gap for intrinsic conductivity results E_g≅0.2eV. Backward laser-induced film transfer (LIFT) was also used for CrSi₂ film deposition from bulk material on Si substrates. Pulsed CO₂ laser was employed for this purpose, because of transparency of silicon at the 10.6μm wavelength. Measurements of the electrical resistance of the deposited films as a function of temperature showed their semiconductor behavior ( E_g=6×10⁻⁴eV). Chromium disilicide films were also deposited by congruent pulsed laser ablation deposition on Si substrates either at room temperature or heated to about 740K. In this last case the deposit exhibits semiconducting properties with E_g≅0.18eV.

Keywords: Silicides; Laser-induced film transfer; Direct current sputtering

Pulsed laser photodeposition of a-Se nanofilms by ArF laser by A.P. Caricato; M. Martino; F. Romano; N. Mirchin; A. Peled (pp. 6517-6521).

Pulsed laser photodeposition from amorphous selenium aqueous colloid solutions using ArF laser radiation at a wavelength of λ=193nm has been investigated. Nanometer thick layers were obtained on UV transparent silica substrates in contact with the solution for various photodeposition parameters. Amorphous Se layers, 20nm thick, were obtained typically by 40 laser pulses of 30ns duration with a fluence of 50mJ/cm². Deposition thresholds for depositing 1nm thick layers were as low as 5 pulses. The deposited nanometer thin surface morphology was analyzed by Evanescent Field Optical Microscopy, Scanning Electron Microscopy and Atomic Force Microscopy. The nanometer thicknesses were evaluated by utilizing the differential evanescent light pattern emanating from the substrates.

Keywords: Nanometer layers; Pulsed laser photodeposition

Fe_xNi100−_x nanometric films deposited by laser ablation on SiO₂/Si substrates by D. Berling; A.P. Caricato; E. Denys; M. Fernández; G. Leggieri; S. Luby; A. Luches; M. Martino; P. Mengucci (pp. 6522-6526).

Fe_xNi100−_x nanometric films were deposited on SiO₂/Si substrates at room temperature using the pulsed laser deposition technique. The targets were Fe–Ni amorphous magnetic foils with composition Fe₅₀Ni₅₀, Fe₃₅Ni₆₅ and Fe₂₂Ni₇₈. Morphological and structural properties of the deposited films were investigated using scanning electron microscopy, Rutherford backscattering spectrometry, grazing incidence X-ray diffraction, and X-ray reflectivity. Electrical and magnetic characteristics of the films were investigated by using the four-point probe and the magneto-optic Kerr effect techniques, respectively. The film properties are strictly dependent on the Fe–Ni compositional ratio.

Keywords: Ferromagnetic materials; Thin film; Pulsed laser ablation; Magnetic properties; Morphological and structural characterizationPACS; 75.50.Bp; 81.15.−z; 79.20.Ds; 75.30.Cr; 07.10.Pz; 78.20.Ls; 75.70.Ak

Pulsed-laser crystallization and epitaxial growth of metal–organic films of Ca-doped LaMnO₃ on STO and LSAT substrates by K. Daoudi; T. Tsuchiya; T. Kumagai (pp. 6527-6530).

Ca-doped LaMnO₃ (LCMO) thin films have been successfully prepared on SrTiO₃ (STO) and [(LaAlO₃)_0.3–(SrAlTaO₆)_0.7] (LSAT) substrates using the excimer laser assisted metal–organic deposition (ELAMOD) process. The crystallization and the epitaxial growth of the amorphous metal–organic LCMO thin films have been achieved using a KrF excimer laser irradiation while the substrates were kept at constant temperature of 500°C. Epitaxial films were obtained using laser fluence in the interval of 50–120mJ/cm². The microstructure of the LCMO films was studied using cross-section transmission electron microscopy. High quality of LCMO films having smooth surfaces and sharp interfaces were obtained on both the STO and the LSAT substrates. The effect of the laser fluence on the temperature coefficient of resistance (TCR) was investigated. The largest values of TCR of the LCMO grown on the LSAT and the STO substrates of 8.3%K⁻¹ and 7.46%K⁻¹ were obtained at different laser fluence of 80mJ/cm² and 70mJ/cm², respectively.

Keywords: PACS; 81.15.−z; 81.15.Np; 73.61.−r; 71.30.+hLCMO; Excimer laser; MOD; TEM; TCR; Bolometer

Mg based photocathodes for high brightness RF photoinjectors by L. Cultrera; G. Gatti; F. Tazzioli; A. Perrone; P. Miglietta; C. Ristoscu; S. Orlanducci; A. Fiori (pp. 6531-6534).

Advanced high brightness radio frequency (RF) gun injectors require photocathodes with a fast response, high quantum efficiency (QE) and good surface uniformity. Metal films deposited by various techniques on the gun back wall could satisfy these requirements. A new deposition technique has been recently proposed, i.e. pulsed laser ablation. Several Mg samples have been deposited by this technique: the emission performance and morphological changes induced on the cathode surface during laser activation are compared and discussed.

Keywords: Magnesium; Pulsed laser deposition; Quantum efficiency; Photocathode

Nanoscopic photodeposited structures analyzed by an evanescent optical method by G. Socol; E. Axente; M. Oane; L. Voicu; A. Petris; V. Vlad; I.N. Mihailescu; N. Mirchin; R. Margolin; D. Naot; A. Peled (pp. 6535-6538).

The evanescent field propagating in waveguides was used to evaluate the profile and growth rate of laser photodeposited a-Se. A pulsed KrF excimer laser was used for deposition. The differential-evanescent light leaking image, was used to analyze the nanostructures in the deposited zones. The relation between the emerging light intensity of the evanescent wave and the optical light intensity propagating in the waveguide was connected to an effective range of the evanescent wave leaking power into the deposited material. The technique provides the nanometric profiles of the ultra-thin photodeposited structures.

Keywords: Nanometer layers; Evanescent wave; Pulsed laser photodeposition; Waveguide method

In-bulk and surface structuring of sapphire by femtosecond pulses by Saulius Juodkazis; Koichi Nishimura; Hiroaki Misawa (pp. 6539-6544).

The actual space–time dependent intensity distribution of a tightly focused (numerical aperture NA = 1.35) Gaussian femtosecond pulse is modeled inside dielectric material. Such focusing is typically used for recording with sub-wavelength resolution inside dielectrics. The multi-pulse structuring inside the bulk and on the surface of sapphire are demonstrated. Formation of nano-cracks and nano-crystals is revealed inside the crystalline sapphire. Ripple formation on the surface is discussed in terms of the efficacy map calculated by theory given in ref. [J.E. Sipe, J.F. Young, J.S. Preston, H.M. van Driel, Laser-induced periodic surface structure. I. Theory, Phys. Rev. B 27 (2) (1983) 1141–1154.].

Keywords: Dielectric breakdown; Femtosecond laser micofabrication; Micro-explosion; Sapphire

Weakly bound buffer layer as a template for metallic nano-clusters growth and film laser-patterning by Elad Gross; Ori Stein; Micha Asscher (pp. 6545-6549).

Growth of metallic nano-clusters and control over their size are critically important for catalysis. Development of film patterning procedures at the nanometer scale has significant impact on future lithography. In this work, we present an approach to grow metallic nano-clusters and control their size using a weakly bound buffer layer as an intermediate substance and a template to control the clusters size at the range 1–15nm.The buffer layer was further employed to create a pattern based on a selective laser ablation procedure. A thicker metallic film deposited on top of pre-patterned buffer layer has been demonstrated as a novel patterning technique at the sub-micron to nanometer scale employing a single laser pulse. The thermal stability of metallic structures prepared this way has been studied at temperature up to 1000K.

Keywords: Metallic nano-clusters; Film laser-patterning; Buffer layer

Formation of periodic structures by the space-selective precipitation of Ge nanoparticles in channel waveguides by Hiroaki Nishiyama; Yoshinori Hirata; Isamu Miyamoto; Junji Nishii (pp. 6550-6554).

Thermally stabilized channel waveguides with Bragg gratings were fabricated by the space-selective precipitation technique of crystalline Ge nanoparticles using KrF excimer laser irradiation. The periodic structures consisting of Ge nanoparticles were formed in Ge–B–SiO₂ thin glass films after exposure to an interference pattern of the laser followed by annealing at 600°C. The channel waveguides with the periodic structures were fabricated by the cladding of the patterned Cr layers on the films. The diffraction peak for the TE-like mode of 11.8dB depth was observed clearly at a wavelength of 1526.4nm, indicating that the periodic structure also served as the optical band-pass filter in optical communication wavelength. The spectral shape, diffraction efficiency, and diffraction wavelength remained unchanged even after annealing at 400°C. Furthermore, a low temperature dependence of the diffraction wavelength – as low as 8.1pm/°C – was achieved. The diffraction efficiency was further enhanced after subsequent annealing at 600°C. The space-selective precipitation technique is expected to be useful for the fabrication of highly reliable optical filters or durable sensing devices operating at high temperature.

Keywords: PACS; 42.70.Ce; 78.67.BfLaser; Refractive index change; Nanoparticles; Grating; Waveguide

Effect of pulse width and fluence of femtosecond laser on the size of nanobump array by Yoshiki Nakata; Noriaki Miyanaga; Tatsuo Okada (pp. 6555-6557).

Conical nanobump arrays were generated on gold thin film processed by interfering femtosecond laser. The transition of the height and diameter as functions of fluence and pulse width was investigated. When the fluence was 87mJ/cm², the height and diameter were not so different at 350fs or shorter pulse width. They decreased at longer pulse width, and no bump could be generated over 1.6ps. The results suggest the decrease of size is due to the diffusion of electron to not-excited region, and due to heat conduction to not heated region or substrate, or change of absorbance of laser. At long pulse width of 2.4ps and relatively higher fluence of 190mJ/cm², nanobump had liquid-like structure as a stop motion of a water drop.

Keywords: PACS; 81.16.−c; 42.25.Hz; 81.07.−b; 68.55.−a; 61.46.+wFemtosecond laser; Interference; Laser modification; Laser ablation; Thin film; Nanomaterial; Nanobump; Array

Enhanced near field mediated nanohole fabrication on silicon substrate by femtosecond laser pulse by Nikolay N. Nedyalkov; Tomoya Miyanishi; Minoru Obara (pp. 6558-6562).

Investigation of the process of nanohole formation on silicon surface mediated with near electromagnetic field enhancement in vicinity of gold particles is described. Gold nanospheres with diameters of 40, 80 and 200nm are used. Irradiation of the samples with laser pulse at fluences below the ablation threshold for native Si surface, results in a nanosized surface modification. The nanostructure formation is investigated for the fundamental ( λ=800nm, 100fs) and the second harmonic ( λ=400nm, 250fs) of the laser radiation generated by ultrashort Ti:sapphire laser system. The near electric field distribution is analyzed by an Finite Difference Time Domain (FDTD) simulation code. The properties of the produced morphological changes on the Si surface are found to depend strongly on the polarization and the wavelength of the laser irradiation. When the laser pulse is linearly polarized the produced nanohole shape is elongated in the E-direction of the polarization. The shape of the hole becomes symmetrical when the laser radiation is circularly polarized. The size of the ablated holes depends on the size of the gold particles, as the smallest holes are produced with the smallest particles. The variation of the laser fluence and the particle size gives possibility of fabricating structures with lateral dimensions ranging from 200nm to below 40nm. Explanation of the obtained results is given on the basis simulations of the near field properties using FDTD model and Mie's theory.

Keywords: PACS; 81.16.−c; 81.65.Cf; 52.38.Mf; 36.40.GkNanostructuring; Plasmons; Gold nanoparticles; Femtosecond laser processing

Stability and fragmentation of organic silicon functionalizations studied by laser desorption mass spectrometry by Dominic Lingenfelser; Peter Hess (pp. 6563-6569).

A method is introduced to investigate organic functionalizations on silicon by laser-induced thermal desorption (LITD), where well-ordered Si(111)-(1×1):H(D) surfaces are used to determine the desorption temperature as a function of laser fluence. To demonstrate the potential of this technique silicon surfaces with ultrathin oxide layers were functionalized with organic end groups. The species desorbed with focused XeCl laser pulses were monitored at an oblique angle and their time-of-flight (TOF) distributions were measured with a quadrupole mass analyzer after electron impact ionization. By assuming a negligible contribution of the oxide and organic layers to the heating effect, the TOF temperatures measured for Si(111)-(1×1):H(D) could be used to determine the mass of the desorbed species. Detailed results are presented for dimethylsilyl (DMS), bromomethyldimethylsilyl (BMDMS), and chloromethyldimethylsilyl (CMDMS) terminated surfaces which were prepared by silanization with suitable chloro and disilazane compounds. While for the DMS termination dimethylsilanol (76u) is desorbed as a single species, clearly identifying the terminating group, in the case of BMDMS and CMDMS further fragmentation of the end group occurs at the surface.

Keywords: PACS; 68.43.Tj; 81.07.Pr; 82.80.RtLaser-induced thermal desorption; Functionalized silicon; TOF mass spectrometry

Patterning of indium–tin oxide on glass with picosecond lasers by Gediminas Račiukaitis; Marijus Brikas; Mindaugas Gedvilas; Tomas Rakickas (pp. 6570-6574).

The results of patterning of the indium–tin oxide (ITO) film on the glass substrate with high repetition rate picosecond lasers at various wavelengths are presented. Laser radiation initiated the ablation of the material, forming grooves in ITO. Profile of the grooves was analyzed with a phase contrast optical microscope, a stylus type profiler, scanning electron microscope (SEM) and atomic force microscope (AFM). Clean removal of the ITO film was achieved with the 266nm radiation when laser fluence was above the threshold at 0.20J/cm², while for the 355nm radiation, the threshold was higher, above 0.46J/cm². The glass substrate was damaged in the area where the fluence was higher than 1.55J/cm². The 532nm radiation allowed getting well defined grooves, but a lot of residues in the form of dust were generated on the surface. UV radiation with the 266nm wavelength provided the widest working window for ITO ablation without damage of the substrate. Use of UV laser radiation with fluences close to the ablation threshold made it possible to minimize surface contamination and the recast ridge formation during the process.

Keywords: PACS; 42.55.Xi; 42.62.Cf; 68.37.Ps; 79.20.DsLaser patterning; Indium–tin oxide

Surface texturing of Si, porous Si and TiO₂ by laser ablation by D. Mills; T. Kreouzis; A. Sapelkin; B. Unal; N. Zyuzikov; K.W. Kolasinski (pp. 6575-6579).

Excimer laser ablation at 308nm has been used to texture the surfaces of a variety of materials of interest for optoelectronic and biotechnological applications. Using a range of pre- and post-processing methods, we are able to produce nano-, micro- and meso-scale features over large areas rapidly in materials such as crystalline Si, porous silicon and TiO₂. Texturing of porous silicon leads to the growth of crystalline dendritic structures, which distinguishes them dramatically from the conical pillars formed from crystalline silicon. Regular arrays of Si microdots are formed by irradiating a Si surface pre-covered with a Cr thin film grating. Nano-crystalline porous TiO₂ films are easily ablated or compacted with laser irradiation. However, at low enough laser fluence, surface roughening without complete loss of porosity is possible.

Keywords: Surface texturing; Laser ablation; Pillar; TiO; ₂; Silicon and porous silicon

Ultrafast-laser-assisted chemical restructuring of silicon and germanium surfaces by Barada K. Nayak; Mool C. Gupta; Kurt W. Kolasinski (pp. 6580-6583).

This article reports a comparative study on texturing in silicon and germanium surfaces after exposure to femtosecond laser irradiation in the gaseous environments of sulfur hexafluoride (SF₆) and hydrogen chloride (HCl). The surface texturing results from the combined effect of laser-assisted chemical etching and laser ablation. Optimized processing conditions have produced features on the order of nanometers in size. We demonstrate for the first time that regular conical pillars can be formed in Ge and that HCl can be used to form regular conical pillars in Si.

Keywords: Femtosecond laser; Surface texturing; Ultrafast lasers; Chemical restructuring; Silicon and germanium surfaces

Ripple formation in the chromium thin film during laser ablation by Kęstutis Regelskis; Gediminas Račiukaitis; Mindaugas Gedvilas (pp. 6584-6587).

The beam of a nanosecond pulse laser tightly focused to a line was applied for the back-side ablation of the chromium thin film on a glass substrate. The stripe ablated with a single laser pulse had sharp edges on both sides and ridges of the melted metal around it. The partially overlapping pulses formed a wide cleaned area with a complicated structure made of the metal remaining from the ridges. Regular structures, ripples, were developed when laser fluence was slightly above the single-pulse removal threshold and the shift between pulses was less than half width of the line ablated with a single laser pulse. The ripples were located periodically (∼4μm) and were orientated perpendicularly to the long axis of the beam spot. Their orientation did not depend on the laser beam polarization. Different models of the ripple formation in the thin metal film were considered, and instability of the moving vapor–liquid–solid contact line during evaporation of thin liquid films appears to be the most probable process responsible for the observed phenomena. Formation of regular gratings with the unlimited line length was experimentally implemented by using the above-mentioned technique.

Keywords: PACS; 47.20.Dr; 68.55.Ac; 89.20.Bb; 42.62.CfLaser ablation; Thin films; Chromium on glass; Gratings; Pattern formation; Ripples

The influence of laser-induced surface modifications on the backside etching process by K. Zimmer; R. Böhme; D. Ruthe; B. Rauschenbach (pp. 6588-6594).

Spectroscopic measurements in the UV/VIS region show reduced transmission through laser-induced backside wet etching (LIBWE) of fused silica. Absorption coefficients of up to 10⁵cm⁻¹ were calculated from the transmission measurements for a solid surface layer of about 50nm. The temperatures near the interface caused by laser pulse absorption, which were analytically calculated using a new thermal model considering interface and liquid volume absorption, can reach 10⁴K at typical laser fluences. The high absorption coefficients and the extreme temperatures give evidence for an ablation-like process that is involved in the LIBWE process causing the etching of the modified near-surface region. The confinement of the ablation/etching process to the modified near-surface material region can account for the low etch rates observed in comparison to front-side ablation.

Keywords: PACS; 81.65.Cf; 81.20.Wk; 81.05.Ke; 79.20.D; 61.80.B; 42.55.L; 42.62.C; 78.40.HaLaser; Ablation; Etching; Fused silica; Liquid; Absorption; Temperature calculations

3D integration of microcomponents in a single glass chip by femtosecond laser direct writing for biochemical analysis by Koji Sugioka; Yasutaka Hanada; Katsumi Midorikawa (pp. 6595-6598).

3D integration of microcomponents in a single glass chip by femtosecond laser direct writing followed by post annealing and successive wet etching is described for application to biochemical analysis. Integration of microfluidics and microoptics realized some functional microdevices like a μ-fluidic dye laser and a biosensor. As one of practical applications, we demonstrate inspection of living microorganisms using the microchip with 3D microfluidic structures fabricated by the present technique.

Keywords: Femtosecond laser; 3D microfabrication; Laser direct writing; Microchip; Biochemical analysis

Three-dimensional photofabrication with femtosecond lasers for applications in photonics and biomedicine by A. Ovsianikov; A. Ostendorf; B.N. Chichkov (pp. 6599-6602).

A status report on rapidly advancing femtosecond laser technology, three-dimensional (3D) microstructuring by multiphoton illumination technique, is given. Taking its origin from multiphoton microscopy, this technique is now becoming an important microfabrication tool. In our work we apply near-infrared Ti:sapphire femtosecond laser pulses (at 800/780nm) for 3D material processing. When tightly focused into the volume of a photosensitive material (or photoresist), they initiate 2PP process by, for example, transferring liquid into the solid state. This allows the fabrication of any computer generated 3D structure by direct laser “recording” into the volume of photosensitive material. 2PP of photosensitive materials irradiated by femtosecond laser pulses is now considered as enabling technology for the fabrication of 3D photonic crystals and photonic crystal templates. In particular, 2PP allows one to introduce defects at any desired locations, which is crucial for the practical applications. Recently, we studied possible applications of 2PP technique in biomedicine. 2PP is a very interesting technique for the fabrication of drug delivery systems, scaffolds for tissue engineering, and medical implants. These and other biomedical applications of 2PP will be reviewed.

Keywords: Femtosecond laser microfabrication; Polymerization; Biomedical engineering; Drug delivery; Computer-aided design; Photonic band gap materials

Rapid prototyping of ossicular replacement prostheses by A. Ovsianikov; B. Chichkov; O. Adunka; H. Pillsbury; A. Doraiswamy; R.J. Narayan (pp. 6603-6607).

Materials used in ossicular replacement prostheses must demonstrate appropriate biological compatibility, acoustic transmission, stability, and stiffness properties. Prostheses prepared using Teflon^®, titanium, Ceravital and other conventional materials have demonstrated several problems, including migration, perforation of the tympanic membrane, difficulty in shaping the prostheses, and reactivity with the surrounding tissues. We have used two-photon polymerization for rapid prototyping of Ormocer^® middle-ear bone replacement prostheses. Ormocer^® surfaces fabricated using two-photon polymerization exhibited acceptable cell viability and cell growth profiles. The Ormocer^® prosthesis was able to be inserted and removed from the site of use in the frozen human head without fracture. Our results demonstrate that two-photon polymerization is able to create ossicular replacement prostheses and other medical devices with a larger range of sizes, shapes and materials than other microfabrication techniques.

Keywords: Two-photon polymerization; Rapid prototyping; Ossicular replacement prosthesis

Novel type of indium oxide thin films sputtering for opto-electronic applications by G. Golan; A. Axelevitch; B. Gorenstein; A. Peled (pp. 6608-6611).

Transparent conductive oxide (TCO) thin films play a significant role in recent optical technologies. Displays of various types, photovoltaic systems, and opto-electronic devices use these films as transparent signal electrodes. They are used as heating surfaces and active control layers. Oxides of TCO materials such as: tin, indium, zinc, cadmium, titanium and the like, exhibit their properties. However, indium oxide and indium oxide doped with tin (ITO) coatings are the most used in this technology.In this work, we present conductive transparent indium oxide thin films which were prepared using a novel triode sputtering method. A pure In₂O₃ target of 2in. in diameter was used in a laboratory triode sputtering system. This system provided plane plasma discharge at a relatively low pressure 0.5–5mTorr of pure argon. The substrate temperature was varied during the experiments from room temperature up to 200°C. The films were deposited on glass, silicon, and flexible polyimide substrates. The films were characterized for optical and electrical properties and compared with the indium oxide films deposited by magnetron sputtering.

Keywords: PACS; 52.77.Dq; 81.15.Cd; 73.61.−rIndium oxide; Plane plasma sputtering; Hot-probe evaluation

The second hyperpolarizability of CdTe nanocrystals using polarization-resolved degenerate four-wave mixing by S.M. Ma; J.T. Seo; Q. Yang; R. Battle; L. Creekmore; K. Lee; B. Tabibi; W. Yu (pp. 6612-6615).

Polarization-resolved forward degenerate four-wave mixing (DFWM) in a nonresonant region revealed the effective third-order nonlinear susceptibility of colloidal CdTe nanocrystals (NCs) with the size near the Bohr radius and various concentrations. The second hyperpolarizabilities,〈γxxxxh〉 and〈γxyyxh〉, of the CdTe NCs were ∼1.15×10⁻⁴¹m⁵/V² and ∼3.01×10⁻⁴²m⁵/V² from the measurement of the concentration-dependent third-order nonlinear susceptibility of CdTe NCs, respectively. The ratio (〈γxyyxh〉/〈γxxxxh〉) of the hyperpolarizabilities was ∼0.26, which indicated a large contribution of an electronic polarization process to the third-order nonlinearity of CdTe NCs.

Keywords: PACS; 42.70.Nq; 78.66.Hf; 78.67.Bf; 78.67.HcCdTe nanocrystals; Nonlinear susceptibility; Hyperpolarizability

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Applied Surface Science (v.253, #15)