Self-similar surface nanodomain structures induced by laser irradiation in lithium niobate

Self-similar surface structures composed of nanodomain rays, which are formed in congruent lithium niobate single crystals under pulsed laser irradiation, are investigated. The computer simulation of the formation of the domain structure is performed using the experimentally revealed rules of ray growth. It is demonstrated that the domain structures formed are fractal objects with a limited range of scaling.     

Implications of transient changes of optical and surface properties of solids during femtosecond laser pulse irradiation to the formation of laser-induced periodic surface structures

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of silicon wafer surfaces by linearly polarized Ti:sapphire femtosecond laser pulses (pulse duration 130 fs, central wavelength 800 nm) is studied experimentally and theoretically. In the experiments, so-called low-spatial frequency LIPSS (LSFL) were found with periods smaller than the laser wavelength and an orientation perpendicular to the polarization. The experimental results are analyzed by means of a new theoretical approach, which combines the widely accepted LIPSS theory of Sipe et al. with a Drude model, in order to account for transient (intra-pulse) changes of the optical properties of the irradiated materials. It is found that the LSFL formation is caused by the excitation of surface plasmon polaritons, SPPs, once the initially semiconducting material turns to a metallic state upon formation of a dense free-electron-plasma in the material and the subsequent interference between its electrical field with that of the incident laser beam resulting in a spatially modulated energy deposition at the surface. Moreover, the influence of the laser-excited carrier density and the role of the feedback upon the multi-pulse irradiation and its relation to the excitation of SPP in a grating-like surface structure is discussed.     

Coloration of a metal surface under pulsed laser irradiation

The regimes of irradiation using nanosecond laser pulses for creation of color images on stainless-steel and titanium surfaces upon laser engraving are studied. Parameters of radiation that correspond to the spectrum of resulting colors on the sample surface are experimentally determined. The spectral analysis of the irradiated area is performed and probe microscopy is used to study the surface relief. Complicated surface relief that results from irradiation indicates the contribution of several optical effects responsible for the surface color under laser engraving.     

Thermal deformation of a solid surface under laser irradiation

A calculation is presented for the deformation of a solid surface when subjected to laser irradiation. The elastic response of the target is given in terms of the stress and displacement field due to thermal expansion, thus allowing an explicit evaluation of the shape of the deformed target surface. Quantitative results are obtained for the height and shape of the induced bump as a function of the laser pulse and target properties. The thermal load threshold is specified for the onset of plastic yield.     

Generation of low-frequency radiation under focused laser irradiation of a conductor

The theory of generation of the low-frequency radiation due to ponderomotive action of a focused laser pulse that is incident on a conductor is developed. Spectral, angular, and energy characteristics of the low-frequency radiation and the spatiotemporal structure of the low-frequency pulse are analyzed.     

Generation of low-frequency radiation under focused laser irradiation of a semiconductor

The excitation of the nonlinear medium based on the zigzag nanotubes is analyzed. The terahertz emission is demonstrated under excitation in spite of an equidistant spectrum of a single nanotube. The system exhibits relatively high absorbance in the absence of pumping due to a relatively large oscillator strength.     

On the mechanism of quartz surface crimping under laser irradiation

The influence of an artificial inhomogeneity on the surface of a solid on the formation of periodic structures under laser irradiation has been studied experimentally. Inhomogeneities of this kind are found to catalyse the arising of periodic structures which appear in the vicinity of inhomogeneities and are of a regular nature. A theory is suggested to explain the principal experimental patterns such as the size of a periodic structure, its only weak dependence on laser radiation intensity, angular dependences, etc.     

Kuramoto—Sivashinsky equation for modulation of surface relief of molten layer and formation of surface periodic microstructures under pulsed laser irradiation of solids

A closed nonlinear two dimensional equation of Kuramoto-Sivashinsky type for the thickness of the laser pulse induced viscous molten layer is derived in the long wavelength and weak nonlinearity approximation. Linear stability analysis shows that under the condition that the temperature gradient at the surface is directed from the surface to the bulk, the surface instability sets in that leads to formation of surface relief structures with wavelength proportional to the thickness of the liquid layer. Computer simulations predict the subsequent formation of lamellar and disordered, quasihexagonal structures of surface relief when the time of irradiation is increased. Obtained results are used for the interpretation of experimental data on formation of lamellar and quasihexagonal surface relief microstructures upon multiple nanosecond pulse laser irradiation of silicon in water confinement.     

Irreproducibility of acoustic emission under cyclic irradiation of solids by an infrared laser pulse

Acoustic emission (AE) caused by a stress wave in solid material samples with various thermophysical properties, i.e., copper, aluminum, and glass upon exposure to СO₂ laser pulses is studied. It is found that the general AE picture (the AE signal arrival time, its amplitude, and others) is not reproduced during repeated experiments following one after another, which indicates irreversible local changes occurred in the material structure. The effect of the critical material energy density on the AE pattern during the laser exposure is demonstrated.     

Formation of nanometer ordered defect-deformational structures induced by energy fluxes in solids

A defect-deformational theory is constructed for the hierarchic formation of nanometer clusters in periodic structures of point defects under the action of an energy flux. The theoretical results obtained are compared with experimental results on the formation of nanometer pore lattices in metals under the action of particle beams and the laser-induced structural modification of semiconductor surfaces. Good agreement between the theory and experiment is obtained. Fiz. Tverd. Tela (St. Petersburg) 39, 2029–2035 (October 1997)     

Long-range effect under low-intensity irradiation of solids

Experimental results of the study of long-range low-intensity irradiation with light effect on the microhardness of silicon plates and aluminum foils (including the irradiation of aluminum/silicon stacks) are presented. These results for silicon agree with the data on irradiation effect on a system of microdefects. Some results on the small-dose long-range effect on the above mentioned materials are also given. The results are discussed using a model of the effect of radiation-induced acoustic waves on defects in solids.     

Single- and multi-pulse formation of surface structures under static femtosecond irradiation

Femtosecond surface structure modifications are investigated under irradiation with laser pulses of 150 fs at 800 nm, on copper and silicon. We report sub-wavelength periodic structures formation (ripples) with a periodicity of 500 nm for both materials. These ripples are perpendicular to the laser polarization and can be obtained with only one pulse. The formation of these ripples corresponds to a fluence threshold of 1 J/cm² for copper and 0.15 J/cm² for silicon. We find several morphologies when more pulses are applied: larger ripples parallel to the polarization are formed with a periodicity of 1 μm and degenerate into a worm-like morphology with a higher number of pulses. In addition, walls of deep holes also show sub-wavelength and large ripples.     

Low-dimensional structures formed by irradiation of laser

Some kinds of low-dimensional nanostructures can be formed by irradiation of laser on the pure silicon sample and the SiGe alloy sample. This paper has studied the photoluminescence (PL) of the hole-net structure of silicon and the porous structure of SiGe where the PL intensity at 706nm and 725nm wavelength increases obviously. The effect of intensity-enhancing in the PL peaks cannot be explained within the quantum confinement alone. A mechanism for increasing PL emission in the above structures is proposed, in which the trap states of the interface between SiO₂ and nanocrystal play an important role.     

Energy-gap distortion in solids under intense laser fields

We discuss, within the framework of the nearly-free electron model, the influence of an intense laser field on the motion of an electron in the effective lattice potential. It is shown that, for moderately intense fields, the band gap decreases linearly with increasing laser intensity.     

Pyrolysis craters produced by laser pulse irradiation on propellant solids

The appearance of craters is discussed, which are observed at the surface of propellant solids after laser pulse heating. The discussion is based on the three-dimensional heat flow equation and an infinitesimal short heat source. The experimental crater dimensions are correlated with the laser pulseenergy and the pyrolysis temperature of the solids.     

Formation of intrinsic oxide nanocrystals on the surface of GaSe under laser irradiation

The process of GaSe surface oxidation is investigated. The work function is revealed to vary by 0.5 eV within a few hours after the formation of a cleaved facet of the semiconductor surface. It is demonstrated that low-energy laser irradiation with a wavelength of 650 nm leads to the generation of an intrinsic oxide on the crystallite surfaces. On account of continuous exposure to laser radiation for 6 h, the work function of the GaSe surface increases by 1 eV, i.e., becomes twice as large as that obtained without irradiation.     

Proton transport in ordered hydrogen-bonded solids

The proton transfer is considered in hydrogenbonded chains with asymmetric double-minimum potentials for proteons. The transport is described by the kinksoliton propagation in an energetically asymmetric double-well potential for a proton. The asymmetry can be caused by the spontaneous polarization in hydrogenbonded ferroelectrics, medium polarization in nonferroelectric hydrogen-bonded solids, crystallographic structure, geometry of the proton neighbourhood and internal electric field in biological macromolecules. The soliton dynamics of the system is analytically studied including the influence of a constant external electric field and damping mechanism. The velocity, energy, momentum and mobility of kinks are calculated.     

Slip on the surface of silicon wafers under laser irradiation:Scale effect

The slip mechanism on the surface of silicon wafers under laser irradiation was studied by numerical simulations and experiments. Firstly, the slip was explained by an analysis of the generalized stacking fault energy and the associated restoring forces. Activation of unexpected {110} slip planes was predicted to be a surface phenomenon. Experimentally,{110} slip planes were activated by changing doping concentrations of wafers and laser parameters respectively. Slip planes were {110} when slipping started within several atomic layers under the surface and turned into {111} with deeper slip.The scale effect was shown to be an intrinsic property of silicon.