Electric breakdown and non-self-sustained electric discharge were triggered and guided by a train of ultrashort sub-TW ultraviolet (UV) pulses overlapped with a long free-running UV pulse of a hybrid Ti:Sapphire-KrF laser facility. Photocurrent sustained by this train is two orders of magnitude higher, and electric breakdown distance is twice longer than those for the discharge triggered by the long UV pulse only. UV filaments of ~100 m length were observed when transporting the laser radiation over the long distance.
Non-self-sustained electric discharge and electric breakdown were triggered and guided by a train of ultrashort sub-TW ultraviolet (UV) pulses overlapped with a long free-running UV pulse of a hybrid Ti:Sapphire - KrF laser facility. Photocurrent sustained by this train is two orders of magnitude higher, and electric breakdown distance is twice longer than those for the discharge triggered by the long UV pulse only. When transporting the laser radiation over the long distance, UV filaments of ~ 100 m length were observed.
Theoretical modeling of photo-induced electron-hole plasma and bandgap dynamics in GaAs at high femtosecond laser intensities ( ∼ TW/cm2) employing a quantum kinetic formalism based on a generalized Boltzmann-type equation, predicts for the first time against expectations, the saturation of plasma densities despite the strong direct bandgap narrowing. Though the transient electronic bandgap renormalization provides a significant positive feedback for all relevant single-photon and impact ionization mechanisms, which is clearly observable at moderate (sub-TW/cm2) laser intensities, the counterintuitive plasma density saturation at higher laser intensities and high plasma densities ( ∼ 1022 cm−3) is dictated by much stronger negative feedback, originating from a highly-nonlinear transient enhancement of the corresponding Auger recombination coefficient for the shrinking bandgap. These theoretical predictions are in semi-quantitative agreement with the results of our time-resolved reflectivity infrared (IR)-pump experiments, which support this newly predicted process of self-limiting ionization dynamics in strongly photo-excited semiconductors, such as GaAs, with induced bandgap shrinkage.
In this work femtosecond laser photo-excitation of GaAs is studied numerically. The transient plasma densities photogenerated
during the pumping IR fs-laser pulses were evaluated having in mind experimental data of time-resolved
reflectivity measurements of transient bandgap shifts.
Theoretical modeling employing quantum kinetic formalism based on a generalized Boltzmann-type equation, including
one/multi-photon photo-excitation, Joule heating and free-carrier absorption, interband excitation, impact ionization,
Auger recombination of electron-hole plasma, thermal exchange with the lattice, etc. is performed.
For the first time the effect of enhancement of ionization by transient bandgap renormalization (BGR) is considered both
experimentally and theoretically. The energy spectra of the electron distribution function and the time dependence of the
electron density are calculated and the key role of BGR in the transient electron-hole plasma dynamics is pointed out.
Gradual evolution of silicon surface topology from one-dimensional to two-dimensional nanogratings and then to isotropic sets of nanospikes was observed by increasing IR and UV femtosecond laser irradiation dose (the variable number of incident laser pulses at the constant laser fluence). The fundamental mechanisms of these topological transformations are discussed.
One-dimensional quasi-periodic structures whose period is much smaller than the wavelength of exciting optical
radiation have been obtained on a titanium surface under the multi-shot action of linearly polarized femtosecond laser
radiation at various surface energy densities. As the radiation energy density increases, the one-dimensional surface
nanogratings oriented perpendicularly to the radiation polarization evolve from quasi-periodic ablative nano-grooves to
regular lattices with sub-wavelength periods (90-400 nm). In contrast to the preceding works for various metals, the
period of lattices for titanium decreases with increasing energy density. The formation of the indicated surface
nanostructures is explained by the interference of the electric fields of incident laser radiation and a surface
electromagnetic wave excited by this radiation, as shown by our transient reflectivity measurements and modeling,
because the length of the surface electromagnetic wave for titanium with significant interband absorption decreases
versus increasing electron excitation of the material.
Mechanisms of nonlinear absorption and ionization of pure atomic argon and molecular nitrogen gases by UV femtosecond
laser pulses were studied using photogalvanic and photoacoustic techniques. The effect of the intermediate Rydberg
resonance, its dynamic Stark perturbation and ponderomotive upshift of the first ionization potential of argon
atoms and nitrogen molecules by the intense laser pulses has been revealed by observing an increase of a power slope of
ion yield from three to four at increasing laser intensity.
Generation of free-carrier plasma and filamentation of the ultra-short laser pulse were investigated and modeled.
Experimental results of filamentation are supported by numerical model which takes into account accumulation
of refractive index modifications due to multi-pulse exposure. A contact acoustic monitoring technique was
employed to perform spatially-resolved in situ detection of micro-plasma formation and filamentation of focused
femtosecond laser pulses with critical and sub-critical powers in glass. The recorded acoustic signals reveal freecarrier
generation mechanisms associated with the formation of plasma and filamentation of the propagating
laser pulses. Optical opacity of the plasma region, which sets in at the irradiance of a few kJ/cm3 (close to
the dielectric breakdown threshold) using pulse focusing optics with numerical aperture NA = 0.75, reveals its
critical character, and allows the estimation of acoustic pressure in the ~GPa range. The pressure depended on
the irradiance as P ~ I0.59. In the case of loose focusing (NA = 0.035) filamentation of fs-pulses occurred at
sub-critical plasma density with P ~ I. Detection and interpretation of these acoustic signatures thus enable
real-time in situ monitoring of optical ionization, pulse filamentation in bulk dielectrics under the irradiation by
femtosecond laser pulses.
Subpicosecond, picometer atomic displacements in &agr;-Te photoexcited by single femtosecond laser pulses have been measured by means of time-resolved optical reflectometry revealing threshold-like coherent quantum emission of single softened fully symmetrical optical A1-phonons and demonstrating absolute detection capability of this technique in studies of coherent phonon dynamics in solids.
Direct SEM examination reveals a complex nanoscale structure of deep narrow central channels within shallow wide
external craters produced by single-shot high-intensity femtosecond laser radiation on Corning 0211 glass and sapphire
surfaces. These internal narrow channels are not expected from ordinary surface melt spallation and expulsion processes
characteristic of the external surface nanocraters, but exhibit nearly the same appearance threshold. Surprisingly, the
nanochannel radiuses rapidly saturate versus incident laser intensity indicating bulk rather than surface character of laser
energy deposition, in contrast to the external craters extending versus laser intensity in a regular manner. These facts
may be explained by channeling of electromagnetic radiation by near-surface ablative filamentary propagation of intense
femtosecond laser pulses in the highly electronically excited dielectrics, by spherical aberrations in the surface
layer, or deep drilling of the samples by short-wavelength Bremsstrahlung radiation of relatively hot surface electron-hole
or electron-ion plasma. The double structure of ablated surface nano-features is consistent with similar structures
observed for bulk damage features fabricated by femtosecond laser pulses at supercritical laser powers, but much lower laser intensities.
Optical damage produced by femtosecond pulsed lasers on dielectric surfaces is extremely precise, allowing the damage mechanisms to be inferred from reproducible damage characteristics. Here nanoscale femtosecond laser ablation is applied to probe the ultrafast dynamics of laser energy deposition including the generation and transport of surface electron-hole or electron-ion plasmas. For shallow surface nano-craters fabricated on quartz and glass surfaces by single 0.53 &mgr;m or 1.05 &mgr;m laser shots, their corresponding well-defined laser intensity thresholds demonstrate pronounced effects of laser wavelength, crystalline state of the dielectric and laser waist radius, indicating equal importance of laser energy deposition and transport phenomena during ablation. Simultaneously, unusually deep surface nanoholes emerge drilled by self-focusing laser beam or forward-scattered highly penetrating short-wavelength radiation from the warm, dense surface plasma.
Dry and steam laser cleaning, DLC and SLC, of nano-and micro-contaminant particles from UV/vis opaque and transparent critical substrates has been studied in front-side laser illumination geometry with the help of time-resolved optical techniques and broadband photoacoustic spectroscopy using a nanosecond 10.6-μm TEA CO2-laser and different absorbing energy transfer media (ETM) fluids. Corresponding basic DLC and SLC mechanisms for removal of nano- and micro-particles from opaque and transparent critical substrates as well as parameters of explosive removal of ETM fluids have been determined and discussed.
Sub- and microsecond relaxation dynamics of superheated surface layers of bulk water cavitating at near-spinodal conditions during heating sub-microsecond long TEA CO2 laser pulses were studied using contact broad-band photoacoustic spectroscopy. Characteristic nanosecond pressure-tension cycles representing steam bubble oscillations were recorded by an acoustic transducer and corresponding oscillation frequencies were measured as a function of incident instantaneous laser fluence during the heating laser pulses. Fundamental oscillation frequencies-9-11 MHz-were found to remain nearly constant in a broad laser fluence range, corresponding to bubble diameters close to thickness of the superheated surface layers. Damped nanosecond and microsecond oscillatory pressure-tension cycles recorded by an acoustic transducer are related to oscillations of steam bubbles of different sizes exhibiting strong dissipative losses and collective (coalescence and percolation) phenomena. These observations demonstrate the apparent ultimate thermodynamic limit of superheating for bulk liquids near their liquid-vapor spinode curves and provide an important insight into basic thermodynamic parameters and spatiotemporal scales of explosive liquid/vapor transformations in absorbing fluids ablated by short laser pulses in the thermal confinement regime.
Using a TEA CO2 laser for explosive surface boiling of bulk water, oscillatory acoustic transients from steam bubbles were recorded using a contact photoacoustic technique. Multiple well-resolved, high-amplitude multi-MHz spectral features representing high-order combination acoustic oscillations of steam bubbles were revealed in spectra obtained by means of numerical Fast Fourier Transformation of these transients. A potential parametric generation mechanism for these high-order combination oscillation modes of steam bubbles is discussed.
The photoacoustic (PA) technique has been employed to a number of new biomedical applications based of highly sensitive detection of laser-induced acoustic waves from individual live cells and single absorbing micro-particles or clusters of nanoparticles. These applications involve both linear and non-linear thermoacoustic phenomena initiated by focused nanosecond single laser pulse and detected with a fast PZT-ceramic acoustic transducer. Particularly, we present the following experimental results: 1) monitoring of linear and non-linear PA responses from red blood cells in suspensions in vitro; 2) detection of PA responses from breast cancer cell targeted with gold nanoparticles; 3) PA study of linear and non-linear interaction of laser with colored polystyrene micro-particles as model single absorbers; 4) monitoring of PA responses from moving absorbers in flow in vitro (PA flow cytometry in vitro); 5) recording of PA responses from blood flow in vivo on rat mesentery as animal model (PA flow cytometery in vivo); and 6) monitoring of sedimentation kinetics of particles and cells. The obtained results demonstrate the high sensitivity, low background, simple detection principle, easy data acquisition, and straightforward interpretation of the PA data.
Dry and steam laser cleaning, DLC and SLC, of nano-and micro contaminant particles from UV/vis opaque and transparent critical substrates has been studied in front-side laser illumination geometry with the help of time-resolved optical techniques using a nanosecond IR CO2-laser and different energy transfer media (ETM) fluids. In the case of SLC, microscopic details of particle-ETM-substrate interactions in pre-deposited micron-thick ETM layers have been revealed preliminarily by means of time-resolved optical microscopy. Fundamental DLC and SLC mechanisms for removal of nano- and micro-particles from opaque and transparent critical substrates have been determined. Optimal conditions for nearly complete laser cleaning have been chosen for different combinations of contaminating nano- and micro-particles and substrates.
A lot of work has been done in the area of laser sterilization using UV lasers whereas this area is not much explored using an IR laser. In this study the cells were catapulted from glass or oxidized silicon substrates by a nanosecond IR CO2 laser. Removal of cells and bacteria was achieved under the micron thick liquid layer pre-deposited on the substrates and lifted off together with biological species at laser fluences exceeding the corresponding boiling thresholds for the liquids used. Catapulting with front-side laser illumination is studied
Several types of center-symmetrical (elliptical) microstructures of multi-nanometer heights are fabricated on a surface of quasi-crystalline graphite ablated by single femtosecond laser pulses with peak intensities in the range of 1-102TW/cm2. Potential underlying physical mechanisms for these high-intensity micro-structuring ablative phenomena are discussed.
Various ablation processes including melt expulsion and debris deposition are demonstrated for intense laser ablation of bulk crystalline Si samples and thin copper films at different laser fluences by single tightly and multiple loosely focused laser pulses, respectively. Implications of our results to high-intensity femtosecond laser nano- and micro-machining of solids are discussed.
KEYWORDS: Particles, Silicon, Gas lasers, Stanford Linear Collider, Pulsed laser operation, Microscopes, Carbon monoxide, Liquids, Semiconducting wafers, Energy transfer
Selective laser removal of micro-particles of one chemical composition from their mixture with micro-particles of another chemical type pre-deposited on hydrophobic or hydrophilic surfaces have been demonstrated by means of steam laser cleaning method realized with nanosecond IR laser and various liquid energy transfer media (ETM). Microscopic imaging of particle mixture deposition, ETM dosing and final particle removal has been performed with the help of timeresolved optical microscopy. Optimal ETM/particle combinations for selective targeting and removal of specific
particles from their mixture on the surfaces have been revealed.
Ionization dynamics based on various ionization and carrier-loss mechanisms is theoretically studied for bulk crystalline and amorphous solid dielectrics irradiated by ultrashort (femtosecond) laser pulses. Multi-photon ionization is found to be predominant at low laser intensities (Ilas ∼ 10 TW/cm2) with small contribution of avalanche process, which is more significant for amorphous, rather than crystalline materials. At higher intensities - Ilas ~ 102 TW/cm2 - ionization is considerably enhanced by transient bandgap renormalization due to ultrafast ac-Stark effect and multi-particle interactions in electron-hole plasma, but is strongly damped by Auger recombination for electron-hole plasma densities Ne > 1020 cm-3 and accompanied by microscopic damage of corresponding dielectric materials.
A new mechanism of ultra-deep (up to tens of microns per pulse, sub-mm total hole depths) plasma-assisted ablative drilling of optically opaque and transparent materials by high-power nanosecond lasers proposed by Kudryashov et al. has been studied experimentally using average drilling rate and photoacoustic measurements. In the drilling experiments, average multi-micron crater depth per laser shot and instantaneous recoil pressure of ablated products have been measured as a function of laser energy at constant focusing conditions using optical transmission and contact photo acoustic techniques, respectively. Experimental results of this work support the theoretical explanation of the ultra-deep drilling mechanism as a number of stages including ultra-deep "non-thermal" energy delivery by a short-wavelength radiation of the surface high-temperature ablative plasma, bulk heating and melting of these materials, accompanied by the following subsurface boiling in the melt pool and resulting melt expulsion off of the target.
Coherent band spectrum renormalization in dielectrics by a strong anisotropic electric field of intense laser pulses is considered in terms of dynamic Franz-Keldysh effect (DFKE). Comparing to permanent DFKE shifts of center-zone (Γ-point) non-degenerate band extrema, DFKE band spectrum renormalization of degenerate center-zone and zone-edge band extrema occurs as their simultaneous shifts and splitting. Theory-group analysis is invoked to construct the DFKE Hamiltonian for typical dispersion curves of valence and conduction bands in dielectric materials.
Pulsed laser cleaning of sub- and micron-sized (0.3-10 μm) monodispersed model spherical polystyrene particles and fused silica particles from fused silica and glass optical surfaces was performed by means of ns TEA CO2 laser. Efficient removal of these absorbing particles has been demonstrated in certain laser fluence range, which is below the threshold for ablative damage of the fused silica and glass substrate. Removal mechanisms of dry and steam laser cleaning of various critical optical surfaces are discussed.
Dominating ionization mechanisms are revealed for bulk dielectrics irradiated by ultrashort (femtosecond) laser pulses, analyzing reported in literature well-established and consistent experimental data dealing with dependences of electronhole plasma density on laser intensity, damage thresholds and bulk damage (surface crater) size on dielectric bandgaps. Multi-photon ionization was found to be predominant at low laser intensities (Ilas<Ibreak≈10-50 TW/cm2), while at higher intensities - Ilas>Ibreak - ionization processes are strongly damped by unknown non-linear mechanisms, one of those could be Auger recombination, and are followed by microscopic damage of dielectric materials. In the latter range of Ilas the most accurate source of experimental information on ionization processes is scaling relationships between damage thresholds for corresponding dielectrics and their dielectric bandgaps, fs-laser pulsewidth, wavelength and polarization, as well as spatial size of damage (surface crater) on bandgap width or laser pulse energy. Theoretical analysis of these experimental data and some experimental scaling relationships provides an important insight into interplay of various ionization mechanisms and Auger recombination, supported by results of our numerical calculations of EHP density vs. Ilas, and enables to explain other separate important experimental facts on high-intensity ionization in bulk dielectrics.
Using our previously reported model of femtosecond laser ionization in bulk solid dielectrics, density of point defects in a focal volume of arbitrary solid dielectric is predicted as a function of laser intensity Ilas. This dependence shows a nonlinear increase with an exponent 3S (where S is the minimum number of laser photons to excite a carrier in a dielectric material over its bandgap) below a certain threshold intensity Ibreak and with an exponent S above the threshold. Assuming that these point defects coalesce within the focal volume and the volume of the resulting spherical void is proportional to the total number of point defects generated (the point defect density times the focal volume), theoretical curves of void diameter vs. Ilas are obtained, exhibiting non-linear increase with the exponents S for Ilas<Ibreak and S/3 for Ilas>Ibreak, respectively. Comparison of these theoretical predictions with experimental data on femtosecond laser microstructuring of diamond demonstrates their good agreement and gives a new insight into damage mechanisms of bulk solid dielectrics irradiated by ultrashort laser pulses.
A new mechanism of ultra-deep drilling and related molten material expulsion during high-power short-pulse laser ablation of metals, semiconductors and dielectrics is proposed. In this mechanism ultra-deep (multi-micron) heat penetration and melting depths in these materials are assumed to result from their bulk absorption of thermal short-wavelength con-tinuous and characteristic radiation emitted by hot near-surface ablative laser plasmas. Multi-microsecond delays for expulsion of subsonic jets of micron-size droplets and for re-radiation of UV bursts from the irradiated targets are ex-plained by subsurface explosive boiling in bulk of the resulting ultra-deep melt pool.
Viscous air drag and diffusive Brownian motion result in unfavorable re-deposition of sub-micron spherical particle contaminants after their dry laser-assisted detachment from critical surfaces. Theoretical modeling and experimental results on particle transportation in air and thin variable liquid layers point out particle size and lift-off velocity as the most important parameters for efficient particle removal from critical substrates, while for smaller particles with lower inertia, lower lift-off distances and higher diffusion rates dry laser cleaning is less effective due to their fast diffusive redeposition back on these substrates. Under these circumstances one excellent option is the steam laser cleaning technique, when contaminating particles lift-off together with explosively boiling pre-deposited layer or separate micronsized droplets of a low-boiling liquid energy transfer medium and travel in the hydro- and gas-dynamic flow produced to much higher distances from the substrate irrespectively on particle size. Mechanical coupling of particles to the lifting-off liquid layer or separate droplets occurs via the known "inertial" mechanism and/or new mechanism of "dragging" contaminating particles off the substrate by the liquid environment, demonstrated for the first time in this work. Nearly 100% cleaning efficiencies and no any indication of re-deposition were observed for different particles in single-shot steam laser cleaning experiments. Another disadvantageous aspect of DLC is nearly linear increase of cleaning laser fluence with increasing inverse radius of contaminating particles. This circumstance may result in damage (melt-ing, ripples, ablation) of the critical surface at high laser fluences necessary for removal of smaller (nanometer-size) particles and, thus, imposes serious limitation on the operation range of DLC. Fortunately, SLC technique may be applied in such instances, providing cleaning at quite low cleaning laser fluences, which are shown to be universal over broad particle size range.
The impact effect of a vapor recoil pressure on single-shot threshold-like laser micro-structuring of Si via the Kelvin-
Helmholtz-type instability has been revealed during transient and postmortem studies of explosive boiling of predeposited micron-size water droplets on a Si wafer surface. The micro-structuring mechanism assumes local excitation of surface capillary waves in a molten surface layer of the material underneath each water droplet by near-GPa recoil pressure of explosively boiling water and the following steam bubble oscillations. Water droplets and produced structural features (micron-size craters, bumps or others) have exhibited high symmetries in their surface arrangements. Similar structures were also observed on dry clean Si surfaces irradiated by single fs-laser pulses resulted from the same "impact" excitation of surface capillary waves in the Coulomb explosion regime.
Non-ablative mechanism of femtosecond laser drilling of bulk dielectrics is proposed which is assuming concentrating and condensation of laser-generated vacancies into a void in a center of a laser waist at presence of picosecond photo-excited dense electron-hole plasma. Picosecond diffusive transport of neutral and/or charged vacancies and interstitials in the laser waist is driven by an inhomogeneous transient stress induced in the region via interactions of the dense electron-hole plasma and point defects with center-zone acoustic phonons. Simultaneously, inhomogeneous bandgap renormalization due to a coherent interaction of the plasma with center-zone optical phonons may contribute to the diffusive transport of charged point defects. This mechanism allows estimates of electron-hole plasma densities required for the "mild"
micro-void fabrication regime, kinetics of void growth and their characteristic sizes as well as characteristic dimensions of surrounding laser-affected zones, consistent with known experimental results.
Phenomenological model of photo-elastic effect in bulk dielectrics damaged by femtosecond laser radiation in the regime of direct laser-writing of waveguides is proposed. Positive and negative changes of refraction index due to densification and stretching in laser-written damage tracks inside silica glasses and fluorite, respectively, are explained in terms of laser-induced point defect generation. This model predicts spatial narrowing or broadening of such tracks in silica glass and fluorite bulk samples, respectively, related to spatial point defect distribution due to interactions of carriers with long-wavelength optical and acoustic vibrations as well as due to interactions of point defects with longwavelength acoustic vibrations. Direct laser writing of perspective different contrast microstructures with lower-index boundaries in fluorite and other dielectrics is discussed.
A model experimental "thin variable thickness transparent liquid layer on opaque (absorbing) solid substrate" geometry has been chosen to study explosive boiling and lift-off of a pre-deposited 2-propanol (isopropanol, IPA) layer on a crystalline Si substrate heated by a nanosecond KrF excimer laser using contact photoacoustic and optical plume transmission techniques. The threshold-like transition from the low-amplitude (sub-MPa) bipolar to the intense (near GPa) unipolar temporal shape of acoustic transients has been observed above the ablation threshold of 0.17 J/cm2 ("universal" for micron-thick IPA layers used) due to sub-nanosecond explosive near-critical expansion and boiling in the nanometer-thick liquid IPA sub-layer near the film/substrate interface heated by conduction from the Si substrate. This pressure build-up exceeding many times the critical pressure of 2-propanol (4.8 MPa) exerted a reactive force on the cooler liquid overlay of the IPA layer and accelerated it to lift-off (ablative plume) velocities from several m/s to several tens m/s depending on the film thickness, while the initial expansion velocity of the superheated IPA sub-layer was about 1 km/s. In a good agreement with experimental data, calculations of the IPA/substrate interface temperatures versus laser fluence have shown near-critical state of the IPA sub-layer near the ablation threshold. These data allow testing the general sub-surface explosive boiling mechanism, characteristic for pulsed laser ablation of both opaque and weakly-absorbing materials.
Analysis of processes affecting transient optical absorption and photogeneration of electron-hole plasma in silicon pumped by an intense NIR or visible femtosecond laser pulse has been performed taking into account the most important electron-photon, electron-electron and electron-phonon interactions and, as a result, two main regimes of such laser-matter interaction have been revealed. The first regime is concerned with indirect interband optical absorption in Si, enhanced by a coherent shrinkage of its smallest indirect bandgap due to dynamic Franz-Keldysh effect (DFKE). The second regime takes place due to the critical renormalization of the Si direct bandgap along Λ-axis of its first Brillouin zone because of DFKE and the deformation potential electron-phonon interaction and occurs as intense direct single-photon excitation of electrons into one of the quadruplet of equivalent Λ-valleys in the lowest conduction band, which is split down due to the electron-phonon interaction.
The dynamics of explosive boiling of a 2-propanol layer of variable thickness on a Si substrate heated by a nanosecond KrF excimer laser was studied using a contact photoacoustic technique. The transition from acoustic generation at a free Si boundary to that at a rigid alcohol/Si boundary accompanied by a sharp increase of acoustic generation efficiency was found above a laser fluence threshold of 0.17 J/cm2 and a liquid layer thickness greater than 0.25 μm due to subnanosecond near-critical explosive boiling of the superheated liquid layer near the hot absorbing Si substrate. The gradual increase of the photoacoustic response of the superheated alcohol with increasing thickness of the liquid film at fluences above the explosive boiling threshold was attributed to the fluence- and time-dependent increase of the area undergoing explosive boiling.
The formation of well-defined craters is a general feature of laser ablation with ultrashort laser pulses, indicative of a sharp ablation threshold. Results of a microscopic characterization of ablation craters on semiconductors after irradiation with single intense ultrashort laser pulses are presented.
Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern. Optically sharp surface occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials.
Laser ablation process of the polycrystalline and foam graphite samples was studied using optoacoustic spectroscopy. High defect density of the foam graphite allows observe extremely high ablation efficiencies and their nonmonotonous dependence on laser fluence with many peaks. It seems photochemical multiphoton evaporation process of the matter in the form of large surface relief fragments takes place besides thermal one.
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