Analysis of the kinetic equation for mass transport induced by short laser pulses

The interaction between high-power short-pulse laser radiation and a metal target is considered. An analysis is made of the transport of material atoms from the surface layer into the bulk of a semi-infinite sample under the action of the stress field of a plane shock wave and a temperature gradient. The results of numerical calculations for a laser power density of 10⁹ W/cm² and a pulse length of 30 ns show good agreement with the results of earlier experiments. Pis’ma Zh. Tekh. Fiz. 23, 84–87 (March 12, 1997)     

Spectral intensity and phase changes of short laser pulses by two-photon absorption

We calculated third order non-linear polarization to estimate the two-photon absorption of non-interacting two-level molecules in the transmission-type degenerate pump–probe geometry. The spectral intensity and the phase changes of the laser pulses when passing through a thin dielectric slab composed of the molecules were considered. We also investigated the effect of the decay rate of the molecules and the chirp of the pulses on their spectral intensity and phase changes.     

Mathematical modeling of damage in unidirectional composites

Solutions are developed for the two-dimensional region containing unidirectional fibers embedded in an elastic matrix with an initial flaw in the form of a transverse notch, a rectangular cut-out, and a circular hole. Subsequent damage due to the presence of the flaw is generated by remote stresses acting parallel to the fibers. This work is an extension of the paper by Goree and Gross [1] in which the flaw was taken in the form of a notch (crack) and the subsequent damage, due to loading, consisted of longitudinal matrix yielding and splitting at the end of the notch. The present study accounts for longitudinal matrix damage as in [1] and, in addition, includes transverse matrix and fiber damage in the vicinity of the flaw for the above three initial shapes. The fibers are taken as linearly elastic, the matrix material as elasticperfectly plastic and the classical shear-lag stress displacement assumptions are used. An ultimate stress failure criterion is used for both the fibers and the matrix; simple tension for the fibers and shear failure for the matrix.

For ductile matrix composites (boron/aluminum) the present results indicate that both longitudinal matrix yielding and transverse notch extension must be included in order for the model to agree with experimental results. Interestingly, the extent of the transverse damage region at failure is shown to be approximately constant, independent of the initial flaw shape or length.

Very little difference is found between the results for the three types of initial damage, i.e. the notch, rectangular cut-out and circular hole. In all cases, the presence of additional damage changes the nature of the stress distribution in the unbroken fibers. For the original Hedgepeth[2] problem of a notched laminate the stresses decay as the square root of the distance from the notch tip. Inclusion of longitudinal or transverse damage significantly reduces the maximum stress concentration in the unbroken fibers and gives a much more uniform stress state. It is shown that this behavior cannot be accounted for by introducing an effective notch length or crack tip damage zone with a square root behavior.     

Research on optical damage to sodium chloride by ultrashort laser pulses

Thresholds of optical damage to sodium chloride by ultrashort laser pulses with a duration of about 40 fs are determined. Experiments were carried out using a terawatt titanium–sapphire laser device. p-polarized laser radiation at a wavelength of 800 nm fell on the specimen surface at an angle of 60°. Optical damage to the surface was observed when the critical electric field strength attained 94 MV/cm.     

Frequency doubling of ultrashort laser pulses in biological tissues

Theoretical and experimental studies of second-harmonic generation (SHG) in biological tissues was performed by use of ultrashort laser pulses (<1 ps). A simplified one-dimensional model for the generation and the propagation of frequency-doubled light inside tissue was developed. This model was tested in vitro against measurements of pig and chicken tissue and human tooth. The experimental results indicate that the intensity of SHG varies significantly among tissue types and between test sites in individual tissue. Possibilities of using this nonlinear tissue property in imaging and diagnostics are discussed.     

Features of the mechanoluminescence of thin metal films excited by long and short laser pulses

We have studied the strain-induced luminescence from thin fine-grained films of various metals under the action of short (subnanosecond) and long (millisecond) laser pulses. The possible mechanism of luminescence excitation is considered, according to which the emission is due to the interaction between grain boundary dislocations and impurities occurring in the intergranular region.     

Numerical simulation of the amplification of a short laser pulse by a ytterbium doped amplifier longitudinally pumped by short pump pulses

A simple and original derivation making it possible to analytically compute the performances achievable with an end pumped ytterbium (Yb) amplifier pumped by a pump pulse and seeded by a laser pulse both shorter than the upper-level lifetime is presented. This may be useful when amplifiers or lasers are pumped by a Ti:sapphire laser. The case of a Yb:YAG amplifier is numerically investigated and it is shown that, in these conditions, the resulting efficiency can reach quantum efficiency.     

Optical damage of the surface of sodium and potassium chlorides by femtosecond laser pulses

Optical damage thresholds for the surface of sodium and potassium chloride crystals irradiated by 80-fs laser pulses have been determined. P-polarized terawatt radiation of a chromium-forsterite laser operating at a wavelength of 1240 nm was incident on a sample surface at an angle of 35°. The optical damage of the crystal surface was observed when the electric field strength reached a critical level of 76 MV/cm for NaCl and 64 MV/cm for KCl.     

Optical harmonic generation from animal tissues by the use of picosecond and femtosecond laser pulses

Second- and third-harmonic generations of femtosecond and picosecond laser pulses have been measured from chicken skin, muscle, and fat tissues. The magnitude of the harmonic signals showed a strong structural dependence with the signal from skin interface being the strongest. The polarization dependence of the signal was also measured and found to be consistent with the fact that the tissue samples were highly scattering random media. The second-harmonic- and third-harmonic-generation conversion efficiencies were found to be in the range of ~10(-7) to ~10(-10).     

Above threshold dissociation in HD+ using frequency chirped laser pulses

We have theoretically studied the dynamics of above threshold dissociation (ATD) in molecular ions HD⁺ using frequency chirped femtosecond laser pulses from numerical solutions of the time-dependent Schrödinger equation by using the three-dimensional time-dependent quantum wave packet method. Energy-dependent distributions of ATD fragments are analyzed by an asymptotic-flow expression in momentum space. Linearly positive and negative frequency chirped laser pulses are adopted. It is found that varying frequency chirped parameters can change branching ratios of the 1sσ _g and 2pσ _u dissociations channels. The concept of a light-induced potential is used to interpret the ATD process. The angular resolved energy distributions of the photofragments are also illustrated.     

Effect of pressure on the microstructure of an austenitic stainless steel shock-loaded by very short laser pulses

Irradiation of metallic targets by a high-energy pulsed laser can generate in materials shock waves with pressure amplitudes of the same order as with conventional shocks from explosives, flyer plate impact etc., but with much shorter pulse durations. Experiments were performed with a 0.6 ns pulsed laser on 304 austenitic stainless steel samples. The effects of induced pressure on the microstructure were investigated by transmission electron microscopy in addition to microhardness measurements and are compared with the conventional results. The twin density and the presence of -phase are particularly studied. In spite of the very short pulses, twins were present in the observed areas whatever the pressure, while -phase embryos were only present in the pressure range 15–25 GPa.     

Bulk and surface laser damage of silica by picosecond and nanosecond pulses at 1064 nm

We measured bulk and surface dielectric breakdown thresholds of pure silica for 14 ps and 8 ns pulses of 1064 nm light. The thresholds are sharp and reproducible. For the 8 ns pulses the bulk threshold irradiance is 4.75 +/- 0.25 kW/microm2. The threshold is approximately three times higher for 14 ps pulses. For 8 ns pulses the input surface damage threshold can be made equal to the bulk threshold by applying an alumina or silica surface polish.     

在短脉冲激光作用下薄膜的损伤机制

计算了不同材料的能带带隙、初始电子密度、激光波长和激光脉宽等参效对薄膜的抗激光损伤闭值的影响,研究了在不同脉冲宽度激光作用下多光子离化和雪崩离化两种损伤机制的竞争．结果表明,在以平均电子能量不变为特征的雪崩电离的建立期间,光电离速度影响初始电子的浓度,从而影响雪崩电离和光电离之间的竞争．激光脉冲的宽度越大,雪崩电离对电子发展的贡献越大,而多光子离化的贡献越小．     

Generation of single short tunable UV pulses using a simple short-cavity dye laser

Generation of single, short, tunable UV pulses using a short-cavity dye laser (SCDL) with a novel simple cavity structure is described. A single 80-ps pulse has been obtained at the 616-nm wavelength from a rhodamine 640 dye laser pumped with a 2-ns N2 laser pulse. By amplifying and subsequent frequency-doubling, a 90-ps 308-nm pulse was generated, which can be used as a short-duration XeCl excimer laser source. The temporal characteristics coupled rate equations.     

Surface damage of extreme-ultraviolet gratings exposed to high-energy 20-fs laser pulses

The damage fluences of gratings for diffraction of ultraviolet radiation, which are used in high-order harmonic generation experiments, have been measured with respect to the fundamental laser beam radiation. We have tested gold and platinum coatings of 40- and 50-nm thickness, respectively, deposited onto fused-silica substrates, after irradiation of high-energy, spatially filtered, 20-fs laser pulses at 780 nm. The damage appears at a fluence of ~0.3 J cm(-2) for gold and at a fluence of ~0.4 J cm(-2) for platinum. Scanning electron microscopy of the irradiated regions revealed different damaging mechanisms for the two coatings.     

Experimental study on GaP surface damage threshold induced by a high repetition rate femtosecond laser

The surface damage threshold of undoped bulk <110> GaP induced by a high repetition rate femtosecond pulse at 1040 nm with a duration of 61 fs was studied. The threshold value was obtained by a linear fit of the incident single pulse fluence and was confirmed with a breakdown test around the threshold level. The result will be useful in high intensity, high repetition rate laser applications and ultrafast processes.     

Generation of continuously wavelength-tunable optical short pulses by use of two self-seeded Fabry-Perot laser diodes and an optical switch

Generation of wavelength-tunable optical short pulses by use of two self-seeded Fabry-Perot laser diodes in a parallel configuration is described. The system supports continuous wavelength tuning in a relatively wide range of 42 nm. The side-mode suppression ratio achieved is 35 dB across the entire wavelength tuning range. The system is convenient for continuous wavelength tuning.