Alexander I. Vodchits

Ultrafast excitation of out-of-plane vibrations and vibrational energy redistribution after internal conversion of 4-nitroaniline

Ultrafast vibrational excitation and energy redistribution in the electronic ground state after internal conversion of 4-nitroaniline (PNA) and isotope labeled analogs is investigated by picosecond anti-Stokes resonance Raman spectroscopy. In PNA, PNA-di-15N and PNA-D4, anti-Stokes Raman lines of overtones and/or combination bands of out-of-plane vibrations display risetimes close to the decay time of the electronically excited state of about 0.5 ps and pronounced excess populations. Compared to such fast dynamics, the strongly Raman active totally-symmetric modes show a considerably slower picosecond rise time. Our results indicate primary excitation of out-of-plane vibrations by internal conversion and secondary excitation of strongly Raman active vibrations by redistribution of the vibrational energy.

Barium-nitrate-based Raman lasers excited with LiF:F2-laser and its second harmonic

Raman lasers on barium nitrate crystal pumped with the radiation of nanosecond LiF:F2 laser and its second harmonic have been developed and optimized. As a result, using simple and cheap all-solid-state laser technique the continuously tunable radiation of the first, second, and third Stokes components of stimulated Raman scattering of LiF laser radiation with maximum conversion efficiencies of about 35, 28, and 10%, respectively, was obtained in the spectral range between 1240 and 1800 nm. Using the second harmonic radiation for pumping barium nitrate allowed us to generate the continuously tunable radiation of its first and second Stokes components in 594-682 nm range with maximum conversion efficiencies of approximately 40-45 and 10-15%, respectively.

Thermal lensing in barium nitrate due to stimulated Raman scattering of nanosecond laser pulses

Barium nitrate crystal are studied using one- and two-beam Z-scan techniques by excitation with the second harmonic radiation of nanosecond Nd:YAG laser and probing with the cw He:Ne laser. For the first time, a thermal lens due to the dissipation of energy of the SRS-excited Ag vibrational mode to the heat is observed and measured.

Stimulated Raman scattering in compressed gases by short laser pulses

Stimulated Raman scattering (SRS) excited by picosecond pulses (3.5 - 4 ps) of a synchronously pumped dye laser has been studied in compressed methane, hydrogen and their mixture. Physical energetic SRS-efficiencies (corrected for the linear losses of the optical elements) up to about 55 - 60% and 35 - 37% for the generation of the first vibrational Stokes radiation were reached in methane at a pressure of 60 bar and at excitation wavelengths near 600 nm and 740 nm, respectively. SRS-efficiencies versus pump pulse energy, pressure of gas and temporal duration of laser pulses were studied at 600 nm in methane. A very rich spectrum of Raman lines (including some vibrational, vibrational-rotational and combination Raman lines) was observed in the mixture of methane (35 bar) and hydrogen (25 bar). The energy efficiency of SRS-conversion to the 1-st rotational Stokes Raman line of hydrogen reached about 20% in the mixture. In contrast, the 1-st vibrational Stokes components of hydrogen and methane were substantially suppressed in this mixture. Our measurements demonstrate that methane is one of the most suitable Raman media for obtaining effective SRS-generation especially at pico- and femtosecond excitation because of its suitable parameters controlling the SRS-process and that the mixtures of compressed gases are rather promising Raman media for extending the tuning range of pico- and femtosecond laser systems and for optimizing the efficiencies of SRS-conversion to the different Raman components.

Nonlinear refraction and absorption in vanadate crystals at 532 nm

The nonlinear optical properties of a number of vanadate crystals were studied using a single-beam Z-scan at 532 nm (with a picosecond Nd:YAG laser (YAG: yttrium aluminum garnet)). Both nonlinear refraction indices and nonlinear absorption coefficients were measured for the crystals. A substantial dependence of the nonlinear optical properties of the vanadates on the cuts of the crystals, the orientation of their axes relative to the polarization vector of the laser radiation, and the ligation with active ions was observed.

Laser coagulation of tissues by 1,6 μm and 2 μm laser radiation

The advantages of applications of diode-pumped solid-state laser based on thulium-doped potassium yttrium tungstate and Raman laser with the wavelengths of &lgr;~2 &mgr;m and &lgr;~1.6 &mgr;m correspondingly for local coagulation of surface tissue defects are considered.

Multi-wavelength quasi-continuous wave all-solid-state system based on Raman lasers

High-repetition-pulse-rate nanosecond laser system is developed. It is based on Raman lasers with barium nitrate and KGW crystals. The minimum Raman threshold of laser generation corresponds to only 0.2-0.4 kW of peak pumping power. The laser system generates the radiation at 22 wavelengths in the 280-1600 nm spectral range with average powers from several mW to 1.4 W. The maximum Raman conversion efficiency reaches 40 %. The minimum spectral width of the generated radiation is equal to 0.1 cm-1. This laser system can be used for spectroscopy studies, medicine, and for other applications.

2 kW threshold quasi-cw narrowband all-solid-state Raman laser generating in UV, visible and near IR ranges for spectroscopy

This contribution reports on experimental study of a barium nitrate based Raman laser system pumped with the second harmonic of a quasi-cw Nd:YAG laser. Five Stokes components generated and frequency doubled cover a spectral range of 280-740 nm with an average power from 10 to 800 mW and a spectral width of 0.2 cm in the visible range. Pulsed lasers generating tunable or multiwave radiation have wide applications in Raman spectroscopy. For the successful use, the developed lasers should meet such requirements as continuous tunability or multiwave operation in a wide spectral range, narrow linewidth, sufficient output power, low divergence of the laser beam, simplicity in operation and low cost. All-solid state lasers meet these requirements rather well. Especially Raman lasers using stimulated Raman scattering (SRS) of light in crystals can be promising laser sources for the spectroscopic applications. Recently, we have developed a cheap and simple Raman laser based on the wellknown barium nitrate crystal which can generate nanosecond narrowband (0.25 cm in IR) continuously tunable radiation in the ranges of 190-1800 nm [1,2]. The repetition rate of this laser was equal to 10 Hz. However, for many applications in spectroscopy, especially in Raman spectroscopy, it is necessary to use laser pulses with higher repetition rates, so-called quasi-cw radiation. High repetition rate can substantially improve the conditions of measurement of Raman spectra and shorten the time for this measurement. Recently, some studies have been performed to develop quasi-cw Raman lasers generating radiation in IR and visible ranges [3-6]. In these studies, comparatively high cost diode-pumped Raman lasers were used. To develop a cheap quasi-cw Raman laser and to extend the spectral range of its generation to UV region we have performed our studies on a barium nitrate based Raman laser system pumped with the second harmonic (SH) radiation of quasi-cw flash-lamp-pumped Nd:YAG laser and on frequency shift of Raman laser radiation using the second harmonic generation (SHG). The results of these studies are presented in this report. The optical scheme of the developed laser source is shown in Fig. 1. For pumping the Raman laser the linear polarized SH radiation at 532 nm from a commercially available quasi-cw (1 kHz) Nd:YAG laser with acousto-optic modulation (model LF2210, SOLAR TII) was used. The pumping pulse width was equal to 110 ns (FWHM). The pumping laser beam passed through an optical isolator (half-wave plate, polarizer and quarter-wave plate) to block back-scattered radiation or smoothly change the pump beam power and its polarisation between linear and circular and then the beam was focused with a lens inside a Raman laser resonator. The Raman laser consisted of two spherical mirrors and a barium nitrate crystal of 70 mm length between them. Spherical mirrors were used to compensate partly the thermal lens effect in the crystal due to the dissipation of Raman excitation to heat. Also, the crystal was mounted in a special cage for axial symmetric heat removing. Our previous studies using two-beam time-resolved z-scan in barium nitrate showed that a considerable thermal lens is induced in it due to SRS of nanosecond laser pulses [7]. Using z-scan data and measurement with the help of a collimated He-Ne laser beam propagating through a Raman laser we could determine the optical power of the thermal lens at 1

Raman amplification in barium nitrate studied with focused laser beams

Raman amplification in barium nitrate crystal is studied using focused laser beams for the different amplification regimes and focusing conditions. The realized method of study allows one to observe the saturation of Raman amplification as a valley in the experimental curve. Also, it is possible to determine Raman gain coefficient using the fitting of the experimental dependences.