The excitation of longitudinal and transversal electromagnetic waves in lithium niobate and tantalate crystals is of interest for establish the conditions of coherent longitudinal and transversal waves generation in media and in vacuum. In this paper the results of laser excitation of transversal and longitudinal polar modes in these crystals are presented. We have measured spontaneous Raman spectra of lithium niobate and tantalate crystals in 0° (forward), 90°, and 180° (backward) scattering geometries. We have observed Raman peaks, related to fundamental transversal and longitudinal A1(Z) and E((X,Y) polar optical modes. In addition, there were pseudoscalar symmetry A2 peaks, forbidden by selection rules in Raman spectra for point group C3v. This was explained by reducing of the point group from C3v to C3 due to the presence of impurities in real crystals. Besides, the acoustic biphonon at low frequency has been observed. High intensity of spontaneous A1(Z)LO and A1(Z)TO Raman satellites gives the opportunity for generation of coherent longitudinal and transversal terahertz waves in lithium niobate and tantalate crystals with the help of Stimulated Raman Scattering under using high-power laser pumping. The presence of pseudoscalar and biphonons mode in low frequency region results in the strong interaction with fundamental soft mode and sharp central peak near the phase transition.
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.
Cheap and simple low-threshold quasi-cw (1 kHz) barium nitrate based Raman laser excited with the second harmonic radiation of flash-lamp pumped Nd:YAG laser is developed and studied. Created laser system allows one to generate the radiation of five Stokes components at 563.4, 598.7, 638.7, 684.5, and 737.7 nm, simultaneously. Using special resonator mirrors with optimized reflections average powers of the first and second Stokes components higher than 1 W have been reached. It corresponds to Raman conversion efficiency of 30%. The radiation of the Stokes components is frequency doubled in a KDP crystal and the second harmonic radiations at 281.7, 299.4, 319.4, 342.3, and 368.7 nm are obtained with average powers of 10 mW and higher. Narrowing the spectral width of the generated radiation up to about 0.2 cm-1 in the visible ranage is demonstrated by applying an etalon inside Raman laser resonator. The laser system made can be used for spectroscopic applications and in other field of science and engineering.
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.
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.
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.
The ultrafast relaxation of induced anisotropy in condensed media (organic liquids and their mixtures) is studied by forced light scattering using broadband dye laser radiation with variable spectral width. The influence of the finite spectral width of an exciting laser pulse on the observed relaxation dynamics of the investigated samples (so-called spectral-filter effect) is demonstrated for the first time in transient spectroscopy with incoherent light.
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.
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