D. S. Chunaev

Picosecond stimulated Raman scattering in crystals

The comparative values of the peak and integral cross sections of spontaneous Raman scattering and the optical dephasing time of molecular vibrations were determined for several oxide crystals by spontaneous Raman spectroscopy. The spectral, time, and energy parameters of stimulated Raman scattering (SRS) were measured for ten crystals using picosecond YLF: Nd laser pumping with a radiation wavelength of 1047 nm. An analysis of the experimental dependence of the threshold energy of pumping SRS on the integral and peak cross sections of spontaneous Raman scattering showed that the SRS gain increment explicitly depended on the integral cross section and was independent of the peak cross section of spontaneous Raman scattering as the ratio between the pumping pulse width (11 ps) and the time of optical dephasing of molecular vibrations changed from 0.42 to 9.3. The gain coefficients of steady-state stimulated Raman scattering under threshold stimulated Raman scattering conditions were determined for all the crystals studied on the basis of the measured threshold SRS pumping energies, the duration and width of the spectrum of pulses, the nonlinear interaction length, the intensity of pumping, and the theoretical dependences that relate the steady-state and transient SRS gain increments. The steady-state SRS gain coefficients obtained in this work fitted well a linear dependence on the peak cross sections of spontaneous Raman scattering, which substantiated the correctness of our analysis and measurements.

Temporal characteristics of picosecond stimulated raman scattering in oxide crystals

The temporal characteristics of stimulated Raman scattering (SRS) under 22-ps laser excitation were studied in eight oxide crystals with a T2 optical-phonon dephasing time variable by up to two orders of magnitude. The measured SRS pulse width was shortened from 13.8 ps for Ba(NO3)2 (T2 = 26.5 ps) to 4 ps for the LiNbO3 (T2 = 0.38 ps) crystal. The dependence of SRS pulse width on the dephasing time was analyzed in the framework of the known SRS theory.

Two-photon absorption of high-power picosecond pulses in PbWO4, ZnWO4, PbMoO4, and CaMoO4 crystals

The nonlinear process of two-photon interband absorption is studied in tungstate and molybdate oxide crystals excited by a sequence of high-power picosecond pulses with a wavelength of 523.5 nm. The transmission of the crystals is measured for the excitation pulse intensity up to 100 GW/cm2. The pulse intensity in the crystals initially transparent at a wavelength of 523.5 nm is strongly limited due to two-photon absorption (TPA), and the reciprocal transmission in PbWO4 and ZnWO4 crystals reaches 50–60. In all crystals, TPA induces long-lived one-photon absorption, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the laser excitation intensity. Absorption dichroism manifests itself in a significant difference in the transmission intensities when the principal orthogonal optical axes of the crystals are excited. The TPA coefficients are determined during the excitation of two optical axes of the crystals. TPA coefficients β for the crystals vary over a wide range, namely, from β = 2.4 cm/GW for PbMoO4 to β = 0.14 cm/GW for CaMoO4, and the values of β can differ almost threefold when different optical axes of a crystal are excited. Good agreement is achieved between the measured intensities limited by TPA and the estimates calculated from the measured nonlinear coefficients. Stimulated Raman scattering (SRS) upon excitation at a wavelength of 523.5 nm is only detected in two of the four crystals under study. The experimental results make it possible to explain the suppression of SRS by its competition with TPA, and the measured nonlinear coefficients are used to estimate this suppression.

Nonstationary Raman amplification of superluminescence in crystals

The sequential excitation of LiF crystals with F2− color centers and KGd(WO4)2 crystals by 1047-nm laser pulses with a duration of 22 ps is investigated. Broadband (275 cm−1) Stokes superluminescence appearing in the fluoride crystal undergoes nonstationary Raman amplification in the oxide crystal. The amplified pulses with a Stokes frequency shift of 767 cm−1, a duration of 7 ps, and a power up to ∼1.1 MW have high spatial and time coherence and exhibit time delay increasing from ∼1 to 4.5 ns with a decreasing pump power. The results are discussed in terms of the cooperative Raman scattering.

Control of radiation spatial coherence under synchronous amplification in crystalline LiF:F2− amplifier

A technique of spatial coherence control, based on the synchronous amplification of a radiation in LiF crystals with F2− color centers, is demonstrated. Spatial radiation distributions of stimulated Raman scattering (SRS) in oxide crystals were investigated under picosecond laser excitation. Low spatial radiation coherence was revealed for both the transient and quasi-stationary SRS. Spatially incoherent SRS was transformed to spatially coherent radiation as a result of phase—locked picosecond synchronous laser pumping of nonlinear Raman and LiF: F2− crystals and the Stokes radiation amplification in the color center crystal.

Two-photon absorption in arsenic sulfide glasses

The two-photon absorption coefficient of light in chalcogenide glass has been measured. CW probe radiation has been used to observe the linear absorption in glass induced by two-photon excitation. The induced absorption lifetime was found to be .

Two-photon absorption in SiO{sub 2}- and (SiO{sub 2} + GeO{sub 2})-based fibres at a wavelength of 349 nm

The nonlinear two-photon light absorption coefficients have been measured in an optical fibre with a quartz glass (SiO2) core and in a fibre with a germanosilicate glass (SiO2 + GeO2) core. The two-photon absorption coefficient β measured at a wavelength of 349 nm in the (SiO2 + GeO2)-based fibre (13.7 cm TW-1) multiply exceeds that for the pure quartz glass optical fibre (0.54 cm TW-1).