Stimulated Raman scattering (SRS) has been observed in more than 100 crystals generating about 2000 different
wavelengths covering the ultraviolet, visible and infrared spectral regions with a mean spacing of 1 nm. Barium nitrate
crystals have been used to demonstrate high Raman shifted output energy up to 156 mJ or high average power of 10 W at
1.197 μm, 1.369 μm and 1.599 μ;m wavelengths with quantum efficiencies of up to 66 %.
Semi-classical theory of Q-switched microchip - lasers with transient and quasi-stationary intracavity Raman conversion
has been developed. Rate wave equations describing generation of Stokes pulses of different orders and their multiwave
mixing have been written and discussed in detail. Theoretical results agree well with experiments for passively Q-switched
microchip - lasers with intracavity Raman conversion in crystals of Ba(NO3)2 and CaMoO4. It is shown, that intracavity Raman conversion in microchip - lasers represents a simple and effective method of generation of short
Stokes pulses with duration as short as 100 ps, energy in the &mgr;J-range and peak power of up to several tens of kW.
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.
The results of further detailed investigations of passive Q-switch Raman microchip lasers based on Nd:LSB crystal with Ba(NO3)2, BaWO4 and KGd(WO4)2 crystals as intracavity Raman media are presented. It is shown that intracavity stimulated Raman scattering (SRS) in microchip lasers is very simple and efficient method for generation of high power pulses with duration comparable to ones reaching under more technically complicated mode-locking regime. Modeling output energy parameters and emission kinetics of Nd:LSB microchip laser with intracavity SRS on the base of enhanced theoretical model of Q-switch Raman microchip lasers operation taking into account cross-section intensity distribution of pump, laser and Stokes modes, thermalization processes of activator ions on upper and lower multiplet levels and features of saturable absorber intracavity bleaching at spatially nonhomogeneous laser mode has shown good agreement with experimental results.
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.
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.
A new theoretical model of SRS for a Bessel pump beam is presented. The experimentally observed regularities of SRS, concerning the formation of output patterns an nearly diffraction-limited axial Stokes beam are explained within the framework of this model. The model represents the Stokes radiation as a superposition of partial Bessel modes if a cylindrical waveguide is formed by a pump beam.
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.
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.
The effective nonlinear coefficients, tuning curves, angular and spectral phase-matching widths are calculated for all possible types of optical parametric generation in the principal planes of biaxial KTP crystal at Nd:YAG ((lambda) p equals 1.064 micrometers ) laser pumping. The plots presented enable the maximal nonmonochromaticity and radiation divergence to be found for any value of the signal wave and different interaction types. We developed a numerical simulation of nonstationary optical parametric generation by solving the cut-off system of coupled equations, which describes the three wave interaction in oscillator cavity, partially filled with KTP crystal. The dependencies obtained demonstrate the dynamics of signal pulse generation development. The optimal crystal and resonator parameters were found to ensure the maximal optical parametric oscillator efficiency.
The pair correlations between the instantaneous intensities of input pump superbroadband (250 cm-1) laser radiation with femtosecond and picosecond noise structures, output depleted pump radiation and Stokes one generated through stimulated Raman scattering in compressed hydrogen have been experimentally studied under the weak and strong pump depletion by means of time-delayed four-wave mixing in Kerr-shutter configuration. Experimental results show that the Stokes radiation is well correlated with the input pump one up to the energy conversion efficiency of about 30%. Considerable suppression of the femtosecond amplitude fluctuations in the depleted pump beam has been observed upon increasing the pump depletion that indicates the possible way for preparing superbroadband light with predominant phase fluctuations using SRS. The picosecond part of cross-correlation function between the depleted pump and Stokes radiation flows has very asymmetric shape that is probably caused by the transiency in the scattering of picosecond noise spikes and by the fact that these spikes in Stokes and depleted pump radiation flows have opposite temporal shapes.
The calculations and experimental investigation of spontaneous scattering quantum noise influence on the statistics of energy backward SRS at the pump radiation focusing have been fulfilled. The analysis of the calculation results shows that the narrow region of pump power exists where bistable regime of BSRS conversion takes place.
We have constructed, theoretically described and studied characteristics of plane-parallel geometry KTP optical parametric oscillator for conversion Nd:YAG laser radiation into eye- safe region (1.57 micrometers ). 50% conversion efficiency and 12.5 mJ generation energy have been attained.
The application of two-color incoherent cross-correlation spectroscopy to study ultrafast processes in organic liquids and solutions is discussed. This kind of time-resolved spectroscopy is based on using two broadband correlated pulses of long duration with different central frequencies. Its time resolution is determined by the cross-correlation time of the intensities of used radiations. The statistical properties of the radiations have been investigated. The developed technique allowed us to perform kinetic measurements with femtosecond resolution on the ordinary nanosecond laser spectrometer modifying it in a comparatively simple way. The present technique was demonstrated and tested by studying subpicosecond Kerr dynamics in a number of organic liquids and their mixtures. An approach is also proposed to study the population relaxation of electronic and vibronic states of organic molecules including non-luminescent ones.
In the last few years radiation sources on the basis of stimulated Raman scattering (SRS) have been sufficiently widely used in spectroscopy. Of great importance therein are high efficiency of radiation conversion to one, usually first Stokes (FSC) or anti-Stokes (FASC) component of SRS, small divergency of converted radiation flux, stability of reproduction of its energy and time characteristics. SRS is a process of amplification of quantum noises. Therefore, quantum noise is one of the sources of fluctuation of temporal, energy, and spectral parameters of converted light. The stability of SRS radiation pulses is strongly influenced by fluctuations of laser radiation parameters. The design features of Raman shifters and properties of scattering medium influence SRS efficiency and converted flux divergency. As a result, fluctuations of converted radiation parameters can reach dozens percent, radiation divergency can be about 10-2 rad, and efficiency of SRS-conversion to one component doesn't usually exceed 10 divided by 20%. In our paper results on experimental study of SRS regimes and geometrical factors influence on statistical properties (fluctuations of energy and instantaneous intensity) of pulses of forward, backward FSC, FASC of SRS and depleted pump as well as on efficiency of SRS process are briefly presented. On the basis of the investigations made we discuss the ways of creating SRS-based radiation sources with high quantum efficiencies (>= 70%) of conversion into FSC, close to diffraction divergency (approximately equals 0.5 mrad) of converted flux and high stability (variation coefficient <EQ 6%) of energy parameters
The application is discussed of saturation resonance Raman technique with nanosecond lasers both in spontaneous and coherent regimes to the investigations of the excited states of metalloporphyrins. It is shown that saturation technique enables us to obtain new information about transient states, additional to the data of direct kinetic measurements. For nickel octaethylporphyrin (OEP) complex, we present spectroscopic evidences for the population of the higher excited electronic state, presumably of the 1B1g origin. For CuOEP dissolved in a number of oxygen (O)-containing solvents, the existence of fast channel of relaxation via the excited charge-transfer (CT) state is also proven on the basis of saturation Raman data. To explain the observed transformations in transient Raman spectra under variation of excitation power, simulations of the population redistribution processes in both systems are performed using quasi-stationary rate equations approach. The advantages of resonance coherent anti-Stokes Raman scattering (RCARS) over spontaneous resonance Raman (RR) spectroscopy is proven unambiguously in such type of investigations.
The steady interest in metalloporphyrins (Me-P) which have a central metal with an unfilled d-electronic shell
is due to the rich variety of photoinduced physico-chemical processes they participate in, and by the possibility of
modelling on the basis of them, the behavior of related native biological complexes.
Me-Ps with an unfilled d-shell of the central atom have typical two-band absorption spectra in the ground electronic
state: intense Soret or B band in the near UV and less intense Q band in the visible. According to Gouterman's fourorbital
model,' the Me-P absorption spectrum arises from the promotion of ir-electrons from the highest occupied
molecular orbitals a, , a2 to the lowest unoccupied orbitals e of the porphyrin conjugated macrocycle. The a,
and a2 orbitals are quasi-degenerate and the electron excited configurations e have the same symmetry. That is
why there is a strong mixing between the two orbital excitations a, -e and a, - e due to the configurational
interaction via the electron-electron repulsion, resulting in a two-band absorption spectrum.
The effect of the central atom is interpreted in this model as a perturbation of the ir-states of the porphyrin ring.
The perturbation is largely determined by the electrons of the unfilled shells of the metal ion (e.g. the md'-shell).
This effect is quite pronounced for the ground electronic state, nevertheless the interactions of excited ir-electronic
configurations with d-electron ones cause more dramatic changes in the spectroscopic properties of Me-P excited
states. New low-lying excited states are possible: the charge-transfer (CT) levels related to the electrodensity
transfering from the ring a, , a2 to the d-orbitals of central metal (ir-d states) or from the d-orbitals to the ring
e(d - ir states), and (d - d) levels bound up with the d-electron excitations. The present paper deals with the
investigations of porphyrin metallocomplexes with nickel (Ni-P) and copper (Cu-P) having electron configurations
3d8 and 3d9 , respectively.
The steady interest to the metalloporphyrins having the central metal with unfilled d-electronic shell is caused
by the rich variety of photoinduced chemical-physical processes they participate in and by the possibility to model
on their basis the behavior of native biological systems.
From the photophysical point of view, copper porphyrins (Cu-P) have two distinct peculiarities, associated with
the presence of a Cu ion:
1) The unpaired electron to the d2_2 orbital of copper (II) ion couples with the normal porphyrin (ir, ir*) excited
states, this "allows" the nonradiative transition 2S ...+ 2T1 with the rate constant of over 1013 1 resulting in the
absence of fluorescence from the Si state.
2) A charge-transfer (CT) state lies close in energy above T1 in noncoordinating solvents (benzene, toluene) and
is thermally accessible, that causes the relatively short relaxation time from T1 (10-100 ns) at room temperature.
The processes of formation of complexes between Cu-P in the ground and excited T1 states and nitrogen-
containing molecules (pyridine, piperidine) have been studied by picosecond absorption spectroscopy.' We have
observed for the first time the phenomenon of the oxygen-containing solvents influence on the energy relaxation
processes in excited Cu-P. The comprehensive study of this effect is presented.
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