Study On The Raman Scattering Properties Of KDP And DKDP Crystals

Potassium dihydrogen phosphate KH2PO4 (KDP) and its deuterated analog K(H1-xDx)2PO4 (DKDP) are the most popular nonlinear optical crystals possessing large nonlinear optical coefficient and high laser damage threshold, which has been widely used as frequency conversion material. In addition, KDP and DKDP are also excellent piezoelectric, eletro-optical crystal materials. In recent years, intertial confinement fusion (ICF) project has been drawn attention to many countries. The ICF is a kind of heat nuclear explosion that can be controlled in the laboratory, which provide a new routine to generate environmentally energy in the future. At present, KDP and DKDP crystal is only one crystal that could meet the requirement of ICF research for its excellent optical property and the ability to grow large size. In ICF system, KDP and DKDP crystal is used as frequency conversion and electro-optical switch materials.In ICF system, as high fluence laser light propagates through large aperture KDP and DKDP crystals, the coupling of spontaneous Raman scattering light with the pump field stimulates the spontaneous Raman scattering and the stimulated Raman scattering will experience high gain across the transverse dimension of crystals, i. e. transverse Raman stimulated scattering (TSRS). TSRS can lead to not only the energy loss from the main beam, but also optical damage in the components. The TSRS effect has limited the improvement of the laser energy. A series of measures are taken at home and abroad to depress the TSRS effect. It is well-known that substituting DKDP crystal for KDP crystal is an effective way to reduce the TSRS gain coefficient in the system. However, the relation between the dueteration degree and the TSRS gain coefficient is not clear, which make it difficult to get the best dueteration degree of DKDP crystal to suppress the TSRS effect. Furthermore, under much higher fluence laser light, the TSRS in DKDP crystal is also notable and the growth of large-scale DKDP crystal is difficult and expensive. Thus, it is meaningful to continue the research on TSRS effect.The stimulated Raman scattering (SRS) originates from the spontaneous Raman scattering and the gain coefficient of TSRS can be estimated by the spontaneous Raman spectrum, which indicate that the SRS is closely related to the spontaneous Raman scattering. In this paper, we theoretically and experimentally study the Raman scattering property of KDP (DKDP) crystal. We firstly classified and assigned the vibration modes of KDP crystal by the factor group theory in detail. In addition, the stimulated Raman scattering (SRS) of KDP crystal was realized by an ultra-cavity single-pass configuration, and the TSRS effect in the third harmonic generation (THG) is well analyzed. The influences of growth method, impurity ions, deuteration degree and temperature on the Raman scattering of KDP/DKDP crystal are well investigated. The main contents of the paper are as follows:1. By the factor group analysis, we firstly get the KDP vibration modes: T= 4A1+5A2+6B1+7B2+13E and classified the modes by the position symmetry analysis in detail. Besides, the angular dependence of the A1 mode in the backward Raman scattering is verify. The angular dependence of the A1 mode indicate that the symmetry of KDP crystal is practically lower than D2d. What is more, the Raman spectra of annealed and unannealed KDP crystal show no obvious difference. Therefore, the angular dependence of the A1 mode is determined by crystal structure and independent of lattice internal stress.The polarized Raman spectra of KDP and DKDP crystals were measured, and the Raman peaks of the spectra are well assigned. Though the observed spectra exist several contradictions with the selection rules, the spectra satisfy the Raman selection rules of D2d basically. 2. The dependence of the SRS gain coefficient on the excitation wavelength and scattering geometry is analyzed according to the theory of stimulated Raman scattering. The SRS of KDP crystal was realized by an ultra-cavity single-pass configuration and the pump source is 532 nm picoseconds laser. Three Stokes lines of the v1 mode (559.43 nm,589.74 nm,623.5 nm) were observed due to its larger SRS gain coefficient. What is more, with the increasing of the order of Raman scattering, the scattering intensity decrease, and the pump threshold display an nonlinear increasing with the order of Raman scattering. Hence, the TSRS of KDP crystal in high power laser system is mainly considered in the first-order SRS of the v1 mode.The SRS gian is related to the excitation wavelength and scattering geometry, which lead to the appearence of several kinds of TSRS in the process of third harmonic generation. In this paper, we quantitatively compared the TSRS effect in the third harmonic generation containing type Ⅰ and type Ⅱ KDP crystals. It is found that at high third frequency convention efficiency and fluence incidence, the TSRS effect in tripling KDP crystal will be severe especially the 3ω pumped TSRS. Once obtained the conversion efficiencies in the third harmonic generation, we can predict the intensity of different kinds of TSRS in situ according to the result and take measures to reduce the probability of laser damage.3. We investigate the influences of gowth method (traditional growth method and rapid growth method), thermal anneal and impurity ions (Cr3+ and Ni2+ ions) on the Raman spectrum and SRS of KDP crystal.The result indicates that these factors have different level of effects on the optical properties (transmittance and light scatters), but have no obvious impact on the Raman spectrum and SRS gain coefficient of KDP crystal. The result indicated that theses factors have different effect on the optical properties of crystal (transmittance and light scatters), but have no obvious influence on the Raman spectrum and SRS gain coefficient of the v1 mode. Due to the SRS is related to the optical properties of crystal (transmittance and light scatters), the gowth method, thermal anneal and impurity ions will influence the TSRS effect of large-scale KDP crystal in the high power laser system.4. The Raman spectra of DKDP crystals with different deuterium concentrations are measured in the X(YY)-X and X(ZZ)-X scattering geometries. With the deuterium concentration increasing, the v1 mode shifts to lower wavenumbers and the shifts in the X{ZZ)-X scattering geometry is larger than that in the X(YY)-X scattering geometry, full width at half maximum (FWHM) increases firstly then decreases, but the peak intensity displays an opposite isotopic effect to the FWHM. According to the coupling theory for three modes, we find that the isotopic Raman shift of the vi mode is the result of the coupling of the v1 mode with the δ(OD) mode combined with the v1and v3 modes coupling, of which the v1/δ(OD) modes coupling play the leading role. Besides, basing on the change of the crystal structure induced by the replacement of H atoms by D atoms in DKDP crystal, we establish the model of scattering superposition for DKDP crystal. According to the model, we calculate the Raman spectra of DKDP crystals which is consistent with the measure results and the variations of FWHM and peak intensity with deuterium concentration are quantitatively explained. And it is confirmed that the isotopic effect of the peak intensity is mainly due to the variation of FWHM with dueteration degree.Using the Raman scattering from water as a reference, the TSRS gain coefficients of DKDP crystals with different deuteration degrees are derived and the measuring error is 15%. It is found that as the deuterium concentration in DKDP crystal increasing, the TSRS gain coefficient decreases to about 40.1% of KDP crystal firstly then increases to about 68.9%. The relation between the Raman shift and the TSRS gain coefficient is established, which favors the measure of TSRS gain coefficient and its homogeneity in situ. The depencence of TSRS gain coefficient on deuteration degree is ascribe to the variation of FWHM with deuteration degree, which indicates that we can take measures to enlarge the FWHM of the v1 mode to depress its TSRS effect. In addition, basing on the analysis of TSRS effect in the chapeter 3, we get the relation between the threshold intensity of TSRS effect in the third harmonic generation and the duteration degree of DKDP crystal.5. Using backward Raman scattering device, we measured the Raman spectra of the v1 mode in KDP and DKDP crystals over the temperature range from 285.3 K to 345.2 K. And the temperature dependence of the Raman shift, FWHM, intensity and SRS gain coefficient were well analyzed.The result reveals that with temperature increasing the v1 mode displays a red-shift and the linewidth broadens in KDP crystal, but the influence of temperature on the Raman scattering of DKDP crystal is related to its deuteration degree. For the DKDP crystal with lower and higher different deuterium concentrations, the variation with temperature of Raman shift and FWHM is similar to that of KDP crystal. But the for other DKDP crystals the temperature dependence of Raman shift and FWHM fluctuates a lot, which is ascribe to the variation of O-D bond with temperature. The scattering intensity of the v1 mode in KDP and DKDP crystal shows no obvious tendency. But the SRS gain coefficient of the v1 mode tended to decrease at higher temperature. For KDP crystal, the gain cofficient at 345.2 K decreases about 12% comparing to 285.3 K. Thus, the use of KDP and DKDP crystals at higher temperature can reduce the SRS intensity in high fluence laser systems to some extent.