- Email: [email protected]
Raman spectroscopy of crystals for stimulated Raman scattering
March 1999
Optical Materials 11 (1999) 307±314
Raman spectroscopy of crystals for stimulated Raman scattering T.T. Basiev a
a,* ,
A.A. Sobol a, P.G. Zverev a, L.I. Ivleva a, V.V. Osiko a, R.C. Powell b,1
Laser Materials and Technology Research Center of GPI, Vavilov Street, 38, Moscow 117942, Russian Federation b Optical Sciences Center, University of Arizona, Tucson, AZ 85721-0094, USA
Abstract Raman frequency shift, line width, integral and peak Raman scattering cross sections were measured in various crystals using spontaneous Raman spectroscopy. The highest Raman gain coecient in steady state Stimulated Raman Scattering (SRS) regime was proved to be in barium nitrate crystal; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium molybdate crystal is proposed as a new highly ecient Raman material. Ó 1999 Elsevier Science B.V. All rights reserved. OCIS: 300.6450; 290.5910; 190.2640
1. Introduction In 1963 Eckhardt et al. [1] described the ®rst observation of stimulated Raman scattering (SRS) in diamond, calcite and a-sulfur single crystals. Since then SRS in solids has become an area of intensive investigation in laser physics. For almost 20 years the natural calcite crystal was the only one solid state material practically and widely used for investigations of SRS due to its high Raman gain, comparatively large size and low cost. The energy of its intense SRS-active vibronic mode is 1086 cmÿ1 and the line width is about 1.2 cmÿ1 [2,3]. In 1980 a number of new synthetic crystals, barium, sodium, and lead nitrates were proposed
* 1
Corresponding author. E-mail: [email protected]. E-mail: [email protected].
as perspective Raman materials [4]. These crystals have intense vibronic modes with the energy of about 1050 cmÿ1 , which correspond to the internal symmetrical vibrations of [NO3 ] molecular ions. The number of various laser con®gurations has shown that the quantum conversion eciency of Ba(NO3 )2 SRS shifter can be up to 80% for nanosecond laser pulses even for IR radiation with much lower gain [5±7]. This has led to the availability of commercialization of solid state Raman lasers with special properties such as high eciency and diraction-limited beam quality at eyesafe wavelengths [8]. However, the SRS threshold for 25 ps pump pulses was found to be 10 times higher than that for nanosecond pulses due to long vibronic relaxation time [9]. This transient SRS was accompanied by wide angular scattering to higher Stokes and anti-Stokes components, that interfered in Ba(NO3 )2 crystal application for Raman lasers pumped by picosecond pulses.
0925-3467/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 8 ) 0 0 0 3 0 - 5
308
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
The diamond crystal has larger Raman frequency shift 1332 cmÿ1 and high gain [1]. But due to its small dimensions and high cost it has not found many practical applications as a Raman material. In 1985 it was found [10,11] that potassium gadolinium tungstate, KGd(WO4 )2 exhibited ecient SRS properties. In spite of the fact that its Raman gain coecient at 1064 nm (6 cm/GW) is twice less than that in barium nitrate, the KGW crystal found many practical applications for frequency shifting of picosecond laser radiation [10,11]. Variations in the Raman scattering cross section and Raman mode line width are responsible for the dierence in the behavior of the above SRS materials in steady state and transient regimes. The comparative investigation of SRS-active vibronic modes in some solids using spontaneous Raman spectroscopy is presented in this paper. The results are important to understand the Raman properties of crystals and to search new nonlinear materials with certain frequency shift and high SRS eciency. The theory of SRS process is well developed and can be found elsewhere [12±14]. Here we would like to outline that depending on the pump laser pulse duration (sp ), two temporal cases can be considered. The ®rst one is a steady state regime, when the pump pulse duration sp is much longer than the vibronic Raman mode dephasing time TR sp TR . The second case is a transient regime, when the pump pulse duration is smaller than the dephasing time sp TR and spectral width of pump laser Dmp is much broader than the Raman line homogeneous broadening ÿ1 DXR pcTR Dmp DXR : In the steady state case if there is no pump depletion, the solution for the Stokes intensity is exponential [15]: IS l IS 0 expfgSS Ip lg:
1
Here IS 0 and IS l are Stokes intensities at the input and output of the Raman medium, Ip is the pump intensity and l the length of the nonlinear crystal. The gSS is a steady state Raman gain coecient which is determined by the Raman properties of nonlinear medium [15]:
gSS
kp k2S N hcpn2S DXR
dr ; dX
2
where N is the number of scattering centers, kp , kS are the pump and Stokes wavelengths, nS , refractive index at kS and dr=dX is the Raman scattering cross section. Thus, we can note that in the steady state case the Raman gain coecient gSS is linearly proportional to the Raman scattering cross section and inversely proportional to the linewidth of Raman transition. The Raman linewidth is determined by a mechanism of vibronic phase relaxation due to the phonon±phonon coupling in the medium and has a strong temperature dependence ÿ1 [16]. The value of dr=dX DXR can be characterized as a peak intensity in the measured spontaneous Raman scattering spectrum and for simplicity we will denote it as ÿ1
Rpeak dr=dX DXR : In the transient case when sp TR and the spectral width of pump laser Dmp is broader than the Raman line width DXR Dmp DXR ; with no pump depletion the analytical expression for the Stokes intensity for large ampli®cation gain was found in Ref. [14]. Following Ref. [15] we can present it as sp IS l / IS 0 exp ÿ TR ( 1 ) kp k2S N dr 2 : 3 exp 2 Ip sp l hn2S dX From Eq. (3) one can see that the Raman gain is proportional to the square root of the multiplication of the pump pulse energy Ip á sp , the crystal length l, and the total integral Raman scattering cross section (dr/dX), but does not depend on the Raman linewidth DXR . Under certain assumptions, the integral Raman scattering cross section can be characterized as an integral value of the Raman line intensity in spontaneous Raman scattering spectrum and we denoted it as Rint dr=dX. Due to variations in crystallographic structure dierent crystals have rather dierent Raman spectra. For SRS laser and shifter development they can be interesting due to changes in SRS
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
309
frequency shift, various values of Raman scattering cross section and line width exhibiting dierent Raman gain. At the same time there can be other nonlinear processes, like pump beam self-focusing, Brillouin scattering, and second harmonic generation, which can be competitive with SRS and restrict applications of this certain material for Raman lasers. Below we will compare only integral and peak Raman scattering intensities Rint and Rpeak for various prospective Raman materials in order to characterize their SRS features for the Raman laser development.
linewidth was determined by the spectral resolution of the system and the widths of the slits. Except narrow lines (DXR < 1 cmÿ1 ), the spectral resolution was about 10% of the measured linewidth value. The peak cross section value Rpeak was measured with the accuracy of 10% for broad lines, while for narrow lines it became higher (20%) due to spectral resolution limitation. While the accuracy for Rint was about 10% for narrow lines and increased to 20±40% due to uncertainty of the spectrum background level.
2. Experimental setup
3. Results and discussions
The experimental setup for spontaneous Raman spectroscopy utilized the excitation by a CW argon ion laser (k 488 nm) and registration by doublespectrometer ``Spex-1403''. A premonochromator was used to cut plasma discharge lines of the laser. The laser intensity at the sample was about 1 W. Backwards scattering scheme was used to increase signal to noise ratio. To compensate the self-polarization of spectrometer, a polarization scrambler was placed in front of the entrance slit. The spectral resolution of the system was 0.2 cmÿ1 . Single crystal as well as polycrystal samples were investigated in our experiments. To compare integral and peak scattering cross sections in different materials the plane-parallel samples were made from single crystals with the thickness either 0.7 or 2 mm. An available diamond single crystal had slope facets and 0.7 mm thickness. The exciting laser intensity and spectrometer slit width had constant values in dierent sets of measurements, the focusing of exciting radiation and the collimating of scattered light were the same in all the experiments. Since the Raman scattering was excited by a polarized beam, for some samples for certain propagation direction (K) we made several experiments with dierent polarization of the excitation light (E) with respect to crystallographic axes. The registered Raman spectra were analyzed with software ®tting program which allowed to obtain values for Raman line width, peak and integral cross sections. The accuracy of the Raman
Among solid state materials which exhibit remarkable SRS properties one can outline ``simple'' crystals which are composed of one or two elements: diamond (C), silicon (Si), and quartz (SiO2 ). These crystals have a covalent type of bonding which allow them to exhibit intense lines in Raman spectra. The diamond crystal is the example of highly packed covalent structure in which each carbon atom is surrounded by four neighbors located in the corners of tetrahedron. There is only one very intense Raman line of F2g symmetry at 1332 cmÿ1 for this crystal. Another type of SRS-active material is a group of molecular ionic crystals. They are composed of a cation atom and an anion complex like [XO3 ] for X N, C, Cl, Br, I atoms or [YO4 ] for Y Si, Ge, W, P, S, Mo, Nb atoms. There are ionic bonds between a cation and molecular group in such crystals, while covalent bonds dominate inside the molecular complexes. The intense Raman modes in these crystals correspond to the symmetrical internal valent vibrations inside molecular [XO3 ] or [YO4 ] complexes. Let us consider in detail the tungstate crystals. All tungstate crystals investigated can be divided into two groups: simple crystals with scheelite structure of C64h space symmetry, and tungstates with monoclinic C62h structure. Raman spectra of simple calcium, strontium and barium tungstate crystals are shown in Fig. 1. Each has one very intense A1g line about 910±925 cmÿ1 which corresponds to the internal symmetrical valent
310
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
Fig. 1. Unpolarized spontaneous Raman spectra of tungstate crystals with scheelite structure: CaWO4 and BaWO4 single crystals, and SrWO4 powdered sample. The geometry of excitation was K^C4 , EkC4 .
vibration in [WO4 ] tetrahedron group. Table 1 shows data on internal Raman frequency shift and Raman mode linewidth in these crystals together with maximum frequency of external lattice mode. From Table 1 it follows that the increase of cation radius in the row Ca2 ! Sr2 ! Ba2 reTable 1 Frequencies XR and linewidths DXR of SRS-active internal A1g vibronic mode and maximal external lattice mode frequencies xmax lat in tungstate crystals with scheelite structure Material
XR (cmÿ1 )
DXR (cmÿ1 )
ÿ1 xmax lat (cm )
CaWO4 SrWO4 a BaWO4 a
910.7 921.5 926.5
4.8 3 2.2
274 240 194
a
Polycrystalline sample.
sults in the increase of internal vibronic mode frequency XR and decrease of its linewidth. In the ®rst approximation the frequency of A1g internal mode of tetragonal group must be determined by interactions inside the tetragonal groups [17]. The sort of cations (Ca2 , Sr2 , or Ba2 ) can in¯uence on this A1g frequency only by changing the size of the crystal unit cell and by covalence cation eect [17]. The moving in the series of tungstates Ca2 ! Sr2 ! Ba2 increases the unit cell and interionic distance inside the molecular group. The degree of covalence bond between the cation and molecular group usually decreases within the series Ca2 ! Sr2 ! Ba2 . These two factors must decrease the frequency of A1g vibration for Sr2 and Ba2 in comparison with Ca2 . As it was seen from Table 1 the opposite result was registered in our experiments. This anomalous phenomenon can be explained by decreasing of interaction between internal and external Raman modes in scheelite structure in the series of Ca2 ! Sr2 ! Ba2 tungstates. Such interaction decreasing together with increasing internal Raman mode frequency XR and decreasing maximum lattice mode frecould be responsible for slowing quency xmax lat down of dephasing processes and decreasing of homogeneous linewidth. The last feature is of special importance as it was shown above the steady state SRS gain coecient is inversely proportional to the linewidth value. For the same value of integral Raman scattering cross section for Ca, Sr, and Ba tungstates, decreasing of the linewidth by the factor 2.2 will increase the peak scattering intensity and Raman gain coecient by the same factor of 2.2. These results allow us to predict similar behavior of molybdate crystals with similar structure. Unpolarized spontaneous Raman scattering spectra of CaMoO4 , SrMoO4 and BaMoO4 crystals under excitation perpendicular to C4 axis (K^C4 ) with the radiation polarized along C4 (EkC4 ) are shown in Fig. 2. From Table 2 one can see that molybdate crystals exhibit similar behavior as tungstate crystals. In the range of metal ion substitutes Ca2 ® Sr2 ® Ba2 , Raman frequency is slightly increased and its linewidth is strongly narrowed from CaMoO4 (5.0 cmÿ1 ) to SrMoO4 (3 cmÿ1 ) and BaMoO4 (2.1 cmÿ1 ) to-
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
311
(TR Gd, Y, Yb, Lu, etc.) ions such as NaTR(WO4 )2 and KTR(WO4 )2 . NaTR(WO4 )2 crystals have scheelite structure similar to that of a simple tungstate. Statistical distribution of Na and TR3 ions over the same lattice sites results in variation of [WO4 ] complex surroundings due to the tungstate lattice host distortions. This causes A1g Raman line inhomogeneous broadening up to 3±7 times larger than that of simple tungstates (Fig. 3). The KTR(WO4 )2 tungstates series has a scheelite structure at temperature above 1000°C and transforms into a monoclinic one on cooling. K and TR3 ions occupy de®nite positions in the monoclinic tungstate ordered structure. Contrary to the scheelite structure, the edge shared [WO6 ] octahedrons form molecular groups in the mono-
Fig. 2. Unpolarized spontaneous Raman spectra of molybdate single crystals: CaMoO4 , SrMoO4 and BaMoO4 . Scattering geometry of excitation was K^C4 , EkC4 .
gether with maximum lattice phonon frequency decreasing. Hence, we can predict 1.7 times increase in Raman gain for SrMoO4 crystal and 2.4 times increase for BaMoO4 with respect to that in CaMoO4 . The complex tungstate series includes cation substances with alkali (Na and K) and rare earth Table 2 Frequencies XR and linewidths DXR of SRS active internal A1g vibronic mode and maximal external lattice mode frequencies xmax lat in molybdate crystals with scheelite structure Material
XR (cmÿ1 )
DXR (cmÿ1 )
ÿ1 xmax lat (cm )
CaMoO4 SrMoO4 BaMoO4
879.3 887.7 892
5.0 2.8 2.1
266 232 186
Fig. 3. Unpolarized spontaneous Raman spectra of tungstates with monoclinic structure: KGd(WO4 )2 single crystal (K^C2 , EkC2 ) and NaY(WO4 )2 powdered sample.
312
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
clinic tungstates [18] with Raman spectra considerably dierent from those of scheelite tungstates. Two intensive Raman lines corresponding to [WO6 ] complex vibrations in the spectra of monoclinic tungstates are registered (Fig. 3). As we have seen above the important parameter to characterize of SRS-active nonlinear material is
the values of Rpeak and Rint . The comparative data measured in all our experiments are presented in Table 3. Since the diamond sample exhibited one of the most intense Raman line, the values for other crystals were normalized to those of the diamond. Since the steady state Raman gain coecient is de®ned by the peak cross section Rpeak the highest
Table 3 Spontaneous Raman scattering parameters of crystals Material
Lattice space Molecular group group
Diamond SiO2
O7h D63
Raman freq. XR (cmÿ1 )
Raman line width DXR (cmÿ1 )
Integral cross section Rint (a.u.)
Peak intensity Scattering geometry of Rpeak (a.u.) excitation K
E
[SiO4 ]
1332.9 464.5
2.7 7.0
100 2.2
100 1.2
kC3 ^C3
^C3 kC3
Nitrates and calcite T6h Ba(NO3 )2 NaNO3 D63d CaCO3 D63d
[NO3 ] [NO3 ] [CO3 ]
1048.6 1069.2 1086.4
0.4 1.0 1.2
21 23 6.0
63 44 10.6
kC4 kC3 kC3
kC4 ^C3 ^ C3
Tungstates CaWO4 SrWO4 a BaWO4 a NaY(WO4 )2 a KGd(WO4 )2 KGd(WO4 )2 KGd(WO4 )2 KGd(WO4 )2 KY(WO4 )2 KY(WO4 )2 KY(WO4 )2 KY(WO4 )2 KYb(WO4 )2 KYb(WO4 )2 KYb(WO4 )2 KYb(WO4 )2
C64h C64h C64h C64h C62h C62h C62h C62h C62h C62h C62h C62h C62h C62h C62h C62h
[WO4 ] [WO4 ] [WO4 ] [WO4 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ]
910.7 921.5 926.5 918 901 901 768 768 905.6 905.6 767.4 767.4 908 908 757 757
4.8 3 2.2 15 b 5.4 5.4 6.4 6.4 7 7 8.4 8.4 7.4 7.4 15 b 15 b
47 ) ) ) 54 43 19 65 50 45 20 64 48 48 25 b 70 b
18.6 ) ) ) 25 22 8.2 29 24 22 9 24 24 24 13.8 25 b
^C4
kC4
^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2
^C2 kC2 ^C2 kC2 ^C2 kC2 ^C2 kC2 ^C2 kC2 ^C2 kC2
Molybdates CaMoO4 SrMoO4 BaMoO4
C64h C64h C64h
[MoO4 ] [MoO4 ] [MoO4 ]
879.3 887.7 892.4
5.0 2.8 2.1
64 55 52
34 51 64
^C4 ^C4 ^C4
kC4 kC4 kC4
Iodate and niobates C66 LiIO3 LiNbO3 C63v LiNbO3 C63v LiNbO3 C63v LaNbO4 C32h
[IO3 ] [NbO6 ] [NbO6 ] [NbO6 ] [NbO4 ]
821.6 872 632 250 805
5.0 21.4 27 28 9
54 44 166 ) 22
25 5 18 22 7.1
kC2 kC3 ^C3 ^C3 ^C2
^C2 ^C3 kC3 kC3 kC2
Phosphates Ca5 (PO4 )3 F Sr5 (PO4 )3 F
[PO4 ] [PO4 ]
964.7 950.3
2.8 2.8
3.8 3.8
^C6 ^C6
kC6 kC6
a b
C26h C26h
Polycrystal sample. Line with inhomogeneous splitting.
3.4 3.4
b
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
gain after the diamond is expected to be in barium and sodium nitrate crystals. In spite the fact that the integral cross sections of SRS-active vibronic modes for nitrate Raman crystals are approximately 2±3 times less than for tungstates, Ba(NO3 )2 and NaNO3 have high gain due to the narrowest SRS-active lines. Their linewidth values were measured in our experiments to be as small as DXBa NO3 2 0:4 cmÿ1 and DXNaNO3 1 cmÿ1 at room temperature which are correspondingly 7 and about 2 times less than that in the diamond. Zverev et al. [16] assigned narrow linewidth of SRS-active mode in Ba(NO3 )2 crystal to lower probability of vibronic relaxation due to the absence of the three phonon splitting relaxation process for this mode. The peak cross section Rpeak of the Raman mode in Ba(NO3 )2 , measured in our experiments, exhibits high value (63% of that in the diamond), that is twice higher than in KGd(WO4 )2 crystal. Due to larger linewidth NaNO3 crystal has lower value of Rpeak of only 44%. Natural calcite demonstrates a smaller value of Rpeak that is only about 10%. Both simple scheelite tungstate crystals such as MWO4 (here M is Ca, Sr, or Ba) and ``complex'' monoclinic tungstates KTR(WO4 )2 have high integral Raman scattering cross section Rint . But due to larger spectral broadening 5.4±15 cmÿ1 those with monoclinic structure exhibit lower values of peak cross sections from 8% to 29% depending on the crystal and orientation. In our experiments crystals of SrWO4 and BaWO4 tungstates were not available as a bulk single crystal for comparative intensity measurements, but due to their narrower linewidth (Table 1) one can predict values of Rpeak as high as 30% and 40% in these materials which is higher than in other tungstates and closer to those in nitrates. As we have seen above molybdate crystals exhibit similar linewidth behavior and due to higher integral cross section they exhibit even larger values of Rpeak . The predicted perceptiveness and high Raman gain in barium molybdate was proved by the peak cross section value measured as high as 64%, the same as for the best nitrate Ba(NO3 )2 . Raman mode (632 cmÿ1 ) in lithium niobate crystal is much broader (27 cmÿ1 ) and in spite of the fact that it has the largest integral cross section (166%) its Rpeak exhibits the value of only 18% of
313
that for diamond. Higher frequency Raman lines in lanthanum and lithium niobates (805 and 872 cmÿ1 ) exhibit weaker Rpeak of 7% and 5%, respectively. Calcium and strontium apatites have higher [PO4 ] vibronic frequency bands but smaller values of peak Raman scattering cross section (Rpeak 3.8%). All these values are small but close to that of CaCO3 crystal, which was successfully used for SRS even for nanosecond pump pulses. Due to large frequency shift (XR 800±960 cmÿ1 ) and broad linewidths (DXR 3±20 cmÿ1 ), we can propose that the above-mentioned niobate and apatite crystals can be useful for SRS of picosecond laser pulses where integral cross sections are more important than peak ones. Integral Raman scattering cross section Rint determines the Raman gain coecient in the transient case under short pulse pumping sp TR , when the wide spectral band Dmp sÿ1 p of pumping is in a good overlap with the broad spectrum of Raman line, pc DX sp 6 1. Comparison of the Rint in dierent materials shows that high values are observed in lithium niobate for 632 cmÿ1 mode (166%), diamond (100%), lithium iodate (54%) and some tungstate crystals (40±60%). Smaller values are observed in sodium and barium nitrate crystals (23% and 21%), calcite (6.0%), calcium and strontium apatites (3.4%) and silica (2.2%) crystals. These data explain why nowadays the KGd(WO4 )2 crystal is one of the most popular Raman material for picosecond operation with the Stokes shift of 901 cmÿ1 . KGW optical elements of large sizes and good optical quality are commercially available at moderate price. Some other Raman materials presented in Table 3 can attract interest due to dierent values of Raman frequency, but they have rather small values of integral and peak cross sections, which lead to much higher SRS thresholds. 4. Conclusion Analysis and comparison of spontaneous Raman spectra of SRS-active modes in crystals have shown that diamond, barium and sodium nitrate crystals must have higher values of Raman gain
314
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
coecient for steady state SRS of nanosecond pulses. The shortening of the pump pulses below 20±100 ps results in the transient behavior of the SRS in the above crystals and increase of SRS threshold. Tungstate crystals with comparatively large Raman linewidth can be proposed for the operation with shorter pump pulses. Moreover, they can also be used for transient SRS experiments due to large integral cross section Rint . Spectroscopic properties observed in new barium and strontium molybdate and tungstate Raman crystals allows to predict high values of their Raman gain for both nanosecond and picosecond laser pulses and to propose them as a perspective nonlinear media for developing nano and picosecond Raman shifters.
[6] [7]
[8]
[9] [10]
Acknowledgements This work was partly supported by the US Civilian Research and Development Foundation and Russian Ministry of Science and Technology under Award N RE2-140.
[11]
[12]
References [1] G. Eckhardt, D.P. Bortfeld, M. Geller, Stimulated emission of Stokes and anti-Stokes Raman lines from diamond, calcite and a-sulfur single crystals, Appl. Phys. Lett. 3 (1963) 137±138. [2] G. Bisson, G. Mayer, Eects Raman stimule's dans la calcite, C.R. Acad. Sci.(Paris) 265 (1967) 397. [3] K. Park, Phys. Lett. 22 (1966) 39; Phys. Lett. A 25 (1967) 490. [4] A.S. Eremenko, S.N. Karpukhin, A.I. Stepanov, SRS of the second harmonic of neodymium laser in nitrate crystals, Sov. J. Quantum Electron. 10 (1980) 113±116. [5] T.T. Basiev, V.N. Voitsekhovskii, P.G. Zverev, F.V. Karpushko, A.V. Lubimov, S.B. Mirov, V.P. Morozov, I.V. Mochalov, A.A. Pavlyuk, G.V. Sinitsyn, V.E. Yakobson, Conversion of tunable radiation from a laser utilising an LiF crystal containing F-2 colour centres by
[13] [14] [15]
[16] [17] [18]
stimulated Raman scattering in Ba(NO3 )2 and KGd(WO4 )2 crystals, Sov. J. Quantum Electron. 17 (1987) 1560±1563. P.G. Zverev, T.T. Basiev, Compact SRS laser on barium nitrate crystal, in: Proceedings of All-Union Conference on Laser Optics, vol. 2, Leningrad, 1993, p. 363. P.G. Zverev, T.T. Basiev, I.V. Ermakov, A.M. Prokhorov, Stimulated Raman scattering in barium nitrate crystal in the external optical cavity, in: Laser Methods of Surface Treatment and Modi®cation, vol. 2498, SPIE ± The International Society for Optical Engineering, 1994, pp. 164±169. J.T. Murray, R.C. Powell, N. Peyghambarian, D. Smith, W. Austin, R.A. Stolzenberger, Generation of 1.5 lm radiation through intracavity solid-state Raman shifting in Ba(NO3 )2 nonlinear crystals, Opt. Lett. 20 (1995) 1017± 1019. P.G. Zverev, J.T. Murray, R.C. Powell, R.J. Reeves, T.T. Basiev, Stimulated Raman scattering of picosecond pulses in barium nitrate crystals, Opt. Commun. 97 (1993) 59±63. K. Andryunas, Y.K. Vishchakas, V. Kabelka, I.V. Mochalov, A.A. Pavlyuk, G.T. Petrovskii, V.P. Syrus, SRS-self conversion of Nd3 laser emission in tungstate crystals, JETP Lett. 42 (1985) 410±412. A.M. Ivanuk, M.A. Ter-Pogosyan, P.A. Shaverdov, V.D. Belyaev, V.L. Ermolayev, H.P. Tikhonova, Picosecond light pulses under intracavity stimulated Raman scattering in the active element of neodymium laser (in Russian), Opt. Spektrosk. 59 (1985) 950. Y.R. Shen, The Principles of Nonlinear Optics, Ch. 10, Wiley, New York, 1984. R.L. Carman, F. Shimizu, C.S. Wang, N. Bloembergen, Theory of Stokes pulse shapes in transient stimulated Raman scattering, Phys. Rev. A 2 (1972) 60±72. Y. Wang, Theory of stimulated Raman scattering, Phys. Rev. 182 (1969) 482±494. W. Kaiser, M. Maier, Stimulated Rayleigh, Brillouin and Raman spectroscopy, in: F.T. Arecchi, E.O. Schulz-Dubois (Eds.), Laser Handbook, Ch. E2, North-Holland, Amsterdam, 1972, pp. 1077±1150. P.G. Zverev, W. Jia, H. Liu, T.T. Basiev, Vibrational dynamic of the Raman-active mode in barium nitrate crystal, Opt. Lett. 20 (1995) 2378±2380. A.N. Lasarev, A.P. Mirgorodskii, I.S. Ignatiev, Vibronic Spectra of Complex Oxides (in Russian), Nauka, Leningrad, 1975. Yu.K. Vishikos, I.V. Mochalov, A.V. Mikhailov, R.F. Klevtsova, A.V. Lyubimov, Crystal structure and Raman scattering in crystals of KGd(WO4 )2 , Litovskii Fizicheskii Journal 28 (1988) 224±234.
Optical Materials 11 (1999) 307±314
Raman spectroscopy of crystals for stimulated Raman scattering T.T. Basiev a
a,* ,
A.A. Sobol a, P.G. Zverev a, L.I. Ivleva a, V.V. Osiko a, R.C. Powell b,1
Laser Materials and Technology Research Center of GPI, Vavilov Street, 38, Moscow 117942, Russian Federation b Optical Sciences Center, University of Arizona, Tucson, AZ 85721-0094, USA
Abstract Raman frequency shift, line width, integral and peak Raman scattering cross sections were measured in various crystals using spontaneous Raman spectroscopy. The highest Raman gain coecient in steady state Stimulated Raman Scattering (SRS) regime was proved to be in barium nitrate crystal; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium molybdate crystal is proposed as a new highly ecient Raman material. Ó 1999 Elsevier Science B.V. All rights reserved. OCIS: 300.6450; 290.5910; 190.2640
1. Introduction In 1963 Eckhardt et al. [1] described the ®rst observation of stimulated Raman scattering (SRS) in diamond, calcite and a-sulfur single crystals. Since then SRS in solids has become an area of intensive investigation in laser physics. For almost 20 years the natural calcite crystal was the only one solid state material practically and widely used for investigations of SRS due to its high Raman gain, comparatively large size and low cost. The energy of its intense SRS-active vibronic mode is 1086 cmÿ1 and the line width is about 1.2 cmÿ1 [2,3]. In 1980 a number of new synthetic crystals, barium, sodium, and lead nitrates were proposed
* 1
Corresponding author. E-mail: [email protected]. E-mail: [email protected].
as perspective Raman materials [4]. These crystals have intense vibronic modes with the energy of about 1050 cmÿ1 , which correspond to the internal symmetrical vibrations of [NO3 ] molecular ions. The number of various laser con®gurations has shown that the quantum conversion eciency of Ba(NO3 )2 SRS shifter can be up to 80% for nanosecond laser pulses even for IR radiation with much lower gain [5±7]. This has led to the availability of commercialization of solid state Raman lasers with special properties such as high eciency and diraction-limited beam quality at eyesafe wavelengths [8]. However, the SRS threshold for 25 ps pump pulses was found to be 10 times higher than that for nanosecond pulses due to long vibronic relaxation time [9]. This transient SRS was accompanied by wide angular scattering to higher Stokes and anti-Stokes components, that interfered in Ba(NO3 )2 crystal application for Raman lasers pumped by picosecond pulses.
0925-3467/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 8 ) 0 0 0 3 0 - 5
308
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
The diamond crystal has larger Raman frequency shift 1332 cmÿ1 and high gain [1]. But due to its small dimensions and high cost it has not found many practical applications as a Raman material. In 1985 it was found [10,11] that potassium gadolinium tungstate, KGd(WO4 )2 exhibited ecient SRS properties. In spite of the fact that its Raman gain coecient at 1064 nm (6 cm/GW) is twice less than that in barium nitrate, the KGW crystal found many practical applications for frequency shifting of picosecond laser radiation [10,11]. Variations in the Raman scattering cross section and Raman mode line width are responsible for the dierence in the behavior of the above SRS materials in steady state and transient regimes. The comparative investigation of SRS-active vibronic modes in some solids using spontaneous Raman spectroscopy is presented in this paper. The results are important to understand the Raman properties of crystals and to search new nonlinear materials with certain frequency shift and high SRS eciency. The theory of SRS process is well developed and can be found elsewhere [12±14]. Here we would like to outline that depending on the pump laser pulse duration (sp ), two temporal cases can be considered. The ®rst one is a steady state regime, when the pump pulse duration sp is much longer than the vibronic Raman mode dephasing time TR sp TR . The second case is a transient regime, when the pump pulse duration is smaller than the dephasing time sp TR and spectral width of pump laser Dmp is much broader than the Raman line homogeneous broadening ÿ1 DXR pcTR Dmp DXR : In the steady state case if there is no pump depletion, the solution for the Stokes intensity is exponential [15]: IS l IS 0 expfgSS Ip lg:
1
Here IS 0 and IS l are Stokes intensities at the input and output of the Raman medium, Ip is the pump intensity and l the length of the nonlinear crystal. The gSS is a steady state Raman gain coecient which is determined by the Raman properties of nonlinear medium [15]:
gSS
kp k2S N hcpn2S DXR
dr ; dX
2
where N is the number of scattering centers, kp , kS are the pump and Stokes wavelengths, nS , refractive index at kS and dr=dX is the Raman scattering cross section. Thus, we can note that in the steady state case the Raman gain coecient gSS is linearly proportional to the Raman scattering cross section and inversely proportional to the linewidth of Raman transition. The Raman linewidth is determined by a mechanism of vibronic phase relaxation due to the phonon±phonon coupling in the medium and has a strong temperature dependence ÿ1 [16]. The value of dr=dX DXR can be characterized as a peak intensity in the measured spontaneous Raman scattering spectrum and for simplicity we will denote it as ÿ1
Rpeak dr=dX DXR : In the transient case when sp TR and the spectral width of pump laser Dmp is broader than the Raman line width DXR Dmp DXR ; with no pump depletion the analytical expression for the Stokes intensity for large ampli®cation gain was found in Ref. [14]. Following Ref. [15] we can present it as sp IS l / IS 0 exp ÿ TR ( 1 ) kp k2S N dr 2 : 3 exp 2 Ip sp l hn2S dX From Eq. (3) one can see that the Raman gain is proportional to the square root of the multiplication of the pump pulse energy Ip á sp , the crystal length l, and the total integral Raman scattering cross section (dr/dX), but does not depend on the Raman linewidth DXR . Under certain assumptions, the integral Raman scattering cross section can be characterized as an integral value of the Raman line intensity in spontaneous Raman scattering spectrum and we denoted it as Rint dr=dX. Due to variations in crystallographic structure dierent crystals have rather dierent Raman spectra. For SRS laser and shifter development they can be interesting due to changes in SRS
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
309
frequency shift, various values of Raman scattering cross section and line width exhibiting dierent Raman gain. At the same time there can be other nonlinear processes, like pump beam self-focusing, Brillouin scattering, and second harmonic generation, which can be competitive with SRS and restrict applications of this certain material for Raman lasers. Below we will compare only integral and peak Raman scattering intensities Rint and Rpeak for various prospective Raman materials in order to characterize their SRS features for the Raman laser development.
linewidth was determined by the spectral resolution of the system and the widths of the slits. Except narrow lines (DXR < 1 cmÿ1 ), the spectral resolution was about 10% of the measured linewidth value. The peak cross section value Rpeak was measured with the accuracy of 10% for broad lines, while for narrow lines it became higher (20%) due to spectral resolution limitation. While the accuracy for Rint was about 10% for narrow lines and increased to 20±40% due to uncertainty of the spectrum background level.
2. Experimental setup
3. Results and discussions
The experimental setup for spontaneous Raman spectroscopy utilized the excitation by a CW argon ion laser (k 488 nm) and registration by doublespectrometer ``Spex-1403''. A premonochromator was used to cut plasma discharge lines of the laser. The laser intensity at the sample was about 1 W. Backwards scattering scheme was used to increase signal to noise ratio. To compensate the self-polarization of spectrometer, a polarization scrambler was placed in front of the entrance slit. The spectral resolution of the system was 0.2 cmÿ1 . Single crystal as well as polycrystal samples were investigated in our experiments. To compare integral and peak scattering cross sections in different materials the plane-parallel samples were made from single crystals with the thickness either 0.7 or 2 mm. An available diamond single crystal had slope facets and 0.7 mm thickness. The exciting laser intensity and spectrometer slit width had constant values in dierent sets of measurements, the focusing of exciting radiation and the collimating of scattered light were the same in all the experiments. Since the Raman scattering was excited by a polarized beam, for some samples for certain propagation direction (K) we made several experiments with dierent polarization of the excitation light (E) with respect to crystallographic axes. The registered Raman spectra were analyzed with software ®tting program which allowed to obtain values for Raman line width, peak and integral cross sections. The accuracy of the Raman
Among solid state materials which exhibit remarkable SRS properties one can outline ``simple'' crystals which are composed of one or two elements: diamond (C), silicon (Si), and quartz (SiO2 ). These crystals have a covalent type of bonding which allow them to exhibit intense lines in Raman spectra. The diamond crystal is the example of highly packed covalent structure in which each carbon atom is surrounded by four neighbors located in the corners of tetrahedron. There is only one very intense Raman line of F2g symmetry at 1332 cmÿ1 for this crystal. Another type of SRS-active material is a group of molecular ionic crystals. They are composed of a cation atom and an anion complex like [XO3 ] for X N, C, Cl, Br, I atoms or [YO4 ] for Y Si, Ge, W, P, S, Mo, Nb atoms. There are ionic bonds between a cation and molecular group in such crystals, while covalent bonds dominate inside the molecular complexes. The intense Raman modes in these crystals correspond to the symmetrical internal valent vibrations inside molecular [XO3 ] or [YO4 ] complexes. Let us consider in detail the tungstate crystals. All tungstate crystals investigated can be divided into two groups: simple crystals with scheelite structure of C64h space symmetry, and tungstates with monoclinic C62h structure. Raman spectra of simple calcium, strontium and barium tungstate crystals are shown in Fig. 1. Each has one very intense A1g line about 910±925 cmÿ1 which corresponds to the internal symmetrical valent
310
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
Fig. 1. Unpolarized spontaneous Raman spectra of tungstate crystals with scheelite structure: CaWO4 and BaWO4 single crystals, and SrWO4 powdered sample. The geometry of excitation was K^C4 , EkC4 .
vibration in [WO4 ] tetrahedron group. Table 1 shows data on internal Raman frequency shift and Raman mode linewidth in these crystals together with maximum frequency of external lattice mode. From Table 1 it follows that the increase of cation radius in the row Ca2 ! Sr2 ! Ba2 reTable 1 Frequencies XR and linewidths DXR of SRS-active internal A1g vibronic mode and maximal external lattice mode frequencies xmax lat in tungstate crystals with scheelite structure Material
XR (cmÿ1 )
DXR (cmÿ1 )
ÿ1 xmax lat (cm )
CaWO4 SrWO4 a BaWO4 a
910.7 921.5 926.5
4.8 3 2.2
274 240 194
a
Polycrystalline sample.
sults in the increase of internal vibronic mode frequency XR and decrease of its linewidth. In the ®rst approximation the frequency of A1g internal mode of tetragonal group must be determined by interactions inside the tetragonal groups [17]. The sort of cations (Ca2 , Sr2 , or Ba2 ) can in¯uence on this A1g frequency only by changing the size of the crystal unit cell and by covalence cation eect [17]. The moving in the series of tungstates Ca2 ! Sr2 ! Ba2 increases the unit cell and interionic distance inside the molecular group. The degree of covalence bond between the cation and molecular group usually decreases within the series Ca2 ! Sr2 ! Ba2 . These two factors must decrease the frequency of A1g vibration for Sr2 and Ba2 in comparison with Ca2 . As it was seen from Table 1 the opposite result was registered in our experiments. This anomalous phenomenon can be explained by decreasing of interaction between internal and external Raman modes in scheelite structure in the series of Ca2 ! Sr2 ! Ba2 tungstates. Such interaction decreasing together with increasing internal Raman mode frequency XR and decreasing maximum lattice mode frecould be responsible for slowing quency xmax lat down of dephasing processes and decreasing of homogeneous linewidth. The last feature is of special importance as it was shown above the steady state SRS gain coecient is inversely proportional to the linewidth value. For the same value of integral Raman scattering cross section for Ca, Sr, and Ba tungstates, decreasing of the linewidth by the factor 2.2 will increase the peak scattering intensity and Raman gain coecient by the same factor of 2.2. These results allow us to predict similar behavior of molybdate crystals with similar structure. Unpolarized spontaneous Raman scattering spectra of CaMoO4 , SrMoO4 and BaMoO4 crystals under excitation perpendicular to C4 axis (K^C4 ) with the radiation polarized along C4 (EkC4 ) are shown in Fig. 2. From Table 2 one can see that molybdate crystals exhibit similar behavior as tungstate crystals. In the range of metal ion substitutes Ca2 ® Sr2 ® Ba2 , Raman frequency is slightly increased and its linewidth is strongly narrowed from CaMoO4 (5.0 cmÿ1 ) to SrMoO4 (3 cmÿ1 ) and BaMoO4 (2.1 cmÿ1 ) to-
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
311
(TR Gd, Y, Yb, Lu, etc.) ions such as NaTR(WO4 )2 and KTR(WO4 )2 . NaTR(WO4 )2 crystals have scheelite structure similar to that of a simple tungstate. Statistical distribution of Na and TR3 ions over the same lattice sites results in variation of [WO4 ] complex surroundings due to the tungstate lattice host distortions. This causes A1g Raman line inhomogeneous broadening up to 3±7 times larger than that of simple tungstates (Fig. 3). The KTR(WO4 )2 tungstates series has a scheelite structure at temperature above 1000°C and transforms into a monoclinic one on cooling. K and TR3 ions occupy de®nite positions in the monoclinic tungstate ordered structure. Contrary to the scheelite structure, the edge shared [WO6 ] octahedrons form molecular groups in the mono-
Fig. 2. Unpolarized spontaneous Raman spectra of molybdate single crystals: CaMoO4 , SrMoO4 and BaMoO4 . Scattering geometry of excitation was K^C4 , EkC4 .
gether with maximum lattice phonon frequency decreasing. Hence, we can predict 1.7 times increase in Raman gain for SrMoO4 crystal and 2.4 times increase for BaMoO4 with respect to that in CaMoO4 . The complex tungstate series includes cation substances with alkali (Na and K) and rare earth Table 2 Frequencies XR and linewidths DXR of SRS active internal A1g vibronic mode and maximal external lattice mode frequencies xmax lat in molybdate crystals with scheelite structure Material
XR (cmÿ1 )
DXR (cmÿ1 )
ÿ1 xmax lat (cm )
CaMoO4 SrMoO4 BaMoO4
879.3 887.7 892
5.0 2.8 2.1
266 232 186
Fig. 3. Unpolarized spontaneous Raman spectra of tungstates with monoclinic structure: KGd(WO4 )2 single crystal (K^C2 , EkC2 ) and NaY(WO4 )2 powdered sample.
312
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
clinic tungstates [18] with Raman spectra considerably dierent from those of scheelite tungstates. Two intensive Raman lines corresponding to [WO6 ] complex vibrations in the spectra of monoclinic tungstates are registered (Fig. 3). As we have seen above the important parameter to characterize of SRS-active nonlinear material is
the values of Rpeak and Rint . The comparative data measured in all our experiments are presented in Table 3. Since the diamond sample exhibited one of the most intense Raman line, the values for other crystals were normalized to those of the diamond. Since the steady state Raman gain coecient is de®ned by the peak cross section Rpeak the highest
Table 3 Spontaneous Raman scattering parameters of crystals Material
Lattice space Molecular group group
Diamond SiO2
O7h D63
Raman freq. XR (cmÿ1 )
Raman line width DXR (cmÿ1 )
Integral cross section Rint (a.u.)
Peak intensity Scattering geometry of Rpeak (a.u.) excitation K
E
[SiO4 ]
1332.9 464.5
2.7 7.0
100 2.2
100 1.2
kC3 ^C3
^C3 kC3
Nitrates and calcite T6h Ba(NO3 )2 NaNO3 D63d CaCO3 D63d
[NO3 ] [NO3 ] [CO3 ]
1048.6 1069.2 1086.4
0.4 1.0 1.2
21 23 6.0
63 44 10.6
kC4 kC3 kC3
kC4 ^C3 ^ C3
Tungstates CaWO4 SrWO4 a BaWO4 a NaY(WO4 )2 a KGd(WO4 )2 KGd(WO4 )2 KGd(WO4 )2 KGd(WO4 )2 KY(WO4 )2 KY(WO4 )2 KY(WO4 )2 KY(WO4 )2 KYb(WO4 )2 KYb(WO4 )2 KYb(WO4 )2 KYb(WO4 )2
C64h C64h C64h C64h C62h C62h C62h C62h C62h C62h C62h C62h C62h C62h C62h C62h
[WO4 ] [WO4 ] [WO4 ] [WO4 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ] [WO6 ]
910.7 921.5 926.5 918 901 901 768 768 905.6 905.6 767.4 767.4 908 908 757 757
4.8 3 2.2 15 b 5.4 5.4 6.4 6.4 7 7 8.4 8.4 7.4 7.4 15 b 15 b
47 ) ) ) 54 43 19 65 50 45 20 64 48 48 25 b 70 b
18.6 ) ) ) 25 22 8.2 29 24 22 9 24 24 24 13.8 25 b
^C4
kC4
^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2 ^C2
^C2 kC2 ^C2 kC2 ^C2 kC2 ^C2 kC2 ^C2 kC2 ^C2 kC2
Molybdates CaMoO4 SrMoO4 BaMoO4
C64h C64h C64h
[MoO4 ] [MoO4 ] [MoO4 ]
879.3 887.7 892.4
5.0 2.8 2.1
64 55 52
34 51 64
^C4 ^C4 ^C4
kC4 kC4 kC4
Iodate and niobates C66 LiIO3 LiNbO3 C63v LiNbO3 C63v LiNbO3 C63v LaNbO4 C32h
[IO3 ] [NbO6 ] [NbO6 ] [NbO6 ] [NbO4 ]
821.6 872 632 250 805
5.0 21.4 27 28 9
54 44 166 ) 22
25 5 18 22 7.1
kC2 kC3 ^C3 ^C3 ^C2
^C2 ^C3 kC3 kC3 kC2
Phosphates Ca5 (PO4 )3 F Sr5 (PO4 )3 F
[PO4 ] [PO4 ]
964.7 950.3
2.8 2.8
3.8 3.8
^C6 ^C6
kC6 kC6
a b
C26h C26h
Polycrystal sample. Line with inhomogeneous splitting.
3.4 3.4
b
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
gain after the diamond is expected to be in barium and sodium nitrate crystals. In spite the fact that the integral cross sections of SRS-active vibronic modes for nitrate Raman crystals are approximately 2±3 times less than for tungstates, Ba(NO3 )2 and NaNO3 have high gain due to the narrowest SRS-active lines. Their linewidth values were measured in our experiments to be as small as DXBa NO3 2 0:4 cmÿ1 and DXNaNO3 1 cmÿ1 at room temperature which are correspondingly 7 and about 2 times less than that in the diamond. Zverev et al. [16] assigned narrow linewidth of SRS-active mode in Ba(NO3 )2 crystal to lower probability of vibronic relaxation due to the absence of the three phonon splitting relaxation process for this mode. The peak cross section Rpeak of the Raman mode in Ba(NO3 )2 , measured in our experiments, exhibits high value (63% of that in the diamond), that is twice higher than in KGd(WO4 )2 crystal. Due to larger linewidth NaNO3 crystal has lower value of Rpeak of only 44%. Natural calcite demonstrates a smaller value of Rpeak that is only about 10%. Both simple scheelite tungstate crystals such as MWO4 (here M is Ca, Sr, or Ba) and ``complex'' monoclinic tungstates KTR(WO4 )2 have high integral Raman scattering cross section Rint . But due to larger spectral broadening 5.4±15 cmÿ1 those with monoclinic structure exhibit lower values of peak cross sections from 8% to 29% depending on the crystal and orientation. In our experiments crystals of SrWO4 and BaWO4 tungstates were not available as a bulk single crystal for comparative intensity measurements, but due to their narrower linewidth (Table 1) one can predict values of Rpeak as high as 30% and 40% in these materials which is higher than in other tungstates and closer to those in nitrates. As we have seen above molybdate crystals exhibit similar linewidth behavior and due to higher integral cross section they exhibit even larger values of Rpeak . The predicted perceptiveness and high Raman gain in barium molybdate was proved by the peak cross section value measured as high as 64%, the same as for the best nitrate Ba(NO3 )2 . Raman mode (632 cmÿ1 ) in lithium niobate crystal is much broader (27 cmÿ1 ) and in spite of the fact that it has the largest integral cross section (166%) its Rpeak exhibits the value of only 18% of
313
that for diamond. Higher frequency Raman lines in lanthanum and lithium niobates (805 and 872 cmÿ1 ) exhibit weaker Rpeak of 7% and 5%, respectively. Calcium and strontium apatites have higher [PO4 ] vibronic frequency bands but smaller values of peak Raman scattering cross section (Rpeak 3.8%). All these values are small but close to that of CaCO3 crystal, which was successfully used for SRS even for nanosecond pump pulses. Due to large frequency shift (XR 800±960 cmÿ1 ) and broad linewidths (DXR 3±20 cmÿ1 ), we can propose that the above-mentioned niobate and apatite crystals can be useful for SRS of picosecond laser pulses where integral cross sections are more important than peak ones. Integral Raman scattering cross section Rint determines the Raman gain coecient in the transient case under short pulse pumping sp TR , when the wide spectral band Dmp sÿ1 p of pumping is in a good overlap with the broad spectrum of Raman line, pc DX sp 6 1. Comparison of the Rint in dierent materials shows that high values are observed in lithium niobate for 632 cmÿ1 mode (166%), diamond (100%), lithium iodate (54%) and some tungstate crystals (40±60%). Smaller values are observed in sodium and barium nitrate crystals (23% and 21%), calcite (6.0%), calcium and strontium apatites (3.4%) and silica (2.2%) crystals. These data explain why nowadays the KGd(WO4 )2 crystal is one of the most popular Raman material for picosecond operation with the Stokes shift of 901 cmÿ1 . KGW optical elements of large sizes and good optical quality are commercially available at moderate price. Some other Raman materials presented in Table 3 can attract interest due to dierent values of Raman frequency, but they have rather small values of integral and peak cross sections, which lead to much higher SRS thresholds. 4. Conclusion Analysis and comparison of spontaneous Raman spectra of SRS-active modes in crystals have shown that diamond, barium and sodium nitrate crystals must have higher values of Raman gain
314
T.T. Basiev et al. / Optical Materials 11 (1999) 307±314
coecient for steady state SRS of nanosecond pulses. The shortening of the pump pulses below 20±100 ps results in the transient behavior of the SRS in the above crystals and increase of SRS threshold. Tungstate crystals with comparatively large Raman linewidth can be proposed for the operation with shorter pump pulses. Moreover, they can also be used for transient SRS experiments due to large integral cross section Rint . Spectroscopic properties observed in new barium and strontium molybdate and tungstate Raman crystals allows to predict high values of their Raman gain for both nanosecond and picosecond laser pulses and to propose them as a perspective nonlinear media for developing nano and picosecond Raman shifters.
[6] [7]
[8]
[9] [10]
Acknowledgements This work was partly supported by the US Civilian Research and Development Foundation and Russian Ministry of Science and Technology under Award N RE2-140.
[11]
[12]
References [1] G. Eckhardt, D.P. Bortfeld, M. Geller, Stimulated emission of Stokes and anti-Stokes Raman lines from diamond, calcite and a-sulfur single crystals, Appl. Phys. Lett. 3 (1963) 137±138. [2] G. Bisson, G. Mayer, Eects Raman stimule's dans la calcite, C.R. Acad. Sci.(Paris) 265 (1967) 397. [3] K. Park, Phys. Lett. 22 (1966) 39; Phys. Lett. A 25 (1967) 490. [4] A.S. Eremenko, S.N. Karpukhin, A.I. Stepanov, SRS of the second harmonic of neodymium laser in nitrate crystals, Sov. J. Quantum Electron. 10 (1980) 113±116. [5] T.T. Basiev, V.N. Voitsekhovskii, P.G. Zverev, F.V. Karpushko, A.V. Lubimov, S.B. Mirov, V.P. Morozov, I.V. Mochalov, A.A. Pavlyuk, G.V. Sinitsyn, V.E. Yakobson, Conversion of tunable radiation from a laser utilising an LiF crystal containing F-2 colour centres by
[13] [14] [15]
[16] [17] [18]
stimulated Raman scattering in Ba(NO3 )2 and KGd(WO4 )2 crystals, Sov. J. Quantum Electron. 17 (1987) 1560±1563. P.G. Zverev, T.T. Basiev, Compact SRS laser on barium nitrate crystal, in: Proceedings of All-Union Conference on Laser Optics, vol. 2, Leningrad, 1993, p. 363. P.G. Zverev, T.T. Basiev, I.V. Ermakov, A.M. Prokhorov, Stimulated Raman scattering in barium nitrate crystal in the external optical cavity, in: Laser Methods of Surface Treatment and Modi®cation, vol. 2498, SPIE ± The International Society for Optical Engineering, 1994, pp. 164±169. J.T. Murray, R.C. Powell, N. Peyghambarian, D. Smith, W. Austin, R.A. Stolzenberger, Generation of 1.5 lm radiation through intracavity solid-state Raman shifting in Ba(NO3 )2 nonlinear crystals, Opt. Lett. 20 (1995) 1017± 1019. P.G. Zverev, J.T. Murray, R.C. Powell, R.J. Reeves, T.T. Basiev, Stimulated Raman scattering of picosecond pulses in barium nitrate crystals, Opt. Commun. 97 (1993) 59±63. K. Andryunas, Y.K. Vishchakas, V. Kabelka, I.V. Mochalov, A.A. Pavlyuk, G.T. Petrovskii, V.P. Syrus, SRS-self conversion of Nd3 laser emission in tungstate crystals, JETP Lett. 42 (1985) 410±412. A.M. Ivanuk, M.A. Ter-Pogosyan, P.A. Shaverdov, V.D. Belyaev, V.L. Ermolayev, H.P. Tikhonova, Picosecond light pulses under intracavity stimulated Raman scattering in the active element of neodymium laser (in Russian), Opt. Spektrosk. 59 (1985) 950. Y.R. Shen, The Principles of Nonlinear Optics, Ch. 10, Wiley, New York, 1984. R.L. Carman, F. Shimizu, C.S. Wang, N. Bloembergen, Theory of Stokes pulse shapes in transient stimulated Raman scattering, Phys. Rev. A 2 (1972) 60±72. Y. Wang, Theory of stimulated Raman scattering, Phys. Rev. 182 (1969) 482±494. W. Kaiser, M. Maier, Stimulated Rayleigh, Brillouin and Raman spectroscopy, in: F.T. Arecchi, E.O. Schulz-Dubois (Eds.), Laser Handbook, Ch. E2, North-Holland, Amsterdam, 1972, pp. 1077±1150. P.G. Zverev, W. Jia, H. Liu, T.T. Basiev, Vibrational dynamic of the Raman-active mode in barium nitrate crystal, Opt. Lett. 20 (1995) 2378±2380. A.N. Lasarev, A.P. Mirgorodskii, I.S. Ignatiev, Vibronic Spectra of Complex Oxides (in Russian), Nauka, Leningrad, 1975. Yu.K. Vishikos, I.V. Mochalov, A.V. Mikhailov, R.F. Klevtsova, A.V. Lyubimov, Crystal structure and Raman scattering in crystals of KGd(WO4 )2 , Litovskii Fizicheskii Journal 28 (1988) 224±234.