2023-10-05
Raman Crystals
All solid-state sources of laser radiation in the spectral regions uncovered by commercial lasers are highly desirable due to the growing number of applications of these devices in medicine, spectroscopy, defence, and research. Stimulating Raman scattering (SRS) allows to shift frequency of laser radiation in those spectral regions. The use of solid state Raman shifters is advantageous due to high conversion efficiency, no phase matching necessity, and easier handling compared to gaseous and liquid Raman cells. Among the most efficient commercial Raman active crystals are baruim nitrate Ba(NO3)2 and potassium gadolinium tungstate KGd(WO4)2 (KGW). The diamond crystal which demonstrates large Raman frequency shift and high gain has not found commercial applications as a Raman material because of its small dimensions and high cost.
SRS temporal regimes and spontaneous Raman scattering parameters of Raman crystals
Depending on the duration of the pump laser pulse, two temporal cases of SRS, steady state and transient, can be considered. The steady-state case is observed when the duration of the pump laser pulse is longer than the vibronic relaxation time. In the steady-state case the Raman gain coefficient is linearly proportional to the Raman scattering cross section and inversely proportional to the linewidth of the Raman transition. The Raman linewidth is determined by a mechanism of vibronic phase relaxation that is due to phonon–phonon coupling in the medium. The value of the Raman scattering cross section can be measured as a maximum in the spontaneous Raman scattering spectrum. In the transient case the transient Raman gain depends on the integral Raman scattering cross section. In this case the Raman gain can be measured as an integral value of the Raman line intensity in a spontaneous Raman scattering spectrum. The spontaneous Raman scattering parameters in diamond, barium nitrate and rare-earth tungstates as well as in the other crystals were measured in [1] and the results are summarized in Table 1.
Since the diamond exhibites one of the most intense Raman lines, the values of integral cross section and peak intensity for the other crystals were normalized to those of the diamond. As it was mentioned above, the integral Raman scattering cross section determines the Raman gain coefficient in the transient case. A comparison of this parameter in different materials shows high values in diamond and tungstate crystals (50–65% of that in diamond). Smaller value is observed in barium nitrate crystal (21%). These data explain why nowadays the KGW crystal is one of the most popular materials for picosecond SRS operation with its Stokes shifts of 901 and 768 cm-1 and high resistance to moisture. Similar characteristics demonstrates another tungstate crystal – potassium yttrium tungstate KY(WO4)2 .
Since the steady-state Raman gain coefficient is defined by the peak cross section, from Table 1 follows that high gain is expected in diamond and barium nitrate crystals. Though the integral cross sections of SRS active vibronic modes of Ba(NO3)2 are 2-3 times less than for tungstates, this crystal has high value of peak intensity due to narrow Raman line. The peak cross section of the Raman mode in Ba(NO3)2 exhibits a high value 60% of that in diamond, which is more than two times higher than that in tungstate crystals.
Comparison of spontaneous Raman scattering spectra of crystals shows that barium nitrate crystals must have higher values of Raman gain coefficient 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 this crystal and in an increase of SRS threshold. The use of tungstate crystals with comparatively large Raman linewidths and large integral cross section can be suggested for operation with picosecond pump pulses.
Physical, optical and SRS properties of Raman crystals
Physical and optical properties of Ba(NO3)2 and KGW Raman crystals are summarized in Table 2.
Barium nitrate Ba(NO3)2 is one of the leading crystals among solid-state Raman shifters in terms of Raman gain coefficient, which is the highest at nanosecond steady-state regime (47 cm/GW@532 nm pump). Barium nitrate also features a moderately broad transparency range (0,33 µm - 1,8 µm) and high damage threshold. Drawbacks of Ba(NO3)2 crystal are low thermal conductivity (1,17 Wm-1K-1) and high thermo-optic coefficient (dn/dT= -20×10-6 K-1), which lead to the thermal lensing effect. The crystal is soft and hygroscopic, therefore should be treated with caution. Barium nitrate is a good Raman shifter for nanosecond applications.
Potassium gadolinium tungstate KGd(WO4)2 (KGW) crystal features good mechanical properties, relatively good thermal conductivity (2,5-3,4 Wm-1K-1) and wide transparency range, which spans from 350 nm to 5 µm. KGW as a Raman crystal features two large Raman modes at 768 cm-1 and 901 cm-1, which are pump polarization dependent. KGW crystal has found many practical applications for frequency shifting, especially for picosecond laser pulses.
Raman wavelength observed in Ba(NO3)2 and KGW crystals are given in the Table 3.
Nd:KGW crystals for self-Raman lasers
In addition, it is possible to use Nd-doped KGW crystals for self-Raman lasers. Self-Raman lasers (SRLs) are the Raman lasers of lowest cost and they allow simple and compact set-ups. In these lasers, the fundamental laser transition and the stimulated Raman scattering (SRS) occur in the same crystal, thus, the amount of optical components in the cavities is smaller than in cavities with separate crystals for fundamental and Stokes wavelengths generation, consequently reducing the intracavity losses and providing higher efficiency. Nd:KGW is an efficient laser media owing to its large emission cross section, broad absorption spectrum, high optical damage threshold, and capability of high Nd3+ doping concentration. Combination of excellent laser properties of Nd3+ ion with efficient Raman conversion in KGW host makes Nd:KGW very attractive material for Q-switched and mode-locked self-Raman lasers, including lasers emitting in the “eye-safe” spectral range for laser rangefinders. At fundamental Nd:KGW laser wavelength of 1351 nm efficient first-Stokes Raman shifting to 1538 nm wavelength could be obtained.
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Optical elements of large size and good optical quality from Ba(NO3)2, KGd(WO4)2, KY(WO4)2 and Nd:KGd(WO4)2 are commercially available at moderate prices from 4Lasers.
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References
1.T.T. Basiev, A.A. Sobol, P.G. Zverev, V.V. Osiko, and R.C. Powel „Comparative spontaneous Raman spectroscopy of crystals for Raman lasers“ Applied Optics, 38, 594-598 (1999).
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