Optical spectroscopy of the RbPb 2 Cl 5 :Dy 3+ laser crystal and oscillation at 5.5 m at room temperature

Transcription

1 2690 J. Opt. Soc. Am. B/ Vol. 24, No. 10/ October 2007 Okhrimchuk et al. Optical spectroscopy of the RbPb 2 Cl 5 :Dy 3+ laser crystal and oscillation at 5.5 m at room temperature Andrey G. Okhrimchuk, 1,3, * Leonid N. Butvina, 1 Evgueni M. Dianov, 1 Irina A. Shestakova, 1 Ninel V. Lichkova, 2 Vladimir N. Zagorodnev, 2 and Alexander V. Shestakov 3 1 Fiber Optics Research Center, General Physics Institute of Russian Academy of Science, 38 Vavilov Str., Moscow , Russia 2 Institute of Microelectronics Technology of Russian Academy of Science, Chernogolovka, Moscow region , Russia 3 Elements of Laser Systems Co., 3 Vvedensky Str., Moscow , Russia *Corresponding author: okhrim@fo.gpi.ru Received May 22, 2007; revised August 1, 2007; accepted August 5, 2007; posted August 13, 2007 (Doc. ID 83279); published September 21, 2007 Oscillation at 5.5 m wavelength has been investigated in the low-phonon, moisture-resistant RbPb 2 Cl 5 :Dy 3+ crystal in free-running mode at room temperature. In line with our knowledge this is the longest laser wavelength for a rare-earth doped crystal that does not require any special precautions for survival. Spectra and kinetics of luminescence of the new laser crystal were investigated under pumping to 1.3 m absorption band, corresponding to lasing experiments. Emission cross sections were determined for mid-ir transitions. It was found that emission cross section on the 6 H 9/2 + 6 F 11/2 6 H 11/2 laser transition at 5.5 m wavelength is as high as cm Optical Society of America OCIS codes: , , INTRODUCTION A solid-state laser operating on electronic transitions in rare-earth ions is very attractive as a bright source for mid-ir (MIR) applications. But in practice, an operation wavelength of such a laser does not exceed 3 m. Progress in technology of low-phonon energy matrixes for rareearth ions is required to overcome this limitation. Promising candidates for this role are crystals of the MePb 2 Cl 5 class, where Me=K or Rb [1,2]. They are moisture resistant, and this circumstance beneficially distinguishes these crystals from other low-phonon chloride and bromide crystals that could be doped with rare-earth ions. The Dy 3+ is a promising active ion for (MIR), because it has series of closely spaced multiplets of cm 1 and intense absorption bands in the near-ir suitable for pumping. Spectroscopic properties of the KPb 2 Cl 5 :Dy KPC:Dy 3+ crystals were extensively studied by Nostrand et al., and it was predicted that MIR oscillation is possible in this crystal at wavelength as long as 9 m [1]. In this study we continue our investigation of oscillation and spectroscopic properties of the rare-earth doped RbPb 2 Cl 5 (RPC) crystals [3]. Our crystal growth experience has shown the best optical quality of this crystal among the above mentioned class [4]. 2. PREPARATION OF SAMPLE AND DETERMINATION OF Dy 3+ ION CONCENTRATION The 6 9 mm diameter colorless cylindrical boules of RPC crystals nominally doped with 1.0 weight % of DyCl 3 were grown by the vertical Bridgman method in the two-zone furnace in a silica tube crucible with a vertical gradient of 20 grad/cm. The crystal growth speed was 1 mm/h. The stoichiometric polycrystalline compound of RPC was synthesized by melting of the high purity RbCl and PbCl 2 mixture, and then the obtained compound was purified by zone refining technique. The charging material for crystal growth was prepared by adding a commercial high-purity DyCl 3 to the purified RPC polycrystalline compound. The details of preparation of raw material were reported in [4]. The RPC crystal is biaxial and belongs to the monoclinic crystal class, like the KPC crystal (Table 1). The optical properties are expected to be anisotropic for this crystal, and the direction of crystal growth commonly needs to be optimized. But at this stage of our investigation we did not have a special seed crystal to start crystal growth in the predetermined direction. Instead, in the tube crucible, with a waist of about 2 3 mm diameter, located near its bottom was used to start growth of a single crystal in a random crystallographic direction. To make samples for spectroscopic investigations and oscillation experiments, the boules of crystals were cut perpendicular to the growth direction, and then the samples were polished. The crystallographic orientations of the polished planes were determined by x-ray method and were found to be different for samples grown in different experiments. The samples of crystals were free of any inclusions and bubbles, but slight perturbations of a refractive index were observed over the volume of crystals. X-ray analysis revealed multiple reflections belonging to the same crys /07/ /$ Optical Society of America

2 Okhrimchuk et al. Vol. 24, No. 10/ October 2007/ J. Opt. Soc. Am. B 2691 Table 1. Basic Properties of the KPb 2 Cl 5 and RbPb 2 Cl 5 Crystals Space-group symmetry Crystal lattice parameters Melting temperature C KPb 2 Cl 5 RbPb 2 Cl 5 Reference Monoclinic - P2 1 /c [2] =90 =90 [2] =90.1 =90.1 =90 =90 a=0.88 nm a=0.89 nm b=0.79 nm b=0.80 nm c=1.24 nm c=1.24 nm [4] Density g/cm this work Refractive indexes n x n y = [1], n z =2.019 = m this work =0.633 m Maximum phonon energy hv max (in Raman spectrum) cm [5] tallographic plane, but dispersing at angles up to 20. This observation indicates the presence of crystal blocks disoriented from one another by these angles. We assume that the refractive index perturbations are a consequence of the block structure of the crystals. The concentration of Dy 3+ ions was determined by the inductively coupled plasma mass spectroscopy. We found a considerable gradient of Dy 3+ concentration along a length of the crystal manifesting that a segregation coefficient of this ion was significantly different from unity. A refractive index n was estimated by measurement of an unpolarized transmittance T of an undoped crystal in the wavelength range of 4 10 m on assumption that there were no absorption or scattering losses in the crystal, but the Fresnel reflection was on the faces only. The crystal faces were high-quality polished and were parallel to each other with an accuracy of 30 arc sec (controlled by a standard technique on the optical goniometer, model G5, LOMO). A refractive index was calculated by the formula n = 1+ 1 T T. The crystallographic orientation of the polished plates of the crystal was 111, so we believe that the obtained index was some average over optical axes (which practically coincide with crystallographic axes, because all elementary cell angles are very close to 90 ). The transmittance was found to be 77.1±0.3 %. A refractive index calculated by Eq. (1) is presented in Table 1. For oscillation experiments the planes of the 30 mm long RPC:Dy 3+ crystal were polished so that final surfaces were parallel to each other with an accuracy of 30 arc sec. The antireflection coatings were not made on the faces of the laser rod. The concentration of the Dy 3+ in the laser crystal was cm APPARATUS FOR SPECTROSCOPIC INVESTIGATIONS The unpolarized absorption spectra were recorded by the LOMO SF20M scanning spectrophotometer in the visible (VIS) and near-ir regions and by the Vector-22 Fourier transform infrared spectroscopy (FTIR) spectrometer in the MIR region. A flashlamp-pumped YAG: Nd laser operating at and m wavelengths simultaneously in the pulsed free-running regime was used for excitation of the crystals to measure the spectra and the kinetics of luminescence. The laser pulse duration was 80 s (FWHM). The 1 m grating monochromator was used for spectral selection of luminescence. The 300, 150, and 75 grating/mm gratings were used to take the luminescence spectra and the kinetics at the first order of reflection of the grating in the 1 3, 2 6, and 4 6 m regions, respectively. An emission of the crystals was detected by the Ge photodiode in the 1 16 m wavelength range and the MCT (mercury cadmium tellurium) photoresistor 77 K in the m range to measure spectra and kinetics of luminescence. The luminescence spectra were spectrally corrected. The blackbody source (LOMO TRSh- 2850) operated at 2850 K was used for spectral calibration of the detection system. The long-wavelength pass interference filters were used to cut a short-wavelength emission from the black body coming in the high orders of reflections of the grating. The 1 2, , 3 5, and m regions were calibrated separately with appropriate filters; the same filters were used when measuring the luminescence spectra. 4. SPECTROSCOPIC INVESTIGATIONS The spectroscopic investigations of the RPC:Dy 3+ crystal have been carried out at room temperature. Unpolarized absorption spectra are presented in Fig. 1, and this is a result of combination of spectra recorded with the VIS near-ir and FTIR spectrometers. The absorption spectrum of RPC:Dy 3+ crystal is found to be close to those of the KPC:Dy 3+ crystal [Fig. 1(a)]. In particular, the multiphonon infrared cutoff is located near 500 cm 1 (20 m wavelength), the same as for the KPC:Dy 3+ crystal. This is in agreement with the equal widths of the vibration spectra of these crystals, as a result of the investigations by Raman spectroscopy [5]. According to [5] both crystals have the same maximum phonon energy equal to 203 cm 1. The RPC:Dy 3+ crystal has the intensive absorption bands in the region of m, which are analogous to the absorption bands in the KPC:Dy 3+ crystal and have almost the same wavelength positions [Fig. 1(a)]. Because of this analogy we accepted the Russell Saunders assignments of energy states of the Dy 3+ 4f electronic shell reported in [1] for the KPC:Dy 3+ crystal. In Fig. 2 the energy-level diagram of the Dy 3+ ion with energies of Stark s multiplets for the RPC crystal is presented. Here we assume that the energy positions of the multiplets with a six-fold spin degeneracy correspond to the centers of absorption bands in the region of m cm 1. Notice, that an energy of the 6 H 5/2 multiplet is not determined, because the absorp-

3 2692 J. Opt. Soc. Am. B/ Vol. 24, No. 10/ October 2007 Okhrimchuk et al. Fig. 1. Unpolarized absorption spectra: (a) comparison of f f transition bands in KPC:Dy 3+ (thin curve) and RPC:Dy 3+ (thick curve) crystals; (b) an entire absorption cross-section spectrum of RPC:Dy 3+ ; near-ir region is shown in details at the inset. tion band due to the 6 H 15/2 6 H 5/2 transition is very weak and not visible in the spectrum. This experimental fact is supported by very low values of corresponding reduced matrix elements [6]. In VIS the RPC:Dy 3+ is transparent up to 0.37 m and has only very weak absorption in the m region, corresponding to the spin-forbidden transitions. The Dy 3+ ion has six intensive emission bands centered at 1.32, 1.75, 2.44, 2.9, 4.3, and 5.5 m when pumping at 1.3 m (to the 6 H 9/2 + 6 F 11/2 multiplets) (Fig. 3). Since the 1.32, 1.75, and 2.9 m bands strongly overlapped with the absorption bands centered at 1.3, 1.72, and 2.82 m correspondingly, we assigned these luminescence bands in relation to the transitions to the ground state. Overlapping of the absorption and emission bands often leads to reabsorption of luminescence photons with following reemission after some delay, characterized by an excitedstate decay lifetime. This phenomenon increases a luminescence lifetime and impedes obtaining of a true relaxation time of an excited state. If it takes place a luminescence lifetime characterizes not only an intrinsic property of electronic transitions between multiplets but also a geometry of the sample involved in this process (plate thickness, etc.). In our case this concerned especially to the absorption and emission bands located at 1.3 m, since they are the strongest bands. To investigate the role of the reabsorption and to find the measurement conditions when it vanished, we cut three samples of the RPC:Dy 3+ crystal from the same boule. One sample was a rod of 15 mm long and 9 mm diameter with parallel polished faces, and another two samples were polished plates with thicknesses of 0.9 and 0.45 mm. We measured kinetics of luminescence in the 1.32 m band. Because of strongly different sizes of samples, we used different geometry of excitation and detection in order to minimize reabsorption of luminescence in each case, but pump intensity was the same in all experiments. For the 15 mm sample a luminescence emission was collected by a lens in the direction perpendicular to a pump laser beam, the last passed through the polished surfaces of the crystal rod. No other special equipment was used for this sample. The measurement setup for the 0.9 and 0.45 mm plates is shown in Fig. 4. The 0.9 and 0.45 mm plates were made from the closely spaced volumes of the crystal boule, separated from each other by no more than 2 mm. Absorption of the plates at the wavelength of the luminescence band maximum did not exceed 5% for the 0.9 mm plate and 2.5% for the 0.45 mm plate. Thus the reabsorption along the pump direction was minimized. A diaphragm was used to block a secondary luminescence emission that originated from a volume of the crystal around the pump beam after reabsorption of a primary luminescence propagated and reflected along the polished planes of the crystal plate being investigated. A diameter of the pump beam spot was slightly smaller the Fig. 2. Energy-level diagram of Dy 3+ ion in RPC crystals with luminescent transitions under excitation to the 6 H 9/2 + 6 F 11/2 multiplet. Energy positions in cm 1 are depicted to the left of the multiplet notations. Fig. 3. Unpolarized luminescence spectra of RPC:Dy 3+ crystals under pumping at 1.3 m. Solid curves, luminescence with 0.51 ms lifetime; dashed curves, 5.6 ms lifetime; dotted curve, 13 ms lifetime. Relative intensities within bands with the same lifetime are corrected for apparatus sensitivity. The spectral resolution for 1.32 m and 2.44 m bands is 15 nm; for 1.75 m and 2.9 m bands, 50 nm; and for 4.3 m and 5.5 m bands, 100 nm.

4 Okhrimchuk et al. Vol. 24, No. 10/October 2007/J. Opt. Soc. Am. B 2693 Fig. 4. Setup for measurement of the luminescence kinetics: 1, sample; 2, diaphragm; 3, pump beam; 4, lens, collecting luminescence; 5, spectrometer with a detector; 6, absorber of the pump beam. diaphragm diameter 1mm. In this way an effect of reabsorption in the directions of the polished plane was also diminished. Measured lifetimes were 0.76, 0.505, and ms for the plates with 15, 0.9, and 0.45 mm thickness, respectively. This series of measurements clearly demonstrates a reabsorption effect in the 15 mm plate, and this effect was absent in the plates with a thickness of 0.9 mm and lesser. Thus the true luminescence lifetime for the RPC:Dy 3+ crystal at 1.32 m was found to be 0.51±0.1 ms. The following investigations of the kinetics have been made with the 0.9 mm plate. The series of the analogous measurements of luminescence kinetics was made for the KPC:Dy 3+ crystal in 1.34 m emission band with samples of the same sizes as the above-mentioned RPC:Dy 3+ plates. Luminescence lifetimes of 0.57, 0.413, and ms were obtained. Thus the true luminescence lifetime for the KPC:Dy 3+ crystal at 1.34 m was found to be 0.41±0.05 ms. The kinetics of luminescence was found to be different in quality for the different bands, but there are three groups of the bands with the same kinetics behavior (Fig. 3). The spectrally resolved kinetics for the 1.32, 2.4, and 5.5 m bands demonstrated single exponential curves beginning immediately after the end of the pump pulse with a decay time of 0.51 ms, and the kinetics for the 1.75 m, 2.9, and 4.3 m bands had an intensity rise for 1.3 ms after the end of the pump pulse, and then it had the single exponential tail; the 1.75 and 4.3 m bands had an equal decay time of 5.6 ms; and the 2.9 m band had a decay time of 13 ms (Table 2). The kinetics curves for the most intensive luminescence band from each above-mentioned group are depicted in Fig. 5. Essentially, we used the energy positions of the luminescence bands and the energy-level diagram (Fig. 2) to assign the bands in relation to transitions between the multiplets. This assignment was carried out by taking into account a remarkable difference in the character of luminescence kinetics for the different bands at the initial Fig. 5. The spectrally resolved kinetics of the RPC:Dy 3+ luminescence. Detection wavelengths, 1.32 m (solid curve), 1.75 m (dash curve), 2.9 m (dotted curve). time interval after a pump pulse (Fig. 5). That is, we considered that the bands with the 0.51 ms decay time kinetics corresponded to the transition originating from the directly excited 6 H 9/2 + 6 F 11/2 multiplet. Then the 1.75 and 4.3 m luminescences run owing to excitation of the 6 H 11/2 multiplet through the 6 H 9/2 + 6 H 11/2 6 H 11/2 transitions and the latter transition took place as long as the 6 H 9/2 + 6 F 11/2 level was populated, and to this reason the kinetics curves of the 1.75 and 4.3 m luminescences had maximums delayed by 1.3 ms relative to the end of a pump pulse. This delay time was in agreement with a decay time of the 6 H 9/2 + 6 F 11/2 state. Analogously, a luminescence of the 2.9 m band originated from the 6 H 13/2 multiplet, and run after its excitation due to the 6 H 9/2 + 6 F 11/2 6 H 13/2 transition. Thus emission transitions between each multiplet, the 6 H 9/2 + 6 F 11/2 and below it, were observed in the RPC:Dy 3+ crystal, because there were no energetic phonons that could effectively depopulate these multiplets. The decay times of the multiplets obtained in this analysis are summarized in Table 2. The luminescence branching ratios i were calculated from the luminescence spectra F spectrally corrected for instrument sensitivity and normalized in photons per second: F i d i =, 2 N F i d i=1 where N is a total number of the electronic transitions from the multiplet and i is a number of the transition Table 2. Relaxation Characteristics of Three Least Excited Multiplets of Dy 3+ in RPC Crystals Initial State Transition Final State Center of Luminescence Band m Luminescence Decay Time lum (ms) Branch Ratio Luminescence Quantum Efficiency 6 H 9/2 + 6 F 11/2 6 H 11/ ± H 13/ H 15/ H 11/2 6 H 13/ ± H 15/ H 13/2 6 H 15/ ±1 1 1

5 2694 J. Opt. Soc. Am. B/ Vol. 24, No. 10/ October 2007 Okhrimchuk et al. from the multiplet. As we mentioned above, we did not investigate polarization properties of the luminescence and absorption spectra in this work; instead we were trying to average the polarization factors to obtain a total emission probability of each excited state. To approach this goal we chose a crystal plate the polished faces of which were close to the 111 crystallographic plane, and a luminescence emission was collected by a lens on the entrance slits of the monochromator in the direction perpendicular to a pump laser beam passing through the plate perpendicular to its polished faces. We believed that at the chosen crystallographic orientation of the sample all main polarization components of luminescence were recorded on equal terms. To prove this assumption, we recorded the spectra at the variable orientations of the sample obtained by turning the sample around the direction of an excitation beam through the angle of 90. The difference in relative intensities of the luminescence bands in these records was no more than 5%. Because of a wide spectral range of luminescence originating from the 6 H 9/2 + 6 F 11/2 state, its spectrum was impossible to record with the same setup owing to overlapping of the grating orders and an insufficient blazing range of the grating outside of its nominal range, so the whole spectrum was recorded by parts, and then the parts of the spectrum were joined. The bands centered at 1.32 m and at 1.75 m were recorded with the 150 grating/mm grating and the 1 m cutoff longwavelength passing filter. The band at 2.44 m was recorded with the same grating and the 1.9 m cutoff filter. The bands at 2.9 m and 4.3 m were recorded with the 150 grating/mm grating and the 1.9 m or3 m cutoff filters, respectively. The band centered at 5.5 m was recorded with the 75 grating/mm grating and the 5 m cutoff filter. The instrument sensitivity spectra were recorded with the same corresponding setups, and then corrected luminescence spectra were computed (Fig. 3). The branching ratios obtained from the luminescence spectra are summarized in Table 2. We used the approach proposed in [1] to determine a quantum efficiency of luminescence. As a result of appropriate solution of the rate equations, which expressed an equality of photons and numbers of nonradiative acts due to the 6 H 9/2 + 6 F 11/2 6 H 11/2 transition to photons emitted from the 6 H 11/2 state at a single pulse of excitation, we obtained for the quantum efficiency of the 6 H 9/2 + 6 F 11/2 state 1 : 1 = g F 2g d 2g F 1g d, 3 where 12, 1g, and 2g are the luminescence branching ratios for 6 H 9/2 + 6 F 11/2 6 H 11/2, 6 H 9/2 + 6 F 11/2 6 H 15/2 and 6 H 11/2 6 H 15/2 transitions, respectively, and F 1g and F 2g are luminescence spectra (in photons/s) corresponding to the 6 H 9/2 + 6 F 11/2 6 H 15/ m and 6 H 11/2 6 H 15/ m transitions. In Eq. (3) we assume that the quantum efficiency of a luminescence originating from the 6 H 11/2 multiplet is equal to unity, since it is practically true for the KPC:Dy 3+ crystal [1]. Here the luminescence spectra were recorded using time-resolved technique and for each excitation pulse a full integral under the kinetic curve was taken. As a result, we obtained 1 = For the KPC:Dy 3+ crystal it was reported 1 =0.793 [1]. A probability of a multiphonon nonradiative decay rate of the 6 H 9/2 + 6 F 11/2 state may be calculated by the formula W mp1 = lum The nonradiative relaxation rates for RPC and KPC crystals were found to be very close to each other and equal to 502 and 505 s 1, respectively. This is no surprise, since the crystals have similar crystal lattice parameters and an equal energy of the most energetic phonon (Table 1). So we assumed that phenomenological B and parameters in the energy gap law: W mp = B exp E 1 exp hv E/hv max max, 5 kt are approximately equal for these crystals, and the multiphonon relaxation rates for other lower states were estimated with the parameters B= s 1 and = cm 1 obtained for KPC crystal in [1]. Then luminescence quantum efficiency was calculated by formula equivalent to Eq. (4): q = 1 lum W mp 1. 6 lum The results of calculations are presented in Table 2. A luminescence quantum efficiency was found to be close to unity for the 6 H 11/2 and 6 H 13/2 states. Thus the assumption, when we deriving Eq. (3), is really true. Emission cross section was computed by Fuchtbauer Ladenburg formula: em = th i 4 8 cn 2 lum 0 F i F i d. 7 The results of treatment of the experimental luminescence spectra by Eq. (7) are presented in Fig OSCILLATION EXPERIMENT For oscillation experiments we had the two-mirror resonator of the m range consisting of the concave highly reflective (HR) mirror of 5 cm curvature radius and the flat output coupler of 3% transmittance; the laser crystal was placed near the output coupler. The cavity length was 4.2 cm. A pump source was a pulsed flashlamp-pumped Nd: YAG laser, operating in free multimode mode at and m simultaneously; the repetition rate was 2.5 Hz. A pump laser beam was focused by the 8 cm focus lens. It passed through the dichroic HR mirror and pumped the laser RPC:Dy 3+ crystal through its face; the waist diameter in the crystal was 300 m. The laser cavity was tuned with a standard technique by alignment of reflections of a green laser beam from the mirrors with position of the pump beam. Thus

6 Okhrimchuk et al. Vol. 24, No. 10/October 2007/J. Opt. Soc. Am. B 2695 Fig. 6. Cross-section spectra of the unpolarized emission. The spectral resolution for 1.32 m and 2.44 m bands is 15 nm; for 1.75 m and 2.9 m bands, 50 nm; for 4.3 m and 5.5 m bands 100 nm. matching of the cavity modes with a pumped crystal volume has been attained. Since the laser crystal did not have AR coating, orientation of its surfaces was also precisely tuned to be perpendicular to a green laser beam to keep an emission reflected from the laser crystal ends in the cavity. After such alignment the mid-ir oscillation was obtained at the center of 5.5- m emission band. The oscillogram of the oscillation pulses was recorded by a cooled MCT detector with a time resolution of 2 s. We observed a series of separated mid-ir oscillation pulses initiated nearly at the maximum of each pump pulse (Fig. 7). We explain a deep modulation of the output by a slow relaxation rate of the low laser level 6 H 11/2 5.6 ms, that is, by self-restriction of the laser transition. The oscillation spectrum was recorded with the scanning grating monochromator and the MCT detector. The center wavelength was found to be 5.54 m, and the width of averaged oscillation spectrum recorded by pulseto-pulse was about 20 nm. In Fig. 8 the dependence of the output pulse energy on the pump-pulse energy is shown. 6. CONCLUSIONS In conclusion, we obtained an effective oscillation in a new laser RPC:Dy 3+ crystal and on a new laser transition for Dy 3+ ion, 6 F 11/2 + 6 H 9/2 6 H 11/2, at room temperature. This crystal is moisture resistant and thus does not require any special precaution for survival. An intense absorption band at 1.3 m is suitable for pumping by Fig. 7. Oscillogram of the 5.5 m oscillation. Fig. 8. Dependence of the output oscillation pulse energy upon the pump pulse energy. YAG:Nd laser and laser diodes. To increase a lasing efficiency, the progress in crystal growth is required to eliminate a blocking structure of the crystal and to improve its optical quality. The oscillation obtained is centered at 5.5 m wavelength being the maximum of the emission cross-section spectrum. Tuning of laser wavelength within the entire 5 6 m amplification band is expected to be realized. This spectral range falls into the transparency window of atmosphere, and at the same time corresponds to absorption bands of numerous chemical compounds, so the solidstate lasers based on this crystal may find applications in environment monitoring and other applications in which an effective light interaction with vibrations of molecules may be explored. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, grant The authors thank V. K. Karandashev for determination of Dy concentrations in the double salt crystals. REFERENCES 1. M. C. Nostrand, R. H. Page, S. A. Payne, L. I. Isaenko, and A. P. Yelisseyev, Optical of properties of Dy 3+ - and Nd 3+ -doped KPb 2 Cl 5, J. Opt. Soc. Am. B 18, (2001). 2. K. Nitsch, M. Dusek, M. Nikl, K. Polak, and M. Rodova, Ternary alkali lead chlorides: crystal growth, crystal structure, absorption and emission properties, Prog. Cryst. Growth Charact. Mater. 30, 1 22 (1995). 3. A. G. Okhrimchuk, L. N. Butvina, E. M. Dianov, N. V. Lichkova, V. N. Zagorodnev, and A. V. Shestakov, New laser transition in the RbPb 2 Cl 5 :Pr 3+ crystal in the m wavelength range, Quantum Electron. 36, (2006). 4. N. V. Lichkova, V. N. Zagorodnev, L. N. Butvina, A. G. Okhrimchuk, and A. V. Shestakov, Preparation and optical properties of rare-earth-activated akali metal lead chloride crystals, Inorg. Mater. 42, (2006). 5. K. Rademaker, W. F. Krupke, R. H. Page, and S. A. Payne, Optical properties of Nd 3+ - and Tb 3+ -doped KPb 2 Br 5 and RbPb 2 Br 5 with low nonradiative decay, J. Opt. Soc. Am. B 21, (2004). 6. A. A. Kaminskii, Crystalline Lasers: Physical Progresses and Operating Schemes (CRC Press, 1996).