PbIn6Te10

New inorganic nonlinear crystals for three-wave parametric processes in the spectral range up to 30 microns have been grown and studied in LNT. The transparency boundary of the long-wavelength region of conventional selenide compounds is limited by multiphonon absorption, which begins around 14-18 microns and decreases to zero transmission near 19 microns for AgGaSe2, 20 microns for GaSe, and 25 microns for CdSe. Unfortunately, the commercially available crystal with the farthest long-wavelength limit of CdSe has only a very small nonlinearity (~ 18 pm/V). Elementary Te, known for more than 35 years, is transparent from 3.5 to 36 microns and has the highest known nonlinear coefficient among inorganic materials, amounting to ~ 600 pm/V [48]. However, due to the high losses similar to elementary Se, its actual application is almost impossible. Somewhat later, single crystals of the chalcopyrite type AgGaTe2 (an analog of AgGaS2 and AgGaSe2 with an expected transmission band boundary near 25 microns) were grown, having sufficiently large dimensions and high optical quality, but the birefringence of this material also proved insufficient for phase interaction. Another chalcopyrite type telluride, LiGaTe2, has sufficient birefringence, but is chemically unstable in air. In 2011, LNT initiated a study of another family, triple tellurides, about which only their structural properties were actually known. H. J. Deiseroth and co-authors reviewed data on X-ray studies of crystals with a nonmetallic type of the β-Mn phase: SnGa6Te10, PbIn6Te10, and PbGa6Te10. They were obtained only in polycrystalline form. These compounds belong to the point group D3 (32) and the spatial group R32 (D37), but only one of them, PbIn6Te10(PIT), has sufficient birefringence to obtain phase synchronism.
PbIn6Te10 single crystals were grown by the Bridgman—Stockbarger method at a rate of 6 mm per day, with a temperature gradient in the crystallization zone of 10-15 °C, optical elements and prisms were made from them.

The PIT transparency region, in which the absorption coefficient does not exceed 0.07 ± 0.03 cm−1, extends from 3 to 20 microns. At an absorption level of 0.3 cm−1, PIT is transparent from 1.7 to 25 microns (Picture 23), while the zero-transmission boundary is around 31 microns. Based on the measured absorption coefficient α(hv) and using the dependences (hv)2 from hv and (hv)1/2 from hv, we directly and indirectly obtained the values of the band gap Eg (directly) = 1.08 eV (1.15 µm) and Eg (indirectly) = 0.96 eV (1.29 µm), respectively (Figure 24 and 25).

The PIT transparency region, in which the absorption coefficient does not exceed 0.07 ± 0.03 cm−1, extends from 3 to 20 microns. At an absorption level of 0.3 cm−1, PIT is transparent from 1.7 to 25 microns (Picture 23), while the zero-transmission boundary is around 31 microns. Based on the measured absorption coefficient α(hv) and using the dependences (hv)2 from hv and (hv)1/2 from hv, we directly and indirectly obtained the values of the band gap Eg (directly) = 1.08 eV (1.15 µm) and Eg (indirectly) = 0.96 eV (1.29 µm), respectively (Figure 24 and 25).

The reflection coefficients of PIT were measured in the spectral range of 1.5–10.4 microns using the autocollimation method, using prisms with a vertex angle of about 12° and an aperture of 12 × 15 mm2. The measurement accuracy was higher than 0.005. We found that PIT is optically positive with a characteristic birefringence of ne − no ~ 0.05. Table 1 shows the Sellmeyer PIT coefficients.
The nonlinear coefficient d11 for PIT was measured by comparing the efficiency of the GWG conversion with the efficiency obtained for ZnGeP2 (ZGP). A femtosecond OPA based on KNbO3 operating at 4.7 microns with a pulse repetition rate of 1 kHz was used as a laser source. Special attention was paid to obtaining spectral and angular susceptibility so that these effects and the spatial drift of the wave caused by birefringence could be ignored. The results can be summarized as follows: d_eff (PIT) = (0.290 ± 0.015)·d₃₆ (ZGP) or d₁₁(PIT) = (0.647 ± 0.034)·d₃₆(ZGP). Bearing in mind that for this process d₃₆(ZGP) = 79 pm/V, we got d₁₁(PIT) = (51 ± 3) pm/V.
The PIT crystal has birefringence sufficient for phase interaction. Its very high nonlinear coefficient of 51 pm/V combined with such a wide transmission can make it a unique material for nonlinear frequency conversion in the middle and far infrared spectral ranges. It can be pumped by means of an Er3+ laser source operating in the spectral range of 2.9 microns, Cr2+ lasers in the spectral range of 2.5 microns (without significant two-phonon absorption), or even, after reducing residual losses, in the range of about 2 microns, like the well-known powerful Ho3+- or Tm3+-laser systems.
At the NLO and ISOM/ODS 2011 International Conference (Kauai, Hawaii, USA, 2011), dedicated to the 50th anniversary of nonlinear optics, our work PbIn6Te10: new nonlinear crystal for three-wave interactions with transmission extending from 1.7 to 25 µm (S. Avanesov, V. Badikov et al., Opt. Mater. Express 1(7), 1286–1291 (2011)) The creation of a nonlinear far-infrared frequency converter was noted among the works reflecting the world's most significant achievements in recent years.