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The first demonstration, to the best of our knowledge, of continuous-wave (cw) difference-frequency generation (DFG) in LiIn[S.sub.2] crystal is reported. Wide spectral coverage (5.5-11.3 [micro]m) has been obtained with angle and wavelength tuning for type II (eoe) critically phase-matched parametric interaction. The phase-matching conditions in cw DFG have been investigated, which allowed us to improve the Sellmeier parameters by use of a two-pole dispersion equation. An effective nonlinear coefficient [d.sub.eff] = 6.9 [+ or -] 0.8 pm/V has been determined at ~7 [micro]m relative to the well-known nonlinear coefficient [d.sub.36] of AgGa[S.sub.2], which yields a power-conversion efficiency of ~12.4 [micro]W/([W.sup.2] cm). We evaluated the high-resolution spectral characteristics of the DFG source by recording [C.sub.2][H.sub.2] and S[O.sub.2] spectra. OCIS codes: 190.4410, 190.4400, 190.2620, 300.6340, 300.6320, 300.6360.

Developments towards a phase locked 3:1 frequency division of a radiation using AgGaS2 and periodically poled lithium niobate crystals are described.

Mid‐infrared (mid‐IR) nonlinear optical (NLO) crystals are indispensable for the mid‐IR lasers generation with tunable wavelengths from 3 to 20 µm. AgGaSe2 is a commercial mid‐IR NLO crystal with the highest figures of merit, but suffers low laser damage threshold (LDT). To achieve the balance of optical transmission, NLO effect, and LDT, it is proposed to molecularly modify the AgGaSe2 structure by introducing the [LiSe4] tetrahedra, and successfully grow large crystals of a new selenide AgLiGa2Se4. The replacement of half of the heavy Ag+ cations with light Li+ increases the band gap to 2.2 eV (vs. 1.7 eV in AgGaSe2). The LDT value in AgLiGa2Se4 increases five times compared to that in AgGaSe2, while keeping a relatively large NLO susceptibility of 26 pm V−1. Moreover, the thermal expansion coefficients in AgLiGa2Se4 are approximately two times lower in absolute value compared with AgGaSe2, which is beneficial to the large crystal growth. All these advantages would make AgLiGa2Se4 a new promising NLO crystal for mid‐IR laser applications. [ABSTRACT FROM AUTHOR]

LiGaSe2 (LGSe) and LiInSe2 (LISe) are wide band-gap nonlinear crystals transparent in the mid-IR spectral range. LiGa0.5In0.5Se2 (LGISe) is a new mixed crystal, a solid solution in the system LGSe-LISe, which exhibits the same orthorhombic structure (mm2) as the parent compounds in the same time being more technological with regard to the growth process. In comparison with LGSe and LISe its homogeneity range is broader in the phase diagram. About 10% of the Li ions in LGISe occupy octahedral positions (octapores) with coordination number of 3. The band-gap of LGISe is estimated to be 2.94 eV at room temperature and 3.04 eV at 80 K. The transparency at the 0-level extends from 0.47 to 13 μm. LGISe crystals exhibit luminescence in broad bands centered near 1.7 and 1.25 eV which is excited most effectively by band-to-band transition. From the measured principal refractive indices and the fitted Sellmeier equations second-harmonic generation from 1.75 to 11.8 μm (fundamental wavelength) is predicted. The nonlinear coefficients of LGISe have values between those of LGSe and LISe. 6LGISe crystals are considered promising also for detection of thermal neutrons. [ABSTRACT FROM AUTHOR]

SrMgF4 has an extremely large bandgap Eg of 12.50 eV as obtained from reflection dispersion. The symmetry of this crystal is monoclinic P21 at room temperature and transforms to the orthorhombic Cmc21 phase near 478 K as the temperature increases. The acentric character of the low-temperature (LT) phase is confirmed by pyroelectric luminescence at T < 440 K. The fundamental absorption edge of the LT phase is located at 122 nm (10.15 eV). A considerable difference between the absorption edge and bandgap Eg is due to the strong exciton absorption. The first-principles electronic structure, refractive indices, nonlinear susceptibility and polarizability were calculated for both LT and high-temperature (HT) phases. Band-to-band transitions are direct for the LT phase but indirect for HT. In spite of relatively low birefringence (~0.017) and nonlinear susceptibility (~0.044 pm V-1, an order lower than that in KDP), SrMgF4 crystals are considered promising for nonlinear optics thanks to their transparency far in the vacuum ultraviolet spectral region. [ABSTRACT FROM AUTHOR]