S. Lobanov

LiInSe2: A biaxial ternary chalcogenide crystal for nonlinear optical applications in the midinfrared

Single crystals of lithium selenoindate (LiInSe2) are grown by the Bridgman–Stockbarger technique up to sizes of 10 mm in diameter and 20 mm in length. The different phases have color from yellow (as grown) to dark red (after annealing) but all have the same wurtzite type structure mm2 with slightly differing cell parameters. The band gap for the yellow phase at 300 K is at 2.86–2.87 eV. The red color is attributed to point defects and can be removed by proper illumination. Sellmeier equations are constructed for the 0.5–11 μm range and their validity is checked with second harmonic generation which provides first estimations of the nonlinear coefficients of LiInSe2. The potential of LiInSe2 is compared to that of the widely spread and technologically mature AgGaS2.

Optical, vibrational, thermal, electrical, damage, and phase-matching properties of lithium thioindate

Lithium thioindate (LiInS2) is a new nonlinear chalcogenide biaxial material transparent from 0.4 to 12 μm that has been successfully grown in large sizes and with good optical quality. We report on new physical properties that are relevant for laser and nonlinear optics applications. With respect to AgGaS(e)2 ternary chalcopyrite materials, LiInS2 displays a nearly isotropic thermal expansion behavior, a 5-times-larger thermal conductivity associated with high optical damage thresholds, and an extremely low-intensity-dependent absorption, allowing direct high-power downconversion from the near-IR to the deep mid-IR. Continuous-wave difference-frequency generation (5–11 μm) of Ti:sapphire laser sources is reported for the first time to our knowledge.

Optical, thermal, electrical, damage, and phase-matching properties of lithium selenoindate

Lithium selenoindate (LiInSe2) is a new nonlinear chalcogenide biaxial crystal, related to LiInS2 and transparent from 0.54 to 10 μm at the 50% level (10 mm thickness), which has been successfully grown in large sizes and with good optical quality. We report on what we believe to be new physical properties that are relevant for laser and nonlinear optical applications and summarize all relevant characteristics, both from the literature and as measured in the present work. With respect to AgGaS(e)2 ternary chalcopyrite materials, LiInSe2 displays a nearly isotropic thermal expansion behavior with three- to five-times-larger thermal conductivities associated with high optical damage thresholds, and low intensity-dependent absorption, allowing direct high-power downconversion from the near-infrared, especially 1064 nm, to the deep mid-infrared. Continuous-wave difference-frequency generation (5.9–8.1 μm) of Ti:sapphire laser sources is reported for the first time as well as nanosecond optical parametric oscillation with a Nd:YAG laser as the pump source at 100 Hz and idler tuning between 4.7 and 8.7 μm.

Growth and characterization of LiInS2 single crystals

Bulk LiInS 2 single crystals were grown using the Bridgman}Stockbarger technique. The crystals were characterized in composition, structure and defects. The composition was determined both in average and local versions, the latter was carried out using the unique di!erential dissolution technique. For all crystals a departure from ideal LiInS 2 stoichiometry, especially for cations, was detected. In the optical absorption spectra a strong band at 360 nm which disappears after annealing in S 2 vapor was, with a high probability, related to sulfur vacancies V S : their oscillator strength is f*1.3 * 10~4. The blue photoluminescence in all crystals is likely to be due to In L* antisite defect with an energy level 0.62 eV below the conduction band. The universal values of forbidden band gap, E ’ are 3.72 and 3.57 eV at 80 and 300 K, respectively, for all crystals studied indicating the similar character of dominant bonds. ( 2000 Elsevier Science B.V. All rights reserved.

LiInSe 2 nanosecond optical parametric oscillator

Optical parametric oscillation using the new lithium selenoindate nonlinear crystal is reported for what is to our knowledge the first time. A 17 mm long, type II phase-matched sample is pumped by a 10 ns Nd:YAG laser. The minimum pump energy threshold is 3 mJ for a signal-resonant configuration. The signal and idler waves are tunable between 1.47 and 1.57 microm, and 3.3 and 3.78 microm, with a total output energy of 170 microJ corresponding to a 2.4% energy conversion at 8 mJ pump, only limited by the AR coatings damage. With optimized crystal quality and coatings, lithium selenoindate should show superior performance as compared with AgGaS(e)2 crystals, owing to its 4x larger thermal conductivity.

Nd:YAG pumped nanosecond optical parametric oscillator based on LiInSe2 with tunability extending from 4.7 to 8.7 µm

LiInSe(2) is one of the few (only 5) non-oxide nonlinear optical crystals whose band-gap (2.86 eV) and transparency allowed in the past nanosecond optical parametric oscillation in the mid-IR without two-photon absorption for a pump wavelength of 1064 nm. However, the first such demonstration was limited to the 3.3-3.78 microm range for the idler and the average idler power did not exceed 2.5 mW. Here we report broadly tunable operation, from 4.7 to 8.7 microm, of an OPO based on LiInSe(2), achieving maximum idler pulse energy of 282 microJ at approximately 6.5 microm, at a repetition rate of 100 Hz (approximately 28 mW of average power).

Widely tunable continuous-wave mid-infrared radiation (5.5–11 µm) by difference-frequency generation in LiInS 2 crystal

The first demonstration, to the best of our knowledge, of continuous-wave (cw) difference-frequency generation (DFG) in LiInS2 crystal is reported. Wide spectral coverage (5.5-11.3 microm) 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 deff = 6.9 +/- 0.8 pm/V has been determined at approximately 7 microm relative to the well-known nonlinear coefficient d36 of AgGaS2, which yields a power-conversion efficiency of approximately 12.4 microW/(W2 cm). We evaluated the high-resolution spectral characteristics of the DFG source by recording C2H2 and SO2 spectra.

Thermal properties of the midinfrared nonlinear crystal LiInSe2

The thermal conductivity and thermal-expansion and thermo-optic coefficients, the knowledge of which is essential for nonlinear optical applications, are measured along the three crystallographic axes of the newly discovered orthorhombic crystal LiInSe2. The latter has a nonlinear susceptibility only a quarter lower than that for the commercially available AgGaS2, but its advantages include ∼4 times higher thermal conductivity, ∼2 times lower thermo-optic coefficients, and the lack of sign inversion in thermal-expansion coefficients.

Orthorhombic crystals of lithium thioindate and selenoindate for nonlinear optics in the Mid-IR

We review the optical and other relevant properties of two lithium indium compounds with wurtzite type structure, LiInS2 and LiInSe2, which occupy a unique position among the mid-IR nonlinear crystals, having band-gaps at 347 nm and 433 nm, respectively. Realized and potentially interesting frequency conversion schemes with these two biaxial crystals are discussed.

Optical transitions due to native defects in nonlinear optical crystals LiGaS2

LiGaS2 (LGS) is a recently developed nonlinear optical crystal widely used for nonlinear conversion in the mid-infrared spectral region, but its applications are significantly influenced by the native defects present in the lattice. In this work, absorption and photoluminescence (PL) spectra are studied after annealing the as-grown LGS crystal in different chemical environments in order to reveal features related to anion vacancy (VS) and cation antisite defect (GaLi). In addition, irradiation with fast electrons produces VS centers and their complexes in LGS. The VS migration and mutual transformations in these defects take place at room temperature whereas their complete annealing occurs at 670 K. Meanwhile, the first-principles calculations are adopted to identify the relevant optical transitions. A good agreement with the experimental results was obtained. It is established that the dominant defects VS result in the optical absorption at 3.43 eV and violet PL emission, while GaLi is responsible for absorption at 2.95 eV. The intense PL peak at 1.92 eV is associated with the self-trapped excitons in LGS. The other weak PL bands are also related to the recombination of electrons or holes with the VS and GaLi defect states.LiGaS2 (LGS) is a recently developed nonlinear optical crystal widely used for nonlinear conversion in the mid-infrared spectral region, but its applications are significantly influenced by the native defects present in the lattice. In this work, absorption and photoluminescence (PL) spectra are studied after annealing the as-grown LGS crystal in different chemical environments in order to reveal features related to anion vacancy (VS) and cation antisite defect (GaLi). In addition, irradiation with fast electrons produces VS centers and their complexes in LGS. The VS migration and mutual transformations in these defects take place at room temperature whereas their complete annealing occurs at 670 K. Meanwhile, the first-principles calculations are adopted to identify the relevant optical transitions. A good agreement with the experimental results was obtained. It is established that the dominant defects VS result in the optical absorption at 3.43 eV and violet PL emission, while GaLi is responsible for ab...