Application of new nonlinear-optical crystals for development of novel methods for nonlinear-optical conversion of solid-state laser radiation into mid-infrared range presents an important task of modern infrared photonics. Significance of this challenge is caused not only by limited choice of solid-state sources of coherent radiation in mid-infrared range, but also by potential applications of such sources in science, technology, medicine, and biology. Efficient method of optical frequency down-conversion is the difference-frequency generation (DFG) allowing the single-pass conversion of the pump and signal optical frequencies lying in the near-IR range into the mid-IR idler wave. The narrowband, frequency stable signal wave for the DFG is generated in our setup by the stimulated Raman scattering (SRS) in a cubically nonlinear crystal (CaCO3, BaWO4, or diamond). In order to present a comparative study, the LiGaSe2 and LiGaS2 crystals with the equal length of 8 mm were used. Narrowband idler waves at the discrete wavelengths of 4.6 / 5.4 / 7.5 / 9.2 μm and high pulse energies in the range 10 - 50 μJ were generated. The measured linewidths were close to the monochromator resolution limit of < 2 cm-1 (~10 nm @ 7.5 μm) and they can be even narrower. It can be supposed that the idler wave linewidth should be comparable with the Raman mode linewidth (ΔνR = 1.2 2.7 cm-1). Generation at 10.8 μJ was achieved in LiGaSe2 only and the output energy was at in the order of ~100 μJ (close to the measuring probe resolution limit).
GaSe crystals that are promising as nonlinear optical converters in the mid- and far-infrared ranges are charac- terized by high Fresnel losses leading to transmittance per surface at the level of 77%. In this study, antireflection microstructures (ARMs) were fabricated on the surface of the GaSe single crystal by single-pulse femtosecond laser ablation. This method makes it possible to increase the transmission up to up to 92%.
LiGaSe2 and LiInSe2 are promising nonlinear crystals for conversion of laser radiation to the mid-IR spectral range
which are transparent down to the visible and UV. We successfully grew a new mixed crystal as a solid solution in the
system LiGaSe2 - LiInSe2, with a composition of LiGa0.5In0.5Se2 which has the same orthorhombic structure (mm2) as
the parent compounds (LiGaSe2 and LiInSe2). The new crystal is more technological with regard to the growth process
in comparison with LiGaSe2 and LiInSe2 since its homogeneity range is broader in the phase diagram. We established
that about 10% of the Li ions are found in octahedral position with coordination number of 3. The band-gap of
LiGa0.5In0.5Se2 is estimated to be 2.94 eV at room temperature. The transparency at the 0-level extends from 0.47 to
13 μm. The dispersion of the principal refractive indices was measured and Sellmeier equations were constructed. The
fundamental wavelength range for the SHG process extends from 1.75 to 11.8 μm. The nonlinear coefficients of
LiGa0.5In0.5Se2 have values between those of LiGaSe2 and LiInSe2.
LiInSe2 is one of the few (in the meanwhile 6) non-oxide nonlinear crystals whose band-gap (2.86 eV) and transparency
enabled 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 3.34-3.82 μm spectral range with a
maximum idler energy of 92 μJ at 3.457 μm for a repetition rate of 10 Hz. Now we achieved broadly tunable operation,
from 4.7 to 8.7 μm, reaching maximum idler pulse energy of 282 μJ at 6.514 μm, at a repetition rate of 100 Hz
(~28 mW of average power).
LiInSe2 is one of the few (in the meanwhile 6) non-oxide nonlinear crystals whose band-gap (2.86 eV) and transparency
enabled 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 3.34-3.82 μm spectral range with a
maximum idler energy of 92 μJ at 3.457 μm for a repetition rate of 10 Hz. Now we achieved broadly tunable operation,
from 4.65 to 7.5 μm, with a single crystal, reaching maximum idler pulse energy of 282 μJ at 6.514 μm, at a repetition
rate of 100 Hz (~28 mW of average power).
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