The orthorhombic lithium ternary chalcogenides with the chemical formula LiBC2, where B=In or Ga and C=S or Se, occupy a special position among the non-oxide nonlinear crystals because they are characterized by the widest band-gaps.1 Thus, in contrast to most of the other chalcogenide mid-IR nonlinear crystals used for down-conversion of near-IR laser sources, they will not suffer two-photon absorption (TPA) at the pump wavelength of 1064 nm (nanosecond or picosecond Nd:YAG based systems) and in some cases even near 800 nm (femtosecond Ti:sapphire lasers/amplifiers).2 In addition, these compounds exhibit increased damage threshold and relatively low refractive index dispersion in the infrared, which is important for frequency conversion of ultrashort laser pulses, and their thermal conductivity is higher compared to their Ag analogues AgBC2, which is an essential advantage at high average powers. Though exhibiting the lowest nonlinear coefficients, e.g. deff~5.5 pm/V for down conversion from 1064 nm to an idler wavelength of 6.45 µm, LiGaS2 (LGS) possesses the widest band-gap among the LiBC2 compounds and recently we established that it is also the compound characterized by highest damage threshold at 1064 nm: with 14 ns pulses at 100 Hz (exposure time of 1 min) the damage threshold of such crystals, both uncoated and with a single layer antireflection- (AR-)coating, was between ~3.5 J/cm2 (turbidity) and ~4 J/cm2 (full damage) in terms of on-axis fluence. In addition, no aging (no surface degradation after a period of ~7 years) was observed in such polished colourless LGS samples.
© 2011 Optical Society of AmericaPDF Article
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