Wide optical transparency range, large nonlinearity coefficient, high optical homogeneity
Non-linear crystals
Currently, BaGa4S7, BaGa4Se7, BaGa2GeS6 and BaGa2GeSe6 crystals have shown impressive results in parametric frequency conversion. The scientific community is scrutinizing their properties in interaction with high-power laser radiation.
Silver-gallium sulfide crystal, also known as AgGaS2 (AGS), shows significant efficiency in near- and mid-infrared spectral range applications.
This is due to its pronounced nonlinear optical properties and high transmission index in the IR region of the spectrum. AgGaS2 facilitates three-wave interactions covering a wide range of wavelengths in the near and mid-IR.
The crystal is often found in infrared generation solutions in the 3 to 10 μm range, especially in the context of optical parametric oscillators (OPO) and optical parametric amplifiers (OPA) pumped by Nd: YAG (1064 nm) or Ti:Sa lasers (800 nm).
In addition, AgGaS2 facilitates frequency mixing, either total frequency generation or difference frequency generation. Moreover, AgGaS2 serves as an efficient frequency doubling crystal for the conversion of infrared radiation similar to that of CO2 lasers at a wavelength of 10.6 μm.
AgGaGeS4
AgGaGeS4 crystal, known as AGGS, is a promising nonlinear crystal belonging to the solid solution family.
It has remarkable properties including a high nonlinear optical coefficient (d31 = 15 pm/V), a wide transmission range from 0.5 to 11.5 μm, and a low absorption coefficient. These unique characteristics make it ideal for converting 1.064 μm Nd:YAG laser frequencies to mid-infrared wavelengths from 4 to 11 μm.
The AGGS crystal also outperforms other crystals in terms of laser damage threshold and phase synchronism. Its high damage threshold makes it suitable for use in stable and high power frequency conversion processes.
AgGaGe5Se12
AgGaGe5Se12 (selenium-germanium-gallium-silver) crystal is a nonlinear optical crystal with rhombic symmetry of solid solution.
It is applicable for solid-state lasers with a frequency shift of 1 μm in the mid-infrared range (2−12 μm). It can be used to produce tunable (4−7.5 μm) femtosecond pulses with a repetition rate of 1 kHz with phase matching for three-wave nonlinear interactions. Pulses with a duration of only five optical cycles (FWHM = 84 fs) are generated near 5 μm with an energy of 0.5 μJ.
Due to the higher damage threshold, larger birefringence and bandwidth, and a greater variety of phase matching schemes, AgGaGe5Se12 can be an alternative to AgGaS2 and AgGaSe2.
BaGa2GeS6
BaGA2GeS6 crystal is a quaternary nonlinear crystal designed for mid-IR. They have excellent nonlinear characteristics comparable to commercial crystals (AgGaS2).
Also, possess chemical stability and have a large forbidden band width. Thus, BaGA2GeS6 crystals are promising for spectroscopic applications in visible, near IR and mid-IR spectral regions (0.38-10 μm).
BaGa2GeSe6
BaGa2GeSe6 crystal has a high damage threshold (110 MW/cm2), a wide spectral transparency range (0.5-18 μm) and high nonlinearity (d11=66±15 pm/V), which makes this crystal very attractive for frequency conversion of mid-IR laser radiation.
It is proved to be the most efficient crystal for second harmonic generation of CO- and CO2- lasers. It is found that broadband two-stage frequency conversion of multilinear CO-laser radiation in this crystal is possible in the wavelength range (2.5-9 μm) with higher efficiency than in ZnGeP2 and AgGaSe2 crystals.
BaGA2GeSe6 crystals are used for nonlinear optical frequency conversion in their transparency range. The wavelengths at which the maximum conversion efficiency can be obtained and the tuning range for generating difference frequencies are found.
Ba2Ga8GeS16
The Ba2Ga8GeS16 compound is isotropic and crystallizes in a non-centrosymmetric space group. The transparency range is (0.42-20 μm) and has a broad forbidden zone (3 eV).
The crystal exhibits strong nonlinear second harmonic generation (SHG) intensity comparable to the reference AgGaS2, with phase matching at 1950 nm laser irradiation. It has a high damage threshold (22 times higher than that of AgGaS2).
BaGa4S7
A new nonlinear optical crystal BaGa4S7 for the mid-infrared (IR) region was grown by the Bridgman-Stockbarger method. Polycrystalline materials of stoichiometric composition were synthesized from BaS, Ga and S as starting materials by solid-phase reactions.
The optical absorption edges in the UV-visible and IR bands are at wavelengths of 350 nm and 13.7 μm (for polished crystals).
From optical measurements of the second harmonic generation on the powders, the NLO coefficient d33 was determined to be 12.6 pm/V. The laser damage threshold of the single crystal was about 1.2 J/cm2 at a wavelength of 1.064 μm. The Vickers hardness value of the crystal is 327.5 HV5, which is equivalent to a Mohs hardness of about 5.
BaGa4Se7
A high quality crystal based on a chalcogenide compound. An acentric rhombic structure was identified in 1983 and an IR NLO effect was reported in 2009.
The crystal was obtained using the Bridgman-Stockbarger method. BaGa4Se7 crystal shows high transmittance over a wide range (0.47−18 μm) except for the absorption peak (15 μm).
CdGa2S4
The growing interest in the wide-gap semiconductor CdGa2S4 (Eg=3.71 eV at 10K) is due to its unique properties.
It is a compound with high photosensitivity, photoluminescence, cadotoluminescence and X-ray luminescence. Also, it has birefringence, optical isotropic point and stimulated emission with a maximum at 2.1 eV (T=80K).
The material is promising for creation of narrow-band tunable optical filters, electro-optical modulators and lasers on its basis.
GaSe
GaSe crystal has been successfully applied in various applications, including efficient frequency doubling (SHG) of CO2 laser radiation and SHG of pulsed CO, CO2 and DF laser radiation (λ = 2.36 μm). In addition, GaSe is capable of converting CO and CO2 laser radiation into the visible range.
GaSe crystal is applied in the fields of optical parametric generation (OPO) in the wavelength range from 3.5 to 18 μm and efficient terahertz wave generation in the range from 100 to 1600 μm.
HgGa2S4
Mercury thiogallate crystal HgGa2S4 has high values of damage threshold and conversion efficiency. Such crystal is applicable for frequency doubling and OРO/OРA in he wavelength range from 1 to 10 μm.
The GWG efficiency of CO2 laser radiation for 4 mm long HgGa2S4 is about 10% pulse duration 30 ns, radiation power density 60 MW/cm2.
HgхCd1-хGa2S4
HgxCd1-xGa2S4 is a mixed nonlinear crystal that is considered as a new material for parametric mid-IR oscillators pumped by Nd:YAG lasers.
Due to the cadmium concentration gradient along the crystal axis, it is possible to tunable the OPO under non-critical phase matching conditions by linearly shifting the crystal. By varying the parameter x from 0.14 to 0.56, it is possible to obtain OPO generation under conditions of noncritical phase synchronism in the spectral range of 2.7-9 μm.
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