Investigation of Mid-Infrared Broadband Second-Harmonic Generation in Non-Oxide Nonlinear Optic Crystals

Abstract

The mid-infrared (mid-IR) continuum generation based on broadband second harmonic generation (SHG) (or difference frequency generation) is of great interest in a wide range of applications such as free space communications, environmental monitoring, thermal imaging, high-sensitivity metrology, gas sensing, and molecular fingerprint spectroscopy. The second-order nonlinear optic (NLO) crystals have been spotlighted as a material platform for converting the wavelengths of existing lasers into the mid-IR spectral region or for realizing tunable lasers. In particular, the spectral coverage could be extended to ~19 µm with non-oxide NLO crystals. In this paper, we theoretically and numerically investigated the broadband SHG properties of non-oxide mid-IR crystals in three categories: chalcopyrite semiconductors, defect chalcopyrite, and orthorhombic ternary chalcogenides. The technique is based on group velocity matching between interacting waves in addition to birefringent phase matching. We will describe broadband SHG characteristics in terms of beam propagation directions, spectral positions of resonance, effective nonlinearities, spatial walk-offs between interacting beams, and spectral bandwidths. The results will show that the spectral bandwidths of the fundamental wave allowed for broadband SHG to reach several hundreds of nm. The corresponding SH spectral range spans from 1758.58 to 4737.18 nm in the non-oxide crystals considered in this study. Such broadband SHG using short pulse trains can potentially be applied to frequency up-conversion imaging in the mid-IR region, in information transmission, and in nonlinear optical signal processing.

LixAg1–xGaSe2: Interplay Between Lithium and Silver in Mid‐Infrared Nonlinear Optical Chalcogenides

“…SHG intensity measured by the Kurtz and Perry power method is proportional to the square of the nonlinear second‐order coefficient d eff ; [ 24 ] this value for AgGaSe 2 is 33 pm V −1 . [ 25 ] The obtained d eff values for different compositions ( x ) are shown in Figure a. It can be seen that nonlinearity correlates well with the degree of the structure distortion f (Table S5, Supporting information).…”

Crystal Growth, Characterization, and Properties of Nonlinear Optical Crystals of LixAg1–xGaSe2 for Mid-Infrared Applications

“…The SHG performance of Li x Ag 1– x GaSe 2 ( x = 0.5, 0.6, 0.8, and 1) is shown in Figure a. The value of AgGaSe 2 whose NLO coefficient is 33 pm/V (@λ F = 2.3 μm) is regarded as about 3.3 times of LiGaSe 2 . The SHG response increases with the increase of the Li content when x ≥ 0.5, and Li 0.8 Ag 0.2 GaSe 2 has reached the maximum value.…”

“…(a) Efficiency of SHG in Li x Ag 1– x GaSe 2 and the value of the structural distortion f (the value of AgGaSe 2 was used as a reference); (b) the dependence of the SHG intensities on particle size ranges of Li 0.8 Ag 0.2 GaSe 2 .…”

“…The SHG performance of Li x Ag 1−x GaSe 2 (x = 0.5, 0.6, 0.8, and 1) is shown in Figure 13a. The value of AgGaSe 2 whose NLO coefficient is 33 pm/V (@λ F = 2.3 μm) 37 is regarded as about 3.3 times of LiGaSe 2 . The SHG response increases with improve the SHG intensity in Li x Ag 1−x GaSe 2 with the same structure and make Li x Ag 1−x GaSe 2 a more excellent NLO crystal compared with AgGaS 2 and AgGaSe 2 .…”