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.
Highlights
The field of mid-infrared (IR) photonics is growing rapidly due to increasing demand for applications such as free space communications, remote sensing, environmental monitoring, thermal imaging, defense, IR countermeasure, medicine, gas sensing, and molecular fingerprint spectroscopy [1,2,3]
The mid-IR spectral regions up to ~4 μm could be readily accessible with the oxide Nonlinear optic (NLO) crystals such as lithium niobate (LiNbO3), lithium tantalate (LiTaO3), and potassium titanyl phosphate (KTiOPO4) [6,7,8,9]
The upper spectral limit of photons generated via parametric generation can be extended to ~19 μm using non-oxide crystals such as chalcopyrite semiconductors, orthorhombic ternary chalcogenides, and orientation-patterned (OP) semiconductors (e.g., OP-GaAs, OP-GaP, OP-ZnSe, and OP-GaN) [4,5,11,12]
Summary
The field of mid-infrared (IR) photonics is growing rapidly due to increasing demand for applications such as free space communications, remote sensing, environmental monitoring, thermal imaging, defense, IR countermeasure, medicine, gas sensing, and molecular fingerprint spectroscopy [1,2,3]. Considering the bandwidth of the continuum is inversely proportional to the group velocity (GV) mismatch between the interacting optical waves, broadband parametric generation is possible through GV matching [12] Such broadband SHG using short pulse trains can potentially be applied to frequency up-conversion imaging in the mid-IR region, in information transmission using optical pulse signals, and in nonlinear optical signal processing [27,28,29]. A broader input bandwidth means that a train of pulses with narrower temporal widths can be used as the F-wave because the temporal width of a pulse has a Fourier transform relationship with its spectral width Another critical advantage of the simultaneous BPM–GV matching scheme is that it allows for the use of long crystal lengths without considering the temporal walk-off between the interacting waves. Potential applications of such broadband light sources will be briefly described
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