Kevin T. Zawilski

Cadmium silicon phosphide (CdSiP2) is a nonlinear optical material widely used in optical parametric oscillators. Intrinsic defects (vacancies and antisites) are responsible for unwanted broad optical absorption bands in these crystals that degrade the performance of the devices. In the present work, optical absorption and electron paramagnetic resonance (EPR) spectra are acquired (at room temperature and 12 K, respectively) from a neutron-irradiated CdSiP2 crystal. After the irradiation, the crystal is highly absorbing from the band edge near 600 nm to beyond 1.3 μm because of overlapping defect-related absorption bands. Heating to 550 °C removes nearly all the absorption induced by the high-energy neutrons. EPR spectra show that a primary effect of the neutrons is the production of phosphorous vacancies. A broad EPR signal attributed to perturbed intrinsic defects is seen before heating the irradiated crystal. After warming to 550 °C, an EPR spectrum representing isolated neutral phosphorous vacancies (VP0) is observed without light and an EPR spectrum from neutral phosphorous-on-silicon antisites (PSi0) can be photoinduced with 633 nm light. The formation of the PSi0 antisites (occurring when phosphorous interstitials created by the neutrons are trapped at silicon vacancies) provides direct evidence that the neutrons displaced phosphorous ions. The VP0 and PSi0 donors were not detected in EPR spectra taken from the as-grown CdSiP2 crystal (i.e., before the neutron irradiation).

Room temperature ferromagnetism observed in polycrystalline MnGeP2 bulk samples was investigated by different experimental techniques. The magnetization measurements show a high temperature ferromagnetic phase with a critical temperature TC = 294.0 K and a low temperature magnetic phase transition at TN = 40.5 K. As these parameters (TC,TN) correspond to those of MnP, a ferromagnetic half-metal, whose composition of the samples was further characterized by x-ray diffraction and Rutherford backscattering measurements. They confirmed the presence of orthorhombic MnP precipitates. Electron paramagnetic resonance (EPR) and ferromagnetic resonance (FMR) measurements in the temperature range of T = 4 K to T = 380 K confirm the attribution of the magnetic phase to MnP and show no additional EPR or FMR signal associated with MnGeP2. We conclude that the magnetic properties of the polycrystalline MnGeP2 bulk samples grown by the horizontal gradient freeze technique are dominated by MnP inclusions and that phase pure chalcopyrite MnGeP2 is not ferromagnetic in contrary to previous reports.

Zinc germanium diphosphide (ZnGeP2) is a ternary semiconductor best known for its nonlinear optical properties. A primary application is optical parametric oscillators operating in the mid-infrared region. Controlled donor doping provides a method to minimize the acceptor-related absorption bands that limit the output power of these devices. In the present study, a ZnGeP2 crystal is doped with selenium during growth. Selenium substitutes for phosphorus and serves as a deep donor. Significant concentrations of native defects (zinc vacancies, germanium-on-zinc antisites, and phosphorous vacancies) are also present in the crystal. Electron paramagnetic resonance (EPR) is used to establish the atomic-level model for the neutral charge state of the selenium donor. The S = 1/2 signal from the neutral donors is produced at 6 K by illuminating with 633 nm light (electrons excited from doubly ionized Zn vacancies convert SeP+ donors to SeP0 donors). A g matrix, with principal values of 2.088, 2.203, and 1.904, is extracted from the angular dependence of the EPR spectrum. The principal-axis direction associated with the 1.904 principal value is close to a Se–Ge bond. This indicates an asymmetric distribution of unpaired spin density around the selenium ion and thus predicts the deep donor behavior.

We study the nanohardness and Young’s modulus of randomly oriented CdSiP₂ (CSP) and ZnGeP₂ (ZGP) single crystals, grown via the horizontal Bridgman method. CSP and ZGP are the only two pnictide chalcopyrites widely used as nonlinear optical crystals in the mid-IR part of the spectrum. Nanoindentation is employed in the continuous stiffness mode (45 Hz, 2 nm) using a Berkovich tip. Nanohardness values of 9.9 ± 0.2 GPa (Knoop hardness of 905 kg/mm²) for CSP and 11.5 ± 0.1 GPa (993 kg/mm²) for ZGP are derived. For Young’s modulus, we obtain 136 ± 2 GPa (CSP) and 150 ± 2 GPa (ZGP). The trend of increasing hardness with bandgap and melting point of the isostructural CSP and ZGP, as deduced from previous measurements, is not confirmed. The results for ZGP are compared to 2GaP, its binary isoelectronic analog, and the values obtained, 11.0 ± 0.3 GPa for the nanohardness and 154 ± 2 GPa for Young’s modulus, indicate good matching within the accuracy limits.

Cadmium germanium diphosphide (CdGeP2) crystals, with versatile terahertz-generating properties, belong to the chalcopyrite family of nonlinear optical materials. Other widely investigated members of this family are ZnGeP2 and CdSiP2. The room-temperature absorption edge of CdGeP2 is near 1.72 eV (720 nm). Cadmium vacancies, phosphorous vacancies, and germanium-on-cadmium antisites are present in as-grown CdGeP2 crystals. These unintentional intrinsic point defects are best studied below room temperature with electron paramagnetic resonance (EPR) and optical absorption. Prior to exposure to light, the defects are in charge states that have no unpaired spins. Illuminating a CdGeP2 crystal with 700 or 850 nm light while being held below 120 K produces singly ionized acceptors (VCd−) and singly ionized donors (GeCd+), as electrons move from VCd2− vacancies to GeCd2+ antisites. These defects become thermally unstable and return to their doubly ionized charge states in the 150–190 K range. In contrast, neutral phosphorous vacancies (VP0) are only produced with near-band-edge light when the crystal is held near or below 18 K. The VP0 donors are unstable at these lower temperatures and return to the singly ionized VP+ charge state when the light is removed. Spin-Hamiltonian parameters for the VCd− acceptors and VP0 donors are extracted from the angular dependence of their EPR spectra. Exposure at low-temperature to near-band-edge light also introduces broad optical absorption bands peaking near 756 and 1050 nm. A consistent picture of intrinsic defects in II-IV-P2 chalcopyrites emerges when the present CdGeP2 results are combined with earlier results from ZnGeP2, ZnSiP2, and CdSiP2.

The last decade has witnessed explosive growth in terahertz radiation fields, ranging from fundamental research to applications. This progress has been marked by the introduction of various terahertz radiation sources and detectors. Here, we review the progress of nonlinear pnictide and chalcogenide ternary crystals for use in the terahertz frequency regime. At the center of this topical review are the various physical and optical properties of numerous emerging pnictide and chalcogenide nonlinear crystals, which are anchored to state-of-the-art coherent terahertz radiation sources based on optical rectification and difference frequency generation, as well as terahertz radiation phase-resolved detectors based on electro-optic sampling. This review aims to provide the foundation necessary to continue advancing pnictide and chalcogenide ternary crystals for the generation, detection, and manipulation of terahertz radiation.

We demonstrate a method to generate wavelength tunable mid-infrared radiation from 3.7–4.3 µm using a nanosecond 1.064 µm Nd:YAG pump at 10 Hz, employing cascaded optical parametric generation in non-critically cut CdSiP₂ crystals.

Barium gallium selenide (BaGa₄Se₇) is a recently developed nonlinear optical material with a transmission window extending from 470 nm to 17 μm. A primary application of these crystals is the production of tunable mid-infrared laser beams via optical parametric oscillation. Unintentional point defects, such as selenium vacancies, cation vacancies (barium and/or gallium), and trace amounts of transition-metal ions, are present in BaGa₄Se₇ crystals and may adversely affect device performance. Electron paramagnetic resonance (EPR) and optical absorption are used to identify and characterize these defects. Five distinct EPR spectra, each representing an electron trapped at a selenium vacancy, are observed at low temperature (there are seven crystallographically inequivalent selenium sites in the crystal). One spectrum is stable at room temperature and is present before illumination. The other four are produced at lower temperatures with 532 nm laser light and are thermally unstable at room temperature. Each S = 1/2 singly ionized selenium vacancy has a large, nearly isotropic, hyperfine interaction with ⁶⁹Ga and ⁷¹Ga nuclei at one neighboring Ga site. A significant portion of the unpaired spin resides in a 4s orbital on this adjacent Ga ion and gives principal values of the hyperfine matrices in the 3350–6400 MHz range. Broad photoinduced optical absorption bands in the visible and near-infrared are assigned to the selenium vacancies.

Chalcopyrite and chalcogenide crystals are investigated for optical rectification and electro-optic sampling. Broadband and narrowband terahertz radiation is produced using ZnGeP₂, CdSiP₂, and AgGaSe₂ chalcopyrite crystals, as well as a BaGa₄Se₇ chalcogenide crystal. Broadband terahertz radiation is detected using a ZnGeP₂ chalcopyrite crystal. We envision these crystal classes being used to generate and detect terahertz radiation in research and commercial applications.