Peter G. Schunemann

We demonstrate a cascaded optical parametric generation architecture based on type-I non-critical phase matching (NCPM) in CdSiP ₂ producing tunable mid-wave infrared radiation in the first atmospheric transmission window. The architecture was implemented with two Nd:YAG pump lasers: a 10 Hz flashlamp-pumped system and a 1 kHz diode-pumped Nd:YAG system, generating up to 108 and 17 µJ at 3.75 µm, respectively. Temperature tuning of the second NCPM CdSiP ₂ stage, with orthogonal crystal c axes to the first stage, enabled continuous wavelength tuning from 3.75 to 4.35 µm.

CdGeP ₂ is a non-centrosymmetric uniaxial crystal whose birefringence is too low for phase-matching in its transparency range; however, it has been successfully applied in THz generation using near-IR laser pump sources. We present measurements of the linear thermal expansion of CdGeP ₂ in the 12-820 K temperature range by X-ray diffraction using powdered samples. The results indicate strong anisotropy with negligible compression along the optical axis.

High-performance nonlinear optical (NLO) materials are essential for shifting ultrafast laser sources deep into the mid-infrared, especially for spectroscopic applications aimed at identifying species in the molecular fingerprint region (6.7–20 μm). Traditional oxide NLO crystals are limited to output wavelengths below 5 μm, but non-oxide semiconductors like ZGP, CSP, OP-GaAs, and OP-GaP offer deep infrared transparency, high nonlinearities, and broadband tunability. We review essential property requirements, compare materials of choice in two distinct classes - bulk birefringent crystals and quasi-phase-matched semiconductors - cite key ultrafast laser demonstrations, and identify emerging new materials and future directions in the field.

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).

We demonstrate optical transient detection (OTD) via photorefractive two-wave mixing using visible femtosecond pulses. This technique utilizes a nonlinear crystal to perform OTD directly in the blue spectrum by employing second-harmonic generation (SHG) of high-repetition-rate infrared pulses. The approach enables measurement of phase variations in dynamic signals by using conventional Si-based detectors, while effectively minimizing stationary background fluctuations. Results show a correlation between input phases and output intensities, and condition for enhanced phase sensitivity under weak phase modulation. Additionally, we demonstrate that synchronization of blue pulses results in low-average-power requirement for OTD using a single nonlinear crystal.

Mid-to-far-infrared (IR) spectral content is recorded using the novel self-balanced and self-phase-corrected electro-optical (EO) sampling arrangement. Self-balancing guarantees that the electric field emerging from the EO crystal yields a signal of zero via a Wollaston prism and balanced photodetector (i.e., without a wave plate following the EO crystal) in the absence of the electric field being sampled. Moreover, self-phase-correction ensures a nearly frequency-independent phase difference between the probe electric field pulse and the electric field pulse generated within the EO crystal via second-order nonlinear optical interactions. The self-balanced and self-phase corrected arrangement has the potential to yield enhancement of EO signals recorded using previously investigated birefringent crystals (implemented within traditional EO sampling geometries) and deliver optimum EO signal strengths when considering unexplored birefringent crystals.

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.

We demonstrate mid-infrared time-domain optical coherence tomography (OCT) with an orientation-patterned GaP optical parametric oscillator. Instantaneous broadband mid-infrared spectra provide reduced scattering for OCT applications including cultural heritage, quality assurance, and security. B-scan calibrations performed across the wavelength tuning range show depth resolutions of 67 µm at 5.1 µm and 88 µm at 10.5 µm. Volumetric imaging inside a plastic bank card is demonstrated at 5.1 µm, with a 1 Hz A-scan rate that indicates the potential of stable broadband OPO sources to contribute to mid-infrared OCT.