CdSiP2 Crystal Research Articles

Residual optical absorption from native defects in CdSiP2 crystals

CdSiP2 crystals are used in optical parametric oscillators to produce tunable output in the mid-infrared. As expected, the performance of the OPOs is adversely affected by residual optical absorption from native defects that are unintentionally present in the crystals. Electron paramagnetic resonance (EPR) identifies these native defects. Singly ionized silicon vacancies (VSi−) are responsible for broad optical absorption bands peaking near 800, 1033, and 1907 nm. A fourth absorption band, peaking near 630 nm, does not involve silicon vacancies. Exposure to 1064 nm light when the temperature of the CdSiP2 crystal is near 80 K converts VSi− acceptors to their neutral and doubly ionized charge states (VSi0 and VSi2−, respectively) and greatly reduces the intensities of the three absorption bands. Subsequent warming to room temperature restores the singly ionized charge state of the silicon vacancies and brings back the absorption bands. Transitions responsible for the absorption bands are identified, and a mechanism that allows 1064 nm light to remove the singly ionized charge state of the silicon vacancies is proposed.

Nanohardness Measurements of CdSiP2 and ZnGeP2 Chalcopyrite-Type Nonlinear Optical Crystals

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.

CdSiP2: An Emerging Crystal for Electro-Optic Sampling from Terahertz to the Infrared

An emerging crystal, CdSiP2, is shown to exhibit a higher nonlinearity than other key electro-optic sampling crystals, while also satisfying the electro-optic sampling phase-matching conditions across the spectral range of ∼15–90 THz. Frequencies up to ∼65 THz are measured using an 800 nm central-wavelength probe electric field pulse and a (110) CdSiP2 crystal; however, a higher central-wavelength (i.e., 1.3 μm) probe pulse can be used to record frequencies up to 90 THz. Given the high nonlinearity and electro-optic phase-matching capabilities of CdSiP2, it is envisioned to emerge as the prominent crystal for detecting electric field waveforms within the high terahertz and infrared spectral regimes.

Tunable and coherent terahertz source based on CdSiP2 crystal via collinear difference frequency generation.

CdSiP2 (CSP) crystals have attracted increasing attention as efficient optical conversion media. Herein, the optical properties of a CSP crystal grown with the vertical Bridgman method are measured by a terahertz time-domain spectrometer (THz-TDS) at 0.2-3 THz. For the first time, to the best of our knowledge, the broadband, tunable, coherent, monochromatic THz radiation from 0.08 to 1.68 THz (3775-178 µm) is generated experimentally via this crystal, which is pumped by a nanosecond Q-switched Nd:YAG laser and an optical parametric oscillator (OPO) and based on difference frequency generation (DFG) technology. The output power and its corresponding conversion efficiency at 0.74 THz are 26.6 mW and 1.4 × 10-7, respectively. Our work demonstrates that the CSP crystal is a potential efficient terahertz DFG candidate for out-of-door applications.

First-principles study of dopant stability and related optical properties in CdSiP2 crystal

CdSiP2 (CSP) with outstanding mid-infrared nonlinear optical properties in the 3–5 μm and 8–12 μm windows has always been expected for application requirement, whereas the inadequate of absorption losses cannot be ignored as practical usage. The defect formation energies, electronic structure characteristics, structure distortion and related optical properties of CSP with some traditional and basic dopants M= (Mg, Al, Cu, Fe, and Mn) substituting Cd or Si sites are investigated by employing density functional theory (DFT). The calculations show that the energetically and thermally stable states in CSP crystals are Mg0, Al1+, Cu4+, Fe4+, Mn4+ at Cd sites and FeSi1+ at SiP2 reference states. Fe and Mn dopants at Cd sites bring abundant valences, including 0, +1, +2 and +4 charged states, and result in a mean absorption spectrum increase. New peaks appear in the density of states as the CuCd4+, FeCd4+, and MnCd4+ take shape, and thus affect the absorption spectrum. According to the calculation results, highly localized d orbitals of dopants should be mainly responsible for the absorption aberrance, because they not only lead to a mean absorption spectrum increase, but also bring new absorption peaks affecting the conversion efficiency along X/Y or Z directions.

Electron paramagnetic resonance and optical absorption study of acceptors in CdSiP2 crystals

Cadmium silicon diphosphide (CdSiP2) is a nonlinear material often used in optical parametric oscillators (OPOs) to produce tunable laser output in the mid-infrared. Absorption bands associated with donors and acceptors may overlap the pump wavelength and adversely affect the performance of these OPOs. In the present investigation, electron paramagnetic resonance (EPR) is used to identify two unintentionally present acceptors in large CdSiP2 crystals. These are an intrinsic silicon-on-phosphorus antisite and a copper impurity substituting for cadmium. When exposed to 633 nm laser light at temperatures near or below 80 K, they convert to their neutral paramagnetic charge states (SiP0 and CuCd0) and can be monitored with EPR. The corresponding donor serving as the electron trap is the silicon-on-cadmium antisite (SiCd2+ before illumination and SiCd+ after illumination). Removing the 633 nm light and warming the crystal above 90 K quickly destroys the EPR signals from both acceptors and the associated donor. Broad optical absorption bands peaking near 0.8 and 1.4 μm are also produced at low temperature by the 633 nm light. These absorption bands are associated with the SiP0 and CuCd0 acceptors.

Synthesis of Polycrystalline CdSiP2 in a Gradient Temperature Field

A procedure for the synthesis of a CdSiP2 compound from the initial elementary components in a gradient thermal field has been developed. The phase and chemical composition of the synthesized and recrystallized material is confirmed by the data of X-ray diffraction analysis and scanning electron microscopy with an energy-dispersive system. The polycrystalline material obtained by the developed method will be used to grow bulk nonlinear optical CdSiP2 crystals.

Critically phase-matched Ti:sapphire-laser-pumped deep-infrared femtosecond optical parametric oscillator based on CdSiP2.

We report a high-repetition-rate femtosecond optical parametric oscillator (OPO) for the deep-infrared (deep-IR) based on type-I critical phase-matching in CdSiP2 (CSP), pumped directly by a Ti:sapphire laser. Using angle-tuning in the CSP crystal, the OPO can be continuously tuned across 7306-8329nm (1201-1369 cm-1) in the deep-IR. It delivers up to 18mW of idler average power at 7306nm and >7 mW beyond 8000nm at 80.5MHz repetition rate, with the spectra exhibiting bandwidths of >150 nm across the tuning range. Moreover, the signal is tunable across 1128-1150nm in the near-infrared, providing up to 35mW of average power in ∼266 fs pulses at 1150nm. Both beams exhibit single-peak Gaussian distribution in TEM00 spatial profile. With an equivalent spectral brightness of ∼5.6×1020photons s-1 mm-2 sr-10.1% BW-1, this OPO represents a viable alternative to synchrotron and supercontinuum sources for deep-IR applications in spectroscopy, metrology, and medical diagnostics.

Terahertz birefringence and absorption of a chalcopyrite CdSiP2 crystal

The terahertz (THz) birefringence and absorption of a chalcopyrite CdSiP2 (CSP) crystal are studied at frequencies between 0.5 and 2.9 THz. Time-domain spectroscopy measurements are used to record the phonon-polariton dispersion and reststrahlen band absorption (centered at 2.03 THz) of CSP's lowest-frequency E mode. The ordinary and extraordinary refractive indices of this material, along with the extinction coefficients, are determined over the investigated frequency range. The uniform CSP birefringence (∼0.08) and the low extinction coefficient values (≤0.008) at frequencies between 0.5 and 1.9 THz suggest the potential for waveplate applications.

Crystal growth and dislocation etch pits observation of chalcopyrite CdSiP2

CdSiP2 is the only crystal that can offer Non-critical Phase Matching (NCPM) for a 1064nm pumped optical parametric oscillation (OPO) with idler output in the 6μm range. In this paper, a large, crack-free CdSiP2 single crystal measuring 18mm in diameter and 65mm in length was successfully grown by the Vertical Bridgman method (MVB) with an explosion-proof quartz ampoule. The results of lattice parameters, element composition and IR transmittance of the as-grown crystal characterized by X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDS) and Fourier transformation infrared spectrometer (FTIR) showed the as grown crystal crystallized well and the absorption coefficients at 4878cm−1 and 2500cm−1 were 0.14cm−1 and 0.06cm−1. Moreover, a new etchant composed of Br2, HCl, HNO3, CH3OH and H2O (1:800:800:400:400 in volume ratio) was prepared and the dislocation etch pits on oriented faces of as-grown CdSiP2 crystal were observed for the first time. It is found the etch pits are in rectangular structure on the (101) face, but in trigonal pyramid structure on (312) face. According to the quantities of the etch pits, the average densities of dislocation were evaluated to be 2.28×105/cm2 and 1.4×105/cm2, respectively.

Growth and polarized Raman spectroscopy investigations of single crystal CdSiP2: Experimental measurements and ab initio calculations

Large crack free single crystals of CdSiP2 with the dimension of 40mm in length and 8mm in diameter were grown by vertical Bridgman method. The bonds vibration properties of the CdSiP2 single crystal were investigated by means of polarized micro-Raman spectroscopy and ab initio calculations. The Raman-active modes and IR-active modes were selected in turn. They were definitively assigned in wave number and symmetry based on classical lattice dynamics calculations. The results indicate that the high phonon frequencies mainly arise from the atomic vibrations of the lighter atoms Si or benching/stretching vibrations of PSi bonds; while, the lower frequencies mainly arise from the atomic vibrations of the heaver atoms Cd or the benching/stretching vibrations of PCd bonds. The conclusions may apply to other II–IV–V2 chalcopyrite compounds such as ZnGeP2, CdGeP2, ZnSiP2.

Defect-related optical absorption bands in CdSiP_2 crystals

When used as optical parametric oscillators, CdSiP2 crystals generate tunable output in the mid-infrared. Their performance, however, is often limited by unwanted optical absorption bands that overlap the pump wavelengths. A broad defect-related optical absorption band peaking near 800 nm, with a shoulder near 1 µm, can be photoinduced at room temperature in many CdSiP2 crystals. This absorption band is efficiently produced with 633 nm laser light and decays with a lifetime of ∼0.5 s after removal of the excitation light. The 800 nm band is accompanied by a less intense absorption band peaking near 1.90 µm. Data from eight CdSiP2 crystals grown at different times show that the singly ionized silicon vacancy (VSi−) is responsible for the photoinduced absorption bands. Electron paramagnetic resonance (EPR) is used to identify and directly monitor these silicon vacancies.

Intrinsic defects and their effects on the optical properties in the nonlinear optical crystal CdSiP2: a first-principles study.

In view of their high nonlinear optical coefficients and good phase-matching properties, CdSiP2 (CSP) crystals are considered as one of the most promising materials in the field of nonlinear optical applications. However, the slight absorption losses around 1.34 μm and 1.78 μm under e-polarized light have been affecting its performance. In this study, first-principles calculations were performed to identify the effects of various charge defects on the absorption properties. Different intrinsic defects in the CSP crystal were calculated using the HSE method and compared according to the specific chemical environments in the experiments. The results show that the point defects of V, Si, and V, which can be spontaneously formed, are dominant in the Cd-poor environment. The combination of Si and V defects is the most favorable cluster in the Cd-poor case because of its relatively low formation energy. Furthermore, the antisite defect SiCd was found to be responsible for the main absorption peaks at 1.34 and 1.78 μm in the experimental spectra, whereas other defects and clusters, such as the defects Si and V, also contribute to these red shifted absorptions. Our results intend to provide a guideline for adjusting the optical absorption in CSP by modifying its defects.

Identification of native defects (vacancies and antisites) in CdSiP2 crystals

Electron paramagnetic resonance (EPR) is used to identify four native defects in single crystals of CdSiP2. This nonlinear optical material is used in optical parametric oscillators to generate tunable output in the mid-infrared. The performance of these frequency-conversion devices is limited when infrared absorption bands associated with native defects overlap a pump wavelength. Cadmium, silicon, and phosphorus vacancies and also silicon-on-cadmium antisites are present in the as-grown undoped CdSiP2 crystals. Using near-band-edge 632.8 nm light from a He-Ne laser, a paramagnetic charge state, and thus an EPR spectrum, is formed at liquid-helium temperatures for three of the four defects. The EPR spectrum from the singly ionized silicon vacancy (VSi−) is present without light and has five hyperfine lines due to equal interactions with the four neighboring 31P nuclei. In contrast, the photoinduced EPR spectrum from the singly ionized cadmium vacancy (VCd−) has a three-line hyperfine pattern due to equal interactions with only two of its four neighboring 31P nuclei. The light-induced spectrum from the singly ionized silicon-on-cadmium antisite (Si Cd+) also has a three-line hyperfine pattern, thus indicating that the unpaired spin interacts primarily with only two 31P neighbors. For the neutral phosphorus vacancy (VP 0), the unpaired spin is primarily localized on the nearest-neighbor silicon ions and the photoinduced EPR spectrum has no resolved 31P hyperfine interactions. The silicon and cadmium vacancies are acceptors, and the silicon-on-cadmium antisite and the phosphorus vacancy are donors.

Differential Thermal Analysis and Crystal Growth of CdSiP2

Abstract Thermal properties of CdSiP2 polycrystalline were studied by differential thermal analysis (DTA) technique with a quartz capillary column. It is found that the melting point and crystallization temperature of CdSiP2 are 1139 and 1126 °C, respectively, and the supercooling degree of CdSiP2 melt is evaluated to be 13 °C. According to the results of DTA, the structure of a furnace and the temperature profile of the crystal growth for CdSiP2 were optimized. A crack-free CdSiP2 crystal with 15 mm in diameter and 40 mm in length was grown by the modified vertical Bridgman (VB) method. The X-ray diffractionmeter (XRD), X-ray energy dispersive microanalysis (EDX) and infrared spectrophotometer (IR) were employed to characterize the properties of as-grown crystal. A new cleavage face of (112) was identified in XRD spectrum. The results of EDX indicate that the crystal is of good stoichiometry. The infrared transmission is up to 55% in the infrared region from 7000 to 1500 cm−1. All the characterization results show that the obtained crystal is integrated in structure and good in optical quality which can be used in devices fabrication.

A modified method to simulate the dispersion properties of infrared nonlinear optical crystals

This paper presents a modified method to study the dispersion properties of infrared nonlinear optical crystals, which takes advantage of the first principles calculations and oscillator model. The phonon effects play an important role in the dispersion properties, and obvious discrepancies between calculated and experimental results are found due to the lack of such effects in first principles optical property calculations. In order to compensate for such deficiency, the oscillator model was introduced. Phonon oscillator terms are added to the calculated dispersion equations, and the phonon oscillator coefficients can be obtained from first principles infrared dielectric property calculations. With the modified method, the simulated dispersion curves of GaAs, CdSiP2, AgGaS2, and ZnGeP2 crystals show excellent agreement with the experimental results.

Investigations on Optical Scatter Particles during CdSiP$lt;inf$gt;2$lt;/inf$gt; Crystals Growth

Polycrystalline charges of CdSiP2 (CSP) were synthesized by direct reaction method using P, Si, and Cd elements as the initial materials. Single crystals with the dimension of 8 mm in diameter and 40 mm in length were successfully grown by vertical Bridgman method. The dimension and the composition of the optical scatter particles in the CSP crystals were identified by SEM measurement. The results show that most of optical scattering particles are round-shaped with dimension of 2-8 μm. The EDS measurement result shows that the atomic percent of Si in the op- tical scattering particles is over 88%. The synthesis mechanism of CSP by single temperature zone method was studied by powder XRD. The results show that there is some unreacted silicon left when synthsized process is carried out be- low 1150℃. By optimizing the synthesis parameters and the Si inclusions were successfully restrained and the high optical quality CSP single crystal was obtained.

Femtosecond-laser pumped CdSiP_2 optical parametric oscillator producing 100 MHz pulses centered at 62 μm

We report the first, to the best of our knowledge, femtosecond-laser-pumped optical parametric oscillator (OPO) based on the newly developed nonlinear crystal, CdSiP₂. The OPO was synchronously pumped by a mode-locked Yb:KYW/Yb:fiber master-oscillator power amplifier, providing 1.053 μm pump pulses with durations of 130 fs at a repetition rate of 100 MHz. The 0.5-mm-thick CdSiP₂ crystal was cut for a type-I noncritical interaction, providing a broad phase-matching bandwidth and ensuring excellent temporal overlap among the pump, signal, and idler pulses. The OPO generated signal pulses with a spectral coverage over 1260-1310 nm and mid-infrared idler pulses with a broad spectral coverage at 5.8-6.6 μm (6.2 THz). With a 2% output coupler (OC), the threshold pump power was 600 mW. At the maximum pump power of 1.6 W, 0.55 W was absorbed due to two-photon absorption and residual linear absorption in the CdSiP₂ crystal, 0.75 W was transmitted, and 53 mW signal power was extracted through the OC. We estimate that the generated idler power was 24 mW.

Vertical Bridgman growth and optical properties of CdSiP2 crystals

Single crystals of CdSiP2 (CSP) with the dimension of 8 mm in diameter and 40 mm in length were successfully grown using vertical Bridgman method. Optical observations showed the presence of macro-defects including flaws and inclusions in the as-grown crystals. The macro-defects can be restrained through controlling the process of polycrystalline synthesis and growth parameters. For the crystals with improved qualities, the high-resolution X-ray diffraction measurement on the (004)-faced plate indicates that the full-width at half-maximum (FWHM) of the rocking curve is 34′′. The phase-matching curves calculated from refractive indices indicate that, for 1.064 μm pumping, the CSP crystal will produce a noncritically phase-matched output at 6.2 μm through a type-I parametric process, and a noncritically phase-matched output at 5.0 μm through a type-II parametric process. For 1.55 μm and 2.00 μm pumping, the crystal yields continuous tunability from 2 to 6.5 μm through a type-I parametric process. The absorption coefficients for e-polarized light at 1.064 μm, 1.55 μm and 2.0 μm of the as-grown CSP crystal are 0.53 cm−1, 0.39 cm−1, and 0.25 cm−1, respectively.

Optical parametric generation in CdSiP_2 at 6125 μm pumped by 8 ns long pulses at 1064 nm

A 21.4 mm long noncritically cut CdSiP2 crystal, pumped by 8 ns pulses at 1064 nm in a double-pass configuration for pump, signal, and idler, generated 523 μJ, 5.8 ns idler pulses at 6.125 μm. The average power of 52.3 mW at the repetition rate of 100 Hz is the highest ever achieved at such wavelengths with direct down conversion from the 1 μm spectral range.