Broadband terahertz pulse emission from ZnGeP_2

Abstract

Optical rectification is demonstrated in (110)-cut ZnGeP 2 (ZGP) providing broadband terahertz (THz) generation. The source is compared to both GaP and GaAs over a wavelength range of 1150 nm to 1600 nm and peak intensity range of 0.5 GW/cm 2 to 40 GW/cm 2 . ZGP peak-to-peak field amplitude is larger than in the other materials due to either lower nonlinear absorption or larger second order nonlinearity. This material is well suited for broadband THz generation across a wide range of infrared excitation wavelengths.Over the last decade, broadband sources of intense THz pulses have received significant attention due to the possibility of accessing a diverse range of light-matter interactions and the potential for creating high-speed electronics. For a recent review of the progress see Ref.[1] and references therein. Large-area zincblende ̅ crystals are commonly used in time-domain terahertz spectroscopy as sources and electrooptic (EO) samplers [2] due to large second-order nonlinear coefficients. A common issue with materials like GaAs and ZnTe is that the optimum wavelength for THz and optical velocity matching is poorly positioned with respect to the onset of two-photon absorption.Here we generate broadband THz pulses in ZnGeP2, a chalcopyrite (pnictide) crystal with ̅ symmetry, by optical rectification. Optical rectification is a second-order nonlinear optical process that generates a transient D.C. polarization, which typically creates THz radiation for short excitation pulses. In previous work, ZnGeP2 has only been used as a source of narrow-band and tuneable THz radiation through parametric down conversion [3,4]. This work verifies that ZGP has a suitable coherence length, high laserdamage threshold [3] and nonlinear figure-of-merit [5] for optical rectification. ZGP's generation efficiency surpasses that of high-quality and high-resistivity wafers of GaAs and GaP across the infrared, including the datacoms C-band around 1550 nm. This study complements the recent resurgence of interest in ZGP as a nonlinear material driven by high-quality growth [6,7].Undoped ZGP single crystals were grown by the horizontal gradient freeze technique [6]. Samples were cut in the (110) plane and double-side polished for optical transmission measurements. Sample thicknesses are 0.33 mm and 0.93 mm for the short and long crystals respectively. The orientation of the crystals was verified by electron paramagnetic resonance (EPR), ensuring that the z-axis is in the plane of the plate. EPR measures the phosphorus hyperfine signal associated with the zinc-vacancy acceptor in asgrown ZGP, differentiating the 100 and 001 directions [8]. ZGP is birefringent due to a ~2% lattice compression in the z-axis, which complicates the THz generation for pump polarizations that are not parallel or perpendicular to this axis. Measurements were performed at normal incidence with the crystal rotated in the plane to maximize the THz signal. The rotation results in the pump being polarized perpendicular to the z-axis of the ZGP crystal, such th...

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“…We use α to denote the polarization of the measured THz field, and for this work orthogonal components at α = β (parallel) and α = β + π/2 (perpendicular) were recorded. Detection was calibrated using a strong OR source [35]. The ω and 2ω pulses transmitted first through the substrate and then the TI to avoid stretching of the THz emission by the substrate [36].…”

Coherent control of injection currents in high-quality films of Bi2Se3

“…The quasi-static electric field of the THz induces birefringence in the ZnTe that is seen by the gate pulse and measured by projecting the orthogonal polarization components onto balanced photodetectors. 22 The signal is extracted using lock-in detection referenced to a mechanical chopper placed close to the laser source, which modulates the light at 250 Hz. The THz signal is mapped out in the time domain by varying the relative time delay between the emitted THz and the gate pulse.…”

Terahertz emission from ZnGeP_2: phase-matching, intensity, and length scalability

“…Power and Length Scalability Figure 2 (a) shows THz transients collected from the 0.33-mm, 0.92-mm and 3.03-mm ZGP(012) crystals, which are pumped with 1.28-μm radiation with intensity < 1 GW/cm 2 . The pump wavelength and intensity ensure the best phase-matching condition and that nonlinear absorption is negligible [24]. ZGP(012) is studied, because ZGP(110) is phase matched at the edge of the OPA tuning range; see Section 4B.…”

“…where z is the position inside the crystal, t is time, opt is the linear optical absorption and  is the 3PA coefficient. 2PA is neglected due to selected excitation wavelength [24].…”

“…OR in chalcopyrite ZnGeP2 (ZGP) shares attractive qualities with zincblende III-V semiconductors [24,25], including much lower FCA below ~3 THz compared with ZnTe and LiNbO3, smaller velocity mismatch in the near IR (NIR), large 2 nd -order nonlinear coefficients, and a 2PA edge at 1.1 μm. Thus, 2PA can be avoided in ZGP at phase-matched pump wavelengths, as is the case in GaP.…”