Terahertz Pulse Generation from GaAs Metasurfaces

Abstract

Ultrafast optical excitation of select materials gives rise to the generation of broadband terahertz (THz) pulses. This effect has enabled the field of THz time-domain spectroscopy and led to the discovery of many physical mechanisms behind THz generation. However, only a few materials possess the required properties to generate THz radiation efficiently. Optical metasurfaces can relax stringent material requirements by shifting the focus onto the engineering of local electromagnetic fields to boost THz generation. Here we demonstrate the generation of THz pulses in a 160 nm thick nanostructured GaAs metasurface. Despite the drastically reduced volume, the metasurface emits THz radiation with efficiency comparable to that of a thick GaAs crystal. We reveal that along with classical second-order volume nonlinearity, an additional mechanism contributes strongly to THz generation in the metasurface, which we attribute to surface nonlinearity. Our results lay the foundation for engineering of semiconductor metasurfaces for efficient and versatile THz radiation emitters.

Nonlinear Terahertz Generation in Semiconductor Metasurfaces

“…Whilst we find a good agreement with the experimental data for one orientation of the MS (red line), we also find that the calculated volume nonlinearity does not show the same dependence on MS orientation as the THz field measured from the MS. This result, combined with our investigation of the THz emission from unpatterned GaAs layers [3], allows us to conclude that a significant proportion of the THz field is generated at the MS interfaces, where the nonlinear properties of the material are altered due to the abrupt change in crystal structure.…”

“…Firstly, we design a MS based on GaAs, which is able to support a wide variety of THz generation mechanisms [3]. The MS is designed to maximize absorption of NIR light when excited at a 45 angle (Fig.…”

Terahertz Pulse Generation with Binary Phase Control in Nonlinear InAs Metasurface

“…THz generation in bulk InAs crystals has been attributed mainly to photocurrent mechanisms, − which offer limited options for controlling polarity of emitted THz pulses. In contrast, the volume optical rectification effect depends sensitively on the material crystallographic orientation, and the excitation polarization. , While the volume nonlinear contribution in InAs has been overshadowed by the photocurrent mechanisms, this polarization dependence can enable control of the THz generation process.…”

“…To investigate the underlying mechanisms, we characterized the radiated THz pulses for different excitation polarizations. This experiment allows us to differentiate between the photocurrent mechanisms and the optical rectification mechanisms . We find that THz pulses generated from the metasurface strongly depend on the incident polarization angle θ (Figure a).…”

“…The pulse amplitude was 3–4 times higher than that for the ZnTe emitter over the entire range of excitation intensities. The experiment demonstrates that the InAs metasurface provides one of the highest optical-to-THz conversion efficiencies. , Using a literature value of conversion efficiency of ∼1.0 × 10 –6 for a 2 mm thick ZnTe crystal (at 0.133 GW/cm 2 of pump excitation), we can estimate that the optical-to-THz conversion efficiency in the InAs metasurface is ∼10 –5 for 0.5 GW/cm 2 of excitation (see details in the Supporting Information). …”