Gapless detection of broadband terahertz pulses using a metal surface in air based on field-induced second-harmonic generation

We investigate second-harmonic generation (SHG) light from a Pt surface in atmosphere under terahertz (THz) pulses. THz pulse modulated SHG intensity, ?I-2?, shows a clear time profile of the THz field, which is similar to that of the conventional electro-optic sampling. The result can be explained by interference between THz field-induced second-harmonic light from air molecules in an optical path and a local oscillator from a Pt surface, whereby heterodyne detection of the THz waveform can be achieved. Using numerical calculations of a wave equation, we discuss the contribution of the Gouy phases of all the pulses, including near-infrared, SHG, and THz pulses, and identification of effective nonlinear susceptibility of the Pt surface. Our method, simply using a polished metal surface and air molecules, does not suffer from phonons or phase matching in solid-state optics and does not require any power supply, bias voltage, or fabrication process, but it offers a simple and gapless sampling method for broadband THz pulses. Here, we demonstrate the gapless detection of a broadband THz pulse in the region of 0.2-20 THz using this method.

Automated summary

In this study, we investigate the second-harmonic generation (SHG) light from a Pt surface under terahertz (THz) pulses in atmosphere. The THz pulse modulated SHG intensity shows a clear time profile of the THz field, similar to conventional electro-optic sampling. Our method offers a simple and gapless sampling technique for broadband THz pulses without the need for power supply, bias voltage, or fabrication process.