Gapless Detection of Broadband Terahertz Pulses Using a Metal Surface in Air-based on Field-induced Second-harmonic Generation

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. 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 and named it the air-metal coherent detection (AMCD) method [1].
[1] S. Tanaka et al., Applied Physics Letters 122, 251101 (2023).