Gapless Detection of Broadband Terahertz Pulses Using a Metal Surface in Air Based on Field Induced Second-Harmonic Generation
Yoshinobu and Matsunaga Groups
Terahertz (THz) time-domain spectroscopy has been attracting much attention in many research areas such as imaging, molecular spectroscopy, and solid-state physics because the energy covers various elementary excitations in solids and molecules. Various methods for detecting the phase-locked THz electric field have been developed, such as electro-optic (EO) sampling and photoconductive antennas. Because most of these detection methods use insulating solid crystals, phonon absorptions and the phase matching condition in the crystals largely disturb the time-domain waveform of the THz pulse, particularly between 5 and 15 THz. To realize a gapless detection for broadband THz pulses, Air-Biased Coherent Detection (ABCD) has been utilized; it is based on the interference between the THz electric field-induced second harmonic (TFISH) light from air molecules and an electric-field-induced second harmonic light by the electrodes with high bias voltage above 1 kV. Recently, to reduce the voltage value, Solid-State-Biased Coherent Detection (SSBCD) using insulators such as silica or diamond instead of air was developed; however, it still requires sub-kV bias and microfabrication processes. Therefore, a much simpler geometry for gapless broadband THz pulse detection is highly demanded.

Fig. 1 (a) Schematic illustration of the experimental setup. (b) The time profile of ∆I2ω measured for the Pt single crystal using the broadband THz pulse. (c) The blue line corresponds to the amplitude spectra obtained by fast Fourier transformation of the time trace of (b). As a reference, the amplitude spectrum detected by the EO sampling method with the GaP crystal was added as the red line.
We have developed a new detection method, termed as Air-Metal Coherent Detection (AMCD), where we utilize a metal surface instead of the vias voltage in ABCD. The schematic of our experimental setup is depicted in Fig. 1(a). Second-harmonic generation (SHG) light from a Pt surface in air under broadband THz pulse irradiation was investigated. An output of the Ti:sapphire regenerative amplifier was divided into two beams for THz generation and for a near-infrared pulse as a fundamental light of SHG lights. Here, the THz pulse was generated from the two-color laser-induced air plasma filamentation, and we confirmed its broad bandwidth up to at least several tens of THz by using mid-infrared-sensitive power meters. Both P-polarized pulses were collinearly focused on the Pt surface in air, and THz pulse-modulated SHG intensity ΔI2ω was measured by a photomultiplier tube. The time profile of ΔI2ω and the amplitude spectrum obtained by fast Fourier transform of the time trace were shown in Figs. 1(b) and 1(c), respectively. For comparison, the spectrum evaluated by using a conventional EO sampling method with a GaP crystal is also added in Fig. 1(c). The spectrum evaluated by EO sampling (red) was restricted only below 7 THz due to the phonon resonances and phase-matching conditions in the GaP crystal. By clear contrast, the spectrum obtained from the time trace of ΔI2ω measurement (blue) detected the broadband frequency components without gaps. Because the THz field inside the metal is sufficiently weak, the TFISH generation in the metal is negligible. As a result, the effect of phonons is absent for the ΔI2ω measurements, enabling a gapless detection of broadband THz pulses in the region of 0.2–20 THz as shown in Fig. 1(c). We also confirmed that this method works well even with a gold mirror instead of the Pt surface.
In this study, a new gapless detection method was developed for broadband THz pulses by using a metal surface in air without any high voltage electrodes. The present AMCD method does not suffer from phonons or phase matching in insulating solid-state optics and does not require any power supply, bias voltage, or fabrication process, but offers a simple and gapless sampling method for broadband THz pulses.
References
- S. Tanaka, Y. Murotani, S. A. Sato, T. Fujimoto, T. Matsuda, N. Kanda, R. Matsunaga, and J. Yoshinobu, Appl. Phys. Lett., 122, 251101 (2023).