Air-aided coherent detection for broadband terahertz wave

Broadband terahertz (THz) waves detection has posed a challenge for the past two decades. In this work, a time domain spectroscopy system for broadband terahertz detection with air serving as the detection medium was developed. To enable coherent detection, crystalline quartz was introduced to generate a local oscillator beam via the second harmonic mechanism, which allows for the reconstruction of the THz pulse waveform. Experimentally, a gapless spectrum extending up to 14 THz with a dynamic range of 60 dB was obtained. Our method provides the same level of performance compared to the traditional air-biased-coherent-detection method and operates without the need for a high-voltage modulator, offering a more user-friendly approach for broadband THz detection. We also demonstrated that the THz power from the spintronic terahertz emitter could reach 0.3 mW, resulting in a peak field of up to 1.95 MV/cm. These findings will contribute to the advancement for THz spectroscopy and remote sensing applications.