Design and performance of reflective ultra-broadband terahertz time-domain spectroscopy with air-biased-coherent-detection

In general, a reflective spectrometer is more suitable for the spectroscopic measurement for highly absorptive samples. We report the design and evaluation of a reflective terahertz time-domain spectroscopy (R-THz-TDS), using air as THz wave emitter and sensor, together with air-biased-coherent-detection (ABCD) method. With an 85 fs pulse amplified laser, we demonstrate a usable bandwidth from 0.5 THz to 12 THz, together with a peak dynamic range (DR) better than 2000:1 and a peak THz electrical field greater than 30 kV/cm. With a 32 fs pulse amplified laser, the usable bandwidth is further expanded to a continuous 35 THz. Several far-infrared optical properties such as phonon resonance and plasma resonance in various samples are reported. We also compared both transmission and reflection measurements. Furthermore, the time-resolved optical pump-THz probe experiment is performed. The evolution of carrier dynamics of GaAs and InSb samples are demonstrated in this study. Finally, the uniqueness and advantage of this R-THz-TDS spectrometer are comprehensively compared with traditional THz-TDS and Fourier transform infrared (FTIR) spectroscopy, including radiation source, detector, DR, bandwidth, resolution, peak power, and other features. In terms of signal-to-noise ratio (SNR), this study provides the SNR variation with frequency in both broadband R-THz-TDS and FTIR spectroscopy.

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