Terahertz unipolar polarimetry by second-harmonic generation in air

Femtosecond laser-based terahertz (THz) sources have gained attention for their potential in various applications. As for any electromagnetic radiation, the wave polarization is a critical parameter, which needs to be under control in a wide class of those applications. However, characterizing the polarization of broadband THz pulses remains challenging due to their limited efficient optics. THz air-biased coherent detection has emerged as a promising candidate. The technique employed is heterodyne detection, utilizing second-harmonic generation induced by THz radiation in laser-induced air plasmas. This approach provides exceptional spectral bandwidth and an unbounded power detection limit, rendering it highly suitable for ultra-broadband and high-power THz sources. It enables a very efficient polarization measurement too. However, recent findings have revealed that the laser-induced air plasma generated in this technique can exhibit birefringence, which in turn adds systematic errors to the polarization-state determination. In this Letter, we propose a simplified approach that utilizes a weak probe beam and avoids high-voltage DC bias-fields. Unlike the terahertz (THz) air-biased coherent detection scheme, our approach yields a unipolar, intensity-proportional signal for second-harmonic generation. The experimental results reported in this Letter show the absence of induced birefringence in air and, hence, demonstrate accurate measurements of the polarization state of ultra-broadband THz pulses. Therefore, our technique may provide valuable results in applications where the polarization state, and not the full electric field waveform, is required for analysis or characterization. Finally, we discuss a possible application of our method to the emergent field of THz singular optics.