Time-domain measurement of coherent transition radiation using a photoconductive antenna with micro-structured electrodes

Abstract

The longitudinal electric field profile of coherent transition radiation (CTR), which is a radially polarized terahertz (THz) pulse from an electron bunch, was measured using a large-aperture photoconductive antenna (PCA) with a diameter of 8.8 mm and micro-structured concentric electrodes to detect THz light pulses. Photo-induced charge carriers were generated on the PCA by the irradiation of femtosecond laser pulses on the electrode plane. Pulsed light irradiation enabled time-resolved measurement of the THz light pulses, and time-domain measurement of CTR was conducted by measuring the electric-field-induced current output from the PCA by sweeping the timing of the laser irradiation. The frequency spectra obtained from the detected longitudinal electric field profiles of the CTR showed a reasonable response signal from the low-frequency region as low as 0.1 THz. In the present demonstration, the large-aperture PCA-detected THz wave from the CTR was from 0.1 to at least 1 THz. THz generation using the PCA was also performed and measured using a Michelson interferometer with a helium-cooled bolometer. Comparison of the THz pulse of the CTR detected using the PCA and that generated using the PCA revealed that time-domain measurement of the THz pulse using the PCA was limited by the response time of the charge carriers in the semiconductor. Enlargement of the aperture of the PCA enhanced the detectable frequency region toward a lower frequency.