300 GHz Photonic Self-Mixing Imaging-System with vertical illuminated Triple-Transit-Region Photodiode Terahertz Emitters

In this paper, we report a phase-sensitive photonic THz two-tone self-mixing imaging system, comprising a self-developed vertical illuminated triple transit region photodiode (TTR-PD) and a commercial square law Schottky barrier diode as THz emitter and THz detector, respectively. Using the two-tone self-mixing approach, the phase information of a device-under-test can be extracted by down-mixing two THz signals inside the SBD, whereas the phase information remains in the output signal. The THz tones are generated by two free-running lasers and the TTR-PD, while one optical signal is externally modulated for double sideband carrier suppression. By means of the self-mixing, the phase noises of the free running lasers are canceled out. Using a second PD for the trigger, lock-in detection allows fast imaging speed, only limited by the integration constant. Beside the imaging system, we present the characteristics of the used vertically illuminated terahertz triple transit region photodiodes with thin depletion zone and small active areas. Numerical analysis by energy-balance model based TCAD simulations show transit-time limitations over 200 GHz, due to the electron field management within the active photodiode layers and the resulting high electron velocities. Therefore, the fabricated TTR-PDs show a flat ± 2dB frequency response within the frequency range from 225 GHz to 305 GHz. By employing the proposed photonic two-tone imaging system with the fabricated TTR-PDs, amplitude and phase difference vector images are taken at 299.5 GHz and 300.5 GHz. Metal and acrylic glass items inside a paper envelope are clearly visible, which demonstrates the potential of the system.