Efficient Optical-to-sub-THz Carrier Frequency Down-Conversion by UTC-PD-Integrated HEMT

We experimentally investigate the carrier frequency down-conversion of optical signals to sub-THz signals by the unitraveling-carrier-photodiode-integrated high-electron-mobility transistor (UTC-PD-integrated HEMT) photonic double-mixer. First, we investigate the effect of high-intensity subcarrier signal input on enhancement of its double-mixing conversion gain. The fabricated UTC-PD-integrated HEMT demonstrates the linear increase in the conversion gain by up to +6.8 dB with increasing the subcarrier signal intensity from +0.63 dBm to +6.63 dBm without saturation, and the conversion gain enhancement of +6.8 dB from -51.0 dB to -44.2 dB is achieved at 110-GHz photomixing frequency. Second, we investigate the effect of the shape and size of the UTC-PD mesa on the conversion gain. We show that further enhancement of the maximum conversion gain, +5 dB, can be achieved by employing the circular-shaped mesa with an optimal diameter close to the diameter of the input Gaussian beam. These results demonstrate that the UTC-PD-integrated HEMT is feasible for efficient carrier frequency down-conversion in the future beyond 5G wireless networks.