An electronic device may include a photonics-based phased antenna array that conveys wireless signals at frequencies greater than 100 GHz. In a transmit mode, the array may transmit signals using the first and second optical signals. In a receive mode, the array may receive signals using the optical signals. In a passive mode, the array may reflect incident wireless signals as reflected signals. Photodiodes in the array may be controlled to exhibit output impedances that are mismatched with respect to input impedances of radiating elements in the array. Different mismatches can be used across the array or as a function of time to impart different phase and/or frequency shifts on the reflected signals. The phase shifts may be used to encode information into the reflected signals and/or to form a signal beam of the reflected signals.