Characterization of ultrafast devices using near-field optical heterodyning

We demonstrate a novel technique for highly localized injection of millimeter waves in ultrafast devices that combines optical heterodyning and near-field optics. The technique relies on evanescent coupling of two interfering lasers to a submicron area of a device by means of a near-field fiber optic probe. Scanning measurements show the dc and ac photoresponses of two ultrafast device structures, namely low-temperature GaAs photoconductive switches and InP-based high electron mobility transistors. The response characteristics were rich in structures that revealed important details of device dynamics.