Study of ultrafast devices using near-field optical heterodyning

Near-field optical heterodyning (NFOH) is a novel technique for the injection of millimeter waves at any arbitrary point of an ultrafast device or circuit with high degree of spatial localization. The technique has the capacity to provide details of the operation of the device being probed, which are inaccessible using conventional means. In essence, the technique involves optical mixing of two lasers in a tiny area of the device using near-field coupling. In this paper, we present the results of more comprehensive measurements, in which we performed a scan across the entire device under test. These results show first, the resolution capabilities of the NFOH technique and second, the kind of details one can obtain about the dynamics of ultrafast devices. Two new categories of devices, namely photoconductive switches and high electron mobility transistors (HEMTs), were tested to illustrate the versatility of the technique