IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 12, NO. 10, OCTOBER 2002 369 Characterization of Ultrafast Devices Using Near-Field Optical Heterodyning M. E. Ali, K. Geary, H. R. Fetterman, S. K. Han, and K. Y. Kang Abstract—We demonstrate a novel technique for highly local- ized injection of millimeter waves in ultrafast devices that com- bines optical heterodyning and near-field optics. The technique relies on evanescent coupling of two interfering lasers to a sub- micron 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 photo- conductive switches and InP-based high electron mobility transis- tors. The response characteristics were rich in structures that re- vealed important details of device dynamics. Index Terms—High electron mobility transistors, mil- limeter-wave generation, near-field optics, optical fiber probes, optical heterodyning, photoconductive switches, photodetectors, phototransistors, ultrafast photoresponse. I. INTRODUCTION N EAR-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 [1]. It has the capacity to provide details of the operation of a probed device which are inaccessible using conventional means. The method involves optical mixing of two lasers in a tiny area of the device using near-field coupling. The lasers are set apart in frequency by a desired amount which is tunable over hundreds of gigahertz. A near-field fiber-optic probe, which has a sub-wavelength aperture, couples the lasers evanescently to the device from a distance of 100 nm. The illumination spot, which is aperture-limited rather than diffrac- tion-limited, has approximately the same size as the aperture and defines the point of optical interaction. The local density of photogenerated carriers at this point oscillates at the difference frequency and produces millimeter waves. The response of the device to such a localized source of millimeter waves is then observed at its terminals. In previous work, the validity of the NFOH technique has been verified up to 100 GHz for heterojunction phototransistors [1]. There, measurements were restricted only to a specific point of the device. In this paper, we present the results of more comprehensive measurements, in which we performed a scan across the entire device under Manuscript received January 10, 2002; revised April 23, 2002. This work was supported by grants from the Air Force Office of Scientific Research (AFOSR) and the University of California Microelectronics Innovation and Computer Re- search Opportunities (CA MICRO). The review of this letter was arranged by Associate Editor Dr. Ruediger Vahldieck. M. E. Ali, K. Geary, and H. R. Fetterman are with the Department of Electrical Engineering, University of California, Los Angeles, CA 90095 USA. S. K. Han and K. Y. Kang are with the Telecommunication Basic Research Laboratory, Electronics and Telecommunications Research Institute, Taejon, Korea. Digital Object Identifier 10.1109/LMWC.2002.804553. Fig. 1. Experimental setup for the near-field optical heterodyne measurements. test. These results show both the resolution capabilities of NFOH and the type of details one can obtain regarding the dynamics of ultrafast devices. Photoconductive switches and high electron mobility transistors (HEMTs) were tested to illustrate the versatility of the technique. II. EXPERIMENTAL SETUP Our experimental arrangement, as shown in Fig. 1, consisted of a laser mixing setup, a near-field scanning system, and a high- frequency probing and measurement setup. The laser mixing setup employed a tunable dye laser and a temperature stable HeNe laser with a difference frequency tuning range of sev- eral hundred GHz around the HeNe wavelength of 632.790 nm. The laser beams were combined using a fiber-optic coupler, one output of which was coupled to a bare-ended fiber with the bare 1531-1309/02$17.00 © 2002 IEEE