Frequency-doubled photons can measure current density

The possibility was anticipated theoretically 16 years ago and could be used in applications as a semiconductor diagnostic.

In 1995 Johns Hopkins University theorist Jacob Khurgin showed that when light of frequency ω shines on a current-carrying semiconductor, some of the light that bounces off or passes through will acquire the doubled frequency 2ω. As a consequence, Khurgin predicted, it should be possible to probe the current in a semiconductor via a measurement of its outgoing frequency-doubled light. That challenge has now been realized by the University of Kansas’s Hui Zhao, his student Brian Ruzicka, and an international team that includes Khurgin. In one experiment, they scanned the 4-µm-diameter focus of a 1800-nm laser beam across a room-temperature gallium arsenide sample to which two gold electrodes had been affixed. The transmitted 900-nm light was detected with a silicon photodiode. The green swath in the figure shows the power at 2ω—proportional to current density—due to the current flowing left to right between the electrodes. Although the direct current measured by Zhao and colleagues is accompanied by an electric field that itself generates frequency doubling, theoretical considerations show that the observed frequency doubling is indeed current induced, not field induced. Moreover, the researchers followed their DC experiment with one in which frequency-doubled radiation measured an oscillating current generated via a procedure called coherent current injection. That technique exploits quantum interference to produce current without having to use any external electric field. (B. A. Ruzicka et al., Phys. Rev. Lett., in press.)—Steven K. Blau