International Journal of Infrared and Millimeter Waves, Vol. 10, No. 7, 1989 OTPICALLY ILLUMINATED DIELECTRIC INTERFACES AS HIGH-SPEED FAR-INFRARED MODULATORS Erich N. Grossman Department of Astronomy University of Texas at Austin Received April 25, 1989 Abstract A new device for the gigahertz modulation of far-infrared radiation is ana- lytically and numerically analyzed. It consists of a thin layer of a high-mobility, direct-bandgap semiconductor, such as GaAs, in which a high-density electron-hole plasma is rapidly created and destroyed, thereby rapidly changing the free-carrier reflectivity of the active layer. Illumination by a high-power, near-infrared laser diode array generates the plasma through intrinsic photoconduction. It is shown that this device acts primarily as an amplitude modulator, and that its efficiency increases sharply with increasing far-IR frequency, in contrast to a Schottky diode, which acts primarily as a phase modulator, and whose efficiency falls off sharply with far-IR frequency. The breakeven frequency lies at about 1.5 THz, depending slightly on the assumed device parameters. The relative advantage of the new device increases rapidly with increasing far-infrared frequency. At an operating frequency of 2.5 Tttz (119 #m), for example, a i GHz modulation bandwidth may be achieved with a single-sideband conversion loss of only -21 db, versus a Schottky's loss of-39 db, assuming a laser diode power of 1 W, which is readily available from recently developed laser diode arrays. I. Introduction A number of important applications exist for a means of conveniently mod- ulating a beam of far-infrared radiation at speeds greater than are possible with mechanical chopping. We particularly have in mind modulation of the output of a far-IR molecular gas laser. Such applications include characterization of high- speed far-IR detectors, signal processing of the far-II% probe beams used for plasma diagnostics, and, potentially most important, generation of coatinuousty tunable sidebands as a source of tunable, CW, far-IR power, a long-sought goal which would create a multitude of new technological possibilities in the far-IR. The re- 803 0195-9271/89/0700-0803506.00/0 9 1989 Plenum Publishing Corporation