Amplitude-, phase-, and frequency modulation of far-infrared radiation by optical excitation of silicon

Abstract

The 118.6 μm radiation of an H 2O-laser is modulated in amplitude, phase, and frequency by optical generation of free carriers in a silicon wafer. Two compact, diode-pumped CW Nd:YAG-lasers (λ = 1.06 μm, P = 40 mW each) are used for optical excitation. They are frequency tunable and can be electrically modulated in output power. 70% maximal effective modulation of the FIR intensity and 74° maximal phase modulation are achieved. Modulation frequencies for amplitude and phase up to 80 kHz have been realized, the degree of modulation decreasing with increasing modulation frequency. FIR frequency modulation is accomplished by generating an optically induced “travelling” grating using two Nd:YAG-lasers. Frequency shifts up to 15 MHz were achieved. With 26 mW effective excitation power the maximal efficiency was 1.5 × 10 −3 with a 3 dB roll-off frequency of 35 kHz for the Si wafer used.