A wide band laser detector using the photodielectric effect in cooled semiconductors

An AM-FM detector is described which makes use of the photodielectric effect to optically tune a microwave oscillator. The device operates at cryogenic temperatures where the photodielectric effect dominates the photoconductive effect in semiconductors such as germanium and silicon. The device is an improvement over the photoconductive detector in several respects although the change in polarization due to photo-induced free charge is derived from the complex photoconductivity. The device consists of an amplifier and a superconducting resonant cavity having a quarter-wave stub terminated in a semiconductor wafer. The unloaded cavities have Q´s in excess of 10⁷which provides the oscillator with exceptional short-term stability. The bandwidth is limited by the lifetime of carriers which is typically one nanosecond or less. Other factors limiting the frequency response are the loaded Q and the time required for a disturbance to propagate around the loop. The sensitivity is proportional to lifetime. Response of 20 Cps per microwatt for high resistivity germanium at 787 Mc/sec. is typical where the source is a GaAs injection diode. The photodielectric detector is shown to surpass those operating in the absorption mode over a wide frequency range.