Coherent Microwave Emission from an Electron-hole Plasma

Coherent microwave radiation, 6.5 to 44 GHz, is generated by InSb at 77°K with an injected electron current transverse to a magnetic field. The maximum output power is about 10 microwatts for input power levels of one to five watts. Grooves cut into the Suhl surface of the rod-shaped InSb samples impose the coherence and determine the frequency range of coherent operation. Wavelength measurements of a surface wave show that the effective groove width is equal to about a half-wavelength. A theory of double-stream interaction in a thin plasma layer with a magnetic field transverse to the current flow predicts instabilities in the observed frequency range. The theory predicts all of the qualitative and several of the quantitative features of the observed emission. Noise emission is predicted and observed at temperatures up to room temperature with appropriate onset magnetic fields. The theoretical analysis and concurrent experimental evidence demonstrate the existence of an instability in a thin-layer plasma in the absence of a magnetic field at 77°K.