Gigahertz Radiation from Magnetic-Field-Free Electron-Hole Plasmas

This work shows that gigahertz radiation is emitted from electron-hole plasmas in $p-\mathrm{I}\mathrm{n}\mathrm{S}\mathrm{b}$ at 77°K whose only external influence is an applied voltage. In order to satisfy the conditions for radiation, the plasma must have a conductance which falls within a limited range. The appropriate plasma conductances require unusually high average electric field strengths $E$ with attendant low average current densities $J$ near threshold, or large $J$ above threshold. The area of the $J-E$ plane associated with radiation is accessible only with plasmas produced in high-quality InSb. More conductance conditions necessary for radiation become accessible by applying a staircase voltage function instead of the usual square wave, because plasma is accumulated from step to step. Some properties of the radiation are described, such as its extreme sensitivity to plasma conductance, its decay characteristics, and a possible correlation between radiation amplitude and current level. The radiation observed at current levels which are far above its current threshold occurs during plasma pinching with two orders of magnitude more plasma current flowing than flows near threshold, namely ∼2 A instead of ∼20 mA. A further extension of accessible conductance conditions that are suitable for producing radiation, particularly an extension to higher electric field strengths at low current densities, is obtained by application of magnetic field strengths of a few hundred gauss. Many data obtained by fulfilling the necessary condition for radiation, namely, the appropriate range of plasma conductance, are summarized. Evidence that this is not a sufficient condition when the plasma conductance is "tuned" by means of a magnetic field is discussed.