Photodielectric Detector using a Superconducting Cavity

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Abstract
Photoinduced free-carrier processes in Si and Ge have been studied in superconducting cavities to exploit their excellent frequency stability and negligible dissipation. Using the narrow bandwidth and very high unloaded Q, it is possible to detect changes in the complex dielectric constant by observing the shift in resonant frequency. Simultaneous measurement of relative power absorption gives additional and confirming data. An increase in frequency of several kHz/mW and a linear power absorption is observed for low values of light. At higher intensity both responses become nonlinear due to the growth of the plasma frequency and nonuniform carrier density distribution. For a 525 Ω·cm sample of n-type Si in a cavity with resonances at 290 and 810 MHz, data analysis showed the thermal carrier density at 4.2°K was 109 cm−3, relaxation time was 1.44×10−10 sec, the Fermi level was 0.00305 eV above the donor level, and the unilluminated plasma frequency was 148 MHz. The product of conversion efficiency and free-carrier lifetime was approximately 10−10 sec, indicating the effect could be used for a wide-bandwidth optical detector. By repeating the experiment at different frequencies, temperatures, and wavelengths, the theoretical agreement holds and considerable insight can be gotten into recombination and trapping processes, lattice collision and mobility effects, and the changes in these phenomena with radiation and stress of many kinds.