Enhanced-Mode Radiation Detection by Superconducting Films

Abstract

In the course of investigating radiant-energy detection by high-resistance superconducting films [≈10 KΩ/square (□)] an unexpected enhanced mode of detection was observed at microwave frequencies. This mode was observed in addition to and differed significantly from the expected thermal or bolometer mode of detection. Since that time detailed measurements have been made of the dependence of the enhanced-mode response on bias current, incident power, temperature, and sample resistance; these are reported here. Correlation of the characteristics of the enhanced mode with the nonlinear VI (voltage-current) characteristics of the film is shown. This suggests that the enhanced mode is the result of a nonlinear response to currents induced in the film by the incident radiation, which explains the short wavelength cutoff associated with the enhanced mode. Physical mechanisms which might cause this response are considered and fabrication techniques for these films are presented in detail. Since the initial report, enhanced-mode responses have been observed with a responsivity per square (r=104 V/W) which is thousands of times greater than the peak bolometer responsivity. At the operating point, the film has a lower output impedance [<50Ω/square(□)] and responds with a time constant less than 0.1 nsec as estimated from heterodyning measurements at 9.4 GHz. At present, enhanced-mode devices with an active area of 1 cm2 can achieve a noise equivalent power of less than 10−13 W Hz−1/2. This compares quite favorably with other millimeter wave detectors such as InSb free carrier detectors, doped Ge detectors, and superconducting point-contact Josephson junction devices.