Terahertz direct detectors based on superconducting hot electron bolometers (Conference Presentation)

Abstract

Superconducting niobium nitride (NbN) hot electron bolometers (HEBs) have been used widely in the astronomical observations with its low noise temperature [1] (a few times of the quantum noise limit) as heterodyne detectors. On the other hand, with high temperature coefficient of resistance (TCR) and low noise characteristics, NbN HEB can be considered as direct detectors. Combined with NbN material’s quick response property (response time:~ps), NbN HEB direct detector can be used in quick terahertz (THz) imaging and weak THz ultrashort pulse signal detection. A direct detector system similar to the microwave stability system[2] in our lab has been constructed. The injected microwave is used to suppress the superconducting current and bias the HEB to an optimum bias point combined with the constant DC voltage source. The bias current changes corresponding to the incident THz signal power is read out by the dynamic signal analyzer. Compared to the thermal bias method which used the heating methods to heat the HEB to its critical temperature, the proposed method can enhance the direct detector’s stability and decrease the consumption of the liquid helium, which is key for the long time observations in the astronomical field and so on. More importantly, we found that our method can enhance the TCR of the NbN HEB from 8.45/K with the thermal bias method to 961/K. We obtained the noise equivalent power (NEP) of 1.4×10-12 W/Hz1/2 at 4.2 K and 0.65 THz. This value is mainly limited by the read out circuit at this moment. The response time of 86 ps is obtained in the separate measurement. Further improvement of NEP can be realized with optimizing the read out circuit.