Superconducting Terahertz Direct Detectors Based On Niobium Nitride Hot Electron Bolometers

Terahertz(THz)wave is one of the important resources of electromagnetic wave and can be expected to be used in many applications.THz technology has been developing very fast in recent two decades.With penetrability to most of the dielectric materials compared to visible light and high spatial resolution compared to microwave(MW),the THz wave would be promising used in security imaging field.High-speed,high sensitivity and high spatial resolution imaging systems need to develop array detectors with high sensitivity.Superconducting detectors have high sensitivity advantage.Superconducting niobium nitride(NbN)hot electron bolometer(HEB)mixers play an important role in THz mixers with high sensitivity and low local oscillator(LO)power requirements,but it would be hard to build array mixers.How to realize array direct detectors merit studying.So we focused on THz direct detector detection characteristics of the superconducting NbN HEB and had done the following worksAlthough relations between mixing performance and direct detection performance of NbN HEBs are not clear,good direct detection performance would be achieved based on NbN HEBs with excellent mixing performance.In order to verify our assumption,we measured mixing performance of many HEB chips and screen out two chips with good TN.Noise temperature of the two chips is 500 K(chipl)and 400 K(chip2).The intermediate frequency(IF)gain bandwidth(GBW)are 1.85 GHz and 2.5 GHz.The best noise temperature we obtained is comparable to the state of the art performance in the world.We focused on investigating current-voltage(I-V)curves responses with heating and high-frequency electromagnetic waves respectively.We found the MW I-V responses are associated with IF GBW.When the injected MW frequency higher than IF GBW,current jumps appear in the low bias region of I-V curves.We also measured the R-T curves with MW injection and compared the R-T curves with the same frequency of MW I-V curves responses and proposed an appropriate MW frequency selection method to pumped NbN HEBs as direct detectors.We constructed the NbN HEB direct detection system and demonstrated its performance.We adopted microwave reflection readout method to construct this setup.The noise equivalent power(NEP)of 1.6 pW/Hz1/2(chipl)and 1.5 pW/Hz1/2(chip2)are obtained which are comparative to the NEP of 1.4 pW/Hz1/2 with bias current readout method.These results are also comparative to the NEP of commercial low-temperature silicon bolometer.We also combined the NbN HEB direct detector and Fourier transform spectrometer(FTS)to measure a mesh filter's transmission spectrum and a THz QCL's emission spectrum.The scanning speed can be accelerated by using the NbN HEB direct detector.The whole scanning time using direct detector can be down to 20 seconds compared to 10 minutes by the Golay cell detector.