Development of High Sensitivity Graphene Terahertz Heterodyne Mixture Detector

650 GHz Antenna Coupled G-FET Terahertz Heterodyne Mixture Detector

Terahertz waves with frequencies between infrared and millimeter waves have broad application prospects in the fields of imaging, radar and communications. The interaction between terahertz waves and matter has important scientific significance. The high-sensitivity terahertz wave detector is the core device for developing terahertz application technology. It is one of the important means and main contents for terahertz scientific research. Terahertz wave detection can be divided into two methods: direct detection and heterodyne detection: direct detection only obtains the intensity or power information of the terahertz wave; and heterodyne detection can obtain the amplitude, phase, and frequency information of the terahertz wave at the same time. Hertz radar, communications, and spectral imaging applications require the core components. The heterodyne detector detects the signal by down-converting the measured signal into an intermediate-frequency signal in the microwave radio frequency band by mixing the measured terahertz signal with the low-noise local coherent terahertz signal. Compared with direct detection, heterodyne detection usually has higher response speed and sensitivity, but the detector structure and circuit are more complex, and higher requirements are placed on the mixing mechanism, efficiency, and materials.

Antenna-coupled field effect transistors support self-mixing detection and heterodyne mixing detection in terahertz bands with frequencies much higher than their cutoff frequencies. The former is an effective method of direct detection, can form a large-scale array detector, and is also the basis for implementing heterodyne mixing detection based on field effect transistors. Currently, CMOS transistors based on CMOS have implemented two (426 GHz) and three (639 GHz) sub-harmonic mixing detections with a local oscillator frequency of 213 GHz, but their high-impedance characteristics limit the improvement of operating frequency and IF bandwidth. .

Graphene FETs for achieving high-sensitivity terahertz self-mixing and heterodyne mixing due to their high electron mobility, highly tunable Fermi energy, bipolar carriers, and their nonlinear transport properties Probing provides a new way. In the earlier period, the Qinhua team of the two key laboratories and Feng Zhihong team successfully obtained a low-impedance, high-sensitivity graphene terahertz detector operating at room temperature. The operating frequency (340 GHz) and sensitivity (~50 pW/Hz1/2) were achieved. The highest level in this class of detectors (Carbon 116, 760-765 (2017)). This cooperation further increased the operating frequency to 650 GHz and realized heterodyne mixing detection.

The G-FET terahertz detector operating at 650 GHz was first integrated with 216, 432 and 650 GHz self-mixing detection by integrating an ultra-hemispherical silicon lens, verifying that the response characteristics of the detector are consistent with the design expectations and self-mixing detection The responsivity and terahertz power were tested and calibrated. On this basis, a heterodyne mixing detection with a local oscillator of 216 GHz and 648 GHz was realized, and a sub-harmonic (432 GHz) and third-order subharmonic (648 GHz) mixing with a local oscillator of 216 GHz was realized. Frequency detection. The mixing losses are 38.4 dB and 57.9 dB, respectively, and the corresponding noise equivalent power is 13 fW/Hz and 2 pW/Hz, respectively. The second subharmonic mixing loss is about 8 dB higher than the 216 GHz heterodyne mixing loss.

(a) Schematic diagram of a quasi-optically coupled heterodyne mixing detection system; (b) Intermediate frequency spectrum of 216 GHz heterodyne mixing detection

The frequency of mixing obtained this time is much higher than the highest working frequency (-200 GHz) of graphene heterodyne detection that has been reported internationally, but the IF signal bandwidth is less than 2 GHz, which is lower than the internationally reported highest IF bandwidth (15 GHz). . In general, the performance of current G-FET heterodyne hybrid detectors is not as good as that of Schottky diode mixers. However, there is great room for improvement in material quality, device design, and process technology. According to Andersson et al.'s prediction, the mixing efficiency of G-FETs can be reduced to 23.5 dB. How to achieve and surpass the performance indicators of Schottky diode mixer detectors is a key issue that needs to be tackled in the future.

(a) Comparison of the effects of transmission imaging on leaves by direct detection at 432 GHz and secondary sub-harmonic detection at 216 GHz, respectively; (b) 2nd sub-harmonic detection on lemon slices using a local oscillator of 216 GHz Perspective imaging

The above figure shows a comparison of transmission imaging based on direct detection of 432 GHz and second harmonic detection. Transmission imaging with sub-harmonic detection shows a higher dynamic range than direct detection, up to 40 dB.

The research work has been supported by the National Natural Science Foundation of China (No. 61271157, 61401456, 61401297, etc.), the National Key Research and Development Plan (2016YFF0100501, 2014CB339800), the Chinese Academy of Sciences Green Promotion Association (2017372), the nanoscale processing platform of the Suzhou Nanotechnology Institute of the Chinese Academy of Sciences, and a test analysis platform. And Nanjing University of superconductivity research institute strong support.