Study On Quasi-Optical Detectors Based On Planar Schottky Diode

Terahertz radiation is frequently referred to as the spectral region within frequency range of0.1-10THz. Due to its unique property variety of applications has been proposed, such as concealed weapon detection, fingerprint spectroscopy, short range radar and secured high-speed data communication. For these applications, THz detector is an essential component.Schottky diodes are widely used nonlinear components for millimetre and terahertz wave applications. They offer low parasitic capacitance and series resistance when used as mixers, multipliers and detectors. Such Schottky detectors can operate at room temperature and have an extremely faster response time compared with other detectors, such as micro-bolometers or Golay cells. There is a long heritage of using point contact Schottky devices with a long wire antenna for far infrared video detection. Air-bridged Schottky structures were demonstrated in the late1980’s as a reliable, high alternative to whisker contacted diodes. This approach gives a mechanically stable structure with low parasitic capacitance and where the anodes are in a reproducible electromagnetic environment.In order to couple the radiation to Schottky diode, waveguide or silicon lens are used. For the waveguide based detectors, the operating frequency is limited by the cut off frequency. In contrast, the detectors by using planar antennas mounted on silicon lenses, which are normally called quasi-optical detectors, exhibiting a broadband response. Based on this advantage, we developed the research on quasi-optical detectors.Firstly, we introduced the basic theory of Gaussian beam, and described the propagation in quasi-optics. By using dual-path quasi-optical system, we tested and verified the Gaussian beam propagation principle.Next, based on the planar Schottky diode fabricated in Rutherford Appleton Laboratory, we studied the characteristic of THz detector. These characteristic include bandwidth, responsivity and noise performance. We analyzed the effects on sensitivity due to different bias current, and designed a detector circuit to achieve the best NEP. In order to provide a broadband performance, we designed a bow-tie antenna with broadband impedance. To achieve a high gain, a silicon lens with high dielectric constant was used to focus the beam. By mounting a planar bow-tie antenna onto the silicon lens, the totally gain of the antenna system is higher than20dB. A hybrid method is used to calculate the far-field pattern of the quasi-optical antenna. This is done by using ray-tracing to illuminate the lens surface with the radiation of bow-tie element (computed via HFSS), and then physical optics is employed to compute the fields across the boundary between lens and the air.Then, the measurements were made based on quasi-optical detectors. By using different setup we measured the noise performance, responsivity and response time. According to the noise measurements, the flicker noise performance versus frequency was talked about; we measured and responsivity on a quasi-optical bench and analyzed the sensitivity of the detector. A conclusion of the detector was made by comparing with similar works reported by other groups. In order to test the response time of the detector, a different setup was used. A compact, high speed, photonic system, based on1.55μm wavelength fibre-optic telecoms components and a waveguide photomixer, provides the THz signal. Together with the detector, this setup provided a new THz transmission link and could be used for THz communication.Finally, we presented a kind of a multi-channel THz detector using lens based bow-tie array to analyze the performance of focal plane imaging.8 bow-tie elements are arranged on the focal plane with carefully design to show a performance of broadband, high gain, well compact and easy assembling. These characteristics of the detector are preferred for detecting weak THz signal. Measured far field shows that the radiation pattern of each element is shifted angularly, by≈90, which can be used for THz imaging. However, because of the diffraction of THz, we faced some problems by using focal plane array. Based on these, several options were provided to improve the performance of focal plane imaging.This work is done by the cooperation of the International Open Lab in Electromagnetic Theory and Applications of BUPT and Rutherford Appleton Laboratory. The cooperation based on talent exchange can help us to catch up with developed countries on THz technology, improve our fundamental technology and innovation ability.