A reflective grating-coupled structure on the silicon substrate was designed to improve the detection efficiency of terahertz detectors for the frequency ranging from 0.26 THz to 0.36 THz. By using finite difference time domain (FDTD) solutions, the simulation and optimized design of the grating-coupled structure were carried out. The results showed that the signal was effectively reflected and diffracted by the reflective grating-coupled structure which significantly enhanced the electric field in the place of the detector. The maximum electric field can be increased by 2.8 times than that of the Fabry-Perot resonator. To verify the design results, the reflective grating-coupled structure was applied in the preparation of the Nb5N6 array detector chip and compared with the Nb5N6 array detector chip with the F-P resonator. The results showed that the maximum voltage responsivity of the Nb5N6 detector with the reflective grating-coupled structure was 2 times larger than the Nb5N6 detector with the F-P resonator. It indicates that the reflective grating-coupled structure can efficiently improve the detection efficiency of THz detectors.

/pdf/reflective-grating-coupled-structure-improves-the-detection-3m65s9sixi.pdf

Reflective grating-coupled structure improves the detection efficiency of THz array detectors

Niobium Nitride (NbN) nanowire is the most popular detection material of superconducting nanowire single photon detectors (SNSPDs) for high repetition rate and high efficiency. However, it has been assumed to be difficult for fabricating SNSPDs with arrays over large area, which are critical components of quantum imaging, linear optical quantum computing, satellite laser ranging, and high-speed quantum key distribution. This paper reported a 4*4 pixel NbN SNSPDs array with an equivalent receiving aperture of 300 micrometer associated with beam compression technology, which is the first NbN SNSPD coupled by 300 micrometer fiber according to our knowledge. The designed pixel was compact with a filling factor of 98.5%, resulting in a high quantum efficiency of 94.5%, a system efficiency of 46% for photons coupled from 300 micrometer fiber without optimizing polarization, and a system time resolution of 92 ps. This work demonstrates the feasibility of a large SNSPDs array, and paves the way for developing efficient photon camera with NbN nanowires.

A 16-pixel NbN nanowire single photon detector coupled with 300 micrometer fiber

The quantum efficiency of photon counters can be measured with standard uncertainty below 1% level using correlated photon pairs generated through spontaneous parametric down-conversion process. Normally a laser in UV, blue or green wavelength range with sufficient photon energy is applied to produce energy and momentum conserved photon pairs in two channels with desired wavelengths for calibration. One channel is used as the heralding trigger, and the other is used for the calibration of the detector under test. A superconducting nanowire single photon detector with advantages such as high photon counting speed (<20 MHz), low dark count rate (<50 counts per second), and wideband responsivity (UV to near infrared) is used as the trigger detector, enabling correlated photons calibration capabilities into shortwave visible range. For a 355nm single longitudinal mode pump laser, when a superconducting nanowire single photon detector is used as the trigger detector at 1064nm and 1560nm in the near infrared range, the photon counting efficiency calibration capabilities can be realized at 532nm and 460nm. The quantum efficiency measurement on photon counters such as photomultiplier tubes and avalanche photodiodes can be then further extended in a wide wavelength range (e.g. 400-1000nm) using a flat spectral photon flux source to meet the calibration demands in cutting edge low light applications such as time resolved fluorescence and nonlinear optical spectroscopy, super resolution microscopy, deep space observation, and so on.

Improved photon counting efficiency calibration using superconducting single photon detectors

High sensitivity room temperature THz direct detectors made of YBa2Cu3O7-x nano-bridges, with a broadband optical coupling (0.1-2THz) are presented. An optical responsivity (RV) up to 45 V/W was measured at 400 GHz. For bolometers with an electrical responsivity of 70 V/W a noise equivalent power (NEP) of 50 pW/Hz0.5 was experimental obtained.

High sensitivity terahertz bolometers as room temperature detectors

A 1 × 16 Nb 5 N 6 microbolometer array for a terahertz (THz) imaging system has been demonstrated. The system consists of an objective lens and an extended hemispherical silicon lens. A finite difference time domain (FDTD) method was used to analyze the imaging system in detail. The calculated field-of-view of the system is about 7° and the half-power beam width is about 160 μm. The microbolometer array chip is attached to the silicon lens for 0.3 THz detection. The preliminary results for the actual system shows that the mutual coupling among these antenna integrated elements can be
ignored when the spacing is larger than 500 μm. The calculated results agree with the experimental data well, which
means that the FDTD method can be used to evaluate and optimize such a compact THz imaging system. This linear
imaging system should find direct application in active THz imaging.

Nb5N6 microbolometer array for a compact THz imaging system

Nb5 N6 microbolometer arrays for terahertz detection

Niobium nitride superconducting nanowire single-photon detectors were fabricated on thermally oxidized silicon substrates with large active areas of 30 μm× 30 μm.To achieve non-constricted detectors,we improved the film growth and electron beam lithography process to fabricate uniform 100-nm wide Nb N nanowires with a fill factor of 50%.The devices showed 72.4% system detection efficiency(SDE) at 100-Hz dark count rate(DCR) and 74-ps timing jitter,measured at the fiber communication wavelength of 1550 nm.The highest SDE which is 81.2% when the DCR is ～ 700 c/s appears at the wavelength of 1650 nm.

High efficiency, large-active-area superconducting nanowire single-photon detectors

A High-Efficiency Broadband Superconducting Nanowire Single-Photon Detector with a Composite Optical Structure

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Nb5 N6 microbolometer arrays for terahertz detection

High efficiency, large-active-area superconducting nanowire single-photon detectors

A High-Efficiency Broadband Superconducting Nanowire Single-Photon Detector with a Composite Optical Structure