We present a readout circuit for 1 × 64 Nb<sub>5</sub>N<sub>6</sub> microbolometer array detector. The intrinsic average responsivity of the detectors in the array is 650 V/W, and the corresponding noise equivalent power (NEP) is 17 pW/√Hz. Due to the low noise of the detector, we design a low noise readout circuit with 64 channels. The readout integrated circuit (ROIC) is fabricated under CMOS process with 0.18μm design rule, which has built-in bias and adjustable numerical-controlled output current. Differential structure is used for each pixel to boost capacity of resisting disturbance. A multiplexer and the second stage amplifier is followed after the ROIC. It is shown that the ROIC achieves an average gain of ~47dB and a voltage noise spectral density of ~9.34nV/√Hz at 10KHz. The performance of this readout circuit nearly fulfills the requirements for THz array detector. This readout circuit is fit for the detector, which indicates a good way to develop efficient and low-cost THz detector system.
In order to effectively improve the coupling efficiency of terahertz (THz) detectors, we design a grating-coupled structure on the high-resistivity silicon substrate for 0.2 THz to 0.35 THz band to enhance the ability of coupling terahertz signals. We simulated the electric field distribution of the grating-coupled structure in surface and inside by using the finite difference time domain (FDTD) method. The electric field in the central area of the silicon surface can be enhanced more than 4 times compared with the non-structure silicon substrate. We also simulated the Fabry-Perot cavity in the frequency range from 0.2 THz to 0.35 THz, and the electric field in the central area of the silicon surface can be improved one time compared with the non-structure silicon substrate. In addition, the electric field distribution on the silicon surface can be changed by adjusting parameters of the grating-coupled structure. When the period of the grating is 560 μm, the width of the gold is 187 μm, and the thickness of the silicon substrate is 720 μm, a 4.7 times electric field could be achieved compared with the non-structure silicon substrate at 0.27 THz and around. So, the simulation result shows that the grating-coupled structure has an obvious advantage compared with the Fabry-Perot cavity at THz coupling efficiency.
Diffractive silicon microlens with ten staircases is designed and analyzed in this paper. The power distribution at the focal plane of the microlens is calculated and frequency dependence and focusing performance of the microlens is also evaluated by a FDTD method The simulation results show the diffractive lens has a good ability of focusing at 0.3 THz and around, and thus it can improve the coupling efficiency of the incident power into the Nb<sub>5</sub>N<sub>6</sub> microbolometers. Development of a focal plane array (FPA) using such devices as detectors is favorable since diffractive microlens array has many advantages, such as light weight, low absorption loss, high resolution, and the most important point is that the microlens array can be easily integrated by ready mass production using standard micro-fabrication techniques.
Superconducting nanowire single-photon detectors (SNSPDs) with a composite optical structure composed of phase-grating and optical cavity structures are designed to enhance system detection efficiency and count rates. Numerical simulation by finite-difference time-domain method shows that the photon absorption capacity of SNSPDs with a composite optical structure can be enhanced significantly by adjusting the parameters of the phase-grating and optical cavity structures. The absorption capacity of the superconducting nanowires reached 69.8% at the wavelength of 850 nm with 0.3 filling factor. When the filling factor was reduced to only 0.08, the absorption capacity is still 48.52%. It greatly decreased the kinetic inductance of SNSPDs, and improved the count rates.
We present the experimental demonstration of a quasioptical terahertz (THz) detector. It is based on the series connection of three Nb 5 N 6 microbolometers. This detector is of high responsivity and broadband response to THz signals. The maximum optical responsivity is 428 V/W at 0.245 THz and the minimum is 102 V/W at 0.367 THz. The thermal time constant of the detector has been demonstrated to be 1.3 μs, which is similar to the ones obtained for single-element microbolometers. These results make arrays of antenna-coupled Nb 5 N 6 microbolometers promising for the development of pixels in THz focal-plane arrays.
A 1 × 16 Nb<sub>5</sub>N<sub>6</sub> 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.
In order to accurately characterize the radio frequency (RF) responsivity of the antenna-coupled detector in the terahertz (THz) band, we introduce an iterative deconvolution algorithm to extract the effective receiving area. We use this method to characterize an antenna-coupled Nb 5 N 6 microbolometer as an example. The effective receiving area is approximately 0.7 mm 2 , which is 7.4 times larger than the physical area of the detector. The RF responsivity of the Nb 5 N 6 microbolometer is 480 V/W at 0.28 THz, including the effect of the antenna coupling and the substrate interference. This work offers an effective way to characterize antenna-coupled THz detectors and to analyze the element-to-element spacing along two orthogonal directions in THz focal plane array chips.
In recent years our team has done a lot of work toward the goal of sensitive, inexpensive detectors for terahertz
detection. In this paper we describe simple fabrication steps and the characterizations of uncooled Nb<sub>5</sub>N<sub>6</sub>
microbolometers for terahertz imaging. The best dc responsivity of the Nb<sub>5</sub>N<sub>6</sub> microbolometer is –760 V/W at the bias
current of 0.19 mA. A typical noise voltage as low as 10 nV/Hz<sup>1/2</sup> yields a low noise equivalent power (NEP) of 1.3×10<sup>-11</sup> W/Hz<sup>1/2</sup> at a modulation frequency above 4 kHz. We constructed a quasi-optical type receiver by attaching this
uncooled Nb<sub>5</sub>N<sub>6</sub> microbolometer to the hyperhemispherical silicon lens. Subsequently, the imaging experiment is
performed using this Nb<sub>5</sub>N<sub>6</sub> microbolometer receiver at a THz imaging system.