Chengtao Jiang

Electrically tunable superconducting terahertz metamaterial with low insertion loss and high switchable ratios

With the emergence and development of artificially structured electromagnetic materials, active terahertz (THz) metamaterial devices have attracted significant attention in recent years. Tunability of transmission is desirable for many applications. For example, short-range wireless THz communications and ultrafast THz interconnects require switches and modulators. However, the tunable range of transmission amplitude of existing THz metamaterial devices is not satisfactory. In this article, we experimentally demonstrate an electrically tunable superconducting niobium nitride metamaterial device and employ a hybrid coupling model to analyze its optical transmission characteristics. The maximum transmission coefficient at 0.507 THz is 0.98 and decreases to 0.19 when the applied voltage increases to 0.9 V. A relative transmittance change of 80.6% is observed, making this device an efficient narrowband THz switch. Additionally, the frequency of the peak is red shifted from 0.507 to 0.425 THz, which means that the device can be used to select the frequency. This study offers an alternative tuning method to existing optical, thermal, magnetic-field, and electric-field tuning, delivering a promising approach for designing active and miniaturized THz devices.

Electrical dynamic modulation of THz radiation based on superconducting metamaterials

We demonstrate an electrically tunable superconducting metamaterial capable of modulating terahertz waves dynamically. The device is based on electromagnetically induced transparency-like metamaterials, and the maximum modulation depth reaches 79.8% in the transmission window. Controlled by an electrical sinusoidal signal, such a device could achieve a modulation speed of approximately 1 MHz. The superior property and simplicity of design make this device promising for the development of high performance THz systems.

SNSPDs on a Magnesium Fluoride Substrate for High System Efficiency and Ultra-Wide Band

We developed a superconducting nanowire single-photon detector (SNSPD) based on a magnesium fluoride (MgF2) substrate with the aim of producing a high system efficiency (SE) and wideband. The proposed SNSPD on a MgF2 substrate could simultaneously provide: 1) low lattice mismatching of the lattice constant between MgF2 and superconducting niobium nitride (NbN) films; 2) a low reflection loss (n = 1.37); and 3) a wide transmission band (0.2-7 μm). Based on the designed structure, a simulation indicated that the maximum absorption efficiency of the designed SNSPD on the MgF2 substrate is 97%. An SE of 46% at 1064 nm was measured without maximizing the polarization. The SNSPD on the MgF2 substrate was examined in a long-haul laser ranging application over four months, expanding the possibilities for application such as for laser ranging of space debris.

The design of Nb5N6 microbolometer detector for 0.3 THz detector

We design and simulate planar antenna structure on the high- resistivity silicon substrate(ρ=1000Ω·cm) for the Nb5N6 micro- bolometer at the frequency range from 0.265 THz to 0.365 THz by CST Studio Suite. We have obtained the center frequency of the antenna at 0.3 THz by optimizing parameters of the antenna structure and the antenna has the very good radiation directivity. And the maximum directivity of the antenna is around 8.634 dBi at 0.3THz. The measured best voltage response of the Nb5N6 micro-bolometer detector is at 0.307 THz. The measured response frequency and the simulated S-parameter are in substantial agreement.

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

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.

A low noise readout integrated circuit for Nb5N6 microbolometer array detector

We present a readout circuit for 1 × 64 Nb5N6 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.

A sensitive coupling structure for terahertz detectors array

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.

Characterizations of diffractive microlens in Nb5N6 microbolometers array for THz detection

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 Nb5N6 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.