Tamm-cavity terahertz detector.

Efficiently fabricating a cavity that can achieve strong interactions between terahertz waves and matter would allow researchers to exploit the intrinsic properties due to the long wavelength in the terahertz waveband. Here we show a terahertz detector embedded in a Tamm cavity with a record Q value of 1017 and a bandwidth of only 469 MHz for direct detection. The Tamm-cavity detector is formed by embedding a substrate with an Nb5N6 microbolometer detector between an Si/air distributed Bragg reflector (DBR) and a metal reflector. The resonant frequency can be controlled by adjusting the thickness of the substrate layer. The detector and DBR are fabricated separately, and a large pixel-array detector can be realized by a very simple assembly process. This versatile cavity structure can be used as a platform for preparing high-performance terahertz devices and opening up the study of the strong interactions between terahertz waves and matter.

Modeling of a multireflector terahertz imaging system using a geometrical optics model based on angular spectrum theory.

We propose a simulation method for a multireflector terahertz imaging system. The description and verification of the method are based on an existing active bifocal terahertz imaging system at 0.22 THz. Using the phase conversion factor and angular spectrum propagation, the computation of the incident and received fields requires only a simple matrix operation. The phase angle is used to calculate the ray tracking direction, and the total optical path is used to calculate the scattering field of defective foams. Compared with the measurements and simulations of aluminum disks and defective foams, the validity of the simulation method is confirmed in the field of view of 50 cm × 90 cm at 8 m. This work aims to develop better imaging systems by predicting their imaging behavior for different targets before manufacturing.

Freestanding narrowband terahertz filters based on aluminum foil.

Terahertz (THz) filters with high transmission coefficient (T) in the passband and frequency selectivity are critical in numerous applications such as astronomical detection and next-generation wireless communication. Freestanding bandpass filters eliminate the Fabry-Pérot effect of substrate, thus providing a promising choice for cascaded THz metasurfaces. However, the freestanding bandpass filters (BPFs) using the traditional fabrication process are costly and fragile. Here, we demonstrate a methodology to fabricate THz BPFs using aluminum (Al) foils. We designed a series of filters with center frequencies below 2 THz and manufacture them on 2-inch Al foils with various thicknesses. By optimizing the geometry, T of the filter at the center frequency is over 92%, and the relative full-width half maxima (FWHM) is as narrow as 9%. The responses of BPFs show that "cross-shaped" structures are insensitive to the polarization direction. The simple and low-cost fabrication process of the freestanding BPFs promise their widespread applications in THz systems.

Simple terahertz metasurface with broadband and efficient functionality.

Pancharatnam-Berry (PB) metasurfaces have demonstrated mighty capability to manipulate electromagnetic (EM) waves, and exhibited potential applications for devices with broadband and efficient functionality. However, it remains a challenge to simultaneously achieve broadband and efficient wavefront manipulation for terahertz (THz) components with simple profiles. Herein, we introduce a simple ultra-thin PB metasurface with superior properties in the THz region. The structure is composed of a simple metallic C-Shaped Split Ring Resonator (CSRR) patterned on a flexible polyimide support layer. It is verified that the circular transmission efficiency is close to the theoretical limit of the single-layer metasurface in the range of 0.6 - 1.2 THz. Furthermore, we design metasurfaces based on the PB meta-atoms with spatially rotated orientation to achieve beam steering and superposition of vortex waves. The results are basically in line with expectations, validating the good performances of our proposal. This simple and easily deployable metasurface will give rise to more possibilities for the design of THz functional devices.

Tunable terahertz electromagnetically induced transparency based on a composite structure superconducting metamaterial.

We experimentally present a tunable electromagnetically induced transparency (EIT)-like response in bright-bright mode resonators. In contrast to previous studies, we used NbN film and a gold film composite structure metamaterial. A significant slow-light effect could be observed at the transmission window, and the maximum group index could reach 100. As a variation in temperature alters the intrinsic ohmic loss of superconducting NbN film, a temperature-dependent transmittance and slow-light effect were observed. To better illustrate the physical mechanism of the two modes, a hybrid coupling model was introduced to fit the experimental transmission spectra and extract the characteristic parameters of sub-resonators. We found excellent agreement with experimental results. Our results provide deeper insight into the metamaterial analogs of an EIT-like response and offer an alternative approach for engineering slow-light devices, bandpass filters, and switches/modulators at terahertz frequencies.

Flexible bilayer terahertz metasurface for the manipulation of orbital angular momentum states.

Metasurfaces employed for generating orbital angular momentum (OAM) beams have drawn tremendous interest since they can offer extensive applications ranging from quantum optics to information processing over the subwavelength scale. In this study, a flexible bilayer metasurface is proposed and experimentally verified in the terahertz (THz) region. Based on Pancharatnam-Berry (P-B) phase, the proposed meta-atom satisfies perfect polarization-flipping at the design frequency and is implemented for the generation of vortex beams under circularly polarized (CP) illumination. Two metasurfaces are designed, fabricated and experimentally characterized with a THz spectral imaging system for linearly polarized (LP) illumination. The transmitted field intensity distribution of y component is petal-shaped of gradually varied pieces with the frequency due to the complementary symmetric structure, indicating OAM state transition between a single vortex beam and superposition of two vortex beams. The measured spectral imaging distributions of amplitude and phase show good agreement with the simulation results. Such designs open a pathway for modulation of THz OAM states and bring more possibilities for flexible metasurfaces in a THz application.

Electrically tunable terahertz switch based on superconducting subwavelength hole arrays.

We experimentally demonstrate an electrically tunable superconducting device capable of switching the extraordinary terahertz (THz) transmission. The planar device consists of subwavelength hole arrays with real-time control capability. The maximum transmission coefficient at 0.33 THz is 0.98 and decreases to 0.17 when the applied voltage only increases to 1.3 V. A relative intensity modulation of 82.7% is observed, making this device an efficient THz switch. Additionally, this device exhibits good narrow-bandpass characteristics within 2 THz, which can be used as a frequency-selective component. This study offers an ideal tuning method and delivers a promising approach for designing active and miniaturized devices in THz cryogenic systems.

Nb5N6 microbolometer for sensitive, fast-response, 2-µm detection.

Room-temperature thermal detection at a wavelength of 2 µm in the short-wave infrared range (1.7-3 µm) was demonstrated for the first time using a Nb5N6 microbolometer. The photothermal responses of two types of Nb5N6 microbolometers were evaluated. By suspending Nb5N6 microwires in the air above the substrate, a reduction in thermal conductance of the device by a factor of 39 was achieved. The measured optical voltage responsivity RO of the Nb5N6 microbolometer reached the value of 61.5 V/W. A noise equivalent power of 8.5 × 10-11 W/√Hz (at 1 kHz) and a detectivity D* = 2.0 × 107 cm√Hz /W with a typical response time as small as 0.17 ms was obtained at a wavelength of 2 µm for a 10 × 30-µm2 device. The performance could be improved further by optimizing the design and operating parameters. This study revealed a simple low-cost technique to develop a large-scale focal plane array in silicon for infrared detection.