Ultrafast All-Optical Switching Modulation of Terahertz Polarization Conversion Metasurfaces Based on Silicon.

With the development of ultrafast optics, all-optical control of terahertz wave modulation based on semiconductors has become an important technology of terahertz wave regulation. In this article, an ultrawideband terahertz linear polarization converter consisting of a double-layered metasurface is first proposed. The polarization conversion ratio of the device is ∼ 100% at 0.2-2.2 THz, and the transmission of copolarization approaches zero in the full band, which demonstrates the ability of high-purity output with rotating input linear polarization of 90° over an ultrawideband. By analysis of the surface current and electric field distribution, the physical mechanism of polarization conversion is elucidated. In addition, the influence of important geometric parameters on the device is discussed and analyzed in detail, which provides theoretical support for the design of high-performance polarization converters. More importantly, by introducing semiconductor silicon to construct an actively controllable metasurface, we design all-optical polarization converters based on a meta-atomic molecularization metasurface and all-dielectric metasurface; the dynamically tunable ultrawideband linear polarization conversion is realized under optical pumping, which solves the inherent problem of the performance of the metasurface polarization converters. Numerical simulation shows that the switching response of the two types of actively controllable devices under optical pumping is about 700 and 1800 ps, respectively, and can manipulate polarized wave conversion ultrafast, which brings new opportunities for all-optical controlled ultrafast terahertz polarization converters. Our results provide a feasible scheme for the development of state-of-the-art active and controllable ultrafast terahertz metasurface polarization converters, which have great application potential in short-range wireless terahertz communication, ultrafast optical switches, the transient spectrum, and optical polarization control devices.

Metasurface Terahertz Perfect Absorber with Strong Multi-Frequency Selectivity.

In this paper, we design a metasurface terahertz perfect absorber with multi-frequency selectivity and good incident angle compatibility using a double-squared open ring structure. Simulations reveal five selective absorption peaks located at 0-1.2 THz with absorption 94.50% at 0.366 THz, 99.99% at 0.507 THz, 95.65% at 0.836 THz, 98.80% at 0.996 THz, and 86.70% at 1.101 THz, caused by two resonant absorptions within the fundamental unit (fundamental mode of resonance absorption, FRA) and its adjacent unit (supermodel of resonance absorption, SRA) in the structure, respectively, when the electric field of the electromagnetic wave is incident perpendicular to the opening. The strong frequency selectivity at 0.836 THz with a Q-factor of 167.20 and 0.996 THz with a Q-factor of 166.00 is due to the common effect of the FRA and SRA. Then, the effect of polarized electromagnetic wave modes (TE and TM modes) at different angles of incidence (θ) and the size of the open rings on the device performance is analyzed. We find that for the TM mode, the absorption of the resonance peak changes only slightly at θ = 0-80°, which explains this phenomenon. The frequency shift of the absorption peaks caused by the size change of the open rings is described reasonably by an equivalent RLC resonant circuit. Next, by adjusting two-dimensional materials and photosensitive semiconductor materials embedded in the unit structure, the designed metasurface absorber has excellent tunable modulation. The absorption modulation depth (MD) reaches ≈100% using the conductivity of photosensitive semiconductor silicon (σSI-ps), indicating excellent control of the absorption spectrum. Our results can greatly promote the absorption of terahertz waves, absorption spectrum tunability, and frequency selectivity of devices, which are useful in the applications such as resonators, bio-detection, beam-controlled antennas, hyperspectral thermal imaging systems, and sensors.

High sensitivity of room-temperature terahertz photodetector based on silicon.

Silicon (Si) is the most important semiconductor material broadly used in both electronics and optoelectronics. However, the performance of Si-based room temperature detectors is far below the requirements for direct detection in the terahertz (THz) band, a very promising electromagnetic band for the next-generation technology. Here, we report a high sensitivity of room temperature THz photodetector utilizing the electromagnetic induced well mechanism with an SOI-based structure for easy integration. The detector achieves a responsivity of 122 kV W-1, noise equivalent power (NEP) of 0.16 pW Hz-1/2, and a fast response of 1.29 µs at room temperature. The acquired NEP of the detector is âˆ¼2 orders lower in magnitude than that of other types of Si-based detectors. Our results pave the way to realize Si-based THz focal plane arrays, which can be used in a wide range of applications, such as medical diagnosis, remote sensing, and security inspection.

High Performance of Room-Temperature NbSe2 Terahertz Photoelectric Detector.

Photoelectric detection is developing rapidly from ultraviolet to infrared band. However, terahertz (THz) photodetection approaches is constrained by the bandgap, dark current, and absorption ability. In this work, room-temperature photoelectric detection is extended to the THz range implemented in a planar metal-NbSe2-metal structure based on an electromagnetic induced well (EIW) theory, exhibiting an excellent broadband responsivity of 5.2 × 107 V W-1 at 0.027 THz, 7.8 × 106 V W-1 at 0.173 THz, and 9.6 × 105 V W-1 at 0.259 THz. Simultaneously, the NbSe2 photoelectric detector (PD) with ultrafast response speed (∼610 ns) and ultralow equivalent noise power (4.6 × 10-14 W Hz-1/2) in the THz region is realized, enabling high-resolution imaging. The figure of merit (FOM) characterizing the detection performance of the device is 2 orders of magnitude superior to that of the reported THz PDs based 2D materials. Furthermore, the THz response speed is 2 orders of magnitude faster than that of the visible due to the different response mechanisms of the device. Our results exhibit promising potential to achieve highly sensitive and ultrafast photoelectric detection.

Ultrabroadband Tellurium Photoelectric Detector from Visible to Millimeter Wave.

Ultrabroadband photodetection is of great significance in numerous cutting-edge technologies including imaging, communications, and medicine. However, since photon detectors are selective in wavelength and thermal detectors are slow in response, developing high performance and ultrabroadband photodetectors is extremely difficult. Herein, one demonstrates an ultrabroadband photoelectric detector covering visible, infrared, terahertz, and millimeter wave simultaneously based on single metal-Te-metal structure. Through the two kinds of photoelectric effect synergy of photoexcited electron-hole pairs and electromagnetic induced well effect, the detector achieves the responsivities of 0.793 A W-1 at 635 nm, 9.38 A W-1 at 1550 nm, 9.83 A W-1 at 0.305 THz, 24.8 A W-1 at 0.250 THz, 87.8 A W-1 at 0.172 THz, and 986 A W-1 at 0.022 THz, respectively. It also exhibits excellent polarization detection with a dichroic ratio of 468. The excellent performance of the detector is further verified by high-resolution imaging experiments. Finally, the high stability of the detector is tested by long-term deposition in air and high-temperature aging. The strategy provides a recipe to achieve ultrabroadband photodetection with high sensitivity and fast response utilizing full photoelectric effect.

Photodetectors of 2D Materials from Ultraviolet to Terahertz Waves.

2D materials are considered to be the most promising materials for photodetectors due to their unique optical and electrical properties. Since the discovery of graphene, many photodetectors based on 2D materials have been reported. However, the low quantum efficiency, large noise, and slow response caused by the thinness of 2D materials limit their application in photodetectors. Here, recent progress on 2D material photodetectors is reviewed, covering the spectrum from ultraviolet to terahertz waves. First the interaction of 2D materials with light is analyzed in terms of optical physics. Then the present methods to improve the performance of 2D material photodetectors are summarized, such as defect engineering, p-n junctions and hybrid detectors, and the issue of serious overestimation of the performance in reported photodetectors based on 2D materials is discussed. Next, a comparison of 2D material photodetectors with traditional commercially available detectors shows that it is difficult to balance the current 2D material photodetectors with regard to having simultaneously both high sensitivity and fast response. Finally, a possible novel EIW mechanism is suggested to advance the performance of 2D material photodetectors in the future.