Broadband terahertz metasurface for multi-functional manipulation based on Ge2Sb2Te5

Tunable multifunctional metasurface has wide application such as optical electromagnetics and material science. In this paper, a terahertz (THz) metasurface based on double graphene split-ring resonators (GSRRs) are theoretically demonstrated, integrating dual-band absorption and plasmon-induced transparency (PIT) filtering effect. The structure is composed of a monolayer of graphene arrays with periodic patterns and a metal ground surface partitioned by a silicon dioxide dielectric layer. When the initial structure of unit cell is three-layer sandwich structure (bottom metal plate), its dual-frequency absorption spectra appears two peaks at 2.50 THZ and 3.38 THz, which are 99.98% and 97.94%, respectively. Then the mechanism of double band absorption is explained by analyzing the distribution of surface current and electric intensity of the absorbent material. When the initial arrangement of the cell is a double layer structure (without the bottom metal plate), the PIT effect will occur when the incident wave is y-polarized. And in a certain range to achieve more than 90% of the transmission. In addition, CST simulations demonstrate that the designed model supports changing the operating frequency by adjusting the Fermi energy of graphene The dual-function terahertz metasurface proposed in this work has broad application prospects in broadband communication, terahertz imaging and industrial sensors.

Broadband terahertz (THz) absorbers are highly desired in detection, modulation, receiving, and imaging devices. We report the design and successful implementation of a novel broadband THz metasurface with a near-perfect absorption. Different from the traditional metal/dielectric/metal three-layer structures, the as-designed THz absorber has one more metal layer and a dielectric spacer on top, both of which are 200 nm thick. Although the total thickness increased by ∼7%, the near-perfect THz absorption band significantly broadened by 4×, achieving a broadband absorption of 270 GHz. Broadband, polarization-insensitive, and near-perfect THz absorptions were also observed over wide incident angles in these meta-absorbers, where the electric field and power loss were mainly concentrated in the additional thin dielectric layer. Such a broadband THz absorption was achieved through electromagnetic coupling between the top and middle metal layers and the resultant overlapping of the resonance frequencies. This strategy can be adapted to other spectrum-shaping devices.

This paper proposes a bi-functional switchable broadband terahertz metasurface ground on U-shaped vanadium dioxide (VO2) and graphene. The proposed design can effectively switch the current working state through a two-parameter regulation mechanism. Specifically, as we fix graphene’s Fermi level at 1 eV, and VO2 is in the form of insulating, the proposed design can be seen as a broadband terahertz absorber. Shifting the Fermi level of graphene can dynamically modulate the amplitude of the broadband absorption spectrum. In other words, by arbitrarily tailoring the Fermi level of graphene, the proposed design can freely switch states, i.e., mode switching from broadband absorption to broadband reflection, in the frequency range of interest. As graphene’s Fermi level is equal to 0.01 eV, and the vanadium dioxide in the structure is in the metallic state; the designed metasurface can be seen as a broadband terahertz linear polarization converter. It can convert the incident linearly polarized terahertz wave into its orthogonal polarization. By varying the conductivity of vanadium dioxide in the simulation, the proposed design can freely tune the current operating state over the operating frequency range, similar to “ON” and “OFF”. Metasurfaces can work efficiently in different frequency ranges by changing the geometric parameters. Therefore, the designed structure has switchable and tunable functions simultaneously, providing additional options for integrated, intelligent, and miniaturized devices.

Switchable metasurfaces with diversified functionalities have become an emerging research area owing to its potential for realizing integrated and miniature meta-devices. Although great efforts have been devoted in this area, switchable metasurfaces with diversified functionalities still require dealing with formidable challenges. In this paper, a switchable broadband terahertz (THz) metasurface with multiple functionalities is proposed utilizing the phase transition property of vanadium dioxide (VO2), and it can switch functionalities by changing the state of VO2 from metal to insulator thermally. The simulation results demonstrate that when VO2 is in metallic state at the temperature of 400 K, the designed metasurface works as a half wave plane with polarization conversion ratio more than 90% at THz frequency band ranging from 0.66 to 1.44 THz. When VO2 is in insulating state at the temperature of 300 K, the metasurface can flexibly control the phase of the THz wave. As a proof of concept, several devices with high performance for realizing anomalous reflection, diffuse scattering, vortex beam and beam-focusing are numerically investigated. The proposed metasurface has potential applications in THz imaging, sensing and other intellectual systems for photonics integration.

Broadband terahertz reconfigurable metasurface based on 1-bit asymmetric coding metamaterial

Terahertz devices is promising in the development of next-generation infrared photodetectors. However, the performances of photodetectors are largely limited by their poor light absorption and small detection range. Here, we designed a vanadium dioxide (VO2) based tunable broadband terahertz (THz) metasurface absorber (VO2-BTMA) for a wider tunable broadband detection range. The results show that the VO2- BTMA is able to maintain more than 90% absorption in the range of 2.21–5.64 THz with an ultra-wide bandwidth of 3.43 THz. When the absorption peaks are at 2.607 THz and 5.064 THz, the absorption rates of VO2- BTMA devices reaches 96.8% and 96.4% respectively. Meanwhile, we found that the conductivity of VO2 can be controlled by changing the temperature, thus affecting the absorption rate of the device and achieving tunability. When the incident angle of terahertz wave increases from 0° to 50°, the absorber can still maintain 80% of the absorption rate, and the absorber has excellent stability.

Multifunctional vanadium dioxide-based terahertz metasurface with ultra-wideband, narrowband switching and polarization selection

We demonstrate efficient terahertz (THz) modulation by coupling graphene strongly with a broadband THz metasurface device. This THz metasurface, made of periodic gold slit arrays, shows near unity broadband transmission, which arises from coherent radiation of the enhanced local-field in the slits. Utilizing graphene as an active load with tunable conductivity, we can significantly modify the local-field enhancement and strongly modulate the THz wave transmission. This hybrid device also provides a new platform for future nonlinear THz spectroscopy study of graphene.

In this study, we introduce a genetic algorithm (GA) into the catenary theory model to achieve automatic and inverse design for terahertz (THz) metasurface absorbers. The GA method was employed by seeking optimal dispersion distributions to achieve broadband impedance matching. A THz dual-metasurface absorber was designed using the proposed approach. The designed metasurface absorber exhibits an absorbance exceeding 88% at 0.21-5 THz. Compared to the traditional design method, the proposed method can reduce time consumption and find the optimal result to achieve high performance. The investigations provide important guidance and a promising approach for designing metasurface-based devices for practical applications.

We present terahertz metasurfaces based on aligned rectangular graphene patches placed on top of a dielectric layer to convert the transmitted linearly polarized waves to circular or elliptical polarized radiation. Our results lead to the design of an ultrathin broadband terahertz quarter-wave plate. In addition, ultrathin metasurfaces based on the arrays of L-shaped graphene periodic patches are demonstrated to achieve broadband cross-polarization transformation in reflection and transmission. The proposed metasurface designs have tunable responses and are envisioned to become the building blocks of several integrated terahertz systems.

Bifunctional metasurface for cross-polarization conversion and ultra-broadband absorption in terahertz range

Terahertz (THz) near-field imaging and spectroscopy provide valuable insights into the fundamental physical processes occurring in THz resonators and metasurfaces on the subwavelength scale. However, so far, the mapping of THz surface currents has remained outside the scope of THz near-field techniques. In this study, we demonstrate that aperture-type scanning near-field microscopy enables non-contact imaging of THz surface currents in subwavelength resonators. Through extensive near-field mapping of an asymmetric D-split-ring THz resonator and full electromagnetic simulations of the resonator and the probe, we demonstrate the correlation between the measured near-field images and the THz surface currents. The observed current dynamics in the interval of several picoseconds reveal the interplay between several excited modes, including dark modes, whereas broadband THz near-field spectroscopy analysis enables the characterization of electromagnetic resonances defined by the resonator geometry.

Terahertz (THz) absorbers have drawn great attention due to their potential applications in high-resolution imaging systems, sensing, and imaging. In particular, metasurface-based THz absorbers have exhibited the exotic advantage in high efficiency and broad bandwidth benefitted from the excellent abilities of metasurface in flexible modulating electromagnetic (EM) waves. However, the interactions between metasurface and EM waves are complex, and the metasurface-based absorbers have many structural parameters to optimize for high performance. Therefore, the absorbers are constrained by the manual design process with limited geometry complexity and tedious parameters sweeping. In this paper, the genetic algorithm (GA) is employed to the design of THz metasurface absorber. The EM responses of metasurface device is calculated by a simple yet powerful analytic method derived from catenary field. The employment of GA can achieve the automatic design process and demand-oriented reverse design for high performance and decreasing time consumption. As a proof-of-concept, the broadband and monolayer metasurface terahertz absorber with absorbance exceeding 80% in the frequency range from 1 to 4 THz is designed by the proposed strategy based on five typical types of metasurface. The investigations of this article present important guidance and a promising approach to design and optimize metasurface-based devices for their practical applications.

Compact and programmable wavefront control is a central task for advancing terahertz (THz) wave spectroscopy, imaging, and wireless communications. Although electrically programmable metasurfaces have exhibited remarkable versatility and significantly promoted THz dynamic device development, realizing two-dimensional (2D), nonvolatile, broadband, and high-resolution wavefront control remains a critical objective. Here, we present an optically programmable metasurface method that potentially overcomes these difficulties by leveraging the reversible phase change of the chalcogenide material Ge2Sb2Te5 (GST). The core innovation lies in the use of selective optical excitation to locally address and induce phase changes in constituent GST patches, enabling reconfigurable and nonvolatile reversal of the meta-atom symmetry. This unique mechanism yields a robust and broadband 0/π phase-switching capability at the meta-atom level, operating with subwavelength resolution and without the need for complex integrated electrodes. By employing spatially patterned optical pumping with predesigned masks as examples, we experimentally demonstrate two distinct 2D coded functionalities: controllable beam steering and tunable beam focusing. Our method establishes a new paradigm for programmable THz metasurfaces, offering a promising pathway for active and flexible THz wavefront engineering critical for systems requiring long-term, stable functionalities.

Although the design of graphene-based tunable broadband terahertz (THz) absorbers has attracted much attention, improving the functionality of the absorbers to adapt to different scenarios is still worth studying. This paper presents an innovative design of a quad-functional metasurface absorber (QMA) in the THz region, which can switch the absorption frequency/band by means of dual voltage/thermal manipulation. By electrically manipulating the chemical potential of graphene, the QMA can switch freely between the narrowband absorption mode ("NAM") and the broadband absorption mode ("BAM"), while thermally manipulating the phase transition of VO2 allows switching between the low-frequency absorption mode ("LAM") and the high-frequency absorption mode ("HAM"). Detailed mechanistic analysis shows that the "NAM" and "BAM" are due to the switching of the fundamental and second order graphene surface plasmon polariton (SPP) resonances, respectively, and the switching between "LAM" and "HAM" is due to the phase transformation of VO2. Furthermore, the QMA is polarization insensitive in all absorption modes and maintains excellent absorption performance at large angular incidence of TE- and TM-polarized waves. All the results indicate that the proposed QMA has great potential for stealth, sensing, switching, and filtering applications.