Dual-band and polarization-independent infrared absorber based on two-dimensional black phosphorus metamaterials

Polarization-Independent Perfect Absorption in Monolayer Black Phosphorus Metasurfaces at Terahertz Frequencies via Critical Coupling

Two-dimensional (2D) materials, which possess rich underlying physical properties that can provide the potential for designing more efficient and compact optoelectronic devices, have attracted great interest among scientists. Due to the atomic-scale thickness and the anisotropy of in-plane conductivity, 2D black phosphorus (BP) exhibits a polarization-dependent absorption spectrum with low absorption, which limits its further development in polarization-independent applications such as light absorbers and sensors. In this paper, a polarization-independent perfect absorber in the terahertz band is proposed, which is composed of a patterned BP monolayer deposited on a lossless photonic crystal (PC) slab with a back reflection mirror. The absorption of the patterned BP monolayer can reach 100% at resonant frequencies through the critical coupling mechanism of guided resonance. Moreover, the absorber exhibits polarization-independent absorption characteristics for vertically incident light, which are attributed to the 4-fold rotational symmetry of the PC substrate and the patterned BP monolayer deposited on it. This work opens up the possibility of fabricating optically polarization-independent devices based on single-layer 2D anisotropic materials.

Mixed finite element numerical mode matching method for designing infrared broadband polarization-independent metamaterial absorbers

Conventional numerical methods have found widespread applications in the design of metamaterial structures, but their computational costs can be high due to complex three-dimensional discretization needed for large complex problems. In this work, we apply a recently developed numerical mode matching (NMM) method to design a black phosphorus (BP) absorber. NMM transforms a complex three-dimensional (3D) problem into 2D numerical eigenvalue problems plus a 1-D analytical propagation solution, thus it can save a lot of computational costs. BP is treated as a 2D surface and represented by the anisotropic surface conductance. With a realistic simulation study, we show that our method is more accurate and efficient than the standard finite element method (FEM). Our designed absorber can achieve an average absorption of 97.4% in the wavelength range of 15 to 23 μm under normal incidence. Then, we investigate the physical mechanism of the absorber, tuning the geometric parameters and electron doping to optimize the performance. In addition, the absorption spectra under oblique incidence and arbitrary polarization are studied. The results confirm that our absorber is polarization-independent and has high absorption at large incident angles. Our work validates the superiority of NMM and provides a new simulation platform for emerging metamaterial device design.

THz Broadband Absorber Based on MoS2 with Split Rings and Archimedean Spiral Structures

The MoS2 surface plasmon resonance structure is proposed as a THz absorber in this work. The absorber adopts a double layer structure of Archimedean spirals stacked with split rings. In 1.2–3.0 THz, the absorption is greater than 92%, and the relative absorption bandwidth reached the value of 85.7%. Due to the circular-like symmetry of the unit, the polarization of the absorber is less sensitive to the incident angle within a certain range. When the incident angle is within 60°, the absorption in the bandwidth is still greater than 85%. The design efficiency is also significantly improved by the combined method of the equivalent circuit and finite difference time domain. Our work provides new directions for the design of terahertz devices, which is of great importance for various fields including terahertz imaging, detection and sensing, and especially in 6G communication systems.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Ultrawideband Terahertz Absorber with Dielectric Cylinders Loaded Patterned Graphene Structure

In this paper, we theoretically designed and numerically analyzed an ultrabroadband meta-absorber with near unity absorptivity that works in terahertz spectrum. A wideband meta-absorber composed of bilayer patterned graphene and dielectric cylinder array with high symmetry was proposed. The wideband absorption mechanism benefited from two aspects. The first one was enhanced surface plasmons based on bilayer patterned graphene. And the second one was the coupling of continuous resonant modes within Fabry-Perot cavities to the enhanced surface plasmons in the graphene. An ultrawide bandwidth with absorptivity over 90% were obtained from 3.2 THz to 9.4 THz. Simulated results showed that the proposed ultra-wideband absorbing structure also possessed high performance of polarization independence, flexible tunability, large incident angle insensitivity, and compact fabrication.

Efficient HIE-FDTD method for designing a dual-band anisotropic terahertz absorption structure

The finite-difference time-domain (FDTD) method is considered to be one of the most accurate and common methods for the simulation of optical devices. However, the conventional FDTD method is subject to the Courant-Friedrich-Levy condition, resulting in extremely low efficiency for calculating two-dimensional materials (2DMs). Recent researches on the hybrid implicit-explicit FDTD (HIE-FDTD) method show that the method can efficiently simulate homogeneous and isotropic 2DMs such as graphene sheet; however, it is inapplicable to the anisotropic medium. In this paper, we propose an in-plane anisotropic HIE-FDTD method to simulate optical devices containing graphene and black phosphorus (BP) sheets. Numerical analysis shows that the proposed method is accurate and efficient. With this method, we present a novel multi-layer graphene-BP-based dual-band anisotropic terahertz absorption structure (GBP-DATAS) and analyze its optical characteristics. Combining the advantages of graphene and BP localized surface plasmons, the GBP-DATAS demonstrates strong anisotropic plasmonic resonance and high absorption rate in the terahertz band.

Tailoring anisotropic absorption in a borophene-based structure via critical coupling

The research of two-dimensional (2D) materials with atomic-scale thicknesses and unique optical properties has become a frontier in photonics and electronics. Borophene, a newly reported 2D material, provides a novel building block for nanoscale materials and devices. We present a simple borophene-based absorption structure to boost the light-borophene interaction via critical coupling in the visible wavelengths. The proposed structure consists of borophene monolayer deposited on a photonic crystal slab backed with a metallic mirror. The numerical simulations and theoretical analysis show that the light absorption of the structure can be remarkably enhanced as high as 99.80% via critical coupling mechanism with guided resonance, and the polarization-dependent absorption behaviors are demonstrated due to the strong anisotropy of borophene. We also examine the tunability of the absorption behaviors by adjusting carrier density and lifetime of borophene, air hole radius in the slab, the incident angle and polarization angle. The proposed absorption structure provides novel access to the flexible and effective manipulation of light-borophene interactions in the visible and shows a good prospect for the future borophene-based electronic and photonic devices.

Broadband absorption using all-graphene grating-coupled nanoparticles on a reflector

In this paper, the hybridized localized surface plasmon resonances (LSPRs) of a periodic assembly of graphene-wrapped nanoparticles are used to design a nanoparticle assisted optical absorber. Bandwidth enhancement of this structure via providing multiple types of plasmonic resonances in the associated unit cell using two densely packed crossly stacked graphene strips is proposed. The designed graphene strips support fundamental propagating surface plasmons on the ribbons, and gap plasmons in the cavity constructed by the adjacent sections. Graphene strips exhibit a hyperbolic dispersion region in the operating spectrum and assist in the bandwidth enhancement. Moreover, since the nanoparticles are deposited on the top strips, real-time biasing of them can be easily conducted by exciting the surface plasmons of the strip without the necessity to electrically connect the adjacent nanoparticles. The overall dynamic bandwidth of the structure, using a two-state biasing scheme, covers the frequencies of 18.16–40.47 THz with 90% efficiency. Due to the symmetry of the structure, the device performs similarly for both transverse electric (TE) and transverse magnetic (TM) waves and it has a high broadband absorption rate regarding different incident angles up to 40°. Due to the presence of 2D graphene material and also using hollow spherical particles, our proposed absorber is also lightweight and it is suitable for novel compact optoelectronic devices due to its sub-wavelength dimensions.

Triple-band black-phosphorus-based absorption using critical coupling

Black phosphorus (BP) is an important two-dimensional material that plays a key role in new photoelectric devices. In this work, a triple-band BP-based absorber was proposed, in which a monolayer BP is coupled with the missing angle rectangular structure. Due to the critical coupling of the guided resonance, the BP absorber achieves a triple-band absorption. The results showed that the absorption spectra at 2901.76 nm, 3810.71 nm, and 4676.97 nm under TM polarization achieve a high absorption of 95.45%, 98.68%, and 98.06%, respectively. In addition, the absorption peak and resonance wavelength can be flexibly adjusted by the electron doping of BP, the geometrical parameters of the structure, and the refractive index of the dielectric substrate. Because of the anisotropy properties of BP, the structure exhibits polarization-dependent absorption characteristics. Thus, the missing angle rectangular structure will provide a potential to design mid-infrared absorbers and shows a significant practical application in many photoelectric devices such as photodetectors, modulators, and optical switches.

Gold-black phosphorus nanostructured absorbers for efficient light trapping in the mid-infrared

We propose a gold nanostructured design for absorption enhancement in thin black phosphorus films in the 3-5 µm wavelength range. By suitably tuning the design parameters of a metal-insulator-metal (MIM) structure, lateral resonance modes can be excited in the black phosphorus layer. We compare the absorption enhancement due to the resonant light trapping effect to the conventional 4n2 limit. For a layer thickness of 5 nm, we achieve an enhancement factor of 561 at a wavelength of 4 µm. This is significantly greater than the conventional limit of 34. The ability to achieve strong absorption enhancement in ultrathin dielectric layers, coupled with the unique optoelectronic properties of black phosphorus, makes our absorber design a promising candidate for mid-IR photodetector applications.

Tunable multi-band terahertz absorber using a single-layer square graphene ring structure with T-shaped graphene strips

We numerically demonstrate a tunable dual-band terahertz metamaterial absorber (MA) with near-unity absorption using single-layer square graphene ring structure with T-shaped graphene strips. By periodically loading four T-shaped graphene strips to the square graphene ring periodic array without additionally increasing the size of MA device, the pre-existing resonant frequency will have a red shift and simultaneously a new resonance will be generated at higher frequency for achieving a dual-band MA. The two absorption peaks can be tuned to the resonant frequencies of interest by varying the parameters of the square graphene ring and T-shaped graphene strips. The operating frequency of the absorption spectrum can be also manipulated by adjusting the chemical potential of graphene, without changing their geometric parameters. Additionally, numerical results show that the proposed MA possesses polarization-independent and incident-angle-insensitive properties. To further extend the proposed structure's application with more absorption peaks, a tri-band MA is investigated through adding four more T-shaped graphene strips based on the dual-band absorber configuration. Therefore, our research work will be a good candidate for the design of various graphene-based tunable multi-band absorbers at different frequency regions with potential applications in optoelectronic devices and systems.

Black phosphorus-based anisotropic absorption structure in the mid-infrared

Black phosphorus (BP), an emerging two-dimensional (2D) material with intriguing optical properties, forms a promising building block in optical and photonic devices. In this work, we propose a simple structure composed of a monolayer BP sandwiched by polymer and dielectric materials with low index contrast, and numerically demonstrate the perfect absorption mechanism via the critical coupling of guided resonances in the mid-infrared. Due to the inherent in-plane anisotropic feature of BP, the proposed structure exhibits highly polarization-dependent absorption characteristics, i.e., the optical absorption of the structure reaches 99.9% for TM polarization and only 3.2% for TE polarization at the same wavelength. Furthermore, the absorption peak and resonance wavelength can be flexibly tuned by adjusting the electron doping of BP, the geometrical parameters of the structure and the incident angles of light. Finally, the perfect absorption is also realized with the multilayer BP by simply adjusting the geometrical parameters. With high efficiency absorption, the remarkable anisotropy, flexible tunability, and easy-to-fabricate advantages, the proposed structure shows promising prospects in the design of polarization-selective and tunable high-performance devices in the mid-infrared, such as polarizers, modulators and photodetectors.

Designing a nearly perfect infrared absorber in monolayer black phosphorus

Black phosphorus (BP) is a type of 2D layered material with a direct bandgap that displays high carrier mobility and strong in-plane anisotropy; it also exhibits potential as a promising optoelectronic material for IR applications. In this paper, we propose a nearly perfect IR absorber composed of a metal film, a spacer with a monolayer BP inside, and a distributed Bragg reflector (DBR). The electric field is confined inside the resonator generated by the metal film and DBR, and the absorption can be enhanced up to nearly 100%, owing to the strong interaction of BP with the confined field. Our results show that the absorption performance of the proposed structure is not only critically dependent on the electron density but also relies on the position of the BP within the spacer. This dependence can be mitigated because the absorption peak wavelength can be tuned by adjusting the angle of the light and the parameters of the DBR. Moreover, the absorber can be served as a reflective linear polarizer based on the anisotropic absorption properties. Our work can be helpful in designing a narrow perfect absorber and polarization-sensitive devices for IR waves.

Anisotropic infrared plasmonic broadband absorber based on graphene-black phosphorus multilayers

Two-dimensional materials (2DMs) such as graphene and black phosphorus (BP) have aroused considerable attentions in the past few years. Engineering and enhancing their light-matter interaction is possible due to their support for localized surface plasmon resonances in the infrared regime. In this paper, we have proposed an infrared broadband absorber consisting of multilayer graphene-BP nanoparticles sandwiched between dielectric layers. Benefiting from the properties of graphene and BP, the absorber exhibits both perfect broadband responses and strong anisotropy beyond individual graphene and BP layers. The absorber is tunable with the variation of geometric parameters as well as the doping levels of graphene and BP. The physical insight is revealed by electric field distributions. Furthermore, the angular robustness for incident wave is investigated. The proposed anisotropic omnidirectional broadband absorber may have promising potential applications in various biosensing, communication and imaging systems.

Tailoring anisotropic perfect absorption in monolayer black phosphorus by critical coupling at terahertz frequencies

A metamaterial perfect absorber composed of a black phosphorus (BP) monolayer, a photonic crystal, and a metallic mirror is designed and investigated to enhance light absorption at terahertz frequencies. Numerical results reveal that the absorption is enhanced greatly with narrow spectra due to critical coupling, which is enabled by guided resonances. Intriguingly, the structure manifests the unusual polarization-dependent feature attributable to the anisotropy of black phosphorus. The quality factor of the absorber can be as high as 95.1 for one polarization while 63.5 for another polarization, which is consistent with the coupled wave theory. The absorption is tunable by varying key parameters, such as period, radius, slab thickness, incident angle, and polarization angle. Furthermore, the state of the system (i.e., critical coupling, over coupling, and under coupling) can be tuned by changing the electron doping of BP, thus achieving various applications. This work offers a paradigm to enhance the light-matter interaction in monolayer BP without plasmonic response, and this easy-to-fabricate structure will provide potential applications in BP-based devices.

Tunable terahertz metamaterial absorber based on Dirac semimetal films

In this paper, the tunable properties of metamaterial absorbers based on 3D Dirac semimetal films (DSFs) in the terahertz (THz) regime are discussed in theory. We consider the absorbers with square-shaped, circular-patch, and cross-shaped resonators. These resonances are theoretically polarization-insensitive at normal incidence because of their 90° rotational symmetry and can achieve perfect absorption in numerical simulation. We then introduce dual-band and broadband absorbers by combining two DSF-based square-shaped (or circular-patch) resonators into one unit cell with different sizes. Unlike with a conventional metal-based absorber, the absorption of a DSF-based absorber can be dynamically tuned by varying the Fermi energy instead of refabricating the structures. Moreover, the DSFs can be regarded as a "Salisbury screen" of an absorber to block the transmission at the THz frequencies, which can be more convenient than graphene in the application of a tunable absorber. Our designs have potential applications in various fields such as sensors, thermal detectors, and imagers.

Strong coupling between magnetic plasmons and surface plasmons in a black phosphorus-spacer-metallic grating hybrid system

We propose a black phosphorus-spacer-metallic grating hybrid system to investigate the strong coupling between black phosphorus surface plasmons (BPSP) and magnetic plasmons (MP) at far-infrared frequencies. We theoretically and numerically illustrate interactions between the BPSP mode and MP mode in the coupling regime, which leads to a prominent Rabi splitting and the formation of multiple hybrid modes. Since the mechanisms of the two resonance modes are completely different, the fields in the system can be selectively localized in the spacer layer or metallic slits by regulating the coupling between such modes. Due to the strong anisotropic in-plane properties of black phosphorus (BP), the coupling between BPSP and MP modes in both armchair and zigzag directions is quite different. This work offers a new paradigm to enhance the light-matter interaction through the coupling of multiple resonance modes, and the proposed device will provide potential applications in constructing easy-to-fabricate BP-based plasmonic devices.

Broadband MoS2-based absorber investigated by a generalized interference theory

In this paper, a broadband absorber utilizing monolayer molybdenum disulfide (MoS₂) is proposed, and a generalized interference theory (GIT) is derived to investigate this absorber. Using the hybrid Lorentz-Drude and Gaussian model of monolayer MoS₂ and the dyadic Green's functions, the propagation properties of monolayer MoS₂ are first investigated. Then, a sandwich-like MoS₂-based absorber design is proposed in the visible regime. The sandwich-like structure is mounted on a fully reflective gold mirror, which forms a Fabry-Perot resonator to strengthen light-matter interactions and enhance the absorption. To numerically calculate the absorption performance of this absorber, the GIT is next derived from interference theory. The numerical results indicate that an absorption ≥ 90% is obtained for a range of wavelengths (λ) from 389 to 517 nm, and this absorber can operate well, even with an angle of incidence up to 60°, which also verifies the prediction of the MoS₂-based absorber mainly operating at λ < 700 nm. Afterward, the operating mechanism of the proposed design is determined using the theory of destructive interference. Finally, the proposed design and derived GIT are validated by a simulation using commercial electromagnetic software. The derived GIT drives the numerical investigation of the multilayer structure with various polarization types and angles of incidence of the waves, and the MoS₂-based absorber can be used in several applications such as photoelectric storage and photoelectric detection.

Design and performance of a terahertz absorber based on patterned graphene

Modern terahertz (THz) technology offers the advantage of enhanced target detection ability with high spatial and temporal resolutions in the THz band, which makes it a formidable threat to stealth targets. Consequently, THz absorbers have outstanding potential as an electromagnetic countermeasure. In this Letter, we design, fabricate, and characterize a THz absorber based on patterned graphene. We present the transfer, photolithography, and etching processes involved in graphene patterning, as well as the experimental measurements of the fabricated absorber. Our simulations show that with an increase in the Fermi energy, the performance of the designed absorber gradually improves and, finally, decreases slightly. Further, the absorption bandwidth first broadens and then narrows slightly. The effective bandwidth with absorption ≥90% ranges from 1.54 to 2.23 THz, with the relative bandwidth (RBW) reaching about 36.6%. Although the measured RBW (from ∼12% to ∼14% and then to ∼8%) slightly deviates from the simulated one, the position of the resonant frequency is well matched between theory and experiment. Moreover, we illuminate the absorption mechanism using the theory of destructive interference. This Letter can significantly contribute to the design, manufacture, and application of patterned graphene-based THz absorbers.

Towards high performance hybrid two-dimensional material plasmonic devices: strong and highly anisotropic plasmonic resonances in nanostructured graphene-black phosphorus bilayer

Plasmonics of two-dimensional materials have attracted increasing attention in the past few years. It provides a platform for strong light-matter interactions and enables a variety of novel applications in the infrared and terahertz ranges. In this paper, we study the plasmonic properties of a graphene-black phosphorus (G-BP) bilayer. It exhibits both strong and highly anisotropic plasmonic responses that performs beyond individual graphene and black phosphorus films. Polarization dependent, anisotropic perfect absorption can be realized in this type of two-dimensional plasmonic nanostructures with moderate doping levels. This type of hybrid architecture opens a new door for high performance two-dimensional material plasmonic devices.

Ultra-wideband high-efficiency reflective linear-to-circular polarization converter based on metasurface at terahertz frequencies

The polarization conversion of electromagnetic (EM) waves, especially linear-to-circular (LTC) polarization conversion, is of great significance in practical applications. In this study, we propose an ultra-wideband high-efficiency reflective LTC polarization converter based on a metasurface in the terahertz regime. It consists of periodic unit cells, each cell of which is formed by a double split resonant square ring, dielectric layer, and fully reflective gold mirror. In the frequency range of 0.60 - 1.41 THz, the magnitudes of the reflection coefficients reach approximately 0.7, and the phase difference between the two orthogonal electric field components of the reflected wave is close to 90° or -270°. The results indicate that the relative bandwidth reaches 80% and the efficiency is greater than 88%, thus, ultra-wideband high-efficiency LTC polarization conversion has been realized. Finally, the physical mechanism of the polarization conversion is revealed. This converter has potential applications in antenna design, EM measurement, and stealth technology.