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

A broadband modulator based on graphene/black phosphorus heterostructure with enhanced modulation depth

We theoretically present a broadband modulator based on graphene/black phosphorus heterostructure which can work over a large waveband from visible (VIS) to mid-infrared (MIR) regions. By utilizing the angle dependence of black phosphorus, surface plasmon polaritons (SPP) modulation can be achieved in VIS regime, while the wavelength is tuned within the near-infrared (NIR) or MIR regions, the enhanced modulation depth can be achieved by few-layer graphene films. Results show that the proposed plasmonic modulator exhibits a broad waveband from 400 nm to 3 μm. In addition, this proposed modulator features high modulation depth (MD), low insertion loss (IL), large 3-dB modulation bandwidth and small power consumption from VIS to MIR regions. Our work may extend the operation waveband of opto-electro devices based on the hybridized 2D materials and would promote their potential future applications.

Silicon-Based On-Chip Tunable High-Q-Factor and Low-Power Fano Resonators with Graphene Nanoheaters

Tunable and low-power microcavities are essential for large-scale photonic integrated circuits. Thermal tuning, a convenient and stable tuning method, has been widely adopted in optical neural networks and quantum information processing. Recently, graphene thermal tuning has been demonstrated to be a power-efficient technique, as it does not require thick spacers to prevent light absorption. In this paper, a silicon-based on-chip Fano resonator with graphene nanoheaters is proposed and fabricated. This novel Fano structure is achieved by introducing a scattering block, and it can be easily fabricated in large quantities. Experimental results demonstrate that the resonator has the characteristics of a high quality factor (∼31,000) and low state-switching power (∼1 mW). The temporal responses of the microcavity exhibit qualified modulation speed with 9.8 μs rise time and 16.6 μs fall time. The thermal imaging and Raman spectroscopy of graphene at different biases were also measured to intuitively show that the tuning is derived from the joule heating effect of graphene. This work provides an alternative for future large-scale tunable and low-power-consumption optical networks, and has potential applications in optical filters and switches.

Resonance fluorescence engineering in hybrid systems consist of biexciton quantum dots and anisotropic metasurfaces

The resonance fluorescence properties in the steady-state regime are investigated for a driven cascaded exciton-biexciton quantum dot coupled to the two-dimensional black phosphorus metasurfaces. It is shown that for the material parameters under consideration, both the elliptic and hyperbolic dispersion patterns of the surface plasmon modes are achievable according to the variation of the carrier concentration. Further study on the Purcell factor indicates unequal enhancements in the spontaneous decay of the orthogonal in-plane dipoles. Motivated by this intriguing phenomenon, we then investigate the steady-state properties of the driven quantum dot, where the populations of the dressed levels are highly tunable by engineering the anisotropy of the surfaces. As a result, the manipulation of the carrier concentration will lead to strong modifications in the resonance fluorescence. Under certain conditions, one can observe the squeezing of two-mode noise spectra with different resonances and polarizations. Although at the expense of declines in the photon-sideband detunings, it is feasible to enhance the two-mode squeezing by gate doping. Our proposal can be easily extended to other hybrid systems containing anisotropic metasurfaces, which are important for the development of quantum information science.

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.

Dynamical manipulation of a dual-polarization plasmon-induced transparency employing an anisotropic graphene-black phosphorus heterostructure

Dynamical tunable plasmon-induced transparency (PIT) possesses the unique characteristics of controlling light propagation states, which promises numerous potential applications in efficient optical signal processing chips and nonlinear optical devices. However, previously reported configurations are sensitive to polarization and can merely operate under specific single polarization. In this work we propose an anisotropic PIT metamaterial device based on a graphene-black phosphorus (G-BP) heterostructure to realize a dual-polarization tunable PIT effect. The destructive interference coupling between the bright mode and dark modes under the orthogonal polarization state pronounced anisotropic PIT phenomenon. The coupling strength of the PIT system can be modulated by dynamically manipulating the Fermi energy of the graphene via the external electric field voltage. Moreover, the three-level plasmonic system and the coupled oscillator model are employed to explain the underlying mechanism of the PIT effect, and the analytical results show good consistency with the numerical calculations. Compared to the single-polarization PIT devices, the proposed device offers additional degrees of freedom in realizing universal tunable functionalities, which could significantly promote the development of next-generation integrated optical processing chips, optical modulation and slow light devices.

Circular dichroism enhancement and dynamically adjustment in planar metal chiral split rings with graphene sheets arrays

Chiral plasmonic nanostructures have become a promising platform for polarization converters and molecular analysis. However, the circular dichroism (CD) of planar chiral plasmonic nanostructures is always weak and difficult for dynamic adjustment. In this work, graphene sheets (GSs) are introduced in planar metal chiral split rings (MCSRs) to enhance and dynamically adjust their CD effect. The chiral split rings consist of rotated big and small split rings. Simulation results show that the plasmonic coupling between MCSRs and GSs can enhance the absorption and CD spectra of MCSRs at two resonant wavelengths. The surface current distributions reveal that the CD signals are due to the localized surface plasmon resonances on the big and small split rings, respectively. The loss distributions illustrate that the increased loss mainly locates in GSs. In addition, the CD spectra of MCSRs/GSs can be dynamically adjusted and influenced by the Fermi energy of GSs, the geometric parameters of MCSRs and, the mediums in the environment. It can be used to detect the environmental temperature and concentration. The results help to design dynamically adjustable chiral nanostructures and promote their applications in environment detection.

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.

Ultrafast and low power all-optical switching in the mid-infrared region based on nonlinear highly doped semiconductor hyperbolic metamaterials

Guided wave modes in the uniaxial anisotropic hyperbolic metamaterials (HMMs) based on highly doped semiconductor instead of metal in the mid-infrared region are investigated theoretically. The heavily doped semiconductor is used to overcome the restrictions of the conventional metal-based structures caused by the lake of tunability and high metal loss at mid-infrared wavelengths. The unit cells of our proposed metamaterial are composed of alternating layers of undoped InAs as a dielectric layer and highly doped InAs as a metal layer. We numerically study the linear and nonlinear behavior of such multilayer metamaterials, for different arrangements of layers in the parallel (vertical HMM) and perpendicular (horizontal HMM) to the input wave vector. The effect of doping concentration, metal to dielectric thickness ratio in the unit cell (fill-fraction), and the total thickness of structure on the guided modes and transmission/reflection spectra of the metamaterials are studied. Moreover, the charge redistribution due to band-bending in the alternating doped and undoped layers of InAs is considered in our simulations. We demonstrate that the guided modes of the proposed hyperbolic metamaterial can change by increasing the intensity of the incident lightwave and entering the nonlinear regime. Therefore, the transition from linear to the nonlinear region leads to high-performance optical bistability. Furthermore, the switching performance in the vertical and horizontal HMMs are inspected and an ultrafast, low power, and high extinction ratio all-optical switch is presented based on a vertical structure of nonlinear highly doped semiconductor hyperbolic metamaterials.

Dynamically adjustable-induced THz circular dichroism and biosensing application of symmetric silicon-graphene-metal composite nanostructures

Induced circular dichroism (ICD) has been used to detect biomolecular conformations through the coupling between chiral molecules and achiral metal nanostructures with the localized surface plasmon (LSP). However, this ICD is always weak and cannot be dynamically adjusted. Here, we put dielectric and graphene nanostructures on a metal-substrate for restricting more light energies and obtaining dynamic adjustable performance. A composite nanostructure array composed of achiral silicon-nanorods on a metal-substrate and graphene-ribbons (ASMG) is theoretically investigated. Two strong ICD signals appear in the THz region. Near-field magnetic distributions of ASMG reveal that the two strong ICD signals are mainly due to the surface plasmon resonances (SPPs) on the metal-substrate and LSP in the graphene nanostructures, respectively. The ICD signals strongly depend on the geometric parameters of ASMG and are dynamically adjusted by just changing the Fermi levels of graphene-ribbons. In addition, left-handed ASMG and right-handed ASMG can be used to identify the chiral molecular solutions with different chiralities. The maximum enhancement factor of the chiral molecular solutions could reach up to 3500 times in the THz region. These results can help to design dynamically adjustable THz chiral sensors and promote their application in biological monitoring and asymmetric catalysis.

Ultra-narrowband visible light absorption in a monolayer MoS2 based resonant nanostructure

Enhance light absorption in two-dimensional (2D) materials are of great importance for the development of many optoelectronic devices such as photodetectors, modulators and thermal emitters. In this paper, a resonant nanostructure based on subwavelength gratings of monolayer molybdenum disulphide (MoS₂) is proposed. It is shown numerically that the excitation of guided modes in the proposed structure leads to perfect absorption in the visible range. The linewidth of the absorption spectrum can be narrow down to 0.1 nm. The resonance wavelength exhibits an almost linear dependence on the incidence angle. The proposed structure provides a method to design ultra-narrowband absorbers and similar designs can be applied to other 2D materials. It may find applications for optical filters, directional thermal emitters, 2D materials based lasers and others.

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.

Graphene plasmonically induced analogue of tunable electromagnetically induced transparency without structurally or spatially asymmetry

Electromagnetically induced transparency (EIT) arises from the coherent coupling and interference between a superradiant (bright) mode in one resonator and a subradiant (dark) mode in an adjacent resonator. Generally, the two adjacent resonators are structurally or spatially asymmetric. Here, by numerical simulation, we demonstrate that tunable EIT can be induced by graphene ribbon pairs without structurally or spatially asymmetry. The mechanism originates from the fact that the resonate frequencies of the bright mode and the dark mode supported by the symmetrical graphene ribbon pairs can be respectively tuned by electrical doping levels, and when they are tuned to be equal the graphene plasmon coupling and interference occurs. The EIT in symmetrical nanostructure which avoids deliberately breaking the element symmetry in shape as well as in size facilitates the design and fabrication of the structure. In addition, the work regarding to EIT in the structurally symmetric could provide a fresh contribution to a more comprehensive physical understanding of Fano resonance.

Remarkably high-Q resonant nanostructures based on atomically thin two-dimensional materials

Planar optical resonant structures with high quality (Q) factors play a crucial role in modern photonic technologies. In this paper, a type of remarkably high-Q resonant nanostructure based on atomically thin two-dimensional (2D) materials is proposed. It is shown theoretically and numerically that with the excitation of leaky modes in the proposed structures, guided mode resonant (GMR) gratings, can achieve resonances with extremely narrow linewidths down to 0.0005 nm and high Q-factors up to millions in the telecom range. The thickness of 2D materials and thus the high-Q resonances can be precisely controlled by changing the layer number of 2D materials, providing a versatile platform for strong light-matter interactions. As an example, dramatic nonlinear reflectance can be realized around the resonance at a power level of a few kW cm-2 with the Kerr effect. This new type of 2D material resonant nanostructure can be employed for a variety of applications ranging from lasers, filters and polarizers to nonlinear optical devices.

Hybrid metal-graphene plasmonic sensor for multi-spectral sensing in both near- and mid-infrared ranges

This paper proposes a hybrid metal-graphene plasmonic sensor which can simultaneously perform multi-spectral sensing in near- and mid-IR ranges. The proposed sensor consists of an array of asymmetric gold nano-antennas integrated with an unpatterned graphene sheet. The gold antennas support sharp Fano-resonances for near-IR sensing while the excitation of graphene plasmonic resonances extend the sensing spectra to the mid-IR range. Such a broadband spectral range goes far beyond previously demonstrated multi-spectral plasmonic sensors. The sensitivity and figure of merit (FOM) as well as their dependence on the thickness of the sensing layer and Fermi energy of graphene are studied systematically. This new type of sensor combines the advantages of conventional metallic plasmonic sensors and graphene plasmonic sensors and may open a new door for high-performance, multi-functional plasmonic sensing.

Tunable and Anisotropic Dual-Band Metamaterial Absorber Using Elliptical Graphene-Black Phosphorus Pairs

We numerically propose a dual-band absorber in the infrared region based on periodic elliptical graphene-black phosphorus (BP) pairs. The proposed absorber exhibits near-unity anisotropic absorption for both resonances due to the combination of graphene and BP. Each of the resonances is independently tunable via adjusting the geometric parameters. Besides, doping levels of graphene and BP can also tune resonant properties effectively. By analyzing the electric field distributions, surface plasmon resonances are observed in the graphene-BP ellipses, contributing to the strong and anisotropic plasmonic response. Moreover, the robustness for incident angles and polarization sensitivity are also illustrated.

Strong circular dichroism enhancement by plasmonic coupling between graphene and h-shaped chiral nanostructure

Circular dichroism (CD) is useful in polarization conversion, negative refraction chemical analysis, and bio-sensing. To achieve strong CD signals, researchers constantly break the symmetry of nanostructures. However, how to further enhance the CD based on a new mechanism has become a new challenge in this field. In this work, a hybrid plasmonic chiral system composed of an array of graphene ribbons (GRs) over h-shaped sliver chiral nanostructures (HSCNs) is theoretically investigated. Results demonstrate that the plasmonic coupling between HSCNs and GRs results in different enhanced absorptions for different circularly polarized lights. The absorbance of right circularly polarized light is enhanced to perfect absorption; the absorption of left circularly polarized light is enhanced weakly. It leads to the CD effect of HSCNs@GRs approaching 88%. The loss distributions of HSCNs and HSCNs@GRs reveal that the absorption is enhanced and transferred from HSCNs to GRs. Moreover, the current distributions of HSCNs@GRs are simplified to equivalent LC resonant circuits, which can qualitatively explain the change of CD signals by tuning geometrical parameters of HSCNs@GRs. The findings of this work provide a new method of enhancing chirality and benefit the design of graphene-based chiral optoelectronic devices.

Coupling effects in single-mode and multimode resonator-coupled system

We have proposed a simple metal-dielectric-metal (MDM) waveguide system side-coupled with single-mode and multimode resonators. This proposed structure can achieve a typical dual plasmon-induced transparency (PIT) effect in the transmission spectra. The two PIT peaks exhibit opposite evolution tendencies with the increase in the depth of stubs. A multimode-coupled mode theory (M-CMT), confirmed by simulated results, is originally introduced to investigate the coupling effects of the proposed structure. Compared to the previous reported multichannel filters, the proposed structure includes obvious advantages, such as structural simplicity and ease of fabrication. In addition, the sensing characteristics of the proposed structure based on PIT effects are discussed numerically. The results demonstrate that the proposed structure is suitable for applications in multichannel filters, optical switches, and sensors.

Optical activity in monolayer black phosphorus due to extrinsic chirality

The phenomenon of optical activity has fundamental importance and widespread applications in polarization optics, analytical chemistry, and molecular biology. In the past two decades, there has been much research on designing metamaterials with strong optical activity, which generally employs chiral plasmonic or dielectric nanostructures with resonant responses. In this Letter, we show theoretically and numerically that strong optical activity can be obtained in unpatterned monolayer black phosphorus (BP) without using resonant structures. The optical activity can be attributed to the extrinsic chirality from the mutual orientation of the BP film with in-plane anisotropy and the incident light. The obtained circular dichroism in this atomically thick material is comparable to that in previously reported chiral metamaterials, and the optical activity is inherently tunable by controlling the Fermi level of monolayer BP.

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.