Graphene-based perfect optical absorbers harnessing guided mode resonances

Low-Threshold and High-Extinction-Ratio Optical Bistability within a Graphene-Based Perfect Absorber

A kind of graphene-based perfect absorber which can generate low-threshold and high-extinction-ratio optical bistability in the near-IR band is proposed and simulated with numerical methods. The interaction between input light and monolayer graphene in the absorber can be greatly enhanced due to the perfect absorption. The large nonlinear coefficient of graphene and the strong light-graphene interaction contribute to the nonlinear response of the structure, leading to relatively low switching thresholds of less than 2.5 MW/cm² for an absorber with a Q factor lower than 1000. Meanwhile, the extinction ratio of bistable states in the absorber reaches an ultrahigh value of 47.3 dB at 1545.3 nm. Moreover, the influence of changing the structural parameters on the bistable behaviors is discussed in detail, showing that the structure can tolerate structural parametric deviation to some extent. The proposed bistable structure with ultra-compact size, low thresholds, high extinction ratio, and ultrafast response time could be of great applications for fabricating high-performance all-optical-communication devices.

Guided-mode resonance with reduced bandwidth in mid-infrared absorption and thermal emission

Narrowband resonance plays an important role in many optical applications, especially for the development of wavelength-selective properties and enhanced light-matter interaction. In this paper, we demonstrate metal-insulator-metal (MIM) waveguide gratings, which exhibit guided-mode resonance (GMR) with reduced bandwidth in mid-infrared absorption and thermal emission. Our fabricated MIM waveguide grating consists of a copper substrate, a lossless ZnSe film, and a top gold stripe grating. Our measurements reveal strong GMRs with a bandwidth of 1.29% of the central wavelength in both mid-infrared absorption and thermal emission spectra. By varying structural parameters of the MIM waveguide grating, strong absorptions and thermal emissions of GMRs are observed and tuned within the 3-5 µm wavelength range. These results manifest the great potential of engineering infrared properties by using GMR and could be useful for spectral control in a variety of infrared devices.

Design of vanadium-dioxide-based resonant structures for tunable optical response

Phase change materials are suitable for tunable photonic devices where the optical response can be altered under external stimuli, such as heat, an electrical or an optical signal. In this scenario, we performed numerical simulations to study the optical properties of a flat unpatterned resonant structure and a grating, both coated with a thin film of vanadium dioxide (VO₂). Our results suggest that it is possible to modulate broadband and narrowband reflectance spectra of the resonant structures in the visible to near-infrared range by more than 40 % when the VO₂ undergoes an insulator-to-metal phase transition. Resonant devices with a tunable spectral response may find application in sensors, filters, absorbers, and detectors.

High Q Resonant Graphene Absorber with Lossless Phase Change Material Sb2S3

Graphene absorbers have attracted lots of interest in recent years. They provide huge potential for applications such as photodetectors, modulators, and thermal emitters. In this letter, we design a high-quality (Q) factor resonant graphene absorber based on the phase change material Sb2S3. In the proposed structure, a refractive index grating is formed at the subwavelength scale due to the periodical distributions of amorphous and crystalline states, and the structure is intrinsically flat. The numerical simulation shows that nearly 100% absorption can be achieved at the wavelength of 1550 nm, and the Q factor is more than hundreds due to the loss-less value of Sb2S3 in the near-infrared region. The absorption spectra can be engineered by changing the crystallization fraction of the Sb2S3 as well as by varying the duty cycle of the grating, which can be employed not only to switch the resonant wavelength but also to achieve resonances with higher Q factors. This provides a promising method for realizing integrated graphene optoelectronic devices with the desired functionalities.

High-Quality Graphene-Based Tunable Absorber Based on Double-Side Coupled-Cavity Effect

Graphene-based devices have important applications attributed to their superior performance and flexible tunability in practice. In this paper, a new kind of absorber with monolayer graphene sandwiched between two layers of dielectric rings is proposed. Two peaks with almost complete absorption are realized. The mechanism is that the double-layer dielectric rings added to both sides of the graphene layer are equivalent to resonators, whose double-side coupled-cavity effect can make the incident electromagnetic wave highly localized in the upper and lower surfaces of graphene layer simultaneously, leading to significant enhancement in the absorption of graphene. Furthermore, the influence of geometrical parameters on absorption performance is investigated in detail. Also, the device can be actively manipulated after fabrication through varying the chemical potential of graphene. As a result, the frequency shifts of the two absorption peaks can reach as large as 2.82 THz/eV and 3.83 THz/eV, respectively. Such a device could be used as tunable absorbers and other functional devices, such as multichannel filters, chemical/biochemical modulators and sensors.

Bi-functional tunable reflector/high-Q absorber design using VO2 assisted graphene-coated cylinder array

In this paper, a bi-functional tunable reflector/absorber device using an assembly of graphene-coated cylindrical wires, backed by a thermally controlled phase change material, is proposed. The reflection coefficient of the graphene-coated wire-grating manifests multiple resonances, originating from the hybridized excitation of localized surface plasmons in the graphene shells. The first plasmonic resonance (with the order of two), in the free-standing configuration, shows tunable near-perfect reflection while the second plasmonic resonance (with the order of three), in the reflector-backed array, exhibits near-perfect absorption. Because of the metal-insulator transition in the phase change material, it is feasible to switch between these two functionalities using a VO₂ back layer. Moreover, the high-quality factor of the absorption band (Q ∼ 128.86) is due to its Fano line shape, leading to a narrow bandwidth. Thus, the absorbing mode can be possibly used for refractive index sensing with the sensitivity of S ∼ 9000 nm/RIU (refractive index unit) and figure of merit of FOM ∼ 104 RIU^-1. In the proposed structure, different optical, material, and geometrical parameters affect the optical response of the operating bands, offering a flexible design.

Guided Mode Resonance in a Low-Index Waveguide Layer

In this paper, we show that the guided mode resonance can exist in a low-index waveguide layer on top of a high-index substrate. With the help of the interaction of diffraction from a metal grating and total internal reflection effects, we verify that the guided mode can be supported in the low-index SU8 layer on a high-index substrate. Simulation and experiment show the resonant wavelength can be simply manipulated by controlling the geometrical parameters of the metal grating and waveguide layer. This structure extends the possibilities of guided-mode resonance to a broader class of functional materials and may boost its use in applications such as field enhancement, sensing and display.

Inverse design of graphene-assisted metallodielectric grating and its applications in the perfect absorber and plasmonic third harmonic generation

In this article, we propose a graphene metamaterial coupled with metallodielectric grating (GMCMG) structures to achieve plasmon induced reflection effects in the reflection spectrums. In order to enhance the light-matter interaction in the graphene, the micro-genetic algorithm is applied in the performance optimization for the GMCMG. Due to the absorption enhancement of graphene and the inverse design of photonic structures, a perfect absorber and an efficient third harmonic generator are obtained by employing optimized GMCMG structures. Compared with previous works, our design scheme provides a simple and efficient method for the optimization of photonic devices and has significant applications in optical modulators, absorbers and sensors.

A suspended metasurface achieves complete light absorption: a 50 nm-thick optical nanomicrophone

Considerably subtle vibrations can be detected by light signals. Commonly, this is achieved based on the phase change of light that can be attributed to the vibration of a movable mirror, which has been used in gravitational wave detection. For a homogeneous dielectric membrane, the thinner the membrane, the greater the membrane vibration amplitude will be with respect to the sound pressure. However, if the membrane is too thin, most of the light will transmit through the membrane and the sensitivity will be reduced. To resolve this contradiction, we have developed a metasurface membrane with a thickness of only 50 nm but a considerably high reflectivity. This membrane is integrated with a 100-nm-thick gold membrane to form a cavity that can achieve perfect absorption of light. The vibration of the metasurface, which records the sound wave information, can change the light absorption. The noise equivalent pressure of the proposed structure is several orders lower than those of the recently reported optoacoustic detectors, and the alternating current signal response can be enhanced by approximately 1500 times compared with that of a membrane without a metasurface. The integration of nanomechanical oscillators and ultrathin membranes with a metasurface may facilitate future ultrasensitive sound and ultrasonic detection and benefit optomechanic design.

Ultra-narrowband light absorption enhancement of monolayer graphene from waveguide mode

Greatly improving the light absorption efficiency of graphene and simultaneously manipulating the corresponding absorption bandwidth (broadband or narrowband) is practically important to design graphene-based optoelectronic devices. In this work, we will theoretically show how to largely enhance the absorption in graphene and efficiently control the absorption bandwidth in the visible region, by the excitation of the waveguide mode for the graphene monolayer to be sandwiched between the gold sphere array and dielectric waveguide structure composed of indium tin oxide (ITO) film on a quartz substrate. It is found that the maximum absorption efficiency can reach as high as about 45% and the full-width at half-maximum (FWHM) of the absorption peak can be tuned from about 1 to 10 nanometers, when the array period of gold spheres or the thickness of ITO film is changed.

Absorption enhancement by a period array of nano-grooves in gold substrate

Comprehensive investigations of absorption enhancement by a period array of nano-grooves in gold substrate are provided. A simple Fabry-Perot model is presented to explore impact factors on absorption enhancement. The impacts of structural parameters on absorption enhancement for an array of nano-grooves are explored and discussed. Our calculations show that complete absorption (about 1) can be obtained with the groove depth fulfilling the Fabry-Perot resonance condition. The effect of groove width on absorption for a trapezoidal groove array is slightly different from that for a rectangular groove array, because of the impact of single-pass loss in the grooves. Surface plasmon resonance that can carry most of the energy away from grooves and propagate along the metal surface could lead to extremely low absorption for a period array of nano-grooves. Our discussions identify two key roles in absorption enhancement for a nano-groove array: the fundamental groove mode resonance and the generation of surface plasma polaritons. In addition, the analysis of absorption enhancement for an array of trapezoidal grooves can also provide a comprehensive contribution of Fabry-Perot resonance and electromagnetic interaction along the bevel edges of the trapezoidal grooves.

Rapid large-scale fabrication of multipart unit cell metasurfaces

Periodic diffractive elements known as metasurfaces constitute platform technology whereby exceptional optical properties, not attainable by conventional means, are attained. Generally, with increasing unit-cell complexity, there emerges a wider design space and bolstered functional capability. Advanced devices deploying elaborate unit cells are typically generated by electron-beam patterning which is a tedious, slow process not suitable for large surfaces and quick turnaround. Ameliorating this condition, we present a novel route towards facile fabrication of complex periodic metasurfaces based on sequential exposures by laser interference lithography. Our method is fast, cost-effective, and can be applied to large surface areas. It is enabled by precise control over periodicity and exposure energy. With it we have successfully patterned and fabricated one-dimensional (1D) and two-dimensional (2D) multipart unit cell devices as demonstrated here. Thus, zero-order transmission spectra of an etched four-part 1D grating device are simulated and measured for both transverse-electric (TE) and transverse-magnetic (TM) polarization states of normally incident light. We confirm non-resonant wideband antireflection (∼800 nm) for TM-polarized light and resonance response for TE-polarized light in the near-IR band spanning 1400-2200 nm in a ∼100 mm² device. Furthermore, it is shown that this method of fabrication can be implemented not only to pattern periodic symmetric/asymmetric designs but also to realize non-periodic metasurfaces. The method will be useful in production of large-area photonic devices in the realm of nanophotonics and microphotonics.

Resonant waveguide grating reflection filter with a quasi-rectangular spectrum under fully conical incidence

We present the design, analysis, and characterization of a filter with a quasi-rectangular spectrum. The spectral features of the filter are achieved by adjusting the incident angle under fully conical incidence. When the incident angle is 75°, the filter with a quasi-rectangular spectrum is presented at the central wavelength of 475 nm. The proposed filter has a bandwidth of 7.3 nm (R>90%), its corresponding Δλ/λ is approximately 1.5%, and the estimated rejection ratio is larger than 10 dB. Furthermore, the quasi-rectangular filtering feature is stable in the incident angle range of 75° to 85°. Our approach reveals the quasi-rectangular spectrum attributes of double resonance peaks merger under fully conical incidence and thus can be used to exploit filter devices.

Dual-band tunable narrowband near-infrared light trapping control based on a hybrid grating-based Fabry-Perot structure

A hybrid grating-based Fabry-Perot structure is proposed to investigate light manipulation in the near-infrared wavelength region. It is found that the electromagnetic energy can be easily trapped in different parts of the system at different polarization states. For TM polarization, numerical results show that two remarkable narrowband absorptance peaks appear owing to the excitation of critical coupling with guided mode resonance and Fabry-Perot resonance. While for TE polarization, only one narrowband absorptance peak is generated because only Fabry-Perot resonance is excited. The near-infrared spectral selectivity of the system can be tuned by changing the geometrical parameters. In addition, the spectral absorptance of the system can be optimized by applying gate voltage on graphene sheet to change graphene chemical potential. This valuable dual-band tunable narrowband absorber is a potential application for high-performance optoelectronic devices.

A Narrow Dual-Band Monolayer Unpatterned Graphene-Based Perfect Absorber with Critical Coupling in the Near Infrared

The combination of critical coupling and coupled mode theory in this study elevated the absorption performance of a graphene-based absorber in the near-infrared band, achieving perfect absorption in the double bands (98.96% and 98.22%), owing to the guided mode resonance (the coupling of the leak mode and guided mode under the condition of phase matching, which revealed 100% transmission or reflection efficiency in the wavelet band), and a third high-efficiency absorption (91.34%) emerged. During the evaluation of the single-structure, cross-circle-shaped absorber via simulation and theoretical analysis, the cross-circle shaped absorber assumed a conspicuous preponderance through exploring the correlation between absorption and tunable parameters (period, geometric measure, and incident angle of the cross-circle absorber), and by briefly analyzing the quality factors and universal applicability. Hence, the cross-circle resonance structure provides novel potential for the design of a dual-band unpatterned graphene perfect absorber in the near-infrared band, and possesses practical application significance in photoelectric detectors, modulators, optical switching, and numerous other photoelectric devices.

Graphene perfect absorber of ultra-wide bandwidth based on wavelength-insensitive phase matching in prism coupling

We proposed perfect absorbers of ultra-wide bandwidths based on prism coupling with wavelength-insensitive phase matching, which consists of three dielectric layers (Prism-Cavity-Air) with monolayer graphene embedded in the cavity layer. Due to inherent material dispersion of the dielectric layers, with the proper choice of the incidence angle and the cavity thickness, the proposed perfect absorbers can satisfy the phase matching condition over a wide wavelength range, inducing enormous enhancement of the absorption bandwidth. The requirement on the material dispersions of the prism and the cavity layer for the wavelength-insensitive phase matching over a wavelength range of the interest has been derived, and it has been demonstrated that the various kinds of materials can meet the requirement. Our theoretical investigation with the transfer matrix method (TMM) has revealed that a 99% absorption bandwidth of ~300 nm with perfect absorption at λ  = 1.51 μm can be achieved when BK7 and PDMS are used as the prism and the cavity layer, respectively, which is ~7 times wider than the conceptual design based on the non-dispersive materials. The full width at half maximum of our designed perfect absorber is larger than 1.5 μm.

Enhanced absorption in a graphene embedded 1D guided-mode-resonance structure without back-reflector and interferometrically written gratings

A theoretical model based on the coupled mode theory is presented to calculate the absorption in a graphene embedded 1D guided-mode-resonance (GMR) structure that does not require a back reflector. The optimized graphene-GMR structure can absorb up to 70% of the incident light which far exceeds the already reported results without using any back-metal reflector or Bragg mirror. The theoretical analysis is valid for binary gratings and pyramidal gratings which are patterned using an interference lithography system. We experimentally validate our theoretical results and analyze the influence of the geometrical parameters to achieve critical coupling for the enhanced absorption.

Flexible control of light trapping and localization in a hybrid Tamm plasmonic system

A hybrid Tamm plasmonic system is proposed to investigate light manipulation at near-infrared frequency. The numerical results reveal that two remarkable absorption peaks are generated due to the different types of resonant modes excited in the structure, which can be well explained theoretically by guided-mode resonance (GMR) and Tamm plasmon polaritons. It is found that the electromagnetic energy can be easily trapped in different parts of the structure. More importantly, strong interaction between the two modes can be achieved by adjusting the structure period or incident angle, resulting in obvious mode hybridization and exhibiting unique energy-transfer characteristics. In addition, the active modulation of GMR-based absorption can be controlled in a continuous type by tuning the polarization angle or in a jump type by adjusting the chemical potential of graphene. This work should be useful for developing many high-performance optoelectronic devices, including sensors, modulators, detectors, etc.

Graphene-Based Cylindrical Pillar Gratings for Polarization-Insensitive Optical Absorbers

In this study, we present a two-dimensional dielectric grating which allows achieving high absorption in a monolayer graphene at visible and near-infrared frequencies. Dielectric gratings create guided-mode resonances that are exploited to effectively couple light with the graphene layer. The proposed structure was numerically analyzed through a rigorous coupled-wave analysis method. Effects of geometrical parameters and response to the oblique incidence of the plane wave were studied. Numerical results reveal that light absorption in the proposed structure is almost insensitive to the angle of the impinging source over a considerable wide angular range of 20°. This may lead to the development of easy to fabricate and experimentally viable graphene-based absorbers in the future.

High fabrication-tolerant narrowband perfect graphene absorber based on guided-mode resonance in distributed Bragg reflector

We propose the narrowband perfect absorbers with enormously high fabrication tolerance, which consists of a low-contrast grating and a finite distributed Bragg reflector (DBR) layer with an ultrathin absorbing medium (graphene). It is numerically shown that the proposed perfect absorber outperforms the previously proposed schemes in fabrication tolerance. According to the rigorous coupled wave analysis (RCWA) and coupled mode theory (CMT) fitting, over a considerably wide range of grating width and thickness, the proposed absorber provides a proper ratio of leakage rate to loss rate while preserving resonant condition, so that almost perfect absorption (>99.9%) can be obtained. This result is attributed to the strong electric field confinement in the DBR region rather than the grating layer owing to lower index of grating compared to DBR. In addition, without degrading the fabrication tolerance, the bandwidth of the proposed absorber can be controlled by the DBR thickness (the number of pairs) and a narrow absorbing bandwidth of sub-nanometer is achieved with 8.5 Si/SiO₂ pair stacked DBR.

Graphene-Based Perfect Absorption Structures in the Visible to Terahertz Band and Their Optoelectronics Applications

Graphene has unique properties which make it an ideal material for photonic and optoelectronic devices. However, the low light absorption in monolayer graphene seriously limits its practical applications. In order to greatly enhance the light absorption of graphene, many graphene-based structures have been developed to achieve perfect absorption of incident waves. In this review, we discuss and analyze various types of graphene-based perfect absorption structures in the visible to terahertz band. In particular, we review recent advances and optoelectronic applications of such structures. Indeed, the graphene-based perfect absorption structures offer the promise of solving the key problem which limits the applications of graphene in practical optoelectronic devices.

Coherent perfect absorption and asymmetric interferometric light-light control in graphene with resonant dielectric nanostructures

Engineering light absorption in graphene has been the focus of intensive research in the past few years. In this paper, we show numerically that coherent perfect absorption can be realized in monolayer graphene in the near infrared range by harnessing resonances of dielectric nanostructures. The asymmetry of the structure results in different optical responses for light illuminated from the top side and the substrate side and enables asymmetric interferometric light-light control. The absorbed and scattered light exhibit interesting nonlinear behavior, allowing switching a strong optical signal output with a weak light. This work may stimulate potential applications including new types of sensors, coherent photodetectors and all-optical logical devices.

Near-absolute polarization insensitivity in grapheme based ultra-narrowband perfect visible light absorber

Strong light-graphene interaction is essential for the integration of graphene to nanophotonic and optoelectronic devices. The plasmonic response of graphene in terahertz and mid-infrared regions enhances this interaction, and other resonance mechanisms can be adopted in near-infrared and visible ranges to achieve perfect light absorption. However, obtaining near-absolute polarization insensitivity with ultra-narrow absorption bandwidth in the visible and near-infrared regimes remains a challenge. In this regard, we numerically propose a graphene perfect absorber, utilizing the excitation of guided-modes of a dielectric slab waveguide by a novel sub-wavelength dielectric grating structure. When the guided-mode resonance is critically coupled to the graphene, we obtain perfect absorption with an ultra-narrow bandwidth (full-width at half-maximum) of 0.8 nm. The proposed design not only preserves the spectral position of the resonance, but also maintains >98% absorption at all polarization angles. The spectral position of the resonance can be tuned as much as 400 nm in visible and near-infrared regimes by tailoring geometrical parameters. The proposed device has great potential in efficient, tunable, ultra-sensitive, compact and easy-to-fabricate advanced photodetectors and color filters.

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.

Monolayer-graphene-based broadband and wide-angle perfect absorption structures in the near infrared

Broadband optical absorption structures in the near infrared by coupling monolayer-graphene with periodical metal structures are proposed and demonstrated numerically. Optical absorption of graphene with over-50%-absorption bandwidth up to hundreds of nanometer caused by magnetic dipole resonances and magnetic coupling effect are investigated in detail, and the demonstrated bandwidths are one order higher than those caused by dielectric guiding mode resonances. In addition, the influences of geometrical parameters of structures are fully analyzed and these demonstrated structures show angular-insensitive absorption for oblique incidence in a large angular range. The demonstrated absorption structures in this work provide new design ideas in the realization of advanced graphene-based optoelectronic devices.

Multiband and Broadband Absorption Enhancement of Monolayer Graphene at Optical Frequencies from Multiple Magnetic Dipole Resonances in Metamaterials

It is well known that a suspended monolayer graphene has a weak light absorption efficiency of about 2.3% at normal incidence, which is disadvantageous to some applications in optoelectronic devices. In this work, we will numerically study multiband and broadband absorption enhancement of monolayer graphene over the whole visible spectrum, due to multiple magnetic dipole resonances in metamaterials. The unit cell of the metamaterials is composed of a graphene monolayer sandwiched between four Ag nanodisks with different diameters and a SiO₂ spacer on an Ag substrate. The near-field plasmon hybridizations between individual Ag nanodisks and the Ag substrate form four independent magnetic dipole modes, which result into multiband absorption enhancement of monolayer graphene at optical frequencies. When the resonance wavelengths of the magnetic dipole modes are tuned to approach one another by changing the diameters of the Ag nanodisks, a broadband absorption enhancement can be achieved. The position of the absorption band in monolayer graphene can be also controlled by varying the thickness of the SiO₂ spacer or the distance between the Ag nanodisks. Our designed graphene light absorber may find some potential applications in optoelectronic devices, such as photodetectors.

Tunable ultra-high-efficiency light absorption of monolayer graphene using critical coupling with guided resonance

We numerically demonstrate a novel monolayer graphene-based perfect absorption multi-layer photonic structure by the mechanism of critical coupling with guided resonance, in which the absorption of graphene can significantly reach 99% at telecommunication wavelengths. The highly efficient absorption and spectral selectivity can be obtained with designing structural parameters in the near-infrared region. Compared to previous works, we achieve the complete absorption of single-atomic-layer graphene in the perfect absorber with a lossless dielectric Bragg mirror, which not only opens up new methods of enhancing the light-graphene interaction, but also makes for practical applications in high-performance optoelectronic devices, such as modulators and sensors.

A proposal of a perfect graphene absorber with enhanced design and fabrication tolerance

We propose a novel device structure for the perfect absorption of a one-sided lightwavve illumination, which consists of a high-contrast grating (HCG) and an evanescently coupled slab with an absorbing medium (graphene). The operation principle and design process of the proposed structure are analyzed using the coupled mode theory (CMT), which is confirmed by the rigorous coupled wave analysis (RCWA). According to the CMT analysis, in the design of the proposed perfect absorber, the HCG, functioning as a broadband reflector, and the lossy slab structure can be optimized separately. In addition, we have more design parameters than conditions to satisfy; that is, we have more than enough degrees of freedom in the device design. This significantly relieves the complexity of the perfect absorber design. Moreover, in the proposed perfect absorber, most of the incident wave is confined in the slab region with strong field enhancement, so that the absorption performance is very tolerant to the variation of the design parameters near the optimal values for the perfect absorption. It has been demonstrated numerically that absorption spectrum tuning over a wider wavelength range of ~300 nm is possible, keeping significantly high maximum absorption (>95%). It is also shown that the proposed perfect absorber outperforms the previously proposed scheme in all aspects.

Monolayer-graphene-based perfect absorption structures in the near infrared

Subwavelength perfect optical absorption structures based on monolayer-graphene are analyzed and demonstrated experimentally. The perfect absorption mechanism is a result of critical coupling relating to a guided mode resonance of a low index two-dimensional periodic structure. Peak absorption over 99% at wavelength of 1526.5 nm with full-width at half maximum (FWHM) about 18 nm is demonstrated from a fabricated structure with period of 1230 nm, and the measured results agree well with the simulation results. In addition, the influence of geometrical parameters of the structure and the angular response for oblique incidence are analyzed in detail in the simulation. The demonstrated absorption structure in the presented work has great potential in the design of advanced photo-detectors and modulators.

Dual-band light absorption enhancement of monolayer graphene from surface plasmon polaritons and magnetic dipole resonances in metamaterials

It is well known that the absorption efficiency of a suspended monolayer graphene in the optical wavelength rang is only 2.3%, which limits its optoelectronic applications. In this work, we numerically demonstrate dual-band absorption enhancement of monolayer graphene at optical frequency, with the maximum absorption efficiency reaching to about 70% under optimum conditions. The dual-band absorption enhancement arises from the excitations of surface plasmon polaritons and magnetic dipole resonances in metamaterials. The monolayer graphene is sandwiched between a periodic array of Ag nanodisks and a SiO₂ spacer supported on an Ag substrate. The resonance wavelengths of two absorption bands arising from surface plasmon polaritons and magnetic dipole resonances can be easily tuned by the array period and the diameter of the Ag nanodisks, respectively. Our designed graphene light absorber may find some potential applications in optoelectronic devices, such as photodetectors.

Achieving ultranarrow graphene perfect absorbers by exciting guided-mode resonance of one-dimensional photonic crystals

Graphene perfect absorbers with ultranarrow bandwidth are numerically proposed by employing a subwavelength dielectric grating to excite the guided-mode resonance of one-dimensional photonic crystals (1DPCs). Critical coupling of the guided-mode resonance of 1DPCs to graphene can produce perfect absorption with a ultranarrow bandwidth of 0.03 nm. The quality factor of the absorption peak reaches a ultrahigh value of 20000. It is also found that the resonant absorption peaks can be tuned by controlling the dispersion line of the guided mode and the period of the grating. When the parameters of the grating and the 1DPCs are suitably set, the perfect absorption peaks can be tuned to any randomly chosen wavelength in the visible wavelength range.

Wavelength-tunable perfect absorber based on guided-mode resonances

We numerically investigate the triple-band perfect absorption in a metal-insulator-metal structure. The absorption peak from the TE-polarized guided-mode resonance (GMR) is highly sensitive to the incident angle. Thus a wavelength-tunable perfect absorber (PA) based on the TE GMR is proposed for the first time. By the precise control of the incident angle, the ∼5  nm narrowband perfect absorber can be modulated linearly about 3 nm/° in our structure. For single frequency light, the intensity tunability of the absorption between 6.2%-99.27% is realized only by changing the incident angle of 5°. The further study focused on TM polarization confirms the possibility to realize a polarization-independent wavelength-tunable PA. Such a PA possesses potential for applications in absorption filter, thermal emitter, surface-enhanced Raman scattering, biosensing, and nonlinear optics.