Realization of mid-infrared broadband absorption in monolayer graphene based on strong coupling between graphene nanoribbons and metal tapered grooves

Bandwidth tunability of graphene absorption enhancement by hybridization of delocalized surface plasmon polaritons and localized magnetic plasmons

The bandwidth-tunable absorption enhancement of monolayer graphene is theoretically studied in the near-infrared wavelengths. The monolayer graphene is placed on the silver substrate surface with a periodic array of one-dimensional slits. Two absorption peaks are found to result from the hybridization of delocalized surface plasmon polaritons and localized magnetic plasmons. The positions of absorption peaks are accurately predicted by a coupling model of double oscillators. The full width at half maximum of absorption peaks is largely tuned from about 1-200 nm by changing the array period of slits. The effect of the slit size on absorption peaks is also investigated in detail. Our work is promising in applications for photoelectric devices.

Metasurface inverse designed by deep learning for quasi-entire terahertz wave absorption

Ultra-broadband and efficient terahertz (THz) absorption is of paramount importance for the development of high-performance detectors. These detectors find applications in next-generation wireless communications, military radar systems, security detection, medical imaging, and various other domains. In this study, we present an ultra-wideband THz wave metasurface absorber (UTWMA) featuring a composite surface microstructure and a multilayer absorbing material (graphene). This UTWMA demonstrates remarkable capabilities by achieving highly efficient absorption levels, reaching 96.33%, within the 0.5-10 THz frequency range. To enhance the efficiency and precision of the design process, we have incorporated artificial neural networks, which enable rapid and accurate parameter selection. Moreover, we have conducted a comprehensive analysis of the absorption mechanism exhibited by the UTWMA at different frequencies. This analysis combines insights from the electric field distribution and effective medium theory. The findings presented in this paper are expected to catalyze further research in the domain of broadband THz technology, particularly in the context of metasurfaces and related fields. Additionally, this work paves the way for the development of compact, supercontinuous THz photovoltaic or photothermal electrical devices.

Compact plasmon modulator based on the spatial control of carrier density in indium tin oxide

To keep pace with the demands of semiconductor integration technology, a semiconductor device should offer a small footprint. Here, we demonstrate a compact electro-optic modulator by controlling the spatial distribution of carrier density in indium tin oxide (ITO). The proposed structure is mainly composed of a symmetrical metal electrode layer, calcium fluoride dielectric layer, and an ITO propagating layer. The carrier density on the surface of the ITO exhibits a periodical distribution when the voltage is applied on the electrode, which greatly enhances the interaction between the surface plasmon polaritons (SPPs) and the ITO. This structure can not only effectively improve the modulation depth of the modulator, but also can further reduce the device size. The numerical results indicate that when the length, width, and height of the device are 14 µm, 5 µm, and 8 µm, respectively, the modulation depth can reach 37.1 dB at a wavelength of 3.66 µm. The structure can realize a broadband modulation in theory only if we select a different period of the electrode corresponding to the propagating wavelength of SPPs because the modulator is based on the scattering effect principle. This structure could potentially have high applicability for optoelectronic integration, optical communications, and optical sensors in the future.

A multi-parameter tunable plasmon modulator

Multi-parameter control of light is a key functionality to modulate optical signals in photonic integrated circuits for various applications. However, the traditional optical modulators can only control one or two properties of light at the same time. Herein, we propose a hybrid structure which can modulate the amplitude, wavelength and phase of surface plasmon polaritons (SPPs) simultaneously to overcome these limitations. The numerical results show that when the Fermi level of graphene changes from 0.3 to 0.9 eV, the variation of optical transmission, wavelength and phase are 32.7 dB, 428 nm and 306°, respectively. The demonstrated structure triggers an approach for the realization of ultracompact modulation and has potential applications in the fields of optical switches, communications and photo-detection.

Ultra-broadband and completely modulated absorption enhancement of monolayer graphene in a near-infrared region

Achieving ultra-broadband and completely modulated absorption enhancement of monolayer graphene in near-infrared region is practically important to design graphene-based optoelectronic devices, however, which remains a challenge. In this work, by spectrally designing multiple magnetic plasmon resonance modes in metamaterials to be adjacent to each other, near-infrared light absorption in monolayer graphene is greatly improved to have an averaged absorption efficiency exceeding 50% in a very broad absorption bandwidth of about 800 nm. Moreover, by exerting an external bias voltage on graphene to change Fermi energy of graphene, the ultra-broadband absorption enhancement of monolayer graphene exhibits an excellent tunability, which has a nearly 100% modulation depth and an electrical switching property. This work is promising for applications in near-infrared photodetectors, amplitude modulators of electromagnetic waves, etc.

A plasmon modulator by directly controlling the couple of photon and electron

The manipulation of surface plasmon polaritons plays a pivotal role in plasmonic science and technology, however, the modulation efficiency of the traditional method suffers from the weak light-matter interaction. Herein, we propose a new method to overcome this obstacle by directly controlling the couple of photon and electron. In this paper, a hybrid graphene-dielectric- interdigital electrode structure is numerically and experimentally investigated. The plasmon is excited due to the confined carrier which is regulated by the potential wells. The frequency of plasmon can be tuned over a range of ~ 33 cm⁻¹, and the obtained maximum extinction ratio is 8% via changing the confined area and the density of carrier. These findings may open up a new path to design the high efficiency all-optical modulator because the electrons can also be driven optically.

Dynamically switchable broadband and triple-band terahertz absorber based on a metamaterial structure with graphene

This paper proposes a terahertz absorber with a simple four-layered structure that can be dynamically switched between broadband and triple-band by controlling the chemical potential of graphene. The proposed absorber owns broadband absorption in the frequency range from 5.28 THz to 7.86 THz with the corresponding absorption efficiency above 90%, when the chemical potential of graphene is 150 meV. By increasing the chemical potential of graphene to 550 meV, the broadband absorption splits into triple-band absorption, with the peak locating at 5.39 THz, 7.01 THz and 8.1 THz, respectively. Detailed investigation shows that the broadband absorption should originate from magnetic resonance, Fabry-Pérot cavity resonance and surface plasmon polariton. The triple-band absorption should arise from the combination of Fabry-Pérot cavity resonance and surface plasmon polariton. Additionally, both broadband absorption and triple-band absorption are insensitive to the incident polarization. This tunable and bifunctional metamaterial structure shows a great potential in terahertz applications, such as detectors, modulators and sensors.

Imaging metasurfaces based on graphene-loaded slot antennas

Spectral imagers, the classic example being the color camera, are ubiquitous in everyday life. However, most such imagers rely on filter arrays that absorb light outside each spectral channel, yielding ∼1/N efficiency for an N-channel imager. This is especially undesirable in thermal infrared (IR) wavelengths, where sensor detectivities are low. We propose an efficient and compact thermal infrared spectral imager comprising a metasurface composed of sub-wavelength-spaced, differently-tuned slot antennas coupled to photosensitive elements. Here, we demonstrate this idea using graphene, which features a photoresponse up to thermal IR wavelengths. The combined antenna resonances yield broadband absorption in the graphene exceeding the 1/N efficiency limit. We establish a circuit model for the antennas' optical properties and demonstrate consistency with full-wave simulations. We also theoretically demonstrate ∼58% free space-to-graphene photodetector coupling efficiency, averaged over the 1050 cm^-1 to 1700 cm^-1 wavenumber range, for a four-spectral-channel gold metasurface with a 0.883 µm by 6.0 µm antenna pitch. This research paves the way towards compact CMOS-integrable thermal IR spectral imagers.

Highly efficient and controllable photoluminescence emission on a suspended MoS2-based plasmonic grating

Being a new class of materials, transition metal dichalcogenides are paving the way for applications in atomically thin optoelectronics. However, the intrinsically weak light-matter interaction and the lack of manipulation ability has lead to poor light emission and tunable behavior. Here, we investigate the fluorescence characteristic of monolayer molybdenum disulfide on a metal narrow-slit grating, where a highly efficient, 471 times photoluminescence enhancement are realized, based on the hybrid surface plasmon polaritons resonances and the decreased influence of substrate. Moreover, the emitted intensity and polarization are controllable due to the polarization-dependent characteristic and anisotropy of grating. The manipulations of light-matter interactions in this special system provide a new insight into the fluorescent emission process and open a new avenue for high-performance low dimensional materials devices designs.

Near-perfect broadband infrared metamaterial absorber utilizing nickel

We propose a thin, compact, broadband, polarization and angle insensitive metamaterial absorber based on a tungsten reflector, a silicon spacer, and a top pattern composed of a double square-like ring resonator utilizing nickel (Ni). In such a structure, a high absorption (above 80%) bandwidth ∼4.8µm from 3.52 up to 8.32 µm corresponding to the relative bandwidth ∼81% can be achieved with deeply subwavelength unit cell dimensions. Here the physical origin of the broadband absorption is associated with low Q-factor dipole modes of the top pattern inner and outer sides functioning as rectangular nanoantennas. Owing to the structural symmetry, the absorber shows a good incidence angle tolerance in the relatively wide range for both transverse electric and transverse magnetic polarizations. The effective parameters of the Ni-based absorber were retrieved using the constitutive effective medium theory, and the absorption characteristics of the effective medium and metamaterial were compared.