Absorption Of Monolayer Graphene Research Articles

Ultrafast active plasmonic switch based on the broadband high absorption of monolayer graphene with high modulation depth

Optical switches based on plasmonic nanostructures are of great interest due to their high speed performance. To improve the broadband switching performance, a plasmonic design based on metal-insulator-metal (MIM) structure and monolayer graphene (as an active layer) is proposed. In this scheme, the light absorption of the monolayer graphene and the optical bandwidth are increased due to magnetic dipole resonance and magnetic coupling effect. The numerical simulation results of the proposed structure reveal that high absorption is achieved at the wavelength of 1.55 ‌μm which is 67% and 93% for the monolayer graphene and the whole structure, respectively. This structure has a high absorption modulation depth which can be reached nearly 100% around the interband transition position in a wide wavelength range from 1 μm to 2.5 ‌‌‌‌‌μm. Also, regarding its short response time of 10 fs, this structure can be used as an ultrafast switch. In addition, the equivalent circuit model of the structure is derived from the transmission line model (TLM) that its results are in a very good agreement with the numerical simulation results.

Actively modulating near-infrared absorption of monolayer graphene in a compound grating-coupled waveguide structure

In this work, we numerically study the tunable light absorption of monolayer graphene at the near-infrared region by the guided mode resonance in a compound grating-coupled waveguide structure. A biased graphene capacitor is placed below the coupled waveguides. The electromagnetically induced transparency phenomenon is demonstrated by the resonant coupling of guided modes. The light absorption of monolayer graphene at the transparency window can be dynamically tuned by shifting the Fermi energy of graphene in capacitor. The absorption modulation depth of monolayer graphene at the transparency window can vary quickly from zero to nearly 100 % around the interband transition of graphene in capacitor. These results have potential applications in the active photodetectors and photoconductive devices.

Ultra-narrowband, electrically switchable, and high-efficiency absorption in monolayer graphene resulting from lattice plasmon resonance

We propose a novel approach to obtain the ultra-narrowband, electrically switchable, and high-efficiency absorption in the monolayer graphene. The monolayer graphene is sandwiched between the silica substrate and a square array of silver nanospheres, which is covered by a very thick silica layer. The fundamental role of the thick silica cover layer is to homogenize the surrounding medium of the silver nanospheres, and thus efficiently exciting the lattice plasmon resonance. The lattice plasmon resonance arising from the coupling between the dipolar plasmon resonance of silver nanospheres and the zero-order diffraction wave at Wood anomaly can enhance the electromagnetic fields at the graphene surface, and thus greatly improve the near-infrared light absorption of the monolayer graphene with the maximum absorption efficiency up to 45%. Owing to the low radiation loss of the lattice plasmon resonance, the absorption linewidth of the monolayer graphene can be largely compressed to only several nanometers (3.2 nm ∼ 7.4 nm). Moreover, the absorption of the monolayer graphene in our proposed nanostructures has a nearly 100% modulation depth to exhibit an excellent switching property. Our work will be helpful for some graphene-based photoelectric devices.

Near-Perfect Narrow-Band Tunable Graphene Absorber with a Dual-Layer Asymmetric Meta-Grating

A near-perfect narrow-band graphene-based absorber was fabricated using a resonant system integrated with an asymmetric meta-grating at a wavelength of 1550 nm. By optimizing the gap between the two grating strips, the absorption of monolayer graphene can be increased to 99.6% owing to the strong field confinement of the bottom zero-contrast grating (ZCG). The position of the absorption spectrum could be adjusted by tailoring the grating period or the thickness of the waveguide layer. Interestingly, absorption spectrum linewidth can be tailored by changing the thickness of the spacer layer. The accidental bound states in the continuum (BICs) are then demonstrated in the structure. Moreover, the designed structure realizes the dynamic adjustment of the absorption efficiency at a specific wavelength, which has excellent potential in integrated optical devices and systems.

A complete numerical analysis of the impact of disorder and defect cavities on achieving complete optical absorption in monolayer graphene with supporting random structures

A complete numerical analysis of the impact of disorder and defect cavities on achieving complete optical absorption in monolayer graphene with supporting random structures

Transfer matrix optimization of a one-dimensional photonic crystal cavity for enhanced absorption of monolayer graphene.

The optical absorption enhancement of graphene is of significant interest due to its remarkable applications in optical devices. One of the most useful methods is placing graphene in an asymmetric Fabry-Perot cavity made of one-dimensional dielectric multilayers forming two mirrors. In that regard, using the transfer matrix method, we have explicitly calculated the required periodicity of the front photonic multilayer mirror to maximize the absorption in the graphene for any given combination of material types and number of layers. Then we studied the equivalence between these structural configurations and those with arbitrary periodicity but with defects, where the equivalence holds when ω=ξω0,ξ∈Z≥0. These defects are introduced via layer position alterations, based on which we propose an optimization algorithm to maximize absorption in structures having a cavity with an arbitrary periodicity. Numerical calculations are given for dielectric material combinations of TiO2/SiO2 and Ta2O5/SiO2, and to understand the behavior of these optimized structures for any general combination of material types, the mapping of their calculated front mirror periodicity for a range of refractive indices of the two material types has been studied.

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.

Tailoring the light absorption of monolayer graphene via accidental quasi-bound states in the continuum

Bound states in the continuum (BICs) have drawn fundamental and technological interests due to their distinct features such as infinite quality factor and extremely localized fields. Recently, it has been shown that the light absorption of graphene can be effectively enhanced by using symmetry-protected quasi-BICs; however, the important role of the counterparts of accidental quasi-BICs for light absorption enhancement of ultrathin films has not been studied, to our knowledge. Herein, light absorption enhancement of graphene is demonstrated through the excitation of accidental quasi-BICs based on a simple silicon grating metasurface (SGM). Highly efficient light absorption of monolayer graphene can be achieved at over-coupled resonance, and the locations of the absorption peaks and their peak values can be dynamically tuned by varying the incident angle. The enhanced light absorption of graphene is originated mainly from the hybrid toroidal dipole and electric quadrupole mode according to the far-field multiple decompositions and near-field distributions of the unit cell of the structure. In addition, the absorption responses of the SGM with graphene are robust to the variation of structural parameters, and their optical performances can be highly modulated as the Fermi level of graphene is altered.

Dual-band perfect graphene absorber with an all-dielectric zero-contrast grating-based resonant cavity

We proposed an all-dielectric resonant cavity based on a zero-contrast grating (ZCG), enabling the perfect absorption of monolayer graphene at the wavelength of 1550 nm band. The mechanism of dual-band perfect absorption, which is verified by the coupled-mode theory (CMT), can be attributed to the critical coupling of Fabry–Perot cavity resonance and guided-mode resonance (GMR) in the ZCG-based cavity. The absorption peaks can be tuned by tailoring the structural parameters, such as the thickness of the waveguide layer in the ZCG, and the cavity length. Interestingly, we find that the critical coupling of the system can be robustly controlled by changing the grating thickness and grating period of ZCG. The proposed structure has potential application prospects for high-performance graphene-based optoelectronic devices.

Broadband, wide-incident-angle, and polarization-insensitive high-efficiency absorption of monolayer graphene with nearly 100% modulation depth at communication wavelength

We theoretically study how to greatly improve the near-infrared light absorption of monolayer graphene through the excitation of magnetic resonance in metamaterials. The absorption maximum of monolayer graphene is able to reach up to about 77% at the communication wavelength of 1550 nm. The conventionally defined absorption bandwidth, i.e., the full width at half maximum (FWHM), is nearly 160 nm. Thanks to the localization nature of magnetic resonance, the broadband high-efficiency absorption of monolayer graphene is insensitive to the incident angle and the light polarization. The absorption maximum, the absorption bandwidth, and the resonance position all have almost no change, when the incident angle is increased to even 60 degrees for both p and s polarizations. By applying an external bias voltage to change Fermi energy, the absorption in graphene can be completely modulated, with a nearly 100% modulation depth. Furthermore, the graphene absorption has an abrupt and large change around the interband transition, which exhibits an electrical switching property. Our work may find potential applications in graphene-based optoelectronic devices such as photodetector and modulator.

Electrically-modulated infrared absorption of graphene metamaterials via magnetic dipole resonance

To increase the absorption efficiency of graphene-based devices, we propose a metamaterial structure comprising of a square metal plate array on graphene, a dielectric spacer, and a metal substrate. This work numerically demonstrates enhancement of the near-infrared absorption of monolayer graphene via the magnetic dipole resonance in the metamaterial under normal-incidence. Furthermore, the absorption efficiency of graphene-based devices can reach up to 96% with proposed structures incorporating three layers of graphene when the Fermi energy is 0.6 eV. Varying the Fermi energy by varying the applied voltage, is shown to enable an increase of the absorption efficiency of graphene-based devices with a reduction of the Fermi energy. Therefore, the absorption wavelength exhibits a linear electrical modulation in a wide range from 1362 to 1787 nm. The designed graphene light absorber has potential applications in optoelectronic devices such as photodetectors and perfect absorbers. • A new resonance peak appeared by adding single layer graphene. • By increasing the number of graphene layers to three, the absorption efficiency can be increased from 32% to 96%. • By changing the Fermi energy of graphene, the maximum tunable wavelength range can reach 425 nm. • The absorption efficiency of all resonance peaks is more than 87%.

Graphene perfect absorber with loss adaptive Q-factor control function enabled by quasi-bound states in the continuum

Numerous device structures have been proposed for perfect absorption in monolayer graphene under single-sided illumination, all of which requires the critical coupling condition, i.e., the balance between the loss of graphene and the leakage rate of the device. However, due to the difficulty of the precise control of the quality of synthesized graphene and unwanted doping in graphene transferred to the substrate, the loss of graphene is rather unpredictable, so that the perfect absorption is quite difficult to achieve in practice. To solve this problem, we designed a novel perfect absorber structure with a loss adaptive leakage rate control function enabled by the quasi-bound states in the continuum (BIC) and numerically demonstrated its performance. Our designed device is based on a slab-waveguide grating supporting both the quasi-BIC and the guided-mode resonance (GMR); the quasi-BIC with an adjustable leakage rate controlled by an incident angle is responsible for absorption, while the GMR works as an internal mirror. Since the proposed device scheme can have an arbitrarily small leakage rate, it can be used to implement a perfect absorber for any kind of ultrathin absorbing media. Due to the simple structure avoiding an external reflector, the device is easy to fabricate.

Theory of nonlinear microwave absorption in an electron-doped and gapped graphene monolayer

Abstract We present a theory of nonlinear microwave absorption in monolayer graphene grown epitaxially on a SiC substrate and electron-doped by electrostatic gating. The intensity-dependent absorption coefficient of normally incident microwave radiation is calculated by using the Boltzmann equation techniques. The Boltzmann transport equation for mobile carriers in the conduction band of the graphene layer is solved to third order in the applied microwave field, including nonlinearities due to both the nonparabolicity of the conduction band and the energy-dependent scattering rate of electrons by acoustic phonons and short-range lattice defects. The intraband absorption coefficient decreases with an increase in the intensity of incident radiation, thus suggesting that electron-doped epitaxial graphene may be exploited as a saturable absorber operating at microwave frequencies. The saturation intensity is calculated as a function of excitation frequency and electron concentration and found to be a few MW/cm 2 over the millimeter wave frequency band considered (30–300 GHz) at a typical doping level of 1012 cm−2.

Highly Efficient Light Absorption of Monolayer Graphene by Quasi-Bound State in the Continuum.

Graphene is an ideal ultrathin material for various optoelectronic devices, but poor light–graphene interaction limits its further applications particularly in the visible (Vis) to near-infrared (NIR) region. Despite tremendous efforts to improve light absorption in graphene, achieving highly efficient light absorption of monolayer graphene within a comparatively simple architecture is still urgently needed. Here, we demonstrate the interesting attribute of bound state in the continuum (BIC) for highly efficient light absorption of graphene by using a simple Si-based photonic crystal slab (PCS) with a slit. Near-perfect absorption of monolayer graphene can be realized due to high confinement of light and near-field enhancement in the Si-based PCS, where BIC turns into quasi-BIC due to the symmetry-breaking of the structure. Theoretical analysis based on the coupled mode theory (CMT) is proposed to evaluate the absorption performances of monolayer graphene integrated with the symmetry-broken PCS, which indicates that high absorption of graphene is feasible at critical coupling based on the destructive interference of transmission light. Moreover, the absorption spectra of the monolayer graphene are stable to the variations of the structural parameters, and the angular tolerances of classical incidence can be effectively improved via full conical incidence. By using the full conical incidence, the angular bandwidths for the peak absorptivity and for the central wavelength of graphene absorption can be enhanced more than five times and 2.92 times, respectively. When the Si-based PCS with graphene is used in refractive index sensors, excellent sensing performances with sensitivity of 604 nm/RIU and figure of merit (FoM) of 151 can be achieved.

Strong Terahertz Absorption of Monolayer Graphene Embedded into a Microcavity.

Terahertz reflection behaviors of metallic-grating-dielectric-metal (MGDM) microcavity with a monolayer graphene embedded into the dielectric layer are theoretically investigated. A tunable wideband reflection dip at about the Fabry–Pérot resonant frequency of the structure is found. The reflectance at the dip frequency can be electrically tuned in the range of 96.5% and 8.8%. Because of the subwavelength distance between the metallic grating and the monolayer graphene, both of the evanescent grating slit waveguide modes and the evanescent Rayleigh modes play key roles in the strong absorption by the graphene layer. The dependence of reflection behaviors on the carrier scattering rate of graphene is analyzed. A prototype MGDM-graphene structure is fabricated to verify the theoretical analysis. Our investigations are helpful for the developments of electrically controlled terahertz modulators, switches, and reconfigurable antennas based on the MGDM-graphene structures.

Mid‐Wave Infrared Polarization‐Independent Graphene Photoconductor with Integrated Plasmonic Nanoantennas Operating at Room Temperature

AbstractGraphene photodetectors operating in the mid‐wave infrared (MWIR) face challenges that include the optical absorption of monolayer graphene being intrinsically low and the carrier lifetime in graphene being very short. Previous reports of graphene photoconductors in the MWIR have sought to overcome these challenges using approaches that include the integration of plasmonic nanoantennas and/or engineered electrodes. However, this has led to the photoresponse of these detectors being strongly polarization dependent. Here, a graphene photoconductor is reported that achieves polarization‐independent and fast response simultaneously via the integration of plasmonic nanoantennas that are termed Jerusalem‐cross antennas (JC‐antennas). Compared to previous works, the JC‐antennas concentrate the incident light onto graphene more efficiently with enhanced polarization‐independent optical absorption. MWIR detection is demonstrated at both room temperature and cryogenic temperatures, with measured responsivity of 14.5 V W−1 (room temperature) and 4400 V W−1 (78 K). Due to the carrier collection by the JC‐antennas and gapless band structure of graphene, the detector also shows significant and broadband photoresponse that extends to visible and near‐infrared wavelengths. The detector shows fast temporal response with a measured rise time of 3 ns, which would be more than sufficient for many practical applications (e.g., imaging).

Tunable plasmonics photodetector in near-infrared wavelengths using graphene chemical doping method

Graphene-based photodetectors have been caught in the spotlight of optoelectronics devices and they are significant candidates for tunable detectors. Unfortunately, little reports are presented for absorption enhancement in near-infrared range. Therefore, here, a graphene-based photodetector with plasmonic structure consisting of Silicon and SiO2 substrate, graphene layer, and metal nano-grating in near-infrared range is proposed. The optical absorption of graphene layer in this structure increases due to the occurrence of plasmonic effects and excitation of surface plasmon polaritons (SPPs) in metal and graphene interface. The optical response of the proposed structure is numerically simulated using the finite-difference time-domain (FDTD) method. To optimize the optical absorption of the structure, the effects of geometrical parameters, the chemical potential of graphene, the number of graphene layers, and the incident angle have been investigated. Based on numerical results, the absorption of monolayer graphene enhanced from 0.023 to nearly 0.70, even to a maximum value of 0.80 in three-layer graphene and the total absorption of optimized structure reached to about 0.98 at telecommunication wavelength (1.55 μm). Moreover, the absorption spectrum of the proposed structure can be tuned by either changing the geometrical parameters of nano-grating or chemical potential of graphene.

Broadband, wide-angle, and polarization-insensitive enhancement of light absorption in monolayer graphene over whole visible spectrum

By precise control of the resonance positions of four magnetic dipole modes in metamaterials, we numerically exhibit a broadband light absorption enhancement of monolayer graphene in the whole visible region. In the wavelength range from 450 to 800 nm, the minimum and maximum absorption efficiencies of monolayer graphene exceed 40% and 70%, respectively, and the averaged absorption efficiency can reach as high as about 65%, which suggests great potentials in various optoelectronic devices. Thanks to the localization nature of magnetic dipole modes, the light absorption in monolayer graphene under oblique incidence is, on the whole, less dependent on the incidence angle and the polarization direction of light. Besides ultra-broad bandwidth, other good performances of the light absorption enhancement of monolayer graphene, such as wide incidence-angle and polarization insensitivity, are also desired for graphene-based optoelectronic devices in some cases.

The ultraviolet absorption of graphene in the Tamm state

Ultraviolet (UV) light trapping by monolayer graphene is of great significance in optoelectronic devices. In this work, to enhance the UV absorption of monolayer graphene, a metallic thin film-type/graphene/Bragg mirror optical structure is developed using transfer matrix methods. Our results showed that the absolute UV absorption of monolayer graphene can reach approximately 83 % at the wavelength of 270 nm, which is a 9.2-fold enhancement compared to the intrinsic value of free-standing graphene (9%). This high absorption is due to the strong field confinement of the Tamm state in the silica spacer layer. The UV absorption of graphene can be tuned by adjusting the nanostructure parameters and incident angles. Furthermore, the proposed structure is less affected by machining errors. Considering the influence of processing precision, the light absorption of graphene in this structure is higher than that in the microcavity of a photonic crystal. The results obtained in this study may find promising applications in high-performance UV–graphene optoelectronic devices, such as modulators and photodetectors, as well as in photocatalysis.

Enhanced absorption of monolayer graphene using a metal–dielectric elliptical cavity array

In this paper, a strong coupling system consisting of a metal–dielectric elliptical cavity array and graphene nanoribbons is demonstrated, which can effectively enhance the light absorption of monolayer graphene in the mid-infrared spectral region. The elliptical cavity excites a strong localized field, which facilitates strong coupling between a cavity mode and a graphene surface plasmon (GSP) mode, leading to ultrabroad Rabi splitting. The classical LC circuit model is used to reveal the resonance relationship of the cavity mode. The coupled harmonic oscillator model is used to confirm the strong coupling phenomenon. The numerical simulation results demonstrate that fluctuations can be effectively tuned by adjusting the filling factor of the graphene nanoribbons. In addition, the broadband absorption of monolayer graphene can be further enhanced by changing the mobility of the graphene. A bandwidth of 6 μm with an average broadband absorption rate of 78.8% is obtained for the monolayer graphene. Such a substantial enhancement in the broadband absorption of graphene can lead to high-performance graphene optoelectronic components in the mid-infrared spectral region.