Fermi Energy Of Graphene Research Articles

Centroid shifts of spatiotemporal vortex pulses reflected and refracted on graphene.

The Goos-Hänchen and Imbert-Fedorov shifts are significant wave phenomena, yet the underlying mechanism governing the spatiotemporal vortex pulses reflected and refracted on graphene remains opaque. In this study, we analytically derive the expressions for the centroid shifts of spatiotemporal vortex pulses by applying the Fresnel-Snell formulas to each plane wave in the incident spatiotemporal vortex pulse spectrum. We demonstrate that the longitudinal shifts are correlated with the angular shifts, and thus, both are subject to resonant enhancement in the vicinity of the Brewster angle. It is possible to tune the resonant enhancement of the shifts by modifying the Fermi energy of graphene. An increase in the vortex topological charge l results in an enhancement of both the angular and longitudinal shifts while the transverse shifts are reduced. The shifts of the intensity distribution, in accordance with the Goos-Hänchen and Imbert-Fedorov shifts, facilitate experimental measurements. The high frequency in the terahertz region will diminish the resonant enhancement of the spatial shifts of the reflected wavepackets. The analysis presented here can be extended with minimal effort to spatiotemporal vortex pulses reflected and refracted on other two-dimensional atomic crystals.

Total resonant terahertz absorption enabled by large-scale adjustable admittance inverse matching in a metal groove with graphene

Abstract We propose a tunable terahertz (THz) perfect absorber based on a metal groove with graphene-loaded dielectric resonator and theoretically study its basic properties. The proposed absorber allows switching between the regimes of perfect absorption at the Fabry-Perot resonance excited near the cut-off frequency of the metal groove and almost total reflection away from the resonance by changing the Fermi energy in graphene. For this purpose, we propose a “bottom up” approach which is based on tuning the admittance of input line (the metal groove in our case) instead of the structure admittance in order to reach the perfect admittance matching condition. We demonstrate that this effect can be realized at arbitrary selected frequency in the entire THz range due to the dispersion of incoming wave in metal groove, which ensures a large-scale tunability of its characteristic admittance. As a result, the total absorption can be realized in the Fabry-Perot resonance even in a simple graphene-loaded dielectric cavity for any admittance of graphene layer, which is advantageous as compared to majority of existing THz absorbers with more complicated design.

Vanadium dioxide and a graphene-based ultra-wideband absorption and polarization conversion switchable terahertz device

A multifunctional terahertz functional device based on vanadium dioxide (VO2) and graphene is proposed, which can realize ultra-wideband absorption and polarization conversion. When the VO2 is in the insulating state, the device can achieve the polarization conversion function to convert the incident wave into the corresponding cross-polarized wave. Polarization conversion ratios (PCRs) can exceed 90% in the 2.1–8 THz frequency range; in the 3–7.5 THz range, the PCR can be more than 95%. When VO2 is in the metallic state and the graphene Fermi energy is 0.9 eV, the device can realize a broadband absorption function. An absorption rate of more than 90% can be achieved over a wide frequency range of 3.3–7.7 THz. In addition, the polarization conversion device can maintain high performance in broadband polarization conversion at incident angles no greater than 40°. The absorber device also exhibits insensitivity to both incident and polarization angles. These advantages which make the proposed multifunctional terahertz functional device have a wide range of applications in the fields of terahertz imaging, sensing, communication, and so on.

Tunable electromagnetically induced transparency modulation using L-shaped complementary graphene metamaterials

In this study, we demonstrate the tunable modulation of the electromagnetically induced transparency (EIT) effect using L-shaped complementary graphene metamaterials. The structure consists of a horizontal line slot and a vertical line slot, representing the bright and dark modes, respectively. The synergistic interaction between the bright and dark modes generates a pronounced transparent window within the transmission spectrum. Owing to symmetry, the EIF effect can be realized in two perpendicular polarization directions. Compared to metallic metamaterials, graphene metamaterials are tunable by controlling the Fermi energy of graphene via the gate voltage rather than by redesigning the structure. The regulation of the Fermi energy level in the complementary graphene metamaterials presented herein is more straightforward than that in the discrete graphene configurations. By tuning the EIT transparent window, we enabled actively controlled sensing capabilities and the realization of slow light effects. This work illuminates potential applications in the development of environmental sensors, slow-light devices, and terahertz modulators.

Tunable graphene plasmonic metadevice for infrared polarization resolved spectroscopy detection

Polarization-resolved spectroscopy imaging holds significant promise in various fields. However, conventional polarization-resolved spectroscopy imaging systems often reply on separate dispersion elements, polarization splitting components, and mechanical scanning or rotating structures, resulting in bulky systems with intricate designs. The pursuit of lightweight and seamlessly integrated polarization spectroscopy imaging systems presents a critical challenge. Here, we propose an integrated polarization spectroscopy detector operating in the mid-infrared range based on a tunable graphene metasurface. Within the graphene metasurface, plasmonic resonances of graphene are excited by a gold nano-grating, allowing simultaneous modulation of spectral and polarization transmission characteristics for infrared light by tuning the Fermi energy of graphene. With the help of neural network based recovery algorithms, this system achieves pixel-level spectral detection (with a resolution below 200 nm), multi-color polarization Stokes vector analysis (with a monochromatic angular error of less than 0.1 rad), and concurrent polarization and spectroscopy detection. This research paves the way for the development of the next generation integrated infrared polarization resolved spectroscopy imaging systems.

Singular photon spin Hall effect in chiral-graphene heterostructures and its application in chirality detection.

In this paper, we consider the singular photonic spin Hall effect (PSHE) in a chiral-graphene-chiral (CGC) heterostructure in the THz band. We investigate the impact of a chiral medium on reflectance spectra and the modulation of the Fermi energy on the surface conductivity of graphene. Our study shows that placing a chiral medium on both sides of a monolayer of graphene results in an enhanced transverse shift (TS) compared to placing a non-chiral medium. Moreover, the direction of the TS of the PSHE can be altered by adjusting the sign of the chirality parameter and the Fermi energy of graphene. Finally, we establish a quantitative relationship between the PSHE and the chirality parameter and the Fermi energy of graphene. By dynamically modulating the PSHE in graphene, it is possible to flexibly detect chirality parameters. This work opens up new avenues for chiral molecular detection and graphene-PSHE dynamic modulation.

Enhanced four-band absorption in a tunable graphene subwavelength grating structure for photodetection

This paper investigates a sub-wavelength multilayer dielectric grating structure based on a simple metasurface, achieving dynamically switchable four-band absorption enhancement. This enhancement spans the visible to near-infrared (NIR) range, with absorption exceeding 90%, representing nearly a 40-fold increase over the intrinsic absorption of a single graphene layer. The contributions of guided-mode resonance (GMR), optical Tamm state (OTS), and photonic crystal defect cavity resonance to the absorption spectra are analyzed separately, offering a method for independent multi-channel signal extraction. Additionally, we explore the impact of structural parameters on the absorption response and examine the broad-spectrum multiband detection mechanism. This mechanism, explained through the integration of resonant cavity perturbation theory and the dynamic tuning of absorption efficiency via graphene's Fermi energy levels, results in an independently tunable optical system for four key operating bands. These properties make the structure suitable for applications such as multiband biomedical photodetectors or components in optoelectronic devices with multiple operating bands.

Multidimensional dynamic control of optical skyrmions in graphene–chiral–graphene multilayers

Abstract Optical skyrmions are topological quasiparticles with a complex vectorial field structure. Their associated characteristics of ultra-small, ultra-fast and topological protection have great application prospects in high density data storage, light matter interaction and optical communication. At present, the research of optical skyrmions is still in its infancy, where the construction and flexible regulation of different topological textures are current research hotspot. Here, we combine the twist degree of freedom of materials and optical skyrmions. Based on graphene–chiral–graphene multilayers structure, we demonstrate the field mode symmetry and hybridization to form Bloch-type graphene plasmons skyrmion lattice. At the same time, by changing chirality parameter, the Fermi energy of graphene and the phase of incident light, multidimensional control of Bloch-type optical skyrmions can be realized. Our work demonstrated that the properties of materials provide the additional dimensions to regulate the topological states, and the combination of different materials structures provides the possibility for dynamic construction and manipulation of multiple topological states, which is expected to find applications in integrated nanophotonics devices.

Fano-resonant mechanism of terajet formation using graphene-covered high-index mesoscale spheres.

Photonic jet in terahertz (THz) frequency range (terajet) plays an important role in modern THz scanning systems to achieve a superresolution beyond the diffraction limit. Based on analytical simulations, we introduce a synergetic effect of a mesoscale dielectric sphere and graphene to improve the focusing properties of a particle. We show that a graphene-covered dielectric sphere is able to enhance the field behind it if the refractive index is high. This conflicts with a generally accepted statement that a jet is generated only for low-index dielectrics with n < 2. We demonstrate the tunability of the terajet characteristics with respect to the graphene Fermi energy and discover a Fano resonance causing the field increase. This design leverages the tuning properties of the graphene allowing dynamic control over the power and size of the generated terajet in real time. With high-index materials, we get the opportunity for integration of terajet-assisted imaging with semiconductor technology.

Graphene-functionalized photonic crystal fibers for detecting the refractive index of a wide range of analyzes

This study looks into a new photonic crystal fiber (PCF) sensor covered in graphene that can detect changes in the analyte's refractive index (RI) in the COMSOL environment. The sensor operates by detecting a peak in light loss that varies in wavelength based on the RI of the analyte. Higher RI analytes shift the peak towards longer wavelengths. It is explored how many factors affect the sensor's performance, such as graphene's Fermi energy, the site inside the fiber, and the thickness of the gold layer. The location of graphene and its Fermi energy have a substantial influence on how the sensor responds. Interestingly, the resonant wavelength has a linear relationship with the RI of the analyte, which allows for precise RI calculation. Increasing gold thickness improves light-material interaction, but subsequent thickening has a detrimental impact, lowering sensor sensitivity. The appropriate thickness varies according to the material under consideration. Finally, the analysis determines the optimal arrangement of graphene for maximum sensitivity by completely covering the analyte holes in the PCF. By finding the right balance between these factors, the suggested sensor can have both high spectral sensitivity and low sensor resolution.

Narrow-band absorption enhancement and modulation of single layer graphene by surface plasmon polaritons in near-infrared region

We theoretically study the very sharp absorption enhancement of the single layer graphene by the surface plasmon polaritons in near-infrared region. We place the single layer graphene on the silver substrate surface with a periodic slit array. Such a structure design is helpful to reduce the difficulty in experimental fabrication. By changing the slit period, we can largely tune the absorption peak of the single layer graphene in the wavelength range from 1000 nm to 2000 nm, covering the well-known communication wavelength of 1550 nm. The absorption efficiency of the single layer graphene varies between about 20 % and 80 %, and the absorption bandwidth is reduced from about 10 nm to 1 nm. Moreover, by changing the Fermi energy of the single layer graphene, we can also completely modulate the absorption peak from a maximum to almost zero and thus realize a nearly 100 % modulation depth. This work has a potential application in electro-optic modulators.

Dynamic tunable and switchable broadband near-infrared absorption modulator based on graphene-hybrid metasurface

The integration of graphene with metasurfaces enables devices with remarkable dynamic tunability, propelling electromagnetic (EM) manipulations to new heights by transitioning from static to dynamic control. In this study, we theoretically investigate a broadband absorption modulator with dynamic tunability based on a graphene hybrid metasurface. The metasurface consists of a monolayer graphene sheet sandwiched between a square silver block and a silica layer. The excitation of the magnetic toroidal dipole (MTD) leads to a significant enhancement of graphene’s electromagnetic absorption. By arranging four blocks as a supercell to support multi-resonance, we achieve a broadband modulator spanning from 1000 to 1210 nm, with graphene absorption exceeding 57 %. Notably, there is no plasmonic hybridization among adjacent components within the super unit. By tuning the Fermi energy of graphene, narrow-band tunability can be achieved at any wavelength within the operation spectrum. Furthermore, the designed device exhibits a perfect modulation depth (∼100 %). We demonstrate the switchability of the proposed device by showcasing its ON/OFF status at representative wavelengths of 1205 nm, 1119 nm, and 1052 nm. Thus, the proposed graphene-based hybrid metasurface fulfills the requirements for broadband tunability and switchability, offering a high ON/OFF ratio, full modulation depth, and a small switch voltage gap. This design holds significant potential for future developments.

Tunable plasmon-induced transparency in anisotropic graphene-black phosphorus photonic device for high-performance sensors and switchers

A novel photonic device composed of graphene and black phosphorus (G-BP) has been proposed, which achieves high-performance plasmon-induced transparency (PIT) effect within the THz range and maintains substantial tunability and anisotropy. The anisotropy of the PIT effect arises from the near-field coupling between two bright modes characterized by distinct effective electron masses of BP, resulting transparency window at 33.35 THz for TE polarization and at 26.92 THz for TM polarization. Through the modulation of Fermi energy in graphene, doping levels of BP and geometric parameters separately, a tunable transparency window is achieved. Notably, the convergence or divergence of the anisotropic transparency windows can be well manipulated with different BP doping levels. Furthermore, the proposed G-BP photonic device exhibits a high sensitivity to changes in the surrounding refractive index and substrates, with a maximum sensitivity of 12.04 THz/RI, rendering it suitable for sensor applications. Overall, the proposed photonic device exhibits notable PIT effects characterized by high anisotropic performance, substantial tunability, great sensitivity, and stability, making it a promising candidate for applications in sensors, polarizers, and switchers.

Active tunable terahertz multifunctional device switching from multiple narrowband perfect absorption to plasma-induced transparency

This paper puts forward a terahertz multifunctional device based on reversible phase transition property of VO2, which can realize active switching from multiple narrowband perfect absorption to plasma-induced transparency (PIT). By means of impedance matching theory and electric field distributions, we analyze the mechanism behind three absorption formants, and the absorption spectrum is consistent with data obtained from coupled mode theory. Three perfect absorption formants at the frequencies of 4.37 THz, 5.27 THz and 6.1 THz are extremely sensitive to the changes in external environment, which possesses a higher sensitivity up to 1.29 THz/RIU and FOM up to 9.45, indicating prominent sensing performance. By regulating the Fermi energy in a unified manner, three absorption formants can be observed within a specific frequency range. When the Fermi energy of three graphene patterns is individually manipulated, trimodal narrowband perfect absorption will transform into one bimodal and two unimodal ones. While the device is under PIT operation mode, two transmission windows are formed at 5.1 THz and 5.92 THz. It can be concluded that dual PIT arises from the mutual coupling between two bright modes and a dark mode formed by patterned graphene, when combined with the transmission spectra and the corresponding electric field distributions. Similarly, the modulation of graphene Fermi energy can realize dynamic tuning of the transmission performance, and single PIT can be achieved when graphene patterns are regulated separately. In summary, the proposed device can meet various functional requirements for multiple perfect absorption and PIT under different conditions and promote developments of terahertz technology in fields of environmental detection, electromagnetic modulation, multi-function devices and other application.

Tunable nonreciprocal metasurfaces based on a nonlinear quasi-bound state in the continuum.

Nonreciprocal devices are essential and crucial in optics for source protection and signal separation. A hybrid grating system consisting of a silicon grating, a graphene layer, and a silicon waveguide layer is employed to create a high-Q quasi-BIC (bound state in the continuum). Then, the high-Q properties of the quasi-BIC are harnessed to enhance the third-order nonlinear effect of silicon, thereby improving the nonreciprocal characteristics of the device. The nonreciprocal transmittance ratio of the device can be tunable by adjusting the graphene Fermi energy level, achieving tunability ranging from 0.0865 to 30.57 dB. It also enables the best performance of the device over a wider range of frequency bands. This study provides a new, to the best of our knowledge, method for designing tunable nonreciprocal devices with a wide range of potential applications.

Tunable terahertz polarization conversion and absorption devices based on metamaterials

Metamaterials, as novel materials, have electromagnetic behaviors that can be realized by rationally designing the unit structure. It plays an important role in the research of terahertz functional devices. Switchable terahertz devices are constructed by flexibly combining the excellent properties of graphene and vanadium dioxide (VO2) with the application potential of terahertz technology. When the VO2 is in the metallic state and the graphene Fermi energy level is 0 eV, the device behaves as a broadband polarization switching device. It can realize cross-polarization conversion for the incident electromagnetic wave in the range of 2–4.2 THz. When the VO2 is in the insulating state and the graphene Fermi energy level is 0.9 eV, the device is an absorption device. The absorption rate can reach more than 99% at 4 THz. In addition, the polarization conversion ratio (PCR) as well as the absorption rate of the device can be tuned independently. This actively tunable switchable terahertz device provides ideas for dynamically controlling terahertz, bringing more possibilities for various applications such as terahertz imaging, stealth, and communication.

Multi-functional and actively tunable terahertz metamaterial absorber based on graphene and vanadium dioxide composite structure

In this paper, a fabrication friendly terahertz (THz) metamaterial hybrid absorber comprised of square-shaped graphene and U-shaped vanadium dioxide is proposed and numerically analyzed. This device can be tuned dynamically to function as a wide-broadband, reduced-broadband, multi-band, or perfect single-band absorber. Switchable qualities are achieved through simultaneous tuning of the electrical properties of graphene and the phase transition properties of vanadium dioxide. The theory behind the absorption is explored using the distribution of the electric field and the theory of impedance matching. It is clear from the simulation results that the suggested structure can perform as a wide-broadband absorber for the transverse electric (TE) wave during the insulating phase of vanadium dioxide. In this case, graphene’s Fermi energy is configured to 0.7 eV, and it has a 0.1 ps relaxation time. Moreover, only changing the relaxation time to 0.5 ps enables the identical structure to perform as a multi-band absorber. On the contrary, the designed structure exhibits a reduced broadband absorption for the TE wave when the vanadium dioxide is thermally tuned to a metallic state and graphene’s Fermi energy is configured to 0 eV with a 0.1 ps relaxation time. Moreover, by inducing the metallic state in vanadium dioxide and adjusting the Fermi energy of graphene to 0.3 eV with a 0.1 ps relaxation time, the structure performs as a single-band perfect absorber. The main advantage of this work is that the proposed absorber device can be operated as a single-band, multi-band, or broadband absorber with two different bandwidths by applying external voltage and changing temperature. Additionally, for both the TE and transverse magnetic (TM) waves, the structure retains a high level of absorption until 40° of incident angle. Finally, due to its multi-functional property, simple geometric structure, and actively adjustable function, the presented structure has possible real-life applications in the terahertz frequency range.

Full-space energy-controlled multi-wavefront generators enabled by hybrid graphene-VO2 terahertz metasurfaces

In terahertz communication, multi-wavefront reconfigurable metasurfaces allowing for energy control are highly desirable for multiple channels and high capacity. However, existing metasurfaces can only produce limited wavefronts in either transmission or reflection space, severely hindering their practical application ranges. Herein, a dual-controlled reconfigurable metasurface integrated with graphene and VO2 is demonstrated for an energy-controlled multi-wavefront generator in full-space. By altering the geometry sizes and rotation angles of the meta-atoms as well as external excitations, the polarization independent phase responses are achieved to actively generate energy-controlled multiple wavefronts in full-space. As a proof-of-concept demonstration, three types of full-space metasurfaces are constructed to respectively realize various meta-deflectors, metalens/meta-deflector/orbital angular momentum (OAM) generator, and multimode OAM generator in full-space by utilizing the spin-decoupling method. Moreover, the intensities of these wavefronts can be actively modulated by adjusting the Fermi energy of graphene. Therefore, such diversified metasurface will possess a promising application prospect in terahertz multifunctional devices, and communication and imaging systems.

Surface Plasmon Waveguide Based on Nested Dielectric Parallel Nanowire Pairs Coated with Graphene

A kind of surface plasmon waveguide composed of two nested cylindrical dielectric parallel nanowire pairs coated with graphene was designed and studied. The dependence of the mode characteristics and the normalized gradient force of the lowest two modes supported by the waveguide on the parameters involved were analyzed by using the multipole method. To ensure rigor, the finite element method was employed to verify the accuracy of the multipole method, thus confirming its results. The results show that the multipole method is a powerful tool for handling this type of waveguide. The real part of the effective refractive index, the propagation length, the figure of merit, and the normalized gradient force can be significantly affected by the operating wavelength, the Fermi energy of graphene, the waveguide geometric parameters, and the refractive index of the inner dielectric nanowire. Due to the employment of nested dielectric nanowire pairs coated with graphene, this waveguide structure exhibits significant gradient force that surpasses 100 nN·μm−1·mW−1. The observed phenomena can be attributed to the interaction of the field with graphene. This waveguide holds promising potential for applications in micro/nano integration, optical tweezers, and sensing technologies.

Tunable ultra-broadband terahertz metamaterial absorbers based on complementary split ring-shaped graphene

In the design of graphene-based tunable broadband terahertz (THz) metamaterial absorbers (MAs), simplifying the gating structure to control the Fermi energy of graphene is a critical requirement for practical applications. To solve this problem, we numerically demonstrate two kinds of tunable ultra-broadband THz MAs based on complementary split ring-shaped graphene. The first absorber exhibits an ultra-broadband absorption performance with an absorptance of above 90% in the frequency range of 2.06–4.24 THz, with a relative absorption bandwidth of 69.2%. By changing the Fermi energy of graphene from 0 to 0.8 eV through bias voltage, the absorptance can be tuned from 32.8% to 99.9%. The ultra-broadband absorption mechanism is based on the surface plasmon polariton resonances caused by the surface charges of complementary split ring-shaped graphene. In addition, to further expand the absorption bandwidth, we cover another dielectric layer on the first absorber to make the second absorber have an increased relative absorption bandwidth of 108.27%.