Polarization-sensitive triple plasmon-induced transparency with synchronous and asynchronous switching based on monolayer graphene metamaterials

Actively tunable plasmon-induced transparency in terahertz based on Dirac semimetal metamaterials

We numerically investigate a tunable plasmon-induced transparency based on bulk Dirac semimetal (BDS) metamaterial in the terahertz band. In the unit cell, the prominent transparent peak appears to be due to the interference between the cut wires (CWs) and split-ring resonators (SRRs). An active modulation via near-field coupling is obtained by varying the Fermi level of the BDS. Introducing photoactive silicon, it will be found that once the intensity of the pump light is adjusted, a tunable transparent peak will appear. Furthermore, by shifting the coupling distance between CWs and SRRs, the depth of the transparent peak will change accordingly. Finally, we place the structure in environments with different refractive indices, which will exhibit excellent sensitivity and facilitate the application of biochemical sensors. This simple and easy-to-fabricate metamaterial structure will have excellent potential applications in modulation, filters, and detection.

Quadruple Plasmon-Induced Transparency and Dynamic Tuning Based on Bilayer Graphene Terahertz Metamaterial

This study proposes a terahertz metamaterial structure composed of a silicon-graphene-silicon sandwich, aiming to achieve quadruple plasmon-induced transparency (PIT). This phenomenon arises from the interaction coupling of bright-dark modes within the structure. The results obtained from the coupled mode theory (CMT) calculations align with the simulations ones using the finite difference time domain (FDTD) method. Based on the electric field distributions at the resonant frequencies of the five bright modes, it is found that the energy localizations of the original five bright modes undergo diffusion and transfer under the influence of the dark mode. Additionally, the impact of the Fermi level of graphene on the transmission spectrum is discussed. The results reveal that the modulation depths (MDs) of 94.0%, 92.48%, 93.54%, 96.54%, 97.51%, 92.86%, 94.82%, and 88.20%, with corresponding insertion losses (ILs) of 0.52 dB, 0.98 dB, 1.37 dB, 0.70 dB, 0.43 dB, 0.63 dB, 0.16 dB, and 0.17 dB at the specific frequencies, are obtained, achieving multiple switching effects. This model holds significant potential for applications in versatile modulators and optical switches in the terahertz range.

Multifrequency on-off modulation and slow light characterization of the patterned black phosphorus metamaterial based on dual plasmon-induced transparency

We present a monolayer patterned black phosphorus (BP) metamaterial for generating a tunable dual plasmon-induced transparency (PIT). We have derived the expression for the theoretical transmittance by introducing the coupled mode theory (CMT), and the calculated results of the expression highly overlap with the simulation results. The quarterly frequency synchronous switch with two different operating bands is designed by the carrier density and scattering rate on the dual PIT modulation effect. Two parameters were selected as important markers to show the performance of the optical switch: the modulation depth (MD) and the insertion loss (IL). The theoretical analysis of this structure shows that the higher modulation depth (5.45d B Collapse

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Effect of symmetry breaking on multi-plasmon-induced transparency based on single-layer graphene metamaterials with strips and rings

A single-layer graphene metamaterial consisting of a horizontal graphene strip, four vertical graphene strips, and two graphene rings is proposed to realize tunable multi-plasma-induced transparency (MPIT) by the coupled mode theory and the finite-difference time-domain method. A switch with three modulation modes is realized by dynamically adjusting the Fermi level of graphene. Moreover, the effect of symmetry breaking on MPIT is investigated by controlling the geometric parameters of graphene metamaterials. Triple-PIT, dual-PIT, single-PIT can be transformed into each other. The proposed structure and results provide guidance for applications such as designing photoelectric switches and modulators.

Polarization-sensitive switchable display through critical coupling between graphene and a quasi-BIC

Enhanced light-matter interaction of a local field is of prime importance in optics as it can improve the performance of nanophotonic devices. Such enhancement can be achieved by utilizing the optical bound states in the continuum (BICs). In this study, a dielectric metasurface is proposed that could enhance the light-matter interactions in graphene. A symmetry-protected BIC was observed in such a metasurface, which could transform into a quasi-BIC with a high quality (Q-) factor when the in-plane symmetry is broken. As the graphene monolayer was introduced into the system, its absorption was enhanced by the quasi-BIC resonance. By optimizing the graphene Fermi energy and the asymmetry parameter of the metasurface to satisfy the critical-coupling condition, a tunable absorber could be achieved. The absorbing intensity could be efficiently modulated by varying the polarization direction of the incident light, the maximum difference of which was up to 95.4%. Also, further investigation showed that such a feature indicates potential application in digital switches and image displays, which could be switched by incident polarization only, and therefore without dependence on an additional structural change.

Double Narrowband Induced Perfect Absorption Photonic Sensor Based on Graphene-Dielectric-Gold Hybrid Metamaterial

Double narrowband induced perfect absorption in the terahertz region is achieved in a graphene-dielectric-gold hybrid metamaterial, whose physical mechanism is analyzed using the coupled-mode theory (CMT), which agreed well with the finite-difference time-domain (FDTD) simulation. This study found that the Fermi level of graphene can be adjusted to improve the absorptivity when the refractive index (RI) n_d of the chosen dielectric cannot achieve a good absorption effect. In addition, the blue shift of absorption spectrum can be used in the design of dual-frequency electro-optical switches, of which the modulation degree of amplitude (MDA) can reach as high as 94.05% and 93.41%, indicating that this is a very promising electro-optical switch. Most significantly, the RI sensing performance is investigated, which shows an ultra-high absorption sensitivity S_A = 4.4°/RIU, wavelength sensitivity S_λ = 9.8°/RIU, and phase shift sensitivity S_φ = 2691°/RIU. At last, an interesting finding is that the two peaks (R1 and R2) of plasmon-induced absorption (PIA) show different polarization characteristics (insensitive or sensitive) to the incident light angle; this polarization-sensitive is particularly important for the PIT/PIA-based optical polarizers. Undoubtedly, this paper is of great significance to the research and design of terahertz photonic devices and sensors.

Dynamically tunable bound states in the continuum supported by asymmetric Fabry-Pérot resonance

The dynamic regulation of quasi-bound states in the continuum (quasi-BIC) is a research hotspot, such as incident angle, polarization angle, temperature, a medium refractive index, and medium position regulation. In this paper, a dual-band ultra-high absorber composed of upper asymmetric graphene strips and lower graphene nanoribbons can generate a symmetry-protected quasi-BIC and Fabry-Pérot resonance (FPR) mode. The band structure further demonstrates the symmetry-protected BIC. Research shows that the absorption system can withstand a relatively wide range of incidence and polarization angles. Interestingly, the quasi-BIC and FPR modes can be modulated by the Fermi levels of the graphene1 and graphene2, respectively, realizing a multifunctional switch with high modulation depth (MD > 94%), low insertion loss (IL < 0.23 dB), and large dephasing time (DT > 4.35 ps). This work provides a new approach for the dynamic regulation of quasi-BIC and stimulates the development of multifunctional switches in the absorber.

Dynamically Tunable and Multifunctional Polarization Beam Splitters Based on Graphene Metasurfaces

Based on coupled-mode theory (CMT) and the finite-difference time-domain (FDTD) approach, we propose a graphene metasurface-based and multifunctional polarization beam splitter that is dynamically tunable. The structure, comprising two graphene strips at the top and bottom and four triangular graphene blocks in the center layer, can achieve triple plasma-induced transparency (PIT). In a single polarization state, the computational results reveal that synchronous or asynchronous six-mode electro-optical switching modulation may be performed by modifying the Fermi levels of graphene, with a maximum modulation degree of amplitude (MDA) of 97.6% at 5.148 THz. In addition, by varying the polarization angle, a polarization-sensitive, tunable polarization beam splitter (PBS) with an extinction ratio and insertion loss of 19.6 dB and 0.35 dB at 6.143 THz, respectively, and a frequency modulation degree of 25.2% was realized. Combining PIT with polarization sensitivity provides a viable platform and concept for developing graphene metasurface-based multifunctional and tunable polarization devices.

Investigation of bound states in the continuum in dual-band perfect absorbers

Enhancing the light-matter interaction of two-dimensional materials in the visible and near-infrared regions is highly required in optical devices. In this paper, the optical bound states in the continuum (BICs) that can enhance the interaction between light and matter are observed in the grating-graphene-Bragg mirror structure. The system can generate a dual-band perfect absorption spectrum contributed by guided-mode resonance (GMR) and Tamm plasmon polarition (TPP) modes. The optical switch can also be obtained by switching the TE-TM wave. The dual-band absorption response is analyzed by numerical simulation and coupled-mode theory (CMT), with the dates of each approach displaying consistency. Research shows that the GMR mode can be turned into the Fabry-Pérot BICs through the transverse resonance principle (TRP). The band structures and field distributions of the proposed loss system can further explain the BIC mechanism. Both static (grating pitch P) and dynamic parameters (incident angle θ) can be modulated to generate the Fabry-Pérot BICs. Moreover, we explained the reason why the strong coupling between the GMR and TPPs modes does not produce the Friedrich-Wintgen BIC. Taken together, the proposed structure can not only be applied to dual-band perfect absorbers and optical switches but also provides guidance for the realization of Fabry-Pérot BICs in lossy systems.

Terahertz multimode modulator based on tunable triple-plasmon-induced transparency in monolayer graphene metamaterials

A simple monolayer graphene metamaterial based on silicon/silica substrates is proposed, and typical triple-plasmon-induced transparency (PIT) is realized in the terahertz band. The physical mechanism is analyzed by coupled mode theory (CMT), and the results of CMT agree well with the finite-difference time-domain simulation. A multimode electro-optical switch can be designed by dynamic tuning, and the modulation degrees of its resonant frequencies are 84.0%, 87.3%, 83.0%, 88.1%, and 76.7%. In addition, triple-PIT gradually degenerates into dual-PIT with a decrease in the length of one bright mode. Interestingly, the group index can reach 770 at Ef=0.8eV, which shows that it can be designed as a slow light device with extraordinary ability. Therefore, the results of this paper are of great significance to the research and design of electro-optical switches and slow light devices in the terahertz band.

Realizing PIT-like transparency via the coupling of plasmonic dipole and ENZ modes

Plasmon induced transparency (PIT), known as the coupling of plasmon modes in metamaterials, has attracted intensive research interests in photonic applications. In this work, a PIT-like transparency is realized via the strong coupling of plasmonic dipole and epsilon-near-zero (ENZ) mode. Two types of metasurfaces, namely the gold nanoantenna and dolmen-like metasurface, are designed with an integrated ENZ material aluminum doped zinc oxide (AZO) film. Simulations with the finite element method (FEM) demonstrate that single and double transparent windows are achieved respectively. The adjustments of the peak position and transmittance of transparent windows via the structure parameters and the AZO film thickness are further investigated. This work provides an alternative coupling scheme of realizing PIT-like transparency with simple metasurface design, and offers great potential for future metamaterial applications.

Triple plasmon-induced transparency and dynamically tunable electro-optics switch based on a multilayer patterned graphene metamaterial

A terahertz-band metamaterial composed of multilayer patterned graphene is proposed and triple plasmon-induced transparency is excited by coupling three bright modes with one dark mode. The Lorentz curve calculated by the coupled-mode theory agrees well with the finite-difference time-domain results. Dynamic tuning is investigated by changing the Fermi level. Multimode electro-optics switching can be designed and achieved, and the amplitude modulations of four resonance frequencies are 94.3%, 92.8%, 90.7%, and 93%, respectively, which can realize the design of synchronous and asynchronous electro-optics switches. It is hoped that these results can provide theoretical support and guidance for the future design and application of photonic and optoelectronic devices.

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

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

Quadruple plasmon-induced transparency of polarization desensitization caused by the Boltzmann function

This study proposes a graphene metamaterial desensitized to the polarized angle to produce tunable quadruple plasmon-induced transparency (PIT). As a tool employed to explain the PIT, n-order coupled mode theory (CMT) is deduced for the first time and closely agrees with finite-difference time-domain (FDTD) simulations according to the quadruple PIT results in the case of n = 5. Additionally, the response of the proposed structure to the angle of polarized light is investigated. As a result, the Boltzmann function satisfied by the response of graphene strips to the polarization direction of incident light is proposed for the first time. Its property of polarization desensitization can be attributed to structural centrosymmetry, and conjugated variety which the Boltzmann functions result in. Therefore, a quintuple-mode modulation based on simultaneous electro-optical switch is realized by tuning Fermi levels within graphene. Its modulation degrees of amplitude and dephasing times are obtained. Given that the slow-light property is an important application of PIT, the n-order group index is thereby obtained. Hence, not only do the insights gained into polarization-desensitization structure provide new ideas for the design of novel optoelectronic devices, but also the results from the n-order CMT offer new research progress and references in theory.

Three-band perfect absorber with high refractive index sensing based on an active tunable Dirac semimetal

In this paper, we designed a three-band narrowband perfect absorber based on bulk Dirac semi-metallic (BDS) metamaterials. The absorber consists of a hollow Dirac semi-metallic layer above, a gold layer below and a photonic crystal slab (PCS) in the middle. The study found that the terahertz wave absorber achieved three perfect absorption rates of more than 95% in the range of 1 to 2.4 THz. The minimum bandwidth (FWHM) is 0.02 THz, and the maximum quality factor (Q) is 106. A reasonable explanation of high absorption can be obtained by impedance matching, electric dipole and other principles. The absorption spectra of the two polarizations show different responses at different incident angles. In addition, we also obtained the influence of the structural parameters of the upper layer of the metamaterial on the absorption performance. We defined the refractive index sensitivity (S) with a maximum sensitivity of 0.1525 THz RIU^-1 and a highest quality factor (FOM) of 4.26 in the refractive index range of 1 to 1.8. The maximum adjustable range is 0.06 THz in the Fermi energy range of 60 to 140 meV. Because of its excellent characteristics, our absorber will have good development prospects in the fields of optical switching, biochemical imaging, and space detection.

A Tunable “Ancient Coin”-Type Perfect Absorber with High Refractive Index Sensitivity and Good Angular Polarization Tolerance

In this paper, we design and present a graphene-based “ancient coin”-type dual-band perfect metamaterial absorber, which is composed of a silver layer, silicon dioxide layer, and a top “ancient coin” graphene layer. The absorption performance of the absorber is affected by the hollowed-out square in the center of the graphene layer and geometric parameters of the remaining nano disk. The optical properties of graphene can be changed by adjusting the voltage, to control the absorption performance of the absorber dynamically. In addition, the centrally symmetric pattern structure greatly eliminates the polarization angle dependence of our proposed absorber, and it exhibits good angular polarization tolerance. Furthermore, the proposed “ancient coin”-type absorber shows great application potential as a sensor or detector in biopharmaceutical, optical imaging, and other fields due to its strong tunability and high refractive index sensitivity.

Based on Ultrathin PEDOT:PSS/c-Ge Solar Cells Design and Their Photoelectric Performance

In recent years, nanostructures have improved the performance of solar cells and are regarded as the most promising microstructures. The optical properties of PEDOT:PSS/c-Ge hybrid solar cells (HSCs) based on the octagon germanium nanoparticles (O-GNPs) were numerically analyzed using the finite-difference time-domain (FDTD) method. The optimal structure of the hybrid solar cell is determined by changing the thickness of the organic layer and structural parameters of nanoparticles to enhance the optical absorption and eventually achieve high broadband absorption. By changing the structure parameter of O-GNPs, we studied its effect on solar cells. The optimization of geometric parameters is based on maximum absorption. The light absorption of our optimized HSCs is basically above 90% between 200 and 1500 nm. PEDOT:PSS is placed on top of O-GNPs to transmit the holes better, allowing O-GNPs to capture a lot of photons, to increase absorbance value properties in the AM1.5 solar spectral irradiated region. The transmittance is increased by adding poly-methyl methacrylate (PMMA). At the same time, the electrical characteristics of Ge solar cells were simulated by DEVICE, and short-circuit current (Jsc), open-circuit voltage (Voc), maximum power (Pmax), filling coefficient (FF) and photoelectric conversion efficiency (PCE) were obtained. According to the optimization results after adjusting the structural parameters, the maximum short-circuit current is 44.32 mA/cm2; PCE is 7.84 mW/cm2; FF is 69%. The results show that the O-GNPs have a good light trapping effect, and the structure design has great potential for the absorption of HSCs; it is believed that the conversion efficiency will be further improved through further research.

Broadband plasmon-induced transparency modulator in the terahertz band based on multilayer graphene metamaterials

In this study, multilayer graphene metamaterials comprising graphene blocks and graphene ribbon are proposed to realize dynamic plasmon-induced transparence (PIT). By changing the position between the graphene blocks, PIT phenomenon will occur in different terahertz bands. Furthermore, PIT with a transparent window width of 1 THz has been realized. In addition, the PIT shows redshifts or blueshifts or disappears altogether upon changing the Fermi level of graphene, and hence a frequency selector from 3.91 to 7.84 THz and an electro-optical switch can be realized. Surprisingly, the group index of this structure can be increased to 469. Compared with the complex and fixed structure of previous studies, our proposed structure is simple and can be dynamically adjusted according to demands, which makes it a valuable platform for ideas to inspire the design of novel electro-optic devices.

Design of Narrow-Band Absorber Based on Symmetric Silicon Grating and Research on Its Sensing Performance

In this paper, using the surface plasmon and Fabry–Pérot (FP) cavity, the design of a symmetric silicon grating absorber is proposed. The time-domain finite difference method is used for simulation calculations. The basic unit structure is a dielectric grating composed of silicon dioxide, metal and silicon. Through the adjustment of geometric parameters, we have achieved the best of the symmetric silicon grating absorber. A narrowband absorption peak with an absorption rate greater than 99% is generated in the 3000–5000 nm optical band, and the wavelength of the absorption peak is λ = 3750 nm. The physical absorption mechanism is that silicon light generates surface plasmon waves under the interaction with incident light, and the electromagnetic field coupling of surface plasmon waves and light causes surface plasmon resonance, thereby exciting strong light response modulation. We also explore the influence of geometric parameters and polarization angle on the performance of silicon grating absorbers. Finally, we systematically study the refractive index sensitivity of these structures. These structures can be widely used in optical filtering, spectral sensing, gas detection and other fields.