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Showing papers on "Coupled mode theory published in 2021"


Journal ArticleDOI
TL;DR: In this paper, a multi-band ideal absorber of monolayer graphene was designed and analyzed through impedance matching and coupled mode theory, and a new method based on critical coupling and guided resonance was proposed theoretically and numerically.

99 citations


Journal ArticleDOI
TL;DR: In this article, a simple graphene metasurface was proposed to achieve obvious graphene plasmon-induced transparency (PIT) phenomenon, which can find that PIT, reflectivity and absorbance can be effectively tuned by the Fermi level.
Abstract: Ultra-high sensitivity sensor has significant application for micro-nano optical devices in terahertz. Here, we propose a simple graphene metasurface, which can achieve obvious graphene plasmon-induced transparency (PIT) phenomenon. We can find that PIT, reflectivity, and absorbance can be effectively tuned by the Fermi level. Moreover, the finite-different time-domain (FDTD) numerical results are well agreement with the coupled mode theory (CMT) results. Interestingly, an ultra-high sensitivity sensor performance based on tunable PIT in terahertz bands can be realized in our proposed metasurface, the sensitivity and Figure of merit (FOM) can reach up to 1.7745 THz/RIU and 23.61, respectively. Hence, these results can provide theoretical guidance for terahertz dynamic integrated photonic devices.

75 citations



Journal ArticleDOI
TL;DR: In this paper, a metal-insulator-metal plasmonic sensor with one rectangular and two square nanorod array resonators that shows a Fano resonance is analyzed and suggested.
Abstract: In this paper, a metal–insulator–metal plasmonic sensor with one rectangular and two square nanorod array resonators that shows a Fano resonance is analyzed and suggested. The finite difference time domain method is used to investigate the output spectra and sensing characteristics. The transmission spectra show a sharp and asymmetric shape, because of the narrow-band spectrum and broad-band one affected by two square resonators and rectangular cavity. The coupled mode theory is used to describe the Fano resonance effect. The Fano resonance shows a notable red shift with an increasing dielectric material refractive index. The results show that with optimizing the physical parameters, the sensitivity is attained 1090 nm/RIU, and water and Ethanol temperature sensitivities are achieved as high as 0.087 nm/ °C and 0.475 nm/ °C, respectively. The corresponding figure of merit value is 2 × 104 RIU−1. The proposed structure can be used in photonic integrated devices to perform the sensing operation.

29 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed a polarization and angle-insensitive multi-band adjustable terahertz absorber, which is composed of a kind of periodic array square graphene ring with a Jerusalem cross graphene sheet.
Abstract: In this paper, a polarization and angle-insensitive multi-band adjustable terahertz absorber is proposed and numerically investigated, which is composed of a kind of periodic array square graphene ring with a Jerusalem cross graphene sheet. The numerical results indicate that the peaks absorptivity reach 99.9%, 99.9%, and 98.5% at 2.02THz, 3.44 THz, and 7.58 THz, respectively. The three resonance frequencies can be dynamically tuned by varying the chemical potential. Its physical mechanism can be analyzed by using Coupled Mode Theory (CMT) and electric field distribution. Moreover, the designed absorber is insensitive to polarization state and has the capability of wide-angle absorptivity. As a sensing application, the sensing characteristics of this monolayer graphene metasurface structure were investigated. In addition, we can achieve multi-spectral absorption peaks by applying bilayer graphene composite structure arrays. The proposed terahertz absorber may benefit branches of science such as refractive sensors, solar absorption, stealth, and other optoelectronic devices.

25 citations


Journal ArticleDOI
TL;DR: In this article, non-reciprocal Rayleigh waves are investigated in a continuous 2D semi-infinite medium bound with an array of space-time modulated spring-mass oscillators.
Abstract: Research on non-reciprocal propagation of waves is of great significance in the field of photonic and phononic crystals for realizing flexible one-way propagation devices with potential engineering applications. Here, non-reciprocal Rayleigh waves are investigated in a continuous two-dimensional (2D) semi-infinite medium bound with an array of space–time modulated spring–mass oscillators. The involved modulation is a wave-like perturbation of the surface of the continuous medium that breaks time-reversal symmetry and reciprocity. To characterize the propagation of Rayleigh waves in such a complex 2D medium with continuous and discrete interface, an analytical study is performed to obtain dispersion-engineered bandgaps by adopting the asymptotic method and coupled mode theory, which is also validated by numerical simulation. Specifically, the non-reciprocal transmission of Rayleigh waves with one-way mode conversion is illustrated, and various relevant physical quantities, including conversion length and band gap size, are quantitatively estimated. This work sheds light on versatile control of Rayleigh wave propagation ranging from sensing and evaluation of engineering structures to guided wave-based damage detection techniques.

24 citations


Journal ArticleDOI
TL;DR: In this paper, the authors introduced a graphene layer into an all-dielectric compound grating waveguide layer supporting quasi-BIC to achieve near-infrared perfect absorption of graphene.
Abstract: Recently, based on the selective excitation of the guided mode, researchers realized quasi-bound states in the continuum (quasi-BICs) in all-dielectric compound grating waveguide structures. In this paper, we introduce a graphene layer into an all-dielectric compound grating waveguide layer supporting quasi-BIC to achieve near-infrared perfect absorption of graphene. The underlying physical mechanism of perfect absorption can be clearly explained by the critical coupling theory derived from temporal coupled-mode theory in a single-mode, one-port system. By changing the Fermi level and the layer number of the graphene, the absorption rate of the system can be flexibly tuned. In addition, by changing the geometric parameter of the compound grating waveguide structure, the radiation coupling rate of the quasi-BIC can also be flexibly tuned. Therefore, the critical coupling condition can be maintained in a broad range of the Fermi level and the layer number of the graphene. The full width at half maximum of the near-infrared perfect absorption peak can be flexibly tuned from 5.7 to 187.1 nm. This bandwidth-tunable perfect absorber would possess potential applications in the design of 2D material-based optical sensors, electrical switchers, and solar thermophotovoltaic devices.

23 citations


Journal ArticleDOI
Yubin Park1, Viktar Asadchy1, Bo Zhao1, Cheng Guo1, Jiahui Wang1, Shanhui Fan1 
TL;DR: In this paper, a semitransparent emitter that fully absorbs normally incident energy from a given direction with zero backward and unity forward emissivity was designed and analyzed for photon-based energy harvesting.
Abstract: Kirchhoff's law of thermal radiation imposes a constraint on photon-based energy harvesting processes since part of the incident energy flux is inevitably emitted back to the source. By breaking the reciprocity of the system, it is possible to overcome this restriction and improve the efficiency of energy harvesting. Here, we design and analyze a semitransparent emitter that fully absorbs normally incident energy from a given direction with zero backward and unity forward emissivity. The nearly ideal performance with wavelength-scale thickness is achieved due to the magneto-optical effect and the guided-mode resonance engineered in the emitter structure. We derive the general requirements for the nonreciprocal emitter using the temporal coupled mode theory and the symmetry considerations. Finally, we provide a realistic emitter design based on a photonic crystal slab consisting of a magnetic Weyl semimetal and silicon.

23 citations


Journal ArticleDOI
Jiaqi Niu1, Yueqi Zhai1, Qingqing Han1, Jingquan Liu1, Bin Yang1 
TL;DR: It is demonstrated that the coupling coefficient between two coplanar metallic split-ring resonators can be tuned to satisfy the Friedrich-Wintgen BIC condition with normal terahertz incidence when metals are modeled as perfect electric conductors.
Abstract: The realization of bound states in the continuum (BICs) in optical systems has been relying mainly on symmetry breaking. In contrast, another mechanism, known as resonance-trapped (or Friedrich-Wintgen) scenario, has been reported in the limited scope of dielectric resonant inclusions or at off-Γ points. In this Letter, we demonstrate that the coupling coefficient between two coplanar metallic split-ring resonators can be tuned to satisfy the Friedrich-Wintgen BIC condition with normal terahertz (THz) incidence when metals are modeled as perfect electric conductors. Temporal coupled-mode theory is applied to validate the results. Experimentally, a BIC-induced cloaking effect has been observed, owing to the intrinsic dissipation loss of the constitutive materials. Our findings suggest an alternative strategy to construct BICs in metallic metasurfaces apart from conventional symmetry-breaking methods.

21 citations


Journal ArticleDOI
TL;DR: In this paper, a graphene metamaterial desensitized to the polarized angle was used to produce tunable quadruple plasmon-induced transparency (PIT), and the Boltzmann function satisfied by the response of graphene strips to the polarization direction of incident light was proposed for the first time.
Abstract: 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.

20 citations


Journal ArticleDOI
TL;DR: In this paper, a reflection-type coupling system composed by two identical linear resonances in a metal-insulator-metal configuration is theoretically proposed using the coupled-mode theory, whose phase diagram can be well controlled upon the coupling changes.
Abstract: Efficient and flexible manipulation of electromagnetic waves using metasurfaces has attracted continuous attention in recent years. However, previous studies mainly apply sole resonance effect to accomplish the task. Here, we show that introducing a meta-coupling effect would reveal further physical insights in the electromagnetic wave control. To demonstrate this, a reflection-type coupling system composed by two identical linear resonances in a metal-insulator-metal configuration is theoretically proposed using the coupled-mode theory, whose phase diagram can be well controlled upon the coupling changes. Such intriguing optical property is verified by a double C-shaped resonator in the terahertz regime, where the coupling effect can be tuned by changing their either relative distance or rotation. More importantly, the reflection phase shift around the working frequency can be efficiently engineered without having to change the dimensions of the resonators. Two efficient anomalous metasurface deflectors are designed and experimentally characterized, whose maximum measured efficiency is more than 70%. The proposed controlling strategy further enriches the designing freedoms of metasurfaces and may find broad applications in realizing efficient and tunable functional devices.

Journal ArticleDOI
TL;DR: In this article, a unique metasurface scheme was proposed to produce extremely high Q-factor Fano resonance of the reconstructive coherent mode in the terahertz (THz) regime, where the surface currents are out of phase for an individual SRR, leading to the cancellation of net dipole moment.
Abstract: High Q-factor resonance has a pivotal role in wide applications for manipulating electromagnetic waves. However, high Q-factor resonance, especially in the terahertz (THz) regime, has been a challenge faced by plasmonic metamaterials due to the inherent ohmic and radiation losses. Here, we theoretically present a unique metasurface scheme to produce extremely high Q-factor Fano resonance of the reconstructive coherent mode in the THz regime. The THz metasurface is composed of periodically arranged vertical symmetric split ring resonators (SRRs), which can produce perfect reconstructive coherent coupling effect in the sense that dipole radiation is destructively suppressed. Under the polarized electric field perpendicular to SRR gap, the surface currents are out of phase for an individual SRR, leading to the cancellation of net dipole moment. The reconstructive coherent mode resonance can occur between each SRR and its neighboring SRRs, accompanied by destructive interference of the scattered fields of each SRR. This is due to the coupling between the localized resonance of individual particles and the Rayleigh anomaly of the array. The proposed metasurface can significantly suppress far-field radiation and perform an extremely high Q-factor beyond 104 level with large modulation depth in the THz region, which pushes the advancement of THz high Q-factor resonance. The extremely high Q-factor of reconstructive coherent mode is tunable by adjusting the geometry parameters. The design strategy is useful to develop ultra-sensitive sensors, narrow-band filters and strong interaction of field-matter in the THz regime.

Journal ArticleDOI
TL;DR: In this article, a semi-classical model for spin-injected vertical-cavity surface-emitting lasers (spin-VCSELs) with local optical anisotropies is presented.
Abstract: We present a semi-classical model for spin-injected vertical-cavity surface-emitting lasers (spin-VCSELs) with local optical anisotropies. Particular focus is put on highly-anisotropic spin-lasers with broad application potential. A generalized matrix formalism for extraction of the laser modes is introduced, which enables to calculate spatial distribution of vectorial modes in arbitrary spin-VCSELs. Time-dependence of such laser modes is further studied using the generalized coupled mode theory (CMT). It is the natural anisotropic generalization of the conventional modedecomposition approach. We use the circularly-polarized optical modes as the basis for CMT, which leads to extension of the well-known spin-flip model (SFM). In contrary to conventional SFM, the only input parameters are the geometric and local optical properties of the multilayer structure and properties of the gain media. The advantages of the theory are demonstrated on design and optimization of spin-VCSEL structure with high-contrast grating. We show that the proposed structures can be used for i) polarization modulation in THz range with tremendous applications for future ultrafast optical communication and ii) as perspective compact THz sources.

Journal ArticleDOI
TL;DR: In this paper, a topological nanophotonic corner state in a second-order topological photonic crystal (PC) cavity was investigated under in-plane excitation and the expectation values of a mirror-flip operation for the Bloch modes of a PC slab were used to characterize the topological phase.
Abstract: In silicon photonics, the cavity mode is a fundamental mechanism to design integrated passive devices for on-chip optical information processing. Recently, the corner state in a second-order topological photonic crystal (PC) rendered a global method to achieve an intrinsic cavity mode. It is crucial to explore such a topological corner state in silicon photonic integrated circuits (PICs) under in-plane excitation. Here, we study both theoretically and experimentally the topological nanophotonic corner state in a silicon-on-insulator PC cavity at a telecommunications wavelength. In theory, the expectation values of a mirror-flip operation for the Bloch modes of a PC slab are used to characterize the topological phase. Derived from topologically distinct bulk polarizations of two types of dielectric-vein PCs, the corner state is induced in a 90-deg-bend interface, localizing at the corner point of real space and the Brillouin zone boundary of reciprocal space. To implement in-plane excitation in an experiment, we fabricate a cross-coupled PC cavity based on the bend interface and directly image the corner state near 1383 nm using a far-field microscope. Finally, by means of the temporal coupled-mode theory, the intrinsic Q factor of a cross-coupled cavity (about 8000) is retrieved from the measured transmission spectra. This work gives deterministic guidance and potential applications for cavity-mode-based passive devices in silicon PICs, such as optical filters, routers, and multiplexers.

Journal ArticleDOI
TL;DR: In this paper, a vector coupled-mode description of the fields using local Frenet-Serret frames that rotate and twist with each of the N cores is presented, focusing on dispersion, polarization states, and transverse field profiles of the helical Bloch modes.
Abstract: The behavior of electromagnetic waves in chirally twisted structures is a topic of enduring interest, dating back at least to the 1940s invention of the microwave travelling-wave-tube amplifier and culminating in contemporary studies of chiral metamaterials, metasurfaces, and photonic crystal fibers (PCFs). Optical fibers with chiral microstructures, drawn from a spinning preform, have many useful properties, exhibiting, for example, circular birefringence and circular dichroism. It has recently been shown that chiral fibers with N-fold rotationally symmetric (symmetry group CN) transverse microstructures support families of helical Bloch modes (HBMs), each of which consists of a superposition of azimuthal Bloch harmonics (or optical vortices). An example is a fiber with N coupled cores arranged in a ring around its central axis (N-core single-ring fiber). Although this type of fiber can be readily modeled using scalar coupled-mode theory, a full description of its optical properties requires a vectorial analysis that takes account of the polarization state of the light, which is particularly important in studies of circular and vortical birefringence. In this paper, we develop, using an orthogonal 2D helicoidal coordinate system embedded in a cylindrical surface at constant radius, a rigorous vector coupled-mode description of the fields using local Frenet–Serret frames that rotate and twist with each of the N cores. The analysis places on a firm theoretical footing a previous HBM theory in which a heuristic approach was taken, based on physical intuition of the properties of Bloch waves. After a detailed review of the polarization evolution in a single spiraling core, analysis of the N-core single-ring system is carefully developed step by step. Accuracy limits of the analysis are assessed by comparison with the results of finite element modeling, focusing in particular on the dispersion, polarization states, and transverse field profiles of the HBMs. We believe this study provides clarity into what can sometimes be a rather difficult field and will facilitate further exploration of real-world applications of these fascinating waveguiding systems.

Journal ArticleDOI
TL;DR: In this article, a momentum-space imaging spectroscopy (MSIS) system is presented, which can directly study the spectral information in momentum space, and the photonic dispersion can be captured in one shot with high energy and momentum resolution.
Abstract: The novel phenomena in nanophotonic materials, such as the angle-dependent reflection and negative refraction effect, are closely related to the photonic dispersions E ( p ) . E ( p ) describes the relation between energy E and momentum p of photonic eigenmodes, and essentially determines the optical properties of materials. As E ( p ) is defined in momentum space (k-space), the experimental method to detect the energy distribution, that is the spectrum, in a momentum-resolved manner is highly required. In this review, the momentum-space imaging spectroscopy (MSIS) system is presented, which can directly study the spectral information in momentum space. Using the MSIS system, the photonic dispersion can be captured in one shot with high energy and momentum resolution. From the experimental momentum-resolved spectrum data, other key features of photonic eigenmodes, such as quality factors and polarization states, can also be extracted through the post-processing algorithm based on the coupled mode theory. In addition, the interference configurations of the MSIS system enable the measurement of coherence properties and phase information of nanophotonic materials, which is important for the study of light-matter interaction and beam shaping with nanostructures. The MSIS system can give the comprehensive information of nanophotonic materials, and is greatly useful for the study of novel photonic phenomena and the development of nanophotonic technologies.

Journal ArticleDOI
TL;DR: N numerically and experimentally demonstrate quasi-bound states in the continuum (BICs) in free-standing metal complementary periodic cross-shaped resonators (CPCRs) at terahertz frequencies and shows the high Q-factor of the quasi-BIC is mainly determined not by the radiative loss but the material loss, as the asymmetry of the structures is unremarkable.
Abstract: We numerically and experimentally demonstrate quasi-bound states in the continuum (BICs) in free-standing metal complementary periodic cross-shaped resonators (CPCRs) at terahertz frequencies. Such induced quasi-BICs arise from the broken symmetry in CPCRs. By slightly breaking mirror symmetry through reducing the length of one arm in CPCRs, the measured Q-factor of the quasi-BIC can reach 102, which is significantly high compared with that of conventional modes. Further study shows that the high Q-factor of the quasi-BIC is mainly determined not by the radiative loss but the material loss, as the asymmetry of the structures is unremarkable. The sharp quasi-BICs realized in the proposed structure may immediately boost the exploration of ultra-narrowband filters and ultra-sensitive sensors at terahertz frequencies.

Journal ArticleDOI
TL;DR: In this paper, the authors proposed a five-step-coupled pyramid-shaped monolayer graphene metamaterial and predicted a dynamically controllable plasmon-induced transparency (PIT) spectral response with four transparency peaks for the first time in the monolayers of the material.
Abstract: Graphene, a new two-dimensional (2D) material, has attracted considerable attention in recent years because of the metallic characteristics at terahertz frequencies. The phase coupling of multilayer graphene-coupled grating structures is normally used to realize multiple plasmon-induced transparency (PIT) spectral responses. However, the device becomes more complicated with the increase in the number of graphene layers. In this work, we propose a five-step-coupled pyramid-shaped monolayer graphene metamaterial and predict a dynamically controllable PIT with four transparency peaks for the first time in the monolayer graphene metamaterial. A tunable multi-switch and good slow light effect is predicted over the wide PIT window, and the maximum modulation depth is high up to 16.89 dB, which corresponds to 97.95%, while the time delay of the induced transparent window is as high as 0.488 ps, where the corresponding group refractive index is 586. The electric field distributions and quantum level theory are used to explain the physical mechanism of the PIT with four transparency peaks. The coupled mode theory (CMT) is employed to establish the mathematical model of the PIT with four transparency peaks, and the consistency between the simulated and the calculated results is nearly perfect. We believe that the pyramid-shaped monolayer graphene metamaterial could be useful in efficient filters, switches, and slow light devices.

Journal ArticleDOI
TL;DR: The first proof-of-concept demonstration of an integrated mode-splitting biosensor insensitive to temperature and refractive index variations of the liquid matrix where the molecules to be detected are embedded is demonstrated.
Abstract: Self-referenced biosensing based on mode-splitting on a microring resonator is experimentally demonstrated. A Bragg grating integrated on the surface of the ring provides coupling between the clockwise and counterclockwise travelling modes of the pristine ring resonator lifting their degeneracy. The amount of mode-splitting is directly related to the reflectivity of the grating and it is only affected by structurally modifying the grating. Environmental perturbations to the surroundings of the gratings, such as temperature and bulk refractive index variations, have a minor effect on the amount of mode-splitting. This principle allows the realization of a self-referenced sensing scheme based on the detection of variations of the mode-splitting induced by structural changes to the grating. In this work, a polymethyl methacrylate (PMMA) Bragg grating is integrated onto a ring resonator in Al2O3. It is shown both theoretically and experimentally that the amount of splitting of a resonance varies minimally under temperature or bulk refractive index perturbations. However, the structural change of attaching a layer of biomolecules inside the grating does affect its reflectivity and the amount of mode splitting present. This result represents the first proof-of-concept demonstration of an integrated mode-splitting biosensor insensitive to temperature and refractive index variations of the liquid matrix where the molecules to be detected are embedded. The reported results pave the road towards the realization of truly self-referenced biosensors.

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate an ultra-compact and fabrication error tolerant silicon three-mode multiplexer by shallowly etching rectangular trenches on a multi-mode interferometer.
Abstract: Mode-division multiplexing can scale the capacity of optical communications and optical interconnects. We demonstrate an ultra-compact and fabrication-error tolerant silicon three-mode multiplexer by shallowly etching rectangular trenches on a multi-mode interferometer. Depending on the selected input port, the TE0 mode is converted to the eigenmodes of the bus waveguide. These modes are coupled to each other owing to the refractive-index perturbation induced by the shallow trenches and finally converted to a selected spatial mode at the output. A three-mode multiplexing device is experimentally demonstrated with a footprint of 2.00 × 17.05 µm2. The bandwidths for the three channels are >70 nm with crosstalk values below –10 dB.

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate a link between radiation control at critical coupling and metasurface-based bound states in the continuum (BIC) physics, and develop a generalized theory to engineer light absorption of 2D materials in coupling resonance metamurfaces.
Abstract: Recent progress in nanophotonics is driven by the desire to engineer light–matter interaction in two-dimensional (2D) materials using high-quality resonances in plasmonic and dielectric structures Here, we demonstrate a link between radiation control at critical coupling and metasurface-based bound states in the continuum (BIC) physics, and develop a generalized theory to engineer light absorption of 2D materials in coupling resonance metasurfaces In a typical example of hybrid graphene–dielectric metasurfaces, we present manipulation of the absorption bandwidth by more than one order of magnitude by simultaneously adjusting the asymmetry parameter of silicon resonators governed by BIC and graphene surface conductivity while the absorption efficiency remains maximum This work reveals the generalized role of BIC in radiation control at critical coupling, and provides promising strategies in engineering light absorption of 2D materials for high-efficiency optoelectronics device applications, eg, light emission, detection, and modulation

Journal ArticleDOI
TL;DR: In this paper, a dielectric grating placed on top of a distributed Bragg reflector was used to investigate the optical Tamm states supported by a Dielectric Grating and showed that under certain conditions the Tamm state may become a bound state in the continuum.
Abstract: We investigate optical Tamm states supported by a dielectric grating placed on top of a distributed Bragg reflector. It is found that under certain conditions the Tamm state may become a bound state in the continuum. The bound state, in its turn, induces the effect of critical coupling with the reflectance amplitude reaching an exact zero. We demonstrate that the critical coupling point is located in the core of a vortex of the reflection amplitude gradient in the space of the wavelength and angle of incidence. The emergence of the vortex is explained by the coupled mode theory.

Journal ArticleDOI
01 Dec 2021-Optik
TL;DR: In this article, a polarization insensitive broadside coupling mechanism in graphene-metamaterials was demonstrated for realization of miniaturized, compact, ultra-sensitive THz thin film sensors with substantially large effective areas for strong THz-matter interaction.

Journal ArticleDOI
Ran-Ran Xie1, Guo-Qing Qin1, Hao Zhang1, Min Wang1, Gui-Qin Li1, Dong Ruan1, Gui-Lu Long1 
TL;DR: In this paper, a phase-controlled dual-wavelength resonance based on whispering-gallery-mode (WGM) microcavities was achieved experimentally, where not only two optical pathways but also a unidirectional coupling between counter-propagating waves were formed.
Abstract: We report a novel, to the best of our knowledge, way to achieve phase-controlled dual-wavelength resonance based on whispering-gallery-mode (WGM) microcavities experimentally. With the help of a feedback waveguide, not only two optical pathways but also a unidirectional coupling between counter-propagating waves are formed, which is the requirement of all-optical analogues of electromagnetically induced transparency and Autler–Townes splitting. By adjusting the accumulating phase introduced from the fiber waveguide, we observe the signal lineshape changes from symmetric to asymmetric, i.e., the resonant transmission and extinction ratio of two splitting modes can be controlled, which brings a new degree of freedom to the WGM resonator system. These results may boost the development of quantum state control and pave the way for reconfiguring devices such as narrow-band filters.

Journal ArticleDOI
TL;DR: In this article, a new and unique design and three-dimensional simulation of a graphene-based integrated half adder by the use of ring resonators and central waveguides have been presented.
Abstract: In the field of optical technologies, modern optical scenarios such as integrated multi-operand optical gates are considered as very useful and effective substrates. In this paper, a new and unique design and three-dimensional simulation of a graphene-based integrated half adder by the use of ring resonators and central waveguides have been presented. The proposed structure includes two separate AND and XOR gates, and the presence of graphene in this design allows to tune the wavelength and to turn on or off the structure at the desired wavelength without changing the design parameters and only by changing the Fermi voltage of graphene. The half adder logic is controlled based on the output power of the CARRY and SUM ports at a central wavelength of 1550 nm. Additionally, the extinction ratio obtained for each of the CARRY and SUM ports has been equal to 21.76 dB and 18.73 dB, respectively. The average quality factor of the output of resonators for XOR and AND gates has been 873.75 and 1116.96, respectively. In the proposed structure, single-layer graphene was used because the graphene plasmonic effect was considered. The Extinction ratio in our work is better than the other works. Finite-difference-time-domain (FDTD) and coupled mode theory (CMT) have been the two methods used to extract and analyze the results of the output spectrum of the proposed structure, and the results of the output spectrum have been exactly the same in the two methods.

Journal ArticleDOI
TL;DR: This work experimentally demonstrates a dual-band asymmetric transmission effect only for one-polarized linear wave in the terahertz band and introduces the coupled-mode theory, which is in good agreement with the experiment in the two bands.
Abstract: The reported dual-band asymmetric transmission is usually an effect of mutual polarization conversion, where one polarized wave is converted to its cross-polarization in the first band while the other polarized wave is converted to its cross-polarization in the second band. In this work, we experimentally demonstrate a dual-band asymmetric transmission effect only for one-polarized linear wave in the terahertz band. It is measured that the cross-polarization transmission coefficient Tyx reaches two peaks of 0.715 and 0.548 at the frequency of 0.74 THz and 1.22 THz, respectively. While the transmission coefficient Txy is lower than 0.2 in the wide-band from 0.5 THz to 1.5 THz. Firstly, the multiple interference model is used to discuss the physical mechanism of the dual-band asymmetric transmission. However, the second band of the calculated spectrum is offset due to the strong near field coupling between the two metal layers. The coupled-mode theory is then introduced and the fitting result of the coupled-mode theory is in good agreement with that of the experiment in the two bands. This research would provide new theoretical instructions in designing and analyzing multiband asymmetric transmission in the terahertz, microwave or the optical bands.

Journal ArticleDOI
TL;DR: In this article, a plasmonic structure consisting of an equilateral triangle-shaped cavity (ETSC) and a metal-insulator-metal (MIM) waveguide is proposed to realize triple Fano resonances.
Abstract: A kind of plasmonic structure consisted of an equilateral triangle-shaped cavity (ETSC) and a metal-insulator-metal (MIM) waveguide is proposed to realize triple Fano resonances. Numerically simulated by the finite difference time domain (FDTD) method, Fano resonances inside the structure are also explained by the coupled mode theory (CMT) and standing wave theory. For further research, inverting ETSC could dramatically increase quality factor to enhance resonance wavelength selectivity. After that, a bar is introduced into the ETSC and the inverted ETSC to increase resonance wavelengths through changing the structural parameters of the bar. In addition, working as a highly efficient narrowband filter, this structure owes a good sensitivity (S = 923 nm/RIU) and a pretty high-quality factor (Q = 322) along with a figure of merit (FOM = 710). Additionally, a narrowband peak with 1.25 nm Full-Width-Half-Maximum (FWHM) can be obtained. This structure will be used in highly integrated optical circuits in future.

Journal ArticleDOI
TL;DR: In this article, the authors considered resonators supporting either a bright mode or a dark mode, introducing an additional degree of freedom for spectral modulation relative to bright modes alone, and demonstrated tunable spectral splitting by changing the separation between resonators.
Abstract: We study the absorptivity of coupled metamaterial resonators in the mid-infrared range We consider resonators supporting either a bright mode or a dark mode, introducing an additional degree of freedom for spectral modulation relative to bright modes alone In a dark-bright coupled resonator system, we demonstrate tunable spectral splitting by changing the separation between resonators We show via coupled mode theory that resonator separation can be mapped to coupling constant We further introduce a dark-dark coupled resonator system, which gives rise to an emissive bright mode only in the presence of inter-resonator coupling The dark-dark system yields a broadband emissivity that decays to zero exponentially with resonator separation, providing a design method for strong thermal emissivity control

Journal ArticleDOI
Marjan Bazian1
TL;DR: A comprehensive review of the principle structure of ADF, coupled mode theory (CMT), types and recent applications in WDMs, accelerometer and bio/chemical sensors is reported.
Abstract: Add–drop filter (ADF) is a key component in optical integrated circuits that can be used in all-optical communication networks and wavelength division multiplexing (WDM) systems. The quality factor, coupling efficiency, transmission efficiency and coupling length are important parameters in add–drop filters. Photonic crystal (PC) optical devices have become popular among researchers because their structure is suitable to embed into optical circuits. This paper covers a comprehensive review of the principle structure of ADF, coupled mode theory (CMT), types and recent applications in WDMs, accelerometer and bio/chemical sensors. Although there are some different categories of photonic crystal ring resonator-based ADF in general, all of them can be divided into photonic to two class of non-circular and circular. This article is reported a comprehensive study about ADF and improvement of these ADF.

Journal ArticleDOI
TL;DR: In this paper, the cross-sectional profiles and spatial distributions of the fields in guided normal modes of two coupled parallel optical nanofibers were studied, and it was shown that the distribution of the magnetic field components with respect to the principal axes is opposite to that of the electric field components.
Abstract: We study the cross-sectional profiles and spatial distributions of the fields in guided normal modes of two coupled parallel optical nanofibers. We show that the distributions of the components of the field in a guided normal mode of two identical nanofibers are either symmetric or antisymmetric with respect to the radial principal axis and the tangential principal axis in the cross-sectional plane of the fibers. The symmetry of the magnetic field components with respect to the principal axes is opposite to that of the electric field components. We show that, in the case of even $\mathcal{E}_z$-cosine modes, the electric intensity distribution is dominant in the area between the fibers, with a saddle point at the two-fiber center. Meanwhile, in the case of odd $\mathcal{E}_z$-sine modes, the electric intensity distribution at the two-fiber center attains a local minimum of exactly zero. We find that the differences between the results of the coupled mode theory and the exact mode theory are large when the separation distance between the fibers is small and either the fiber radius is small or the wavelength of light is large. We show that a slight difference between the radii of the nanofibers leads to strong asymmetry of the intensity distributions of the guided normal modes.