Asymmetric Fano Resonance Research Articles

Tunable Fano resonances at 2 μm waveband based on a single microring resonator

Abstract Development of silicon photonic devices for the 2 μm waveband is urgent since the 2 μm waveband has been considered as a potential communication window for next-generation optical communications. Asymmetric Fano resonances are very promising in the development of devices such as optical switches, sensors, and modulators for the 2 μm waveband. In this paper, a Fano resonance at the 2 μm waveband is theoretically realized by using a silicon-based microring resonator and the thermo-optic effect is employed to tune it. Utilizing this single microring resonator structure instead of the present coupled microring resonator structures, we achieve the Fano resonance with an extinction ratio of 30.5 dB and a slope ratio of 69.3 dB nm−1 at the 2 μm waveband. The metal heater and graphene heater are designed and optimized to achieve the Fano tuning. The thermal-optical tuning efficiencies are 0.46 nm mW−1, 0.61 nm mW−1 and 1 nm mW−1 for the metal heater, the metal heater with air adiabatic slots, and the graphene heater, respectively. This work provides an effective scheme for the formation and tuning of the Fano resonance at the 2 μm waveband, which is conducive to the development of devices and optical communication systems at the 2 μm waveband.

Ultrahigh-[formula omitted] Fano resonance in a cavity-waveguide coupled system based on second-order topological photonic crystals with elliptical holes

We present a novel configuration of second-order topological photonic crystal (SOTPC) comprising interconnected dielectric material with elliptical air holes. Based on this SOTPC design, we construct a topological corner state (TCS) cavity and a topological edge state (TES) waveguide. Through the coupling of the TCS cavity with the TES waveguide, we establish a topological cavity-waveguide coupled system (TCWCS) that exhibits an ultrasharp asymmetric topological Fano resonance. Numerical simulations demonstrate that the proposed TCWCS achieves an exceptionally high quality factor exceeding 108, surpassing previously reported results by several orders of magnitude. Moreover, the refractive index sensing application implemented with TCWCS shows a sensitivity of 278 nm/RIU and a figure of merit surpassing 107 1/RIU. Notably, our simulations reveal that the lineshape and quality factor of the Fano resonance, as well as the index sensing performance in the TCWCS, are highly robust against structural defects. The demonstrated capabilities of the TCWCS open up avenues for further exploration and development in the field of topological photonics based on our proposed SOTPC.

Multiple Fano resonances by MIM waveguide coupling whispering gallery mode resonator and application in Refractive index sensing

In this study, a baffled metal–insulator–metal (MIM) waveguide coupled with a whispering gallery mode resonator (WGMR) is proposed. This structure could excite quadruple Fano resonances. The asymmetric Fano resonance transmittance spectrum and the electric field at the resonance peak were numerically simulated using the finite difference time domain method. The obtained data were fitted using the multimode interference coupled mode theory. The number of Fano resonance peaks was tuned by the outer radius of the WGMR. With other geometric parameters unchanged, the number of Fano resonance peaks increased with increasing outer radius of the WGMR. Thus, it achieved multiple Fano resonances by adjusting solely the radius of the WGMR. When the ring width was fixed, the structure could excite multiple Fano resonances within a certain outer radius range. This structure was used for refractive index sensing and its sensitivity and figure of merit (FOM) were 3004 nm/RIU and 3.14 × 104 in a gas environment, respectively. Therefore, the proposed structure can excite multiple Fano resonances and achieve high sensitivity and FOM, providing a theoretical basis for micro and nano applications.

Fano resonance and enhanced sensing in the excitation of the surface phonon polariton

The surface phonon polariton is a collective oscillation mode of phonons and incident electromagnetic waves in polar dielectric materials. Compared with the surface plasmon polariton, it has low loss and can be applied to the mid-infrared band. A surface phonon resonance sensor based on waveguide-coupling is proposed. The sensor structure is a typical Kretschmann configuration consisting of a germanium (Ge) prism, a silicon carbide (SiC) layer, an indium selenide (In2Se3) film, a titanium dioxide (TiO2) film, and the surrounding dielectric. The reflectivity possesses significant asymmetric Fano resonance dips. In sensing applications, the waveguide-coupling structure yields a sensitivity by intensity of 11278RIU−1 and a figure of merit of 10344RIU−1. Our investigation provides an alternative method for refractive index sensing, thus opening up opportunities for the design of various phonon devices based on Fano resonance.

Dissipative coupling in a Bragg-grating-coupled single resonator with Fano resonance for anti-PT-symmetric gyroscopes.

Anti-parity-time-symmetric Hamiltonians show an enhanced sensitivity to external perturbations that can be used for high-performance angular velocity sensing. Dissipative coupling is a valuable way for realizing anti-PT-symmetric Hamiltonians with optical resonators and is usually obtained by means of auxiliary waveguides. Here, we model and experimentally show the dissipative coupling between two counterpropagating modes of a single resonator, by means of a Bragg-grating placed in the feeding bus. The proposed solution enables the possibility of accurately designing the dissipative coupling strength in integrated non-Hermitian gyroscopes, thus providing high flexibility in the design of the proposed sensor. Moreover, we theoretically and experimentally demonstrate that the dissipative coupling between two counterpropagating modes of the same resonant cavity can give rise to an asymmetric Fano resonance.

Symmetric and asymmetric Fano resonances in a broken axial symmetry metasurface of split ring resonators

In the paper, a metasurface is investigated, the unit cell of which consists of two resonators. We show that the breaking of the axial symmetry of the unit cell in such a structure leads to the appearance of two Fano resonances instead of one that is characteristic of the symmetric unit cell. The two resonances are different, one of which is symmetric and the other is asymmetric. This is confirmed by calculating the phases of currents in the adjacent resonators. Modeling results are confirmed by experimental investigation of the metasurface spectrum in a microwave frequency range showing two resonances measured experimentally. A detailed study of the currents flowing in the adjacent resonators of the unit cell demonstrates that currents at the Fano resonance dips flow either in phase or in antiphase with respect to the external electric field, whereas in peaks this difference is π/2. This confirms the interference nature of the Fano effect.

Photonic transistor based on a coupled-cavity system with polaritons.

We investigate the transmission of probe fields in a coupled-cavity system with polaritons and propose a theoretical schema for realizing a polariton-based photonic transistor. When probe light passes through such a hybrid optomechanical device, its resonant point with Stokes or anti-Stokes scattered effects, intensity with amplification or attenuation effects, as well as group velocity with slow or fast light effects can be effectively controlled by another pump light. This controlling depends on the exciton-photon coupling and single-photon coupling. We also discover an asymmetric Fano resonance in transparency windows under the strong exciton-photon coupling, which is different from general symmetric optomechanically induced transparency. Our results open up exciting possibilities for designing photonic transistors, which may be useful for implementing polariton integrated circuits.

Glycerol concentration sensor based on the MIM waveguide structure

Glycerol is widely used in medicine, industry and skin care products. This study investigated a high-sensitivity glycerol concentration sensor based on double Fano resonances in a metal-insulator-metal (MIM) waveguide structure, established a coupling model of a baffle waveguide (BW) and a circular split ring resonator (CSRR), and generated asymmetric double Fano resonances in the waveguide structure. The Fano resonance transmittance reached 0.82, and the linear relationship between the refractive index (RI) and the glycerol concentration was obtained using the sensitivity of the Fano resonance spectrum. The application of the proposed sensor for glycerol concentration detection revealed that the Fano resonance wavelength was redshifted with the RI and that the sensing sensitivity reached 1153.85 nm/refractive index unit (RIU); therefore, the quick detection of the corresponding glycerol concentration can be realized. This proposed structure has significance in the research of optical sensors and optical switches.

Plasmonic Nanosensors and Metasensors Based on New Physical Mechanisms

Plasmonics can bind light to their surface while increasing its intensity. The confinement and enhancement of light allows high–density, independent, subwavelength sensor elements to be constructed in micrometer–sized arrays. Plasmonic nanostructures have been widely used in the sensing field because of their fast, real–time and label–free characteristics. Numerous plasmonic metasensors have been configured for next–generation technologies since the emergence of metamaterials and metasurfaces. Among these applications, the development of high–sensitivity sensors based on new physical mechanisms has received tremendous interest recently. This review focuses on high–sensitivity plasmonic nanosensors and metasensors based on new physical mechanisms, especially based on Fano resonance and the exceptional point (EP). The asymmetric Fano resonance generated by the interference of different resonance modes has a narrower bandwidth, while an EP occurs whenever two resonant modes coalesce both in their resonant frequency and their rate of decay or growth. Both physical mechanisms could tremendously improve the sensitivity of the plasmonic sensors. We summarize the working principles, the latest development status and the development trends of these plasmonic nanosensors and metasensors. It is believed that these new sensing mechanisms can inspire more fruitful scientific research.

Whispering gallery mode resonator based on dual microspheres in an expanded hollow sphere cavity of capillary tube

A whispering gallery mode microsphere resonator is proposed and demonstrated. The device is based on dual microspheres embedded in an expanded hollow sphere cavity formed by splicing a single-mode fiber and a capillary tube. The whispering gallery mode resonance is excited by the light traveling along the hollow sphere cavity wall and directed out by use of a tapered optical fiber in closing contact with one of the microspheres. The device has a good quality factor (Q) of 2.072 × 103, and a number of asymmetric Fano resonance lines. It is compact in structure, low in cost, simple in fabrication, and convenient in operation.

Carbon nanotube resonator enhancement of Majorana fermions induced slow light in a hybrid semiconductor/superconductor device

We theoretically investigate the Rabi-like splitting in the absorption spectrum of a single electron spin mediated by Majorana fermions in a hybrid semiconducting nanowire/superconductor system. The absorption spectra can display the symmetrical splitting phenomenon, and their related optical propagation such as fast light and slow light are investigated in different parametric regimes. When we consider implanting a spin into the carbon nanotube (CNT) resonator, the Rabi-like splitting in the absorption spectrum changes into asymmetric Fano resonance, which is accompanied by the rapid steep dispersion promising the slow- or fast-light effect, and even reaching tunable fast-to-slow light propagation (or vice versa) with controlling different parameter regimes. Moreover, we also investigate the role of the CNT resonator, which behaves as a phonon cavity enhancing the fast and slow light effect.

Active Fano resonance switch using dual-layer graphene in an embedded dielectric metasurface.

We propose an active optical Fano switch (OFS) based on an embedded dielectric metasurface (EDM) including dual-layer graphene (DLG). An EDM is a dielectric grating overlapped by two cladding layers, and it excites a Fano resonance. DLG is positioned inside the upper cladding layer to maximize light-graphene interaction. Thus, with a small change of the chemical potential (µc) of graphene, a resonance wavelength is tuned to switch the OFS on and off. First, a red-parity asymmetric Fano resonance is realized, and a sharp asymmetric lineshape is achieved by controlling the structural parameters of the EDM and the interaction between the Fano resonance and additional weak Fabry-Perot interference for efficient switching. The distance of a peak-to-dip wavelength (Δλp-d) and the change of chemical potential (Δµc) for switching is analyzed by varying the duty cycle (DC) and grating thickness (tg) of the EDM. Furthermore, switching contrast as a figure of merit (FoM) is analyzed. With DC of 0.5 and tg of 70 nm, the OFS requires Δλp-d of 7.3 nm and Δµc of 0.25 eV. The FoM of 0.97 is achieved. By adjusting the two parameters, the switching condition is tuned. In the case of a blue parity, the effect of the two parameters exhibits a similar trend to that of the red parity. The FoM, however, is lower due to the reversed parity.

Refractive index sensor based on a ring with a disk-shaped cavity for temperature detection applications.

In this study, we proposed a novel refractive index sensor structure, comprising a metal-insulator-metal (MIM) waveguide and a circular ring containing a disk-shaped cavity (CRDC). The finite element method was used to theoretically analyze the sensor characteristics. The simulation results showed that the disk-shaped cavity is the key to the asymmetric Fano resonance, and the radius of the CRDC has a significant influence on the performance of the sensor. A maximum sensitivity and figure of merit (FOM) of 2240 nm/RIU and 62.5, respectively, were realized. Additionally, the refractive index sensor exhibits the potential of aiding in temperature detection owing to its simple structure and high sensitivity of 1.186 nm/ºC.

CROW-based Fano structures for all optical switching devices.

In this paper, an improved optical Fano switch based on coupled resonator optical waveguides (CROWs) is presented. The new topological design is employed to achieve steeper and highly asymmetric Fano resonances (FRs). Physically, in the proposed structures, due to the increase in the effective refractive index at the center of the CROW, a confined mode arises in the continuum background according to the variational theorem and leads to FR. The results show that in CROW-based Fano switches, the Fano spectrum is improved by tuning the number of nanocavities. The ratio between the slope ratio and linewidth shows an improvement of 55.25% from single to CROW5. As an important application of FR, an ultra-compact device with a CROW-based Fano structure is demonstrated. The results of the numerical finite difference time domain simulation agree well with the theoretical coupled mode theory.

Fano Resonance Hybrid Waveguide-Coupled Plasmonic Sensor Using Transparent Conductive Oxide in the Near-Infrared Range

We proposed an ultra-sensitive refractive index sensor by using indium-doped cadmium oxide as a plasmonic material operating in near-infrared based on Fano resonance. The proposed sensor has a hybrid multilayer waveguide structure that supports both a long-range surface plasmon polariton (LRSPP) mode and a dielectric waveguide (DWG) mode. The design strategy of the structure parameters of the inner layers is elaborated in detail through the numerical analysis of the two modes. By suitably tailoring the thickness of the coupling layer, a strong mode coupling between the two modes could be achieved, leading to a sharp asymmetric Fano resonance. With the designed optimal physical parameters, our proposed sensor could achieve a maximum intensity sensitivity of 19,909 RIU−1, a 193-fold enhancement than that of a conventional long-range SPR (LRSPR) based scheme. The proposed design can be a promising platform for biochemical sensing in the near-infrared region.

VO2-metallic hybrid metasurfaces for agile terahertz wave modulation by phase transition

The combination of VO2 and metasurfaces has opened an attractive route to dynamically control terahertz (THz) waves based on the giant conductivity change. However, the high-precision control of microfabrication and single performance of conductivity change limit the multifunctional application of VO2-based metasurfaces. Here, we proposed a VO2-metallic hybrid metasurface by in situ depositing high-quality VO2 thin films onto a metasurface composed of asymmetric Fano resonance units. It exhibits agile frequency and amplitude modulation for THz transmission across tuning the dielectric constant and conductivity of VO2. The metallic metasurface is designed as a matrix to achieve high transmission at 0.61 and 0.78 THz due to the split-ring resonance. During the thermally triggered phase transition of VO2, we found that the resonance frequency and amplitude can be tuned dominantly by the change of dielectric constant and conductivity, respectively. In particular, the increased dielectric constant enables red shift of the frequency by around 0.48 THz and the conductivity increases lead to a giant THz amplitude modulation of 88%. These results provide a route for developing VO2-based THz smart devices combined with functional metasurfaces and hold great promise for applications in THz sensor and modulation.

Crosstalk-free all-optical switching enabled by Fano resonance in a multi-mode photonic crystal nanocavity.

We demonstrate all-optical switching using a multi-mode membranized photonic crystal nanocavity exploiting the free-carrier induced dispersion in InP and the sharp asymmetric lineshape of Fano resonances. A multi-mode cavity is designed to sustain two spatially overlapping modes with a spectral spacing of 18 nm. The measured transmission spectrum of the fabricated device shows multiple asymmetric Fano resonances as predicted by optical simulations. The capabilities of the device are benchmarked by comparing a wavelength conversion from 1538.2 nm to 1565.2 nm with a single-mode wavelength conversion at 1566.2 nm on the same device. The results show an improvement in signal quality with a 5.6 dB power penalty reduction at the receiver as well as in energy efficiency with a reduction of the pump power from 534 fJ/bit to 445 fJ/bit.

Magnetization dynamics in the YIG/Au/YIG magnon valve

The magnetization dynamics of an yttrium iron garnet (YIG)/Au/YIG magnon valve was investigated using broadband ferromagnetic resonance. The material characterizations of YIG/Au/YIG were performed using cross-sectional scanning transmission electron microscopy, x-ray diffraction spectroscopy, x-ray photoemission spectroscopy, Raman spectroscopy, and UV–visible spectroscopy. Asymmetric Fano resonance in the YIG/Au (60 nm)/YIG magnon valve structure was observed experimentally, and the two coupled oscillators model was used to describe the source of the Fano resonance qualitatively. We also provide a quantitative description of the Fano resonance and extract the Fano factor, which is an important feature that can be used to define the interaction sign. This represents the first attempt to apply the Fano resonance to magnetization dynamics. The spin wave resonance modes excited by the Au nanoparticles (NPs) surface plasmons were also observed in a YIG/Au NPs/YIG structure. Our findings confirm the occurrence of magnetic Fano resonance in the YIG/Au/YIG magnon valve and pave the way toward the development of quantum information devices based on magnon valves.

Refractive index sensor based on Fano resonance in a ring with a rectangular cavity structure

This paper presents an innovative plasmonics-based refractive index sensor structure that consists of a circular ring containing a rectangular cavity (CRRC) and a metal–insulator–metal (MIM) waveguide with two rectangular stubs. The narrowband mode of the CRRC interacts with the broadband mode of the bus waveguide stub to generate sharp and asymmetric Fano resonance. The transmission characteristics were simulated using the finite element method. A maximum refractive-index sensor sensitivity of 2300 nm/RIU and figure of merit (FOM) of 57.5 were achieved. The proposed structure is simple yet highly sensitive and has the potential to be useful in detecting electrolyte concentration in blood. The results of this study provide a foundation for developing highly sensitive plasmonic sensors biomedical applications.

Absorption and Self-Calibrated Sensing Based on Tunable Fano Resonance in a Grating Coupled Graphene/Waveguide Hybrid Structure

A grating coupled graphene surface plasmon and planar waveguide structure is investigated. A sharp asymmetric profile Fano resonance attributed by the coupling between the graphene plasmon and the waveguide mode can be observed. The physical origin of Fano resonance are analyzed by electric field distribution, and the tunable Fano resonances with different line profiles are demonstrated by varying the Fermi level and the incident angle. The further studies demonstrate the Fano resonance has a potential application in light absorption and the absorption can reach 76% when graphene carrier mobility is 0.5 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /(Vs). In addition, the Fano resonance could potentially be applied as a self-referenced refractive index sensor due to the decoupled property of Fano resonance and a sensitivity of 5.21 μm/RIU can be achieved. Thus, the Fano resonance we proposed in the work would have great prospects in developing excellent light absorption and refractive index sensor functional devices.