Resonant Mode Coupling Research Articles

The bound states in the continuum of coupled resonance modes and multiple-function applications in grating-cavity structures

The bound states in the continuum (BICs) are desired for high-performance optical devices. The structures composed of gratings and cavities were investigated. Grating waveguide modes (GWMs) are excited by the grating in structures. When the symmetry of the structure is broken, the positive and opposite GWMs couple and generate upper- and lower-coupled modes. Two coupled modes show different properties, such as the Fabry-Pérot (F-P) BIC in lower-coupled mode and the symmetry-protected (SP) BICs in upper-coupled mode. The special properties indicate the two coupled modes are desirable for multifunctional applications. The radiation from the quantum emitter (QE) was enhanced 3030 times through the quasi-BIC of lower-coupled mode. The maximal group delay reached 6.1 ps by the quasi-BIC of the lower-coupled mode. The quasi-BIC of the upper-coupled mode generates high-performance absorption. Two coupled resonance modes are sensitive to polarization, which can be used for optical switching with a maximal amplitude modulation depth of up to 99.8% with an insertion loss of less than 0.008 dB. These results offer hope for developing multifunctional and high-performance devices through coupled resonance modes in the grating-cavity structures.

Cavity-enhanced narrowband spectral filters using rare-earth ions doped in thin-film lithium niobate

On-chip optical filters are fundamental components in optical signal processing. While rare-earth ion-doped crystals offer ultra-narrow optical filtering via spectral hole burning, their applications have primarily been limited to those using bulk crystals, restricting their utility. In this work, we demonstrate cavity-enhanced spectral filtering based on rare-earth ions in an integrated nonlinear optical platform. We incorporate rare-earth ions into high quality-factor ring resonators patterned in thin-film lithium niobate. By spectral hole burning at 4 K in a critically coupled resonance mode, we achieve bandpass filters ranging from 7 MHz linewidth, with 13.0 dB of extinction, to 24 MHz linewidth, with 20.4 dB of extinction. By reducing the temperature to 100 mK to eliminate phonon broadening, we achieve an even narrower linewidth of 681 kHz, which is comparable to the narrowest filter linewidth demonstrated in an integrated photonic device, while only requiring a small device footprint. Moreover, the cavity enables reconfigurable filtering by varying the cavity coupling rate. For instance, as opposed to the bandpass filter, we demonstrate a bandstop filter utilizing an under-coupled ring resonator. Such versatile integrated spectral filters with high extinction ratio and narrow linewidth could serve as fundamental components for optical signal processing and optical memories on-a-chip.

Coupling of Surface Plasmon Polaritons and Hyperbolic Phonon Polaritons on the Near-Field Radiative Heat Transfer Between Multilayer Graphene/hBN Structures

ABSTRACT Near-field radiative heat transfer (NFRHT) between multilayer graphene/hBN heterostructures has been demonstrated to exceed the blackbody limit due to the coupling mechanism of surface plasmon polaritons and hyperbolic phonon polaritons, opening the door to applications in thermal management, thermophotovoltaics, and nanoscale metrology. Recent studies have shown that adding vacuum layers within multilayer structures can effectively promote surface modes and thus enhance NFRHT. However, the influence of vacuum layers on NFRHT between multilayer graphene/hBN heterostructures has not been investigated. Moreover, the influence of vacuum layers on coupled resonance modes excited in multilayer structures is worth discussing. In this work, we study the NFRHT based on multilayer graphene/vacuum/hBN/vacuum structures. The results show that as the gap distance increases from 20 nm to 100 nm, the NFRHT of three-cell and six-cell configurations is enhanced, while that of unit-cell configuration is suppressed. The potential mechanism can be identified as the excitation of surface plasmon-phonon polaritons (SPPPs) and hyperbolic plasmon-phonon polaritons (HPPPs) in multilayer structures. The enhancement factor of the six-cell configurations is up to 4.82 when the gap distance is 80 nm. Moreover, the influences of the chemical potential of graphene and the layer thickness on the NFRHT are discussed. The interesting results in this work indicate the perspectives for future near-field research involving coupling of SPPPs and HPPPs, and shed new light on high-performance devices introducing vacuum layers based on near-field radiative heat transfer.

High Electric Field Enhancement Induced by Modal Coupling for a Plasmonic Dimer Array on a Metallic Film

A giant electric field on a subwavelength scale is highly beneficial for boosting the light–matter interaction. In this paper, we investigated a hybrid structure consisting of a hemispheric dimer array and a gold film and realized resonant mode coupling of the surface lattice resonance (SLR) and surface plasmon polariton (SPP). Mode coupling is demonstrated by observing anti-crossing in reflection spectra, which corresponds to Rabi splitting. Although the resonance coupling does not enter the strong coupling regime, an improved quality factor (Q~350) and stronger electric field enhancement in the gap region of the dimer (i.e., hot spot) in our hybrid structure are obtained compared to those of the single dimer or dimer array only. Remarkably, the magnitude of electric field enhancement over 500 can be accessible. Such high field enhancement makes our hybridized structure a versatile platform for the realization of ultra-sensitive biosensing, low-threshold nanolasing, low-power nonlinear optical devices, etc.

Three-dimensional surface lattice plasmon resonance effect from plasmonic inclined nanostructures via one-step stencil lithography

Abstract Plasmonic nanostructures allow the manipulation and confinement of optical fields on the sub-wavelength scale. The local field enhancement and environmentally sensitive resonance characteristics provided by these nanostructures are of high importance for biological and chemical sensing. Recently, surface lattice plasmon resonance (SLR) research has attracted much interest because of its superior quality factor (Q-factor) compared to that of localized surface plasmon resonances (LSPR), which is facilitated by resonant plasmonic mode coupling between individual nanostructures over a large area. This advantage can be further enhanced by utilizing asymmetric 3D structures rather than low-height (typically height < ∼60 nm) structure arrays, which results in stronger coupling due to an increased mode volume. However, fabricating 3D, high-aspect ratio, symmetry-breaking structures is a complex and challenging process even with state-of-the-art fabrication technology. Here, we report a plasmonic metasurface of 3D inclined structures produced via commercial TEM grid–based stencil lithography with a Q-factor of 101.6, a refractive index sensitivity of 291 nm/RIU, and a figure of merit (FOM) of 44.7 in the visible wavelength range at a refractive index of 1.5 by utilizing the 3D SLR enhancement effect, which exceeds the performance of most LSPR systems (Q < ∼10). The symmetry-breaking 3D inclined structures that are fabricated by electron beam evaporation at an angle increase the polarizability of the metasurface and the directionality of the diffractively scattered radiative field responsible for SLR mode coupling. Additionally, we explore the role of spatial coherence in facilitating the SLR effect and thus a high-Q plasmonic response from the nanostructures. Our work demonstrates the feasibility of producing 3D inclined structure arrays with pronounced SLR enhancement for high biological sensitivity by utilizing the previously unexplored inclined stencil lithography, which opens the way to fabricate highly sensitive plasmonic metasurfaces with this novel simple technique.

Unlocking Optical Coupling Tunability in Epsilon‐Near‐Zero Metamaterials Through Liquid Crystal Nanocavities

AbstractEpsilon‐near‐zero (ENZ) metamaterials represent a powerful toolkit for selectively transmitting and localizing light through cavity resonances, enabling the study of mesoscopic phenomena and facilitating the design of photonic devices. In this experimental study, it demonstrates the feasibility of engineering and actively controlling cavity modes, as well as tuning their mutual coupling, in an ENZ multilayer structure. Specifically, by employing a high‐birefringence liquid crystal film as a tunable nanocavity, the polarization‐dependent coupling of resonant modes with narrow spectral width and spatial extent is achieved. Surface forces apparatus (SFA) allowed to continuously and precisely control the thickness of the liquid crystal (LC) film contained between the nanocavities and thus vary the detuning between the cavity modes. Hence, it is able to manipulate nanocavities anti‐crossing behaviors. The suggested methodology unlocks the full potential of tunable optical coupling in epsilon‐near‐zero metamaterials and provides a versatile approach to the creation of tunable photonic devices, including bio‐photonic sensors and/or tunable planar metamaterials for on‐chip spectrometers.

Enhanced optical sensing with metal-insulator-metal nanohole arrays integrated with silver nanoparticles

The field of plasmonic sensing has witnessed rapid advancements in recent years. This work proposes an innovative methodology that employs metal–insulator-metal (MIM) nanohole arrays integrated with ring-shaped silver nanoparticles (AgNPs) to serve as an effective plasmonic refractive index sensor. The hybrid substrates are prepared using a combination of nanosphere lithography (NSL), reactive ion etching, and an enhanced plasmonic site-selective photocatalytic reaction, ensuring large-area fabrication, high throughput, and cost-effectiveness. The resonance peaks observed in the visible-NIR spectra are attributed to the coupling of surface plasmon resonance modes between MIM structures and AgNPs. The resonance wavelength exhibits sensitively to changes in the surrounding medium within the refractive index range of 1 to 1.5, with a remarkable sensitivity of 835 nm/RIU. Our findings reveal the significant potential of this technology for advancing optical sensing applications.

Self-aligned single-electrode actuation of tangential and wineglass modes using PMN-PT

Considering the evolution of rotation sensing and timing applications realized in micro-electro-mechanical systems (MEMS), flexural mode resonant shapes are outperformed by bulk acoustic wave (BAW) counterparts by achieving higher frequencies with both electrostatic and piezoelectric transduction. Within the 1–30 MHz range, which hosts BAW gyroscopes and timing references, piezoelectric and electrostatic MEMS have similar transduction efficiency. Although, when designed intelligently, electrostatic transduction allows self-alignment between electrodes and the resonator for various BAW modes, misalignment is inevitable regarding piezoelectric transduction of BAW modes that require electrode patterning. In this paper transverse piezoelectric actuation of [011] oriented single crystal lead magnesium niobate–lead titanate (PMN–PT) thin film disks are shown to excite the tangential mode and family of elliptical compound resonant modes, utilizing a self-aligned and unpatterned electrode that spans the entire disk surface. The resonant mode coupling is achieved by employing a unique property of [011] PMN–PT, where the in-plane piezoelectric coefficients have opposite signs. Fabricating 1-port disk transducers, RF reflection measurements are performed that demonstrate the compound mode family shapes in the 1–30 MHz range. Independent verification of mode transduction is achieved using in-plane displacement measurements with Polytec’s laser Doppler vibrometer (LDV). While the tangential mode achieves a 40o/s dithering rate at 335 kHz resonant frequency, the n = 2 wine-glass mode achieves 11.46 nm tip displacement at 8.42 MHz resonant frequency on a radius of 60 μm disk resonator in air. A single electrode resonator that can excite both tangential and wine-glass modes with such metrics lays the foundation for a BAW MEMS gyroscope with a built-in primary calibration stage.

Self-injection locking of a low-noise erbium-doped random fiber laser by a random fiber grating ring.

We demonstrate a self-injection locking (SIL) in an Er-doped random fiber laser by a high quality factor (high-Q) random fiber grating ring (RFGR) resonator, which enables a single-mode narrow-linewidth lasing with ultra-low intensity and frequency noise. The RFGR resonator includes a fiber ring with a random fiber grating to provide random feedback modes and noise suppression filters with self-adjusted peak frequency adaptable to small perturbations allowing single longitudinal mode over 7000 s with frequency jitter below 3.0 kHz. Single-mode operation is accomplished by carefully controlling phase delays and mode coupling of resonant modes between main ring and RFGR with a side-mode suppression ratio of 70 dB and narrow linewidth of 1.23 kHz. The relative intensity noise is -140 dB/Hz above 100 kHz and the frequency noise is 1 Hz/Hz1/2 above 10 kHz.

Characterization of the material and electrical properties depending on the Mg:Ag ratio as a cathode for TEOLED

In this study, Mg:Ag mixed alloy films were comprehensively analyzed as a potential transparent cathode, depending on the deposition ratio of Mg:Ag atoms. It was determined that the work function, refractive index, and film formation characteristics varied depending on the deposition ratio of Mg:Ag, and affected electro-optical characteristics such as transmittance and sheet resistance. In addition, the applicability of the proposed transparent cathode for top emitting organic light-emitting diodes (TEOLEDs) was analyzed by comprehensively considering the TEOLED performance in relation to the Mg:Ag deposition ratio. The TEOLED performance was optimized by adjusting the Mg:Ag transmittance, sheet resistance and appropriate work function, and the emission spectrum could be controlled by localized surface plasmon resonance (LSPR) and cavity mode coupling effect.

Effect of ultrathin layer of MoS2 on resonance mode coupling and hybridization of AlGaAs nanoscale Mie-resonator: a simulation study

light–matter interactions, specifically the interaction between Mie resonance modes originated from all-dielectric nano-resonators and exciton modes from the semiconducting transition metal di-chalcogenides (TMDCs) recently become an important field of study due to its application in nanophotonic devices and quantum information processing. Here, we performed finite element method (FEM) based numerical simulations on isolated Al x Ga(1-x)As (x: alloy composition) core - MoS2 ultrathin nanoshell, to study the interaction between Optical Mie resonance modes and exciton modes. The interaction between magnetic dipole (MD) modes originated from the Mie-active dielectric core and excitonic response from the thin semiconductor nano-shell takes place and appears as resonance mode coupling and hybridization in the scattering efficiency spectra. The resultant spectrum was elucidated using a semi-classical coupled mode theoretical model (CMT) and the coupling constant value was estimated, followed by the evaluation of anti-crossing spectral behavior and Rabi splitting. Furthermore, we found that all the properties of the spectrum or the resonance coupling are sensitive to the core radius, alloy composition of the core, shell thickness, and the refractive index of the surrounding medium. By systematically tailoring these parameters, one can tune the quenching dip or line width of the resonance modes. The insights from these simulations not only provided the basis for fundamental research on strong nanoscale light–matter interaction but will also be quite beneficial in fabricating high-efficiency optoelectronic and smart nanophotonic devices related to photon-exciton interactions.

Research on vibration characteristics of the longitudinal-radial composite piezoelectric ultrasonic transducer

The transducer with high power capacity and extensive radiating area is required in some industrial processes like ultrasonic sonochemistry, ultrasonic cleaning and ultrasonic defoaming and so on. In order to achieve the desired effects, the longitudinal-radial composite piezoelectric ultrasonic transducer is proposed in this paper. The transducer is composed of a longitudinal sandwich piezoelectric transducer, a stepped ultrasonic horn and a metal sphere. The longitudinal vibrations of the sandwich piezoelectric transducer and the stepped ultrasonic horn excite the radial vibration of the metal sphere so that the longitudinal-radial coupled vibration of the transducer is achieved. The equivalent circuits for a sphere in radial vibration and for the entire system in longitudinal-radial coupled vibration have been developed. To validate the proposed equivalent circuits, the vibration characteristics of the transducer are investigated using the equivalent circuit method, numerical method, and experiment. The resonance frequencies obtained from the equivalent circuit method, numerical method and experiment are 29992 Hz, 29990 Hz and 30590 Hz, respectively, and the simulated and experimental results indicate that the frequencies correspond to the designed longitudinal-radial coupled resonance mode of the transducer. The measured frequency and displacement distribution showed good agreement with the analytical and numerical calculation results.

Study of resonant mode coupling in the transverse-mode-conversion based resonator with an anti-symmetric nanobeam Bragg reflector.

The traveling-wave like Fabry-Perot (F-P) resonators based on transverse-mode-conversion have been extensively studied as on-chip filters. However, the incomplete transverse mode conversion will lead to the coupling between two degenerated resonant modes, which brings additional loss and may further induce the resonance splitting. In this paper, we take the transverse-mode-conversion based resonator with anti-symmetric nanobeam Bragg reflector as an example and study the resonant mode coupling in both the direct-coupled and side-coupled resonators. The coupled mode equations are used to model the incomplete transverse mode conversion of Bragg reflector. The resonant mode coupling can be effectively suppressed by carefully designing the phase shifter length and adding the tapered holes. The insertion loss of less than -1 dB can be achieved in the simulation using the two methods. This work is believed to benefit the design of mode-conversion based resonators with low insertion loss and non-splitting line shape.

Coupled Thin Film Hydrogenated Amorphous Silicon Microresonator Arrays

Mechanically coupled resonators can potentially improve the sensing capabilities of MEMS resonators. This work presents novel weakly and moderately coupled MEMS bridge resonators with up to four degrees-of-freedom (DOF), with thin-film hydrogenated amorphous silicon thin films as structural layers fabricated on glass substrates by surface micromachining at temperatures no greater than 175 °C. The electrostatically actuated coupled resonators exhibited flexural fundamental resonance frequencies near 1 MHz. Coupled resonance modes were simulated by Finite Element Method and characterized using optical and electrical sensing. Laser Doppler vibrometry was used to scan the coupled mode shapes and confirm resonance mode attribution. Coupled resonators with 4-DOF exhibited motional resistances ~25 times lower than single resonators (1-DOF) actuated with identical voltages. Common (in-phase) and independent (out-of-phase) actuation of the coupled resonators was implemented, resulting in distinct dynamic behaviors of the coupled array. The fabrication process presented in this work is easily scalable to higher degrees-of-freedom and to large-area substrates, which makes it a good fit for large-scale sensing applications. [2022-0161]

Controllable EIT-like mode splitting in a chiral microcavity.

Two coupled resonance modes can lead to exotic transmission spectra due to internal interference processes. Examples include electromagnetically induced transparency (EIT) in atoms and mode splitting in optics. The ability to control individual modes plays a crucial role in controlling such transmission spectra for practical applications. Here we experimentally demonstrate a controllable EIT-like mode splitting in a single microcavity using a double-port excitation. The mode splitting caused by internal coupling between two counter-propagating resonances can be effectively controlled by varying the power of the two inputs, as well as their relative phase. Moreover, the presence of asymmetric scattering in the microcavity leads to chiral behaviors in the mode splitting in the two propagating directions, manifesting itself in terms of a Fano-like resonance mode. These results may offer a compact platform for a tunable device in all-optical information processing.

Resonant mode coupling approximation for calculation of optical spectra of stacked photonic crystal slabs. Part II

We propose the further development of the resonant mode coupling approximation for the calculation of optical spectra of stacked periodic nanostructures in terms of the scattering matrix. We previously showed that given the resonant input and output vectors as well as background scattering matrices of two subsystems, one can easily calculate those for the combined system comprising two subsystems. It allows us to write a resonant approximation for the combined system and speed up the calculation significantly for typical wave scattering problems. This approach is best suitable for isolated resonances when the approximation of a constant background scattering matrix is reasonable. In this article, we aim at expanding the applicability of the resonant mode coupling approximation by utilizing more complicated approximations for the background matrices. In particular, we show that consideration of energy-dependent correction terms for the background matrices remarkably reduces the calculation error of the resonant parameters. Here we first consider a linear approximation of the background scattering matrices which is subsequently used as a base for a piecewise-linear approximation. We show that the latter allows one to keep the approximation error negligibly small with only a few sample points and to apply the resonant mode coupling approximation within almost arbitrary large energy ranges. Finally, we consider the approximation of background matrices by arbitrary matrix functions and propose a technique to derive resonant poles in this case. The methods described here could be considered an alternative approach for calculating the optical spectra of stacked systems.

Resonant mode coupling approximation for calculation of optical spectra of stacked photonic crystal slabs. Part I

We develop the resonant mode coupling approximation to calculate the optical spectra of a stack of two photonic crystal slabs in terms of the scattering matrix. The method is based on the derivation of input and output resonant vectors in each slab in terms of the Fourier modal method in the scattering matrix form. We show that using the resonant mode coupling approximation of the scattering matrices of the upper and lower slabs, one can construct the total scattering matrix of the stack. The formation of the resonant output and input vectors of the stacked system is rigorously derived by means of an effective Hamiltonian. We demonstrate that the proposed procedure dramatically decreases the computation time without sufficient loss of accuracy. We believe that the proposed technique can be a powerful tool for fast-solving inverse scattering problems using stochastic optimization methods such as genetic algorithms or machine learning.

Laser Constructing Short‐Range Disordered Metagratings for Visible Near‐Infrared Polarization‐Independent Absorption

AbstractSubwavelength‐structured metasurfaces working in visible and near‐infrared bands present a high challenge in large‐scale device fabrication. In this study, a scalable, high‐efficient, and low‐cost laser‐induced nanopatterning technique is exploited to fabricate a kind of short‐range disordered metagratings, which enables broadband polarization‐independent absorption in the visible to near‐infrared wavelength. The short‐range disorder of the laser‐induced nanogratings originates from the laser‐induced thermal effect and can be spontaneously organized during laser nanopatterning. The unique disorder can break the unidirectional characteristics of the nanogratings, which empowers the metagratings to excite coupled resonance modes. This helps to achieve near‐perfect absorption in the visible to near‐infrared band (400–1100 nm) with an excellent angular tolerance (up to 60°) and average absorptivities of 96.3% and 93.5% under the TM and TE modes, respectively. These low‐cost metagratings can potentially inspire wide applications in the fields of solar cells, sensing, and thermal emitters.

Diabolical points in coupled ridge resonators

In this paper, the resonant optical properties of two coupled ridges on the surface of a single-mode dielectric slab waveguide are theoretically investigated. The diabolical points of a structure can be achieved by near-field coupling and far-field interference of resonance modes in the ridges. The uniform unity transmission and 2 π spectral phase span are demonstrated around the diabolical points. By using the temporal coupled-mode theory, we analyze the generating conditions of diabolical points in coupled ridges and the corresponding group delay around diabolical points. The influences of optical loss and excitation beam width on the optical response at diabolical points of a structure are also discussed. Diabolical points in the waveguide system have potential applications in dispersion compensation and phase-only modulators in integrated optical circuits.

Ultra-Broadband Perfect Absorber based on Titanium Nanoarrays for Harvesting Solar Energy.

Solar energy is a clean and renewable energy source and solves today's energy and climate emergency. Near-perfect broadband solar absorbers can offer necessary technical assistance to follow this route and develop an effective solar energy-harvesting system. In this work, the metamaterial perfect absorber operating in the ultraviolet to the near-infrared spectral range was designed, consisting of a periodically aligned titanium (Ti) nanoarray coupled to an optical cavity. Through numerical simulations, the average absorption efficiency of the optimal parameter absorber can reach up to 99.84% in the 200-3000 nm broadband range. We show that the Ti pyramid's localized surface plasmon resonances, the intrinsic loss of the Ti material, and the coupling of resonance modes between two neighboring pyramids are highly responsible for this broadband perfect absorption effect. Additionally, we demonstrate that the absorber exhibits some excellent features desirable for the practical absorption and harvesting of solar energy, such as precision tolerance, polarization independence, and large angular acceptance.