high-Q Modes Research Articles

Mode Splitting of High-Q Whispering-Gallery Modes in a Microring Resonator Coated With a Fluorescent High-Refractive-Index Film

A microring resonator coated with a fluorescent high-refractive-index film provides a structure that supports whispering-gallery modes (WGMs) via the contrast in the refractive index. In the past, number, size, and refractive index of nanoparticles were analyzed by the degree of resonant mode splitting in the cavity. Compared with the small mode shift, this splitting is easier to detect and has strong anti-interference ability. However, there are several mechanisms for the mode splitting due to the different sizes of nanoparticles. The magnitudes of different mode splittings are quite different, which easily leads to confusion in spectral analysis. In this work, the mechanisms of resonant mode splitting are studied by the FDTD method combined with light field distributions and spectra. Field distributions of the symmetric mode and the asymmetric mode relative to the position of the nanoparticle for the fundamental order and second radial order were simulated. We first distinguished the splitting of the fundamental mode (~0.010 THz) due to the backscattering of the particles from the separation between the fundamental mode and the high-order mode (~1 THz) for the same particle (~100 nm). In addition, fundamental mode splitting was used to estimate the size and number of particles, and these sizes were consistent with those in the design. This work demonstrates the potential application of microring cavities based on WGMs in single particle detection.

Birefringent whispering gallery cavities designed by linear transformation optics.

It was reported that whispering gallery cavities designed by conformal transformation optics can support high-Q resonant modes with emission directionality. Intrinsically, these cavities have gradient index profiles implementing conformal mappings in physical space. In this paper, using the linear coordinate transformation, we propose another design scheme of whispering gallery cavities with (piecewise-) homogeneous, anisotropic index profile. We numerically show that so-designed cavities are also able to support high-Q whispering gallery modes with directional far-field emission patterns. We verify such characteristics by using a phase space representation (called the Poincaré Husimi function) of the intracavity wave function.

Pushing the limit of high-Q mode of a single dielectric nanocavity

High index dielectric nanostructure supports different types of resonant modes. However, it is very challenging to achieve high-Q factor in a single subwavelength dielectric nanoresonator due to non-hermtian property of the open system. Here, we present a universal approach of finding out a series of high-Q resonant modes in a single nonspherical dielectric nanocavity by exploring quasi-bound state in the continuum. Unlike conventional method relying on heavy computation (ie, frequency scanning by FDTD), our approach is built upon leaky mode engineering, through which many high-Q modes can be easily achieved by constructing avoid-crossing (or crossing) of the eigenvalue for pair leaky modes. The Q-factor can be up to 2.3*10^4 for square subwavelength nanowire (NW) (n=4), which is 64 times larger than the highest Q-factor (Q=360) reported so far in single subwavelength nanodisk. Such high-Q modes can be attributed to suppressed radiation in the corresponding eigenchannels and simultaneously quenched electric(magnetic) at momentum space. As a proof of concept, we experimentally demonstrate the emergence of the high-Q resonant modes (Q=380) in the scattering spectrum of a single silicon subwavelength nanowire.

Fano-Resonant Hybrid Metamaterial for Enhanced Nonlinear Tunability and Hysteresis Behavior.

Artificial resonant metamaterial with subwavelength localized filed is promising for advanced nonlinear photonic applications. In this article, we demonstrate enhanced nonlinear frequency-agile response and hysteresis tunability in a Fano-resonant hybrid metamaterial. A ceramic cuboid is electromagnetically coupled with metal cut-wire structure to excite the high-Q Fano-resonant mode in the dielectric/metal hybrid metamaterial. It is found that the significant nonlinear response of the ceramic cuboid can be employed for realization of tunable metamaterials by exciting its magnetic mode, and the trapped mode with an asymmetric Fano-like resonance is beneficial to achieve notable nonlinear modulation on the scattering spectrum. The nonlinear tunability of both the ceramic structure and the ceramic/metal hybrid metamaterial is promising to extend the operation band of metamaterials, providing possibility in practical applications with enhanced light-matter interactions.

Edge mode engineering for optimal ultracoherent silicon nitride membranes

Due to their high force sensitivity, mechanical resonators combining low mechanical dissipation with a small motional mass are highly demanded in fields as diverse as resonant force microscopy, mass sensing, or cavity optomechanics. “Soft-clamping” is a phononic engineering technique by which mechanical modes of highly stressed membranes or strings are localized away from lossy regions, thereby enabling ultrahigh-Q for ng-scale devices. Here, we report on parasitic modes arising from the finite size of the structure, which can significantly degrade the performance of these membranes. Through interferometric measurements and finite-element simulations, we show that these parasitic modes can hybridize with the localized modes of our structures, reducing the quality factors by up to one order of magnitude. To circumvent this problem, we engineer the spectral profile of the parasitic modes in order to avoid their overlap with the high-Q defect mode. We verify via a statistical analysis that the quality factors of devices fabricated with this modal engineering technique are consistently closer to the value predicted by dissipation dilution theory. We expect this method to find applications in a broad range of contexts such as optomechanical cooling, resonant force microscopy, swept-frequency sensing, or hybrid quantum networks.

Circular Dielectric Rod With Conformal Strip of Graphene as Tunable Terahertz Antenna: Interplay of Inverse Electromagnetic Jet, Whispering Gallery and Plasmon Effects

In this article, we consider the radiation characteristics of a THz antenna made of a circular dielectric rod decorated with conformal graphene strip and illuminated by the field of a line magnetic current. The strip has arbitrary angular size and location and its surface impedance is characterized with Kubo theory. Our mathematically accurate analysis uses a dedicated hypersingular integral equation for the current induced on the strip. Discretization of this equation is carried out by the Nystrom-type method, which has a guaranteed convergence. We study the dependences of the powers radiated and absorbed in this configuration and also the directivity of antenna emission, in wide frequency range from 0 to 10 THz. They show very interesting interplay between the broadband inverse photonic-jet effect of lens-like dielectric rod and two types of resonances: on the moderate-Q plasmon modes of graphene strip and on the extremely high-Q whispering-gallery modes of the circular rod.

Eigenmode operation of piezoelectric resonant gyroscopes

The theory of eigenmode operation of Coriolis vibratory gyroscopes and its implementation on a thin-film piezoelectric gyroscope is presented. It is shown analytically that the modal alignment of resonant gyroscopes can be achieved by applying a rotation transformation to the actuation and sensing directions regardless of the transduction mechanism. This technique is especially suitable for mode matching of piezoelectric gyroscopes, obviating the need for narrow capacitive gaps or DC polarization voltages. It can also be applied for mode matching of devices that require sophisticated electrode arrangements for modal alignment, such as electrostatic pitch and roll gyroscopes with slanted electrodes utilized for out-of-plane quadrature cancellation. Gyroscopic operation of a 3.15 MHz AlN-on-Si annulus resonator that utilizes a pair of high-Q degenerate in-plane vibration modes is demonstrated. Modal alignment of the piezoelectric gyroscope is accomplished through virtual alignment of the excitation and readout electrodes to the natural direction of vibration mode shapes in the presence of fabrication nonidealities. Controlled displacement feedback of the gyroscope drive signal is implemented to achieve frequency matching of the two gyroscopic modes. The piezoelectric gyroscope shows a mode-matched operation bandwidth of ~250 Hz, which is one of the largest open-loop bandwidth values reported for a mode-matched MEMS gyroscope, a small motional resistance of ~1300 Ω owing to efficient piezoelectric transduction, and a scale factor of 1.57 nA/°/s for operation at atmospheric pressure, which greatly relaxes packaging requirements. Eigenmode operation results in an ~35 dB reduction in the quadrature error at the resonance frequency. The measured angle random walk of the device is 0.86°/√h with a bias instability of 125°/h limited by the excess noise of the discrete electronics.

Resonator-assisted single-molecule quantum state detection

We propose a state-sensitive scheme to optically detect a single molecule without a closed transition, through strong coupling to a high-Q whispering-gallery mode high-Q resonator. A background-free signal can be obtained by detecting a molecule-induced transparency in a photon bus waveguide that is critically coupled to the resonator, with a suppressed depumping rate to other molecular states by the cooperativity parameter $C$. We numerically calculate the dynamics of the molecule-resonator coupled system using Lindblad master equations, and develop analytical solutions through the evolution of quasi-steady states in the weak-driving regime. Using Rb$_2$ triplet ground state molecules as an example, we show that high fidelity state readout can be achieved using realistic resonator parameters. We further discuss the case of multiple molecules collectively coupled to a resonator, demonstrating near-unity detection fidelity and negligible population loss.

Lithium niobate photonic-crystal electro-optic modulator

Modern advanced photonic integrated circuits require dense integration of high-speed electro-optic functional elements on a compact chip that consumes only moderate power. Energy efficiency, operation speed, and device dimension are thus crucial metrics underlying almost all current developments of photonic signal processing units. Recently, thin-film lithium niobate (LN) emerges as a promising platform for photonic integrated circuits. Here, we make an important step towards miniaturizing functional components on this platform, reporting high-speed LN electro-optic modulators, based upon photonic crystal nanobeam resonators. The devices exhibit a significant tuning efficiency up to 1.98 GHz V−1, a broad modulation bandwidth of 17.5 GHz, while with a tiny electro-optic modal volume of only 0.58 μm3. The modulators enable efficient electro-optic driving of high-Q photonic cavity modes in both adiabatic and non-adiabatic regimes, and allow us to achieve electro-optic switching at 11 Gb s−1 with a bit-switching energy as low as 22 fJ. The demonstration of energy efficient and high-speed electro-optic modulation at the wavelength scale paves a crucial foundation for realizing large-scale LN photonic integrated circuits that are of immense importance for broad applications in data communication, microwave photonics, and quantum photonics.

High-Q dark hyperbolic phonon-polaritons in hexagonal boron nitride nanostructures.

The anisotropy of hexagonal boron nitride (hBN) gives rise to hyperbolic phonon-polaritons (HPhPs), notable for their volumetric frequency-dependent propagation and strong confinement. For frustum (truncated nanocone) structures, theory predicts five, high-order HPhPs, sets, but only one set was observed previously with far-field reflectance and scattering-type scanning near-field optical microscopy. In contrast, the photothermal induced resonance (PTIR) technique has recently permitted sampling of the full HPhP dispersion and observing such elusive predicted modes; however, the mechanism underlying PTIR sensitivity to these weakly-scattering modes, while critical to their understanding, has not yet been clarified. Here, by comparing conventional contact- and newly developed tapping-mode PTIR, we show that the PTIR sensitivity to those weakly-scattering, high-Q (up to ≈280) modes is, contrary to a previous hypothesis, unrelated to the probe operation (contact or tapping) and is instead linked to PTIR ability to detect tip-launched dark, volumetrically-confined polaritons, rather than nanostructure-launched HPhPs modes observed by other techniques. Furthermore, we show that in contrast with plasmons and surface phonon-polaritons, whose Q-factors and optical cross-sections are typically degraded by the proximity of other nanostructures, the high-Q HPhP resonances are preserved even in high-density hBN frustum arrays, which is useful in sensing and quantum emission applications.

Air-Mode Photonic Crystal Micro-Ring Resonator With Enhanced Quality Factor for Refractive Index Sensing

Intense light-matter interaction is critical for enabling high-sensitivity refractive index sensors, high-efficiency light sources, and high-performance nonlinear devices. A bowtie-hole photonic crystal micro-ring resonator is proposed and numerically demonstrated to enhance the light-matter interaction effectively through achieving strong spatial and temporal light confinement, simultaneously. Through engineering the photonic crystal unit cell, the bowtie-hole photonic crystal is proved to possess the ability to localize light with a highly tight spatial confinement. By optimizing the bowtie-hole radius and the bowtie angle, the Q factor of the air mode in the resonator is improved to 1.42 × 105, which denotes an excellent temporal confinement. With the advantages addressed, the refractive index sensing based on the high-Q air mode in the bowtie-hole photonic crystal micro-ring resonator is investigated. For the air mode in the bowtie-hole photonic crystal micro-ring resonator, the Q factor is comparable with that in the reported research, meanwhile the sensing sensitivity has a 2-fold enhancement.

Quality factor boosting of bulk acoustic wave resonators based on a two dimensional array of high-Q resonant tanks

An innovative resonance excitation methodology of a two-dimensional array of uniformly patterned structure into extremely high-quality factor (Q) mode using engineered frequency-matched thin piezoelectric film on substrate (TPoS) transducer is demonstrated in this Letter. The passive Q-enhancement can be attained lithographically without adding to the power requirement of the system. A 500 nm thick piezoelectric material facilitates the inter-electrical-mechanical domain coupling, and the displacement generated at the transducer periphery drives the patterned 10 μm thick single crystal silicon plate into various resonant modes. Using this topology, unexplored modes can be generated as well by the forced vibration feature of the interacting resonant tanks. Utilizing a multiport port excitation scheme, multiple modes are explored, while the primary focus is maintained on the Lamé mode of the array. In vacuum for an operational frequency of 58.56 MHz, an exceptionally high-quality factor of 58 276 is attained with a motional resistance of 1250 Ω and this is the record-high figure of merit (f × Q = 3.4 × 1012) attained for aluminum nitride-based TPoS based resonators. This successful demonstration of the energy coupling scheme paves a path for the study of existing and investigation of unexplored high-Q modes spanning over a wide range of frequency, making these devices a potential candidate for radio frequency front-end applications. The CMOS compatibility of aluminum nitride makes it an attractive solution for achieving low phase noise monolithically integrated CMOS–microelectromechanical systems oscillators.

All-Optical Switching and Multiple Logic Gates Based on Hybrid Square–Rectangular Laser

All-optical switching and multiple logic gates are experimentally demonstrated using AlGaInAs/InP hybrid square-rectangular lasers. Controllable bistability induced by saturable absorption in the square microcavity is achieved around the threshold with the square cavity in an open circuit state. Based on the bistability, all-optical switching operation is realized by injecting set/reset optical signals with the wavelengths around the lasing mode of 1529.9 nm and another high-Q mode at 1560 nm. Furthermore, mode competitions between three resonant modes around 1541 nm with an interval of 1.2 nm are used to realize all-optical multiple logic gates of NOT, NOR, and NAND functions under low-power optical pulses, with injecting currents to the square and rectangular sections. The static extinction ratio of 34, 40 and 24 dB are obtained for NOT, NOR, and NAND gates, respectively, and the dynamic extinction ratio over 10 dB of NOT function at 15 Gb/s, NOR and NAND functions at 2 Gb/s are demonstrated. With the merits of multifunctional operations, large optical injection wavelength tolerant ranges, low power consumption, small footprint and suitability for on-chip integration, the device offers a potential solution for all-optical signal processing in photonic integration circuits.

Plasmonic sensors with an ultra-high figure of merit

We propose and numerically demonstrate a high-quality hybridized resonant platform, composed of a one-dimensional metal grating and a dielectric cavity. Under a moderate oblique illumination, an ultra-high spectral quality (Q) factor of 1375 is achieved, which shows orders of magnitude larger than that of the system under normal excitation. The high-Q mode results from the strong coupling effect between the surface plasmon polariton of the metal grating and the photonic mode of the dielectric cavity. Based on this hybridized plasmonic resonator, a high-performance sensing scheme with the spectral sensitivity (S) up to 800 nm/RIU (refractive index unit, RIU) is further introduced. Moreover, the figure of merit reaches 1337, indicating a new record for both the localized or propagating surface plasmons based sensors. These features could find applications in sensing and detecting devices, plasmonic switches, and light flow modulators.

Analogue of electromagnetically induced transparency with high-Q factor in metal-dielectric metamaterials based on bright-bright mode coupling.

Based on bright-bright mode coupling, we numerically demonstrated the analogue of electromagnetically induced transparency (EIT) with a high quality factor (Q) in a stacked metal-dielectric metamaterial (MM) in the near-infrared regime. The optical coupling between a high-Q toroidal dipole mode supported by a silicon rod array and a low-Q dipole mode supported by a silver strip array was investigated from the near-field to the far-field regimes. We realized and significantly enhanced the long-range coupling between the two resonance modes through the MM-induced Fabry-Pérot (FP) effect. EIT with a Q factor greater than 1×104 could be achieved even when the two resonant structures were approximately a wavelength apart. These findings may open new avenues for realizing high-Q EIT, which is useful for photonic devices and biosensing applications. The proposed method can be extended to microwaves and terahertz waves.

Plasmonic Sensing on Symmetric Nanohole Arrays Supporting High-Q Hybrid Modes and Reflection Geometry.

Refractometric sensors utilizing surface plasmon resonance (SPR) should satisfy a series of performance metrics, bulk sensitivity, thin-film sensitivity, refractive-index resolution, and high-Q-factor resonance, as well as practical requirements such as manufacturability and the ability to separate optical and fluidic paths via reflection-mode sensing. While many geometries such as nanohole, nanoslit, and nanoparticles have been employed, it is nontrivial to engineer nanostructures to satisfy all of the aforementioned requirements. We combine gold nanohole arrays with a water-index-matched Cytop film to demonstrate reflection-mode, high-Q-factor (Qexp = 143) symmetric plasmonic sensor architecture. Using template stripping with a Cytop film, we can replicate a large number of index-symmetric nanohole arrays, which support sharp plasmonic resonances that can be probed by light reflected from their backside with a high extinction amplitude. The reflection geometry separates the optical and microfluidic paths without sacrificing sensor performance as is the case of standard (index-asymmetric) nanohole arrays. Furthermore, plasmon hybridization caused by the array refractive-index symmetry enables dual-mode detection that allows distinction of refractive-index changes occurring at different distances from the surface, making it possible to identify SPR response from differently sized particles or to distinguish binding events near the surface from bulk index changes. Due to the unique combination of a dual-mode reflection-configuration sensing, high-Q plasmonic modes, and template-stripping nanofabrication, this platform can extend the utility of nanohole SPR for sensing applications involving biomolecules, polymers, nanovesicles, and biomembranes.

Features of the Simultaneous Generation of Low-Q and High-Q Modes in Heterolasers Based on Quantum Dots with a Long Incoherent Relaxation Time of Optical Dipole Oscillations

The features of the multimode steady-state generation of superradiant heterolasers, in which quantum dots with a long incoherent relaxation time serve as an active medium, while low-Q combined Fabry–Perot cavities with a distributed feedback of counter-propagating waves serve as cavities, are studied. It is shown that due to the quantum-coherent dynamics of optical dipole oscillations and population inversion of the working levels in a quantum-dot ensemble with strong inhomogeneous broadening of the spectral line, the simultaneous lasing of modes with various degrees of phasing and/or correlation and with qualitatively different dynamic behavior, notably, quasi-stationary, metastable, self-modulation, pulse-periodic, and quasi-chaotic is possible.

Giant and highly reflective Goos-Hänchen shift in a metal-dielectric multilayer Fano structure.

We experimentally demonstrated a giant Goos-Hänchen (GH) shift in a metal-dielectric multilayer Fano structure. The observed GH shift was 0.176 mm, which corresponded to (GH shift/λ) = 493, where λ is the incident wavelength. A unique feature of this giant GH shift was that it occurred without attenuation, i.e., reflectivity ∼1, due to Fano interference between surface plasmon polariton and high-Q dielectric waveguide mode. The Q-value is determined by the coupling loss. Therefore, we can enhance the GH shift to an arbitrarily large value by controlling the coupling strength. The unique feature whereby the giant GH shift occurs without attenuation has great potential for real-world applications, such as optical switching, optical filters, and sensors, where the reduction of reflected beam intensity is currently a major drawback.

Ultranarrow filter based on Fano resonance in a single cylindrical microresonator for single-longitudinal-mode fiber lasers.

A novel single-longitudinal-mode (SLM) fiber laser with a single cylindrical microresonator (CMR) as the filter is proposed. The CMR has a nanoscale variation in radius, in which high-Q localized modes interfere with continuum radiative modes, generating a peak-like transmission spectrum based on the Fano resonance effect. The peak linewidth is sufficiently small to select only one SLM oscillation in a fiber cavity. In an Er-doped fiber laser coupled with the CMR, we achieve a stable SLM laser operation by optimizing the coupling location of the tapered fiber. Stable lasing at 1568 nm with a 3-dB linewidth of ∼15 kHz is obtained. The laser slope efficiency reduction introduced by the CMR for SLM lasing is limited to 1.4%. This scheme can be a competitive approach for the development of compact, highly efficient, and cost-effective SLM fiber lasers.

Analysis of terahertz wave nonlinear reflection by an array of double silicon elements placed on a metal substrate

We demonstrate simulation results of nonlinear reflection of intensive terahertz radiation from a silicon-on-metal metasurface involving an excitation of a high-Q trapped mode resonance. Conditions are presented to observe the effects of bistability and hysteresis of nonlinear reflection.