Ultrahigh Quality Factor Research Articles

Symmetry-protected dual quasi-bound states in the continuum with high tunability in metasurface

We propose a split ring metasurface supporting simultaneously symmetry-protected dual bound states in the continuum (BIC) under normal excitation. Such two BICs can be classified as symmetry protected toroidal dipole (TD) BIC and magnetic dipole (MD) BIC, and the asymmetrically distributed holes can convert these two BICs into TD quasi-BIC and MD quasi-BIC with ultrahigh Q-factors, respectively. Two eigenmodes reveal the existence of symmetry protected BICs with an infinite lifetime and exhibit the intrinsic TD and MD characters by multipole analysis. In addition, it is shown that the two quasi-BICs can be switched on and off at orthogonal incident polarization orientations simultaneously. Further, such two quasi-BICs can be efficiently modulated by varying the Fermi energy of the graphene covered on the dielectric metasurface, indicating that the TD and MD quasi-BICs in the proposed metasurface have potential applications in designing novel quasi-BIC-based optical modulator and switching.

Alumina coating for dispersion management in ultra-high Q microresonators

Silica optical microspheres often exhibit ultra-high quality factors, yet their group velocity dispersion, which is crucial for nonlinear optics applications, can only be coarsely tuned. We experimentally demonstrate that group-velocity dispersion of a silica microsphere can be engineered by coating it with conformal nanometric layers of alumina yet preserving its ultra-high optical quality factors (∼107) at telecom wavelengths. Using the atomic layer deposition technique for the dielectric coating, which ensures nm-level thickness control, we not only achieve a fine dispersion tailoring but also maintain a low surface roughness and material absorption to ensure a low optical loss. Numerical simulations supporting our experimental results show that the alumina layer thickness is a promising technique for precise tuning of group-velocity dispersion. As an application, we demonstrate the generation of Kerr optical frequency combs, showing that the alumina coatings can also sustain the high optical intensities necessary for nonlinear optical phenomena.

High-Resolution Chipless Tag RF Sensor

In this article, a new coupled structure based on microwave planar resonators is proposed to demonstrate the capability of ultrahigh quality factor performance using a chipless passive resonator. The platform is based on two passive split-ring resonators (SRRs), one as a reader, and the other one as the tag. The reader resonator is designed to operate at 2.6-GHz resonance frequency and is coupled to an active feedback loop with a microwave amplifier to compensate for the resonator’s loss and increase the loaded quality factor of the response. The bandwidth of the feedback amplifier is modified such that the amplifier’s gain is higher at the resonance frequency of the tag than that of the reader by adding a capacitor between emitter and collector of the bipolar junction transistor (BJT) amplifier. The tag is designed at 1.6 GHz and is located at a 2.5-mm vertical distance from the reader. As a result, the overall quality factor of about 75 000 is achieved for the tag performing the sensing. The presented technique provides a great practical solution for highly sensitive, noninvasive, and real-time sensing applications. The proposed sensing tag is integrated with a microfluidic chip to highlight its capability for small volume sensing and lab-on-a-chip applications. The sensitivity potential of the design is verified by detecting the concentration of acetone in deionized water. The average sensitivity of the presented sensor is more than 35 kHz/(1% of acetone concentration variation), which is offering extremely high sensitivity of the structure considering the very small volume of the exposed material under the test and the distance between the sensor and the sample.

Nonlinear radiation oscillator theory for symmetric and anti-symmetric damping graphene coupled metamaterials

We develop a novel nonlinear radiation oscillator theory, which can describe nonlinear optical responses in symmetric and anti-symmetric damping graphene coupled metamaterials. An ultra-high quality factor of induced transparency can be realized in the proposed model. The results show that the spectral response and quality factor can be effectively tuned by the resonant detuning, the phase difference, the damping factor as well as the field intensity of the pump light. Moreover, the quality factor of induced transparency is increased by more than 60 times when the field intensity of the pump light and the damping factor are changed. Especially, the single induced transparency window splits into double induced transparency windows as the damping factors of dark mode and bright mode show anti-symmetric damping case. These results may pave the way for designing the high performance of nano-optical devices in terahertz.

Cascaded Stokes and anti-Stokes laser based on an optical resonator with a self-assembled organic monolayer.

Due to their high circulating intensities, ultra-high quality factor dielectric whispering gallery mode resonators have enabled the development of low threshold Raman microlasers. Subsequently, other Raman-related phenomena, such as cascaded stimulated Raman scattering (CSRS) and stimulated anti-Stokes Raman scattering (SARS), were observed. While low threshold frequency conversion and generation have clear applications, CSRS and SARS have been limited by the low Raman gain. In this work, the surface of a silica resonator is modified with an organic monolayer, increasing the Raman gain. Up to four orders of CSRS are observed with sub-milliwatt (mW) input power, and the SARS efficiency is improved by three orders of magnitude compared to previous studies with hybrid resonators.

A Tunable High-Q Microwave Detector for On-Column Capillary Liquid Chromatography

We report a novel approach to ultrahigh quality factor ( $Q$ ) in the split-ring resonator with an active feedback loop, which utilizes a phase shifter to tune equivalent negative resistance for accurate loss compensation. Based on the proposed technique, a microwave detector is designed for on-column liquid chromatography. With lossy solvents loaded, the measured $Q$ is as high as $4.0\times 10^{6}$ at ~ 3.89 GHz. We demonstrate the highly sensitive and contactless measurement of glucose–water solution in a capillary tube under stop–flow and plug–unplug modes, respectively. With the stop–flow mode, different concentrations between 40 and 150 mg/dL are measured, which covers the normal blood-glucose levels. The minimum detectable quantity of glucose is ~18.8 ng with an effective sample volume of ~47 nL. The plug–unplug mode provides fast and flexible sampling and analysis. Further investigations are required for quantification of material-under-test permittivity and promotion of sensitivity.

A novel method to fabricate on-chip ultra-high-Q microtoroid resonators

Fabricating an optical microtoroid with ultra-high quality factor is an indispensable procedure both for scientific research and engineering. Here, HF/HNO3 wet etching is specially tailored to form silicon pillars for ultra-high quality factor microtoroids instead of xenon difluoride (XeF2) or reactive ion etching (RIE) methods. This is a low-cost, easily maintained and time-saving method. The method uses a very simple etching device and is robust to temperature variations and humidity changes at room temperature(20–25 °C). Anisotropy in isotropic HF/HNO3 etching, which may limit the quality factor, is minimized in the experiment. Quality factor dependence on silicon orientation is experimentally investigated and theoretically explained with group theory, which shows that [111] oriented silicon wafers are superior to [100] and [110] in the wet etching method. The maximum quality factor observed at 1550 nm waveband in the experiment is 1.05 × 108. We believe this technique will have great influence both on laboratory and future production use.

Polarization-independent absorption enhancement in graphene for light incidence in conical mounting

A terahertz graphene tunable absorber consisting of monolayer graphene on a dielectric grating and a substrate separated by a waveguide layer is designed and investigated. The absorber exhibits polarization-independent absorption enhancement at resonance for fully conical light incidence, which is attributed to the simultaneous excitation of two identical guided modes in the waveguide layer. Moreover, the absorber possesses an ultranarrow absorption profile with an ultrahigh quality factor. To intuitively confirm the potential physical mechanism of such phenomena, the electromagnetic field distributions for both polarized light are described. Finally, the influences of Fermi level and relaxation time on the absorption properties are investigated, which present the flexible tunability of the proposed device. It is believed that the conclusions will enable the development of optoelectronic devices by simply combining graphene with a guided-mode resonance structure.

Ultrahigh-Q and Polarization-Independent Terahertz Metamaterial Perfect Absorber

Ensuring a good trade-off between high-quality factor (Q-factor) and polarization independency is a key challenge for designing practicable terahertz metamaterial devices. We propose a symmetric composite aluminum-structured metamaterial absorber to achieve high Q-factor beyond 80 and near-unity absorbance of arbitrary polarization waves in the terahertz regime. Ultrahigh Q-factor reaches 84, and polarization-independent absorption is as high as 99% for resonant frequency tuning from 7.58 to 8.97 THz, covering 14% of the standard THz gap. The geometric effect of the symmetric sublattice on resonant frequency tuning is analyzed. The large Q-factor and strong absorption by oblique incidence is discussed. Designed high-Q metamaterial perfect absorber has various applications, including terahertz hyperspectral imaging, filtering, and sensing.

Perovskite nanowire lasers on low-refractive-index conductive substrate for high-Q and low-threshold operation

Abstract Over the last five years, inorganic lead halide perovskite nanowires have emerged as prospective candidates to supersede standard semiconductor analogs in advanced photonic designs and optoelectronic devices. In particular, CsPbX3 (X = Cl, Br, I) perovskite materials have great advantages over conventional semiconductors such as defect tolerance, highly efficient luminescence, and the ability to form regularly shaped nano- and microcavities from solution via fast crystallization. However, on the way of electrically pumped lasing, the perovskite nanowires grown on transparent conductive substrates usually suffer from strong undesirable light leakage increasing their threshold of lasing. Here, we report on the integration of CsPbBr3 nanowires with nanostructured indium tin oxide substrates possessing near-unity effective refractive index and high conductivity by using a simple wet chemical approach. Surface passivation of the substrates is found out to govern the regularity of the perovskite resonators’ shape. The nanowires show room-temperature lasing with ultrahigh quality factors (up to 7860) which are up to four times higher than that of similar structures on a flat indium tin oxide layer, resulting in more than twofold reduction of the lasing threshold for the nanostructured substrate. Numerical modeling of eigenmodes of the nanowires confirms the key role of low-refractive-index substrate for improved light confinement in the Fabry–Pérot cavity which results in superior laser performance.

Ultra-High-Q Gallium Nitride SAW Resonators for Applications With Extreme Temperature Swings

In this paper, we present ultra-high quality factor ( ${Q}$ ) SAW resonators fabricated on aluminum gallium nitride on gallium nitride on silicon (AlGaN/GaN/Si) heterostructures with ${Q}$ exceeding 6000 at room temperature. We characterize their temperature response in a broad range of temperature (−196°C to 500°C or 77 K to 773 K). The effect of doping on the ${Q}$ and temperature coefficient of frequency (TCF) of the GaN-based SAW resonators is analyzed, for the first time, using un-intentionally doped GaN (UID-GaN), carbon doped GaN (C-doped), and Si doped GaN. The ${Q}$ value for UID-GaN and C-doped GaN is similar and decreases from 2000 to 1000 as the temperature is increased from 77 K to 773 K. The ${Q}$ value of Si-doped GaN is higher than UID and C-doped GaN by a factor of 3 (6622 at 77 K) and decreases with increase of temperature. This value of ${Q}$ at 1.8 GHz is the highest reported amongst aluminum nitride (AlN) or GaN-based SAW resonators. For extreme high temperatures (≥573 K) the TCF value is half the TCF value of UID and C-doped GaN, which shows the possibility of engineering the TCF by tuning the doping concentration. For low temperatures (≤150 K) C-doped and UID-GaN show a turn over point in TCF curve which shows their promise for cold clocks. [2020-0163]

Symmetry-broken silicon disk array as an efficient terahertz switch working with ultra-low optical pump power**Project supported by the National Key R&D Program of China (Grant No. 2017YFA0701005) and the National Natural Science Foundation of China (Grant Nos. 11974221, 91750201, 61927813, and 61775229). Z. Han also acknowledges the support from the Taishan Scholar Program of Shandong Province, China (Grant No. tsqn201909079)

The advancement of terahertz technology in recent years and its applications in various fields lead to an urgent need for functional terahertz components, among which a terahertz switch is one example of the most importance because it provides an effective interface between terahertz signals and information in another physical quantity. To date many types of terahertz switches have been investigated mainly in the form of metamaterials made from metallic structures and optically-active medium. However, these reported terahertz switches usually suffer from an inferior performance, e.g., requiring a high pump laser power density due to a low quality factor of the metallic metamaterial resonances. In this paper, we report and numerically investigate a symmetry-broken silicon disk based terahertz resonator array which exhibits one resonance with ultrahigh quality factor for normal incidence of the terahertz radiations. This resonance, which can never be excited for regular circular Si disks, can help to realize a superior terahertz switch with which only an ultra-low optical pump power density is required to modify the free carrier concentration in Si and its refractive index in the terahertz band. Our findings demonstrate that to realize a high terahertz transmittance change from 0 to above 50%, the required optical pump power density is more than 3 orders of magnitude smaller than that required for a split-ring resonator (SRR) based terahertz switch reported in the literature.

Dynamic Focusing with High-Quality-Factor Metalenses.

Metasurface lenses provide an ultrathin platform in which to focus light, but weak light-matter interactions limit their dynamic tunability. Here we design submicron-thick, ultrahigh quality factor (high-Q) metalenses that enable dynamic modulation of the focal length and intensity. Using full-field simulations, we show that quality factors exceeding 5000 can be generated by including subtle, periodic perturbations within the constituent Si nanoantennas. Such high-Q resonances enable lens modulation based on the nonlinear Kerr effect, with focal lengths varying from 4 to 6.5 μm and focal intensities decreasing by half as input intensity increases from 0.1 to 1 mW/μm2. We also show how multiple high-Q resonances can be embedded in the lens response through judicious placement of the perturbations. Our high-Q lens design, with quality factors 2 orders of magnitude higher than existing lens designs, provides a foundation for reconfigurable, multiplexed, and hyperspectral metasurface imaging platforms.

Coherent oscillations of a levitated birefringent microsphere in vacuum driven by nonconservative rotation-translation coupling.

We demonstrate an effect whereby stochastic, thermal fluctuations combine with nonconservative optical forces to break detailed balance and produce increasingly coherent, apparently deterministic motion for a vacuum-trapped particle. The particle is birefringent and held in a linearly polarized Gaussian optical trap. It undergoes oscillations that grow rapidly in amplitude as the air pressure is reduced, seemingly in contradiction to the equipartition of energy. This behavior is reproduced in direct simulations and captured in a simplified analytical model, showing that the underlying mechanism involves nonsymmetric coupling between rotational and translational degrees of freedom. When parametrically driven, these self-sustained oscillators exhibit an ultranarrow linewidth of 2.2 μHz and an ultrahigh mechanical quality factor in excess of 2 × 108 at room temperature. Last, nonequilibrium motion is seen to be a generic feature of optical vacuum traps, arising for any system with symmetry lower than that of a perfect isotropic microsphere in a Gaussian trap.

Load-induced dynamical transitions at graphene interfaces

The structural superlubricity (SSL), a state of near-zero friction between two contacted solid surfaces, has been attracting rapidly increasing research interest since it was realized in microscale graphite in 2012. An obvious question concerns the implications of SSL for micro- and nanoscale devices such as actuators. The simplest actuators are based on the application of a normal load; here we show that this leads to remarkable dynamical phenomena in microscale graphite mesas. Under an increasing normal load, we observe mechanical instabilities leading to dynamical states, the first where the loaded mesa suddenly ejects a thin flake and the second characterized by peculiar oscillations, during which a flake repeatedly pops out of the mesa and retracts back. The measured ejection speeds are extraordinarily high (maximum of 294 m/s), and correspond to ultrahigh accelerations (maximum of 1.1×1010 m/s2). These observations are rationalized using a simple model, which takes into account SSL of graphite contacts and sample microstructure and considers a competition between the elastic and interfacial energies that defines the dynamical phase diagram of the system. Analyzing the observed flake ejection and oscillations, we conclude that our system exhibits a high speed in SSL, a low friction coefficient of 3.6×10-6, and a high quality factor of 1.3×107 compared with what has been reported in literature. Our experimental discoveries and theoretical findings suggest a route for development of SSL-based devices such as high-frequency oscillators with ultrahigh quality factors and optomechanical switches, where retractable or oscillating mirrors are required.

Silicon‐Waveguide‐Integrated High‐Quality Metagrating Supporting Bound State in the Continuum

AbstractThe photonic bound state in the continuum (BIC) is a spatially bounded eigen state that can be realized in the form of a supercavity mode with an ultrahigh quality factor. The high‐quality supercavity resonance can be supported by photonic crystal slabs, asymmetric metasurfaces, and high‐contrast gratings. However, these schemes all suffer from bulky device size and complex experimental setup. Herein, a silicon‐waveguide‐integrated high‐quality metagrating is proposed and demonstrated as a new platform to manipulate the supercavity mode. The shallow‐ridge metagrating is directly embedded in the silicon slab waveguide, so the input slab mode can be coupled into the resonant ridge mode, which can be further transformed into the supercavity mode by utilizing the intra‐waveguide Fabry–Perot interference. Such an effect has been experimentally verified by the fabricated devices. The metagrating operating near the supercavity regime is also experimentally realized with a high quality factor ≈5200. As an application, such high‐quality metagrating is exploited to realize the temperature sensing with a high temperature sensitivity ≈77 pm K−1. The proposed integrated metagrating provides a novel approach to harness the BIC, paving ways for a new class of BIC‐based nanophotonic devices with high performance, chip‐scale footprint, and long‐term stability.

Packaged Microbubble Resonator for Versatile Optical Sensing

Microbubble whispering-gallery resonator with the intrinsic microfluidic channel is a promising platform for biochemical sensing due to their ultrahigh quality ( Q ) factor and ultralow detection limit. In this work, we realize a packaged microbubble resonator (PMBR) of ultrahigh- Q factor ( $\sim\!3.52\times 10^6$ ) in a moisture curable polymer for practical applications. The long-term stability and thermal response of a PMBR are measured. Further, we demonstrate the dynamic hydrogel gelation transition from hydrophilic to hydrophobic state by detecting the resonant wavelength shift and mode broadening in a filled PMBR simultaneously. The refractive index changes of hydrogel during the transition is explicitly modelled. Our work shows the versatility of PMBR for practical optical sensing and may open a new route to microfluidic dynamic reaction.

An ultrahigh Q-factor dual-band terahertz perfect absorber with a dielectric grating slit waveguide for sensing

The terahertz (THz) perfect absorber with ultra-narrow bandwidth at near-unity absorbance has been a challenging issue due to mutual restrictions of elemental requirements in the design. Here, we propose an extremely sharp dual-band THz perfect absorber with an ultrahigh Q-factor and near-unity absorbance by incorporating a Fabry–Perot micro-cavity into a dielectric grating slit waveguide. Full-wave simulation demonstrates that the proposed structure not only achieves ultra-narrow band perfect absorption with an ultrahigh Q-factor of 4600 in the THz regime, but it also shows extraordinary sensing performance with a record figure of merit (FOM) of 2415 at THz frequencies. The optimized guided mode resonance leads to the ultrahigh Q-factor through suppression of the absorption and radiation losses. The enhanced interaction between the cavity localized fields and the analytes significantly improves the THz sensing capability. Our design approach shows a promising future in simple and inexpensive THz biochemical sensors.

Real-time monitoring of hydrogel phase transition in an ultrahigh Q microbubble resonator

The ability to sense dynamic biochemical reactions and material processes is particularly crucial for a wide range of applications, such as early-stage disease diagnosis and biomedicine development. Optical microcavities-based label-free biosensors are renowned for ultrahigh sensitivities, and the detection limit has reached a single nanoparticle/molecule level. In particular, a microbubble resonator combined with an ultrahigh quality factor ( Q ) and inherent microfluidic channel is an intriguing platform for optical biosensing in an aqueous environment. In this work, an ultrahigh Q microbubble resonator-based sensor is used to characterize dynamic phase transition of a thermosensitive hydrogel. Experimentally, by monitoring resonance wavelength shift and linewidth broadening, we (for the first time to our knowledge) reveal that the refractive index is increased and light scattering is enhanced simultaneously during the hydrogel hydrophobic transition process. The platform demonstrated here paves the way to microfluidical biochemical dynamic detection and can be further adapted to investigating single-molecule kinetics.

Controllable coupling between an ultra-high-Q microtoroid cavity and a graphene monolayer for optical filtering and switching applications.

Whispering-gallery-mode optical microresonators have found impactful applications in various areas due to their remarkable properties such as ultra-high quality factor (Q-factor), small mode volume, and strong evanescent field. Among these applications, controllable tuning of the optical Q-factor is vital for on-chip optical modulation and various opto-electronic devices. Here, we report an experimental demonstration with a hybrid structure formed by an ultra-high-Q microtoroid cavity and a graphene monolayer. Thanks to the strong interaction of the evanescent wave with the graphene, the structure allows the Q-factor to be controllably varied in the range of 3.9 × 105 ∼ 6.2 × 107 by engineering optical absorption via changing the gap distance in between. At the same time, a resonant wavelength shift of 32 pm was also observed. Besides, the scheme enables us to approach the critical coupling with a coupling depth of 99.6%. As potential applications in integrated opto-electronic devices, we further use the system to realize a tunable optical filter with tunable bandwidth from 116.5 MHz to 2.2 GHz as well as an optical switch with a maximal extinction ratio of 31 dB and response time of 21 ms.