high-Q Resonator Research Articles

Broadband infinite-Q plasmons enable intense Smith–Purcell radiation

With the rapid development of nanophotonics for enhancing free-electron radiation, bound states in the continuum (BICs) have emerged as a promising approach for emitting intense Smith–Purcell radiation (SPR) with enhanced intensity. However, current BIC-based methods are limited to single-frequency operation, thereby restricting their applications requiring spectral and angular tunability, such as particle detectors and light sources. To overcome this limitation, this work proposes an approach for constructing plasmonic BICs over a broad spectral range in symmetry-broken systems. By leveraging the high-Q resonances near the BICs, we achieve intense SPR with broadband tunability, potentially improving the radiation intensity by six orders compared to traditional methods. Experimentally, we validate the construction of BIC using plasmonic antennas and achieve broadband demonstration. Our proposed concept can be extended to other plasmonic or guided-wave systems, paving the way toward compact and efficient free-electron sources in hard-to-reach frequency regimes.

Switchable high-Q electromagnetically induced transparency based on the Ge2Sb2Te5 nanodisk dimers

Bound states in the continuum (BICs) based on metasurfaces have gained significant attention recently to enhance the strength of light-matter interaction. However, most BIC metasurfaces possess fixed optical properties once they have been fabricated. In this study, we introduce the concept of BICs to the phase-change metasurfaces composed of Ge2Sb2Te5 nanodisk dimers. The Ge2Sb2Te5 nanodisk dimers can support a low-Q transverse magnetic dipole resonance, a high-Q toroidal dipole resonance and a high-Q longitudinal magnetic dipole resonance. By adjusting geometric parameter, the high-Q longitudinal magnetic dipole resonance can be converted into a BIC mode. Notably, when Ge2Sb2Te5 is in the amorphous phase, high-Q electromagnetically induced transparency (EIT)-like resonances can be achieved due to the interaction between a low-Q transverse magnetic dipole mode and either a high-Q toroidal dipole mode or a high-Q longitudinal magnetic dipole mode. However, the EIT resonances are switched off when Ge2Sb2Te5 is transformed into the crystalline phase. Furthermore, the high-Q EIT resonances enable the realization of an ultrahigh group refractive index, and can be tuned through the phase transition of Ge2Sb2Te5. The switchable high-Q EIT resonances hold potential applications in slow-light devices, optical modulators, and biosensors.

Pushing photon-pair generation rate in microresonators by Q factor manipulation.

Photon pairs generated by employing spontaneous nonlinear effects in microresonators are critically essential for integrated optical quantum information technologies, such as quantum computation and quantum cryptography. Microresonators featuring high-quality (Q) factors can offer simple yet power-efficient means to generate photon pairs, thanks to the intracavity field enhancement. In microresonators, it is known that the photon-pair generation rate (PGR) is roughly proportional to the cubic power of the Q factor. However, the upper limit on PGR is also set by the Q factor: a higher Q factor brings a longer photon lifetime, which in turn leads to a lower repetition rate allowing for photon flow emitted from the microresonator, constrained by the Fourier-transform limit. Exceeding this limit will result in the overlap of photon wave packets in the time domain, thus degrading the quantum character of single-photon light beams. To push the limit of PGR in a single resonator, we propose a method by harnessing the resonance linewidth-manipulated microresonators to improve the maximum achievable photon repetition rate while keeping the power efficiency. The maximum achievable PGR and power efficiency are thus balanced by leveraging the combination of low and high-Q resonances.

Lightweight, High-Q and High Temperature Stability Microwave Cavity Resonators Using Carbon-Fiber Reinforced Silicon-Carbide Ceramic Composite

Lightweight, High-Q and High Temperature Stability Microwave Cavity Resonators Using Carbon-Fiber Reinforced Silicon-Carbide Ceramic Composite

Alternative Approach to Design and Optimization of High-Q Ring Resonators for Membrane-Free Acoustic Sensors.

Membrane-free acoustic sensors based on new principle and structure are becoming a research hotspot, because of many advantages, e.g., their wide bandwidth and high sensitivity. It is proposed that a membrane-free acoustic sensor employs a semi-buried optical waveguide ring resonator (SOWRR) as a sensing element. Using air as the upper cladding medium, the excited evanescent field in the air cladding medium would be modulated by acoustic wave. On this basis, the acoustic sensing model is established. Taking high Q factor and resonance depth as design criteria, the optimal design parameters are given. The optimal values of the air/SiO2: Ge/SiO2 waveguide resonator length and coupling spacing are obtained as 50 mm and 5.6 μm, respectively. The Q factor of the waveguide resonator of this size is as high as 8.33 × 106. The theoretical simulation indicates that the frequency response ranges from 1 Hz to 1.58 MHz and that the minimum detectable sound pressure is 7.48 µPa using a laser with linewidth of 1 kHz. Because of its advantages of wide bandwidth and high sensitivity, the membrane-free sensor is expected to become one of the most promising candidates for the next-generation acoustic sensor.

Dynamic gravitational excitation of structural resonances in the hertz regime using two rotating bars

With the planning of new ambitious gravitational wave observatories, fully controlled laboratory experiments on dynamic gravitation become more and more important. Such experiments can provide new insights in potential dynamic effects such as gravitational shielding or energy flow and might contribute to bringing light into the mystery still surrounding gravity. Here we present a laboratory-based transmitter-detector experiment using two rotating bars as transmitter and a 42 Hz, high-Q bending beam resonator as detector. Using a precise phase control to synchronize the rotating bars, a dynamic gravitational field emerges that excites the bending motion with amplitudes up to 100 nm/s or 370 pm, which is a factor of 500 above the thermal noise. The two-bar design enables the investigation of different transmitter configurations. The detector movement is measured optically, using three commercial interferometers. Acoustical, mechanical, and electrical isolation, a temperature-stable environment, and lock-in detection are central elements of the setup. The moving load response of the detector is numerically calculated based on Newton’s law of gravitation via discrete volume integration, showing excellent agreement between measurement and theory both in amplitude and phase. The near field gravitational energy transfer is 1025 times larger than the one of the propagating gravitational wave component.

High-Q trenched aluminum coplanar resonators with an ultrasonic edge microcutting for superconducting quantum devices

Dielectric losses are one of the key factors limiting the coherence of superconducting qubits. The impact of materials and fabrication steps on dielectric losses can be evaluated using coplanar waveguide (CPW) microwave resonators. Here, we report on superconducting CPW microwave resonators with internal quality factors systematically exceeding 5 × 106 at high powers and 2 × 106 (with the best value of 4.4 × 106) at low power. Such performance is demonstrated for 100-nm-thick aluminum resonators with 7–10.5 um center trace on high-resistivity silicon substrates commonly used in Josephson-junction based quantum circuit. We investigate internal quality factors of the resonators with both dry and wet aluminum etching, as well as deep and isotropic reactive ion etching of silicon substrate. Josephson junction compatible CPW resonators fabrication process with both airbridges and silicon substrate etching is proposed. Finally, we demonstrate the effect of airbridges’ positions and extra process steps on the overall dielectric losses. The best quality factors are obtained for the wet etched aluminum resonators and isotropically removed substrate with the proposed ultrasonic metal edge microcutting.

High-coherence hybrid-integrated 780 nm source by self-injection-locked second-harmonic generation in a high-Q silicon-nitride resonator

Visible, high-coherence optical sources are important to a wide range of applications spanning spectroscopy to precision timing. Integration of these sources on a semiconductor chip is a necessary step if the systems that use these devices are to be made compact, portable, and low power. Here, by self-injection-locking a 1560 nm distributed feedback semiconductor laser to a high-Q silicon-nitride resonator, a high-coherence 780 nm second-harmonic signal is generated via the photogalvanic-induced second-order nonlinearity. A record-low frequency noise floor of 4Hz2/Hz is achieved for the 780 nm emission. The approach can potentially generate signals over a wide range of visible and near-visible bands, and thereby help transition many table-top systems into a fieldable form.

An analytical method for phase noise evaluation in a microwave VCO stabilized by an interferometry structure

This study proposes a set of mathematical formulations to model a microwave interferometric structure to stabilize a free-running oscillator. The microwave interferometric structure employed a carrier suppression system to lock the voltage-controlled oscillator (VCO) frequency to a high-Q resonator and clean up the phase noise spectrum. Analytical relations were derived by modeling the system's stabilization loop components with an equivalent system transfer function. This model optimized the feedback loop circuits for a reliable frequency-locked structure with suitable spectral purity using well-known linear system methods. Thus, the phase noise of an oscillator's output signal locked to a high-Q cavity resonant frequency could be predicted and optimized. Furthermore, a prototype cavity-stabilized oscillator (CSO) system with commercial components wasimplemented to validate the proposed model. The results demonstrated that approximately 40 dB phase noise reduction at 10 kHz offset frequency was obtained at X-band, exhibiting good agreement with the proposed mathematical model. Moreover, the prototype structure's feedback circuits were designed, and their performance was validated using simulation and measurement data.

Efficiently spatial field localization enabled second-harmonic and sum-frequency generation in an etchless LiNbO3 layer by guided resonant quasi-bound states in the continuum

High-Q resonances on metasurfaces have yielded a feasible approach for nonlinear frequency conversion in recent years. In this work, we propose a highly efficient nonlinear metasurface based on an etchless lithium niobate (LiNbO3) layer by combining periodic silicon nanodisks. These nanodisks introduce two kinds of capabilities for achieving ultra-sharp spectral lineshapes owing to the high-Q resonances of bound states in the continuum and simultaneously attain a significantly enhanced spatial field localization within the LiNbO3 cavity owing to guided resonant behaviors. This distinctive mode engineering yielded a total efficiency of conversion of 2.8% in the second-harmonic generation and 1% for sum-frequency generation when the peak intensity of the fundamental pump was as low as 1 MW/cm2. Our method eliminates challenges to the manufacture of etched LiNbO3 thin films and allows for the efficient conversion of frequency. It is suitable for various applications, such as integrated, nonlinear nanophotonic chips, and high-speed communication.

All-dielectric asymmetric planar metasurface based dual-parametric sensor

All-dielectric metasurfaces are a rapidly growing area of research because they have lower ohmic losses and sharper resonances than their metallic equivalents. In this work, we have reported a novel all-dielectric metasurface design for dual parameter (refractive index and temperature) sensing applications. The design consists of a square GaAs block on the top of a quartz substrate with a square hole etched on it. An outer cut to the GaAs block is introduced to break the symmetry. The asymmetry enables a sharp Fano resonance mode with 90% spectral contrast and extremely high quality (Q) factor of value 2.8×104 in the near-infrared (NIR) window. The multipolar nature of the high-Q resonance is studied using the cartesian multipolar decomposition technique, which reveals the electric dipolar nature of the resonance. To examine the refractive index capability of the proposed device, we varied the refractive index of the background medium and found that the device is highly sensitive and results in a very high Figure of Merit (FoM) 5384 RIU−1, owing to the very narrow linewidth of the resonance. The same design is also used for temperature sensing, which shows a notable improvement in sensitivity (72.4 pm/∘C) compared to various existing methods available. The in-depth analysis of the resonance in an optimized all-dielectric metasurface and its potential for sensing presented through this paper can be significant in designing high-performance optical sensors.

Metabolometer Based on Toroidal Response

Toroidal metamaterials stand out by extremely high-Q resonances. Their radiation losses are suppressed, and fields in the metamolecules are extremely high and sensitive to the additional losses. In this work, we introduce a novel concept of metabolometer. It is based on the combination of a microwave high-Q factor toroidal metamaterial as readout device with embedded micro-pad superconductor as an absorber of terahertz (THz) radiation. We establish that a pad with 20 kΩ/sq sheet resistance reduces metamaterial Q-factor and changes the stop-band level by as much as –50 dB at 1.5 GHz. Importantly, this sensitivity to the additional losses requires no galvanic connection to the absorber. This allows one to detect THz heating of superconducting pad via the change in metamaterial transmission spectrum. We consider the absorber as a superconducting hafnium film because of its nonlinear response at 1.5 GHz below {{T}_{c}} = 400 mK. Respectively, we estimate the losses in hafnium over temperature at the metamaterial resonant frequency using Mattis–Bardeen theory. This approach can significantly improve the future design of the terahertz/millimeter-wave detectors.

Tailoring the bound states in the multi-channel nonlinear plasmonic metasurfaces

Metasurfaces offer a paradigm shift in the nonlinear optics, controlling and enhancing of nonlinear effects at the nanoscale, and offering new opportunities for applications in nonlinear optics, plasmonics and electromagnetics. Particularly, nonlinear optical conversion efficiency is limited due to the inherent losses in metallic materials. Nonlinear metasurfaces based on bound states in the continuum(BIC) can effectively increase the Q value and are expected to further enhance the nonlinear efficiency. In this paper, a tri-capacitance-like metasurface supporting both Friedrich–Wintgen quasi-BIC and symmetry-protected quasi-BIC is proposed to generates multiple high-Q resonances in the spectrum. The coupling between perfect BIC and quasi-BIC mode in the multi-channel optical system corresponds to the storage and release of electromagnetic waves, respectively. At the same time, the electromagnetic field is further enhanced at the resonant frequency, which can significantly improve the second order nonlinear efficiency. In addition, due to the different generation mechanism, the frequency and amplitude of the two resonances are easy to tune independently, which facilitates the sum and difference frequency generation (SFG and DFG). Therefore, the proposed nonlinear metasurfaces in this paper are of wide research value in the fields of nonlinear optics, multi-channel communication and memory.

Coherent memory for microwave photons based on long-lived mechanical excitations

Mechanical resonators, due to their capability to host ultralong-lived phonon modes, are particularly attractive for quantum state storage and as memory elements in conjunction with quantum computing and communication networks. Here we demonstrate absorptive-type coherent memory based on long-lived mechanical excitations. The itinerant coherent microwave field is captured, stored, and retrieved from a mechanical memory oscillator which is pre-cooled to the ground state. The phase space distribution allows us to distinguish between coherent and thermal components and study their evolution as a function of storage time. Our device exhibits attractive functions with an energy decay time of T1 = 15.9 s, a thermal decoherence rate of Γth = 2.85 Hz, and acquires less than one quantum noise during the τcoh = 55.7 ms storage period. We demonstrate that both the amplitude and phase information of microwave coherent states can be recovered, indicating the coherence of our memory device. These results suggest that high-Q mechanical resonators and long coherence time phonons could be ideal candidates for the construction of long-lived and on-demand microwave quantum memories.

AlGaAs soliton microcombs at room temperature.

Soliton mode locking in high-Q microcavities provides a way to integrate frequency comb systems. Among material platforms, AlGaAs has one of the largest optical nonlinearity coefficients, and is advantageous for low-pump-threshold comb generation. However, AlGaAs also has a very large thermo-optic effect that destabilizes soliton formation, and femtosecond soliton pulse generation has only been possible at cryogenic temperatures. Here, soliton generation in AlGaAs microresonators at room temperature is reported for the first time, to the best of our knowledge. The destabilizing thermo-optic effect is shown to instead provide stability in the high-repetition-rate soliton regime (corresponding to a large, normalized second-order dispersion parameter D2/κ). Single soliton and soliton crystal generation with sub-milliwatt optical pump power are demonstrated. The generality of this approach is verified in a high-Q silica microtoroid where manual tuning into the soliton regime is demonstrated. Besides the advantages of large optical nonlinearity, these AlGaAs devices are natural candidates for integration with semiconductor pump lasers. Furthermore, the approach should generalize to any high-Q resonator material platform.

On-chip topological THz biosensors

On-chip terahertz (THz) biosensors have enormous potential in advancing the development of integrable devices for real-time, label-free, and noninvasive detection of proteins, DNA, and cancerous tissue. However, high absorption of THz waves by water necessitates evanescent field-based biosensing. The conventional on-chip THz biosensors with small mode confinement volumes and scaling sensitivity to defects severely limit the interaction of analyte with the electromagnetic field. Here, we reveal a topological waveguide cavity system with topologically protected propagating interfacial modes, exhibiting evanescent waves with an out-of-plane field extent of 0.3λ0, where λ0 is the wavelength corresponding to the cavity resonance frequency. Our experiments involving biomolecule detection and leaf-hydration monitoring show that the near-field of high-Q topological cavity resonances accurately detects minute frequency shifts over extended periods, facilitating real-time sensing and monitoring of biological matter. Implementation of topologically protected evanescent fields in waveguide-cavity systems will enhance on-chip THz biosensing.

Spin-valley Rashba monolayer laser.

Direct-bandgap transition metal dichalcogenide monolayers are appealing candidates to construct atomic-scale spin-optical light sources owing to their valley-contrasting optical selection rules. Here we report on a spin-optical monolayer laser by incorporating a WS2 monolayer into a heterostructure microcavity supporting high-Q photonic spin-valley resonances. Inspired by the creation of valley pseudo-spins in monolayers, the spin-valley modes are generated from a photonic Rashba-type spin splitting of a bound state in the continuum, which gives rise to opposite spin-polarized ±K valleys due to emergent photonic spin-orbit interaction under inversion symmetry breaking. The Rashba monolayer laser shows intrinsic spin polarizations, high spatial and temporal coherence, and inherent symmetry-enabled robustness features, enabling valley coherence in the WS2 monolayer upon arbitrary pump polarizations at room temperature. Our monolayer-integrated spin-valley microcavities open avenues for further classical and non-classical coherent spin-optical light sources exploring both electron and photon spins.

Hydroxyl ion absorption in on-chip high-Q resonators.

Thermal silica is a common dielectric used in all-silicon photonic circuits. Additionally, bound hydroxyl ions (Si-OH) can provide a significant component of optical loss in this material on account of the wet nature of the thermal oxidation process. A convenient way to quantify this loss relative to other mechanisms is through OH absorption at 1380 nm. Here, using ultra-high-quality factor(Q-factor) thermal-silica wedge microresonators, the OH absorption loss peak is measured and distinguished from the scattering loss baseline over a wavelength range from 680 nm to 1550 nm. Record-high on-chip resonator Q-factors are observed for near-visible and visible wavelengths, and the absorption limited Q-factor is as high as 8 billion in the telecom band. Hydroxyl ion content level around 2.4 ppm (weight) is inferred from both Q measurements and by secondary ion mass spectroscopy (SIMS) depth profiling.

Reliable micro-transfer printing method for heterogeneous integration of lithium niobate and semiconductor thin films

High-speed Pockels modulation and second-order nonlinearities are key components in optical systems, but CMOS-compatible platforms like silicon and silicon nitride lack these capabilities. Micro-transfer printing of thin-film lithium niobate offers a solution, but suspending large areas of thin films for long interaction lengths and high-Q resonators is challenging, resulting in a low transfer yield. We present a new source preparation method that enables reliable transfer printing of thin-film lithium niobate. We demonstrate its versatility by successfully applying it to gallium phosphide and silicon, and provide an estimate of the transfer yield by subsequently printing 25 lithium niobate films without fail.

Scalable high Q-factor Fano resonance from air-mode photonic crystal nanobeam cavity

Abstract Fano resonance from photonic crystal nanobeam cavity (PCNC) is important building block for large-scale photonic integrated circuits (PICs) to enable photonic switches and sensors with superior characteristics. Nevertheless, most state-of-the-art demonstrations rely on electron beam lithography (EBL) and operate in dielectric mode. Hence, we theoretically, numerically and experimentally present the characteristics of Fano resonance from optical interference between the discrete state of air-mode PCNC and the continuum mode of side-coupled line-defect waveguide with partially transmitting element (PTE) using deep ultraviolet (DUV) lithography for the first time. Experimentally high average Q-factor of ∼1.58 × 104 is achieved for 30 measured devices, which indicates the feasibility of mass manufacture of high-Q Fano resonance from air-mode PTE-PCNC. Additionally, the thermo-optic bi-stability and thermal tuning characterizations of the proposed device are discussed. This work will contribute to building ultra-compact lab-on-chip resonance-based photonic components.