Large Quality Factor Research Articles

Performance Limits of Luminescent Solar Concentrators Tested with Seed/Quantum-Well Quantum Dots in a Selective-Reflector-Based Optical Cavity.

Luminescent solar concentrators (LSCs) can serve as large-area sunlight collectors for photovoltaic devices. An important LSC characteristic is a concentration factor (C), which is defined as the ratio of the output and the input photon flux densities. This parameter can be also thought of as an effective enlargement factor of a solar cell active area. On the basis of thermodynamic considerations, the C-factor can reach extremely high values that exceed those accessible with traditional concentrating optics. In reality, however, the best reported values of C are around 30. Here we demonstrate that using a new type of high-emissivity quantum dots (QDs) incorporated into a specially designed cavity, we are able to achieve the C of ∼62 for spectrally integrated emission and ∼120 for the red portion of the photoluminescence spectrum. The key feature of these QDs is a seed/quantum-well/thick-shell design, which allows for obtaining a high emission quantum yield (>95%) simultaneously with a large LSC quality factor (QLSC of ∼100) defined as the ratio of absorption coefficients at the wavelengths of incident and reemitted light. By incorporating the QDs into a specially designed cavity equipped with a top selective reflector (a Bragg mirror or a thin silver film), we are able to effectively recycle reemitted light achieving light trapping coefficients of ∼85%. The observed performance of these devices is in remarkable agreement with analytical modeling, which allows us to project that the applied approach should allow one to boost the spectrally integrated concentration factors to more than 100 by further improving light trapping and/or increasing QLSC.

Boron nitride nanoresonators for phonon-enhanced molecular vibrational spectroscopy at the strong coupling limit

Enhanced light-matter interactions are the basis of surface-enhanced infrared absorption (SEIRA) spectroscopy, and conventionally rely on plasmonic materials and their capability to focus light to nanoscale spot sizes. Phonon polariton nanoresonators made of polar crystals could represent an interesting alternative, since they exhibit large quality factors, which go far beyond those of their plasmonic counterparts. The recent emergence of van der Waals crystals enables the fabrication of high-quality nanophotonic resonators based on phonon polaritons, as reported for the prototypical infrared-phononic material hexagonal boron nitride (h-BN). In this work we use, for the first time, phonon-polariton-resonant h-BN ribbons for SEIRA spectroscopy of small amounts of organic molecules in Fourier transform infrared spectroscopy. Strikingly, the interaction between phonon polaritons and molecular vibrations reaches experimentally the onset of the strong coupling regime, while numerical simulations predict that vibrational strong coupling can be fully achieved. Phonon polariton nanoresonators thus could become a viable platform for sensing, local control of chemical reactivity and infrared quantum cavity optics experiments.

Electromagnetic energy stored in inhomogeneous scattering systems.

We analytically study the time-averaged electromagnetic energy stored inside scatterers containing inclusions of arbitrary shapes. Assuming the low density of inclusions, we derive the expression for the energy-transport velocity through disordered media without relying on the radiative transfer equation. Moreover, this expression is independent of the shape of scatterers. In addition, we obtain a relation between the dwell and absorption times associated with inclusions by considering the relationship between the internal energy and absorption cross-section. An approximation for the electromagnetic energy stored inside a disordered medium in terms of the transport mean free path and the packing fraction is also derived. This expression suggests that the enhanced electromagnetic energy within the host medium is achieved for inclusions exhibiting negative scattering asymmetry parameters. As a result, disordered media with enhanced backscattering is expected to exhibit large quality factors.

Design rules for combined label-free and fluorescence Bloch surface wave biosensors.

We report on the fabrication and physical characterization of optical biosensors implementing simultaneous label-free and fluorescence detection and taking advantage of the excitation of Bloch surface waves at a photonic crystal's truncation interface. Two types of purposely designed one-dimensional photonic crystals on molded organic substrates with micro-optics were fabricated. These crystals feature either high or low finesse of the Bloch surface wave resonances and were tested on the same optical readout system. The experimental results show that designing biochips with a large resonance quality factor does not necessarily lead in the real case to an improvement of the biosensor performance. The conditions for optimal biochip design and operation of the complete bio-sensing platform are established.

High power density in a piezoelectric energy harvesting ceramic by optimizing the sintering temperature of nanocrystalline powders

Piezoelectric energy harvesting is the research hotspot in the field of new energy, and its core is to prepare piezoelectric ceramics with high transduction coefficient (d33×g33) and large mechanical quality factor (Qm) as well. In addition, the miniaturization of the piezoelectric energy harvester also requires the material to have a submicron fine grain structure. In this work, submicron-structured ternary system, MnO2-doped Pb(Zn1/3Nb2/3)O3-Pb(Zr0.5Ti0.5)O3 was constructed by pressureless sintering of nanocrystalline powders, which has been synthesized for the first time by high-energy ball milling route thereby evading the calcination stage. The microstructure and the energy harvesting characteristics were tailored through changing the sintering temperature. It was found that 1000°C sintered fine-grained specimen (mean grain size ∼0.95μm) showed the maximum d33×g33 value of 9627×10−15m2/N, meanwhile Qm was as large as 774, which was almost seven times larger than pure counterpart. In the mode of the cantilever-type energy harvester, a high power density of 1.5μW/mm3 were obtained for 1000°C sintered specimen at a low resonance frequency of 90Hz and acceleration of 10m/s2, which were further increased to 29.2μW/mm3 when the acceleration increased to 50m/s2, showing the potential applications as a next generation high power multilayer energy harvester.

Nanoimaging of resonating hyperbolic polaritons in linear boron nitride antennas

Polaritons in layered materials—including van der Waals materials—exhibit hyperbolic dispersion and strong field confinement, which makes them highly attractive for applications including optical nanofocusing, sensing and control of spontaneous emission. Here we report a near-field study of polaritonic Fabry–Perot resonances in linear antennas made of a hyperbolic material. Specifically, we study hyperbolic phonon–polaritons in rectangular waveguide antennas made of hexagonal boron nitride (h-BN, a prototypical van der Waals crystal). Infrared nanospectroscopy and nanoimaging experiments reveal sharp resonances with large quality factors around 100, exhibiting atypical modal near-field patterns that have no analogue in conventional linear antennas. By performing a detailed mode analysis, we can assign the antenna resonances to a single waveguide mode originating from the hybridization of hyperbolic surface phonon–polaritons (Dyakonov polaritons) that propagate along the edges of the h-BN waveguide. Our work establishes the basis for the understanding and design of linear waveguides, resonators, sensors and metasurface elements based on hyperbolic materials and metamaterials.

Hyperbolic metamaterial nanoresonators make poor single-photon sources

We study the optical properties of quantum dipole emitters coupled to hyperbolic metamaterial nano-resonators using a semi-analytical quasinormal mode approach. We show that coupling to metamaterial nano-resonators can lead to significant Purcell enhancements that are nearly an order of magnitude larger than those of plasmonic resonators with comparable geometry. However, the associated single photon output $\beta$-factors are extremely low (around 10$\%$), far smaller than those of comparable sized metallic resonators (70$\%$). Using a quasinormal mode expansion of the photon Green function, we describe how the low $\beta$-factors are due to increased Ohmic quenching arising from redshifted resonances, larger quality factors, and stronger confinement of light within the metal. In contrast to current wisdom, these results suggest that hyperbolic metamaterial nano-structures make poor choices for single photon sources.

Determination of Low Loss in Isotopically Pure Single Crystal 28Si at Low Temperatures and Single Microwave Photon Energy

The low dielectric losses of an isotopically pure single crystal 28Si sample were determined at a temperature of 20 mK and at powers equivalent to that of a single photon. Whispering Gallery Mode (WGM) analysis revealed large Quality Factors of order 2 × 106 (dielectric loss ~5 × 10−7) at high powers, degrading to 7 × 105 (dielectric loss ~1.4 × 10−6) at single photon energy. A very low-loss narrow line width paramagnetic spin flip transition was detected with extreme sensitivity in 28Si, with very small concentration below 1011 cm−3 (less than 10 parts per trillion) and g-factor of 1.995 ± 0.008. Such determination was only possible due to the low dielectric photonic losses combined with the long lifetime of the spin transition (low magnetic loss), which enhances the magnetic AC susceptibility. Such low photonic loss at single photon energy combined with the narrow line width of the spin ensemble, indicate that single crystal 28Si could be an important crystal for future cavity QED experiments.

Thin cylindrical slot in an optical microdisk cavity for sensing biomaterials

In this paper, we propose and investigate a thin cylindrical slot etched into a disk shape optical microcavity (MC) aiming for sensing biomaterials in a label-free style. Supporting whispering gallery modes (WGMs), with remarkably large quality factor to modal volume ratio (Q/Vm) of the optical MC structures that penetrate in the slot region, enables us to perform sensing. Three different geometries for the side walls of host microdisk cavities, including vertical, 60° wedged, and half-circular cross section, are selected for investigations. In each individual case, the radial position, width, and height of the thin cylindrical slot are varied. The electromagnetic (EM) field intensity distributions (mode mapping profiles) of the WGMs show funneling of the intensified fields into the slot area that possessing nearly the same high Q values. Tuning the slot position, width, and depth for a suitably chosen WGM, sensing could be optimized for different biomaterials. Sensitivity value as high as 75 nm/RIU is calculated for the half-circular side wall microdisk. The proposed WGM-based slotted microdisk, as a state-of-the-art device which can operate, such as lab-on-chip structure, would function as a sensitive biosensor, even down to the single biomolecule levels.

Regulation and control of insulated layers for intergranular insulated Fe/SiO2 soft magnetic composites

In this work, the intergranular insulated Fe/SiO2 soft magnetic composite cores with tunable insulating layer thickness were prepared by a modified Stober method combined with the spark plasma sintering technology. Most of the conductive Fe particles could be coated uniformly by insulated SiO2 using the modified Stober method, and the high compact and intergranular insulated cores could be obtained quickly by the spark plasma sintering process. The intergranular insulated Fe/SiO2 composite cores exhibited much higher electrical resistivity, lower core loss, better frequency stability of permeability and large higher quality factor than that of raw Fe core without insulated SiO2. The thickness of SiO2 insulating layer, electrical and magnetic properties of intergranular insulated Fe/SiO2 composite cores could be readily controlled by adjusting the tetraethyl orthosilicate concentration. The thickness of SiO2 insulating layer and resistivity of Fe/SiO2 composite cores first increased and then dropped with increasing the tetraethyl orthosilicate concentration, while the permeability and core loss changed in the opposite direction. Fe/SiO2 composite core showed the optimal performance when the tetraethyl orthosilicate concentration was 0.135 mol l−1, which exhibited better frequency stability at high frequencies, much higher electrical resistivity, higher quality factor and lower core loss.

Strong resonances on periodic arrays of cylinders and optical bistability with weak incident waves

A one-dimensional periodic array of circular dielectric cylinders surrounded by air is a simple structure on which guided modes above the light line, also called bound states in the continuum (BICs), may exist. Recent studies reveal that such an array supports not only antisymmetric standing waves which are symmetry-protected BICs, but also propagating Bloch BICs and symmetric standing waves. Near a BIC, there is a family of resonant modes (depending on the Bloch wave number $\ensuremath{\beta}$) with arbitrarily large quality factors. Using a perturbation method, we show that the quality factor of the resonant mode typically depends on $\ensuremath{\beta}$ like $1/{(\ensuremath{\beta}\ensuremath{-}{\ensuremath{\beta}}_{*})}^{2}$, where ${\ensuremath{\beta}}_{*}$ is the Bloch wave number of the BIC, but near a symmetric standing wave $({\ensuremath{\beta}}_{*}=0)$, the quality factor blows up like $1/{\ensuremath{\beta}}^{4}$. This indicates that strong resonances can be more easily induced near a symmetric standing wave. As an application, we numerically study optical bistability for the periodic array assuming the cylinders have a Kerr nonlinearity. With the nonlinear effects enhanced by the resonances, it is possible to have optical bistability for weak incident waves. The numerical results confirm that the weakest incident wave for optical bistability is realized through the resonances near the symmetric standing waves.

Hybrid confinement of optical and mechanical modes in a bullseye optomechanical resonator.

Optomechanical cavities have proven to be an exceptional tool to explore fundamental and applied aspects of the interaction between mechanical and optical waves. Here we demonstrate a novel optomechanical cavity based on a disk with a radial mechanical bandgap. This design confines light and mechanical waves through distinct physical mechanisms which allows for independent control of the mechanical and optical properties. Simulations foresee an optomechanical coupling rate g0 reaching 2π × 100 kHz for mechanical frequencies around 5 GHz as well as anchor loss suppression of 60 dB. Our device design is not limited by unique material properties and could be easily adapted to allow for large optomechanical coupling and high mechanical quality factors with other promising materials. Finally, our devices were fabricated in a commercial silicon photonics facility, demonstrating g0/2π = 23 kHz for mechanical modes with frequencies around 2 GHz and mechanical Q-factors as high as 2300 at room temperature, also showing that our approach can be easily scalable and useful as a new platform for multimode optomechanics.

High-Efficiency Transmission of a Wireless Power Transmission System for Low-Frequency Using REBCO Double-Pancake Coils

Wireless power transmission has been investigated to realize more efficient and more convenient noncontact power transmission systems for electric tramways, electric vehicles, portable telephones, and so on. Since the magnetic resonance type wireless power transmission system is often used in frequency regions of megahertz, there are some problems, such as the increase of wire resistance due to skin effect and a switching loss in converter. Therefore, it is expected to realize high-power wireless power transmission in low-frequency regions of kilohertz. In a wireless power transmission system using copper coils, however, the transmission efficiency decreases with the decrease of resonance frequency because quality factor decreases with the decrease of the resonance frequency. Therefore, we fabricated a model system using high-temperature superconducting (HTS) double-pancake coils composed of REBCO tape and investigated the transmission efficiency characteristics to evaluate the possibility of application of the HTS coil to a high-efficiency wireless power transmission system operated in the low-frequency region of kilohertz. In the copper coil system, the transmission efficiency increases with the resonance frequency. In the HTS coil system, however, a high-efficiency transmission was obtained, even at a low-resonance frequency, due to a large quality factor at low frequency. Moreover, in the wireless power transmission system using HTS coils, the transmission efficiency was high not only at the resonance frequency but also around the resonance frequency. From these results, we believe that HTS coil can realize the high-efficiency wireless power transmission in a low-frequency region of kilohertz.

Nanomechanical motion transduction with a scalable localized gap plasmon architecture.

Plasmonic structures couple oscillating electromagnetic fields to conduction electrons in noble metals and thereby can confine optical-frequency excitations at nanometre scales. This confinement both facilitates miniaturization of nanophotonic devices and makes their response highly sensitive to mechanical motion. Mechanically coupled plasmonic devices thus hold great promise as building blocks for next-generation reconfigurable optics and metasurfaces. However, a flexible approach for accurately batch-fabricating high-performance plasmomechanical devices is currently lacking. Here we introduce an architecture integrating individual plasmonic structures with precise, nanometre features into tunable mechanical resonators. The localized gap plasmon resonators strongly couple light and mechanical motion within a three-dimensional, sub-diffraction volume, yielding large quality factors and record optomechanical coupling strength of 2 THz·nm−1. Utilizing these features, we demonstrate sensitive and spatially localized optical transduction of mechanical motion with a noise floor of 6 fm·Hz−1/2, representing a 1.5 orders of magnitude improvement over existing localized plasmomechanical systems.

Graphene-based reconfigurable terahertz plasmonics and metamaterials

This work discusses and compares two proposed practical approaches for realizing graphene-based reconfigurable terahertz metamaterials, namely: graphene-only plasmonic structures, and graphene/metal hybrid structures. From rigorous theoretical analysis, full-wave electromagnetic numerical simulations, as well as supporting experiments, several reconfigurable structures are analyzed and compared in terms of their: (i) Quality-factor, (ii) Extinction-ratio, (iii) Unit-cell dimensions, and (iv) Resonance-frequency tunability-range. From this analysis it is observed that at terahertz frequencies, although typically possessing larger unit-cell dimensions and being limited by a restricted resonance-frequency tunability-range, reconfigurable metamaterials based on graphene/metal hybrid structures can provide much larger quality-factors, extinction levels, and, when reconfigured, smaller extinction-level degradation than graphene-only plasmonic structures. As a result, when analyzed in the context of reconfigurable terahertz metamaterials, graphene might result attractive as a reconfigurable media providing tunability to otherwise passive metallic structures rather than as a reconfigurable plasmonic material per-se.

Time-Domain Analysis of Wireless Power Transfer System Behavior Based on Coupled-Mode Theory

In this paper, coupled-mode theory (CMT) is used to obtain a transient solution analytically for a wireless power transfer system (WPTS) when unit energy is applied to one of two resonators. The solutions are compared with those obtained using equivalent circuit-based analysis. The time-domain CMT is accurate only when resonant coils are weakly coupled and have large quality factors, and the reason for this inaccuracy is outlined. Even though the time-domain CMT solution does not describe the WPTS behavior precisely, it is accurate enough to allow for an understanding of the mechanism of energy exchange between two resonators qualitatively. Based on the time-domain CMT solution, the critical coupling coefficient is derived and a criterion is suggested for distinguishing inductive coupling and magnetic resonance coupling of the WPTS.

Effects of time-delayed feedback on the properties of self-sustained oscillators.

Most self-sustained oscillations in biological systems and in technical applications are based on a feedback loop, and it is usually important to know how they will react when an external oscillatory force is applied. Here we investigate the effects that the introduction of a time delay in the feedback can have in the entrainment properties of self-sustained oscillators. To do this, we derive analytic expressions for the periodic trajectories and their asymptotic stability, for a generic external oscillatory force. This allows us to show that, for large quality factors, the resonance frequency does not depend on the feedback delay. When the external force is harmonic, it is shown that the largest entrainment range does not correspond to the time delay that gives the maximal response of the unforced oscillator. In fact, that delay gives the shortest entrainment range.

Enhanced Phase Sensitivity in Plasmonic Refractive Index Sensor Based on Slow Light

Fano spectrum exhibits an intriguing feature of narrow asymmetrical resonance profile, which can be used in designing an ultra sensitive refractive index sensor. Here, in this letter, an ultra compact surface plasmon sensor is investigated. The sensor is based on Metal-Dielectric-Metal (MDM) waveguide geometry and connected to a pair of stub resonators. The distance between resonators is adjusted in such a manner that their increased coupling leads to an asymmetrical profile resonance, which is known as Fano resonance. The Fano resonance is very sensitive to any change in refractive index of the material. To measure the sensitivity of the device, stubs are filled with liquid/gaseous material under test. The structure is numerically simulated by the Finite difference time-domain method and the value of sensitivity is obtained as high as S = 510 nm/refractive index unit with a narrow line width of 19 nm and large quality factor Q ≈ 80. Fano resonance is generally accompanied with sharp dispersion, which leads to the generation of slow light. To account an effect of slow light in the waveguide phase, sensitivity is also analyzed as a function of a group delay. The large value of phase sensitivity is reported, which is equal to S O = 41.9 rad for per unit change in refractive index unit. The phase sensitivity rises linearly with the increasing value of group delay or decreasing value of group velocity. Thus, the device is well suited for designing on-chip optical sensors, optical buffers, switches, modulators, and so on.

Sensitivity analysis for photonic crystal microcavities

Resonant modes in photonic crystal microcavities with large quality factors and small mode volumes are important in many applications, but they are very sensitive to geometric and physical parameters of the structure. In this paper, we develop an efficient method for computing the partial derivatives of the complex resonant frequency with respect to parameters such as radii, refractive indices, and positions of the circular cylinders, for two-dimensional photonic crystal microcavities. Like the adjoint variable method for sensitivity analysis, our method is capable of rapidly calculating the partial derivatives with respect to a large number of geometric and material parameters. The method is efficient, since it takes advantage of the many identical unit cells in photonic crystal devices and the analytic solutions for circular cylindrical structures.

Optically-detected nonlinear oscillations of single crystal silicon MEMS accelerometers

Purpose The purpose of this study is to address the nonlinear oscillations of single-crystal silicon micro-electromechanical systems (MEMS) accelerometers subjected to mechanical excitation. Methodology/approach The nonlinear behavior was detected and analyzed by using experimental, analytical and numerical approaches. Piezoelectric shaker as a source of mechanical excitation and differential laser Doppler vibrometer in combination with a micro system analyzer were used in the experimental effort. Two types of devices considered included nonencapsulated samples and samples encapsulated in nitrogen gas compressed between two glasses. Numerical and analytical investigations were conducted to analyze the nonlinear response. A novel method has been suggested to calculate the nonlinear parameters. The obtained experimental, numerical and analytical results are in good agreement. Findings It has been found that the nonlinearity leads to a shift in frequencies and generates higher harmonics, but, most importantly, reveals new phenomena, such as the jump and instability of the vibration amplitudes and phases. Originality/value It has been shown that under the constant excitation force, the MEMS device can work in both linear and nonlinear regions. The role of the beat phenomenon has been also addressed and discussed. It has been found that the attributes of the nonlinear response are strongly dependent on the level and duration of the excitation. It is concluded that the nonlinear response of the systems is strongly dependent on the level of the excitation energy. It has been also concluded that larger quality factors are able to enhance dramatically the nonlinear effects and vice versa.