Optical Resonance Modes Research Articles

High-specificity molecular sensing on an individual whispering-gallery-mode cavity: coupling-enhanced Raman scattering by photoinduced charge transfer and cavity effects.

Optical whispering-gallery-mode (WGM) cavities have gained considerable interest because of their unique properties of enhanced light-matter interactions. Conventional WGM sensing is based on the mechanisms of mode shift, mode broadening, or mode splitting, which requires a small mode volume and an ultrahigh Q-factor. Besides, WGM sensing suffers from a lack of specificity in identifying substances, and additional chemical functionalization or incorporation of plasmonic materials is required for achieving good specificity. Herein, we propose a new sensing method based on an individual WGM cavity to achieve ultrasensitive and high-specificity molecular sensing, which combines the features of enhanced light-matter interactions on the WGM cavity and the "fingerprint spectrum" of surface-enhanced Raman scattering (SERS). This method identifies the substance by monitoring the Raman signal enhanced by the WGM cavity rather than monitoring the variation of the WGM itself. Therefore, ultrasensitive and high-specificity molecular sensing can be accomplished even on a low-Q cavity. The working principles of the proposed sensing method were also systematically investigated in terms of photoinduced charge transfer, Purcell effect, and optical resonance coupling. This work provides a new WGM sensing approach as well as a strategy for the design of a high-performance SERS substrate by creating an optical resonance mode.

Multi-band optical resonance of all-dielectric metasurfaces toward high-performance ultraviolet sensing.

All-dielectric sensors featuring low-loss resonances have been proposed instead of plasmonic-based sensors. However, reported dielectric-based sensors generally work in the visible and near-infrared regions and detect the intensity variation of resonant modes because the electromagnetic energy is mainly confined inside dielectric nanoparticles. It is a challenge to adjust the hotspots from the inside to the surface of the all-dielectric metasurface. In this study, highly uniform Si3N4 all-dielectric metasurfaces have been successfully fabricated as sensing platforms by utilizing nanosphere self-assembly and plasma enhanced chemical vapor deposition techniques. Experimental and simulated results demonstrate that proposed Si3N4 all-dielectric metasurfaces exhibit multiple optical resonant modes in the ultraviolet and visible wavelength and present distinct field-confinement in the gaps of nanoparticles. The hotspots have been successfully adjusted to the surface of Si3N4 nanoparticles. Delightedly, Si3N4 all-dielectric metasurfaces show characteristic wavelength shifts with variation of the refractive index, and the sensitivity can reach 707 nm per RIU for trace detection as sensing substrates. Proposed Si3N4 all-dielectric metasurfaces are promising to act as high-sensitive sensing substrates in the ultraviolet and visible wavelength with the ease of high-throughput fabrication.

Magnetic-Field-Driven Reconfigurable Microsphere Arrays for Laser Display Pixels.

Reconfigurable microlaser arrays are essential to the construction of display panels where the individual pixel should be highly tunable in resonance mode, optical polarization, and lasing wavelength upon external control signals. Here we demonstrate a facile yet reliable approach to fabrication of organic microlaser pixels, in which the assembly of microsphere arrays on each pixel is controlled according to the near-field magnetostatic confinement. The geometrical configuration of diamagnetic microspheres could be readily modulated with the near-field potential traps by using the external field to alternate the saturation magnetization of the underneath micromagnet. The motion of microspheres can be modulated among several states upon applied field, and the reconfigurable microsphere array is thus achieved with high spatial precision and rapid temporal response. Moreover, both isolated and coupled spheres serve as low-threshold microlasers with tunable optical resonance modes, whereas the switching between the vertical and horizontal alignments of coupled spheres manipulates the polarization of lasing outputs. By repeating the magnetostatic confinement on the same substrate, the full-color laser display pixels with magnetically tunable color expression capability are successfully achieved.

Large-area metal-dielectric heterostructures for surface-enhanced raman scattering

Metal-dielectric heterostructures have shown great application potentials in physics, chemistry and material science. In this work, we have designed and manufactured ordered metal-dielectric multiple heterostructures with tunable optical properties, which can be as large as the order of square centimeters in size. We experimentally realized that the surface-enhanced Raman scattering signal of the periodic multiple heterostructures increased 50 times compared with the silicon nanodisk-gold film arrays, which is attributed to the large-scale hotspots and high efficient coupling between the optical cavities and surface plasmon resonance modes. More importantly, the substrate also features a good uniformity and an excellent reproducible fabrication, which is very promising for practical applications.

Epsilon-near-zero substrate-enabled strong coupling between molecular vibrations and mid-infrared plasmons.

As the strong light-matter interaction between molecular vibrations and mid-infrared optical resonant modes, vibrational strong coupling (VSC) has the potential to modify the intrinsic chemistry of molecules, leading to the control of ground-state chemical reactions. Here, by using quartz as an epsilon-near-zero (ENZ) substrate, we have realized VSC between organic molecular vibrations and mid-infrared plasmons on metallic antennas. The ENZ substrate enables sharp mid-infrared plasmonic resonances (Q factor ∼50) which efficiently couple to the molecular vibrations of polymethyl methacrylate (PMMA) molecules with prominent mode splitting. The coupling strength is proportional to the square root of the thickness of the PMMA layer and reaches the VSC regime with a thickness of ∼300 nm. The coupling strength also depends on the polarization of the incident light, illustrating an additional way to control the molecule-plasmon coupling. Our findings provide a new, to the best of our knowledge, possibility to realize VSC with metallic antennas and pave the way to increase the sensitivity of molecular vibrational spectroscopy.

Improvement of high-frequency magnetic properties in antiferromagnetic coupling trilayers by easy-plane magnetocrystalline anisotropy

Antiferromagnetic coupling ferromagnet/nonmagnet/ferromagnet structure trilayers with high resonance frequencies have been attracted much attention. However, their initial permeability is very low as the equivalent antiferromagnetic coupling field is an in-plane equivalent field. Therefore, how to improve the initial permeability and keep the high resonance frequencies is a worthy problem. In this work, an antiferromagnetic coupling Co82Ir18(35 nm)/Ru(0.35 nm)/Co82Ir18(35 nm) trilayer with easy-plane magnetocrystalline anisotropy was fabricated by direct current magnetron sputtering. The high-frequency magnetic properties and the tuning of optical mode ferromagnetic resonance in the trilayer were investigated using vector network analyzer with a shorted micro-strip method. The resonance frequency of the trilayer has a large increase (from 3.80 GHz to 8.16 GHz, the increment is 4.36 GHz) compared with the Co82Ir18(70 nm) single film due to the high-frequency optical mode resonance in the antiferromagnetic coupling ferromagnet/nonmagnet/ferromagnet structure. Moreover, through the introduction of easy-plane magnetocrystalline anisotropy, the calculated results reveal that initial permeability of our trilayer is the largest among other antiferromagnetic coupling Co-based trilayers without easy-plane magnetocrystalline anisotropy at the same resonance frequency (8.16 GHz), where the high-frequency magnetic properties of antiferromagnetic coupling trilayers are improved. Meanwhile, the tuning of optical mode ferromagnetic resonance reveals that the switch field about 161 Oe (about 1/2 of the single film) is observed which can switch the system from optical mode to acoustic mode. The results of this work indicate that it is effective to enhance the high-frequency magnetic properties of antiferromagnetic coupling trilayers by introducing the easy-plane magnetocrystalline anisotropy, which is a new method in improving the applied ability of antiferromagnetic coupling trilayers.

Self-Hybridized Exciton-Polaritons in Sub-10-nm-Thick WS2 Flakes: Roles of Optical Phase Shifts at WS2/Au Interfaces.

Exciton–polaritons (EPs) can be formed in transition metal dichalcogenide (TMD) multilayers sustaining optical resonance modes without any external cavity. The self-hybridized EP modes are expected to depend on the TMD thickness, which directly determines the resonance wavelength. Exfoliated WS2 flakes were prepared on SiO2/Si substrates and template-stripped ultraflat Au layers, and the thickness dependence of their EP modes was compared. For WS2 flakes on SiO2/Si, the minimum flake thickness to exhibit exciton–photon anticrossing was larger than 40 nm. However, for WS2 flakes on Au, EP mode splitting appeared in flakes thinner than 10 nm. Analytical and numerical calculations were performed to explain the distinct thickness-dependence. The phase shifts of light at the WS2/Au interface, originating from the complex Fresnel coefficients, were as large as π/2 at visible wavelengths. Such exceptionally large phase shifts allowed the optical resonance and resulting EP modes in the sub-10-nm-thick WS2 flakes. This work helps us to propose novel optoelectronic devices based on the intriguing exciton physics of TMDs.

Characterization, Luminescence and Optical Resonant Modes of Eu-Li Co-Doped ZnO Nano- and Microstructures

ZnO nano- and microstructures co-doped with Eu and Li with different nominal concentrations of Li were grown using a solid vapor method. Different morphologies were obtained depending on the initial Li content in the precursors, varying from hexagonal rods which grow on the pellet when no Li is added to ribbons to sword-like structures growing onto the alumina boat as the Li amount increases. The changes in the energy of the crystallographic planes leading to variations in the growth directions were responsible for these morphological differences, as Electron Backscattered Diffraction analysis shows. The crystalline quality of the structures was investigated by X-ray diffraction and Raman spectroscopy, showing that all the structures grow in the ZnO wurtzite phase. The luminescence properties were also studied by means of both Cathodoluminescence (CL) and Photoluminescence (PL). Although the typical ZnO luminescence bands centered at 3.2 and 2.4 eV could be observed in all cases, variations in their relative intensity and small shifts in the peak position were found in the different samples. Furthermore, emissions related to intrashell transitions of Eu3+ ion were clearly visible. The good characteristics of the luminescent emissions and the high refraction index open the door to the fabrication of optical resonant cavities that allow the integration in optoelectronic devices. To study the optical cavity behavior of the grown structures, µ-PL investigations were performed. We demonstrated that the structures not only act as waveguides but also that Fabry–Perot optical resonant modes are established inside. Quality factors around 1000 in the UV region were obtained, which indicates the possibility of using these structures in photonics applications.

Enhanced single photon emission in silicon carbide with Bull’s eye cavities

The authors demonstrate a Bull’s eye cavity design that is composed of circular Bragg gratings and micropillar optical cavity in 4H silicon carbide (4H-SiC) for single photon emission. Numerical calculations are used to investigate and optimize the emission rate and directionality of emission. Thanks to the optical mode resonances and Bragg reflections, the radiative decay rates of a dipole embedded in the cavity center is enhanced by 12.8 times as compared to that from a bulk 4H-SiC. In particular, a convergent angular distribution of the emission in far field is simultaneously achieved, which remarkably boost the collection efficiency. The findings of this work provide an alternative architecture to manipulate light–matter interactions for achieving high-efficient SiC single photon sources towards applications in quantum information technologies.

Tunable optical whispering gallery mode in a magnetic microsphere suspended in a ferrofluid

Recent developments in microfluidic technologies have led to a growing interest in whispering gallery mode (WGM) optical resonators. In this report, we showed that in the case of TE waves, it is possible to induce such a mode in magnetizable microspheres suspended in a ferrofluid by applying static magnetic fields. The refractive index of a ferrofluid is dependent on the applied magnetic field. Considering this and using a quantum mechanical analog, radial distribution of the pseudo-potential is calculated for different fields and two different visible wavelengths. It is shown that within a certain range of the applied field, potential well is generated and WGM can be generated. Characteristics of the generated potential well are discussed. The finding may be useful for sensing applications in biotechnology and chemical technology.

Ultrasound Sensing Using Packaged Microsphere Cavity in the Underwater Environment.

The technologies of ultrasound detection have a wide range of applications in marine science and industrial manufacturing. With the variation of the environment, the requirements of anti-interference, miniaturization, and ultra-sensitivity are put forward. Optical microcavities are often carefully designed for a variety of ultra-sensitive detections. Using the packaged microsphere cavity, we fabricated an ultrasound sensor that can work in an underwater environment. During practical detection, the optical resonance mode of the cavity can work with real-time response accordingly. The designed structure can work in various complex environments and has advantages in the fields of precision measurement and nano-particle detection.

Optical modes in the perovskite CsPbX3 semicircular and circular microcavity

In this paper, we used the finite element method to numerically study the possible optical resonance modes in the all-inorganic metal halide perovskite (CsPbX3, [Formula: see text], Br, I) semicircular and circular microcavities. For the semicircular microcavity, the quasi-three-period mode has the highest quality factor among quasi-whispering gallery mode (WGM), two-period mode and four-period mode. The quality factors of the four resonance modes all increase linearly for the larger cavity size. In addition, the circular perovskite microcavities present the WGM patterns with extremely high-quality factors, which indicates that the circular perovskite microcavity has the potential application in the optical microcavity or laser device.

Nonlinear Light Amplification Governed by Structural Asymmetry

AbstractManeuvering the structural asymmetry in metasurfaces plays an essential role in nonlinear nanophotonics, particularly in sub‐wavelength‐scale harmonic generation. Here, a highly efficient and reproducible plasmon‐enhanced second‐harmonic generation platform for exploring these quantitative contributions of structural asymmetries to the amplification of inherently weak nonlinear responses is experimentally designed. It is discovered that such structural asymmetries can not only quantitatively alter the well‐characterized surface susceptibility, but also contribute to the spectral matching and the spatially symmetrical transition of optical resonant modes in plasmon‐enhanced nonlinear polarizations. Furthermore, this study strives to establish a theoretical model to quantify these structural asymmetry‐induced modifications, indicating the consistency with proposed experimental results. These studies may offer a strategy to the design of efficient nonlinear optical nanodevices with extending applications.

Catalysis by Dark States in Vibropolaritonic Chemistry.

Collective strong coupling between a disordered ensemble of N localized molecular vibrations and a resonant optical cavity mode gives rise to two polariton and N-1≫2 dark modes. Thus, experimental changes in thermally activated reaction kinetics due to polariton formation appear entropically unlikely and remain a puzzle. Here we show that the overlooked dark modes, while parked at the same energy as bare molecular vibrations, are robustly delocalized across ∼2-3 molecules, yielding enhanced channels of vibrational cooling, concomitantly catalyzing a chemical reaction. As an illustration, we theoretically show an ≈50% increase in an electron transfer rate due to enhanced product stabilization. The reported effects can arise when the homogeneous linewidths of the dark modes are smaller than their energy spacings.

Generalized theory of optical resonator and waveguide modes and their linear and Kerr nonlinear coupling

We derive a general theory of linear coupling and Kerr nonlinear coupling between modes of dielectric optical resonators from first principles. The treatment is not specific to a particular geometry or choice of mode basis, and can therefore be used as a foundation for describing any phenomenon resulting from any combination of linear coupling, scattering, and Kerr nonlinearity, such as bending and surface roughness losses, geometric backscattering, self- and cross-phase modulation, four-wave mixing, third-harmonic generation, and Kerr frequency comb generation. The theory is then applied to a translationally symmetric waveguide in order to calculate the evanescent coupling strength to the modes of a microresonator placed nearby, as well as the Kerr self- and cross-phase modulation terms between the modes of the resonator. This is then used to derive a dimensionless equation describing the symmetry-breaking dynamics of two counterpropagating modes of a loop resonator and prove that cross-phase modulation is exactly twice as strong as self-phase modulation only in the case that the two counterpropagating modes are otherwise identical.

Morphology, Luminescence, and Optical Properties of Tb‐ and Li‐Codoped ZnO Elongated Nano‐ and Microstructures

A variety of morphologies of ZnO elongated nano‐ and microstructures doped with Li and Tb are obtained by a vapor–solid method. The amount of Tb incorporated depends on the morphology which is controlled by the amount of Li in the initial mixture and the growth conditions. X‐ray diffraction and Raman experiments show the good crystalline quality of the samples. A small quantity of Tb3+ ions seems to be present in the structures, as both cathodoluminescence and photoluminescence suggest. The intensity of the ZnO and Tb3+ intrashell emission bands is strongly influenced by the amount of Li due to the different positions that it can occupy in the ZnO lattice. This fact leads to variations in the defect structure that change the concentration of native defects and the rare earth surroundings. The elongated structures also show waveguiding behavior and Fabry–Perot optical resonant modes with good quality factor.

Perpendicular magnetization anisotropy induced dynamical coherence reduction in stripe domain film

We investigated the magnetization dynamics of the 350 nm permalloy film with in plane domain (IPD), stripe domain (SD), and labyrinth domain (LD) patterns. Experimental and micromagnetic simulation results showed that the change in magnetic domain structure from IPD to LD was due to the increasing perpendicular magnetic anisotropy (PMA) of the film. The magnetization dynamics indicated that the resonant modes of the film strongly depended on the magnetic domain structure. IPD films presented a uniform precession mode. The film with well-regular SD exhibited clear acoustic and optical resonance modes, and the formation of LD suppressed both resonance modes. Finally, the dynamics of magnetization dependent on the domain structure in these films were discussed by using the phenomenological resonance models.

Spectral structure of electromagnetic scattering on arbitrarily shaped dielectrics

Spectral analysis is performed on the Born equation, a strongly singular integral equation modeling the interactions between electromagnetic waves and arbitrarily shaped dielectric scatterers. Compact and Hilbert--Schmidt operator polynomials are constructed from the Green operator of electromagnetic scattering on scatterers with smooth boundaries. As a consequence, it is shown that the strongly singular Born equation has a discrete spectrum, and that the spectral series $ \sum_\lambda|\lambda|^2|1+2\lambda|^4$ is convergent, counting multiplicities of the eigenvalues $ \lambda$. This reveals a shape-independent optical resonance mode corresponding to a critical dielectric permittivity $ \epsilon_r=-1$.

Electric-field tunable rotation of optical mode ferromagnetic resonance in FeCoB/Ru/FeCoB/PMN-PT multilayers

FeCoB/Ru/FeCoB trilayers with a uniaxial magnetic anisotropy were deposited on (011)-cut lead magnesium niobate-lead titanate (PMN-PT) single crystal ferroelectric substrates using a compositional gradient sputtering (CGS) method. Ultrahigh optical mode resonance frequencies from 11.76 to 18.96 GHz at zero magnetic field were achieved due to the strong interlayer exchange coupling between FeCoB films through Ru spacer. In this study, two orthogonal magnetic anisotropic fields (one is CGS induced anisotropy field HKCGS and the other is the piezoelectric stress-induced anisotropy field HKME) were set to realize electric-field (E-field) controlled switch of magnetic moment configuration through their competition. As a result, the optical mode resonance intensity can make reversible 90° rotation under applied E-field, while the advantages of ultrahigh frequency optical mode resonance in trilayers are still maintained because the interlayer exchange coupling is not destroyed by magnetoelectric coupling. The ultrahigh resonance frequency and E-field tunable and reversible FMR rotation in these trilayers provide a potential route to guide microwave flow in multifunctional microwave and spintronic devices.

Flow Speed Sensor Based on Optical Microresonators

A compact flow velocity sensor based on dielectric optical microresonators is demonstrated. In this novel LiDAR-based sensor concept, velocity Doppler shifts from Mie-scattered light are determined using whispering gallery mode (WGM) optical resonators. The microresonators could replace Fabry–Perot interferometers and other optical frequency discriminators often employed in remote sensing applications, thereby significantly reducing the size and weight of the measurement system, making it suitable for flow diagnostics for both earthbound and airborne platforms. Two sets of experiments are carried out to assess the feasibility of the sensor concept. In the first-level proof-of-concept experiments, the tangential velocity of a solid rotating disk is measured using a sphere microresonator, with encouraging results. In follow-up experiments, velocity measurements near the exit of an air jet nozzle are carried out. The air is seeded with water droplets, and the Mie-scattered light is analyzed using on-chip ring resonators. Both single-point and jet profile measurements are made. The results demonstrate the feasibility of a WGM microresonator-based flow sensor.