Morphology-dependent Resonances Research Articles

Measurement of impinging and boiling droplet temperature by two-color/two-dye laser-induced fluorescence technique using continuous-wave laser

The 2-color/2-dye laser-induced fluorescence technique is a non-intrusive method for measuring temperature in millimeter-scale fluids. This study determines optimal conditions for fluorescein disodium (FL) and Sulforhodamine 640 (SRh640) with a low-cost continuous-wave laser to reduce dye saturation and morphology-dependent resonances. The signal images of both dyes, captured by two high-speed cameras, were clearly distinguishable from the background with high contrast under specific concentration conditions. As a result, three concentration combinations were selected based on preliminary experiments, and the fluorescence signal ratio was analyzed to establish a temperature relationship between 32 and 71 °C. The intensities of FL and SRh640 increases and decrease with the temperature, respectively. Errors resulting from laser light attenuation in single-signal images were reduced using the ratiometric method. Solution 1 (CFL = 1.55 × 10−4 / CSRh640 = 3.2 × 10−6 M) exhibited the highest sensitivity at 0.718 (%/ °C) and a strong linear relationship (r2 = 0.997), so it was determined to be suitable concentration for measuring the droplet temperature. The proposed method estimated the temporal temperature variations of ethanol droplets impinging on a heated surface at TW = 80, 150 and 200 °C. At TW = 80 °C, contact boiling initiated without a recoil process, and the heat transfer elevated the droplet's temperature near the surface, leading to bubble formation and growth. As surface temperature increases, the film boiling behavior can be observed. The temporal evolution of the average heat transfer rate was analyzed.

TMB+-mediated etching of urchin-like gold nanostructures for colorimetric sensing

The morphology-dependent localized surface plasmon resonance of gold nanostructures has been widely utilized for designing sensors. One method relies on the color change of gold nanoparticles upon etching. In previous work, TMB2+ oxidized from 3,3′,5,5′-tetramethylbenzidine (TMB) was found to etch gold nanorods (AuNRs), leading to a spectrum of different colors. However, the preparation of TMB2+ needs the addition of a strong acid and other harsh conditions. Herein, a new colorimetric biosensing platform was developed using urchin-like gold nanoparticles (AuNUs). Compared with AuNRs, the etching of AuNUs can happen under mild conditions by TMB+ at pH 6, protecting enzymes and proteins from denaturation. The role of CTAB surfactant was dissected, and its bromide ions were found to be involved in the etching process. Based on these observations, a one-step colorimetric detection of H2O2 was realized by using horseradish peroxidase and H2O2 to oxidize TMB. Within 30 min, this system achieved a detection limit of 80 nM H2O2. This work offered fundamental insights into the etching of anisotropic gold nanostructures and optimized the etching conditions. These advancements hold promise for broader applications in biosensing and analytical chemistry.

Mie scattering with 3D angular spectrum method.

Mie theory is a powerful method to model electromagnetic scattering from a multilayered sphere. Usually, the incident beam is expanded to its vector spherical harmonic representation defined by beam shape coefficients, and the multilayer sphere scattering is obtained by the T-matrix method. However, obtaining the beam shape coefficients for arbitrarily shaped incident beams has limitations on source locations and requires different methods when the incident beam is defined inside or outside the computational domain or at the scatterer surface. We propose a 3D angular spectrum method for defining beam shape coefficients from arbitrary source field distributions. This method enables the placement of the sources freely within the computational domain without singularities, allowing flexibility in beam design. We demonstrate incident field synthesis and spherical scattering by comparing morphology-dependent resonances to known values, achieving excellent matching and high accuracy. The proposed method has significant benefits for optical systems and inverse beam design. It allows for the analysis of electromagnetic forward/backward propagation between optical elements and spherical targets using a single method. It is also valuable for optical force beam design and analysis.

Droplet thermometry based on an optimized two dye two-color laser-induced fluorescence concept

In this work two-color laser-induced fluorescence (2c-LIF) is applied to calibrate and measure the temperature in a micrometric monodisperse ethanol droplet chain. A dye mixture of fluorescein disodium (FL) and sulforhodamine 101 (SRh) is used, in order to form a sensitive signal ratio of one temperature dependent and one independent fluorophore. Spectral and planar fluorescence detection via a fiber-coupled spectrometer and a camera system, consisting of two sCMOS cameras, is set up around the droplet chain. Additionally, absorption measurements of the dye mixture in ethanol are conducted using a photo-spectrometer to analyze the temperature sensitivity of the dyes as well as potential re-absorption and fluorescence cross-talk effects. The spectral setup allowed an investigation of the wavelength range in which morphology dependent resonances (MDR) occur at the phase boundary of the droplet. Thus, the optical filters, determining the color channels in the camera system, are chosen to avoid detection of the lasing signal and providing maximal temperature sensitivity at the same time. The calibrated signal ratios are applied in temperature measurements of evaporating heated droplets in the droplet chain, showing the cooling with larger distance from the nozzle.

Morphology-dependent resonance induced optical forces in a multiple-sphere system.

Optical fields and forces can be greatly enhanced for a microparticle when the whispering gallery modes (WGMs) are excited. In this paper, by solving the scattering problem using the generalized Mie theory, the morphology-dependent resonances (MDRs) and resonant optical forces derived from the coherent coupling of WGMs are investigated in multiple-sphere systems. When the spheres approach each other, the bonding and antibonding modes of MDRs emerge and correspond to the attractive and repulsive forces, respectively. More importantly, the antibonding mode is good at propagating light forward, while the optical fields decay rapidly for the bonding mode. Moreover, the bonding and antibonding modes of MDRs in the PT-symmetric system can persist only when the imaginary part of the refractive index is small enough. Interestingly, it is also shown that for a PT-symmetric structure, only a minor imaginary part of the refractive index is required to generate a significant pulling force at MDRs, making the whole structure move against the light propagation direction. Our work deepens the understanding of the collective resonance behavior of multiple spheres and paves the way for potential applications in particle transportation, non-Hermitian systems, integrated optical devices, etc.

Linking microwave heating of aqueous spheres to morphology-dependent resonances

It was recently suggested that the sparking of grape dimers in microwave ovens is due to the interaction of morphology-dependent resonances in aqueous spheres. However, evidence for microwave resonances in individual grape-sized aqueous spheres has remained weak and is open to interpretation. In this work, we provide new experimental evidence for size-dependent resonances in hydrogel spheres via calorimetric measurement of the electromagnetic energy absorbed by hydrogel spheres under microwave irradiation. Using finite-element simulations, we predict the resonant behavior of grape-sized aqueous spheres and further explore the differences between mode intensities in free-space and various in situ positions of a microwave oven. The lowest morphology-dependent resonance—a magnetic dipolar mode—is experimentally confirmed, appearing at the predicted diameter of ∼1.35 cm. Finally, experimental evidence for higher order modes in larger spheres is suggestive but remains unresolved.

Two-Color Laser-Induced Fluorescence Thermometry In Micrometric Ethanol Droplets Using A Fluorescein And Sulforhodamine 101 Dye Mixture

In this work, two-color laser-induced fluorescence (2c-LIF) is applied to measure the temperature in micrometric ethanol droplets, containing a novel dye mixture of fluorescein (FL) and sulforhodamine 101 (SRh). A monodisperse droplet generator is used to generate 100 µm sized droplets with temperatures from 293 K to 343 K. The addition and excitation of the dye mixture allow the simultaneous detection of the fluorescence signal with two detection systems. A fiber coupled spectrometer and an imaging system with two sCMOS cameras provide both, spectral and spatial information. Measuring both signals at the same time, lasing effects (or morphology-dependent resonances) in the droplet can be clearly recognized in the spectra and circumvented in further measurements.  Additionally, absorption measurements are conducted, which demonstrate the difference in temperature sensitivity between the two dyes and show potential re-absorption effects. At high temperatures, the photon absorption of FL increases strongly while SRh shows a steady absorption at all temperatures. By measuring the emission spectra, the implied temperature dependence can be observed for FL. On the contrary, the emission signal of SRh is constant at any temperature measured. The wavelength regions with highest sensitivity are elaborated and respective filters are chosen for the color channels in the imaging system. No lasing effects were observed in these filter regions in planar and spectral measurements. Temperature-dependent signal ratios of the two channels are obtained for the detected spectra and images. The resulting calibration curves are compared and discussed. A much steeper slope and thus larger sensitivity is achieved by measuring with the spectrometer. Both calibration curves show only very little standard deviations, which proves the applicability and reliability of the setup, the method and the utilized dyes, for application in technical spray systems.

Polarization-dependent LIF/Mie ratio for sizing of micrometric ethanol droplets doped with Nile red.

The present study deals with droplet sizing based on laser-induced fluorescence (LIF) and Mie scattering for varied polarization of the utilized laser (parallel or perpendicular). The polarization-dependent LIF/Mie ratio is studied for micrometric droplets (25-60 µm) produced with a droplet generator. The investigations were carried out with the dye Nile red dissolved in ethanol and ethanol/iso-octane mixtures. A spectral absorption and fluorescence characterization at various dye and ethanol concentrations is carried out in a cuvette in order to identify reabsorption effects. The LIF|| droplet images (index ||: parallel polarization) show a more homogeneous intensity distribution in the droplets and slightly stronger morphology-dependent resonances (MDRs) in comparison to LIF⊥ (index ⊥: perpendicular polarization). The spectral LIF emissions reveal a dependence of the MDR on the ethanol admixture. The larger the ethanol content, the lower the MDR peak, which is also shifted further to the red part of the spectrum. The Mie droplet signal images are mainly characterized by two distinct glare points, one at the entrance of the laser light (reflection) and one at the exit (first-order refraction). The Mie⊥ images show a more pronounced entrance glare point, in comparison to Mie||, where the exit glare point is more pronounced. These observations are in accordance with the theory. The calibration curve of the micro droplet signals revealed a volumetric trend of the LIF signals and a slightly higher LIF⊥ signal and sensitivity in comparison to LIF||. The signal Mie⊥ follows roughly a quadratic trend on average, while Mie|| follows a linear trend. Consequently, the calculated LIF⊥/Mie⊥ ratio shows a linear trend, whereas the LIF||/Mie|| ratio shows a quadratic trend, which confirms theoretical calculations. A numerical simulation of the Mie signal at various detection angles shows a good agreement with the experimental data at large apertures.

Green synthesis and photoluminescence properties of gold nanoparticles with irregular shapes

Inspired by several conventional ideas, the green phytosynthesis of gold nanoparticles (AuNPs) has many advantages over known chemical and physical methods of preparing AuNPs. With no need for special conditions, one can use this approach for large-scale synthesis. Here, we report the green synthesis of AuNPs with irregular shapes by the bio-reduction of tetrachloroauric acid by the extract of Juniperus communis L. The resulting AuNPs were characterized by UV-Vis spectroscopy, TEM, AFM, SAED, and EDX microanalysis. In the background of the unique morphology of AuNPs, we registered the blue-to-green photoluminescence with a broad plasmon signal that indicates the contributions of AuNPs with different sizes and shapes. The phytosynthesized AuNPs are promising candidates for the application in biomedicine and morphology-dependent surface plasmon resonance sensors.

Numerical investigation of the resonance modes for the dielectric optical micro-spheres by using radial basis function method

In this paper, we present a numerical meshless method, which is based on the radial basis functions (RBF), to solve the morphology-dependent resonance (MDR) modes of a dielectric micro-sphere. To check the accuracy of the considered RBF method, we calculated the transverse electric (TE) and transverse magnetic (TM) modes of a perfect sphere that has an analytical characteristic equation. Besides, we employed the traditional mesh-dependent finite difference (FD) method to study the same problem. When we compared the results obtained by the three methods, we observe that the RBF method’s results are in very good agreement with the analytical and FD method’s results. We conclude that the presented meshless method may also be applicable to prolate and oblate spheroidal geometries, which do not have an analytical equation to calculate the modes.

Optical theorem of an infinite circular cylinder in weakly absorbing media

The conventional optical theorem cannot consider losses of host media. As a fundamental problem of light scattering, the generalized optical theorem of an infinite cylinder embedded in a weakly absorbing host is studied in this paper. Using the analytical way, we derive the generalized far-field and distance-independent extinction efficiencies per unit length of the cylinder under normally incident p- and s-polarized waves, which reduce to the conventional formulas in the transparent host media. For large cylinders, the increasing absorption of the host medium leads to an increasing amplitude of interference oscillation and the emergence of negative extinction, which is similar to that for spheres. Furthermore, the absorption of the host medium is proved to destroy the morphology-dependent resonance structure in extinction and suppress the electric field resonance inside the cylinder. For small cylinders in a weakly absorbing host medium, we present the conditions for negative extinction and quantitatively analyze the differences in extinction between the absorbing host medium and the nonabsorbing counterpart. It is found that the ratio of the extinction from the generalized theory to conventional formulas depends only on the refraction indices of the cylinder and the host medium. The results for small cylinders are rather sensitive to the state of polarization of the light. By illustrating the significant differences between the generalized optical theorem and the conventional theory with a specific case of a Ge cylinder in polyethylene, we show the non-negligible impacts of the absorption of the host media on the optical extinction of the cylinder for the two polarizations.

Whispering-gallery-mode sensors for biological and physical sensing

The term whispering gallery mode (WGM) was first introduced to describe the curvilinear propagation of sound waves under a cathedral dome. The physical concept has now been generalized to include light waves that are continuously reflected along the closed concave surface of an optical cavity such as a glass microsphere. The circular path of the internally reflected light results in constructive interference and optical resonance, a morphology-dependent resonance that is suitable for interferometric sensing. WGM resonators are miniature micro-interferometers that use the multiple-cavity passes of light for very sensitive measurements at the microscale and nanoscale, including single-molecule and ion measurements. This Primer introduces various WGM sensors based on glass microspheres, microtoroids, microcapillaries and silicon microrings. We describe the sensing mechanisms, including mode splitting and resonance shift, exceptional-point-enhanced sensing and optomechanical and optoplasmonic signal transductions. Applications and experimental results cover in vivo and single-molecule sensing, gyroscopes and microcavity quantum electrodynamics. We also discuss data analysis methods and the limitations of WGM techniques. Finally, we provide an outlook for molecule, in vivo and quantum sensing.

Morphology-dependent resonance enhanced nonlinear photoacoustic effect in nanoparticle suspension: a temporal-spatial model.

The morphology-dependent resonances (MDRs) hotspot, ubiquity formed between the pairs of nanoparticles in close vicinity, has garnered considerable recent attention. By extending this phenomenon to pulse-laser irradiated nanoparticle suspension, we demonstrate that such collective optical/thermal enhancement can give rise to the nonlinear photoacoustic (PA) generation. In this study, a temporal-spatial analytical expression is derived to quantitatively describe the nonlinear PA signal generation from nanoparticles, incorporating the Grüneisen increase at the microscopic individual particle level and MRDs enhancement at the macroscopic suspension level. The dependence of PA nonlinearity on the critical contributors, including the laser pulse width, the particle size, and the statistical interparticle spacing, is quantitatively discussed. The theory is well validated with the finite element method (FEM) and experimentally proved with semiconducting polymer nanoparticles (SPN) suspension. This work may pave a new direction towards effective MDR based nonlinear PA contract agent design.

Morphology-Dependent Resonances in Two Concentric Spheres with Variable Refractive Index in the Outer Layer: Analytic Solutions

In many applications constant or piecewise constant refractive index profiles are used to study the scattering of plane electromagnetic waves by a spherical object. When the structured media has variable refractive indices, this is more of a challenge. In this paper, we investigate the morphology dependent resonances for the scattering of electromagnetic waves from two concentric spheres when the outer shell has a variable refractive index. The resonance analysis is applied to the general solutions of the radial Debye potential for both transverse magnetic and transverse electric modes. Finally, the analytic conditions to determine the resonance locations for this system are derived in the closed form of both modes. Our numerical results are provided with discussion.

Morphology-dependent resonances in homogeneous and core-shell nonspherical particles

We formulate a perturbation theory for determining the frequencies of quasinormal modes in dielectric nonspherical core-shell particles. The theory is based on the extended boundary condition method (also known as the null-field method), which is used to represent the internal field in terms of a matrix $Q$. In this framework, the excitation of a morphology-dependent resonance corresponds to a vanishing eigenvalue of the $Q$ matrix, which can be determined using perturbation techniques to yield explicit formulas for the corresponding eigenfrequency. We specialize our results to the case of a core-shell spherical particle that has been deformed into a spheroid and provide a comparison between numerical simulations and the perturbation theory. Excellent agreement is found for the small perturbations considered here. Further, we investigate the eigenfrequencies of core-shell particles for an extensive range of parameters.

Analysis of Mie resonances using the Debye series.

Scattering of light from homogeneous spherical particles can exhibit sharp resonances as functions of particle size, wavelength, or refractive index. Such resonances, usually known as morphology-dependent resonances or whispering gallery modes, have been exhaustively studied using Mie theory. This paper demonstrates that the Debye series expansion provides a succinct and easily understood representation of these resonances: For example, the Debye coefficient Rn121 determines (a) the exact conditions for resonance, (b) the number of terms required to replicate the Mie result, and (c) the Q-factor of the resonance.

Internal morphology-dependent resonances of a coated spherical particle

Morphology-dependent resonances (MDRs) for a coated spherical particle are important because of their extensive applications. However, MDRs for coated spherical particles are more complicated than those for homogeneous particles. In this paper, a general expression is proposed for calculating the scattering efficiency of a specific layer of coated particle. Reformulations of internal scattering efficiency are made, which provides a new, to the best of our knowledge, method to investigate the resonance in the core and in the shell independently. MDRs in the core can be shown in four cases on the outmost scattering efficiency curve. To investigate the reason of the resonance, the relationships between MDRs in the core, outmost scattering efficiency, and partial wave are analyzed. It is numerically shown that the expressions are reliable and efficient, which provides a theoretical fundamental for studying of resonances of multilayered particles and for measurement.

Morphology-dependent resonances in laser-induced fluorescence images of micrometric gasoline/ethanol droplets utilizing the dye nile red.

The present study deals with the solvent-dependent morphology-dependent resonances (MDR) in the laser-induced fluorescence (LIF) signal of monodisperse gasoline droplets (30µm-60µm) generated with a droplet generator. To investigate the influence of an ethanol addition to gasoline and the respective LIF signal of the dye nile red dissolved in these fuel blends, a reference gasoline fuel is blended with various ethanol concentrations from E0 (gasoline) to E100 (pure ethanol). A spectral fluorescence characterization of the investigated fuel mixtures at various concentrations is carried out in a micro cell in order to identify the dye and ethanol concentration influence of the respective fuel mixtures. The absorption and emission spectra of the fuel mixtures show a Stokes shift with increasing ethanol concentration towards larger wavelengths. The coefficient of variation (COV) of the fluorescence signals of spherical droplets was utilized to characterize the MDR effects within the droplet LIF images. The investigations revealed an increase of MDR contribution in terms of the COV of LIF signals with larger droplet diameters. For small droplets, no monotonic trend was found for contribution of MDR in the LIF signal as a function of the ethanol concentration. For larger droplets (e.g., 50µm-60µm), a lower contribution of MDR in LIF signals was observed with increasing ethanol content. For E80 and most of the studied ethanol blends, the normalized integrated COV values exhibited maxima at certain droplet sizes (40µm, 47.5µm, and 55µm), which indicate the presence of distinct MDR effects.

Nonreciprocal Morphology-Dependent Resonance in Stacked Spinning Microresonators

As is well known, Sagnac-Fizeau light dragging effect may serve as the fundamental nonreciprocal mechanism for photonic devices. In this study, we theoretically analyze nonreciprocal morphology-dependent resonance in spinning stacked microresonators (SSμRs) based on this effect. A theoretical model is set up to analyze the impact of Sagnac-Fizeau effect on resonance characteristics of SSμRs. Two typical morphology-dependent resonances, i.e., Fano resonance and mode splitting resonance, have been discussed for the microresonators operating at different rotation frequencies. The simulation results presented in this work would provide significant guidance for the design of nonreciprocal photonic devices based on non-magneto-optical approach. Our proposed SSμRs are expected to find promising applications in unidirectional microcavity laser, quantum light chiral control and quantum optical communications.

Tuning morphology-dependent localized surface plasmon resonance in quasi-metallic tungsten oxide nanostructures for enhanced photocatalysis

Tunable localized surface plasmon resonance in quasi-metallic W18O49 was achieved by morphology control and the urchin-like nanospheres showed blue-shifted extinction as well as enhanced photoreduction properties towards Cr(vi).