arXiv subjects

Ilia L. Rasskazov

Publications and source records attributed to Ilia L. Rasskazov.

18 recordsLinked to original sources

arXiv2026

Super-Resonance: Interference-Driven Suppression of Radiative Decay Across Wave Physics

Super-resonance has been coined independently for collective modes of acoustic cavity arrays (Tolstoy), extreme high-$Q$ states of dielectric resonators in photonics, and magnon-polaron bound states in magnonics, and is now applied interchangeably to physically distinct phenomena. We organise this review around the one usage admitting a precise, field-agnostic definition: a mode whose radiative linewidth is driven toward zero by destructive interference among its radiation channels, $γ_{\rm rad}/γ_{\rm rad}^{\rm cl}\to0$, while the bare single-channel coupling remains large. The canonical realisation is the Friedrich-Wintgen scenario of two modes sharing a continuum. The same non-Hermitian eigenvalue structure recurs in Tolstoy's acoustic arrays, photonic bound states in the continuum and quasi-BIC supercavity modes, anapoles, dark modes of coherent perfect absorbers, parity-time-symmetric devices near exceptional points, magnonic dark modes, and topological edge modes. We survey these realisations field by field and distil transferable design rules and figures of merit. We then disambiguate super-resonance from two families sharing the "super-" prefix: coherent enhancement, in which coupling to the continuum is collectively increased rather than suppressed (Dicke super-radiance, superscattering, whispering-gallery modes); and integer-commensurability locking (mean-motion, wave-particle, and Floquet resonances). Amplification, as in black-hole super-radiance, is likewise distinguished.

physics.optics↗

arXiv2026

Scattering correction for infrared spectra of biological cells using computational infrared microspectroscopy and deep learning

IR microspectroscopy of single biological cells is challenged by strong light scattering, which produces baseline effects and peak distortions in the IR spectra and hinders the direct extraction of chemical information. Current methods for scattering correction typically rely on Mie theory and are accurate only under the assumption that the cell can be approximated by a sphere. Here, we present a framework for the scattering correction of IR absorbance spectra that is based on 3D ellipsoid models and provides efficient scattering correction for both suspended (spherical) and adhered (flattened) cells. Our approach combines deep learning approaches with computational IR microspectroscopy based on the finite-difference time-domain (FDTD) method. The FDTD method generates a synthetic library of realistic training spectra, while the deep learning model enables fast spectral inversion. We demonstrate scattering correction in silico using numerical cell phantoms of cervical cancer cells (HeLa) and show that the true absorption spectra can be inferred from IR absorbance spectra. We further show that the 3D cell dimensions can be recovered from the IR absorbance spectra, highlighting that the inherent light scattering could be exploited to realize the full analytical potential of IR spectroscopy. We anticipate that deep learning-based scattering corrections can be readily extended to increasingly complex sample geometries owing to the flexibility of the FDTD method to model arbitrary geometries.

physics.optics↗

arXiv2024

Intuitive understanding of extinction of small particles in absorbing and active host media within the MLWA

In an absorbing or an active host medium characterized by a complex refractive index $n_2=n_2'+{\rm i}n_2''$, our previously developed modified dipole long-wave approximation (MLWA) is shown to essentially overly with the exact Mie theory results for spherical nanoparticle with radius $a\lesssim 25$ nm ($a\lesssim 20$ nm) in the case of Ag and Au (Al and Mg) nanoparticles. The agreement for Au and Ag (Al and Mg) nanoparticles, slightly better in the case of Au than Ag, continues to be acceptable up to $a\sim 50$ nm ($a\sim 40$ nm), and can be used, at least qualitatively, up to $a\sim 70$~nm ($a\sim 50$ nm) correspondingly. A first order analytic perturbation theory (PT) in a normalized extinction coefficient, $\barκ=n_2''/n_2'$, around a nonabsorbing host is developed within the dipole MLWA and its properties are investigated. It is shown that, in a suitable parameter range, the PT can reliably capture the effect of host absorption or gain on the extinction efficiency of various plasmonic nanoparticles.

physics.optics↗

arXiv2024

Generation of nearly pure and highly directional magnetic light in fluorescence of rare earth ions

A thorough analysis of the emission via the magnetic dipole (MD) transition, called magnetic light below, of trivalent rare-earth ions in or near dielectric homogeneous spheres has been performed. In the search for enhancement of fluorescence from magnetic light, one faces the difficult task of identifying the regions where the combined fluorescence due to multiple electric dipole (ED) transitions becomes negligible compared to the fluorescence of the MD transition. We have succeeded in identifying a number of configurations with dielectric sphere parameters and a radial position of a trivalent rare-earth emitter wherein the branching ratio of the MD transition approaches its limit of one, implying that transitions from a given initial level (e.g., $^5$D$_0$-level of Eu$^{3+}$) are completely dominated by the MD transition. The dimensionless directivity of the MD emission, the radiative decay rates, and the fluorescence of the magnetic light can be increased by a factor of more than $25$, $10^3$, and $10^4$, respectively.

physics.optics↗

arXiv2024

Large fluorescence enhancement via lossless all-dielectric spherical mesocavities

Nano- and microparticles are popular media to enhance optical signals, including fluorescence from a dye proximal to the particle. Here we show that homogeneous, lossless, all-dielectric spheres with diameters in the mesoscale range, between nano- ($\lesssim 100$~nm) and micro- ($\gtrsim 1$ $μ$m) scales, can offer surprisingly large fluorescence enhancements, up to $F\sim 10^4$. With the absence of nonradiative Ohmic losses inherent to plasmonic particles, we show that $F$ can increase, decrease or even stay the same with increasing intrinsic quantum yield $q_0$, for suppressed, enhanced or intact radiative decay rates of a fluorophore, respectively. Further, the fluorophore may be located inside or outside the particle, providing additional flexibility and opportunities to design fit for purpose particles. The presented analysis with simple dielectric spheres should spur further interest in this less-explored scale of particles and experimental investigations to realize their potential for applications in imaging, molecular sensing, light coupling, and quantum information processing.

physics.optics↗

arXiv2023

Clausius-Mossotti Relation Revisited: Media with Electric and Magnetic Response

A reexamination of the Clausius-Mossotti relation in which material with both electric and magnetic responses yields surprising results. Materials with indices near zero and with real parts less than zero, that is the real part of both the permeability and permittivity are negative, are found to emerge from the interaction of electric and magnetic responses in a self-consistent theory. The new results point the way to artificial and natural materials with exotic responses. A simulation with 10^10 particles shows good agreement with the analytical results.

physics.optics↗

arXiv2023

Experimental demonstration of superdirective spherical dielectric antenna

An experimental demonstration of directivities exceeding the fundamental Kildal limit, a phenomenon called superdirectivity, is provided for spherical high-index dielectric antennas with an electric dipole excitation. A directivity factor of about 10 with a total efficiency of more than 80\% for an antenna having a size of a third of the wavelength was measured. High directivities are shown to be associated with constructive interference of particular electric and magnetic modes of an open spherical resonator. Both analytic solution for a point dipole and a full-wave rigorous simulation for a realistic dipole antenna were employed for optimization and analysis, yielding an excellent agreement between experimentally measured and numerically predicted directivities. The use of high-index low-loss ceramics can significantly reduce the physical size of such antennas while maintaining their overall high radiation efficiency. Such antennas can be attractive for various high-frequency applications, such as antennas for the Internet of things, smart city systems, 5G network systems, and others. The demonstrated concept can be scaled in frequency.

physics.class-ph↗

arXiv2022

Metal-Dielectric-Enhanced Upconversion: Going "Meso"

A novel concept of a \textit{mesocavity}, based on a simple yet counter-intuitive idea of covering metal nanoparticles with unusually thick ($\gtrsim 100$ nm) high-index dielectric shells, is demonstrated in technologically relevant and largely sought-after case of an upconversion (UC) enhancement. For readily available combinations of materials and geometrical parameters of metal-dielectric core-shell mesocavity, record high values of UC enhancement of $\gtrsim 10^6$ can be achieved on the cavity surface. For emitter inside the high-index dielectric shell with refractive index $n=2.7$, the Lorentz local-field correction can provide an additional enhancement factor for UC of $\approx 92$, resulting in UC enhancement of $\gtrsim 10^8$, which is three orders of magnitude larger than it has ever been reported for individual particles. Our mesocavity concept facilitates a beneficial synergy of plasmonic and whispering gallery-mode resonance functionalities within an intermediary region between nanoscale ($\lesssim 100$ nm) and microscale ($\gtrsim 1$ $μ$m).

physics.optics↗

arXiv2022

Is there a proper figure of merit for a plasmonic structure involved in metal-enhanced fluorescence?

A suitability of a nanostructure for a metal-enhanced fluorescence (MEF) is usually assessed from the value of the maximal fluorescence enhancement factor, $\mathcal{F}$, which it can generate. However, $\mathcal{F}$ is an ambiguous quantity which may dramatically depend on the intrinsic quantum yield, $q_0$, of the emitter in a free space. Here we suggest $\mathcal{F}=\mathcal{F}_{q_0=1}$ taken in the limiting case of $q_0=1$ as a proper figure of merit for a plasmonic structure involved in MEF. Any other realistic $\mathcal{F}$ in the MEF regime can be recovered from $\mathcal{F}_{q_0=1}$ by a simple $1/q_0$ scaling.

physics.optics↗

arXiv2021

Harnessing superdirectivity in dielectric spherical multilayer antennas

Small form-factor, narrowband, and highly directive antennas are of critical importance in a variety of applications spanning wireless communications, remote sensing, Raman spectroscopy, and single photon emission enhancement. Surprisingly, we show that the classical directivity limit can be appreciably surpassed for electrically small multilayer spherical antennas excited by a point electric dipole even if limiting ourselves to purely dielectric materials. Experimentally feasible designs of superdirective antennas are established by using a stochastic optimization algorithm combined with a rigorous analytic solution.

physics.app-ph↗

arXiv2021

Excitation of a homogeneous dielectric sphere by a point electric dipole

Electrically small dielectric antennas are of great interest for modern technologies, since they can significantly reduce the physical size of electronic devices for processing and transmitting information. We investigate the influence of the resonance conditions of an electrically small dielectric spherical antenna with a high refractive index on its directivity and analyze the dependence of these resonances on the effectively excited modes of the dielectric sphere.

physics.optics↗

arXiv2021

Clustering Diffused-Particle Method for Scattering from Large Ensembles of Electromagnetically Polarizable Particles

The Foldy-Lax equation is generalized for a medium which consists of particles with both electric and magnetic responses. The result is used to compute fields scattered from ensembles of particles. The computational complexity is reduced by hierarchical clustering techniques to enable simulations with on the order of 10^10 particles. With so many particles we are able to see the transition to bulk media behavior of the fields. For non-magnetic materials, the observable index, permittivity, and permeability of the effective bulk medium are in good agreement with the Clausius-Mossotti relation. The fields simulated for particles with both electric and magnetic responses are in good agreement with new analytical results for a generalized effective medium theory.

physics.optics↗

arXiv2021

Extraordinary fluorescence enhancement in metal-dielectric core-shell nanoparticles

Contrary to a paradigm of metal-enhanced fluorescence, unusually thick dielectric coatings can be very favorable to achieve extreme values of averaged fluorescence enhancement factor $\bar F\gtrsim 3000$ for emitters located on the surface, or in the interior, of the shell of Au@dielectric spherical core-shell particles under realistic conditions, even for the emitters with 100% intrinsic quantum yield. Thick dielectric coatings facilitate high-quality transverse electric (TE) multipole ($\ell=7$) resonances which are shown as the major cause for the reported extraordinary values of $\bar F$.

physics.optics↗

arXiv2020

Remarkable Predictive Power of the Modified Long Wavelength Approximation

The modified long-wavelength approximation (MLWA), a next order approximation beyond the Rayleigh limit, has been applied usually only to the dipole $\ell=1$ contribution and for the range of size parameters $x$ not exceeding $x\lesssim 1$ to estimate far- and near-field electromagnetic properties of plasmonic nanoparticles. Provided that the MLWA functional form for the $T$-matrix in a given channel $\ell$ is limited to the ratio $T\sim iR/(F+D-iR)$, where $F$ is the familiar size-independent Fröhlich term and $R\sim {\cal O}(x^{2\ell+1})$ is a radiative reaction term, there is a one-parameter freedom in selecting the dynamic depolarization term $D\sim {\cal O}(x^2)$ which preserves the fundamental feature of the MLWA that its predictions coincide with those of the Mie theory up to the order ${\cal O}(x^2)$. By exploiting this untapped design freedom, we demonstrate on a number of different metals (Ag, Al, Au, Mg), and using real material data, that the MLWA may surprisingly yield very accurate results for plasmonic spheres both for (i) $x$ up to $\gtrsim 1$ and beyond, and (ii) higher order multipoles ($\ell>1$), essentially doubling its expected range of validity. Because the MLWA obviates the need of using spherical Bessel and Hankel functions and allows for an intuitive description of (nano)particle properties in terms of a driven damped harmonic oscillator parameters, a significantly improved analysis and understanding of nanoparticle scattering and near-field properties can be achieved.

physics.optics↗

arXiv2020

STRATIFY: a comprehensive and versatile MATLAB code for a multilayered sphere

We present a computer code for calculating near- and far-field electromagnetic properties of multilayered spheres. STRATIFY is one-of-a-kind open-source package which allows for the efficient calculation of electromagnetic near-field, energy density, total electromagnetic energy, radiative and non-radiative decay rates of a dipole emitter located in any (non-absorbing) shell (including a host medium), and fundamental cross-sections of a multilayered sphere, all within a single program. The developed software is typically more than 50 times faster than freely available packages based on boundary-element-method. Because of its speed and broad applicability, our package is a valuable tool for analysis of numerous light scattering problems, including, but not limited to fluorescence enhancement, upconversion, downconversion, second harmonic generation, surface enhanced Raman spectroscopy. The software is available for download from GitLab https://gitlab.com/iliarasskazov/stratify

physics.optics↗

arXiv2020

An intriguing branching of the maximum position of the absorption cross section in Mie theory explained

A potential control over the position of maxima of scattering and absorption cross-sections can be exploited to better tailor nanoparticles for specific light-matter interaction applications. Here we explain in detail the mechanism of an appreciable blue shift of the absorption cross-section peak relative to a metal spherical particle localized surface plasmon resonance (LSPR) and remaining scattering and extinction cross sections. Such a branching of cross sections maxima requires a certain threshold value of size parameter x~0.7 and is a prerequisite for obtaining high fluorescence enhancements, because the spectral region of high radiative rate enhancement becomes separated from the spectral region of high non-radiative rate enhancement. A consequence is that the maximum of the absorption cross section cannot be used as the definition of the LSPR position for x>0.7.

physics.optics↗

arXiv2020

The critical role of shell in enhanced fluorescence of metal-dielectric core-shell nanoparticles

Large scale simulations are performed by means of the transfer-matrix method to reveal optimal conditions for metal-dielectric core-shell particles to induce the largest fluorescence on their surfaces. With commonly used plasmonic cores (Au and Ag) and dielectric shells (SiO2, Al2O3, ZnO), optimal core and shell radii are determined to reach maximum fluorescence enhancement for each wavelength within 550~850 nm (Au core) and 390~500 nm (Ag core) bands, in both air and aqueous hosts. The peak value of the maximum achievable fluorescence enhancement factors of core-shell nanoparticles, taken over entire wavelength interval, increases with the shell refractive index and can reach values up to 9 and 70 for Au and Ag cores, within 600~700 nm and 400~450 nm wavelength ranges, respectively, which is much larger than that for corresponding homogeneous metal nanoparticles. Replacing air by an aqueous host has a dramatic effect of nearly halving the sizes of optimal core-shell configurations at the peak value of the maximum achievable fluorescence. In the case of Au cores,the fluorescence enhancements for wavelengths within the first near-infrared biological window (NIR-I) between 700 and 900 nm can be improved twofold compared to homogeneous Au particle when the shell refractive index ns > 2. As a rule of thumb, the wavelength region of optimal fluorescence (maximal nonradiative decay) turns out to be red-shifted (blue-shifted) by as much as 50 nm relative to the localized surface plasmon resonance wavelength of corresponding optimized core-shell particle. Our results provide important design rules and general guidelines for enabling versatile platforms for imaging, light source, and biological applications.

physics.optics↗

arXiv2019

Electromagnetic energy in multilayered spherical particles

We obtain exact analytic expressions for (i) the electromagnetic energy radial density within and outside a multilayered sphere and (ii) the total electromagnetic energy stored within its core and each of its shells. Explicit expressions for the special cases of lossless core and shell are also provided. The general solution is based on compact recursive transfer-matrix method and its validity includes also magnetic media. The theory is illustrated on the examples of electric field enhancement within various metallo-dielectric silica-gold multilayered spheres. User-friendly MATLAB code which includes the theoretical treatment, is available as a supplement to the paper.

physics.optics↗