arXiv subjects

Asaf Farhi

Publications and source records attributed to Asaf Farhi.

At least 19 recordsLinked to original sources

arXiv2026

Time-Domain Excitation of Finite-Lifetime Resonances and Their Exceptional Points

Resonances associated with complex-frequency poles are ubiquitous across physics and can arise in any open system, ranging from subwavelength particles and cavities to biological structures. When two such resonances coalesce, they form exceptional points (EPs), non-Hermitian singularities known to produce unusual spectral and dynamical behavior. However, the dynamics of the response of such resonances and exceptional points to complex frequency drive remained largely unexplored. Here, we experimentally observe the temporal response of complex-frequency resonances and theoretically study this for exceptional points. We unveil a universal transient phenomenon of open cavities driven at complex frequencies: the system's initial response grows linearly, with enhanced growth at exceptional points (EPs), even though the system is passive and the excitation decays. Closed-form theory for general resonators, extended to higher-order modes, predicts efficient power transfer with $t$ and $t^2$ scaling for complex single poles and exceptional points (EPs), respectively, at all times. We demonstrate these effects in subwavelength optical scatterers and experimentally in an electrical circuit analogue, with excellent agreement, and explore configurations that capture EP-enhanced growth.

physics.optics↗

arXiv2026

Driven Atoms and Molecules as Coherent Attosecond Waveform Processors

Advancing temporal resolution in computation, signal modulation, and quantum measurement is fundamentally constrained in optical resonator platforms by a trade-off between operation time and temporal resolution. We show that driven atomic and molecular systems perform temporal integration of resonant pulses via passive coherent absorption or stimulated emission, enabling attosecond resolution together with long operation time. We derive an analytic description of this mechanism through Bloch equations and time-dependent Schrodinger equation, finding quantitative agreement. We further identify feasible atomic transitions and excitation schemes accessible with current technology, and describe implementations for temporal differentiation and waveform generation at attosecond resolution. These results establish driven quantum transitions as a framework for attosecond-scale waveform processing beyond conventional resonator approaches, with potential applications in computation, optical switching, system control, and data transfer.

physics.optics↗

arXiv2026

Universal framework for anisotropic particles with resonance laws and splitting

Nanophotonics enables precise control over light-matter interactions, though most established design frameworks for subwavelength nanoparticles rely on isotropic materials. Uniaxial and biaxial particles -- common in natural and engineered systems -- introduce new degrees of freedom coupling geometry and material properties, unlocking multispectral and directional response in previously unexplored spectral regions. We present a universal full-wave framework for eigenmodes and resonances in such nanoparticles. Closed-form solutions reveal axial-permittivity sum rules and anisotropy-induced symmetry breaking, producing resonance splitting and novel radiation patterns. Generalizing to ellipsoids enables geometric tuning of multispectral response, while analytic quality factors elucidate mode localization and loss. Full-wave simulations of h-BN and $α$-MoO3 particles confirm the theory. This framework unifies the understanding of anisotropic nanostructures across optics, magnetism, and thermal transport, opening pathways to a new generation of photonic devices with tunable multispectral response and controlled emission with direct applications in sensing and imaging

physics.app-ph↗

arXiv2024

Localized Resonant Phonon Polaritons in Biaxial Nanoparticles

The discovery of localized plasmon polariton resonances has been pivotal in enabling tunability of the optical resonance. Recently, extensive research efforts have aimed to expand these achievements to other polaritonic states that exhibit less loss and in other spectral regions. However, these efforts were limited to isotropic or uniaxial structures, and an eigenmode theory was derived only for isotropic particles. Here, we present a breakthrough in synthesizing biaxial nanostructures that exhibit localized hyperbolic phonon resonances with high Q-factors in the mid-infrared. Furthermore, we develop a theory that predicts high-order resonances in anisotropic particles with coupling between the axial permittivites. Finally, we confirm the theoretical predictions through near-field measurements, which demonstrate the existence of both the first and higher-order resonant modes. Our findings provide the foundation for designing a new generation of anisotropic resonators with various applications in the mid-IR range. Our analysis applies to other fields, such as quasi-magnetostatics and heat conduction.

physics.optics↗

arXiv2023

Generating and processing optical waveforms using spectral singularities

We show that a laser at threshold can be utilized to generate the class of coherent and transform-limited waveforms $\left(vt-z\right)^{m}e^{i\left(kz-ωt\right)}$ at optical frequencies.We derive these properties analytically and demonstrate them in semiclassical time-domain laser simulations. We then utilize these waveforms to expand other waveforms with high modulation frequencies and demonstrate theoretically the feasibility of complex-frequency coherent-absorption at optical frequencies, with efficient energy transduction and cavity loading. This approach has potential applications in quantum computing, photonic circuits, and biomedicine.

physics.optics↗

arXiv2023

Efficient General Waveform Catching by a cavity at a Virtual Absorbing Exceptional Point

State transfer and photon detection are fundamental processes that have direct implications in fields such as quantum computing and photonic circuits. However, while naturally emitted photons decay exponentially in time, to perfectly capture a photon its envelope should increase exponentially to match the time-reversed response of the absorbing cavity. Here we show that a cavity at a virtual absorbing exceptional point captures additional temporal orders of an incoming waveform, resulting in efficient passive state transfer and photon detection. This approach paves the way for state transfer at optical frequencies and efficient detection of a spontaneously emitted photon.

quant-ph↗

arXiv2022

Excitation of absorbing exceptional points in the time domain

We analyze the time-domain dynamics of resonators supporting exceptional points (EPs), at which both the eigenfrequencies and the eigenmodes associated with perfect capture of an input wave coalesce. We find that a time-domain signature of the EP is an expansion of the class of waveforms which can be perfectly captured. We show that such resonators have improved performance for storage or transduction of energy. They also can be used to convert between waveforms within this class. We analytically derive these features and demonstrate them for several examples of coupled optical resonator systems.

physics.optics↗

arXiv2022

Purcell factors and Forster resonance energy transfer in proximity to helical structures

Both spontaneous emission and resonant energy transfer can be enhanced significantly when the emitter is placed in the vicinity of metallic or crystal structures. This enhancement can be described using the electromagnetic Green tensor and is determined by the dominant surface modes of the structure. Here we use the eigenpermittivity formalism to derive the spontaneous emission and FRET rates in the quasistatic regime in a two-constituent medium with an anisotropic inclusion. We then apply our results to a helical structure supporting synchronous vibrations and evaluate the contribution of these modes, which are associated with a strong and delocalized response. We show that this contribution can result in large Purcell factors and long-range FRET, which oscillates with the helix pitch. These findings may have implications in understanding and controlling the interactions of molecules close to helical structures such as the microtubules.

physics.optics↗

arXiv2020

Coupling electrodynamic fields to vibrational modes in helical structures

Helical structures like alpha helices, DNA, and microtubules have profound importance in biology. It has been suggested that these periodic arrangements of constituent units could support collective excitations similarly to crystalline solids. Here, we examine the interaction between such constructs and oscillating dipoles, and evaluate the role of the helicity in the coupling between electrodynamic fields and vibrations. Based on a vibrational and eigenfunction analyses we discover a group of modes of coherent oscillations that give rise to a strong and delocalized response, selectivity in frequency, and typical interaction range. To describe the field scattering due to the structure vibrations we consider an anisotropic permittivity with a helical periodicity, which applies to all vibration types and close dipole locations. This new type of resonances identified here may help explain the role of electrodynamic fields in the diverse functionality of cytoskeletal microtubules in the cellular environment.

physics.optics↗

arXiv2019

Three-dimensional-subwavelength field localization, time reversal of sources, and infinite-asymptotic degeneracy in spherical structures

High-resolution field localization in three dimensions is one of the main challenges in optics and has immense importance in fields such as chemistry, biology, and medicine. In order to generate the time reversed signal of a monochromatic source \emph{discrete} sources that are modulated according to the wave amplitude on a spherical envelope are required, rendering it applicable only in acoustics. Here we approach these challenges by introducing a spherical layer with a resonant permittivity, which naturally generates the spatially \emph{continuous} time-reversed signal of an atomic and molecular multipole transition at the origin. We start by utilizing a spherical layer with a resonant TM $l=1$ permittivity situated in a uniform medium to generate a free-space-subwavelength focal spot at the origin. We remove the degeneracy of the eigenfunctions of the composite medium by situating a point current source (or polarization) directed parallel to the spherical layer, which generates a focal spot at the origin \emph{independently} of its location. The free-space focal spot has a full width at half maximum of $0.4λ$ in the lateral axes and $0.58λ$ in the axial axis, which is tighter by a factor of $\sqrt{2}$ in each dimension in excitation-collection mode, overcoming the $λ/2$ far-field resolution limit in three dimensions. We then explore two directions to localize electric field with deep-subwavelength resolution in three dimensions using this setup. In addition, we show that spherical structures exhibit a new type of degeneracy in which an infinite number of eigenvalues asymptotically coalesce. This high degeneracy results in a variety of optical phenomena such as strong scattering and enhancement of absorption and emission from an atom or molecule by orders of magnitude compared with a standard resonance.

physics.optics↗

arXiv2017

Generating an electromagnetic multipole by oscillating currents

Based on the relation between a plane phased array and plane waves we show that a spherical current layer or a current sphere proportional to a multipole electric field and situated in a uniform medium generates the same multipole field in all space. We calculate TE and TM multipoles inside and outside the spherical layer. The $l=1$ TM multipoles are localized at the origin with a focal spot with full width at half maximum of $0.4λ$ in the lateral axes and $0.58λ$ in the vertical axis. The multipole fields near the origin are prescriptions for the current distributions required to generate those multipole fields. A spherical layer can couple to a multipole source since the oscillation of the electrons in the layer due to the multipole field generates the multipole field in all space, which in turn can drive the multipole currents. Exciting a multipole in a polarizable sphere or spherical layer can couple it to another polarizable sphere or spherical layer.

physics.optics↗

arXiv2017

The electric field of a point charge in a spherical inclusion structure

A point charge in the presence of a metallic nanoshpere is a fundamental setup, which has implications for Raman scattering, enhancement of spontaneous emission of a molecule by an antenna, sensing, and modeling a metallic tip in proximity to a nanoparticle. Here, we analytically expand the electric field of a point charge in an $ε_{2}$ host medium in the presence of an $ε_{1}$ sphere using the sphere eigenstates, where $ε_1$ and $ε_2$ can take any values. Only the $m=0$ spherical harmonics are employed in the expansion and the calculation of the potential and the electric field is very simple. The electric field is strongly enhanced when $ε_{1}/ε_2$ is close to an $(ε_{1}/ε_2)_l$ eigenvalue of a dominant mode, which is determined by the point charge location and the measurement point. An electric field exists inside the sphere when $ε_{1}/ε_2$ is close to a $(ε_{1}/ε_2)_l$ resonance even when $ε_1$ is a conductor. Low order modes generate an electric field far away from the interface, where the $l=1$ mode with a resonance at $ε_1=-2ε_2$ generates a field at the sphere center. The high order modes which are associated with high spatial frequencies become more dominant when the point charge approaches the sphere surface or when the physical parameters are close the high order modes resonances. When $ε_{1}/ε_2$ is smaller or larger than the eigenvalues of the dominant modes, the modes interfere constructively and generate a strong signal at an angular direction equal to that of the source. The spectral information at the sphere surface may be utilized to calculate the point charge location without knowing its magnitude.

physics.optics↗

arXiv2016

A novel method for calculating relative free energy of similar molecules in two environments

Calculating relative free energies is a topic of substantial interest and has many applications including solvation and binding free energies, which are used in computational drug discovery. However, there remain the challenges of accuracy, simple implementation, robustness and efficiency, which prevent the calculations from being automated and limit their use in computational drug discovery. Here we present an exact and complete decoupling analysis in which the partition functions of the compared systems decompose into the partition functions of the common and different subsystems. This decoupling analysis is applicable to submolecules with coupled degrees of freedom and to any potential function, enabling to remove less terms in the transformation. Then we show mathematically, in the context of partition function decoupling, that the two compared systems can be simulated separately, eliminating the need to design a composite system. We demonstrate the decoupling analysis and the separate transformations in an MD calculation of relative free energy for a general force field which is in agreement with experiments. We present a unified soft core technique that will ensure the monotonicity of the numerically integrated function. Finally, we show that when the systems have rugged energy landscape they can be equilibrated without introducing another sampling dimension. The concepts presented in the article have implications in accuracy, efficiency, simplicity and robustness of free energy calculations.

physics.chem-ph↗

arXiv2016

Eigenstates of the full Maxwell equations for a two-constituent composite medium and their application to a calculation of the local electric field of a time dependent point electric dipole in a flat-slabs microstructure

An exact calculation of the local electric field ${\bf E}({\bf r})$ is described for the case of a time dependent point electric dipole ${\bf p}e^{-iωt}$ in the top layer of an $ε_2$, $ε_1$, $ε_2$ three parallel slabs composite structure, where the $ε_1$ layer has a finite thickness $2d$ but the $ε_2$ layers are infinitely thick. For this purpose we first calculate all the eigenstates of the full Maxwell equations for the case where $μ=1$ everywhere in the system. The eigenvalues appear as special, non-physical values of $ε_1$ when $ε_2$ is given. These eigenstates are then used to develop an exact expansion for the physical values of ${\bf E}({\bf r})$ in the system characterized by physical values of $ε_1(ω)$ and $ε_2(ω)$. Results are compared with those of a previous calculation of the local field of a time dependent point charge in the quasi-static regime. Numerical results are shown for the local electric field in practically important configurations where attaining an optical image with sub-wavelength resolution has practical significance.

physics.optics↗

arXiv2015

Calculation of molecular free energies in classical potentials

Free energies of molecules can be calculated by quantum computations or by normal mode classical calculations. However, the first can be computationally impractical for large molecules and the second is based on the assumption of harmonic dynamics. We present a novel, accurate and complete calculation of molecular free energies in standard classical potentials. In this method we transform the molecule by relaxing potential terms which depend on the coordinates of a group of atoms in that molecule and calculate the free energy difference associated with the transformation. Then, since the transformed molecule can be treated as non interacting systems, the free energy associated with these atoms is analytically or numerically calculated. This two-step calculation can be applied to calculate free energies of molecules or free energy difference between (possibly large) molecules in a general environment. We suggest the potential application of free energy calculation of chemical reactions in classical molecular simulations.

physics.chem-ph↗

arXiv2015

Free energy calculation of a molecule by removing VDW and Coulomb interactions in a transformation and treating the molecule as non interacting systems

Free energy calculations in molecular simulations have a variety of applications including determining the strength of molecular processes such as solvation and binding. It has been recently shown that when removing the VDW and Coulomb potential terms of a group of atoms in a molecule by performing a transformation, the molecule can be treated as non interacting systems in the free energy calculation. This treatment is applicable both when the molecule is in vacuum and in liquid and enables a very simple calculation of the free energies associated with the potentials that depend on the relative spherical coordinates of these atoms. Here we demonstrate the method in the free energy calculation of a Methanethiol molecule and compare the results to these obtained by MD simulations in vacuum and in water. The comparison shows agreement between the results and faster computation when using the method by factors varying between 5000 and 10^(12) for the same computational resources.

physics.chem-ph↗

arXiv2014

Exact analysis of a Veselago lens in the quasi-static regime

The resolution of conventional optical lenses is limited by the wavelength. Materials with negative refractive index have been shown to enable the generation of an enhanced resolution image where both propagating and non-propagating waves are employed. We analyze such a Veselago lens by exploiting some exact one dimensional integral expressions for the quasi-static electric potential of a point charge in that system. Those were recently obtained by expanding that potential in the quasi-static eigenfunctions of a three-flat-slabs composite structure. Numerical evaluations of those integrals, using realistic values for physical parameters like the electric permittivities of the constituent slabs and their thickness, reveal some surprising effects: E.g., the maximum concentration of the electric field occurs not at the geometric optics foci but at the interfaces between the negative permittivity slab and the positive permittivity slabs. The analysis provides simple computational guides for designing such structures in order to achieve enhanced resolution of an optical image.

physics.optics↗

arXiv2013

A general, efficient and robust method to calculate free energy difference between systems

Calculating free energy differences is a topic of substantial interest and has many applications including molecular docking and hydration, solvation, and binding free energies which is used in computational drug discovery. However, in equilibrium methods the compared molecules are required to have large phase space overlap, which is usually not satisfied for two random systems, and there remain the challenges of robustness and automation. Here a highly efficient and robust method, that enables a wide range of comparisons, will be introduced, demonstrated and compared. In this method instead of transforming between one system into the other to perform the calculation each system is transformed into its replica with the different long range energy terms relaxed, which is inherently correlated with the original one, in order to eliminate the partition function difference arising from these terms. Then, since each transformed system can be treated as non interacting systems, the remaining difference in the (originally highly complex) partition function will cancel out.

physics.chem-ph↗