arXiv subjects

Tigran V. Shahbazyan

Publications and source records attributed to Tigran V. Shahbazyan.

At least 19 recordsLinked to original sources

arXiv2025

Universal optical polarizability for plasmonic nanostructures

We develop an analytical model for calculation of optical spectra for metal nanostructures of arbitrary shape supporting localized surface plasmons (LSPs). For plasmonic systems with characteristic size below the diffraction limit, we obtain an explicit expression for optical polarizability that describes the lineshape of optical spectra solely in terms of the metal dielectric function and LSP frequency. The amplitude of the LSP spectral band is determined by the effective system volume that, for long-wavelength LSPs, can significantly exceed the physical volume of metal nanostructure. Within the quasistatic approach, we derive the exact LSP Green's function and establish general spectral properties of LSPs, including the distribution and oscillator strength of the LSP states. These results can be used to model or interpret the experimental spectra of plasmonic nanostructures and to tune their optical properties for various applications.

cond-mat.mes-hall↗

arXiv2025

Plasmon-induced coherence, exciton-induced transparency and Fano interference for hybrid plasmonic systems in strong coupling regime

We present an analytical model describing the transition to strong coupling regime for an ensemble of emitters resonantly coupled to a localized surface plasmon in a metal-dielectric structure. The response of a hybrid system to an external field is determined by two distinct mechanisms involving collective states of emitters interacting with the plasmon mode. First is the near-field coupling between the bright collective state and the plasmon mode which underpins energy exchange between the system components and gives rise to exciton-induced transparency minimum in scattering spectra in the weak coupling regime and to emergence of polaritonic bands as the system transitions to strong coupling regime. The second mechanism is the Fano interference between the plasmon dipole moment and plasmon-induced dipole moment of bright collective state as the hybrid system interacts with the radiation field. The latter mechanism is greatly facilitated by plasmon-induced coherence in a system with characteristic size below the diffraction limit as the individual emitters comprising the collective state are driven by same alternating plasmon near field and therefore all oscillate in phase. This cooperative effect leads to scaling of the Fano asymmetry parameter and of the Fano function amplitude with the ensemble size and therefore it strongly affects the shape of scattering spectra for large ensembles. Specifically, with increasing emitters numbers, the Fano interference leads to a spectral weight shift towards the lower energy polaritonic band.

physics.optics↗

arXiv2025

A non-Lorentzian model for strong exciton-plasmon coupling

We develop a non-Lorentzian analytical model for quantum emitters (QE) resonantly coupled to localized surface plasmons (LSP) in metal-dielectric structures. Using the explicit form of LSP Green function, we derive non-Lorentzian version of semiclassical Maxwell-Bloch equations that describe LSPs directly in terms of metal complex dielectric function rather than via Lorentzian resonances. For a single QE resonantly coupled to an LSP, we obtain an analytical expression for effective optical polarizability of the hybrid system which, in the Lorentzian approximation, recovers the results of the classical coupled oscillators model. We demonstrate that non-Lorentzian effects originating from temporal dispersion of the metal dielectric function affect significantly the optical spectra as the hybrid system transitions to the strong coupling regime. Specifically, in contrast to results of Lorentzian models, the main spectral weight in the system scattering spectra is shifted toward the lower energy polaritonic band, consistent with the experiment.

cond-mat.mes-hall↗

arXiv2025

Roadmap on Nonlocality in Photonic Materials and Metamaterials

Photonic technologies continue to drive the quest for new optical materials with unprecedented responses. A major frontier in this field is the exploration of nonlocal (spatially dispersive) materials, going beyond the local, wavevector-independent assumption traditionally made in optical material modeling. On one end, the growing interest in plasmonic, polaritonic and quantum materials has revealed naturally occurring nonlocalities, emphasizing the need for more accurate models to predict and design their optical responses. This has major implications also for topological, nonreciprocal, and time-varying systems based on these material platforms. Beyond natural materials, artificially structured materials--metamaterials and metasurfaces--can provide even stronger and engineered nonlocal effects, emerging from long-range interactions or multipolar effects. This is a rapidly expanding area in the field of photonic metamaterials, with open frontiers yet to be explored. In the case of metasurfaces, in particular, nonlocality engineering has become a powerful tool for designing strongly wavevector-dependent responses, enabling enhanced wavefront control, spatial compression, multifunctional devices, and wave-based computing. Furthermore, nonlocality and related concepts play a critical role in defining the ultimate limits of what is possible in optics, photonics, and wave physics. This Roadmap aims to survey the most exciting developments in nonlocal photonic materials, highlight new opportunities and open challenges, and chart new pathways that will drive this emerging field forward--toward new scientific discoveries and technological advancements.

cond-mat.mes-hall↗

arXiv2023

Purcell factor for plasmon-enhanced metal photoluminescence

We present an analytical model for plasmonic enhancement of metal photoluminescence (MPL) in metal nanostructures with characteristic size below the diffraction limit. In such systems, the primary mechanism of MPL enhancement is excitation of localized surface plasmons (LSP) by recombining carriers followed by a photon emission due to LSP radiative decay. For plasmonic nanostructures of arbitrary shape, we obtain a universal expression for MPL Purcell factor that describes the plasmonic enhancement of MPL in terms of metal dielectric function, LSP frequency, and the system volume. We find that the lineshape of MPL spectrum is affected by the interference between direct carrier recombination processes and those mediated by plasmonic antenna which leads to a blueshift of MPL spectral band relative to LSP resonance in scattering spectra observed in numerous experiments.

cond-mat.mes-hall↗

arXiv2022

Light absorption by weakly rough metal surfaces

We study light absorption by weakly rough metal surfaces with the roughness amplitude and correlation length smaller than the skin depth in metal. We develop a systematic perturbative approach for calculation of the absorptance in such systems and find that roughness-related absorptance variations are determined by an interplay between several system parameters which can result, in particular, in a greater absorption for smaller roughness amplitudes. We show that, for small-scale roughness, the absorptance variations are mainly caused by roughness-induced increase in effective volume of the surface layer, in which the incident light is predominantly absorbed. We argue that such absorptance fluctuations between different samples, even though not related to any electron scattering processes, can appear as sample-to-sample variations of the Drude scattering rate reported in recent measurements of the metal dielectric function.

cond-mat.mes-hall↗

arXiv2022

Spontaneous decay of a quantum emitter near a plasmonic nanostructure

We develop a theory for spontaneous decay of a quantum emitter (QE) situated near metal-dielectric structure supporting localized surface plasmons. If plasmon resonance is tuned close to the QE emission frequency, the emission is enhanced due to energy transfer from the QE to a localized plasmon mode followed by photon emission by plasmonic antenna. The emission rate is determined by intimate interplay between the plasmon coupling to radiation field and the Ohmic losses in metal. Here we develop plasmon Green's function approach that includes plasmon's interaction with radiation to obtain explicit expressions for radiative decay rate and optical polarizability of a localized plasmon mode in arbitrary plasmonic nanostructure. Within this approach, we provide consistent definition of plasmon mode volume by relating it to plasmon mode density, which characterizes the plasmon field confinement, and recover the standard cavity form of the Purcell factor, but now for plasmonic systems. We show that, for QE placed at a "hot spot" near a sharp tip of a small metal nanostructure, the plasmon mode volume scales with the metal volume while being very sensitive to the proximity to the tip. Finally, we derive the enhancement factor for radiated power spectrum for any nanoplasmonic system and relate it to the Purcell factor for spontaneous decay rate. We illustrate our results by numerical example of a QE situated near gold nanorod tip.

cond-mat.mes-hall↗

arXiv2022

Non-Markovian effects for hybrid plasmonic systems in the strong coupling regime

We study the role of non-Markovian effects in the emission spectrum of a quantum emitter resonantly coupled to a surface plasmon in a metal-dielectric structure as the system transitions to strong coupling regime. By using a recent quantum approach to interacting plasmons that incorporates the effects of host material's optical dispersion and losses in the coupling parameters, we obtain analytically the emission spectrum for a plasmonic system of arbitrary shape with characteristic size below the diffraction limit. In the weak coupling regime, the dispersion-induced non-Markovian effects are weak and do not significantly affect the spectral shape of the emission peak. In contrast, in the strong coupling regime, the non-Markovian effects lead to dramatic changes in the emission spectra by causing inversion of spectral asymmetry, as compared to classical and quantum models based on the Markov approximation, which results in a strong enhancement of the lower frequency polaritonic band, consistent with the experiment.

cond-mat.mes-hall↗

arXiv2021

Transition to strong coupling regime in hybrid plasmonic systems: Exciton-induced transparency and Fano interference

We present a microscopic model describing the transition to strong coupling regime for an emitter resonantly coupled to a surface plasmon in a metal-dielectric structure. We demonstrate that the shape of scattering spectra is determined by an interplay of two distinct mechanisms. First is the near-field coupling between the emitter and the plasmon mode which underpins energy exchange between the system components and gives rise to exciton-induced transparency minimum in scattering spectra prior the transition to strong coupling regime. The second mechanism is Fano interference between the plasmon dipole and the plasmon-induced emitter's dipole as the system interacts with the radiation field. We show that the Fano interference can strongly affect the overall shape of scattering spectra, leading to the inversion of spectral asymmetry that was recently reported in the experiment.

cond-mat.mes-hall↗

arXiv2021

Interacting quantum plasmons in metal-dielectric structures

We develop a consistent quantum description of surface plasmons interacting with quantum emitters and external electromagnetic field. Within the framework of macroscopic electrodynamics in dispersive and absorptive medium, we derive, in the Markov approximation, the canonical Hamiltonian, commutation relations, and coupling parameters for the plasmon modes in metal-dielectric structures of an arbitrary shape whose characteristic size is well below the diffraction limit. We then develop a new quantum approach bridging the macroscopic and canonical schemes which describes the interacting plasmons in terms of bosonic modes with linear dispersion whose coupling to the electromagnetic field and quantum emitters is mediated by the classical plasmons. By accurately accounting for medium optical dispersion and losses in the interactions of surface plasmons with light and localized electron excitations, this approach can serve as a framework for studying non-Markovian effects in plasmonics.

cond-mat.mes-hall↗

arXiv2020

Exciton-induced transparency in hybrid plasmonic systems

We present a microscopic model for exciton-induced transparency (ExIT) in a hybrid system comprised of an emitter resonantly coupled to a surface plasmon in a metal-dielectric structure. We obtain an effective optical polarizability of such a system with coupling between the system components expressed in terms of energy transfer rates. We demonstrate that, in the weak coupling regime, the underlying mechanism of ExIT is the energy exchange imbalance between the plasmon and the emitter in a narrow frequency region. We derive in analytic form a frequency-dependent function that accurately describes the shape and amplitude of the transparency window in scattering spectra, supported by numerical calculations.

cond-mat.mes-hall↗

arXiv2019

Exciton-plasmon energy exchange drives the transition to strong coupling regime

We present a model for exciton-plasmon coupling based on an energy exchange mechanism between quantum emitters (QE) and localized surface plasmons in metal-dielectric structures. Plasmonic correlations between QEs give rise to a collective state exchanging its energy cooperatively with a resonant plasmon mode. By defining carefully the plasmon mode volume for a QE ensemble, we obtain a relation between QE-plasmon coupling and a cooperative energy transfer rate that is expressed in terms of local fields. For a single QE near a sharp metal tip, we find analytically the enhancement factor for QE-plasmon coupling relative to QE coupling to a cavity mode. For QEs distributed in an extended region enclosing a plasmonic structure, we find that the ensemble QE-plasmon coupling saturates to a universal value independent of system size and shape, consistent with the experiment.

cond-mat.mes-hall↗

arXiv2019

Cooperative emission mediated by cooperative energy transfer to a plasmonic antenna

We develop a theory of cooperative emission mediated by cooperative energy transfer (CET) from an ensemble of quantum emitters (QE) to plasmonic antenna at a rate equal to the sum of individual QE-plasmon energy transfer rates. If the antenna radiation efficiency is sufficiently high, the transferred energy is radiated away at approximately the same cooperative rate that scales with the ensemble size. We derive explicit expressions, in terms of local fields, for cooperative Purcell factor and enhancement factor for power spectrum valid for plasmonic structures of any shape with characteristic size smaller than the radiation wavelength. The radiated power spectrum retains the plasmon resonance lineshape with overall amplitude scaling with the ensemble size. If QEs are located in a region with nearly constant plasmon local density of states (LDOS), e.g., inside a plasmonic nanocavity, we demonstrate that the CET rate scales linearly with the number of excited QEs, consistent with the experiment, and can be tuned in a wide range by varying the excitation power. For QEs distributed in an extended region saturating the plasmon mode volume, we show that the cooperative Purcell factor has universal form independent of the system size. The CET mechanism incorporates the plasmon LDOS enhancement as well, giving rise to possibilities of controlling the emission rate beyond field enhancement limits.

cond-mat.mes-hall↗

arXiv2019

Cooperative energy transfer controls the spontaneous emission rate beyond field enhancement limits

Quantum emitters located in proximity to a metal nanostructure individually transfer their energy via near-field excitation of surface plasmons. The energy transfer process increases the spontaneous emission (SE) rate due to plasmon-enhanced local field. Here, we demonstrate significant acceleration of quantum emitter SE rate in a plasmonic nano-cavity due to cooperative energy transfer (CET) from plasmon-correlated emitters. Using an integrated plasmonic nano-cavity, we realize up to six-fold enhancement in the emission rate of emitters coupled to the same nano-cavity on top of the plasmonic enhancement of the local density of states. The radiated power spectrum retains the plasmon resonance central frequency and lineshape, with the peak amplitude proportional to the number of excited emitters indicating that the observed cooperative SE is distinct from super-radiance. Plasmon-assisted CET offers unprecedented control over the SE rate and allows to dynamically control the spontaneous emission rate at room temperature enabling an SE rate based optical modulator.

cond-mat.mes-hall↗

arXiv2017

Mode volume, energy transfer, and spaser threshold in plasmonic systems with gain

We present a unified approach to describe spasing in plasmonic systems modeled by quantum emitters interacting with resonant plasmon mode. We show that spaser threshold implies detailed energy transfer balance between the gain and plasmon mode and derive explicit spaser condition valid for arbitrary plasmonic systems. By defining carefully the plasmon mode volume relative to the gain region, we show that the spaser condition represents, in fact, the standard laser threshold condition extended to plasmonic systems with dispersive dielectric function. For extended gain region, the saturated mode volume depends solely on the system parameters that determine the lower bound of threshold population inversion.

cond-mat.mes-hall↗

arXiv2017

Surface-assisted carrier excitation in plasmonic nanostructure

We present a quantum-mechanical model for surface-assisted carrier excitation by optical fields in plasmonic nanostructures of arbitrary shape. We derive an explicit expression, in terms of local fields inside the metal structure, for surface absorbed power and surface scattering rate that determine the enhancement of carrier excitation efficiency near the metal-dielectric interface. We show that surface scattering is highly sensitive to the local field polarization, and can be incorporated into metal dielectric function along with phonon and impurity scattering. We also show that the obtained surface scattering rate describes surface-assisted plasmon decay (Landau damping) in nanostructures larger than the nonlocality scale. Our model can be used for calculations of plasmon-assisted hot carrier generation rates in photovoltaics and photochemistry applications.

cond-mat.mes-hall↗

arXiv2016

Landau damping of surface plasmons in metal nanostructures

We develop a quantum-mechanical theory for Landau damping of surface plasmons in metal nanostructures larger that the characteristic length for nonlocal effects. We show that the electron surface scattering, which facilitates plasmon decay in small nanostructures, can be incorporated into the metal dielectric function on par with phonon and impurity scattering. The derived surface scattering rate is determined by the plasmon local field polarization relative to the metal-dielectric interface and is highly sensitive to the system geometry. We illustrate our model by providing analytical results for surface scattering rate in some common shape nanostructures.

cond-mat.mes-hall↗

arXiv2016

Local density of states for nanoplasmonics

We obtain the local density of states (LDOS) for any nanoplasmonic system in the frequency range dominated by a localized surface plasmon. By including the Ohmic losses in a consistent way, we show that the plasmon LDOS is proportional to the local field intensity normalized by the absorbed power. We obtain explicit formulas for the energy transfer (ET) between quantum emitters and plasmons as well as between donors and acceptors situated near a plasmonic structure. In the latter case, we find that the plasmon-assisted ET rate is proportional to the LDOS product at the donor and acceptor positions, obtain, in a general form, the plasmon ET enhancement factor, and establish the transition onset between Forster-dominated and plasmon-dominated ET regimes.

cond-mat.mes-hall↗