arXiv subjects

Tigran V. Shahbazyan

Publications and source records attributed to Tigran V. Shahbazyan.

33 records · Page 2Linked to original sources

arXiv2016

Coulomb and quenching effects in small nanoparticle-based spasers

We study numerically the effect of mode mixing and direct dipole-dipole interactions between gain molecules on spasing in a small composite nanoparticles with a metallic core and a dye-doped dielectric shell. By combining Maxwell-Bloch equations with Green's function formalism, we calculate lasing frequency and threshold population inversion for various gain densities in the shell. We find that gain coupling to nonresonant plasmon modes has a negligible effect on spasing threshold. In contrast, the direct dipole-dipole coupling, by causing random shifts of gain molecules' excitation frequencies, hinders reaching the spasing threshold in small systems. We identify a region of parameter space in which spasing can occur considering these effects.

cond-mat.mes-hall↗

arXiv2013

Cooperative amplification of energy transfer in plasmonic systems

We study cooperative effects in energy transfer (ET) from an ensemble of donors to an acceptor near a plasmonic nanostructure. We demonstrate that in cooperative regime ET takes place from plasmonic superradiant and subradiant states rather than from individual donors leading to a significant increase of ET efficiency. The cooperative amplification of ET relies on the large coupling of superradiant states to external fields and on the slow decay rate of subradiant states. We show that superradiant and subradiant ET mechanisms are efficient in different energy domains and therefore can be utilized independently. We present numerical results demonstrating the amplification effect for a layer of donors and an acceptor on a spherical plasmonic nanoparticle.

cond-mat.mes-hall↗

arXiv2013

Theory of plasmon-enhanced metal photoluminescence

Metal photoluminescence (MPL) originates from radiative recombination of photoexcited core holes and conduction band electrons. In metal nanostructures, MPL is enhanced due to the surface plasmon local field effect. We identify another essential process in plasmon-assisted MPL - excitation of Auger plasmons by core holes - that hinders MPL from small nanostructures. We develop a microscopic theory of plasmon-enhanced MPL that incorporates both plasmon-assisted enhancement and suppression mechanisms and derive the enhancement factor for MPL quantum efficiency. Our numerical calculations of MPL from Au nanoparticles are in excellent agreement with the experiment.

cond-mat.mes-hall↗

arXiv2012

Long-range plasmon-assisted energy transfer over doped graphene

We demonstrate that longitudinal plasmons in doped monolayer graphene can mediate highly efficient long-range energy transfer between nearby fluorophores, e.g., semiconductor quantum dots. We derive a simple analytical expression for the energy transfer efficiency that incorporates all the essential processes involved. We perform numerical calculations of the transfer efficiency for a pair of PbSe quantum dots near graphene for inter-fluorophore distances of up to 1 $μ$m and find that the plasmon-assisted long-range energy transfer can be enhanced by up to a factor of $\sim10^4$ relative to the Förster's transfer in vacuum.

cond-mat.mes-hall↗

arXiv2012

Fluorescence quenching near small metal nanoparticles

We develop a microscopic model for fluorescence of a molecule (or semiconductor quantum dot) near a small metal nanoparticle. When a molecule is situated close to metal surface, its fluorescence is quenched due to energy transfer to the metal. We perform quantum-mechanical calculations of energy transfer rates for nanometer-sized Au nanoparticles and find that non-local and quantum-size effects significantly enhance dissipation in metal as compared to those predicted by semiclassical electromagnetic models. However, the dependence of transfer rates on molecule's distance to metal nanoparticle surface, $d$, is significantly weaker than the $d^{-4}$ behavior for flat metal surface with a sharp boundary predicted by previous calculations within random phase approximation.

cond-mat.mes-hall↗

arXiv2011

Resonance energy transfer near metal nanostructures mediated by surface plasmons

We develop a unified theory of plasmon-assisted resonance energy transfer (RET) between molecules near a metal nanostructure that maintains energy balance between transfer, dissipation, and radiation. We show that in a wide range of parameters, including in the near field, RET is dominated by plasmon-enhanced radiative transfer (PERT) rather than by a nonradiative transfer mechanism. Our numerical calculations performed for molecules near the Ag nanoparticle indicate that RET magnitude is highly sensitive to molecules' positions.

cond-mat.mes-hall↗

arXiv2010

Plasmon-mediated superradiance near metal nanostructures

We develop a theory of cooperative emission of light by an ensemble of emitters, such as fluorescing molecules or semiconductor quantum dots, located near a metal nanostructure supporting surface plasmon. The primary mechanism of cooperative emission in such systems is resonant energy transfer between emitters and plasmons rather than the Dicke radiative coupling between emitters. We identify two types of plasmonic coupling between the emitters, (i) plasmon-enhanced radiative coupling and (ii) plasmon-assisted nonradiative energy transfer, the competition between them governing the structure of system eigenstates. Specifically, when emitters are removed by more than several nm from the metal surface, the emission is dominated by three superradiant states with the same quantum yield as a single emitter, resulting in a drastic reduction of ensemble radiated energy, while at smaller distances cooperative behavior is destroyed by nonradiative transitions. The crossover between two regimes can be observed in distance dependence of ensemble quantum efficiency. Our numerical calculations incorporating direct and plasmon-assisted interactions between the emitters indicate that they do not destroy the plasmonic Dicke effect.

cond-mat.mes-hall↗

arXiv2009

Size-dependent effects in solutions of small metal nanoparticles

A new theoretical approach for the calculation of optical properties of complex solutions is proposed. It is based on a dielectric matrix with included small metallic inclusions (less than 3 nm) of spherical shape. We take into account the mutual interactions between the inclusions and the quantum finite-size effects. On the basis of the effective medium model, TDLDA and Kohn-Sham theories, some analytical expressions for the effective dielectric permittivity of the solution are obtained.

cond-mat.soft↗

arXiv2008

Cooperative emission of light by an ensemble of dipoles near a metal nanostucture: The plasmonic Dicke effect

We identify a new mechanism for cooperative emission of light by an ensemble of N dipoles near a metal nanostructure supporting a surface plasmon.The cross-talk between emitters due to virtual plasmon exchange leads to a formation of three plasmonic super-radiant modes whose radiative decay rates scales with N, while the total radiated energy is thrice that of a single emitter. Our numerical simulations indicate that the plasmonic Dicke effect survives non-radiative losses in the metal.

cond-mat.mtrl-sci↗

arXiv2008

Vibrational modes of metal nanoshells and bimetallic core-shell nanoparticles

We study theoretically spectrum of radial vibrational modes in composite metal nanostructures such as bimetallic core-shell particles and metal nanoshells with dielectric core in an environment. We calculate frequencies and damping rates of fundamental (breathing) modes for these nanostructures along with those of two higher-order modes. For metal nanoshells, we find that the breathing mode frequency is always lower than the one for solid particles of the same size, while the damping is higher and increases with reduction of the shell thickness. We identify two regimes that can be characterized as weakly damped and overdamped vibrations in the presence of external medium. For bimetalllic particles, we find periodic dependence of frequency and damping rate on the shell thickness with period determined by mode number. For both types of nanostructures, the frequency of higher modes is nearly independent of the environment, while the damping rate shows strong sensitivity to outside medium.

cond-mat.mtrl-sci↗

arXiv2006

Spectroscopy of vibrational modes in metal nanoshells

We study the spectrum of vibrational modes in metal nanoparticles with a dielectric core. Vibrational modes are excited by the rapid heating of the particle lattice that takes place after laser excitation, and can be monitored by means of pump-probe spectroscopy as coherent oscillations of transient optical spectra. In nanoshells, the presence of two metal surfaces results in a substantially different energy spectrum of acoustic vibrations than for solid particles. We calculated the energy spectrum as well as the damping of nanoshell vibrational modes. The oscillator strength of fundamental breathing mode is larger than that in solid nanoparticles. At the same time, in very thin nanoshells, the fundamental mode is overdamped due to instantaneous energy transfer to the surrounding medium.

cond-mat.mes-hall↗

arXiv2006

Microscopic theory of surface-enhanced Raman scattering in noble-metal nanoparticles

We present a microscopic model for surface-enhanced Raman scattering (SERS) from molecules adsorbed on small noble-metal nanoparticles. In the absence of direct overlap of molecular orbitals and electronic states in the metal, the main enhancement source is the strong electric field of the surface plasmon resonance in a nanoparticle acting on a molecule near the surface. In small particles, the electromagnetic enhancement is strongly modified by quantum-size effects. We show that, in nanometer-sized particles, SERS magnitude is determined by a competition between several quantum-size effects such as the Landau damping of surface plasmon resonance and reduced screening near the nanoparticle surface. Using time-dependent local density approximation, we calculate spatial distribution of local fields near the surface and enhancement factor for different nanoparticles sizes.

cond-mat.mes-hall↗

arXiv2005

Polarization and Aharonov-Bohm oscillations in quantum-ring magnetoexcitons

We study interaction and radial polarization effects on the the absorption spectrum of neutral bound magnetoexcitons confined in quantum-ring structures. We show that the size and orientation of the exciton's dipole moment, as well as the interaction screening, play important roles in the Aharonov-Bohm oscillations. In particular, the excitonic absorption peaks display A-B oscillations both in position and amplitude for weak electron-hole interaction and large radial polarization. The presence of impurity scattering induces anticrossings in the exciton spectrum, leading to a modulation in the absorption strength. These properties could be used in experimental investigations of the effect in semiconductor quantum-ring structures.

cond-mat.mes-hall↗

arXiv2004

Finite-Size Effects in Surface-Enhanced Raman Scattering from Molecules Adsorbed on Noble-Metal Nanoparticles

We study the role of strong electron confinement in surface-enhanced Raman scattering from molecules adsorbed on small noble-metal particles. We describe a new source of Raman signal enhancement which originates from different behavior of sp-band and d-band electron densities near the particle boundary. In small particles, a spillover of sp-electron wave-functions beyond the classical radius gives rise to a thin layer with diminished population of d-electrons. In this surface layer, the screening of sp-electrons by d-band electron background is reduced. We demonstrate that the interplay between finite-size and underscreening effects results in an increase of the surface plasmon local field acting on a molecule located in a close proximity to the particle boundary. Our calculations, based on two-region model, show that the additional enhancement of Raman signal gets stronger for smaller nanoparticles due to a larger volume fraction of underscreened region.

cond-mat.mes-hall↗

arXiv2004

Ultrafast Auger spectroscopy of quantum well excitons in a strong magnetic field

We study theoretically the ultrafast nonlinear optical response of quantum well excitons in a perpendicular magnetic field. We address the role of many-body correlations originating from the electron scattering between Landau levels (LL). In the linear optical response, the processes involving inter-LL transitions are suppressed provided that the magnetic field is sufficiently strong. However, in the nonlinear response, the Auger processes involving inter-LL scattering of two photoexcited electrons remain unsuppressed. We show that Auger scattering plays the dominant role in the coherent exciton dynamics in strong magnetic field. We perform numerical calculations for the third-order four-wave-mixing (FWM) polarization which incorporate the Auger processes nonperturbatively. We find that inter-LL scattering leads to a strong enhancement and to oscillations of the FWM signal at negative time delays. These oscillations represent quantum beats between optically-inactive two-exciton states related to each other via Auger processes.

cond-mat.mes-hall↗