Arman S. Kirakosyan

Vibrational modes of metal nanoshells and bimetallic core-shell nanoparticles

We theoretically study the 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 a reduction in the shell thickness. We identify two regimes that can be characterized as weakly damped and overdamped vibrations in the presence of external medium. For bimetallic particles, we find periodic dependence of frequency and damping rate on the shell thickness with period being determined by the mode number. For both types of nanostructures, the frequency of higher modes is nearly independent of the environment, while the damping rate shows a strong sensitivity to the outside medium.

Ultrafast response of surface electromagnetic waves in an aluminum film perforated with subwavelength hole arrays.

The ultrafast dynamics of surface electromagnetic waves photogenerated on aluminum film perforated with subwavelength arrays of holes was studied in the visible spectral range by the technique of transient photomodulation with approximately 100 fs time resolution. We observed a pronounced blueshift of the resonant transmission band that reveals the important role of plasma attenuation in the optical response of nanohole arrays. The blueshift is inconsistent with plasmonic mechanism of extraordinary transmission and points to the crucial role of interference in the formation of transmission bands. The transient photomodulation spectra were successfully modeled within the Boltzmann equation approach for the electron-phonon relaxation dynamics, involving nonequilibrium hot electrons and quasiequilibrium phonons.

Surface plasmon lifetime in metal nanoshells

The lifetime of localized surface plasmon plays an important role in many aspects of plasmonics and its applications. In small metal nanostructures, the dominant mechanism restricting plasmon lifetime is size-dependent Landau damping. We performed quantum-mechanical calculations of Landau damping for the bright surface plasmon mode in a metal nanoshell. In contrast to the conventional model based on the electron surface scattering, we found that the damping rate decreases as the nanoshell thickness is reduced. The origin of this behavior is traced to the spatial distribution of plasmon local field inside the metal shell. We also found that, due to interference of electron scattering amplitudes from nanoshells two metal surfaces, the damping rate exhibits pronounced quantum beats with changing shell thickness.

Ultrafast dynamics of surface electromagnetic waves in nanohole array on metallic film

We study the ultrafast dynamics of surface electromagnetic waves photogenerated on aluminum film perforated with subwavelength holes array by means of transient photomodulation with ∼100 fs time resolution. We observed a pronounced blueshift of the resonant transmission band that reveals the important role of plasma attenuation in the dynamics and that is inconsistent with plasmon–polariton mechanism of extraordinary transmission. The transient photomodulation spectra were successfully modeled within the Boltzmann equation approach for the electron–phonon relaxation dynamics, involving non-equilibrium hot electrons and quasi-equilibrium phonons.

Spectroscopy of vibrational modes in metal nanoshells

In this work 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 the 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.

Ultrafast dynamics of surface electromagnetic waves in subwavelength nanohole array on metallic film

The ultrafast dynamics of surface electromagnetic waves photogenerated on the surfaces of an Al film perforated with 2D subwavelength hole array was studied by the pump-probe correlation spectroscopy. The time-resolved differential transmission exhibits a fast rise on subpicosecond time scale followed by a plateau with subsequent slow decay. This dynamics is accompanied by a blue shift in transient spectra of the anomalous transmision band. A theoretical model is developed that explains both time-resolved and spectrally-resolved data.

Ultrafast Dynamics of Surface Electromagnetic Modes in Subwavelength Nanohole Array

Ultrafast dynamics of surface electromagnetic waves in Aluminum film perforated with subwavelength hole array is studied using pump-probe correlation spectroscopy. The measured transient spectra point to the important role of dissipation in the extraordinary transmission. Article not available.

Coherent oscillations of vibrational modes in core-shell nanoparticles

We study coherent oscillations of radial breathing modes in metal nanoparticles with a dielectric core. Vibrational modes are impulsively excited by a rapid heating of the particle lattice that occurs after laser excitation, while the energy transfer to a surrounding dielectric leads to a damping of the oscillations. In nanoshells, the presence of two metal surfaces leads to a substantially different energy spectrum of acoustic vibrations. The lowest and first excited modes correspond to in-phase (n=0) and out-of-phase (n=1) contractions of shell-core and shell-matrix interfaces respectively. We calculated the energy spectrum as well as the damping of nanoshell vibrational modes in the presence of surrounding medium, and found that the size-dependences of in-phase and anti-phase modes are different. At the same time, the oscillator strength of the symmetric mode is larger than that in solid nanoparticles leading to stronger oscillations in thin nanoshells.