Coherent Oscillations of Breathing Modes in Metal Nanoshells
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Coherent Oscillations of Breathing Modes in Metal Nanoshells Arman S. Kirakosyan1,2 , Tigran V. Shahbazyan1 1 Department of Physics, Jackson State University, Jackson, MS 39217, USA 2 Department of Physics, Yerevan State University, 1 Alex Manoogian St., Yerevan, 375025, Armenia ABSTRACT 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 anti-phase (n=1) contractions of shellcore 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. INTRODUCTION Acoustic vibrational modes in nanoparticles are impulsively excited by a rapid heating of the lattice that takes place after laser excitation. After initial period of rapid expansion, a nanoparticle undergoes radial contractions and expansions around the new equilibrium. The periodic change in nanoparticle volume translates into a modulation in time of the surface plasmon resonance (SPR) energy that dominates nanoparticle optical absorption spectrum. The spectrum of vibrational modes manifests itself via coherent oscillations of differential transmission at SPR energy measured using ultrafast pump-probe spectroscopy [1, 2]. Since the size of laser spot is usually much larger than nanoparticle diameter, the initial expansion is homogeneous so that predominantly the fundamental (n = 0) breathing mode, corresponding to oscillations of nanoparticle volume as a whole, is excited. The lowest excited (n = 1) mode has weaker oscillator strength (≈ 1/4 of that for n = 0), and has also been recently observed [3]. When nanoparticle is embedded in a dielectric medium, the oscillations are damped due to the transfer of latice energy to acoustic waves in surrounding dielectric. In solid particles, the size dependences of eigenmodes energy and decay rate are similar – both are inversely proportional to nanoparticle radius [4]. Here we study the vibrartional modes of metal nanoshells. These recently manufactured metal particles with dielectric core [5] attracted much interest due to unique tunability of their optical properties. By varying the shell thickness during the manufacturing process, the SPR can be tuned in a wide energy interval [6]. Recent pump-probe measurements of vibrational modes dynamics in gold nanoshells submerged in water revealed characteristic oscillation p
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