High-harmonic generation by nonlinear resonant excitation of surface plasmon modes in metallic nanoparticles

The nonlinear electron dynamics in metallic nanoparticles is studied using a hydrodynamic model that incorporates most quantum many-body features, including spill-out and nonlocal effects as well as electron exchange and correlations. We show that, by irradiating the nanoparticle with a chirped laser pulse of modest intensity (autoresonance), it is possible to drive the electron dynamics far into the nonlinear regime, leading to enhanced energy absorption and complete ionization of the nanoparticle on a time scale of the order of 100 fs. The accompanying radiated power spectrum is rich in high-order harmonics.

Figure 1

Schematic view of the ion and electron densities according to (5) for different values of the spill-out parameter ${\sigma }_0$.

Figure 2

Linear dipole (a) and breathing (b) frequencies for gold nanoparticles as a function of $N^{-1/3}$. Blue circles and red squares represent respectively the results with and without correlations. The straight lines are linear fits.

Figure 3

Autoresonant excitation of a gold nanoparticle with $N=200$, for two values of the laser intensity, $I_0=4.5\times {10}^{10}\mathrm{W}/{\mathrm{cm}}^2$ (below threshold, red curves) and $I_0=5.4\times {10}^{10}\mathrm{W}/{\mathrm{cm}}^2$ (above threshold, blue curves). The top panels show the time evolution of the dipole $d\left(t\right)$ (a) below threshold and (b) above threshold. (c) Laser frequency (black straight line) and instantaneous dipole frequency of the electron gas for the below threshold (red) and above threshold (blue) cases. (d) Energy absorbed by the electron gas, for both cases.

Figure 4

Frequency spectrum of the total radiated power in a gold nanoparticle with $N=200$, for two cases, below the autoresonance threshold (a) and above the threshold (b).