Harmonic generation from laser-irradiated clusters

The harmonic emission from cluster nanoplasmas subject to short, intense infrared laser pulses is analyzed by means of particle-in-cell simulations. A pronounced resonant enhancement of the low-order harmonic yields is found when the Mie plasma frequency of the ionizing and expanding cluster resonates with the respective harmonic frequency. We show that a strong, nonlinear resonant coupling of the cluster electrons with the laser field inhibits coherent electron motion, suppressing the emitted radiation and restricting the spectrum to only low-order harmonics. A pump-probe scheme is suggested to monitor the ionization dynamics of the expanding clusters.

Figure 1

Normalized harmonic power $P\left(\omega \right)=\mid A\left(\omega \right)∕N{\mid }^2$ vs harmonic order for an ${\mathrm{Ar}}_{17256}$ cluster at laser intensities $2.5\times {10}^{14}$ (a), $2.5\times {10}^{15}$ (b), and $7.5\times {10}^{17}\mathrm{W}{\mathrm{cm}}^{-2}$ (c). The spectrum (d) is for an ${\mathrm{Ar}}_{92096}$ cluster at the intensity $7.5\times {10}^{17}\mathrm{W}{\mathrm{cm}}^{-2}$.

Figure 2

Time-frequency diagrams corresponding to the spectra of Fig. 1 with the normalized power ${\log }_{10}P$.

Figure 3

TF spectrograms for two PIC electrons of the ${\mathrm{Ar}}_{17256}$ cluster irradiated by the laser intensity $5\times {10}^{16}\mathrm{W}{\mathrm{cm}}^{-2}$. The dimensionless electron distance from the cluster center, $r\left(t\right)∕R_0$, is indicated by the solid line.

Figure 4

(Color) Dipole power ${\log }_{10}{P}_{y,z}\left(\omega \right)$ along $y,z$ vs time delay $t_d$ and frequency $\omega ∕{\omega }_{\mathrm{l}}$ with the pump-pulse intensities (a), (b) $2.5\times {10}^{15}\mathrm{W}{\mathrm{cm}}^{-2}$ [cf. Fig. 2b], (c), (d) $7.5\times {10}^{17}\mathrm{W}{\mathrm{cm}}^{-2}$ [cf. Fig. 2c], and the probe-pulse intensity $2.5\times {10}^{12}\mathrm{W}{\mathrm{cm}}^{-2}$ with $m=3$ (a), (c) and 5 (b), (d).