Predicted Ultrafast Dynamic Metallization of Dielectric Nanofilms by Strong Single-Cycle Optical Fields

We predict a dynamic metallization effect where an ultrafast (single-cycle) optical pulse with a $\lesssim 1\mathrm{V}/\AA$ field causes plasmonic metal-like behavior of a dielectric film with a few-nm thickness. This manifests itself in plasmonic oscillations of polarization and a significant population of the conduction band evolving on a $\sim 1\mathrm{fs}$ time scale. These phenomena are due to a combination of both adiabatic (reversible) and diabatic (for practical purposes irreversible) pathways.

Figure 1

(a) Energy spectrum of the nanofilm as a function of the adiabatically applied electric field. The occupied valence subbands are shown in red, the empty conduction subbands are in blue. (b) The diabatic coupling matrix element ${\dot{\Theta }}_{ik}$ between band-edge subbands [see Eq. (3)] for different pulse amplitudes ${\mathcal{E}}_0$, as indicated on the panel.

Figure 2

Polarization $\mathcal{P}$ and conduction-band population $n_c$ as functions of time $t$ for various ${\mathcal{E}}_0$ and $\tau$. Normalized pulse field $\mathcal{E}(t)/{\mathcal{E}}_0$ is shown by blue line. Normalized population $n_c(t)/n$ ($\mathrm{\text{scaled}}\times 100$) is displayed by the red curve; the same in DA is shown by the dashed gray curve. Relative polarization $\mathcal{P}/{\mathcal{E}}_0$ is displayed by the green line; the same in AA is given by the dashed yellow line. The pulse length is $\tau =0.85\mathrm{fs}$ for (a)–(c) and $\tau =3.4\mathrm{fs}$ for (d).

Figure 3

Effective permittivity ${\epsilon }_0$ as a function of pulse amplitude ${\mathcal{E}}_0$ (a) and inverse pulse duration $1/\tau$ (b). The label AA denotes a result of the adiabatic approximation.

Figure 4

Relative residual population of the conduction band $n_{cr}/n$ as a function of the pulse length $\tau$. The green curve is the result of the full numerical computation, the blue curve is the analytical result with two band-edge subbands, and the red with four. Arrows show the adiabatic phase $\phi$. (a) ${\mathcal{E}}_0=0.94\mathrm{V}/\AA$ and (b) ${\mathcal{E}}_0=1.2\mathrm{V}/\AA$.