Field enhancement of intense laser pulses in a subwavelength plasma aperture

TL;DR

The paper addresses how subwavelength plasma apertures can enhance and control ultrashort, intense laser fields beyond linear plasmonics. It employs three-dimensional particle-in-cell simulations of carbon nanotube plasmas to track field excitation at the aperture entrance, coupling into guided modes, and propagation through the plasma channel. The key finding is a non-resonant field enhancement that saturates near a plasma density of $20n_c$, with peak transverse fields up to about $1.5$–$2.2$ times the incident field and a back-scattered longitudinal component at the ends that interferes to concentrate energy along the axis. Importantly, this mechanism persists across wall thickness, radius variations up to ~$200$ nm, and relativistic intensities ($a_0\approx 1$), enabling potential plasma-based dichroic filters at extreme fields and generalization to planar nanoholes.

Abstract

The interaction of intense, ultra-short laser pulses with nanostructures offers promising avenues for spatiotemporal light control. While enhanced optical transmission through subwavelength apertures has been extensively studied in the linear regime, its extension to ultrashort, high-intensity pulses remains largely unexplored. Here we demonstrate, through three-dimensional particle-in-cell simulations, significant field enhancement of intense laser pulses in subwavelength plasma apertures. The enhancement exhibits a non-resonant character, remaining robust across a wide range of plasma densities and saturating above approximately $20n_c$, while showing minimal dependence on wall thickness. Analysis of the Poynting vector reveals that energy concentration arises from interference between the incident field and back-scattered longitudinal field components. This size-dependent enhanced transmission in plasma apertures enables potential applications such as plasma-based dichroic filters operating at extreme intensities.

Figures (6)

• Figure 1: Field enhancement in a carbon nanotube ($r_a = 300$ nm) irradiated by a laser pulse ($I_0 = 10^{16}$ W/cm$^2$). (a) Temporal profile of the transverse electric field $E_y$ at the nanotube center (solid blue) compared to the vacuum reference (dashed red), normalized to the incident field amplitude $E_0$. (b) Spatial distribution of $E_y$ in the $x$-$y$ plane at $t = 2.39$ fs, with dashed lines indicating the nanotube boundary. (c) Time evolution of $E_y$ along the polarization axis $(x, z) = (0, 0)$. (d) Corresponding time evolution of the longitudinal field $E_x$ along the same axis.
• Figure 2: Dependence of field enhancement on nanotube geometry. (a) On-axis temporal profile of $E_y$ versus axial position for $r_a = 300$ nm. (b) Maximum normalized field amplitude along the laser axis for different inner radii, with light blue indicating positions inside the nanotube. The $r_a = 0$ case corresponds to a solid rod.
• Figure 3: Effect of collisional ionization on field enhancement in a carbon nanotube ($r_a = 300$ nm). (a) Temporal evolution of the normalized electric field $E_y/E_0$ at the nanotube center. (b) Corresponding average charge state as a function of time. (c) Spatial profile of the maximum field amplitude along the longitudinal direction. Solid blue curves: simulations without collisional ionization; dashed red curves: with collisional ionization included.
• Figure 4: Dependence of ionization dynamics and field enhancement on laser intensity and nanotube geometry. (a) Temporal evolution of average charge state for different laser intensities. (b) Spatial profile of maximum field amplitude for corresponding intensities. (c) Average charge state versus time for nanotubes with different wall thicknesses. (d) Maximum field amplitude versus position for different wall thicknesses. (e) Spatial profile of the maximum normalized field amplitude $E_{\text{max}}/E_0$ along the nanotube axis for $I_0 = 10^{16}$ W/cm$^2$ (non-relativistic regime). Different curves correspond to nanotubes of varying length $L$. (f) Corresponding spatial profiles for $I_0 = 2\times10^{18}$ W/cm$^2$ (relativistic regime, $a_0 \approx 1$). The dashed vertical lines indicate the entrance ($x=0$) and exit positions of the nanotubes.
• Figure 5: Role of preionization and collisional effects in field enhancement. (a) Maximum normalized field amplitude along the longitudinal direction for tubular plasmas with different initial charge states. (b) Comparison of field enhancement with and without elastic electron-ion collisions.