Metal Nanoplasmas as Bright Sources of Hard X-Ray Pulses

We report significant enhancements in light coupling to intense-laser-created solid plasmas via surface plasmon and “lightning rod” effects. We demonstrate this in metal nanoparticle-coated solid targets irradiated with 100 fs, 806 nm laser pulses, focused to intensities $\sim {10}^{14}–{10}^{15}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$. Our experiments show a 13-fold enhancement in hard x-ray yield (10–200 keV) emitted by copper nanoparticle plasmas formed at the focal volume. A simple model explains the observed enhancement quantitatively and provides pointers to the design of structured surfaces for maximizing such emissions.

Figure 1

Linear absorption spectra of polished and nanoparticle coated targets. Inset: scanning electron micrograph of SNP-coated target. The solid lines show Drude model fits.

Figure 2

Bremsstrahlung emission at $10°$. Inset: variation of integrated emission with input laser intensity.

Figure 3

(a) Enhancement factor at $\lambda =806\mathrm{n}\mathrm{m}$ as a function of $a/b$ for different metals, for $b=7\mathrm{n}\mathrm{m}$. $L_{\perp }^{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}$ for $ϵ=10+i30$ (plasma $ϵ$) is shown as stars. Inset: spheroidal model for nanoparticle. (b) Variation of dielectric functions of bulk Cu and ENP with intensity.

Figure 4

Comparison of bremsstrahlung emission at $45°$. Solid lines are temperature fits. Inset: Enhancement of emission with a preplasma formed at various delays. The solid line is a guide to the eye.