# Ultrafast photoionization and excitation of surface-plasmon-polaritons on diamond surfaces

## TL;DR

This study investigates ultrafast photoexcitation of diamond surfaces leading to transient surface plasmon-polaritons and permanent surface ripples, combining theoretical modeling with experimental laser-induced surface structuring.

## Contribution

First combined theoretical and experimental analysis of ultrafast photoinduced surface plasmon-polaritons and ripple formation on diamond surfaces.

## Key findings

- Transient surface metallization occurs at high laser intensities.
- Surface ripples with sub-wavelength periods are formed and evolve with laser shots.
- Dielectric permittivity changes are key to ripple formation.

## Abstract

Ultrafast plasmonics of novel materials has emerged as a promising field of nanophotonics bringing new concepts for advanced optical applications. Ultrafast electronic photoexcitation of a diamond surface and subsequent surface plasmon-polaritons (SPPs) excitation are studied both theoretically and experimentally - for the first time. After photoexcitation on the rising edge of the pulse, transient surface metallization was found to occur for laser intensity near 18 TW/cm$^2$ due to enhancement of the impact ionization rate; in this regime, the dielectric constant of the photoexcited diamond becomes negative in the trailing edge of the pulse thereby increasing the efficacy with which surface roughness leads to inhomogeneous energy absorption at the SPP wave-vector. These transient SPP waves imprint permanent fine and coarse surface ripples oriented perpendicularly to the laser polarization. The theoretical modeling is supported by the experiments on the generation of laser-induced periodic surface structure on diamond surface with normally incident 515-nm, 200-fs laser pulses. Sub-wavelength ($\Lambda \approx 100$ nm) and near wavelength ($\Lambda \approx 450$ nm) surface ripples oriented perpendicularly to the laser polarization emerged within the ablative craters with the increased number of laser shots; the spatial periods of the surface ripples decrease with the increasing exposure following known cumulative trends. The comparison between experimental data and theoretical predictions makes evident the role of transient changes of the dielectric permittivity of diamond during the initial stage of periodic surface ripple formation upon irradiation with ultrashort laser pulses.