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

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. Following photoexcitation on the rising edge of the pulse, transient surface metallization was found to occur for laser intensity near 18 TW/cm2 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 (Λ ≈ 100 nm) and near wavelength (Λ ≈ 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 moderately with the increasing exposure. The comparison between experimental data and theoretical predictions demonstrates 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.