  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.
