Surface Plasmon Polariton

Laser-induced-periodic-surface-structures (LIPSS),also termed ripples, were first observed by Birnbaum [1] on some semiconductor surfaces. Since then, LIPSS have been reported on various materials [2–7]. In many cases, after irradiation at Zero angle of incidence, the period of the observed structure is close to the wavelength of the incident radiation, with ripples oriented perpendicular to the direction of the laser polarization. The possible cause of the ripples arising is from a standing wave that is produced due to the interference of the incident laser light with scattered waves from a surface disturbance [8]. Consequently, the melting threshold of the surface is periodically exceeded, and rapid resolidification of a thin layer of molten material may give rise to a corrugated structure.

Taking into account their typical dimensions, two main types of ripples are distinguished. The first type is so-called wavelength ripples (coarse ripples, low spatial frequency LIPSS or LSFL) with spatial periods in the range between 0.4 and 1 times the laser wavelength. By studying the angular dependence of the ripple periods it was demonstrated that the ripple formation is related to sub-surface linear interference processes involving scattered light and plasmon-polaritons. To explain the ripple periods, theoretical models were established on the basis of the Drude theory by Sipe and co-workers [9]. It was shown that the free carriers in metals as well as light-induced carriers in semiconductors and other dielectrics change the optical parameters of the material (refractive index, reflectivity, absorption and excitation coefficients).Starting from random distributions of scattering point defects, local collective oscillations of the electron clouds (Plasmon) with dipole-type emission characteristics (compare [10]) can be excited if the required k-vector selection rule resulting from Plasmon dispersion relations is fulfilled. They decay after a few micron and interfere