On the role of surface plasmon polaritons in the formation of laser-induced periodic surface structures upon irradiation of silicon by femtosecond-laser pulses

Abstract

The formation of nearly wavelength-sized laser-induced periodic surface structures (LIPSSs) on single-crystalline silicon upon irradiation with single or multiple femtosecond-laser pulses (pulse duration τ=130 fs and central wavelength λ=800 nm) in air is studied experimentally and theoretically. In our theoretical approach, we model the LIPSS formation by combining the generally accepted first-principles theory of Sipe and co-workers with a Drude model in order to account for transient intrapulse changes in the optical properties of the material due to the excitation of a dense electron-hole plasma. Our results are capable to explain quantitatively the spatial periods of the LIPSSs being somewhat smaller than the laser wavelength, their orientation perpendicular to the laser beam polarization, and their characteristic fluence dependence. Moreover, evidence is presented that surface plasmon polaritons play a dominant role during the initial stage of near-wavelength-sized periodic surface structures in femtosecond-laser irradiated silicon, and it is demonstrated that these LIPSSs can be formed in silicon upon irradiation by single femtosecond-laser pulses.

Laser-induced periodic surface structures: Fingerprints of light localization

“…The values m * opt = 0.18 and τ D = 1.1 fs for femtosecond-laser-excited silicon were chosen in accordance with the article of Bonse et al 25 The complex refractive index of the excited material n * = √˜ * is used to compute the η maps while r = Re(˜ * )/ 0 and σ = Im(˜ * )ω are used for the FDTD-η maps. type-r features evolve with the depth as follows: they lose their intensity, their spreading, and get progressively closer to the center.…”

“…25 That is, λ = 800 nm, θ = 0, F = 0.1, and s = 0.4. The optical properties of the excited silicon are computed via the complex permittivity˜ * =˜ + ˜ Drude where ˜ Drude is given by…”

“…This leads to an even more important consideration: neither the z location nor N e are known since the method used suffers some of the problems of the Sipe-Drude model. 12,[25][26][27] The electron density in the conduction band is assumed to be homogeneous and constant in the material. This is clearly not the case under ultrashort pulse processing because, even for N e = 0 m −3 , the absorbed energy is inhomogeneous.…”

“…21 The transient changes of the material properties during a laser pulse, and the influence of the pulse duration itself, are not taken into account in the theory. This problem was partly solved by Dufft, 12 Bonse, [25][26][27] and their coworkers. They used a combination of the Drude model with the η theory, also referred as to the SipeDrude model, to explain how the transient optical properties of ZnO and silicon can affect the LFSL characteristics.…”

“…2. It is an η map corresponding to a beam at normal incidence θ = 0 with a wavelength of λ = 800 nm, the optical properties of silicon at room temperatureñ = 3.692 + 0.065i, 25 a set (F,s) = (0.1,0.4), and a polarization parallel to the y axis. In Fig.…”