Implications of transient changes of optical and surface properties of solids during femtosecond laser pulse irradiation to the formation of laser-induced periodic surface structures
Graphical abstract
Research highlights
▶ Fs laser pulses transiently turn the silicon into a metallic state. ▶ LIPSS formation is caused by the interference of the laser pulse with a surface plasmon. ▶ LIPSS affect the energy coupling for subsequent laser pulses during the feedback stage.
Introduction
During the past decades, the investigation of laser-induced periodic surface structures (LIPSS, frequently called ripples) has gained remarkable attraction since it bears potential for the development of laser-based industrial nanopatterning processes for example in medical, tribological, photovoltaic or decorative applications.
Upon irradiation of solid surfaces with multiple linearly polarized fs laser pulses the formation of two distinct types of LIPSS has been observed, so-called low spatial frequency LIPSS (LSFL) and high spatial frequency LIPSS (HSFL) [1], [2], [3], [4], [5], [6], [7], [8], [9]. LSFL have a spatial period close to the irradiation wavelength and it is generally accepted that they are formed due to optical interference of the incident laser radiation with a surface-electromagnetic wave which is created during the irradiation [1], [3], [10]. In contrast, the HSFL exhibit spatial periods significant smaller than the irradiation wavelength. Their origin is still quite controversially discussed in the literature [2], [3], [4], [5], [7], [9], [11], [12], [13], [14] and they are not within the scope of this work.
Using nowadays widely available Ti:sapphire fs-lasers at moderate (kHz) pulse repetition rates, LSFL are the dominant LIPSS type in silicon and they are usually observed after a few laser pulses at laser fluences slightly above the ablation threshold. For normal incident radiation, they have spatial periods ΛLSFL somewhat smaller than the central laser wavelength (λ = 800 nm) and they typically show an orientation perpendicular to the polarization of the laser beam [1]. Different experimental studies reported values ΛLSFL between ∼0.6λ and λ (see Refs. [1], [4], [6], [7], [15]). Several groups have recently studied the origin of the fs-laser generated LSFL in silicon and have proposed, that the excitation of surface plasmon polaritons (SPPs) generated in the near-surface fs-laser-generated (quasi) free electron plasma plays a crucial role during the early stage of the LSFL formation [15], [16], [17], [18].
In this work, we approach the fs-LSFL phenomenon in silicon by combining the generally accepted LIPSS theory of Sipe et al. [10] with a Drude model in order to account for the intra-laser-pulse changes of the optical properties due to the generation of a high-density free-electron plasma in the conduction band of the laser-excited silicon. Complementing one of our previous publications [15], explicit results of the carrier-dependent refractive index and extinction coefficient are presented. From this point of view we shed light on the role of surface plasmon polaritons excitation and their incorporation in optical feedback mechanisms acting during the LSFL formation upon multi-pulse irradiation.
Section snippets
Experimental
A chirped pulse regenerative laser amplifier system (Spectra Physics, Spitfire) providing linearly polarized laser pulses of τ = 130 fs duration at λ = 800 nm central wavelength (corresponding to a single photon energy hν = 1.55 eV) was used at a pulse repetition frequency of 10 Hz for irradiation of single-crystalline silicon wafers in air. The polished (1 1 1) oriented silicon wafer surfaces (n-doped) were placed perpendicular to the laser beam direction close to the focal plane of a lens of 60 mm focal
Results and discussion
Fig. 1 shows an example of the silicon wafer which has been irradiated by five subsequent fs-laser pulses at a peak fluence level of 0.47 J/cm2. After the irradiation, the wafer was mechanically separated along a line through the irradiation spot (via breaking). The scanning electron micrograph of the tilted sample surface reveals periodic lines (LSFL) within the central part of the ablation crater which are oriented parallel to the wafer edge. These LSFL lines have been formed perpendicular to
Conclusions
The formation of LSFL in single-crystalline silicon upon irradiation with NIR femtosecond laser pulses (τ = 130 fs, λ = 800 nm) in air was studied theoretically and experimentally. The origin of the LSFL lies in the transient changes of the optical properties leading to the excitation of surface plasmon polaritons, which can interfere with the incident laser beam and then can lead to a modulated energy deposition into the material. This fluence dependent effect occurs when the initially
Acknowledgements
The authors would like to thank B. Strauß (BAM VI.4, Berlin, Germany) for taking the SEM image. This work was supported by the German Science Foundation (DFG) under Grant Nos. KR 3638/1-1 and RO 2074/7-1.
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