Femtosecond laser-induced periodic surface structures revisited: A comparative study on ZnO

Abstract

Laser-induced periodic surface structures (LIPSS) (ripples) with different spatial characteristics have been observed after irradiation of single-crystalline zinc oxide surfaces with multiple linearly polarized femtosecond pulses (150–200 fs, 800 nm) in air. For normal incident laser radiation, low spatial frequency LIPSS (LSFL) with a period (630–730 nm) close to the wavelength and an orientation perpendicular to the laser polarization have been found in the fluence range between ∼0.7 and ∼0.8 J/cm2 and predominantly for pulse numbers up to N=100. For lower fluences (0.5–0.7 J/cm2), a sharp transition from the LSFL features toward the formation of high spatial frequency LIPSS (HSFL) appears at any given pulse number below N=100. The HSFL are always parallel to the LSFL, exhibit spatial periods between 200 and 280 nm, and completely substitute the LSFL for pulse numbers N>100. Additionally, the influence of the angle of incidence has been studied experimentally for both LIPSS types revealing a different behavior. Experimental evidence for surface scattered second harmonic generation is presented in the regime of HSFL formation. Moreover, we will show that the HSFL structures on ZnO surfaces can be fully explained by an extension of the existing LIPSS theories if the photoexcitation of the dielectric material (affecting its transient optical properties) is considered in the frame of a simple Drude model along with the second harmonic generation at the irradiated surface. Based on our analysis, the current models of femtosecond laser-induced LIPSS are revisited and an explanation is proposed why HSFL are observed predominantly in the subpicosecond range for below band-gap excitation of dielectrics and semiconductors.

Laser-induced periodic surface structures: Fingerprints of light localization

“…5 In comparison, type d have been rarely investigated. 12,19,23 The reason for this is that LIPSSs matching these periodicities have not been observed until the application of pico-and subpicosecond lasers.…”

“…However, the η theory predicts the presence of features which could explain the HSFLs parallel to the polarization. 12,19,23 HSFL is not the only phenomenon which renewed the interest in LIPSSs. For example, with the use of picosecond and femtosecond lasers, LSFLs with a periodicity smaller than the laser wavelength have been observed on different materials.…”

“…Ripples which are either orthogonal [7][8][9][10][11][12][13][14][15] or parallel [16][17][18][19] to the polarization, with a periodicity significantly smaller than the laser light, have been observed for laser pulse durations in the picosecond and femtosecond regime. These are often referred to as high spatial frequency LIPSSs (HSFLs), and as for LSFLs, they were observed on metals, 18,20 semiconductors, [7][8][9][10][11][12]16,17 and dielectrics as well. [13][14][15]19 The influence of polarization, angle of incidence, and wavelength of a laser beam on LSFL formation strongly indicates that the phenomenon is mainly governed by the electromagnetic field.…”

“…The origin of HSFL is still under debate and several theories have been proposed to explain their formation, such as self-organization, 13,16 second-harmonic generation, 7,8,12 or the η theory extended with a modification of the optical properties. 12,19,23 It must be noted that the η theory was created to explain LIPSS formation at a time when HSFLs had not yet been observed.…”

“…12,19,23 It must be noted that the η theory was created to explain LIPSS formation at a time when HSFLs had not yet been observed. However, the η theory predicts the presence of features which could explain the HSFLs parallel to the polarization.…”