The formation of laser-induced periodic surface structures (LIPSS) on two different dielectrics of K9 glass and fused silica upon irradiation in ambient conditions and in vacuum with multiple femtosecond (fs) laser pulse sequences at different pulse durations (35 fs, 260 fs, and 500 fs) was studied experimentally. Three types of LIPSS, so-called high-spatial-frequency LIPSS (HSFL), low-spatial-frequency LIPSS (LSFL), and supra-wavelength periodic surface structures (SWPSS) with different spatial periods and orientations were identified. The appearance was characterized with respect to the experimental parameters of laser fluence and number of laser pulses per spot. The crater morphologies — including nanoripples, periodic microgrooves, quasiperiodic microspikes, and central smooth zone — were observed by scanning electron microscope (SEM). The supra-wavelength structures exhibit periodicities, which are markedly, even multiple times, higher than the laser excitation wavelength. The SWPSS were formed with a broader range of laser fluences, upon the longer laser pulse durations (260 fs and 500 fs) and/or on the lower band-gap dielectrics (K9 glass), due to the deeper effective light penetration depths and thicker viscous surface layers formation. The HSFL were observed on the higher band-gap dielectrics (fused silica) and within a certain narrow laser parameter window. The formation mechanisms of LIPSS were also discussed.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The fs laser processing has gained remarkable attraction due to its multiple diverse applications [1–7], ranging from nanogratings [2–4] to optoeletronics [5–7], microfluidics, and biomedicine, etc. The formation of laser-induced periodic surface structures (LIPSS) on the surface of semiconductors [6–9], metals , and dielectrics [3,4,11–17], etc can be generated by using fs laser direct irradiation [1–18]. The LIPSS are classified to three types, due to the spatial periods [19–32]. One type of LIPSS is the supra-wavelength structures (SWPSS), which are formed by the melting and subsequent capillary effects (or the capillarity convection effect) [20–23]. The periods of SWPSS are in great range from supra-wavelength to even several wavelengths [22,23]. Another type of LIPSS is the low spatial frequency LIPSS (LSFL). The LSFL formation mechanisms are generally accepted as the interference of the incident laser beam with a surface electromagnetic wave (SEW) [11,13,17,19] and/or surface plasmon polaritons (SPPs) [30–32]. The third type of LIPSS is the high spatial frequency LIPSS (HSFL). The formation mechanisms of HSFL are controversially discussed, such as self-organization , nanoplasmonic excitations [3,4], and second-harmonic generation [27–29], etc.
The surface spatial periods of the LIPSS under the irradiation with fs laser pulses has been found to depend upon various experimental parameters such as laser fluence [11,30], number of pulses [31,32], incident laser wavelength [17,20], band gap of the material , pulse duration [24,26,34], and the ambient environment [33,35,36]. G. D. Tsibidis et al. have recently reported the SWPSS on fused silica under different wavelength of 1026 nm and 513 nm . U. Chakravarty et al. have studied the nano-ripple formation on different band-gap semiconductor surfaces, and found that narrower nano-ripples were formed from wide bandgap semiconductors . C. S. Nathala et al reported that the periods of LSFL on titanium was independent on the pulse duration, while the HSFL periods decreased with the increase of pulse durations . Although, many studies have been proceeded on effects of laser pulse durations on the laser ablation efficiency, quality, and morphology [24,26,34], the effects of laser pulse durations on periods of LIPSS formed on dielectrics were rarely reported.
In this paper, the effects of fs laser pulse durations (35 fs, 260 fs, 500 fs) and environments (in air and in vacuum) on the LIPSS on different band gap dielectrics of K9 glass and fused silica were investigated under different laser fluence and number of pulses. We focused on the different band gap of dielectrics under the irradiation of 800 nm fs laser pulses. Three types of LIPSS on fused silica surface under 260 fs in vacuum were observed at different laser fluence regimes simultaneously. It was also observed on fused silica in vacuum that the HSFL transformed to LSFL with increasing the number of laser pulses.
A commercial chirped-pulse-amplification (CPA) Ti: sapphire laser system (Spectra physics) was used to generate a linearly polarized laser pulse. The pulse duration of 35 fs was stretched up to 260 fs and 500 fs by inserting dispersive ZF6 glasses with different length. The laser has a central wavelength of 800 nm. The repetition rate was 10 Hz. A neutral density filter was functioned as the energy attenuator to adjust the laser energy. The pulses were focused by a 150-mm-focal-length lens onto the surface of samples with a incidence angle of about zero degree. The samples were placed in a chamber, which was mounted on a motorized x and y transition stage. Experiments were performed in air (ambient environment) and high vacuum (1.1 × 10−3 Pa) condition. The K9 glass and fused silica samples with a dimension of 15 mm × 15 mm × 5 mm and a root-mean-square (rms) roughness less than 1 nm were studied. The periodicities of the structures and morphologies of ablated craters were examined by using scanning electron microscope (SEM, Phenom Pure).
3. Results and discussion
3.1 Fluence dependence
Figures 1(a) and 1(b) show the results of the LIPSS periodicity of K9 glass, and fused silica with 30 pulses irradiation under different laser pulse durations in air and in vacuum, respectively. The LIPSS periods increased with the increase of the laser fluence. The SWPSS and LSFL were observed on K9 glass surface under irradiation of all the pulse durations of 35 fs, 260 fs, and 500 fs. Furthermore, the LSFL were observed on fused silica surface under irradiation of all the pulse durations of 35 fs, 260 fs, and 500 fs. The SWPSS were observed on the fused silica surface under irradiation of 260 fs, and 500 fs laser, while the HSFL were found on the fused silica surface under irradiation of 35 fs, and 260 fs laser. The supra-wavelength structures exhibit periodicities, which are markedly, even multiple times, higher than the laser excitation wavelength (λ = 800 nm). The SWPSS were formed with a wider range of laser fluences, upon the longer laser pulse durations (260 fs and 500 fs) and/or on the lower band-gap dielectrics (K9 glass). For example, as shown in Figs. 1(a) and 1(b), on the condition of 260 fs laser irradiation in vacuum, the SWPSS were found on K9 glass surfaces with a range of laser fluences of 2.9 to 23 J/cm2, which was wider than that of laser fluences of 2.5 to 7.7 J/cm2 on the fused silica surfaces. Moreover, the HSFL were observed on the higher band-gap dielectrics (fused silica). For the HSFL, a high increase rate of the average period with increasing fluence is found, while the LSFL periods exhibit a moderate increase rate . In the contrast, the SWPSS periods increase slowly with increase of fluence. Especially at 500 fs pulse duration, the SWPSS periods has the slowest increase rate for laser fluence < 21 J/cm2.
As shown in Fig. 1(b), the HSFL were found on the fused silica surface under irradiation of 35 fs in air, and 260 fs in vacuum. At a fixed number of laser pulses, a transition from HSFL to LSFL occurs when a critical fluence threshold is exceeded [11,19,34]. The transition threshold fluence of fused silica were about 2.5 J/cm2, and 2.0 J/cm2 under irradiation of 35 fs in air [19,37], and 260 fs in vacuum, respectively.
3.2 Pulse duration dependence
As shown in Fig. 1, the period of SWPSS in 500 fs was smaller than that of 260 fs at the same laser fluence both in K9 glass and fused silica. At the same fluence of 1.8 J/cm2 (Fig. 1(b)), the period of 260 fs on fused silica in vacuum was 258 nm, which was also smaller than that of 35 fs (374 nm period on fused silica in air). The results of decrease of HSFL period with increase of pulse duration were also reported on the titanium . Thus, the periods formation was related to not only the laser fluence, but also the pulse duration. That means the laser intensity, which affect the multiphoton ionization processes, should be considered.
The SWPSS only can be formed when the viscous layer becomes sufficiently thick . Increasing multiphoton absorption efficiency obviously can reduce the viscous layer thickness, even resulting in a smoothly resolidified surface morphologies without LIPSS formation. On another hand, the effective optical penetration depths () affect the viscous layer thickness intensively. Table 1 listed the values of with various pulse durations on K9 glass and fused silica surface in vacuum [24,36]. The equivalent Gaussian beam radius (ω0) focused on the sample surface were also deduced and listed in Table 1 . Firstly, the K9 glass has lower multiphoton absorption efficiency and larger than that of fused silica. The SWPSS on K9 glass can be formed with a broader range of laser fluences than those of fused silica. Secondly, at longer pulse durations (260 or 500 fs), with “deeper” light penetration than that of 35 fs, the SWPSS can also be formed under laser irradiation with a broader range of laser fluences. Furthermore, at longer pulse durations, a lower number of absorbed photons at color centers may allow mechanical relaxation processes in the glass material leading to the LIPSS formation [24,26]. Thirdly, the focused laser beam radius (ω0) may also affect the SWPSS formation due to the different local laser intensity distribution on the samples. Therefore, the SWPSS were formed by using of a broader range of laser fluences on the lower band-gap dielectrics and/or with the longer laser pulse durations.
As shown in Fig. 1, the supra-wavelength periodic structures on the K9 glass were formed with a wider range of laser fluences than on the fused silica. The supra-wavelength periodic structures on the fused silica were also formed with a wider range of laser fluences under 513 nm than 1026 nm femtosecond laser irradiation [20,21]. The band gap energy of K9 glass is about 4.0 eV , which is lower than that of fused silica (7.8 eV) . When K9 glass was studied, simultaneous absorption of three laser photons at 800 nm can easily induce free conduction band electrons (CBEs). These CBEs can subsequently gain kinetic energy by linear absorption from laser beam. So, decrease the laser wavelength or the band-gap of the dielectrics were helpful to increase the free CBEs production, resulting in the viscous layer thickness increase and SWPSS formation.
3.3 Environment dependence
Figures 2 and Fig. 3 show the SEM of damage sites on K9 glass, and fused silica surface after irradiation with 20 or 30 pulses of different fluences between 2.1 J/cm2 to 23 J/cm2 in air and in vacuum with pulse duration of 260 fs, respectively.
The crater morphologies including nano-ripples (LSFL and HSFL in Fig. 5 and Fig. 6), periodic microgrooves (SWPSS in Figs. 2(b)–2(f), and Figs. 3(b) and 3(e)), quasi-periodic micro-spikes (Fig. 3(d)), and central smooth zone (Fig. 2(a)) were observed by SEM. The environments affected the period characters and the morphologies, especially at the low and the high laser fluence regimes. As shown in Fig. 1(b) of 260 fs on fused silica, when the laser fluence was as low as 1.8 J/cm2 in vacuum, the HSFL was formed (about 258 nm), while LSFL (about 700 nm) was observed as low as 1.7 J/cm2 in air. Furthermore, at the high laser fluence of 19 J/cm2, 23 J/cm2 of 30 pulses irradiated on K9 glass (Fig. 1(a), 260 fs), the SWPSS were found in vacuum, while the central smooth zones were formed in air condition (e.g., Fig. 2(a), 23 J/cm2 of 30 pulses). Especially in high laser fluences, the laser-induced air breakdown, or laser-plasma interaction affected the energy deposition and the material ablation, resulting in different period structures and crater morphologies [24,39].
The period of supra-wavelength structures (SWPSS) varied with laser parameters, e.g. fluences, pulse numbers, pulse durations, and environments as shown in Fig. 1, Fig. 2, and Fig. 3. The laser polarization orientation was shown in Fig. 2(f) in the horizontal direction. As shown in Fig. 2 and Fig. 3, the orientation of the SWPSS were not always parallel to the laser polarization orientation [20,21,24,26]. The hydrodynamical mechanism were used to describe the molten material dynamics and re-solidification processes [20,21,26]. As a result of the inhomogeneous heating (e.g. caused by laser intensity distribution, surface roughness, laser-induced defects etc.) [24,26], the produced temperature field induced surface stresses that drive a shear flow of the molten layer parallel to the thermal gradient. The morphological profile (as well as the period structure orientation) was produced as a result of phase transition, fluid transport, and a re-solidification process. As shown in Fig. 2(e) (4.9 J/cm2, 20 pulses, 260 fs, in vacuum), the ablated crater has an approximate axial symmetry topographic. The single headed arrow indicates a symmetry axial direction. The molten layer depth may be deviated from Gaussian distribution, as well as the laser intensity distribution focused on the local surface along the symmetry axial direction. As a result of the inhomogeneous heating, the produced temperature field induces surface stresses that drive shear flow of the molten layer along the symmetry axial direction, also parallel to the thermal gradient instead of the polarization direction. Further experimental and theoretical studies will be carried out in the future to verify the SWPSS formation mechanisms.
3.4 Pulse number dependence
The periodicity dependence of LIPSS on number of pulses on fused silica surface with 260 fs laser irradiation in vacuum were shown in Fig. 4. The period increased with increasing irradiation pulse number. Again, a sharp transition from HSFL (with periods between 200 and 300 nm) to LSFL (with periods between 600 and 800 nm) can be observed for pulse number of exceeding a critical value . The orientation of these low spatial frequency microstructures was parallel to the laser beam polarization. The period of the LIPSS varied with pulse number was mainly related to the incubation effects, which are typically originating from the laser-induced defects and color centers, for changing of the material absorption [11,37,40]. Our recent work has reported the experimental evidence of fs laser-induced defects and color centers on fused silica surface . Furthermore, the production of CBEs with sequences of pulses may modify the material dielectric constant and the refractive index, and increase the laser absorption [31,41].
Figure 5 and Fig. 6 show the SEM images of damage sites on fused silica under various number of pulses (N) with laser fluence (F) of 2.1 J/cm2, and 1.8 J/cm2 with pulse duration (τ) of 260 fs in vacuum, respectively. As shown in Fig. 5, due to the inhomogeneous laser intensity, the central crater area was covered by LSFL, while the HSFL were additionally formed in the surrounded crater edge. The LSFL and the HSFL are perpendicular to each other. The orientation of LSFL is parallel to the laser beam polarization. When the pulse number increased from 10 to 30, the central crater area covered by LSFL was increased, while the area of HSFL was decreased . The crater morphology characteristics further supports the importance of incubation effects, the dependence of “threshold values” on number of pulses. Therefore, the lower local fluence value for the formation of LSFL is attributed to a higher number of pulses. On another hand, the surface roughness increased with the increasing of irradiation . As shown in Figs. 6(c) and 6(d), the initially random and sparsely distributed trenches (nanoholes, nanoparticles, or nanoprotrusions) were formed on fused silica, which act as a scatterer, and interference of scattered wave and laser pulses lead to LSFL formation with increasing of laser pulse number .
The LIPSS periods changed with the laser fluence, number of pulses, and pulse trains etc [17,19,31,42]. The LSFL are typically formed for higher laser fluences or number of pulses on silica-based glasses [11,12]. The HSFL was observed only in few glasses [11,15,16]. Additionally, higher number of applied pulses are required and the HSFL are formed only within a certain narrow laser parameter window [43,44]. The LSFL orientation was parallel to the laser beam polarization, which was formed as the result of interference between an incident wave and a surface scattered wave. However, the orientation of the HSFL (at 260 fs in vacuum) rotates by 90° to be perpendicular to that of LSFL and the laser polarization, which was shown in Fig. 5 and Fig. 6. Another orientation of the HSFL (at 35 fs in air reported in ) was parallel to that of LSFL and the laser polarization.
One possible mechanism for the HSFL formation may be related to the third harmonic generation . The HSFL formation was due to the interference between the generated third harmonic with a surface electromagnetic wave (SEW) and/or SPPs at the sample surface. It is well known that surface nonlinearities can lead to the generation of strong third harmonic frequencies [45–48]. Laser ablation dominated by SHG is expected to play a crucial role in the formation of HSFL on the surfaces of semiconductors where SHG is significant [27–29]. However, the higher-order process and the application of a higher-intensity laser beam may give a large induced third-order polarization field (P(3ω) ᵡsurface(3)E3(ω)) on the centro-symmetric fused silica surface. Here, ᵡsurface(3) is the third order surface nonlinear susceptibility. Due to the surface-enhanced effect, the ᵡsurface(3) is at least 3 orders of magnitude larger than that of ᵡbulk(3) (SiO2) values . Excitation of SPPs by the higher harmonics reduces the LIPSS spacing by 3 times for third harmonic frequencies respectively. This could explain the very fine HSFL structures observed in this work. Based on the Sipe-Drude model and considering the carrier dependence of the optical properties of fs-laser excited fused silica, the LIPSS period and the orientation have been explained experimentally and theoretically [11,19,31]. The LIPSS period is from ~λ to λ/n, a transition between a metal-like and a dielectric material behavior. The third harmonic wavelength of 800 nm light in fused silica is about 177 nm (λ/3n(λ/3)). In this study, the period of HSFL formed on fused silica was 189 to 304 nm (Fig. 4), which is in the range of 177 to 304 nm (1.14 × λ/3). Some super-third-harmonic wavelength structures were formed. The orientation of the LIPSS have been associated with the excitation of SPPs. When the free-electron densities in the conduction band of the solid exceed ~3 × 1021 cm−3, the orientation of LIPSS would be perpendicular (⊥) to the polarization . Consequently, the material turns from a dielectric state into a strongly absorbing and high reflective metal-like state . At the lower laser fluence regime, the material damage and ablation are mainly due to the conduction band electronics production by photoionization and avalanche ionization. The interference of the generated third-harmonic radiation and the excitation of short-wavelength SPPs could explain the HSFL periods and orientations [11,19,30,31,34]. On the other hand, the absorption of fundamental may lead to thermal poling, and producing an effective second order nonlinearity within fused silica . The HSFL (// to polarization) formation on fused silica surface under 35 fs pulse duration irradiation in air may be related to the second harmonic generation by the thermal-poling enhancing second order nonlinearity [37,49]. Thus, the HSFL formation could be described by the interference mechanisms, the third (second) harmonic generation, and the incubation effects.
Another possible mechanism of cavitation-instability  could explain the HSFL formation on the fused silica surface. Under fs laser irradiation, some nanobubbles, nanovoids, nanocavity formed on rough surface, surface defect, or color centers to produce a lot of initial trenches. As shown in the Fig. 6(a), there are some nanovoids at the crater edges. The interference of the incoming light with the excited SPPs , or inhomogeneity-scattered light waves [50,51], could be the reason for the HSFL formation. However, the actual mechanism of the HSFL still remains as an open question, requiring more experimental and theoretical studies in the future [4,14,19,50,51].
Three types of laser-induced periodic surface structures (LIPSS), so-called high-spatial-frequency LIPSS (HSFL), low-spatial-frequency LIPSS (LSFL), and supra-wavelength periodic surface structures (SWPSS) were observed with different laser fluence regimes on K9 glass and/or fused silica surface under 800 nm femtosecond laser irradiation. The effects of laser fluences, pulse durations, environments, and number of pulses on LIPSS periods were investigated. At a fixed number of laser pulses, a transition from HSFL to LSFL on fused silica occurs with a critical fluence threshold of about 2.5 J/cm2, and 2.0 J/cm2 under irradiation of 35 fs in air, and 260 fs in vacuum, respectively. The SWPSS were formed with a broader range of laser fluences, with the longer laser pulse durations (260 fs and 500 fs) laser irradiation and/or on the lower band-gap dielectrics (K9 glass). The environments affected the morphologies and the period characters, especially at the low and the high laser fluence regimes. The period of the LIPSS varied with pulse number was mainly related to the incubation effects. The initially formed random and sparsely distributed nano-roughness (nanoholes, nanoparticles, nanoprotrusions) gets periodically structured LSFL with increase in number of laser pulses. These results were helpful for further sufficient elucidation of LIPSS formation processes, which could contribute to the improvement of the spatial controllability and the precise formation of a period structure, resulting in the acceleration of development in such applications.
National Natural Science Foundation of China (NSFC) (61705205, 51701087).
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