1. Introduction

Modern technology and industrial applications generate an increasing demand of surface nano-structuring and functionalization with simple processing strategies and for reduced structural sizes. Especially in the past decade, the femtosecond laser used in micro or nano-structures processing, which can offer unique advantages and provide a nanometer-scale precision, has given rise to considerable interest [1,2]. One promising way is based on laser-induced periodic surface structures (LIPSS), which are formed in a single-step process upon irradiation of solids by linearly polarized laser pulses. Laser-induced periodic surface structures (LIPSS), are a universal phenomenon that occurs on solids upon irradiation with linearly polarized laser radiation [3–6]. LIPSS usually emerge as a surface relief composed of (quasi-) periodic lines, which exhibit a clear correlation to the wavelength and polarization of the incidence laser [5]. Based on this characteristic, the LIPSS can be used in the areas of polarimetric measurements [7], optical waveguides [8] and optical information storage [9]. Therefore, it has become one of the frontier scientific and technological directions that are currently receiving much attention.

The origin of the surface periodic structures (LIPSS) is still an outstanding issue, and fundamental understanding of LIPSS processes is important in order to enable improved prediction and optimization of laser processes. At lower femtosecond laser shots, laser-induced low spatial frequency LIPSS (LSFL) with the orientation perpendicular to the laser polarization is a universal phenomenon, and the classical ripples are widely accepted as a result of the interaction between the incident light and the surface scattering wave [10,11]. Another scenario called self-organized model [12,13] hold that after the femtosecond laser irradiates the surface of the material with multiple laser pulses, the high spatial frequency LIPSS (HSFL), laser-induced orthogonal periodic structures (LIOPS) or nanopillar structures have spatial periods significantly smaller than the irradiation wavelength can be observed. Besides, with increasing of laser shots, the traditional LIPSS disappear gradually under successive laser pulse irradiation when the crater deepens [14]. Instead, the laser-induced periodic annular surface structures (LIPASS) can be formed in the center of the damage area [14]. Despite the intensive studies, the fundamental process of periodic nanostructure formation is still insufficiently understood. As well known to all, pump-probe technique is a useful method to study the laser-materials processing dynamics [15–18]. Several optical probing techniques have been developed for studying laser-material interactions. Such as S. Höhm et al. use a trans-illumination femtosecond time-resolved (0.1ps-1ns) pump-probe diffraction approach to reveal the generation dynamics of near-wavelength-sized LIPSS showing a transient diffraction at specific spatial frequencies [15]. And Jia reports the images of temporal evolution of LSFL formation by a collinear pump-probe imaging approach which reveals the influence of pre-existing surface structures on the subsequent ripples formation [16].

Although the modified theory and pump-probing technology can provide a self-consistent explanation for the formation of either LSFL [11], sub-wavelength structures [13] or LIPASS [14], they cannot consistently encompass formation of all these structures. A complete picture of the physical scenario of the LSFL, sub-wavelength structures and LIPASS formation in the ps range is still missing. Moreover, these LIPSS structures have been extensively studied in many materials, such as semiconductors, metals, and dielectrics [3–5], but few people investigated these structures in the fields of infrared optics and optoelectronics applications [19]. Besides, chalcogenide glass has characteristics of wide transparent transmission window (0.6–8μm), high refractive index (n), and low resistivity [20], which make it a promising material for infrared applications, such as optical communication [21], optical sensing and optical recording devices [22]. In this case, investigation the fabrication mechanism of these surface periodic nano-structures on chalcogenide glass not only can promote our understanding of laser solid-matter interaction, but also would contribute to the attractive field of infrared application areas.

Since, in this paper, we investigated the formation of different LIPSS dynamics reflected from the chalcogenide glasses during the femtosecond laser processing as a function of the pulse number and the incident fluence. A probing beam is applied to ensure the real-time and in situ monitoring of surface modification under different pulse numbers. As the result, a clear correlation is established between the pump beam pulse numbers and the surface morphologies. Especially, we have shown that large areas parallel LIPSS patterns can be written on chalcogenide glasses by a careful balance of pulse energy and scan velocity. Translating these conditions to a stepwise moving-spot excitation approach and synchronizing it with reflection pump-probe setup allows us to unravel the dynamics of the LIPSS formation process on chalcogenide glasses.

2. Material and methods

The As₂S₃ based chalcogenide glass can transmit from 700 nm up to ~7 μm, and the magnitude of As₂S₃ nonlinear index of refraction is large as compared to other chalcogenide or fluoride glasses and through the Kramer-Kröning relation [23]. As₂S₃ chalcogenide can be widely used in the infrared application areas. In this case, we select As₂S₃ chalcogenide as the sample in this experiment. High-purity (99.999%) chemical elements of arsenic and sulfur were used as starting materials to prepare As₂S₃ glasses [24,25]. The raw materials were then placed in the silica ampoule, and the standard purification procedures were employed to further purify the starting materials. Then, the sealed silica ampoule was kept at 800 °C for 12 h in a rocking furnace to ensure homogeneity and then quenched in water to avoid crystallization. The as-prepared glass rods were further annealed at 30 °C below transformation temperature T_g for 3 h to minimize internal stress and then cooled to room temperature.

To experimentally observe the transient of the laser-induced periodic structures, the fabrication of different surface periodic structures that we investigated is illustrated in Fig. 1(a), and the detailed setup information is shown in [26]. The reflection pump-probing setup reveals the evolution of different morphologies during the irradiation processes, and three typical surface morphologies can be fabricated with the accumulation of the laser pulse numbers, which are low spatial frequency LIPSS (LSFL), laser-induced orthogonal periodic structures (LIOPS) and the laser-induced periodic annular surface structures (LIPASS) with high spatial frequently LIPSS (HSFL) on the rim. In this experiment, the pump-probing setup revealed the evolution of different morphologies during the ablation processes. The pump-probing imaging system was set up as shown in [26]. Ti sapphire laser pulses of 800 nm wavelength and 150 fs temporal width impinged As₂S₃ targets. The laser beam was focused by a × 5 infinity corrected, non-achromatic long working distance objective lens at normal incidence. For the reflection probing system, the 473 nm continuous laser (output power: 150 mW) has been focused onto the center of the irradiated region at normal incidence. The As₂S₃ specimens are positioned at the focal plane of the probing laser whose location is defined by knife-edge beam profiling. The laser processing beam and probe beam are also measured under knife-edge method. The intensity of the reflection probe signal was measured by a fast photodiode coupled to an oscilloscope. The oscilloscope is used to record the actual delay time of the processing laser signal and the probing laser signal. To ensure true representation, at least four signals are examined at each delay setting. In addition, by changing the laser scanning velocity, sub-wavelength grating structures can be fabricated on the As₂S₃ surface, as shown in Fig. 1(b). In this experiment, the reflection pump-probing setup can help elucidate the transient fabrication of the surface periodic structures, the surface tension induced mass transport, and the effect of the surface plasmon effect and their dependence on the imparted laser fluences and pulse numbers.

 

Fig. 1 (a) Schematic diagram of the reflection pump-probing setup for the formation of laser-induced periodic surface structures (LIPSS) on As₂S₃, pump laser: femtosecond laser with the central wavelength of 800 nm, probing laser: CW laser with the central wavelength of 473nm. Experience results are presented by SEM and the upper right corner of each SEM figures are the schematic diagrams of each typical structure. (b) Schematic of sub-wavelength grating structures on As₂S₃ under the laser velocity speed of 4 mm/s.

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The morphologies of the formed surface structures are characterized by a SEM system (LEO 1530) at an operating voltage of 20 kV. The detailed morphologies of the surface structures are analyzed by atomic force microscopy (AFM) using a SPI4000/SPA-400 system (Seiku Instruments) operating at tapping mode.

3. Results and discussion

According to previous studies, the formation fluence of LIPSS is slightly lower than the material ablation threshold (F_th) that estimated as 7.19 mJ/cm². Next, we describe the fabrication of the LIPSS under the ablation threshold of As₂S₃, and we depict distinct stages of such modification in Fig. 2. The SEM images directly depict the general effects of pulse numbers on the LIPSS morphological evolution at laser fluence of 6.20 mJ/cm², and the morphology of the LIPSS exhibited a strong dependence on pulse numbers (N = 2, 5, 10, 20, 50). According to the structural features, different typical structures can be fabricated after different pulse numbers. As the pulse numbers increasing, the surface morphology evolved from LSFL (N = 2), to LIOPS (N = 5) to nanopillars (N = 20), and to dip with periodic annular structures (N = 50). The representative structures are shown in Figs. 2(f)–2(j), respectively. After 2 laser shots, well aligned surface LSFL with period around 700 nm can be observed (Fig. 2(f)), and the orientation of LSFL is preferentially perpendicular the laser polarization. With increase of laser shots number N = 5, the LSFL in the center area of the ablated site become less significant and finally dissolve, and this central area is instead covered by LIOPS (period of 250 nm) with obvious spatial periodicity (Fig. 1(g)). Progressive decrease of the LIOPS area towards the center is observed with further increase of the laser shots number N = 10, and the nanopillars become dominant morphology in the central area after 20 laser shot (Figs. 2(h, i)), which orientation is preferentially along the laser polarization. Besides, after 20 pulses irradiation, the high spatial frequency LIPSS (HSFL) with periods of 200 nm and orientations perpendicular to the laser polarization are distributed at the periphery of the crater, while the nanopillar structures are distributed in the central region, as shown in Figs. 2(h) and 2(i). Interestingly, after N = 50 shots, a dip is observed in the center of the damage, which is surrounded by LIPASS (Fig. 2(j)), and the period of these annular fringes is about 0.7 μm. Moreover, HSFL appears at the rim of the dip, which is perpendicular to the polarization (Fig. 2(j)).

 

Fig. 2 Scanning electron microscopy (a-e), the magnified SEM images (f-j) depict the characteristic stages of the morphological evolution of As₂S₃ after different irradiation pulse shots: (a) 2, (b) 5, (c) 10, (d) 20, (e) 50 under fluence of 6.20mJ/cm², respectively. The red arrow indicates the pump laser polarization direction.

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We also investigate evolution of the laser-induced surface structures with different laser fluences (5.16 mJ/cm², 8.26 mJ/cm²). After analysis, we know that five different typical structures with certain orientation can be formed with different laser shots. Next, we mainly investigate these periods of polarized LIPSS versus pulse accumulation at different laser fluences, as shown in Fig. 3. At laser shots two and five, only traditional LSFL with period around 700 nm, height of 100 nm and the orientation perpendicular to the polarization are obtained in the central area, as shown in Fig. 4(a). Interestingly, three-dimensional LIOPS with two orthogonal strips (period of 250 nm) and height of 200 nm can be fabricated at the bottom of the surface LSFL, while the surface LSFL are mostly retained because of the lower laser fluence caused by the larger pulse delays and lower laser shots (N<5), as shown in Fig. 4(b). Some references have reported the LIOPS structures can be fabricated after femtosecond laser irradiation, such as crystalline silicon and fused silica [27,28]. But on the As₂S₃ glasses, the results of our experiment are reported for the first time. With increasing of laser shots (more than 10 shots), these surface LSFL and LIOPS are instead of nanopillars with periods almost equal to that of the HSFL (250 nm), and the orientation of this structures, which is preferentially along the laser polarization. Besides, these nanopillars are located in the dip, which height is about 500-600 nm, as shown in Figs. 4(c) and 4(d). Surface showing a period structure of about 250 nm on the periphery of the irradiated area (Type HSFL) with the orientation perpendicular to the polarization, and the central nanopillars are replaced by LIPASS (independent of the polarization, over 800 nm depth) under successive laser pulse irradiation when the crater deepens, as shown in Fig. 2(j).

 

Fig. 3 Periods of LIPSS versus pulse accumulation at different laser fluences (5.16 mJ/cm², 6.20 mJ/cm² and 8.26 mJ/cm²). Representative SEM images are the different typical LIPSS structures (LSFL in red wireframe, HSFL in green wireframe, LIOPS in blue wireframe and Nanopillars in black wireframe). The red arrow indicates the pump laser polarization direction. All scale bars are 2µm.

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Fig. 4 Surface characteristics of LIPSS at 6.20 mJ/cm² laser fluence irradiation with different laser shots. (a-d) SEM images, (e-h) AFM images and (i-l) depth and period of the LIPSS at 3, 5, 10 and 20 laser shots, respectively.

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In order to understand the detailed evolution process of the LIPSS morphologies under different laser shots, the reflection pump-probing experiments are carried out, as shown in Fig. 5. Pump-probing technology can capture in situ reflectivity of films after pulsed laser irradiation [29], and the normal reflection traces are normalized with respect to their initial values before laser irradiation. From Fig. 5, we notice that under first pump laser irradiation, the reflection signal does not change much, and then the reflection signal drops quickly (about 40 ps), stabilizes at lower reflection state (lower than 65%) and then slowly decays (more than 100 ps). This is primarily due to the energy transfer from free electrons to the lattice, resulting in the formation of the LSFL structures [30,31]. In this condition, when the first laser irradiate on the surface, surface plasmon wave can be formed on the surface, and the interference phenomenon appears after subsequent laser irradiate on the surface, which result in the redistribution of the laser fluence on the surface. The laser fluence is high enough, so the melted zone is well defined by the interference phenomenon. A sharp transition between the liquid and solid phases was formed, and surface ripple structures can be formed by the full-melting phenomenon. The nature of the generated electromagnetic field structures and their relation to the simple “surface-scattered wave” model for periodic surface damage are discussed. The period of LSFL can be defined as $\Lambda \text{=}\lambda \text{/((}\lambda \text{/}{\lambda }_s\text{)}\pm \text{sin}\theta \text{)}$ [32], Where λ is the wavelength of the incident laser, the λ_s on a material interface is given by ${\lambda }_s=\lambda {((\epsilon \text{'}+{\epsilon }_d)/\epsilon \text{'}{\epsilon }_d)}^{1/2}$ and ${\epsilon }_d={(n_{A{s}_2{S}_3}+k_{A{s}_2{S}_3})}^2$ [33,34]. In our result, we obtain the simple relationship Λ = λ_s (about 800 nm) at normal incidence. With the increasing of laser shots, up to 5 shots, the reflective signal gives a different scenario, which drops slightly (<5%) after each pump laser shot. Besides, in the AFM images, we know that the depth of the LIOPS (200 nm) after 5 laser shots is larger than that of the LSFL (100 nm) after two shots. In order to gain deeper insight into the mechanism of LIOPS induced on the As₂S₃ surface by femtosecond laser, we have calculated the electric field distributions on the surface LSFL by using a commercial-grade simulator (Lumerical FDTD), which is shown in Fig. 6 [35]. It can be seen that the subsequent ablation occurred preferentially at the enhancement spots of each ripples, leading to the formation of a LIOPS at the bottom of the LSFL [27,28]. From the FDTD simulation result we know that the subsequent incident electric field can be redistributed due to the preexisting LSFL, resulting in the split of the ridges of LSFL and the generation of new strips at the bottom of the primary LSFL. In this case, the new bottom HSFL become the dominant morphology, resulting in the nanopillars at the bottom of the dip. The formation of the LIOPS is the transition between the LSFL and HSFL with an orientation parallel to the laser polarization [12,13]. Progressive decrease of the nanopillars area towards the center is observed with further increase of the laser shots number, and finally vanish after N > 50. A dip is observed in the center of the damage, which is surrounded by several annular periodic fringes, and the reflection signal drops at lower reflection state (20%) [14]. The period of these annular fringes is about 700 nm, which is due to the interference of the incident laser field and the reflected laser field on the crater surface.

 

Fig. 5 Reflection pump-probing signals of fs laser-induced periodic surface structures with different laser shots and the corresponding morphologies of surface periodic structures at different evolution stages. The scattered plots represent the evolution of the reflection signal with different laser shots, and the red curve represents the fs laser pulse with a pulse width of 150 fs.

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Fig. 6 (a) Schematic structure of the initially formed grooves on the As₂S₃. The calculated electric field distributions of the schematic structures (l = 4μm, w = 0.3μm, d = 0.4μm and h = 0.16μm) illuminated by horizontal laser polarization directions. (b) The calculated electric field distributions of the schematic structures.

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Large area nano-grating structures have a wide range of applications in many respects, such as super-hydrophobic [36] and photonic crystals in infrared wavelength [20], and laser line scanning is an effective method for fabricating large-area nano-grating structures [36,37]. In this case, fundamental understanding of laser-induced nano-grating processes is important in order to enable improved prediction and optimization of laser processes. Next, we systematically investigate the effect of different laser scanning velocity on the formation of nano-grating structures. Besides, to confirm the formation mechanism of the line-scanning-induced structures, we also compare the reflected signal of LIPSS at single point irradiation and the structures under line-scanning. In the previous section, we have investigated the reflectivity of LIPSS structures under different laser shots at the same point (laser fluence of 5.16 mJ/cm²), marked as black line in Fig. 7(a). In addition, the reflectivity of the surface structures under different line scanning velocity is also measured as the same method, which is marked as the red line, and the typical SEM images are also listed in Fig. 7(a). For the larger scanning velocity (10 mm/s), it is equivalent to a single-pulse laser irradiation effect, and hole structures are formed on the surface. With decrease of scanning velocity, the reflectivity also drops gradually, which tendency is similar with that of LIPSS under one point irradiation. By continuously scanning step-by-step, the adjacent laser tracks are partially overlapped to each other and formed continuous long ripples perpendicular oriented to the linear polarization vector of the laser beam (scanning velocity 5 mm/s), as shown in Fig. 7(b). Figure 7(c) shows a 3D-AFM image of the ripples. Ripples with a significantly smaller spacing than the irradiation laser wavelength (800 nm) are observed. The corresponding cross-sectional profile is shown in Fig. 7(d). Evidently, the periodic surface structure has a very clear contour with an average spatial periodicity of 780 ± 10nm. The average height of the ripples is 145 ± 10 nm. With further lower writing velocity, the morphology of the gratings is replaced by the laser-induced orthogonal periodic structures (LIOPS), which morphology is similar with that of the single point irradiation. In this condition, it’s demonstrated that the slightly drop of the reflection signal after each laser shot is due to the transition between the LSFL and HSFL with an orientation parallel to the laser polarization, finally resulting the formation of the LIOPS under the surface LSFL (after ten laser shots).

 

Fig. 7 (a) Reflection signal comparison of single-point laser induced LIPSS (black line) and the laser line-scanning induced morphology (red line), and the SEM images in the figure are the corresponding SEM images under certain laser scanning velocity. (b-d) The SEM, 3D-AFM and the corresponding cross-sectional profile of the continuous long ripples with the laser scanning velocity of 5 mm/s. The irradiation of the laser fluence is 5.16 mJ/cm².

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4. Conclusion

We report an optical method capable of simultaneously generating laser-induced self-assembled periodic structures and resolving spatially and temporally their formation process. Pump-probing technology can capture in situ reflectivity of films after pulsed laser irradiation. And the interference phenomenon appears after two shots laser irradiate on the surface, which result in the formation of LSFL on the surface. With the increasing of laser shots, up to 5 shots, It can be seen that the subsequent ablation occurred preferentially at the enhancement spots of each ripples, leading to the formation of a LIOPS at the bottom of the LSFL. As the laser shots going on, we infer that the split of LSFL leads to the formation of nanopillars, and we conclude the formation of the nanopillars under the surface LSFL (after ten laser shots) is due to the transition between the LSFL and HSFL with an orientation parallel to the laser polarization. A dip is observed in the center of the damage, which is surrounded by several annular periodic fringes after 50 laser shots, which is due to the interference of the incident laser field and the reflected laser field on the crater surface. Finally, a direct writing technique for fabrication of nano-gratings on chalcogenide glasses is reported.