1. Introduction

Exposure of material surfaces to laser pulses was reported to induce self-organised structures which are known as laser-induced periodic surface structures (LIPSS) also termed as ripples. Their existence has been reported in semiconductors for several decades now, the first time dating back to 1965 [1].

In the femtosecond regime most of these periodic structures (fs-LIPSS) were shown to have periods substantially smaller than the wavelength of the writing beam and were thus called nanogratings. Of great interest is the fact that these structures are formed without any use of beam shaping or holography, making them suitable for micro and nanomachining thus opening potential applications in optical data storage and photonic devices.

Despite the fact that the fs-LIPSS phenomenon has been studied on various materials, from metals such as nickel [2], ceramics [3], insulators [4] and semiconductors [5], most of the effort in the glassy materials has been concentrated on creating nanostructures on silica [6,7]. The resulting nanograting formation at the surface or in the bulk of silica was reported to generally have spatial periods significantly smaller than the irradiation wavelength (Λ< λ/2) (called high-spatial-frequency LIPSS (HSFL)) as well as plane alignment perpendicular to the electric field [8,9].

Nevertheless, it was observed for strongly absorbing materials such as metals and semiconductors, after exposure to linearly polarized radiation at normal incidence which the period of the observed LIPSS is close to the wavelength of the incident radiation [10]. The generally accepted mechanism responsible for the formation of this type of wavelength ripple relies on the optical interference between the incident radiation and an electromagnetic wave which is scattered along the surface during the irradiation [11,12]. These particular wavelength related ripple structures, which are usually called low spatial frequency LIPSS (LSFL), are rather insensitive to the laser pulse duration. This model was quite successful to describe the formation of LSFLs, especially for long pulses but it could not explain most of the HSFL formation in the case of femtosecond laser irradiation.

Höhm et al. [13] reported recently the formation of two different types of fs-LIPSS on a silica surface. In quartz, LSFL with periods between 460 and 900 nm and an orientation parallel to the laser beam polarization were observed for fluences above 6 J/cm² (for ten successive pulses). Below that fluence, HSFL with spatial periods between 170 and 450 nm were rather found to be perpendicular to the laser beam polarization.

In chalcogenide glasses, it has been demonstrated recently that nanogratings and nanoholes can be fabricated using direct femtosecond laser writing at the surface of As₂S₃ bulk glass [14]. Nanogratings with a period of 180 nm have been achieved using multi-pulse laser irradiation at a power close to the breakdown threshold of the material. Most reports on the fs-laser interaction with chalcogenide glass involved As-based chalcogenides with emphasis on the fabrication of waveguides [15]. In fact, only a few investigations have systematically examined the photoresponse of Ge-based glass to fs-laser irradiation [16].

In this letter, we investigate the formation of self-organized periodic structures on the ablated surface of bulk Ge-S based glass. The laser-induced periodic surface structures obtained have a period of about 720 nm, close to the wavelength of the laser which is 806 nm. We present what we believe is the first study describing the dependence of both the laser fluence and irradiation time on the formation of such photo-induced structures on the surface of Ge-S glass.

2. Experiment

The production of the glass sample started with high-purity Ge, Ga, As and S precursors (5N) which were weighed and introduced into fused quartz ampoules to form a Ge₂₅Ga₁As₉S₆₅ (noted as Ge-S based glass in the text) composition. After vacuum sealing, a heating process up to 900 °C was started and maintained at this temperature for 8 h in a rocking furnace in order to permit reaction of the precursors. Then, the melt was cooled in water and the sample obtained was annealed below the glass transition temperature (~330 °C) during 6 hours. Afterwards the glass rod was removed from the ampoule, cut into slices and polished so as to obtain 2 mm thickness samples.

A Ti-sapphire regenerative amplifier system (Coherent, model Legend-HE) that produced pulses with a maximum energy of 3.5 mJ at 1 kHz repetition rate with a central wavelength of 806 nm was used to expose the Ge-S glass sample. Temporal width of the Fourier-transform limited pulses was measured to be ~34 fs and the laser beam diameter was measured to be ~8.5 mm (at 1/e²) at the laser output. The beam was then focused using a 50 cm focal length plano-convex spherical lens. The sample was placed at a distance of 45 cm from the lens so as to obtain a converging exposure beam with a beam waist w of about 400 µm at the surface of the Ge-S based sample. The laser beam was impinging the sample at normal incidence. Exposed surfaces were analysed by scanning electron microscopy (SEM) (FEI, model Quanta 3D FEG) and to further confirm the periodic structure peak-valley shapes, an atomic force microscope (AFM) (NanoScope V-Veeco) was used to analyze their surface topography.

3. Results and discussion

We first determined the ablation threshold of our material by measuring the diameter of the ablated spot D as a function of the incident pulse energy, E_in. Assuming a Gaussian beam profile, the square of D can in fact be expressed as a function of E_in and E_th, the pulse energy threshold for ablation [17]:

 

Fig. 1 Squared diameter of the ablated craters as a function of the incident pulse energy on the Ge-S glass sample for (a) N = 10 and (b) N = 50 successive pulses.

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Also note that the sample was further exposed for longer pulse numbers and then presented lower ablation threshold values (e.g. 0.07 J/cm² for N = 1000). The surface temperature of the Ge-S sample was monitored during the exposure using a thermal camera (Jenoptik, Variocam) with a close-up lens which provides a spatial resolution of 50µm. We observed that the temperature of the sample would significantly increase for such longer exposures up to the point where the glass would even soften. For example, at a peak pulse fluence of 0.14 J/cm², the glass transition temperature (~380 °C) is reached after N ~6000 successive pulses, proving that heat accumulation in the sample occurs. In the remainder of the paper, we present results where the number of successive pulses was short enough (N ≤ 50) so that the maximum sample temperature does not exceed 50 °C for any peak pulse fluence tested.

The formation of self-organized structures was actually observed within a processing window defined by a pulse fluence ranging between 0.2 and 0.8 J/cm², and corresponding to N ≤ 50 pulses. Above 100 successive pulses, the periodic structures begin to disappear. Figure 2 shows the evolution of the surface morphology as a function of the pulse number. With only 5 pulses (Fig. 2(b)), a random structure can be observed but after 50 pulses (Fig. 2(c)) a series of ripples with periods near the laser wavelength appear on the ablated surface. Also, we have observed the formation of periodically spaced dots or nanochannel structures on the top of the ripples (Fig. 2(c)) which was also reported in silica glass in Ref [13].

 

Fig. 2 SEM images of the Ge-S glass surface after exposure to an increasing number of successive pulses. (a) Unexposed surface, (b) N = 5 at a peak pulse fluence of 0.28 J/cm², (c) N = 50 at 0.42 J/cm², and (d) N = 100 at 0.42 J/cm².

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Clearly, the degree of order increases with increasing numbers of pulses, indicating the need for some energy accumulation to produce the self-organized periodic structures. This accumulation behaviour has been reported on fused silica [22] and supports the importance of incubation in the fs-LSFL formation. This incubation effect promotes and increases the degree of absorption resulting, for subsequent laser pulses, in a larger number of carriers in the conduction band.

However, in the case of chalcogenide glasses the effect of temperature accumulation is strong because they have a low thermal diffusion coefficient, D, which is typically ~10⁻³cm²s⁻¹ [23]. Consequently, as the number of pulses increases (N > 100 pulses), heat accumulation occurs which result in glass softening [24] so that the periodic structure is no longer observed (Fig. 2(d)).

Within the appropriate laser conditions described above, self-organized periodic surface structures are observed on the ablated surface of the Ge-S glass sample. Figure 3(a) shows the profile of the crater after exposure to N = 50 at a peak pulse fluence of 0.42 J/cm², producing a maximum crater depth of around 22 µm. The SEM picture in Fig. 3(b) shows the resulting full image of the ablated region. Due to the Gaussian shape of the laser beam profile, the central crater area is surrounded by an annular region (noted in Fig. 3(b)) where LSFLs are formed (at smaller local fluence values). Looking at the annular region with increased magnification of the SEM (Fig. 3(c)), one observes regular ripple-like structures.

 

Fig. 3 SEM images of an ablated region exposed to N = 50 successive pulses at a peak pulse fluence of 0.42 J/cm². (a) Profile of ablated region. (b) Full image of the exposed region. The diameter of the affected area is ≈620 µm. The ring represents the region in which straight self-organized structures are formed. (c) Zoom on the self-organized structures observed inside the ring. The arrows indicate the polarization of the electric field.

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Figure 4 shows an AFM image of a square (8 × 8 μm²) area within the inner shell, and a typical height profile, as measured across the main direction of the ripples. The latter clearly demonstrates that the ripples have a topological origin with height differences between neighbouring maxima and minima of 100 to 300 nm, with a spacing of around Λ ≈720 nm, a value near the wavelength of the laser (i.e. λ = 806 nm).

 

Fig. 4 AFM image of typical nanogratings formed on the surface of Ge-S based glass. The surface was exposed to N = 50 successive pulses at a peak pulse fluence of 0.42 J/cm².

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Once the ripple structures are formed (typically after 10 pulses), their amplitude remains approximately constant irrespective of the number of subsequent laser pulses until the surface starts softening. It is interesting to note that the same laser-induced periodic surface structure with a period close to the laser wavelength and orientation parallel to the laser beam polarization was recently observed on silica glass by Höhm et al. [13]. The phenomenon has been attributed to an energy transfer mechanism resulting from the interaction of the incident laser beam with the electromagnetic wave scattered at the surface, as originally proposed by Sipe et al. [11]. So, Höhm et al. combined the LIPSS theory of Sipe and associates (also referred to as efficiency factor theory) with the Drude model to support their observation. In our case, although no variation of the ripple period has been observed as a function of the number of pulses and the fluence, we have observed similar surface morphology after irradiation in our chalcogenide glass system. This would suggest that such a behavior is somehow generic of a broad range of glass materials possessing different band gaps. Interestingly though, Fig. 5 shows the center of the ablated spot where micro-domains appear resulting from an exposure of N = 50 successive pulses at a peak pulse fluence of 0.42 J/cm². It can be noted that the structures are still regular but they are assembled in a more complex fashion meaning that even though long-range order is absent, short-range order is somehow kept. Also note that the resulting 2D quasi-periodic pattern is no longer related to the orientation of the electric field and cannot be as readily interpreted on the basis of the Höhm et al. model.

 

Fig. 5 SEM image of a typical micro-domain region exposed to N = 50 successive pulses a at peak pulse fluence of 0.42 J/cm².

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4. Conclusion

Self-organized structures oriented parallel to the electric-field vector of the linearly polarized femtosecond laser radiation were observed on the surface of the Ge-S based glass.

The resulting ablative formation of craters and laser-induced periodic surface structures was investigated in detail by means of AFM and SEM. Depending on the number of laser pulses applied to the same spot, a characteristic evolution from random to a series of generally aligned peaks-and-valleys periodic structures was observed.

The ripple period was found to be constant over most of the exposed region (long-range order), even when the structures were forming micro-domains (short-range order). The period of the resulting ripples was found to be close to the wavelength of the femtosecond laser (Λ ≈720 nm, λ = 806 nm). It was shown that by varying the laser conditions, the ripples appear when the number of pulses is rather small (N ≤ 50 in Ge-S based composition) and that increasing the number of pulses improves the uniformity of the ripples, up to a certain limit (~50 successive pulses) after which signs of organization disappear.