1. Introduction

Following the discovery by Davis et al. [1] on refractive index change in glass materials, the production of optical components such as couplers [2], waveguides [3] and Fresnel zone plates [4] using femtosecond laser (fs-laser) has received a great interest. The control of pulse energy and more generally of laser beam parameters, enables the inscription of tailorable photonic structures in glass materials via local modifications of the refractive index over a range extending typically from 10⁻⁴ to 10⁻² [5]. Permanent optical waveguides operating in the mid-infrared can be successfully produced in chalcogenide glasses (ChGs) via the inscription of tracks of positive refractive index changes produced by simply moving the glass sample across the focus of a fs-laser beam [6]. Direct waveguide writing has already been demonstrated in different As-S based glasses [7]. Spectral broadening in fs-laser written waveguides was also reported in Ga-La-S glass [8]. In the case of Ge₂₅Sb₁₀S₆₅ and As₂S₃, Romanova et al. [9] proposed that permanent change of refractive index would be associated with fs pulse induced heating of the glass sample up to the glass transition temperature Tg, followed by rapid cooling and re-solidification of the illuminated region. Now, the vast majority of reports on fs-laser interaction with ChGs have been focused on As-based ChGs with emphasis on the fabrication of waveguides, nanogratings and nano-holes through fs-laser irradiation [10, 11]. To the best of our knowledge, no investigation has been reported on Ge-S binary glasses photosensitivity arising from the exposure to intense near-infrared femtosecond laser pulses.

In this paper, the photo-induced modification of the refractive index (Δn), of GeS_2.2 glass exposed to 800 nm femtosecond pulses as a function of exposure conditions (incident pulse energy and translation speed) is reported. The Δn measurements were performed with the quantitative phase microscopy technique allowing to infer the spatial distribution of the refractive index.

2. Experiment

Binary Ge-S glasses were chosen among the family of chalcogenide glasses due to their low toxicity and extremely low absorption in comparison with the As-containing glasses [12]. Homogeneous GeS_2.2 glasses were obtained by conventional melt-quenching technique using high-purity Ge, and S (all of 5 N) as raw materials. The mixture was melted in silica ampoules at 900 °C for 10 h in a rocking furnace and then quenched in air. An annealing treatment at 400°C then followed for several hours in order to eliminate any residual internal stress. Glass plates were prepared from the bulk glasses and were polished on both sides.

Planar GeS samples were irradiated with femtosecond pulses generated by a chirped-pulse-amplification (CPA) Ti:sapphire laser system (Coherent RegA) operating at a repetition rate of 100 kHz. Pulses have a central wavelength of 792 nm and their temporal FWHM was measured to be ~60 fs at the laser output and estimated at ~70 fs on the sample. The beam was focused in the bulk at a depth of 300 μm by a 40X (f = 4.5 mm, 0.55 NA) aspheric microscope objective. Samples were translated, across the focal point, perpendicular to the laser beam at different translation speed using a motorized mechanical stage (Newport XML210). A cylindrical lens telescope was used to produce an astigmatic beam and shape the focal volume in such way as to obtain waveguides with circular cross sections [13]. The size of the resulting elliptical beam at focus was evaluated to 8μm (2w_y) by 1μm (2w_x). For further analysis, the incident laser fluence F can be simply inferred from the pulse energy E_p, the repetition rate τ_rep and the translation speed v using the equation:

After the inscription process, the end faces of the sample were polished and the photo-inscribed structures were examined under a phase contrast optical microscope (Olympus IX71). The quantitative phase microscopy (QPM) method was used to obtain the radial refractive index profiles of the waveguides [14]. The QPM commercial software (Iatia ltd.) proceeds from slightly defocused bright field images of the waveguide to extract a corresponding phase image. The radial refractive index profile is retrieved by applying an inverse Abel transform on this phase image [15].

3. Results and discussion

The transmission spectrum of a glass sample of GeS_2.2 with 10 mm in thickness was measured at room temperature in the spectral range 3–15 μm with a spectral resolution of 2.0 cm⁻¹ using a FT-IR spectrometer (Perkin Elmer, Frontier). The glass presents good optical quality and transparency from 3 to 10 μm in the infrared region as shown in Fig. 1 .

 

Fig. 1 Transmission spectrum of a 10 mm thick sample of GeS_2.2

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The characteristic temperatures of the glass have been determined by differential scanning calorimetry (DSC 404 Pegasus-Netzsch). The glass transition temperature is 400 ± 2°C, and no crystallization peak was observed below 600 °C indicating the high thermal stability against devitrification. The linear refractive index of GeS_2.2 is n = 2.15 and its nonlinear index is n₂ = 3.9 x10⁻¹⁸ m²/W [16] which is about 200 times higher than the value of silica‐glass.

The photoinduced refractive index modification, as inferred from the QPM technique, is shown as a function of the exposure parameters in Fig. 2 . Figure 2(a) presents the radial profile of the refractive index modification induced for different input pulse energies at a constant translation speed of 0.05 mm/s while Fig. 2(b) presents the radial profile for different translation speeds at constant input pulse energy of 600 nJ. Typical microscope pictures of the corresponding refractive index change patterns are presented (bottom) for each case.

 

Fig. 2 Radial refractive index change profiles at, (a) constant translation speed of 0.05mm/s for different pulse energies and, (b) constant pulse energy of 600nJ for different translation speeds. Typical microscope pictures of the corresponding patterns are presented (bottom) for each case.

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A significant increase in the diameter of the photoinduced structures is observed for high input pulse energy and a constant translation speed of 0.05 mm/s, as shown in Fig. 2(a). The induced refractive index change remains positive and almost constant in the range 2-3.5x10⁻³ for the whole range of pulse energies tested at low translation speed (i.e. 0.05 mm/s) with a slight trend toward decreasing at high pulse energies. One can conclude from Fig. 2(a) that it is possible to control the size of a positive and potentially waveguiding structure from ~15 to ~40 μm in diameter by adjusting the exposure energy when the translation speed is low.

However, a different behavior is observed when the translation speed is increased for a constant input pulse energy of 600 nJ, as shown in Fig. 2(b). Accordingly, a change in the sign of refractive index is observed in the center of the exposed area (r = 0) when the translation speed is increased above 0.5 mm/s so that the refractive index becomes mainly negative for large translation speeds. Figure 3 summarizes the refractive index change evaluated in the center of the exposed area as a function of the input pulse energy for different translation speeds.

 

Fig. 3 Refractive index changes at the center of the exposed area as a function of pulse energy for different translation speeds.

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From Fig. 3, the two previously described photoinduced refractive index change regimes at low and high translation speeds are summarized. At low translation speed, the refractive index change is positive (within this range) with a slight trend for decreasing with pulse energy. The maximum positive refractive index change reaches ~3 x10⁻³ at pulse energy of 200-800 nJ and a translation speed of 0.05 mm/s. As the translation speed is increased for a fixed input pulse energy, the refractive index change becomes negative with a maximum value of −7.5x10⁻³ at pulse energy of 1000 nJ and a translation speeds of the order of 5 mm/s or faster. One also notes that the negative refractive index change photoinduced at 50mm/s is smaller than that produced at 5mm/s which seems to point to an inversion of the trend for the process at high translation speed. We believe this behavior can be understood on the basis of the decrease of the laser fluence at high translation speed which ultimately limits the maximum refractive index change attainable. This interpretation is consistent with Fig. 2(b) where one can observe a smaller modification volume at 50mm/s than at 5mm/s for pulse energy of 600nJ. In any case though, refractive index changes are negative for large translation speeds (v > 5 mm/s) and high energies (E > 400 nJ). Under such conditions, heat accumulation effects [17] are observed and the material undergoes a strong cast that ends with a complex redistribution of the material during cooling. When a femtosecond pulse is tightly focused in a transparent material, the energy is deposited in a small volume around the focus due to multiphoton absorption [18]. Indeed, the thermal diffusivity of common transparent dielectrics is of the order of ~10⁻³ cm²/s which is ~100 times smaller than metals. Thus, localized heat accumulation in transparent dielectric materials becomes significant when the dielectrics are subjected to the high repetition rate irradiation, whose pulse-to-pulse interval is shorter than the diffusion time of the material, which is generally of the order of a few μs. This can be explained by the size of the photowritten channels at high energy which can limit the refractive index change. For the structures written with a faster travel speed, the amplitude of refractive index change reaches a maximum of −7.5x10⁻³. The thermal effects are more limited for the photowritten channels at high speed; the index change is confined to a small volume (10-15 μm in diameter) which provides a more pronounced index change.

The diameters of the photoinduced structures were also measured as a function of both the input pulse energy and the translation speed (Fig. 4 ).

 

Fig. 4 Diameters of the refractive index structures photoinduced in the GeS_2.2 glass samples as a function of the input pulse energy for different translation speeds.

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It is observed that at large translation speeds (e.g. 50 mm/s), the diameter of the photoinduced structures is essentially fixed to 8 μm, independently of the input pulse energy. As the translation speed decreases, the photoinduced structure diameter increases almost linearly with the translation speed. To better understand this tendency, the same data were plotted as a function of the translation speed for different input pulse energies as represented in Fig. 5 . Note that the translation speed axis is in a log scale.

 

Fig. 5 Diameter of the refractive index structures photoinduced in the GeS_2.2 glass sample as a function of the translation speed for different input pulse energies.

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As shown in Fig. 5, for a travel speed at or below 5 mm/s and a relatively low energy (300 nJ), the size of the photowritten structures is larger than the beam diameter at the focal spot. This is characteristic of the change of the material through the effect of heat accumulation. In this case, there is melting of the material due to the nonlinear absorption of the laser beam. The material is cooled thereafter and then a densification of the material occurs. Channels with 30 μm in diameter are obtained with an energy as low as 500 nJ, suggesting that the material has a low thermal diffusivity (0.23 mm²/s) [19]. For a travel speed of 50 mm/s, the diameters of the channels are comparable or smaller than the estimated size of the focal spot and do not depend on the pulse energy. The formation of these channels can be attributed to structural changes through nonlinear excitation mechanisms [20].

To confirm our ability to control the cross section of the photoinduced structures using the cylindrical telescope as described in section 2, the glass sample was diced transversally to the channels and the cross sections were imaged using a phase microscope. The cross sections of the structures obtained are presented in Fig. 6 for an input energy of 500 nJ and translation speeds of 0.05, 0.5 and 5 mm/s.

 

Fig. 6 Cross sections of the photoinduced structures obtained at pulse energy of 500 nJ and translation speeds of (a) 0.05, (b) 0.5 and (c) 5 mm/s.

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It is noted that an almost circular photoinduced structure can be obtained at intermediate pulse energy (500 nJ) and translation speed (0.5 mm/s). A more elliptical structure is produced at low translation speed in agreement with our expectations since the telescope was rather optimised for faster speed conditions.

The results presented above described two distinct behaviors, essentially depending on the laser fluence. At low laser fluence (i.e. low pulse energy and/or high translation speed), the photoinduced refractive index change is mostly negative and spatially localized. In contrast, at high laser fluence, the photoinduced refractive index change is mostly positive and delocalized. It should be noted that a similar flip in the sign of the refractive index change was observed in other chalcogenide glass compositions and was also observed to be dependent on the photon energy [21–23].

In the first case, we can assume that at low pulse energy there was no evidence of heat accumulation (i.e. non-thermal process) and the light pulse absorbed by the glass sample is responsible only for change the local chemical bonds leading to a material densification. In the second case, as the laser fluence increases, localized cumulative heating is believed to somehow enhance photo-thermochemical reaction in the material.

In fact, similar densification through femtosecond laser irradiation was observed in fused silica (SiO₂) [24]. Indeed, the densification of silica after exposure to fs pulses is well known and has been linked to structural modifications in the material due the formation of smaller SiO₄-ring structures that lead to a gradual densification effect [25].

Nevertheless, the difference between oxygen and chalcogen elements leads to differences in crystal chemistry of oxides and chalcogenides glasses for nearly all element combinations so that we cannot assume that similar processes are occurring in both cases. For instance, the difference in electric polarizability (i.e. 3.88 × 10⁻²⁴ cm³ for O²⁻ as compared to 10.2 × 10⁻²⁴ cm³ for S²⁻ [26]) is of great structural importance.

Chalcogenide glasses have higher polarizabilty properties due to free pair-electron and have the ability to change the electron cloud by an electric field. A change in the temperature induces variation in the density of the material through transfer of energy from the photo-generated electron plasma transfers to the lattice. This will expand the electron cloud uniformly in all direction. As the number of atoms in the glass remains constant, the density decrease as the volume increase and we can estimate the polarizability (α) using the Lorentz-Lorenz equation:

Contrary to Ge-S glasses, a metastable light-induced expansion associated with negative photorefraction (Δn = 0.10) has been reported in GeSbS films [28]. In that case, the observed changes in the physical and optical properties were interpreted on the basis of a decrease of the glass network connectivity which occurs during light induced processes in the glass film. The main difference between the two cases can thus be attributed to a structural difference between GeS_2.2 (mainly formed by Ge-S tetrehdra) and GeSbS (Ge-S tetraehdara and SbS₃ pyramids connecting with sulfurs) [29]. The same negative refractive index change (Δn = −1.5x10⁻³) was also observed during writing channels in the bulk of As₂S₃ glass by femtosecond laser at high repetition rate [30]. The authors observed that the morphology of the channels is complex and mainly dominated by a central negative depth even for a moderated energy. They assumed that under this high repetition rate regime (76 MHz), the increase of the ultrafast temperature induces a pressure wave initiated in the focus and expanding to the outside leading to a decrease of the density in the core, i.e, a negative Δn. The reason for this different value can be explained in terms of glass structure. Germanium and arsenic sulfide glasses have different sulfur sites resulting in a different character of s-p hybridization. While the structure of As₂S₃ glass is dominated by As-S-As-S rings with bridging and nonbridging sulfur atoms [31], the structure of stoichiometric germanium sulfide glasses is dominated by GeS tetrahedra [32] and the structure of Ge-enriched germanium sulfide glasses is dominated by Ge-S-Ge-S chains [33]. The degree of s-p hybridization, which is determined by a specific structure of the glass network (namely by a structure of sulfur sites) is to determine the magnitude and the sign of the volume change and photorefraction [34].

4. Conclusion

We have characterized the refractive index changes induced in GeS binary glasses by exposure to femtosecond IR laser pulses. Spatially resolved measurements reveal two distinctive behaviour: high laser fluence which leads to positive refractive index changes with values up to Δn = 3.5 × 10⁻³ whereas low fluence leads to a maximum negative index change of −7.5x10⁻³. Input pulse energy was seen to greatly influence to control the waveguide diameter (from ~8 to ~50 μm). We proposed that the changes in the refractive index can be understood in terms of light-induced changes of the electronic polarizability through temperature increase due to local absorption of the laser pulse energy. These results clearly illustrate the potential of Ge-S glasses for the development of MidIR waveguides and related integrated MidIR photonic circuits.