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The photosensitivity of GeSx binary glasses in response to irradiation to femtosecond pulses at 800 nm is investigated. Samples with three different molecular compositions were irradiated under different exposure conditions. The material response to laser exposure was characterized by both refractometry and micro-Raman spectroscopy. It is shown that the relative content of sulfur in the glass matrix influences the photo-induced refractive index modification. At low sulfur content, both positive and negative index changes can be obtained while at high sulfur content, only a positive index change can be reached. These changes were correlated with variations in the Raman response of exposed glass which were interpreted in terms of structural modifications of the glass network. Under optimized exposure conditions, waveguides with positive index changes of up to 7.8x10−3 and a controllable diameter from 14 to 25 μm can be obtained. Direct inscription of low insertion losses (IL = 3.1 – 3.9 dB) waveguides is demonstrated in a sample characterized by a S/Ge ratio of 4. The current results open a pathway towards the use of Ge-S binary glasses for the fabrication of integrated mid-infrared photonic components.
© 2014 Optical Society of America
1. Introduction
Over the last two decades, the femtosecond laser has emerged as a powerful tool for micro-processing of optical materials [1]. Intensity in the order of 10 TW/cm2 can be readily reached by femtosecond pulses focused in transparent material, resulting in a laser-matter interaction dominated by a combination of nonlinear absorption processes. This allows to selectively deposit energy in a confined volume around the focus [2,3]. Thus, three-dimensional embedded devices can be written in a one-step fabrication process simply by moving a transparent sample through the focus of a fs-laser beam. Recently, advanced photonic devices such as a μ-fluidic lab-on-chip [4], interconnection devices for astrophotonics [5,6], and multi-photon quantum interferometers [7,8] were successfully fabricated in bulk glass using femtosecond laser direct inscription. Even though major improvements were made, the optical characteristics of tridimensional devices inscribed with a femtosecond laser pales in comparison to those of planar photonic lightwave circuits (PLCs), the current benchmark technology in integrated optics. More specifically, insertion loss and minimum bend radius of individual waveguides are limited by the morphology and magnitude of the photo-induced refractive index change. Although the index modification strongly depends on the chemical composition of the material, the formation of low-loss waveguides with high index contrast (~10−2) has been achieved to date almost exclusively in silicate-based glasses [5,9]. There is thus a strong incentive for developing other optical materials with enhanced photosensitivity to femtosecond laser pulses.
Lately, there has been a growing interest in the fabrication of photonic devices operating in the mid-infrared for applications in biology and medicine [10], environmental monitoring [11], thermal imaging and optical sensing [12]. In this wavelength range, Si-based planar devices exhibit significant transmission losses. As a matter of fact only a few materials have proven to be suitable hosts for the direct inscription of low-loss single mode tridimensional waveguides [13–15]. Among those, chalcogenide glasses (ChG) are prime candidates for the integration of optical devices owing to their wide transparency window in the infrared and to their strong photosensitivity [16]. Up to now, most of the studies on femtosecond laser irradiation of ChG [17–19] put the emphasis on Ga-La-S and As-S based glasses although the mechanisms involved in the inscription process are not yet fully understood. Given the large variety of ChG glasses, predicting the outcome of the exposure to fs pulses of a given type of ChG is elusive. In order to reach a better understanding of the mechanisms that give rise to the permanent modification of the refractive index, further study of the photosensitivity of other ChG to fs-pulses is needed.
Recently, we presented a detailed study of the photosensitivity of GeS2.2 chalcogenide glass to IR femtosecond laser pulses. We showed that the photo-induced index change could be positive or negative depending on laser fluence and that a refractive index contrast of −7.5x10−3 could be reached [20]. The aim of the present work was two-fold. We sought to evaluate the influence of the glass composition on the GeSx material’s photosensitivity and form embedded waveguides designed for single mode operation in the IR domain. Glass samples with different Ge–S ratio were irradiated with 800 nm femtosecond pulses under a wide range of exposure conditions. After irradiation, the refractive index change (Δn) of the photo-induced tracks was measured and correlated modifications to the glass network were investigated by means of a micro-Raman analysis. The results allowed for a better insight of the modification mechanisms arising from the exposure to intense near-infrared femtosecond laser pulses. Careful adjustment of the exposure conditions enabled the direct inscription of an optimized waveguide designed for potential single-mode operation in the mid-infrared with low propagation losses (evaluated at ≤1.1 dB/cm at 633 nm). Thus, we present the first glassy material in which large waveguides with strong index contrast could be directly inscribed using a single passage of the femtosecond laser beam. This allowed for the fast processing of low-loss 3D waveguides operating in the IR, giving Ge-S glass a significant edge over other potential hosts investigated to date.
2. Experiment
2.1 Glass preparation and characterization
Binary Ge-S glasses were chosen among the family of chalcogenide glasses due to their low toxicity and extremely low absorption in comparison with glasses containing As [16]. Homogeneous Ge-Sx (x = 2.2, 3, 4) glasses were obtained by the 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. Subsequently, the samples obtained were annealed at glass transition temperature during their cooling process to room temperature in order to eliminate any residual internal stress. The resulting glasses were cut and polished on both sides. The transmission spectra of the three samples were 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 visible spectra were recorded on a Cary 500 double beam spectrophotometer (Varian). The glasses presents good optical quality and similar transmission spectra which extend up to λ = 12 μm. The spectra of GeS2.2 and GeS4 in the IR range and the spectrum of GeS4 in the visible range are presented in Fig. 1.
Fig. 1 a) IR transmission spectra of the GeS2.2 and GeS4 samples. b) Visible transmission spectrum of the GeS4 sample (thickness of both samples is 7 mm).
The glass transition temperature Tg and crystallization temperature Tx, density ρ, linear refractive index n (measured at λ = 633nm) and bandgap energy Eg of the prepared glass samples used in the experiment are presented in Table 1.
Table 1. Selected physical properties of GeSx binary glass samples.
With the addition of sulfur, the glass transition temperature decreased. It is accepted that the glass transition temperature Tg represents the strength or rigidity of the glass structure and we can envisage that sulfur contributes to decrease the average bond energy. This also has the effect of increasing the thermal stability (Tx-Tg) and slightly reducing the density of the glasses. Furthermore, it can be seen that the optical band gap Eg increases monotonously with an increase of S content. The blue shift of the absorption edge and change in thermal properties all indicate that the nature of the bonding in the glass is modified, probably becoming less covalent [21].
Raman spectra were recorded using back-scattering geometry in the frequency range between 100 and 600 cm−1. The spectra were collected with a LABRAM 800HR Raman spectrometer (Horiba Jobin Yvon) coupled to an Olympus BX30 microscope. The excitation light source was the 632 nm line of a He-Ne laser (Melles Griot). The laser beam was focused with a 100X long working distance objective, generating a sub-micron spot size containing a total power at the sample of approximately 5-10mW. The frequency uncertainty was estimated at ± 2 cm−1. Figure 2 shows the Raman spectra for the different concentration of GeSx glasses prior to fs-laser irradiation.
Fig. 2 Raman spectra of a) GeS2.2, b) GeS3 and c) GeS4. The structures of main bands are identified. Blue spheres denote Ge atoms and yellow ones S atoms.
The spectra reveal systematic changes in the germanium-sulfide structural network versus the sulfur content. The obtained results are similar to the previous report done by Takebe [22]. The bands at 340, 370 and 430 cm−1 have been attributed to, respectively, A1 symmetric breathing of corner-sharing [(GeS4)] tetrahedral units, Ac1 companion vibrations in edge-sharing [(GeS4)] tetrahedral units [23], and an S-S stretching coming from cluster edge dimers [24]. The peak at around 475 cm−1, more pronounced in the Raman spectra of the S-rich glass, may be attributed to the S8 (A1) ring vibration mode of sulfur. The shoulder at 485 cm−1 has been related to vibration of the S (A1) chain [25]. In the lower frequency region (100-250 cm−1) there are bands at 150 cm−1 and 220 cm−1 assigned to the A1 mode and bending E2 mode which are related to S8 rings [26].
2.2 Glass exposure to femtosecond pulses
Planar Ge-S samples (3 x 7 x 10 mm3) were irradiated with femtosecond pulses generated by a chirped-pulse-amplification (CPA) Ti:sapphire laser system (Coherent RegA). The system operates at a central wavelength of 792 nm and a repetition rate of 100 kHz. The temporal FWHM of the pulses 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 speeds vx using a motorized mechanical stage (Newport XML210). A cylindrical lens telescope with a demagnification factor of M = 1/8 was used to produce an astigmatic beam and shape the focal volume in such way as to obtain waveguides with circular cross sections [27]. The size of the resulting elliptical beam at focus was evaluated at 6.2 μm (2wy) by 1 μm (2wx) by 4.8μm (2zRx).
After the inscription process 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 [28]. 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 [29].
3. Results and discussion
3.1 Photosensitivity of GeSx glasses to 800 nm pulses
The refractive index change (Δn) induced in the three Ge-S samples was first investigated. The samples were translated transversally with respect to the laser beam at velocities of 0.05, 0.5, 5 and 50 mm/s while the pulse energy was varied between 12.5 nJ and 1 μJ. Afterward, the resulting tracks were examined and radial index profiles were retrieved using the QPM technique. Mainly for clarity, but also because of a different material response, the results at low (0.05 mm/s) and high (0.5 – 50 mm/s) translation speeds are presented separately.
The Δn measured in the center of the exposed region as function of laser fluence along with the phase contrast images of the longitudinal section and refractive index profiles of typical tracks are presented in Fig. 3. The net fluence F can be inferred from the pulse energy Ep, the repetition rate ΓR, the beam size in the y direction wy and the translation speed vx using the following:
Fig. 3 Refractive index change of tracks inscribed with a translation speed of v = 50 mm/s, 5 mm/s and 0.5 mm/s. a) Δn at the center of the exposed area as a function of net fluence for GeS2.2 and GeS4. b) Phase contrast images of the longitudinal section and the corresponding refractive index profiles (identified by rectangles A, B and C in a)) of tracks inscribed in the three samples with the same net fluence and different translation speeds.
As it was shown for GeS2.2 and other types of glasses [20,30–32] the sign and morphology of the induced index change depend mostly on net fluence. A distinct behavior is associated with each glass composition.
For GeS2.2, the Δn changes from positive to negative with increasing pulse energy independently of the translation speed. This behavior has been explained in ref. 20 and is suggested to be primarily related to a light-induced change in the electronic polarizability of the glass network due to the temperature [33]. A maximum index change of + 1.6 x 10−3 is measured for an inscription made with a net fluence of 1.23 kJ/cm2 (300 nJ, 0.5 mm/s) while a minimum of −7 x 10−3 is reached at a net fluence of 41 J/cm2 (1 μJ, 50 mm/s).
In the range of translation speeds 0.5 - 50 mm/s, irradiation of the GeS3 sample results in the formation of faint tracks with a weak Δn that can be either positive or negative and range between + 1.15 and −0.8 x 10−3.
It is shown that the resulting index change in GeS4 is always positive independently of the irradiation conditions. The index change grows with increasing pulse energy and reaches a maximum of + 4.1 x 10−3 for a moderate translation speed of 5 mm/s and a pulse energy of 400 nJ (F = 164 J/cm2). For a translation speed of 0.5 mm/s, even though the net fluence increases ten-fold, the Δn is smaller (≤ 2.7 x 10−3) than for inscriptions made with a speed of 5 mm/s. The main reason is that the Δn of waveguides inscribed under high net fluence is induced on a larger surface because of the heat accumulation effect that contributes to significantly increase the size of the structures inscribed in the thermal irradiation regime [34].
Also, for a translation speed of 5 mm/s or less and high pulse energies (typically 500 nJ or more), a complex index change is induced in all three samples. The onset for the formation of this type of modification is identified in Fig. 3(a) (Irregular Δn). Such complex index modifications are characterized by a succession of closely spaced bead-like structures formed along the translation direction that transposes in a Δn modulation along the transverse and/or longitudinal direction. A similar phenomenon has been observed in other glass types for waveguides inscribed in the thermal irradiation regime and has been attributed to an interplay between heat-induced material modifications and defocusing of the writing beam [35].
Finally, it is shown that for a given net fluence, the overall trend consists of the Δn being pushed from negative to positive with increasing sulfur content into the glass composition. As depicted in Fig. 3(b) for three different net fluences, the Δn evolves from negative in GeS2.2 to positive in GeS4. Whereas a weak Δn is induced in the GeS3 sample, its value in the center of the exposed region is invariably inferior to that observed in GeS4 but superior to GeS2.2.
Next, we turn to the inscription made at a low translation speed of 0.05 mm/s. The Δn measured in the center of the exposed region as function of F is presented in Fig. 4.
Fig. 4 The Δn at the center of the exposed area for tracks inscribed with a translation speed of v = 0.05 mm/s as a function of net fluence for the three samples.
At a translation speed of 0.05 mm/s, 2000 pulses irradiate the focal volume for a translation distance corresponding to wx = 1 μm. Under these exposure conditions, the resulting Δn is positive independently of the glass composition. Also, the GeS3 and GeS4 samples exhibit a very similar response. Indeed, only a narrow range of pulse energy (< 100 nJ, 4.1 kJ/cm2) allows for the formation of tracks with smooth Δn in both samples. The use of a pulse energy of 500 nJ (F = 21 kJ/cm2) or more results in cracks and damage in both samples. For the GeS2.2 sample, the Δn is positive, decreases at high pulse energy and the processing window is extended to 1 μJ (41 kJ/cm2). No cracks or damage are observed in the GeS2.2 sample. Maximum Δn values of + 3.4 x 10−3, + 3.0 x 10−3 and 3.45 x 10−3 were measured in the GeS4, GeS3 and GeS2.2 samples respectively.
Next, the pulse energy threshold for observable modification was determined, which is defined as the smallest input pulse energy needed to yield an observable index change in the glass sample. These measurements, depicted in Fig. 5, were conducted for GeS2.2 and GeS4 glass compositions as function of the translation speed.
Fig. 5 Pulse energy threshold for observable modification as a function of the translation speed for GeS2.2 and GeS4 glass compositions.
The pulse energy threshold is decreasing with the S / Ge ratio, suggesting an increase of the photosensitivity of the glass with a greater concentration of sulfur. Indeed, the GeS4 composition is rich in sulfur and the smallest measurable pulse energy (12.5 nJ) was sufficient to induce an observable modification independently of the translation speed. Also, the energy threshold is larger in GeS2.2 and increases with translation speed as the net fluence decreases accordingly. This behaviour was expected since the photo-induced index change depends on both the pulse energy and scan speed for a given repetition rate in the multipulse regime. The pulse energy threshold, as defined above, should not be confused with the modification threshold, which is a single pulse effect and as such, should not depend on the translation speed [36].
Large channels with a size that surpasses that of the focal volume are formed at high net fluence. This is characteristic of the thermal irradiation regime where the heat accumulation effect dominates the inscription process. In order to characterize this irradiation regime, the size of the induced tracks was investigated. The diameter of the waveguides as a function of pulse energy for the three different Ge-S compositions is shown in Fig. 6.
Fig. 6 Size of the induced tracks as a function of the pulse energy, for a translation speed of a) v = 50 mm/s and b) v = 5 mm/s.
For a translation speed of 50 mm/s, there is no evidence of thermal effect in GeS2.2 and GeS3 as all tracks are smaller or equal to 10 μm, which is about 1.5 times the size of the focussed beam along the y direction. In GeS4, the thermal regime kicks in between 100 and 200 nJ (4 and 8 J/cm2), as a sharp increase of the diameter from 7 to 14 μm is observed. A maximum diameter of 17 μm is measured for a track inscribed at a pulse energy of 600 nJ (25 J/cm2). For translation speeds of 5 mm/s, the thermal regime is initiated in all three glass compositions for pulse energies inferior to 300nJ. The energy threshold is almost similar for the GeS3 and GeS4 compositions and is lower than that of the GeS2.2. For slower speeds (not shown on Fig. 6), the size of the waveguides grows with increasing net fluence and reaches a maximum value of 90 μm for an inscription made in GeS4 with a translation speed of 0.5 mm/s. We observe that glasses with greater concentrations of sulfur are more prone to thermal effects. This is expected since the glass transition temperature (Tg) of GeS decrease significantly with an increasing sulfur concentration as shown in Table 1.
3.2 Raman analysis of exposed regions
In summary, the overall increase of the photosensitivity depends on the sulfur content and consequently is linked to glass structure. After the inscription process, a micro-Raman analysis of the exposed regions of the GeS2.2 and GeS4 samples was performed. The Raman spectra of tracks inscribed in GeS2.2 and GeS4 glasses using a pulse energy of 600 nJ and different speeds are shown in Fig. 7.
Fig. 7 Raman spectra of irradiated channels in a) GeS2.2 and b) GeS4 glasses which were written using translation speeds of 0.05 (—-) and 50 mm/s (—-) and a pulse energy of 600 nJ.
For the GeS2.2 glass composition, it was demonstrated that the induced Δn remains positive and ranges between 2 and 3.5x10−3 for most of the pulse energies tested at low translation speed (0.05 mm/s). As the translation speed is increased above 0.5 mm/s, the refractive index becomes mainly negative (as depicted in Fig. 3(a)). Independently of the net fluence, a blue shift of the main bands is observed indicating that intramolecular covalent bonds are broken through the absorption of high energy photons. Particularly, the band shift from 485 to 495 cm−1 has been attributed to a diminution of polysulfide anions from S6 to S3 [37]. Also, the modest increase of the band centered at 490 cm−1 reflects an increase in the number of S chains. Moreover, at low translation speed we observed an increase in the intensity of the bands located at 370 and 430 cm−1. This indicates an increase of edge-sharing [(GeS4)] tetrahedra and S dimers which are assumed to exist in the so-called out-rigger raft structure proposed by Phillips [24]. In other words, laser irradiation affects the bonding configurations in the germanium-rich sulfide glass by breaking the corner-shared tetrahedral units. This suggests that the remaining S mostly creates [(GeS4)] edge-sharing units and S–S chains leading to an increase of the 370 and 430 cm−1 bands.
Figure 8 schematizes the structural modifications of the GeS2.2 glass network that results from the irradiation of the material with fs pulses.
Fig. 8 Structural modification of the glass network of GeS2.2 pre- and post-irradiation under high net fluence.
Bearing in mind that the covalent bond lengths of Ge-S corner-sharing tetrahedron distance (3.41 Å) is longer than those of edge-sharing tetrahedra (2.91 Å) [38] and of the S pairs that compose the dimers (2.1 Å) [39], we can suppose that refractive index increase induced under high net fluence in Ge-S glasses with a low sulfur content results from a microstructural densification of the glass network involving the formation of shorter bond lengths. Under low net fluence, it has been shown that the refractive index of the GeS2.2 sample decreases when exposed to fs pulses. The origin of the index decrease is unclear as the Raman analysis reveals a similar overall trend. A similar blue shift of the main bands is observed and appears slightly greater than those of unexposed material. Also, the increase of the 370 cm−1 band is negligible. Therefore, we suggest that the decrease of the refractive index can be related to the presence of sulfur defects that results from Ge-S bond breaking. Some sulfur remains free in the glass network as evidenced by the shift of S chains under low net fluence.
Next, we turn to the GeS4 glass composition. Despite the fact that the irradiation of the GeS4 sample always yields a positive refractive index, we observed two distinct behaviors depending essentially on net fluence (as depicted in Figs. 3 and 4). At a high translation speed of 50 mm/s, the net fluence is low and the Δn is localized and relatively weak (0.8 x 10−3). In contrast, a translation speed of 0.05 mm/s yields the formation of large tracks typical of the thermal irradiation regime. A fivefold increase in the magnitude of the Δn is observed when the translation speed decreases from 50 mm/s to 0.05 mm/s.
For inscriptions made with a low net fluence (see Fig. 7(b)), the intensity of the band at 150 cm−1 as well as the bands at 220, 430 and 475 cm−1 decreases dramatically compared to the non–irradiated glass. However, the main band at 340 cm−1 increases and shifts slightly to lower wavenumbers indicating an increase in edge-sharing [(GeS4)] tetrahedral units. Simultaneous decrease of the bands at 150, 215, 430 and 475 cm−1 indicates that the laser exposure decreases the number of corner-sharing [(GeS4)] tetrahedral units and S8 (A1) rings. These structure variations may suggest that the laser irradiation tends to form a glass network connected mainly by [(GeS4)] tetrahedra which can be related to the modest increase of the refractive index.
As the scan speed decreases, the material reaches high temperatures in the vicinity of the focal volume due to the extended exposure time and the low thermal diffusivity (0.23 mm2/s) of the Ge-S glass [40]. As expected, some Ge–S covalent bonds break under those irradiation conditions, which results in a decrease of the band at 340 cm−1 and 220 cm−1 as shown in the Raman spectra (depicted in Fig. 7(b)). When such bonds break, two neighboring S atoms become free and they can form new S–S bonds connected through the edge-sharing tetrahedral units. This assumption is confirmed by the increase of the band at 430 cm−1 which is assigned to vibrational modes of edge-sharing bi-tetrahedra and S-S stretching coming from cluster edge dimers [24].
In summary, we observed the dependence of the photo-induced Δn on the laser fluence in both glass compositions. At low net fluence, there was no evidence of heat accumulation and the light pulse absorbed by the glass sample is responsible for breaking local chemical bonds. In GeS4, the high amount of sulfur allows the formation of new bonds resulting in an increase of the edge-sharing tetrahedra and a densification of the glass network. In GeS2.2, the newly freed S are not sufficiently numerous to recombine and remain present as defects in the glass network. At high net fluence, localized cumulative heating is believed to increase the population of edge-sharing [(GeS4)] tetrahedra and favours the formation of S–S dimers in both samples.
3.3. Optimized waveguides inscribed in GeS4
Irradiation of the GeS4 sample showed that good quality waveguides could be inscribed with a net fluence ranging between 80 and 164 J/cm2 and a translation speed of 5 mm/s. Waveguides and anti-waveguides presented in section 3.1 exhibit an elliptic cross section that varies between a/b = 1.03 and 6.7 depending on the irradiation conditions and glass composition. In order to minimize the ellipticity of the resulting waveguides in this range of irradiation conditions, waveguides are inscribed slightly closer to the input surface (z = 270 μm) and the astigmatic difference parameter [27] has been adjusted by translating one of the two cylindrical lenses that make up the demagnification telescope (M = 1/8). To determine the optimal configuration of the telescope, waveguides were inscribed at different positions of the translated lens. The end faces of the sample were then polished and observed to assess the cross sections of the waveguides.
Four waveguides were inscribed using the optimal telescope configuration and net fluence range (i.e. translation speed of 5 mm/s and pulse energy ranging between 200 and 500 nJ). Figure 9 shows the Δn at the center and the diameter of the optimized waveguides as a function of the input pulse energy.
Fig. 9 Δn measured at the center of the exposed region and the diameter of optimized waveguides plotted versus pulse energy. Translation speed v = 5 mm/s.
A significant Δn increase is observed compared to waveguides inscribed with a misadjusted demagnification telescope and same exposure conditions. This is caused by more efficient energy deposition on a quasi-circular cross section. The ellipticity (a/b) of the waveguides is reduced and varies between 1.05 and 1.65. The index change grows from + 6.5x10−3 for a moderate translation speed of 5 mm/s and a pulse energy of 200 nJ to a maximum of + 7.8x10−3 for a pulse energy of 400 nJ (F = 164 J/cm2).
Assuming a step-index profile and circular symmetry, the cut-off wavelength of the waveguides for single mode guiding varies between λ = 3.75 and 5.8 μm. For this reason, it was not possible to evaluate propagation loss of the waveguides in the single mode regime. After inscription, the end faces of the sample were repolished and the insertion losses were evaluated using a 633 nm He-Ne laser beam coupled into the waveguide with a 5X (f = 25 mm) microscope objective. The power transmitted out of the waveguide was launched in a multimode optical fiber (Thorlabs GIF625) and measured with a power meter (Newport 818). Propagation losses of the waveguides were estimated by comparing the transmitted output power with the input power. The near-field intensity profile of the waveguides was measured by imaging the end surface onto a CCD device (U-Eye SE) using a 30X (f = 6.2 mm) objective. All optimized waveguides showed insertion losses ranging between 3.1 and 4 dB. The best result was obtained for a waveguide inscribed with a pulse energy of 200 nJ and a translation speed of 5 mm/s corresponding to a net fluence of 82 J/cm2. The phase contrast image of both longitudinal and cross sections, the near field intensity profile of the 633 nm guided light and the refractive index profile of the waveguide are shown in Fig. 10.
Fig. 10 a) Microscope image of the longitudinal section, b) image of the cross section, c) Δn profile and d) near-field mode profile of a waveguide inscribed with a translation speed of 5 mm/s and E = 200 nJ. IL: insertion loss, PL: propagation loss.
The waveguide has a refractive index change of 6.5 x 10−3 and a diameter of 17.7 μm. A total insertion loss of 3.1 ± 0.2 dB was measured for the 6.7 mm long waveguide. By taking into account the 42% power loss (as shown in Fig. 1(b)) originating from the Fresnel reflections (12.5% at both facets) and attenuation of bulk GeS4 glass (17% at λ = 633 nm), the upper bound for the propagation losses of the waveguide is estimated at 1.1 ± 0.2 dB/cm. This underlines the possibility of low-loss waveguiding in bulk sulfur-rich Ge-S glass based on the optimization of the thermal inscription process through laser writing parameters. Moreover, since the size and the Δn of waveguides and anti-waveguides evidenced in this work are controllable, we believe that it is possible to inscribe single-mode low-loss waveguides operating at various wavelengths in the mid-infrared outside of the transparency window of standard optical materials.
4. Conclusion
We have presented an extensive study of femtosecond laser-induced refractive index modifications in GeSx glasses. Irradiation of glasses with a different Ge/S ratio results in dissimilar refractive index changes (Δn) under the same exposure conditions. We showed that the Δn is always positive in GeS4 while it can be negative or positive in GeS3 and GeS2.2 and that the dominant trend is an increase of the photosensitivity of the Ge-S glass with the sulfur content. The GeS4 composition is highly photosensitive and sensitive to thermal effects, allowing for the direct inscription of waveguides with controllable Δn and size. Except for tracks inscribed at a translation speed of 0.05 mm/s, irradiation of GeS3 generally results in a weak Δn making it a less appealing composition. The GeS2.2 composition is less photosensitive than GeS4 but is nevertheless a versatile material in which both waveguides and anti-waveguides with strong Δn and a controllable diameter can be formed. Changes in the Raman spectra suggest that femtosecond pulse irradiation had an effect on the network structure of Ge-S glasses, inducing apparent modifications in the molecular arrangement. It is clear from Raman measurements that the increase in the refractive index is associated with an increase in the population of edge-sharing tetrahedra in both GeS2.2 and GeS4, whereas the index decrease observed primarily in GeS2.2 is believed to be caused by the presence of S defects in the glass network. Furthermore, it is shown that the shape and magnitude of the index profile strongly varies with irradiation conditions. Careful optimization of writing parameters allowed for the inscription of low-loss waveguides in the GeS4 sample under high net fluence. These results demonstrate the feasibility of the integration of Ge-S glasses in 3D photonic circuits operating in the mid-IR. More specifically, GeS glass is a prime material candidate for spectral imaging in the 2-9 μm window.
Acknowledgments
This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), Canada Excellence Research Chair in Photonic Innovations, Ministère du Développement économique, de l'Innovation et de l'Exportation (MDEIE), Fonds de recherche du Québec - Nature et technologies (FQRNT).
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