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Abstract
Ultrafast-laser-inscribed type-II waveguides inside the chalcogenide glass IG2 and the morphology of the inscribed tracks are presented for the first time to our knowledge. Self-organized periodic structures induced by the ultrafast laser in the tracks are characterized. The near-infrared waveguiding properties are studied for various pulse energies, durations, repetition rates, and polarization states, as well as different separations between the two written tracks. A propagation loss of ∼ 2.3 dB/cm is demonstrated in a type-II waveguide formed in IG2.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Photonic circuits are attractive because of their versatility in functionalization, integration, miniaturization, scalability, robustness, and alignment-free operation. Because of absorption lines in molecular spectra and existence of atmospheric transmission windows, the development of sources and photonics in the mid-infrared region is particularly important, with applications in medicine and biology [1,2], industry [3], remote sensing [4], environmental monitoring [5], and astronomy [6,7]. Near- and mid-infrared photonics also allow for spectroscopy of chemical and organic molecules, and especially for medical imaging, in particular within the biological window for tissue penetration at wavelengths between 650 and 1350 nm [8].
The chalcogenide glasses are attractive for applications in the near- and mid-infrared regions because of their transparency arising from phonon suppression by heavy elements, tunability of their optical and thermal properties via changes in composition, high photosensitivity, and high ultrafast Kerr nonlinearity, making them desirable for all-optical signal processing [9–11]. Another domain of application is third harmonic generation from 1064 nm to the ultraviolet [12]. One of the challenges for these applications is that chalcogenide glasses are typically fragile and brittle because of the weak covalent bonding between elements, which then leads to a relatively small bandgap (∼1 - 4 eV) and low glass transition temperature (100–600 °C) compared to their oxide counterparts. Therefore, common manufacturing approaches such as lithography, chemical etching, and fiber drawing are of limited use for these materials. Additionally, these approaches cannot simply be applied to fabricate three-dimensional structures. Ultrafast-laser inscription (ULI) is a flexible and versatile tool for producing a large range of photonic structures [13,14]. It is implemented by tightly focusing an ultrashort optical pulse, typically with sub-ps duration, to reach sufficient intensities for local modification of the material’s properties via multiphoton absorption.
IG2 (Ge33As12Se55) is a commercially available infrared glass transmitting from ∼ 1–12 µm [15], a range that covers the two atmospheric transmission windows (3–5 and 8–12 µm), making it suitable for remote sensing and astrophotonics. The only reported studies of ULI in Se-based glasses focusing on IG2, Ge22As20Se58 or As2Se3 correspond to volumetric grating and type-I waveguide inscription, with propagation losses of ∼ 1 - 1.5 dB/cm at 7.8 µm or 2.94 µm, and maximal refractive index change of the order of 10−2 [16–20]. Type-II waveguides rely on the stress-induced increase in refractive index between two inscribed tracks. There is no investigation of type-II waveguides inside IG2, despite potential practical advantages compared to type-I waveguides, which rely on a positive refractive-index change induced by one or several passes of the focused ultrafast laser beam. Type-I waveguides might require multiple scans or astigmatic shaping of the focal spot for single-scan fabrication [21]. Type-II waveguides can usually be effectively written with a broader range of ULI parameters. They are generally more thermally stable [22]. Additionally, the central propagation region is not directly altered by the ultrafast laser pulse in type-II waveguides, thus preserving the intrinsic material’s properties. There is no extensive investigation of morphological evolution of the laser inscribed tracks inside chalcogenide glasses in general, which is critical in revealing the underneath formation mechanism and providing the means to waveguide performance optimization.
In this article, we demonstrate the fabrication of type-II waveguides in IG2 with a large range of writing parameters (energy, duration, repetition rate, polarization state, and track separation). In Section 2, we describe the experimental setups used for waveguide writing and characterization. In Section 3, we present the morphological characterization of the ultrafast-laser-inscribed tracks for type-II waveguides inside IG2, as a function of the writing parameters, revealing the creation of periodic structures. In Section 4, the near-infrared waveguiding properties are presented, demonstrating consistent single-mode operation with relatively low propagation loss.
2. Experimental setups
2.1 Waveguide writing
The waveguide-writing setup is based on a femtosecond laser (Monaco, Coherent) operating at 1030 nm, with pulse duration τp ranging from 350 fs to 5 ps, and repetition rate fr adjustable between 10 and 500 kHz. A 0.55-NA 50× Plan Apo objective (Mitutoyo) focuses the beam to a ∼ 1.3-µm spot size typically located approximately 200 µm below the substrate’s surface. Pulse energies Ep between 10 and 100 nJ are sufficient to inscribe pairs of tracks suitable for waveguiding. Using circularly polarized ultrashort pulses can reduce the Kerr-effect [23,24] and has been shown to produce type-I waveguides with lower loss [25]. For most experiments, a quarter-wave plate at 1030 nm is used to induce a circular polarization state, unless stated otherwise. The sample is mounted on a computer-controlled air-bearing stage allowing translation in three spatial dimensions. For all the data presented in this article, the sample translation speed relative to the static focused laser beam is 10 mm/s.
The IG2 glass substrates [10 × 5 × 2.5 mm3, with edges defining an (x,y,z) coordinate system] are polished on 4 faces: top/bottom faces (10 × 5 mm2) for waveguide writing through the top surface, and input/output faces (5 × 2.5 mm2) for coupling in and out of the written waveguides. The two tracks defining the type-II waveguide were written parallel to the longest dimension, i.e., in the x direction, at a constant depth z. The waveguides were characterized without any post-inscription polishing of the input or output surface.
2.2 Waveguide characterization
The inscribed tracks are directly imaged using a Differential Interference Contrast microscope in a transmission configuration. Planes corresponding to a constant depth z or a constant longitudinal position x are imaged, allowing either to characterize a top view or a cross-section of the written tracks. The illumination source is a halogen bulb with a spectrum extending into the infrared to allow for good-quality characterization of IG2 substrates, which have high absorption in the visible wavelength range.
The near-IR laser for the waveguide characterization is a continuous wave (cw) 1064 nm Nd:YVO4 laser (Fig. 1). The aspheric CaF2 focusing lens has a focal length 20 mm, and the collection lens is a microscope objective which has a focal length of 10 mm and an NA of 0.4. An output iris of diameter 1.5 mm is located 200 mm away from the back end of the microscope objective to block the scattered light propagating towards the power meter and the beam profiler. The mode field diameter (MFD) is calculated from the measured mode.
The waveguide propagation loss is calculated by taking the ratio of output power of the guided mode and input power of the focused beam, taking into account the coupling loss (0.3 - 0.4 dB), the transmission loss due to IG2 absorption at 1064 nm over 10 mm (0.4 dB), and Fresnel reflection at the input and output faces (1.9 dB). The coupling loss is determined via calculating the overlap integral of the focused input beam (waist ∼ 15 µm) and the mode profile.
The multiple internal reflections due to the high-index contrast across the two coupling faces lead to wavelength-dependent Fabry-Perot resonances [26]. The relatively low reflectivity of the two faces and the waveguide propagation loss lead to a low-finesse Fabry-Perot interferometer with a transmission that is approximately a sinusoidal function of the internal propagation phase, which is proportional to the propagation constant, the transmission length, and the optical frequency. This dependence introduces an uncertainty on the measured transmission. This uncertainty can be calculated at a given wavelength as a function of the single-pass loss, i.e. the combination of the waveguide propagation loss and the 0.4-dB transmission loss due to absorption. The calculated uncertainties are of the order of 1, 0.4, and 0.2 dB for losses of 3, 6, and 10 dB, respectively. Because the characterization laser is not longitudinally singlemode, the actual experimental uncertainty is smaller since the laser modes experience different transmission properties. We have quantified the experimental uncertainty by measuring, for four type-II waveguides, the variation in propagation loss induced by changes of the internal optical phase accumulated by propagation between the input and output faces. This phase is scanned over a range much larger than 2π by modifying the substrate’s temperature by a few degrees. For these four waveguides, we have measured uncertainties equal to 0.6, 0.7, 0.2, and 0.05 dB for propagation losses of 2.3, 3.4, 5.7, and 5.8 dB, respectively. These experimental uncertainties are smaller than the calculated uncertainties and give an estimate of the error bars on the propagation losses reported in this article.
The step-index approximation of the refractive index change Δn of a given waveguide is estimated from the profile of the fundamental guided mode as Δn = NA2/(2n), where n is the material’s optical index and NA is the numerical aperture given by NA = 2λ/(πMFD).
3. Ultrafast-laser-inscribed track morphology
3.1 Dependence on pulse energy and duration
Figure 2 presents microscope images of the tracks written inside the IG2 substrate with pulse energy from 9.3 to 74 nJ and pulse duration from 350 fs to 5 ps. For these tracks, the polarization of the ultrafast laser is circular, and the repetition rate is equal to 500 kHz. The laser is focused 200 µm below the surface (except for α1–α3, for which it is focused at 150 µm). The top view of single tracks and cross-section of the Type-II waveguide demonstrate the impact of writing energy on the morphology of the tracks. The pulse energy is critical for track formation: if the energy is too low, no tracks are observable on the microscope images. At this repetition rate, the energy threshold for track formation with a single scan is approximately 9 nJ and 20 nJ for a 350-fs and 5-ps pulse durations, respectively. At higher energies, self-organized periodic structures are consistently observed. Near the energy threshold, the tracks are composed of discrete separated circular structures for all pulse durations. At higher energies, the circular structures merge with each other. For the 350-fs and 1-ps pulse duration, this merging behavior gives rise to periodic grating-like structures with linear features approximately perpendicular to the scanning direction of the substrate (although the features appear at a small angle in some cases, e.g., α6, α7, and α8). For the 5-ps pulse duration, the merging yields an ensemble of more randomly shaped structures with an irregular size and pitch (α17 and α18). This shows a transition from a single-period behavior to a more chaotic behavior at longer pulse durations. Furthermore, a transition to a periodic grating-like structure is not observed. In all these cases, the scale of the observed features is larger than 1 µm. This is significantly larger than the distance between the focal spots of successive laser pulses, which is given by the ratio of the scanning speed (10 mm/s) and the repetition rate (500 kHz), i.e. 20 nm.
Fig. 2. Morphology of the tracks (upper row: top view, and bottom row: cross-section view of waveguides with two tracks separated by 25 µm) written in IG2 with different pulse energies Ep = 9.3 - 74 nJ, with respect to pulse duration τp = 350 fs, 1 ps and 5 ps. The laser pulses are circularly polarized, and the repetition rate is 500 kHz. The focusing depth in the material is 150 µm for α1–α3 and 200 µm for all other tracks.
The characteristics of the tracks and periodic structures are quantified in Fig. 3. As the pulse energy increases from 20 to 74 nJ, the track length and width increase approximately linearly [Figs. 3(a) and 3(b)], indicating a simple linear relationship between the size of the region where the IG2 glass is being modified and the deposited laser pulse energy, with no dependence on the pulse duration. Higher energies generally induce an upward (toward the surface) displacement of the tracks. The bottom of the track stays near the geometrical focus (within ∼ 5 µm), and the top of the track is displaced linearly upward towards the direction of the laser source (cross-section images in Fig. 2). Track formation closer to the surface at higher energy is consistent with self-focusing occurring during the propagation of the focused pulse. The estimated critical power for self-focusing is PCrit = 3.77λ2/(8π n0 n2) [27]. Using λ = 1030 nm, n0 = 2.59, and n2 = 20×10−18 m2/W, which is estimated from the nonlinear index measured on a chalcogenide glass with similar stoichiometry [28], one finds PCrit = 3 kW. This value is significantly smaller than the power of the pulses used for ULI in this work, e.g., 26 kW at Ep = 9 nJ. A movement of the focus toward the surface is therefore expected at higher energies.
Fig. 3. Feature size of the tracks written in IG2, for (a) track length, (b) track width, and (c) pitch of periodic structures, as a function of pulse energy, for pulse duration τp equal to 350 fs (solid, blue), 1 ps (dotted, red), and 5 ps (dashed, black).
The pitch of the periodic structures strongly depends on the pulse duration [Fig. 3(c)]:
- - at 350 fs, the pitch first decreases from 3.6 µm at 9.3 nJ to ∼ 1.8 µm around 20 - 30 nJ, before increasing slightly to 2.2 µm at 74 nJ. A grating-like structure is observed for energies higher than 20 nJ, with a period that only shows weak energy dependence.
- - at 1 ps, the grain-like structure persists to energies as high as 30 nJ, and the transition to the grating-like structure occurs at energies between 30 and 40 nJ. For higher energies, the track morphology and pitch are similar to those observed at 350 fs.
- - at 5 ps, the grain-like structure is observed at all tested pulse energies. The circular structures are consistently larger than at 350 fs and 1 ps, and the average pitch remains larger than 5 µm. Moreover, more random feature size and shape are observed at higher energies (50 nJ and 74 nJ), for which the average pitch is reported.
3.2 Dependence on pulse energy and repetition rate
Operating at different repetition rates allows for testing the impact of thermal accumulation and potentially better control over the physical sizes and morphology of the modification zones. At a given pulse energy and scanning rate, increasing the repetition rate increases the local absorbed energy and the resulting heat. Figure 4 shows the morphology of the tracks written in IG2 with 350-fs circularly polarized pulses at repetition rates equal to 10, 100, 200 and 500 kHz, for energies ranging from 10 to 50 nJ. Considering the focal spot size and spatial pulse separation, the deposited energy density ranges from 7.5 nJ/µm2 (10 nJ, 10 kHz) to 1,900 nJ/µm2 (50 nJ, 500 kHz).
Fig. 4. Morphology of the tracks (upper row: top view, and bottom row: cross-section view of waveguides with two tracks separated by 25 µm) written in IG2 with different pulse energies Ep = 10 - 50 nJ, pulse duration τp= 350 fs, and repetition rate fr = 10, 100, 200 and 500 kHz. The laser pulses are circularly polarized.
There are no tracks observed at a writing energy equal to 10 nJ for repetition rates up to 200 kHz, whereas this energy is approximately the track-formation threshold at 500 kHz. This indicates that heat accumulation plays a significant role in track formation and type-II waveguide inscription in IG2. At a slightly higher pulse energy (12.5 nJ), there are faint discrete segments observable at the lowest repetition rate (10 kHz). Periodic structures are clearly observed for fr ≥ 200 kHz (Ep = 12.5 nJ, 15.5 nJ and 25 nJ) and fr ≥ 100 kHz (Ep = 37 nJ and 50 nJ), indicating a dependence of this phenomenon on the overall deposited energy. Other writing conditions lead to tear-like structures with a well-defined shape but more random occurrence (β5 and β9).
At a fixed energy, the track length and width approximately increase linearly with the repetition rate [ Figs. 5(a) and 5(b)]. This indicates that the size of the region where the IG2 glass is modified increases with the amount of deposited energy density. The pitch of the periodic modulation has been plotted on Fig. 5(c), except for β4, β5, β6, b9 and β13. Operation at 10 kHz leads to isolated material’s modification with a 1-µm period for the higher writing energies ≥ 15.5 nJ, which is consistent with the distance between successive laser pulses within the sample scanned at 10 mm/s. For the same repetition rate, the material modification observed at 12.5 nJ has a period close to 4 µm, which is also observed at 200 kHz for the same energy. Other writing conditions as shown in Fig. 5(c) lead to periodic structures with period ∼ 1.5 - 2.4 µm, which increases with the repetition rate, but does not significantly depend on the pulse energy over the tested ranges.
Fig. 5. Feature size of the tracks written in IG2, for (a) track length, (b) track width, and (c) pitch of periodic structures, as a function of repetition rate, for pulse energy equal to 12.5 nJ (blue solid line), 15.5 nJ (red dotted line), 25 nJ (black dashed line), 37 nJ (green dashed dotted line), and 50 nJ (light-blue solid line).
3.3 Dependence on polarization state
The morphology of the tracks written with a linearly polarized laser (polarization perpendicular to the track lines, i.e., along the y axis) and circularly polarized laser (Sub-sections 3.1 and 3.2) are in general similar, aside from tracks written at the highest energy (Fig. 6). For these data sets, the pulse duration is 350 fs and the repetition rate is 500 kHz. At Ep = 76 nJ, two different periods are observed, with the coarser grating having twice the period of the finer grating (γ5). Such modulation is not observed on the tracks written with a circular polarization state at Ep = 73.7 nJ (α8) or on another track written at 60 nJ with a linear polarization state. The length, width, and pitch of the tracks written with a circularly and linearly polarized laser are similar [Figs. 7(a), 7(b), and 7(c)]. There are subtle differences between the pitches observed with the two polarization states, in particular the pitch is smaller for the linear polarization state at pulse energies less than 25 nJ. As previously stated, two modulations are observed at the highest energy for the linear polarization. This might be caused by local melting which erases the smaller-scale features.
Fig. 6. Morphology of the tracks (upper row: top view, and bottom row: cross-section view of waveguides with two tracks separated by 25 µm) written in IG2 with different pulse energies Ep = 20–74 (76) nJ, for waveguides α4–α8 (circular polarization) and γ1– γ5 (linear polarization along the y axis). The pulse duration is 350 fs, and the repetition rate is 500 kHz.
Fig. 7. Feature sizes of the tracks written in IG2, for (a) track length, (b) track width, and (c) pitch of periodic structures, as a function of pulse energy, for a circular (blue, solid) and linear polarization state along the y-axis (dashed dotted, green).
3.4 Physical interpretation of the observed track morphology
The morphology of the inscribed tracks depends on the pulse energy, the pulse duration, and the repetition rate in the tested parameters range, but not significantly on the polarization state, indicating that it is mostly driven by the intensity of the focused beam and the local energy accumulation due to successive pulses.
Regarding the grain-like structures that are associated with lower pulse energies (α1 - α3, α9 – α10) or longer pulse durations (α14 - α18), as seen on Fig. 2, similar phenomena have been observed in fused silica and BK7 [29–31]. These discrete features are attributed to an optical defocusing effect [29,31], including optical aberration arising from refractive index change [32] or scattering from the newly formed structures [33]. In areas where the propagation of the focused ultrafast pulse is significantly impacted by the material’s modifications induced by the previous pulses, the light intensity can decrease to a level where two-photon absorption is significantly decreased, hindering further modification. This interpretation was experimentally confirmed by performing multi-scan laser inscription near the threshold energy with 350-fs circularly polarized pulses (Fig. 8). For these results, the IG2 substrate is scanned at the default speed of 10 mm/s along the x direction. Multiple scans are performed, with a separation equal to 0.3 µm in the y direction. The discrete structures, which appear circular with diameter ∼ 2.4 - 3 µm, are consistently formed, regardless of the fact that the focused pulses were separated by 20 nm in the x direction (set by the scanning speed, 10 mm/s, and repetition rate, 500 kHz) and 0.3 µm in the y direction. As the pulse energy increases from 9.3 to 10 nJ [Figs. 8(a) and 8(b), respectively], the spheres have a larger average diameter, ∼ 2.4 and 3 µm respectively, and are more closely packed. In both cases, each scan was performed from bottom to top and successive scans were performed from left to right, referring to directions in Fig. 8. The leftmost line is free of disruption from the neighboring structures, and has a denser distribution of circular structures than subsequent lines: the average period of the observed features from the leftmost to the rightmost line ranges from 3.6 to 4.2 µm [Fig. 8(a)], and from 2.8 to 3.6 µm [Fig. 8(b)], indicating that interaction of the ultrafast pulses with the material along previous lines disrupts the interaction along subsequent lines.
Fig. 8. Morphology of the features written in IG2 (top view) by multiple successive scans in the x direction, separated by 0.3 µm in the y direction, with pulse energy equal to (a) 9.3 nJ (40 scans), and (b) 10 nJ (30 scans).
Grain-like structures have been observed in fused silica and explained by the thermal accumulation of pulses with small energy [31]. As the temperature increases, the heat-affected zone gradually grows and the absorption point shifts upwards [31,34], while bubble nucleation from the melt becomes possible which start to coalesce with each other and rise to the top of the melt. An alternative mechanism of void formation is based on densification from the outer region and rapid quenching which leads to rarefaction in the center [32,35], but the morphology of the resulting features appears significantly different. In [32], the periodic disruptions in fused silica are not very orderly, consisting of several small voids forming a large cavity of several tens of micrometers. In [35], the features formed within porous silica are less regular micro-voids, with period increasing from 10 to 40 µm as the energy density increases. We conjecture that the observed features indeed include voids and regions where the melted glass has undergone mechanical and chemical modifications, but further experimental study and modeling are required.
The grating-like structure (pitch ∼ 2 µm) associated with shorter pulse duration and higher pulse energy (α4 – α8, α11- α13 on Fig. 2) is morphologically different from the grain-like structure, and its pitch behaves differently with respect to energy [Fig. 3(c)]. This transition is observed at ∼ 20 nJ for τp = 350 fs and 30 ∼ 40 nJ for τp = 1 ps. Grating-like periodic structures have been observed in fused silica [29] and in borosilicate glass [36]. A proposed explanation is the periodic change in the depth where the optical energy is absorbed [36,37]. As the pulse-energy deposition accumulates and the temperature increases, the absorption point dominated by avalanche ionization rises gradually towards the top surface away from the geometrical focus. When it gets too far from the geometric focus, the optical intensity is too low to yield significant local multiphoton absorption. Then the beam is refocused downwards near the geometrical focus and the two-photon absorption is reignited. The cycle repeats itself, resulting in a closely packed periodic structure. The grating-like structures are not observed at τp = 5 ps. For this pulse duration, the system remains in the regime dominated by the optical defocusing, for which the grain-like structures are always present. Incidentally, the observed phase change around ∼ 40 nJ from the regular to the irregular pattern is an indication of the period-doubling bifurcation in chaotic systems, typical of the Feigenbaum phenomenon [31,38].
We have observed consistent waveguiding within the region located between the two tracks (Section 4). This indicates that the optical index within this region is larger than in the surrounding areas. No waveguiding within the tracks themselves has been observed at 1064 nm for the presented fabrication parameters. Possible physical mechanisms that explain these observations include a rarefaction in average density within the tracks because of thermal expansion or formation of bubbles, and a stress-induced increase in the optical index within the region between the two tracks because of the mechanical changes due to track formation.
4. Waveguiding properties
4.1 Dependence on pulse energy and duration
Figure 9 shows examples of the guided modes measured at 1064 nm overlapped with the images of the type-II waveguide tracks in the same imaging plane. For all these cases, the track separation is 25 µm. On the first row, the pulse energy is 40 nJ. The first three sub-figures correspond to a circular polarization state with pulse duration equal to 350 fs, 1 ps, and 5 ps, while the fourth one corresponds to 350 fs with linear polarization along the y-axis. On the second row, the pulse duration is 1 ps, the pulses are circularly polarized, and pulse energies equal to 20, 30, 40, 50, and 74 nJ are used. The figures demonstrate the good-quality profiles of the modes guided between the two tracks written by ULI. For the track separations investigated in this article, which are 30 µm or smaller, we did not observe any multimoding behavior for waveguiding at 1064 nm: essentially the same output mode profile between the two tracks has been obtained when launching the input beam at different angles and/or positions relative to the front face of the substrate. At 35 µm, higher-order modes, such as a two-lobed TEM10 mode, have been observed.
Fig. 9. Guided modes at 1064 nm overlapped with the images of type-II waveguide tracks in the same imaging plane. The first row corresponds to a pulse energy of 40 nJ, and pulse duration equal to (a) 350 fs, (b) 1 ps, and (c) 5 ps, with a circular polarization, and (d) 350 fs with a linear polarization along the y axis. The second row corresponds to pulse energy equal to (e) 20 nJ, (f) 30 nJ, (g) 40 nJ, (h) 50 nJ, and (i) 74 nJ, with a pulse duration of 1 ps and circular polarization. The track separation is 25 µm.
The propagation loss, mode field diameter, and refractive index change have been determined for the type-II waveguides written in different experimental conditions with a track separation equal to 25 µm (Fig. 10). For 350 fs with circular polarization, the propagation loss first drops from 6.1 dB/cm to a minimum of 2.7 dB/cm around ∼ 30 nJ, then increases to ∼ 5.6 dB/cm [Fig. 10(a)]. The energy range for propagation loss of the order of 3 dB/cm is relatively large. The mode field diameter does not significantly depend on the pulse duration [Fig. 10(b)], which is consistent with the observation that the track dimensions are similar at 350 fs, 1 ps, and 5 ps [Figs. 3(a) and 3(b)]. Higher writing energies generally lead to a smaller MFD, which is consistent with a stronger confinement imposed by the two tracks. The increased loss at the low energy end is due to insufficient light confinement because of a small effective Δn. The index change generally increases linearly with pulse energy [Fig. 10(c)]. The increased loss at high energy is attributed to the reduction in effective track separation (down to ∼ 15 µm) leading to an over-confinement and scattering from the spatially extended tracks. The propagation loss for longer pulse durations (1 ps, 5 ps) is slightly higher than at 350 fs. For τp = 5 ps, the increased loss for energies exceeding 40 nJ corresponds to a sub-phase transition where the tracks features become irregular (Fig. 2, α17 and α18). Such irregularity is likely to lead to increased optical scattering within the waveguide. Similarly, the refractive index change is not strongly duration-dependent, but increases approximately linearly with the energy, which again indicates the stronger induced confinement. Refractive index changes between 2 × 10−4 and 5 × 10−4 are observed.
Fig. 10. (a) Propagation loss (PL), (b) MFD, and (c) effective refractive index contrast Δn of type-II waveguides written in IG2, as a function of pulse energy, for pulse duration equal to 350 fs (solid, blue), 1 ps (dotted, red), and 5 ps (dashed, black), respectively, with circular polarization. The track separation is 25 µm.
4.2 Dependence on pulse energy and repetition rate
The laser repetition rate is critical for the type-II waveguide performance, showing the importance of the locally accumulated energy from the overlap of many successive pulses. As the repetition rate drops below 200 kHz, there is a general trend of deterioration of waveguiding properties. In particular, waveguiding was only observed at the highest tested energy (50 nJ) when operating at 10 kHz, while energies higher than 25 nJ were required when operating at 100 kHz and 200 kHz [ Fig. 11(a)]. This dependence on the repetition rate is in agreement with the time scale of heat diffusion [39,40], which can be calculated as τ = d2/D, where d is the diameter of the focal spot, and D = κ/(ρCp) is the thermal diffusion coefficient calculated using the thermal conductivity κ, thermal conductivity ρ, and specific heat capacity Cp. Using κ = 0.24 W/mK, ρ = 4.41 g/cm3, and Cp = 0.33 J/gK for IG2 [15], τ is of the order of 10 µs, i.e., the critical frequency defining the transition between athermal and thermal regime is approximately 100 kHz. Figure 11(a) shows that, in the thermal regime, similar propagation loss (below 4 dB/cm) is obtained at the repetition rates of 200 and 500 kHz, with pulse energy between 25 and 37 nJ. At 500 kHz with enough heat accumulation, a propagation loss below 4 dB/cm is obtained for a wider range of energies, except when the energy is too low, which arises from poorer confinement due to lower index contrast. The mode size does not show a particular trend at lower repetition rate for poorly confined modes, but slightly decreases with pulse energy at 500 kHz [Fig. 11(b)]. For this repetition rate, the effective refractive index change Δn generally increases with pulse energy [Fig. 11(c)].
Fig. 11. (a) Propagation loss, (b) MFD, and (c) effective refractive index contrast Δn of type-II waveguides written in IG2, as a function of pulse energy, for different repetition rates. The pulse duration is 350 fs, the polarization is circular, and the track separation is 25 µm.
4.3 Dependence on polarization
Whereas writing tracks with a circular and linear polarization state leads to similar morphologies (Subsection 3.3), the behavior of their corresponding propagation losses is quite different [ Fig. 12(a)]. As the pulse energy increases from 10 nJ, the loss for linear polarization is more sensitive to pulse energy. It sharply drops to a local minimum around 20 nJ, even smaller than that for the circular polarization, then increases rapidly at larger energies. This might be due to differences in the stress induced in IG2 for pulses with different polarizations. However, the mode field diameter [Fig. 12(b)] and effective Δn [Fig. 12(c)] are similar for the two polarization states over the tested energies. Higher energies lead to smaller MFD and larger Δn.
Fig. 12. (a) Propagation loss, (b) MFD, and (c) effective refractive index contrast Δn of type-II waveguides written in IG2, as a function of pulse energy, for a circular (blue markers) and linear (green markers) polarization state. The pulse duration is 350 fs, the repetition rate is 500 kHz, and the track separation is 25 µm.
4.4 Dependence on track separation
Type-II waveguides have been formed with different track separations, using 350-fs circularly polarized pulses with a 500-kHz repetition rate (Fig. 13). The propagation loss generally decreases at larger track separations, being in general the lowest for the 30-µm spacing [Fig. 13(a)]. The propagation loss increases by ∼ 1 dB and ∼ 3 dB for the 25-µm and 20-µm track spacing, respectively. For a given track separation, the propagation loss is not strongly energy-dependent below 50 nJ, but increases slowly after 30 nJ and significantly at 74 nJ. The minimal propagation loss is 2.3 dB/cm for a track separation of 30 µm and pulse energy of 20 nJ. The mode-field diameter generally scales like the track spacing and decreases with energy [Fig. 13(b)]. This is consistent with confinement caused by the inscribed tracks, with larger track sizes and smaller spacings leading to a smaller mode. MFDs varying from 11 to 22 µm have been observed with the parameters used in this study. Finally, Fig. 13(c) shows that Δn generally increases with pulse energy and decreases with track separation, which is consistent with the larger induced stress.
Fig. 13. (a) Propagation loss, (b) MFD, and (c) effective refractive index contrast Δn of type-II waveguides written in IG2, as a function of pulse energy, for track separations equal to 20, 25, and 30 µm. The pulse duration is 350 fs, the polarization is circular, and the repetition rate is 500 kHz.
5. Conclusions
In conclusion, type-II waveguides have been inscribed with an ultrafast laser in the commercially available chalcogenide glass IG2 for the first time to our knowledge. The evolution of morphology, features size, and the waveguiding properties of the waveguides have been studied for a range of track separation (20–30 µm), irradiation pulse energies (9.3 - 76 nJ), pulse duration (350 fs - 5 ps), and repetition rates (10 - 500 kHz) with circular and linear polarization states. Self-organized periodic structures with pitch ranging from 1.8 to 7.4 um have been identified in the chalcogenide glass. The shape and scale of these structures change drastically with the writing parameters, ranging from discrete circular grain-like structures to grating-like structures. For propagation at 1064 nm, better propagation is observed for tracks written at higher repetition rates, shorter pulse durations, and with larger spacings. In general, waveguiding is better for tracks with periodic grating-like structures, compared to periodic grain-like structures, and degrades further for randomly sized grains. A propagation loss of ∼ 2.3 dB/cm has been measured in a type-II waveguide with tracks separated by 30 µm written with 20-nJ, 350-fs circularly polarized pulses at a 500-kHz repetition rate.
The realization of type-II waveguides in a chalcogenide glass opens the way to a large range of applications. Further studies to support these applications include the realization and characterization of singlemode, tapered, and multimode waveguides in the near-infrared and mid-infrared. These waveguides can for example enable the fabrication of photonic lanterns in the mid-infrared [41].
Funding
National Aeronautics and Space Administration (80NSSC20C0027, 80NSSC21C0638).
Disclosures
The authors declare that there is no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. A. R. Adams, C. T. Elliott, A. Krier, B. N. Murdin, R. W. Waynant, I. K. Ilev, and I. Gannot, ““Mid–infrared laser applications in medicine and biology,” Philosophical Transactions of the Royal Society of London. Series A: Mathematical,” Physical and Engineering Sciences 359(1780), 457–458 (2001). [CrossRef]
2. F. Toor, S. Jackson, X. Shang, S. Arafin, and H. Yang, “Mid-infrared Lasers for Medical Applications: introduction to the feature issue,” Biomed. Opt. Express 9(12), 6255–6257 (2018). [CrossRef]
3. Lucia Fernández-Carrasco, D. Torrens-Martín, L. M. Morales, and Sagrario Martínez-Ramírez, “Infrared spectroscopy in the analysis of building and construction materials,” in Infrared Spectroscopy - Materials Science, Engineering and Technology, T. Theophanides, ed. (2012).
4. B. M. Walsh, H. R. Lee, and N. P. Barnes, “Mid infrared lasers for remote sensing applications,” J. Lumin. 169, 400–405 (2016). [CrossRef]
5. U. Willer, M. Saraji, A. Khorsandi, P. Geiser, and W. Schade, “Near- and mid-infrared laser monitoring of industrial processes, environment and security applications,” Optics and Lasers in Engineering 44(7), 699–710 (2006). [CrossRef]
6. L. Labadie and O. Wallner, “Mid-infrared guided optics: a perspective for astronomical instruments,” Opt. Express 17(3), 1947–1962 (2009). [CrossRef]
7. A. Arriola, S. Gross, M. Ams, T. Gretzinger, D. Le Coq, R. P. Wang, H. Ebendorff-Heidepriem, J. Sanghera, S. Bayya, L. B. Shaw, M. Ireland, P. Tuthill, and M. J. Withford, “Mid-infrared astrophotonics: study of ultrafast laser induced index change in compatible materials,” Opt. Mater. Express 7(3), 698–711 (2017). [CrossRef]
8. A. M. Smith, M. C. Mancini, and S. Nie, “Second window for in vivo imaging,” Nat. Nanotechnol. 4(11), 710–711 (2009). [CrossRef]
9. A. B. Seddon, “Chalcogenide glasses: a review of their preparation, properties and applications,” J. Non-Cryst. Solids 184, 44–50 (1995). [CrossRef]
10. M. D. Pelusi, V. G. Ta’eed, L. Fu, E. Magi, M. R. E. Lamont, S. Madden, D. Y. Choi, D. A. P. Bulla, B. Luther-Davies, and B. J. Eggleton, “Applications of highly-nonlinear chalcogenide glass devices tailored for high-speed all-optical signal processing,” IEEE J. Sel. Top. Quantum Electron. 14(3), 529–539 (2008). [CrossRef]
11. B. J. Eggleton, B. Luther-Davies, and K. Richardson, “Chalcogenide photonics,” Nat. Photonics 5(3), 141–148 (2011). [CrossRef]
12. J. Gao, M. A. Vincenti, J. Frantz, A. Clabeau, X. Qiao, L. Feng, M. Scalora, and N. M. Litchinitser, “Near-infrared to ultra-violet frequency conversion in chalcogenide metasurfaces,” Nat. Commun. 12(1), 5833 (2021). [CrossRef]
13. D. Choudhury, J. R. Macdonald, and A. K. Kar, “Ultrafast laser inscription: perspectives on future integrated applications,” Laser Photonics Rev. 8(6), 827–846 (2014). [CrossRef]
14. T. Dezhi, W. Zhuo, X. Beibei, and Q. Jianrong, “Photonic circuits written by femtosecond laser in glass: improved fabrication and recent progress in photonic devices,” Adv. Photonics 3(02), 1–24 (2021). [CrossRef]
15. https://www.vitron.de/files/VITRON_IG-2_Datenblatt_Jan_2015.pdf
16. David Lee, David G. MacLachlan, Helen L. Butcher, Richard A. Brownsword, Damien Weidmann, Colin R. Cunningham, H. Schnetler, and Robert R. Thomson, “Mid-infrared transmission gratings in chalcogenide glass manufactured using ultrafast laser inscription,” in Proc. SPIE 9912, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II, 99122X (2016).
17. H. L. Butcher, D. G. MacLachlan, D. Lee, R. R. Thomson, and D. Weidmann, “Demonstration and characterization of ultrafast laser-inscribed mid-infrared waveguides in chalcogenide glass IG2,” Opt. Express 26(8), 10930–10943 (2018). [CrossRef]
18. H. L. Butcher, D. G. MacLachlan, D. Lee, R. R. Thomson, and D. Weidmann, “Ultrafast laser-inscribed mid-infrared evanescent field directional couplers in GeAsSe chalcogenide glass,” OSA Continuum 1(1), 221–228 (2018). [CrossRef]
19. J. M. Morris, M. D. Mackenzie, C. R. Petersen, G. Demetriou, A. K. Kar, O. Bang, and H. T. Bookey, “Ge22As20Se58 glass ultrafast laser inscribed waveguides for mid-IR integrated optics,” Opt. Mater. Express 8(4), 1001–1011 (2018). [CrossRef]
20. W. Ma, L. Wang, P. Zhang, W. Zhang, B. Song, and S. Dai, “Femtosecond laser direct writing of diffraction grating and its refractive index change in chalcogenide As2Se3 film,” Opt. Express 27(21), 30090–30101 (2019). [CrossRef]
21. T. Gretzinger, S. Gross, M. Ams, A. Arriola, and M. J. Withford, “Ultrafast laser inscription in chalcogenide glass: thermal versus athermal fabrication,” Opt. Mater. Express 5(12), 2862–2877 (2015). [CrossRef]
22. S.-E. Wei, Y. Wang, H. Yao, M. Cavillon, B. Poumellec, G.-D. Peng, and M. Lancry, “Thermal stability of type II modifications by IR femtosecond laser in silica-based glasses,” Sensors 20(3), 762 (2020). [CrossRef]
23. A. H. Nejadmalayeri and P. R. Herman, “Ultrafast laser waveguide writing: lithium niobate and the role of circular polarization and picosecond pulse width,” Opt. Lett. 31(20), 2987–2989 (2006). [CrossRef]
24. D. N. Schimpf, T. Eidam, E. Seise, S. Hädrich, J. Limpert, and A. Tünnermann, “Circular versus linear polarization in laser-amplifiers with Kerr-nonlinearity,” Opt. Express 17(21), 18774–18781 (2009). [CrossRef]
25. M. Ams, G. D. Marshall, and M. J. Withford, “Study of the influence of femtosecond laser polarisation on direct writing of waveguides,” Opt. Express 14(26), 13158–13163 (2006). [CrossRef]
26. R. Regener and W. Sohler, “Loss in low-finesse Ti:LiNbO3 optical waveguide resonators,” Appl. Phys. B 36(3), 143–147 (1985). [CrossRef]
27. A. Brodeur and S. L. Chin, “Ultrafast white-light continuum generation and self-focusing in transparent condensed media,” J. Opt. Soc. Am. B 16(4), 637–650 (1999). [CrossRef]
28. C. Quémard, F. Smektala, V. Couderc, A. Barthélémy, and J. Lucas, “Chalcogenide glasses with high non linear optical properties for telecommunications,” J. Phys. Chem. Solids 62(8), 1435–1440 (2001). [CrossRef]
29. R. Graf, A. Fernandez, M. Dubov, H. J. Brueckner, B. N. Chichkov, and A. Apolonski, “Pearl-chain waveguides written at megahertz repetition rate,” Appl. Phys. B 87(1), 21–27 (2007). [CrossRef]
30. J. Thomas, R. Bernard, K. Alti, A. K. Dharmadhikari, J. A. Dharmadhikari, A. Bhatnagar, C. Santhosh, and D. Mathur, “Pattern formation in transparent media using ultrashort laser pulses,” Opt. Commun. 304, 29–38 (2013). [CrossRef]
31. Y. Bellouard and M.-O. Hongler, “Femtosecond-laser generation of self-organized bubble patterns in fused silica,” Opt. Express 19(7), 6807–6821 (2011). [CrossRef]
32. S. Richter, S. Döring, F. Burmeister, F. Zimmermann, A. Tünnermann, and S. Nolte, “Formation of periodic disruptions induced by heat accumulation of femtosecond laser pulses,” Opt. Express 21(13), 15452–15463 (2013). [CrossRef]
33. K. Cvecek, I. Miyamoto, and M. Schmidt, “Gas bubble formation in fused silica generated by ultra-short laser pulses,” Opt. Express 22(13), 15877–15893 (2014). [CrossRef]
34. P. K. Sahoo, T. Feng, and J. Qiao, “Dynamic pulse propagation modelling for predictive femtosecond-laser-microbonding of transparent materials,” Opt. Express 28(21), 31103–31118 (2020). [CrossRef]
35. H. Ma, R. A. Zakoldaev, A. Rudenko, M. M. Sergeev, V. P. Veiko, and T. E. Itina, “Well-controlled femtosecond laser inscription of periodic void structures in porous glass for photonic applications,” Opt. Express 25(26), 33261–33270 (2017). [CrossRef]
36. I. Miyamoto, Y. Okamoto, R. Tanabe, Y. Ito, K. Cvecek, and M. Schmidt, “Mechanism of dynamic plasma motion in internal modification of glass by fs-laser pulses at high pulse repetition rate,” Opt. Express 24(22), 25718–25731 (2016). [CrossRef]
37. I. H. W. Nordin, Y. Okamoto, A. Okada, T. Takekuni, and T. Sakagawa, “Effect of focusing condition on molten area characteristics in micro-welding of borosilicate glass by picosecond pulsed laser,” Appl. Phys. A 122(5), 492 (2016). [CrossRef]
38. M. J. Feigenbaum, “Quantitative universality for a class of nonlinear transformations,” J. Stat. Phys. 19(1), 25–52 (1978). [CrossRef]
39. S. Juodkazis, H. Misawa, and I. Maksimov, “Thermal accumulation effect in three-dimensional recording by picosecond pulses,” Appl. Phys. Lett. 85(22), 5239–5241 (2004). [CrossRef]
40. A. Benayas, W. F. Silva, A. Ródenas, C. Jacinto, J. Vázquez de Aldana, F. Chen, Y. Tan, R. R. Thomsom, N. D. Psaila, D. T. Reid, G. A. Torchia, A. K. Kar, and D. Jaque, “Ultrafast laser writing of optical waveguides in ceramic Yb:YAG: a study of thermal and non-thermal regimes,” Appl. Phys. A 104(1), 301–309 (2011). [CrossRef]
41. T. A. Birks, I. Gris-Sánchez, S. Yerolatsitis, S. G. Leon-Saval, and R. R. Thomson, “The photonic lantern,” Adv. Opt. Photon. 7(2), 107–167 (2015). [CrossRef]
