Ultrafast laser-induced refractive index changes in ... - OSA Publishing

Ultrafast laser-induced refractive index changes in Ge15As15S70 chalcogenide glass C. D’Amico,1 C. Caillaud,2 P. K. Velpula,1 M. K Bhuyan,1 M. Somayaji,1 J.-P. Colombier,1 J. Troles,2 L. Calvez,2 V. Nazabal,2 A. Boukenter,1 and R. Stoian,1* 1

Laboratoire Hubert Curien, UMR 5516 CNRS, Université de Lyon, Université Jean Monnet, 42000 St. Etienne, France 2 Chemical Sciences Institute of Rennes, UMR 6226 CNRS, University of Rennes I, 35042 Rennes, France *[email protected]

Abstract: Understanding and controlling laser-induced refractive index modifications in bulk chalcogenide glasses is important for a range of photonics applications targeting the mid-infrared spectral region. We focus here on material engineering aspects and pulse spatio-temporal design characteristics that are able to induce and maintain positive refractive index changes in laser-irradiated Sulfur-based chalcogenide glass, mandatory for 3D photonic design. Specifically we study the photoinscription process of a Ge-doped Sulfur-based chalcogenide glass, Ge15As15S70, irradiated by focused ultrafast near-infrared laser pulses where Ge doping plays a determinant role in generating high-contrast positive index changes. By means of aposteriori and real-time in situ observations we show that positive refractive index changes (type I) are the result of a restructuring of the glass matrix and a photo-induced contraction process initiated by twophoton electronic excitation leading to bond softening, molecular mobility, structural changes and rearrangements. Oppositely, negative refractive index changes (type II) could be associated with two different processes: photo-expansion at higher intensities and hydrodynamic evolution initiated by plasma generation and laser heating, with thermomechanical relaxation and stress unload. Alongside the role of Ge in setting various degrees of the matrix connectivity, the structural arrangement developed under different thermal history schemes for glass preparation is equally important as it defines to which extent further structural flexibility is possible. Thus we indicate the role of glass matrix metastability in generated high-contrast refractive index changes and we show that a higher degree of relaxation is an impediment for contrasted positive index changes, while these are developing in unrelaxed glasses, where several degrees of structural flexibility exist. Alongside dynamic time-resolved imaging experiments probing the development and relaxation of excitation, we also show, via static Raman analysis of the modified regions, that significant structural changes are induced by laser irradiation and we discuss the potential processes involved. ©2016 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (140.3440) Laser-induced breakdown; (140.7090) Ultrafast Laser; (160.2750) Glass and other amorphous materials.

References and links 1. 2. 3.

B. J. Eggleton, “Chalcogenide photonics: fabrication, devices and applications. Introduction,” Opt. Express 18(25), 26632–26634 (2010). J. Hu, J. Meyer, K. Richardson, and L. Shah, “Feature issue introduction: mid-IR photonic materials,” Opt. Mater. Express 3(9), 1571–1575 (2013). V. Ta’eed, N. J. Baker, L. Fu, K. Finsterbusch, M. R. E. Lamont, D. J. Moss, H. C. Nguyen, B. J. Eggleton, D. Y. Choi, S. Madden, and B. Luther-Davies, “Ultrafast all-optical chalcogenide glass photonic circuits,” Opt. Express 15(15), 9205–9221 (2007).

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Received 4 Mar 2016; revised 29 Apr 2016; accepted 29 Apr 2016; published 13 May 2016 1 Jun 2016 | Vol. 6, No. 6 | DOI:10.1364/OME.6.001914 | OPTICAL MATERIALS EXPRESS 1914

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.

A. Zakery and S. Elliott, “Optical properties and applications of chalcogenide glasses: a review,” J. Non-Cryst. Solids 330(1-3), 1–12 (2003). J. S. Sanghera and I. D. Aggarwal, “Active and passive chalcogenide glass optical fibers for IR applications: a review,” J. Non-Cryst. Solids 256–257, 462–467 (2008). X. Gai, T. Han, A. Prasad, S. Madden, D.-Y. Choi, R. Wang, D. Bulla, and B. Luther-Davies, “Progress in optical waveguides fabricated from chalcogenide glasses,” Opt. Express 18(25), 26635–26646 (2010). L. Labadie, G. Martin, N. C. Anheier, B. Arezki, H. A. Qiao, B. Bernacki, and P. Kern, “First fringes with an integrated-optics beam combiner at 10μm. A new step towards instrument miniaturization for mid-infrared interferometry,” Astron. Astrophys. 531, A48 (2011). J. Lucas, “Infrared glasses,” Cur. Op. Sol. St. Mat. Sci. 4, 181–187 (1999). J. A. Savage, “Optical properties of chalcogenide glasses,” J. Non-Cryst. Solids 47(1), 101–115 (1982). B. Bureau, X. H. Zhang, F. Smektala, J.-L. Adam, J. Troles, H.-li Ma, C. Boussard-Plèdel, J. Lucas, P. Lucas, D. Le Coq, M. R. Riley, and J. H. Simmons, “Recent advances in chalcogenide glasses,” J. Non-Cryst. Solids 345– 346, 276–283 (2004). J. S. Sanghera, C. M. Florea, L. B. Shaw, P. Pureza, V. Q. Nguyen, M. Bashkansky, Z. Dutton, and I. D. Aggarwal, “Non-linear properties of chalcogenide glasses and fibers,” J. Non-Cryst. Solids 354(2-9), 462–467 (2008). H. Kanbara, S. Fujiwara, K. Tanaka, H. Nasu, and K. Hirao, “Third-order nonlinear optical properties of chalcogenide glasses,” Appl. Phys. Lett. 70(8), 925–927 (1997). K. S. Bindra, H. T. Bookey, A. K. Kar, B. S. Wherrett, X. Liu, and A. Jha, “Nonlinear optical properties of chalcogenide glasses: Observation of multiphoton absorption,” Appl. Phys. Lett. 9(13), 1939–1941 (2001). T. Cardinal, K. A. Richardson, H. Shim, A. Schulte, R. Beatty, K. Le Foulgoc, C. Meneghini, J. F. Viens, and A. Villeneuve, “Non-linear optical properties of chalcogenide glasses in the system As-S-Se,” J. Non-Cryst. Solids 256–257, 353–360 (1999). G. Boudebs, F. Sanchez, J. Troles, and F. Smektala, “Nonlinear optical properties of chalcogenide glasses: comparison between MachZehnder interferometry and Z-scan techniques,” Opt. Commun. 199(5-6), 425–433 (2001). R. R. Thomson, A. K. Kar, and J. Allington-Smith, “Ultrafast laser inscription: an enabling technology for astrophotonics,” Opt. Express 17(3), 1963–1969 (2009). N. Cvetojevic, N. Jovanovic, J. Lawrence, M. Withford, and J. Bland-Hawthorn, “Developing arrayed waveguide grating spectrographs for multi-object astronomical spectroscopy,” Opt. Express 20(3), 2062–2072 (2012). N. Jovanovic, P. G. Tuthill, B. Norris, S. Gross, P. Stewart, N. Charles, S. Lacour, M. Ams, J. S. Lawrence, A. Lehmann, C. Niel, J. G. Robertson, G. D. Marshall, M. Ireland, A. Fuerbach, and M. J. Withford, “Starlight demonstration of the Dragonfly instrument: an integrated photonic pupil-remapping interferometer for highcontrast imaging,” Mon. Not. R. Astron. Soc. 427(1), 806–815 (2012). R. R. Thomson, T. A. Birks, S. G. Leon-Saval, A. K. Kar, and J. Bland-Hawthorn, “Ultrafast laser inscription of an integrated photonic lantern,” Opt. Express 19(6), 5698–5705 (2011). K. Tanaka, “Photoexpansion in As2S3 glass,” Phys. Rev. B 57(9), 5163–5167 (1998). K. Tanaka, “Structural phase transitions in chalcogenide glasses,” Phys. Rev. B Condens. Matter 39(2), 1270– 1279 (1989). H. Fritzche, “Photo-induced fluidity of chalcogenide glasses,” Solid State Commun. 99(3), 153–155 (1996). V. M. Lyubin and V. K. Tikhomirov, “Novel photo-induced effects in chalcogenide glasses,” J. Non-Cryst. Solids 135(1), 37–48 (1991). J. P. De Neufville, S. C. Moss, and S. R. Ovshinsky, “Photostructural transformations in amorphous As2Se3 and As2S3 films,” J. Non-Cryst. Solids 13(2), 191–223 (1974). G. Chen, H. Jain, M. Vlcek, S. Khalid, J. Li, D. A. Drabold, and S. R. Elliott, “Observation of light polarizationdependent structural changes in chalcogenide glasses,” Appl. Phys. Lett. 82, 606–608 (2003). H. Jain, S. Krishnaswami, A. C. Miller, P. Krecmer, S. R. Elliott, and M. Vicek, “In situ high-resolution X-ray photoelectron spectroscopy of light-induced changes in As-Se glasses,” J. Non-Crys. Solids 274, 115–123 (2000). O. M. Efimov, L. B. Glebov, K. A. Richardson, E. Van Stryland, T. Cardinal, S. H. Park, M. Couzi, and J. L. Bruneel, “Waveguide writing in chalcogenide glasses by a train of femtosecond laser pulses,” Opt. Mater. 17(3), 379–386 (2001). C. Meneghini and A. Villeneuve, “As2S3 photosensitivity by two-photon absorption: holographic gratings and self-written channel waveguides,” J. Opt. Soc. Am. B 15(12), 2946–2950 (1998). A. M. Ljungstr¨om and T. M. Monro, “Light-Induced Self-Writing Effects in Bulk Chalcogenide Glass,” J. Lightwave Technol. 20(1), 78–85 (2002). M. Hughes, W. Yang, and D. Hewak, “Fabrication and characterization of femtosecond laser written waveguides in chalcogenide glass,” Appl. Phys. Lett. 90(13), 131113 (2007). N. D. Psaila, R. R. Thomson, H. T. Bookey, S. Shen, N. Chiodo, R. Osellame, G. Cerullo, A. Jha, and A. K. Kar, “Supercontinuum generation in an ultrafast laser inscribed chalcogenide glass waveguide,” Opt. Express 15(24), 15776–15781 (2007). O. Caulier, D. Le Coq, L. Calvez, E. Bychkov, and P. Masselin, “Free carrier accumulation during direct laser writing in chalcogenide glass by light filamentation,” Opt. Express 19(21), 20088–20096 (2011). M. A. Hughes, W. Yang, and D. W. Hewak, “Spectral broadening in femtosecond laser written waveguides in chalcogenide glass,” J. Opt. Soc. Am. B 26(7), 1370–1378 (2009).

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34. S. Juodkazis, T. Kondo, H. Misawa, A. Rode, M. Samoc, and B. Luther-Davies, “Photo-structuring of As2S3 glass by femtosecond irradiation,” Opt. Express 14(17), 7751–7756 (2006). 35. A. Ródenas, G. Martin, B. Arezki, N. Psaila, G. Jose, A. Jha, L. Labadie, P. Kern, A. Kar, and R. Thomson, “Three-dimensional mid-infrared photonic circuits in chalcogenide glass,” Opt. Lett. 37(3), 392–394 (2012). 36. A. Zoubir, M. Richardson, C. Rivero, A. Schulte, C. Lopez, K. Richardson, N. Hô, and R. Vallée, “Direct femtosecond laser writing of waveguides in As2S3 thin films,” Opt. Lett. 29(7), 748–750 (2004). 37. T. Anderson, N. Carlie, L. Petit, J. Hu, A. Agarwal, J. J. Viens, J. Choi, L. C. Kimmerling, K. Richardson, and M. Richardson, “Refractive index modifications in Chalcogenide films induced by sub-bandgap near-IR femtosecond pulses,” Paper CThS6 CLEO conference (2007). 38. P. Masselin, D. Le Coq, A. Cuisset, and E. Bychkov, “Spatially resolved Raman analysis of laser induced refractive index variation in chalcogenide glass,” Opt. Mater. Express 2(12), 1768–1775 (2012). 39. R. Stoian, M. Boyle, A. Thoss, A. Rosenfeld, G. Korn, I. V. Hertel, and E. E. B. Campbell, “Laser ablation of dielectrics with temporally shaped femtosecond pulses,” Appl. Phys. Lett. 80(3), 353 (2002). 40. M. K. Bhuyan, P. K. Velpula, J. P. Colombier, T. Olivier, N. Faure, and R. Stoian, “Single-shot high aspect ratio bulk nanostructuring of fused silica using chirp-controlled ultrafast laser Bessel beams,” Appl. Phys. Lett. 104(2), 021107 (2014). 41. C. D’Amico, G. Cheng, C. Mauclair, J. Troles, L. Calvez, V. Nazabal, C. Caillaud, G. Martin, B. Arezki, E. LeCoarer, P. Kern, and R. Stoian, “Large-mode-area infrared guiding in ultrafast laser written waveguides in Sulfur-based chalcogenide glasses,” Opt. Express 22(11), 13091–13101 (2014). 42. J. Zarzycki, Glasses and the vitreous state (Cambridge University Press, 1991). 43. L. Boesch, A. Napolitano, and P. B. Macedo, “Spectrum of Volume Relaxation Times in B2O3,” J. Am. Ceram. Soc. 53(3), 148–153 (1970). 44. P. Macedo and A. Napolitano, “Effects of a distribution of volume relaxation times in the annealing of BSC glass,” J. Res. Natl. Bur. Stand. 71(3), 231–238 (1967). 45. L. Calvez, Z. Yang, and P. Lucas, “Reversible giant photocontraction in chalcogenide glass,” Opt. Express 17(21), 18581–18589 (2009). 46. P. Lucas and E. A. King, “Calorimetric characterization of photoinduced relaxation in GeSe9 glass,” J. Appl. Phys. 100(2), 023502 (2006). 47. S. Juodkazis, T. Kondo, H. Misawa, A. Rode, M. Samoc, and B. Luther-Davies, “Photo-structuring of As2S3 glass by femtosecond irradiation,” Opt. Express 14(17), 7751–7756 (2006). 48. H. Hisakumi and K. Tanaka, “Optical microfabrication of chalcogenide glasses,” Science 270(5238), 974–975 (1995). 49. H. Hisakumi and K. Tanaka, “Giant photoexpansion in As2S3 glass,” Appl. Phys. Lett. 65(23), 2925 (1994). 50. Chalcogenide Glasses, Preparation, Properties and Applications, X. H. Zhang, and J. L. Adam, eds. (Woodhead Publishing, 2014). 51. R. Stoian, K. Mishchik, G. Cheng, C. Mauclair, C. D’Amico, J. P. Colombier, and M. Zamfirescu, “Investigation and control of ultrafast laser-induced isotropic and anisotropic nanoscale-modulated index patterns in bulk fused silica,” Opt. Mater. Express 3(10), 1755 (2013). 52. R. Shuker and R. W. Gammon, “Raman-Scattering Selection-Rule Breaking and the Density of States in Amorphous Materials,” Phys. Rev. Lett. 25(4), 222–225 (1970). 53. G. Lucovsky, F. L. Galeener, R. C. Keezer, R. H. Geils, and H. A. Six, “Structural interpretation of the infrared and Raman spectra of glasses in the alloy system Ge1-xSx,” Phys. Rev. B 10(12), 5134–5146 (1974). 54. S. Sugai, “Stochastic random network model in Ge and Si chalcogenide glasses,” Phys. Rev. B Condens. Matter 35(3), 1345–1361 (1987). 55. G. Lucovsky, “Optic Modes in Amorphous As2S3 and As2Se3,” Phys. Rev. B 6(4), 1480–1489 (1972). 56. O. M. Efimov, L. B. Glebov, K. A. Richardson, E. Van Stryland, T. Cardinal, S. H. Park, M. Couzi, and J. L. Bruneel, “Waveguide writing in chalcogenide glasses by a train of femtosecond laser pulses,” Opt. Mater. 17(3), 379–386 (2001). 57. G. Lucovsky, R. J. Nemanich, S. A. Solin, and R. C. Keezer, “Coordination dependent vibrational properties of amorphous semiconductors alloys,” Solid State Commun. 17(12), 1567–1572 (1975). 58. B. G. Aitken and C. W. Ponader, “Physical properties and Raman spectroscopy of GeAs sulphide glasses,” J. Non-Cryst. Solids 257, 143–148 (1999). 59. J. D. Musgraves, P. Wachtel, B. Gleason, and K. Richardson, “Raman spectroscopic analysis of the Ge-As-S chalcogenide glass-forming system,” J. Non-Cryst. Solids 386, 61–66 (2014). 60. P. J. S. Ewen and A. E. Owen, “Resonance Raman scattering in As-S glasses,” J. Non-Cryst. Solids 35, 1191– 1196 (1980).

1. Introduction Optical access to the mid-infrared (MIR) spectrum puts forward a strong interest for a range of photonic-based applications including astronomy and space sensing, organic tracing or thermal detection. The challenge of MIR accessibility and fabrication of adapted complex optical functions working in the MIR spectral domain puts forward chalcogenide technology as a promising solution [1–10]. Chalcogenide glasses (ChGs) represent an important class of materials for MIR applications particularly due to their transparency and nonlinear strength [11–15]. Equally, the requirements of embedded, mechanically stable miniature optical

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devices recommends laser fabrication as a technique of remarkable potential for developing miniaturized 3D embedded photonic micro-circuitries. The optical performance relies then on defining upgraded material characteristic and optical design, including a passage from two to three dimensions and, in this context, 3D ultrafast laser processing of transparent materials offers already interesting perspectives [16–19]. Highly contrasted refractive index domains within guiding structures can be obtained by localized laser-induced changes following laser scan; this allows creating light control functions inside bulk materials where the embedded nature assures for intrinsic phase stability. The performance of such light guiding structures (waveguides) in a given spectral range depends on the balance between index contrast, profile, and dimension, thus optimal material response to light interaction is determinant. This implies that the physical interaction, material response and the characteristic dimensional scale have to be accurately controlled for achieving desired mode transport. Typically ChG materials show photosensitivity and intrinsic metastability, which marks the reaction to external radiation [20–26]. The structural flexibility allows tuning optical properties, namely gap and phonon energy, or nonlinearities, by elemental constituents. These features, coupled to a relatively high two photon absorption yield at current ultrafast laser radiation IR wavelengths can allow the development of efficient volume 3D photoinscription methods [27–35]. The induced refractive index change has a nonlinear nature and it is determined, depending on glass composition, by molecular, structural, and thermomechanical rearrangements [36–38]. Here we study the response of Ge15As15S70 glass irradiated by high intensity ultrafast laser pulses in comparison to As2S3, and indicate the facility of generating positive index changes in the former. While asking the question which are the factors that enable the index change form a compositional and structural viewpoint, we discuss the possibility that photo-contraction, structural rearrangements, photo-expansion processes induced by carrier excitation via two-photon absorption and heat-induced mechanical relaxation play an important role in the photo-inscription process of this glass. We first discuss the conditions for type I positive index changes and the stability of the structural modifications. The index modifications are linked to a laser-induced re-structuring process and rearrangement of the glass matrix following bond softening. The rearrangement of the matrix depends on the initial degree of structural relaxation. The thermal history of the glass preparation fix the level of flexibility in rearranging the structure, where high fictive temperature glass allows for more efficiency in densely repacking the structure. Above a certain threshold we show that plasma generation and local laser heating, able to initiate hydrodynamic expansion, is important for the generation of highly contrasted type II void-like negative index changes. Single-pulse type II trace in this chalcogenide glass can be obtained significantly more efficiently by focusing non-diffractive Bessel laser beams due to their increased nonlinear stability. Using time-resolved microscopy on sub-ps and ps scales we confirm a thermomechanical scenario for the type II case, relaxing by pressure wave emission. The results are further confirmed by means of Raman analysis of type I regions photo-inscribed in irradiated and non-irradiated samples, indicating that significant structural changes are induced in the modified region. 2. Experimental conditions An amplified Ti:Sapphire femtosecond laser system delivering 800 nm light pulses with a maximum power of 600 mW and a duration of 160 fs at a repetition rate of 100 kHz was employed as irradiation source. As2S3 and Ge15As15S70 parallelepipedic samples with transparency cut-off respectively at 580 nm and 520 nm were irradiated and their response to laser radiation was studied. Exposure doses were controlled by electromechanical shutters. The beam was focused inside the target principally by large numerical aperture microscope objectives (NA = 0.42–0.45, effective NA due to truncation limited to NA = 0.4). The source is equipped with a pulse envelope control unit in time based on programmable spectral phase modulation in liquid crystal arrays so the response of the samples can be studied as a function of the temporal duration of the writing pulse [39]. Static and dynamic longitudinal writing configurations with translation parallel to the laser axis were used in direct focusing

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geometries. An Olympus BX41 positive optical transmission (OTM) and phase-contrast microscope (PCM) inserted in the irradiation setup was used to image the interaction region in side-view geometry. Positive refractive index changes relative to the background matrix are appearing dark on gray background, while white zones indicate negative index variations. The guiding properties of the photo-inscribed waveguides were verified upon injection with IR light. Time-resolved studies were conducted using an 800 nm amplified Ti:Sapphire femtosecond laser system delivering light pulses with a maximum power of 3 W and a duration of 50 fs at a repetition rate of 1 kHz allowing equally for extraction of single pulses or controllable pulse sequences. A Bessel-Gauss beam was used for excitation with the incoming irradiation pulse crossing an axicon lens. Details are given in [40]. The conical intersection image after the axicon (16° angle, tight focusing) was demagnified and imaged using a 4f afocal system inside the glass material. Non-diffractive excitation allows for a relatively lower yield of nonlinearities with respect to Gaussian propagation rendering thus stable excitation conditions. 3. Comparison As2S3 – Ge15As15S70 A very insightful observation appears from studying on a comparative basis the optical response of As2S3 and Ge15As15S70 under femtosecond laser irradiation. In Fig. 1 we show typical photoinscription traces that can be obtained in the bulk by translating the sample in the longitudinal direction during laser irradiation. Depicted on the left (Fig. 1(a)) is the response of As2S3 and on the right (Fig. 1(b)) the response of Ge15As15S70 for an averaged laser power of 2 mW at 100 kHz repetition rate and two different translation speeds: 100 µm/s (up) and 1000 µm/s (down). (a) As40S60

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As can be clearly seen, As2S3 and Ge15As15S70 samples show almost an opposite response to the femtosecond laser flux, with an increased facility to photoinscribe positive index changes for the latter. By analyzing the response of both samples over a wide range of translation speeds and laser powers, three main conclusions can be given:

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- As2S3 presents mainly type II negative refractive index modifications; positive index type I modifications are observed for very low power or high translation speed in a relatively narrow range and with a very poor contrast (see Fig. 1(a)) - Ge15As15S70 presents mainly type I refractive positive index modifications; type II modifications are observed only at relatively high laser doses (not shown here); to obtain appreciable type II modifications for an average laser powers of 4 mW, translation speeds of 10 µm/s or lower are necessary in the current focusing conditions. - The laser dose processing window for type I traces in Ge15As15S70 is very large: for an average laser power of 4 mW, type I traces are always obtained while changing the translation speed by almost two orders of magnitude, from 50 µm/s up to 1000 µm/s. This corresponds to a positive refractive index variation range going from 0.25 × 10−3 to above 10−3 (see the graph of Fig. 1(c)). The refractive index is estimated from mode measurements in corresponding waveguiding structures. Corresponding values of peak mode values are indicative of the guiding quality and the mode confinement.. To characterize the optical properties, waveguides photo-inscribed in both samples were injected with 800 nm light. No guided light has been observed in the As2S3 sample within single traces. This is due to the fact that light cannot be guided in negative index type II structures and the refractive index contrast for type I structures is too weak to obtain a confined mode, with a reasonable normalized frequency value. This indicates that the index contrast for type I structures photo-inscribed in As2S3 is smaller than 10−4. However, an opposite behavior is observed in the Ge15As15S70 sample. Here a very large processing window for type I structures permits obtaining well-guided single modes for different values of power and translation speed. This allows tuning the intensity and the diameter of the guided mode along the waveguide, as can be seen in Fig. 1(c). A first conclusion can be drawn here. Doping with Ge allows creating glasses (as Ge15As15S70) with a large window for type I modifications, starting from glasses (as As2S3) which have very narrow window for type I modifications under femtosecond laser beam irradiation and therefore can only give few and rather complex solutions to guiding light, like for example depressed cladding concepts. Controlling the induced refractive index variation via elemental composition gives an important flexibility in engineering materials for a large spectrum of needs and applications in photonics, by generating optimized optical functions. For example tuning the refractive index change values in the 10−3 – 10−4 domain is important for determining the guiding conditions and the single-mode or multimode performance, but equally in engineering the evanescently coupling coefficient in optical systems based on evanescently-coupled waveguide arrays [41]. 4. Ultrafast photo-inscription of thermally-conditioned higher/lower-density Ge15As15S70 In this section we make a comparison between the photo-inscription process in lower and higher density (higher and lower enthalpy respectively) chalcogenide glasses obtained by changing the annealing time during the preparation phase. We will discuss the role of the thermal history in the photo-inscription process. 4.1 The role of thermal history After observing the role of Ge in establishing a wide positive index change, a question relies on the as-generated structural flexibility of the glass matrix. How does the connectivity driven by Ge relates to the structural flexibility designed by the thermal history of the glass? Several levels of structural arrangements and rigidity can be obtained by the thermal history during glass preparation and we have opted to compare two distinct regimes; a short annealing close to the glass transition temperature Tg (in the following called the “reference sample”), and a

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long re-annealing (in the following called “re-annealed sample”), both followed by a slow ramped cooling. This provides respectively a high and low value of enthalpy (see Fig. 2). In detail, the Ge15As15S70 is obtained by mixing elements in a furnace at a temperature of 800 °C for 15 hours and rapid cooling to Tg–15 °C. An annealing of 30 minutes allows a partial removal of constraints, followed by a slow cooling at room temperature. This process generates unrelaxed glasses (the reference sample) with characteristic volumes higher than the volume corresponding to relaxed glasses, and therefore a lower density. The re-annealed sample follows an additional temperature treatment. A long annealing (70 h) at Tg–15 °C allows to erase the previous history and determines a rather complete relaxation of the glass. Typical relaxation times in this range below Tg are in the order of tens of minutes, significantly smaller than the heat treatment cycle. Thus a relaxed glass with a minimal volume (low enthalpy) and higher density is obtained. Two samples with different intrinisic relaxation degrees and therefore densities are thus obtained. Enthalpy reference sample

T < Tg

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re-annealed sample Photo-induced relaxation

Time Fig. 2. Schematic of the thermal history of annealed chalcogenide glasses and its influence on the enthalpy of the microscopic glass structure.

In conclusion a process that does not allow structural relaxation generates a glass sample with high enthalpy and volume, i.e. lower density. Re-annealing enables a structural relaxation process that corresponds closely to a structural equilibrium at the annealing temperature, little affected by the cooling time as the relaxation times becomes prohibitively long [49]. In other words, from a standard normal thermodynamic behavior of a chalcogenide glass, the longer the annealing time at temperatures under Tg, the lower its final volume, i.e. higher its density [42]. The glass refractive index, which depends on its density, will depend therefore on its thermal history. A glass annealed long time at a temperature close to Tg will have therefore a higher refractive index than a glass annealed during a short time; global refractive index variations of the order of 10−4 to 10−3 can be reached by annealing [43, 44]. The enthalpy represents a first evaluation of structural flexibility allowing evaluating the strength of additional laser-induced modification on a structure with different degrees of relaxation, up to the point where in principle no further structural changes and relaxation can be induced. The structural state reached by controlled annealing will play therefore a role in additional laser-assisted processes as the laser excitation can relaunch a heating-cooling cycle or additional molecular reorganization. In re-annealed samples, where the glass structure is relaxed, it should be much harder to induce photo-contraction, and furthermore the opposite effect of photo-expansion should be observed, especially at higher intensities [45]. 4.2 Measurement In order to verify this point, we performed ultrafast laser photoinscription of type I waveguides in a reference Ge15As15S70 sample, annealed only 30 min at Tg–15 °C and then slowly cooled to room temperature, and in one re-annealed during 72 hours at Tg −15 °C and then slowly cooled to the room temperature.

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Fig. 3. Comparison of the index change results (PCM) of the waveguide photoinscription process with two different scan speeds in the reference (a, b) and longtime re-annealed Ge15As15S70 (c, d) samples by 2 mW (@100 kHz) and 160 fs laser pulses. For comparison, parts (e) and (f) show the photoinscription traces in a reference As2S3 sample. Figures (g) and (h) show the variation of the transverse relative refractive index profile (in grey levels) of the traces corresponding to the couple of figures (a, c) and (b, d) respectively. The decrease in the gray level correspond to index increase. Note the higher contrast obtained in the reference Ge15As15S70.

The photoinscription process was performed in both samples under the same experimental conditions (numerical aperture of the focusing objective, laser pulse duration, and an averaged laser power of 2 mW) and the response of the two samples was evaluated by injecting IR light in the waveguides and checking the guided modes in the near-field. As can be seen in Fig. 3 the response of the reference and re-annealed samples was significantly different. The reference sample (Figs. 3(a) and 3(c)) showed the expected response of index increase, very similar to that described in [41]. On the contrary the photoinscription of efficient type I waveguides in the re-annealed sample was more difficult to perform and only lower index contrast type I traces could be photo-inscribed inside; this is demonstrated by the fact that it was impossible to obtain a confined guided mode for the waveguides photo-inscribed in the re-annealed sample (Figs. 3(b) and 3(d)), which normally indicates a low refractive index contrast. In Figs. 3(g) and 3(h) we plot the contrast profile (grey level) of type I waveguides photo-inscribed in the reference sample and in the re-annealed sample, in the same experimental conditions indicated in Figs. 3(a-d)). As can be seen, the region where the laser pulse is more intense (the center of the pulse) corresponds always to a negative refractive index variation (indicated with an arrow in the figures) when the sample is re-annealed, and this effect is more pronounced at higher intensities and higher laser doses, as expected (Fig. 3(g)). This behavior is in good agreement with the results reported in [45], i.e. irradiating the surface of high enthalpy chalcogenide glass with low intensity laser pulses results in a large photocontraction, while high intensity irradiation can generate the exactly opposed effect, i.e. photoexpansion, in low enthalpy samples. In Figs. 3(e) and 3(f) we also reported, for

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comparison, the response in terms of index change of a reference As2S3 sample exposed to a photo-inscription process where 2 mW laser pulses and low 60 µm/s (3e) and high 1000 µm/s (3f) scanning speeds are used. We can clearly see that laser-induced densification in As2S3 glass is hard to obtain, as if the molecular arrangement of this sample was already relaxed in a configuration of very low enthalpy. By comparing Figs. 3(c), 3(e), 3(d) and 3(f), we can see that the response of the re-annealed Ge15As15S70 and the response of the reference As2S3 are similar in a certain respect. We can qualitatively conclude that, when re-annealed during long time the response of Ge15As15S70 glasses to laser irradiation tends to match the response of reference As2S3 glasses (not re-annealed). Based on the scenario depicted above, this gives an indication about the role of Ge doping. As2S3 in fact present a quite planar structure; the insertion of Ge introduces a 3D connectivity and new degrees of freedom, allowing for higher flexibly and so a higher relaxation potential. We could think that is more difficult to relax a planar structure such as As2S3 than a 3D connected structure such as Ge15As15S70. In this kind of photo-inscription process the index variation can be totally or partially erased by re-annealing at a temperature close to the glass transition temperature, which for our sample has been measured as Tg = (204 ± 2) °C. However, the thermal stability of the waveguide obtained in our study has been evaluated at different temperature and no significant changes within the light mode transport have been observed until one hour annealing at 190 °C, indicating a quite good thermal stability. 4.3 Discussion We have therefore observed that the laser effect depends on the degree of relaxation, the more relaxed the glass the more difficult is to obtain high contrast positive index changes. This indicates that the laser role is to locally trigger states with a different degree of relaxation with respect to the pristine sample, namely to foster further relaxation in unrelaxed or partially relaxed matrices (with a result in index increase) or to induce structural constraints in relaxed matrices up to thermomechanical effects. The question is then how the laser pulse is able, via the deposited energy/excitation to trigger molecular mobility and structural rearrangement [46]. We discuss below two paths, an electronic driven softening process in the presence of low temperature yields, and later in the text we will discuss the probability of thermal drives for molecular reorganization. In our experimental conditions, laser pulses are focused into the sample and reach peak intensities for which high two-photon absorption takes place; where two photon absorption coefficient lies usually in the 1-2 cm/GW range [47]. Carrier plasma is therefore locally generated, i.e. a number of electrons is promoted from the valence band to the conduction band by two-photon absorption. When electrons are excited to the conduction band, bonds of the glass matrix are potentially weakened and molecular mobility is allowed even at room temperatures where normally it is prohibited by slow kinetics. The lifetime of the electron plasma generated by a single pulse (few tens of nanoseconds [32]) is too short to induce an efficient molecular mobility with only one laser pulse, however, laser pulse after laser pulse, a relaxation channel is generated through which the glass structure is allowed to rearrange even at room temperature The irradiation process can be seen therefore as a way to remove the kinetic constraint to the thermodynamically driven relaxation. These results are in agreement with previous works reported in the literature. Hisakuni and Tanaka [48, 49] reported on photo-induced glass softening effect in amorphous As2S3. They have shown the possibility of athermally photo-induced fluidity, i.e. the possibility of introducing degrees of freedom and structural mobility via paths which are not exactly temperature driven in the glass matrix under laser irradiation. More recently Lucas and King [46] reported on the observation of fast light-activated relaxation in GeSe9 chalcogenide glass illuminated with sub-band-gap light from a Ti-Sapphire laser. However it will be seen below that, in conditions of type I writing, the single shot level of excitation is rather low, with carrier densities presumably below 1019 cm−3, keeping thus the single shot effect in molecular reorganization low, and, at the same time, a low accompanying thermal yield, improbable to sustain molecular reorganization.

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All these results suggest nevertheless that ultrafast optically induced relaxation processes are possible at room temperature under ultrafast laser irradiation or generally where laserinduced heat yield is low, and this process is much efficient in chalcogenide glasses with a glass matrix quenched in a configuration of high enthalpy (or high fictive temperature). The verification of a low temperature yield will be given later in the text. 5. Static and Time resolved imaging of photo-written type I/type II changes 5.1 Static results: type I and type II permanent changes in single and multi-shot regimes We study here the photo-inscription of type I and type II permanent changes induced by focusing ultrashort non-diffractive (Bessel) beams into the sample. We choose Bessel beams because they are quasi-non-propagative self-healing beams and therefore they allow locally deposing higher energy densities with reduced nonlinear effect due to propagation, as observed for example with Gaussian laser beams. Experimental observations show that when high energy and low energy laser pulses are focused into the sample, no permanent type I local changes are observed in single shot regime both with femtosecond and picosecond Bessel beams; type I modification are observed only with low energy Bessel pulses (femtosecond or picosecond) after several laser shots (see Fig. 4(b)). In Fig. 4(b) we can clearly see for example that for 100 nJ - 80 fs Bessel pulses one needs a number of laser shots equal to N = 100 before starting to observe a very low contrasted type I change (threshold), and the index contrast increases with the number of laser shots. This indicates that, in femtosecond or in picosecond regime, an efficient photo-induced densification at low energy can be observed only after many laser shots (cumulative effect), and it is a further indication of the fact that local densification observed in correspondence to type I modified regions has not a thermomechanical origin. This is supported by numerical simulations, which are discussed in the next paragraph. Type II permanent change, on the contrary, can be observed in single shot regime and is associated with the generation of relatively dense electron plasmas as showed and discussed in the paragraph 5.3.

(a) 1 µJ; 4 ps

Type II

N=1

(b) 100 nJ; 80 fs

Type I

N = 10000

∆n>0

N = 1000 N = 100 N = 10 30µm

N=1

∆n0

0

∆n