Femtosecond direct writing of waveguides in non-oxide glasses

Arnaud Zoubir, Martin Richardson, Cedric Lopez, Laeticia Petit, Kathleen A. Richardson, Clara Rivero, Alfons Schulte

School of Optics/CREOL, University of Central Florida, 4000 Central Florida Blvd, Orlando FL 32818-2700

Tel: 407 823 6819, Fax: 407 823 6880, Email: azoubir@mail.ucf.edu

ABSTRACT

We describe the fabrication of waveguides in optical materials using a femtosecond laser. The direct laser writing technique has the unique advantage of allowing volume structures to be fabricated. We investigate several writing schemes in non-oxide glasses and characterize the photo-induced modifications of the optical properties. These changes are linked to structural changes in the glass matrix, as revealed by Raman spectroscopy. Keywords: Direct writing, femtosecond, material processing, waveguides, chalcogenide glasses, PMMA

1. INTRODUCTION The potential of femtosecond lasers for use in the fabrication of waveguides in transparent materials by the direct writing approach has already been demonstrated 1-5. Rapid progress has been made in the last few years in the development of direct-write, femtosecond laser micro-structuring and waveguide writing techniques in various materials, particularly semiconductor and other photo-sensitive glasses. This approach could well become a disruptive technology in photonic device fabrication, perhaps even leading to the development of devices that are difficult to fabricate by any other technique. Although much still needs to be learned about the photo-induced physical processes involved, it is generally admitted that these phenomena are linked to structural rearrangements at the atomic level2,5, initiated by highly nonlinear processes, such as multi-photon ionization. Consequently, smaller features can be potentially be obtained and volume structures can be fabricated in bulk materials. Initially, the intensities required to produce photo-induced changes called for amplified laser systems1,2. However, recent studies have demonstrated the fabrication of photonic devices using simple femtosecond laser oscillators3-5. More deterministic energy deposition and greater processing speeds can thus be achieved. Moreover, the added complexity associated with amplification schemes can be avoided. Many promising results have been reported in oxide glasses, and more particularly in fused silica. Several elemental photonic devices have been demonstrated such as fiber gratings6, directional couplers7 and optical interleavers8. Although the importance of fused silica is evident, other materials can be of importance for several applications. For instance, the low nonlinear index of fused silica tends to limit its use to strictly passive device applications. At the same time, polymeric optical materials show attractive features such as low cost, ease of fabrication and processing, and the ability to tune their composition in order to obtain the desired optical properties9. Moreover, the high electro-optic coefficient and the ability to dope the polymeric matrix10 are attractive features for active waveguide applications. The ability to fabricate the elemental components and the possibility of using different materials open promising perspectives for the fabrication of integrated optical circuits.

Photon Processing in Microelectronics and Photonics III, edited by P. R. Herman, J. Fieret, A. Piqué,T. Okada, F. G. Bachmann, W. Hoving, K. Washio, X. Xu, J. J. Dubowski, D. B. Geohegan, F. Träger,Proc. of SPIE Vol. 5339 (SPIE, Bellingham, WA, 2004) · 0277-786X/04/$15 · doi: 10.1117/12.538483

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In this paper, we report on the fabrication of waveguiding structures in several materials, including polymeric materials such as poly-methylmethacrylate (PMMA) and chalcogenide glass materials. We describe different writing scenarios using a special laser oscillator operating in the low-repetition-rate high-pulse-energy regime. We characterize the fabricated structures by measuring the near-field intensity distribution at the waveguide output and we postulate an explanation for the observed refractive index distribution in PMMA. From the measured waveguide Raman spectra, we identify the photo-induced structural changes in the chalcogenide glass matrix responsible for the change in refractive index.

2. DIRECT LASER WRITING SETUP

Traditional Ti:Sapphire laser cavity designs operate at a repetition rate around 100 MHz. They consequently suffer from small pulse energy values (<5 nJ/pulse). We have developed a special laser in order to meet the energy requirements for laser micromachining. The laser source used in this experiment is a Kerr-lens modelocked Ti:Sapphire laser, producing nearly transform-limited 30 fs pulses centered at 800 nm, with approximately 40 nm of bandwidth (FWHM). The laser repetition rate of the laser is reduced to 25 MHz range by increasing the oscillator cavity length using a 2-m long 1:1 telescope (Figure 1). This hybrid repetition rate value allows higher pulse energy, while maintaining fast processing speeds. The laser exhibits good pulse-to-pulse stability (~5%) with 500 mW of maximum average power, which yields pulse energies around 20 nJ. Fig. 1. Extended cavity schematic of the unamplified Ti:Sapphire oscillator, where FL = 12.5cm focal length lens; CM1, CM2 = 10cm R.O.C. mirrors; CM3, CM4 = 2m R.O.C. mirrors; OC= 12% output coupler; HR=high reflector; M1, M2 = chirped mirrors. The laser beam with a 2.6 M2 value is focused to a tight spot by a microscope objective. The sample is translated with respect to the focused laser beam by a computer-controlled motorized translation stage that is programmed to accomplish a predetermined path in the three dimensions of space. The laser writing setup also includes a variable neutral density filter in order to adjust the pulse energy. The number of pulses can be varied by adjusting the translation speed of the moving stages.

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3. FEMTOSECOND MICRO-STRUCTURING OF CHALCOGENIDE GLASS The chalcogenide glass (ChG) family exhibits several interesting properties that can be exploited for the fabrication of integrated photonic devices. In particular, their excellent infrared transparency, large third-order susceptibility, and low phonon energies11, make chalcogenide glass films good candidates for the fabrication of for all-optical switches12 and integrated optical elements13. Optical waveguides in chalcogenide glass have been fabricated by several techniques including photolithography, ion implantation and laser beam writing14. The materials investigated in this study are 90%(GeS2)-10%(Sb2S3) and arsenic trisulfide (As2S3). As2S3, like other chalcogenide glasses, has a semiconductor band structure. Despite its optical bandgap Eg at 2.35 eV (λ~517 nm), photosensitive properties have been demonstrated over a broad energy range, from γ-radiation down to infrared, below the bandgap energy. Sub-bandgap irradiation has already been successfully exploited to write holographic gratings and self-written channel waveguides through two-photon-induced processes at 800 nm13. These materials have bond structures that are amenable to re-orientation. These photo-induced structural changes have been extensively studied in the case where the photon energy was equal to or greater than the material bandgap energy. In this study, the writing laser has a wavelength of 800 nm. Therefore, the photon energy is below the bandgap energy and the material is essentially transparent to the writing beam. This feature is extremely interesting as the beam can penetrate inside the material enabling the fabrication of buried waveguides, or even three-dimensional structures in bulk materials. 3.1 Micro-structuring of As2S3 thin films Using the modelocked Ti:Sapphire laser described above, relief and volume gratings were fabricated in arsenic trisulfide thin films. The writing studies performed herein, were made on 2-µm-thick thin films of As2S3 which were thermally evaporated from bulk glass starting materials. Bulk glass materials were prepared using the protocol described in Ref. 15. Thin films were deposited using well established conditions described in detail in Ref. 14. Recently, a thorough Fig. 2. Surface profile of (a) the phase and (b) relief grating on the As2S3 film produced with 30 fs laser pulses from the extended cavity Ti:Sapphire oscillator. The writing laser intensity is 0.25 GW/cm2 for (a) and 40 GW/cm2 for (b). The typical number of pulses is ~106 per focal spot

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analysis of film optical properties has been described in Ref. 16. The sample was processed in two regimes: Firstly, the intensity was kept below the ablation threshold, generating a volume grating resulting from photo-darkening. This process is accompanied by a slight photo-expansion, as observed through an interferometric microscope. In the second regime, intensities above the ablation threshold produced a relief grating with grooves of 2-µm depth (Fig. 2). The line spacing was 20µm and was limited to the spot size of the focused beam (approximately 10µm, using a 0.28 NA, 15X reflective microscope objective) Using a Zygo NewView interferometer, we were able to perform interferometric measurements of the refractive index change by measuring an increase of the optical path over the exposed area. The Zygo interferometer is traditionally used for surface profile measurements. However, if the bottom surface of the film is imaged (instead of the top surface), the light going through a region of the film having a different refractive index will travel a different optical path, resulting in a fringe shift in the interferogram pattern. Hence, the refractive index change occurring in the exposed region can be calculated from this fringe shift and from the precise value of the film thickness measured with the same instrument in its traditional use. Using this technique, we measured refractive index changes in the order of 4x10-2. This value is consistent with reported values in the case of bandgap and sub-bandgap femtosecond laser writing in prior bulk ChG studies17, and is significantly higher than the typical values reported for oxide glasses. 3.2 Waveguide fabrication in bulk 90%(GeS2)-10%(Sb2S3) and in AS2S3 thin films As illustrated in the writing experiments performed below ablation threshold, a regime exists where the optical properties of the material can be altered, without drastically damaging the mechanical structure of the glass matrix. The obtained structures, as shown in Fig. 2a, exhibit rather smooth features with no heat affected zone and associated cracks surrounding the processed area. In addition, the exposed region exhibits an increase in refractive index. This regime is particularly advantageous for the fabrication of waveguides. Fig. 3. shows a single channel waveguide fabricated in bulk 90%(GeS2)-10%(Sb2S3). In this case, the sample was translated along the direction of the beam. In this configuration, also called longitudinal writing, the cross-section of the guide is circular and the waveguide length is limited by the microscope objective working distance, in this case 5 mm.

In transverse writing, the sample is translated in a direction perpendicular to the laser beam. The translation speed is varied between 0.1 and 20 mm/s allowing the number of pulses incident on the laser focal spot area to be varied for different laser intensities. The laser intensity is adjusted with a variable metallic neutral

Fig. 3. Cross section of a longitudinal waveguide written in bulk 90%(GeS2)-10%(Sb2S3)

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density filter, placed before the microscope objective. The experiment is performed with a pulse rate of 104 - 106 pulses per focal spot and intensities in the range of 0-0.3 GW/cm2. The refractive index change ∆n induced inside the waveguide is measured with the same technique used for the volume gratings. Measurements are made of its dependence on the laser irradiating intensity and the integrated absorbed laser flux density. The latter is shown in Fig 4(a) for As2S3 film. The value of ∆n increases approximately logarithmically when the integrated laser flux density is increased by decreasing the translation speed. The ∆n value shows a much sharper dependence on the pulse intensity for a constant translation speed as shown in Fig. 4(b). Clearly evident is the presence of an onset threshold for index modification, in this case at ~0.15 GW/cm2 for As2S3. Since the exposure photon energy is well below the absorption edge of the material, this result strongly suggests that the mechanisms responsible for the structural and photo-induced refractive index changes rely on non-linear optical processes.

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Waveguides as long as 2 cm have been fabricated, this time only limited by the range of the translation stage and the sample size. The waveguides are visualized with a differential interference contrast microscope having a magnification of 20X that identified the photo-induced change (Fig.5(a-c)). Characterization of the guiding properties of the waveguides produced is made by butt-coupling a 785 nm single-mode pigtailed laser diode to the cleaved waveguides. The near-field pattern of the coupled modes is then imaged onto a CCD (Fig. 5(d-f)). Coupling into As2S3 thin films is limited by the high refractive index of the material (n=2.45) which produces high Fresnel losses at both ends of the waveguide and mode-matching a single mode fiber to a rectangular waveguide. However, more than 70% of the light exiting the film end face (channel and film) was confined in the channel waveguide. Only a small fraction of the light either did not couple due to mode mismatch, or was decoupled due to scattering. The measured ratio of power coupled in the two output branches of the Y-coupler was near 50%-50%. Fig. 5. Differential interference contrast microscope images of (a) the unexposed As2S3 films (b) a single-channel waveguide and (c) a Y-coupler. The insets (d), (e), (f) are CCD images of the near-field pattern of the coupled modes 3.3 Photo-induced structural changes in As2S3 Details of the material re-structuring processes can in principal be obtained by a number of techniques, though the difficulty of spatially localizing the diagnostic to the size of the waveguide (typically 10 µm) limits its sensitivity. Perhaps the diagnostic that shows the strongest promise is spatially-resolved Raman spectroscopy which provides spectral signatures of the bond structure in the modified region. From this

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diagnostic, with waveguides written in thin films of As2S3, there are strong indications that the structural changes are of the type where As-S bonds are disrupted to form new As-As and S-S bonds thereby changing the material structure and refractive index. Near-IR Raman spectroscopy shows that sub-bandgap femtosecond irradiation has an impact on the low frequency vibrations from 175 to 250 cm-1, attributed to the As4S4 molecular clusters and the As-As homopolar bonds, present in the freshly deposited films (Figure 6).

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4. WAVEGUIDE WRITING IN POLYMERS: ANNULAR WAVEGUIDES IN PMMA Controlling the local change of the refractive index of a polymer such as PMMA with a laser would permit the manufacturing of integrated-optical waveguides in polymer integrated devices. In particular, polymethylmethacrylate (PMMA) is an inexpensive and widely used polymer for the cores of communications grade polymer optical fibers. The current techniques used for polymeric waveguide fabrication include photolithography and reactive ion etching18,19, photobleaching20,21 and high energy ion implantation21.These methods are based on conventional planar technologies, and require numerous processing steps and/or involve the design and fabrication of a mask before waveguides can be fabricated. By contrast, direct-write techniques such as laser direct-writing23 electron beam lithography24,25 and proton beam writing26,27 have the advantage of being maskless, allowing one-step fabrication processes. Femtosecond laser direct-writing has recently become an effective method in the fabrication of photonic structures in glasses1-8. Bends, Y-branches and directional couplers have been demonstrated in PMMA using this technique22. However, the photonic devices fabricated by the direct-write techniques were obtained in PMMA thin films, and are therefore restricted to planar configurations.

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In this experiment, bulk samples of PMMA were exposed to the same laser beam used previously. The beam is focused by a 10X 0.25-NA microscope objective and the PMMA sample is translated longitudinally with respect to the focal spot. Fig. 7 shows an image of the fabricated waveguides observed with a differential interference contrast (DIC) optical microscope.

Fig. 7. (a) cross-sectional image and (b) longitudinal image of the laser fabricated waveguides obtained with a differential interference contrast microscope. Although a positive refractive index change is observed in many materials such as fused silica1-4, 6-8 and chalcogenide glass5,12, femtosecond exposure in PMMA produces a negative refractive index change at the center of the exposed region, which appears darker in the DIC micrograph (Fig. 7). However, a positive index change is observed in the region surrounding the focus spot, which appears lighter in the DIC micrograph. This results in an annular refractive index distribution consistent with what has been observed in some materials such as phosphate glass (Schott IOG-1)28. In the case of PMMA, this feature may be caused by the thermal expansion in the focus, leading to an increased optical density in the surrounding region by thermally induced stress. Although no waveguiding was observed in the center of the waveguide, light can be guided in this higher index ring by total internal reflection. Fig. 8 shows the near-field intensity distribution at the output of the waveguide, captured by a Spiricon CCD camera.

Fig. 8. Near-field intensity distribution at the output of the waveguide captured by a CCD camera

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5. SUMMARY

As more experience is gained in the interaction of femtosecond laser emission with transparent optical materials, several factors become evident. At intensities above the ablation threshold, to the first order, most materials are ablated in the same manner. Although ablation rates may vary from one material to another, the basic mechanism is the same for all. However, below the ablation threshold rests an array of fascinating complex processes that are highly variable dependent on material and irradiation conditions. The precise understanding of most of these structural changes remains to be understood, but already some tantalizing possibilities exist. Control the refractive index with wavelength-scale precision in three dimensions will make possible the fabrication of many unique optical waveguide components in transparent media. Acknowledgements The authors gratefully acknowledge the collaboration of N. Ho and R. Vallee from the University of Laval. This work is supported by NSF, contract # DMR-9912975, and the State of Florida. References: 1. K. Hirao, K. Miura, “Writing waveguides and gratings in silica and related materials by a femtosecond laser”, J. Non-Cryst. Sol. 239, pp. 91-95 (1998). 2. J. W. Chan, T. Huser, S. Risbud, D. M. Krol, “Structural changes in fused silica after exposure to focused femtosecond laser pulses”, Opt. Lett. 26, No. 21, pp. 1726-1728 (2001) 3. C. B. Schaffer, A. Brodeur, J. F. Garca, E. Mazur, “Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy”, Opt. Lett. 26, pp. 93-95 (2001). 4. K. Minoshima, A. M. Kowalevicz, I. Hartl, E. P. Ippen, J. G. Fujimoto, “Photonic device fabrication in glass by use of nonlinear materials processing with a femtosecond laser oscillator”, Opt. Lett. 26, pp. 1516-1518 (2001). 5. A. Zoubir, M. Richardson, C. Rivero, C. Lopez, N. Ho, R. Vallee, and K.A. Richardson, Technical Digest. Summaries of papers presented at the Conference on Lasers and Electro-Optics. Conference Edition (IEEE Cat. No.02CH37337), 2002, pp. 125-126 6. Y. Kondo, K. Nouchi, T. Mitsuyu, M. Watanabe, P. G. Kazansky, K. Hirao, “Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses”, Opt. Lett. 24, No. 10, pp. 646-648 (1999) 7. A. M. Streltsov, N. F. Borrelli, “Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses”, Opt. Lett. 26, pp. 42-43 (2001) 8. C. Florea, K. A. Winick, “Fabrication and characterization of photonic devices directly written in glass using femtosecond laser pulses”, J. Lightwave Technology 21, No. 1, pp. 246-253 (2003) 9. J. Si, J. Qiu, J. Zhai, Y. Shen, K. Hirao, “Photoinduced permanent gratings inside bulk azodye-doped polymers by the coherent field of a femtosecond laser”, Appl. Phys. Lett. 80, pp. 359-361 (2002) 10. L. H. Slooff, A. van Blaaderen, A. Polman, G. A. Hebbink, S. I. Klink, F. C. J. M. Van Veggel,, D. N. Reinhoudt, J. W. Hofstraat, “Rare-earth doped polymers for planar optical amplifiers”, J. Appl. Phys. 91, pp. 3955-3980 (2002) 11. F. Smektala, C. Quemard, L. Leneindre, J. Lucas, A. Barthelemy, C. De Angelis, “Chalcogenide glasses with large non-linear refractive indices”, J. Non-Cryst. Sol. 239, 139-142 (1998) 12. J. M. Harbold, F. O. Ilday, F. W. Wise, J. S. Sanghera, V. Q. Nguyen, L. B. Shaw, I. D. Aggarwal, “Highly nonlinear As S Se glasses for all-optical switching”, Opt. Lett. 27, Issue 2, 119-121 (2002)

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13. C. Meneghini, A. Villeneuve, “As2S3 photosensitivity by two-photon absorption: holographic gratings and self-written channel waveguides”, J. Opt. Soc. Am. B 15, 2946-2950 (1998) 14. J-F. Viens, C. Meneghini, A. Villeneuve, T.V. Galstian, E.J. Knystaunas, M.A. Duguay, K.A. Richardson, T. Cardinal, “Fabrication and Characterization of Integrated Optical Waveguides in Sulfide Chalcogenide Glasses”, J. Lightwave Technology 17 (1999) 15. T. Cardinal, K.A. Richardson, H. Shim, G. Stegeman, A. Schulte, R. Beathy, K. LeFoulgoc, C. Meneghini, J. F. Viens and A. Villeneuve, “Nonlinear Optical Properties of Chalcogenide Glasses in the System As-S-Se,”, J. Non-Cryst. Solids 256 & 257, 353 (1999) 16. J. M. Laniel, J-M. Ménard, K. Turcotte, A. Villeneuve, R. Vallée, C. Lopez, K. Richardson, “Refractive index measurements of planar chalcogenide thin film,”, , J. Non-Cryst. Sols. 328, 1-3, 183-919, (2003) 17. O.M. Efimov, L.B Glebov, K. A. Ricahrdson, E. van Stryland, T. Cardinal, S. H. park, M. Couzi & J. L. Bruneel, J. Opt. Materials. 17,3, 379-386, (2001) 18. M. Hikita, Y. Shuto, M. Amano, R. Yoshimura, S. Tomaru, and H. Kozawaguchi, Appl. Phys. Lett. 63, 1161 ~1993 19. W. Wang, D. Chen, H. R. Fetterman, Y. Shi, W. H. Steier, and L. R.Dalton, Appl. Phys. Lett. 65, 929 ~1994]. 20. M. B. J. Diemeer, F. M. M. Suyten, E. S. Trommel, A. McDonach, J. M. Copeland, L. W. Jenneskens, and W. H. G. Horsthuis, Electron. Lett. 26, 379 (1990) 21. Y. Shi, W. H. Steier, L. Yu, M. Chen, and L. R. Dalton, Appl. Phys. Lett.58, 1131 (1991). 22. D. M. Ruck, S. Brunner, K. Tinschert, and W. F. X. Frank, Nucl. Instrum. Methods Phys. Res. B 106, 447 (1995) 23. L. Eldada, C. Xu, K. M. T. Stengel, L. W. Shacklette, and J. T. Yardley, J.Lightwave Technologyl. 14, 1704 ~1996!. 24. H. Nakayama, O. Sugihara, and N. Okamoto, Appl. Phys. Lett. 71, 1924 (1997). 25. W. H. Wong, J. Zhou, and E. Y. B. Pun, Appl. Phys. Lett. 78, 2110 (2001) 26. F. Watt, J. A. van Kan, and T. Osipowicz, MRS Bull. 25, 33 (2000). 27. T. C. Sum, A. A. Bettiol, H. L. Seng, I. Rajta, J. A. van Kan, and F. Watt, Nucl. Instrum. Methods Phys. Res. B 210, 266 (2003) 28. J. W. Chan, T. R. Huser, S. H. Risbud, J. S. Hayden, D. M. Krol, “Waveguide fabrication in phosphate glasses using femtosecond laser pulses”, Appl. Phys. Lett. 82, pp. 2371-2373 (2003)

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