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Virtual Journal for Biomedical Optics

Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editors: Andrew Dunn and Anthony Durkin
  • Vol. 9, Iss. 5 — Apr. 29, 2014
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Nonlinear characterization of robust multimaterial chalcogenide nanotapers for infrared supercontinuum generation

Soroush Shabahang, Guangming Tao, Michael P. Marquez, Honghua Hu, Trenton R. Ensley, Peter J. Delfyett, and Ayman F. Abouraddy  »View Author Affiliations


JOSA B, Vol. 31, Issue 3, pp. 450-457 (2014)
http://dx.doi.org/10.1364/JOSAB.31.000450


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Abstract

We present the results of an investigation of the nonlinear characteristics of a new class of robust, multimaterial, all-solid chalcogenide nanotapers prepared from high-index-contrast chalcogenide fibers. The fiber is drawn from a preform produced by multimaterial coextrusion and consists of chalcogenide core and cladding (which dictate the optical properties) and a built-in thermally compatible polymer jacket that provides mechanical stability to the fibers and nanotapers. We measure the nonlinear refractive indices both in the bulk chalcogenide glasses using the Z-scan method and directly in the nanotapers from spectral broadening resulting from self-phase modulation using both picosecond and femtosecond pulses. Such robust nanotapers offer many opportunities for dispersion engineering to optimize nonlinear optical fiber applications such as infrared supercontinuum generation. Low-power femtosecond pulses (100W peak power, corresponding to 40pJ energy per pulse) centered at 1.55 μm wavelength launched into the nanotapers generated a supercontinuum extending over a full spectral octave, 1–2 μm. A computational model that takes into account the relevant linear and nonlinear optical parameters provides simulations that are in good agreement with the supercontinuum measurements.

© 2014 Optical Society of America

1. INTRODUCTION

Chalcogenide glasses (ChGs) stand out as the only family of optical materials that are transparent across both the near- and mid-infrared (MIR) and that also can be thermally drawn continuously and stably into extended optical fibers [1

1. J. S. Sanghera, I. D. Aggarwal, L. B. Shaw, L. E. Busse, P. Thielen, V. Nguyen, P. Pureza, S. Bayya, and F. Kung, “Applications of chalcogenide glass optical fibers at NRL,” J. Optoelectron. Adv. Mater. 3, 627–640 (2001).

,2

2. B. J. Eggleton, B. Luther-Davies, and K. Richardson, “Chalcogenide photonics,” Nat. Photonics 5, 141–148 (2011).

]. Crucially, ChGs exhibit higher optical nonlinearities than rival infrared glasses. For example, the ChG As2Se3 has approximately 1 order of magnitude higher nonlinear refractive index n2 compared to tellurite glasses, 2 orders of magnitude higher than fluoride (ZBLAN) glasses, and 3 orders of magnitude higher than silica glass [3

3. A. Zakery and S. R. Elliott, Optical Nonlinearities in Chalcogenide Glasses and their Applications (Springer-Verlag, 2007).

], while maintaining a wider transparency window than tellurite, fluoride, or silicate glasses. A wide range of applications for ChG fibers can benefit from this combination of useful optical characteristics ranging from high-speed optical communication that requires ultrafast all-optical processing and switching [4

4. M. Asobe, “Nonlinear optical properties of chalcogenide glass fibers and their application to all-optical switching,” Opt. Fiber Technol. 3, 142–148 (1997). [CrossRef]

10

10. B. J. Eggleton, T. D. Vo, R. Pant, J. Schröder, M. D. Pelusi, D. Y. Choi, S. J. Madden, and B. L. Davies, “Photonic chip based ultrafast optical processing based on high nonlinearity dispersion engineered chalcogenide waveguides,” Laser Photon. Rev. 6, 97–114 (2012). [CrossRef]

] to MIR supercontinuum generation (SCG) [11

11. L. B. Shaw, P. A. Thielen, F. H. Kung, V. Q. Nguyen, J. Sanghera, and I. Aggarwal, “IR supercontinuum generation in As–Se photonic crystal fiber,” in Proceedings of Advanced Solid State Photonics (2005), paper TuC5.

,12

12. R. R. Gattass, L. B. Shaw, V. Nguyen, P. Pureza, I. D. Aggarwal, and J. S. Sanghera, “All-fiber chalcogenide-based mid-infrared supercontinuum source,” Opt. Fiber Technol. 18, 345–348 (2012). [CrossRef]

] for spectroscopy and countermeasures (complementing SCG in tellurite [13

13. P. Domachuk, N. A. Wolchover, M. Cronin-Golomb, A. Wang, A. K. George, C. M. B. Cordeiro, J. C. Knight, and F. G. Omenetto, “Over 4000 nm bandwidth of mid-IR supercontinuum generation in sub-centimeter segments of highly nonlinear tellurite PCFs,” Opt. Express 16, 7161–7168 (2008). [CrossRef]

,14

14. G. Qin, X. Yan, C. Kito, M. Liao, T. Suzuki, A. Mori, and Y. Ohishi, “Highly nonlinear tellurite microstructured fibers for broadband wavelength conversion and flattened supercontinuum generation,” J. Appl. Phys. 107, 043108 (2010). [CrossRef]

] and fluoride [15

15. Z. Chen, A. J. Taylor, and A. Efimov, “Coherent mid-infrared broadband continuum generation in non-uniform ZBLAN fiber taper,” Opt. Express 17, 5852–5860 (2009). [CrossRef]

,16

16. O. P. Kulkarni, V. V. Alexander, M. Kumar, M. J. Freeman, M. N. Islam, F. L. Terry Jr., M. Neelakandan, and A. Chan, “Supercontinuum generation from ∼1.9 to 4.5 μm in ZBLAN fiber with high average power generation beyond 3.8 μm using a thulium-doped fiber amplifier,” J. Opt. Soc. Am. B 28, 2486–2498 (2011). [CrossRef]

] fibers).

The use of fibers in nonlinear optical applications offers obvious advantages in increasing the nonlinear interaction length, obviating the need for optical alignment, in addition to mechanical stability, which are critical advantages in harsh or unstable environments. Nevertheless, well-known difficulties in processing ChGs hamper their utilization in the fiber form factor. Indeed, current fabrication approaches face challenges in controlling the core diameter, the core-to-cladding diameter ratio and index contrast, and the fiber outer diameter. Despite the decades-long development of ChG fibers [17

17. J. S. Sanghera, L. B. Shaw, and I. D. Aggarwal, “Applications of chalcogenide glass optical fibers,” C.R. Chim. 5, 873–883 (2002). [CrossRef]

], harnessing their high optical nonlinearity has been curtailed by their poor power-handling capabilities [18

18. C. Florea, L. Busse, J. Sanghera, B. Shaw, and I. R. Aggarwal, “A simple phenomenological study of photodarkening in As2S3 glasses,” Opt. Mater. 34, 1389–1393 (2012). [CrossRef]

,19

19. I. V. Fekeshgazi, K. V. Mai, N. I. Matelesko, V. M. Mitsa, and E. I. Borkach, “Structural transformations and optical properties of As2S3 chalcogenide glasses,” Semiconductors 39, 951–954 (2005). [CrossRef]

] and large normal group velocity dispersion (GVD) [20

20. R. E. Slusher, G. Lenz, J. Hodelin, J. Sanghera, L. B. Shaw, and I. D. Aggarwal, “Large Raman gain and nonlinear phase shifts in high-purity As2Se3 chalcogenide fibers,” J. Opt. Soc. Am. B 21, 1146–1155 (2004). [CrossRef]

,21

21. S. Shabahang, G. Tao, J. J. Kaufman, and A. F. Abouraddy, “Dispersion characterization of chalcogenide bulk glass, composite fibers, and robust nanotapers,” J. Opt. Soc. Am. B 30, 2498–2506 (2013). [CrossRef]

]. To overcome these obstacles in ChG fibers, novel approaches are required, particularly to achieve broadband SCG. For example, cascaded Raman frequency shifting [12

12. R. R. Gattass, L. B. Shaw, V. Nguyen, P. Pureza, I. D. Aggarwal, and J. S. Sanghera, “All-fiber chalcogenide-based mid-infrared supercontinuum source,” Opt. Fiber Technol. 18, 345–348 (2012). [CrossRef]

,22

22. M. Duhant, W. Renard, G. Canat, T. N. Nguyen, F. Smektala, J. Troles, Q. Coulombier, P. Toupin, L. Brilland, P. Bourdon, and G. Renversez, “Fourth-order cascaded Raman shift in AsSe chalcogenide suspended-core fiber pumped at 2 μm,” Opt. Lett. 36, 2859–2861 (2011). [CrossRef]

,23

23. L. B. Shaw, R. R. Gattass, J. S. Sanghera, and I. D. Aggarwal, “All-fiber mid-IR supercontinuum source from 1.5 to 5 μm,” Proc. SPIE 7914, 79140P (2011). [CrossRef]

] has been used in conjunction with long pump pulses (tens of picoseconds to nanoseconds) to produce spectral broadening via stimulated Raman scattering. While the long pump pulses help reduce the deleterious impact of large GVD, the generated supercontinua are expected to be incoherent [24

24. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78, 1135–1184 (2006). [CrossRef]

].

To date, three strategies have been explored to overcome the high ChG material GVD by balancing it with counteracting waveguide GVD. The first strategy relies on high-index-contrast composite fibers, such as ChG/silica step-index fibers [25

25. N. Granzow, S. Stark, M. Schmidt, A. Tverjanovich, A. L. Wondraczek, and P. Russell, “Supercontinuum generation in chalcogenide-silica step-index fibers,” Opt. Express 19, 21003–21010 (2011). [CrossRef]

], ChG/polymethyl methacrylate (PMMA) hybrid microwires [26

26. R. Ahmad and M. Rochette, “High efficiency and ultra broadband optical parametric four-wave mixing in chalcogenide-PMMA hybrid microwires,” Opt. Express 20, 9572–9580 (2012). [CrossRef]

,27

27. C. Baker and M. Rochette, “High nonlinearity and single-mode transmission in tapered multimode As2Se3-PMMA fibers,” IEEE Photon. J. 4, 960–969 (2012). [CrossRef]

], and ChG-core/tellurite-cladding microstructured fibers [28

28. C. Chaudhari, M. Liao, T. Suzuki, and Y. Ohishi, “Chalcogenide core tellurite cladding composite microstructured fiber for nonlinear applications,” J. Lightwave Technol. 30, 2069–2076 (2012). [CrossRef]

]. The new material incorporated with the ChG in such heterostructures typically sets a limitation; for example, the high material loss of silica glass and PMMA at long wavelengths is a drawback in MIR applications. The second strategy makes use of dispersion-engineered photonic crystal fibers (PCFs) [11

11. L. B. Shaw, P. A. Thielen, F. H. Kung, V. Q. Nguyen, J. Sanghera, and I. Aggarwal, “IR supercontinuum generation in As–Se photonic crystal fiber,” in Proceedings of Advanced Solid State Photonics (2005), paper TuC5.

,29

29. L. Brilland, F. Smektala, G. Renversez, T. Chartier, J. Troles, T. N. Nguyen, N. Traynor, and A. Monteville, “Fabrication of complex structures of holey fibers in chalcogenide glass,” Opt. Express 14, 1280–1285 (2006). [CrossRef]

,30

30. W. Gao, M. Liao, X. Yan, C. Kito, T. Kohoutek, T. Suzuki, M. E. Amraoui, J. C. Jules, G. Gadret, F. Désévédavy, F. Smektala, and Y. Ohishi, “Visible light generation and its influence on supercontinuum in chalcogenide As2S3 microstructured optical fiber,” Appl. Phys. Express 4, 102601 (2011). [CrossRef]

] and suspended-core fibers [31

31. M. El-Amraoui, G. Gadret, J. C. Jules, J. Fatome, C. Fortier, F. Désévédavy, I. Skripatchev, Y. Messaddeq, J. Troles, L. Brilland, W. Gao, T. Suzuki, Y. Ohishi, and F. Smektala, “Microstructured chalcogenide optical fibers from As2S3 glass: towards new IR broadband sources,” Opt. Express 18, 26655–26665 (2010). [CrossRef]

34

34. M. Liao, X. Yan, G. Qin, C. Chaudhari, T. Suzuki, and Y. Ohishi, “Controlling the chromatic dispersion of soft glass highly nonlinear fiber through complex microstructure,” J. Non-Cryst. Solids 356, 2613–2617 (2010). [CrossRef]

], but these technologies have not yet reached the level of maturity of their silica-PCF counterparts. A third strategy utilizes bare ChG fiber tapers for dispersion engineering through an appropriate tapering ratio, while ensuring broadband single-mode guidance to produce SCG with a high-quality spatial profile [35

35. E. C. Mägi, L. B. Fu, H. C. Nguyen, M. R. E. Lamont, D. I. Yeom, and B. J. Eggleton, “Enhanced Kerr nonlinearity in sub-wavelength diameter As2Se3 chalcogenide fiber tapers,” Opt. Express 15, 10324–10329 (2007). [CrossRef]

,36

36. D. D. Hudson, S. A. Dekker, E. C. Mägi, A. C. Judge, S. D. Jackson, E. Li, J. S. Sanghera, L. B. Shaw, I. D. Aggarwal, and B. J. Eggleton, “Octave spanning supercontinuum in an As2S3 taper using ultralow pump pulse energy,” Opt. Lett. 36, 1122–1124 (2011). [CrossRef]

]. Indeed, As2Se3 tapers with a 1.2 μm waist diameter demonstrated an enhanced nonlinearity of 62,000 times that of a standard silica single-mode fiber [35

35. E. C. Mägi, L. B. Fu, H. C. Nguyen, M. R. E. Lamont, D. I. Yeom, and B. J. Eggleton, “Enhanced Kerr nonlinearity in sub-wavelength diameter As2Se3 chalcogenide fiber tapers,” Opt. Express 15, 10324–10329 (2007). [CrossRef]

] (see also Refs. [36

36. D. D. Hudson, S. A. Dekker, E. C. Mägi, A. C. Judge, S. D. Jackson, E. Li, J. S. Sanghera, L. B. Shaw, I. D. Aggarwal, and B. J. Eggleton, “Octave spanning supercontinuum in an As2S3 taper using ultralow pump pulse energy,” Opt. Lett. 36, 1122–1124 (2011). [CrossRef]

39

39. F. Luan, J. Van Erps, M. D. Pelusi, E. Mägi, T. Iredale, H. Thienpont, and B. J. Eggleton, “High-resolution optical sampling of 640 Gbit/s data using dispersion-engineered chalcogenide photonic wire,” Electron. Lett. 46, 231–232 (2010). [CrossRef]

]). While silica tapers have long been a source for SCG [40

40. T. A. Birks, W. J. Wadsworth, and P. St. J. Russell, “Supercontinuum generation in tapered fibers,” Opt. Lett. 25, 1415–1417 (2000). [CrossRef]

], bare ChG tapers are extremely difficult to handle due to the inferior mechanical properties of ChGs compared to silica, which limits the utility of this approach despite its promise [36

36. D. D. Hudson, S. A. Dekker, E. C. Mägi, A. C. Judge, S. D. Jackson, E. Li, J. S. Sanghera, L. B. Shaw, I. D. Aggarwal, and B. J. Eggleton, “Octave spanning supercontinuum in an As2S3 taper using ultralow pump pulse energy,” Opt. Lett. 36, 1122–1124 (2011). [CrossRef]

,41

41. A. Marandi, C. W. Rudy, V. G. Plotnichenko, E. M. Dianov, K. L. Vodopyanov, and R. L. Byer, “Mid-infrared supercontinuum generation in tapered chalcogenide fiber for producing octave-spanning frequency comb around 3 μm,” Opt. Express 20, 24218–24225 (2012). [CrossRef]

,42

42. C. W. Rudy, A. Marandi, K. L. Vodopyanov, and R. L. Byer, “Octave-spanning supercontinuum generation in in situ tapered As2S3 fiber pumped by a thulium-doped fiber laser,” Opt. Lett. 38, 2865–2868 (2013). [CrossRef]

].

Recently, we developed a new extrusion-based technique for fabricating hybrid ChG/polymer fiber preforms that enable drawing extended lengths of robust ChG fibers that incorporate a thick built-in protective thermoplastic polymer jacket that is integral to the fiber structure [43

43. G. Tao, S. Shabahang, E.-H. Banaei, J. J. Kaufman, and A. F. Abouraddy, “Multimaterial preform coextrusion for robust chalcogenide optical fibers and tapers,” Opt. Lett. 37, 2751–2753 (2012). [CrossRef]

]. This fabrication strategy resolves the traditional concerns of the mechanical fragility of ChGs without compromising their optical performance. In a hybrid fiber produced using this process, the optical properties are dictated by the ChG index-guiding structure, while the mechanical robustness derives from a millimeter-diameter built-in polymer jacket. In contrast to most alternative approaches, this process offers flexibility in choosing the geometric parameters (such as the core and cladding diameters) and gives access to a wide range of core and cladding ChG combinations and, thereby, provides control over their index contrast. Uniquely, the thermal compatibility between the ChG and the polymer allows for fiber tapering without first removing the polymer jacket, resulting in robust tapers that are easily handled and manipulated. Concomitantly, using ChGs jointly with a high core-to-cladding index contrast leads to strong confinement of the optical mode to the taper core [43

43. G. Tao, S. Shabahang, E.-H. Banaei, J. J. Kaufman, and A. F. Abouraddy, “Multimaterial preform coextrusion for robust chalcogenide optical fibers and tapers,” Opt. Lett. 37, 2751–2753 (2012). [CrossRef]

], thereby enhancing the nonlinearity and enabling control over GVD. Furthermore, these nanotapers obviate the need for in situ tapering of ChG fibers [41

41. A. Marandi, C. W. Rudy, V. G. Plotnichenko, E. M. Dianov, K. L. Vodopyanov, and R. L. Byer, “Mid-infrared supercontinuum generation in tapered chalcogenide fiber for producing octave-spanning frequency comb around 3 μm,” Opt. Express 20, 24218–24225 (2012). [CrossRef]

,42

42. C. W. Rudy, A. Marandi, K. L. Vodopyanov, and R. L. Byer, “Octave-spanning supercontinuum generation in in situ tapered As2S3 fiber pumped by a thulium-doped fiber laser,” Opt. Lett. 38, 2865–2868 (2013). [CrossRef]

] and the complications associated with precarious handling of traditional bare ChG tapers. We have recently reported the first observations of octave-spanning infrared SCG in such robust all-solid nanotapers with strong field confinement [44

44. S. Shabahang, M. P. Marquez, G. Tao, M. U. Piracha, D. Nguyen, P. J. Delfyett, and A. F. Abouraddy, “Octave-spanning infrared supercontinuum generation in robust chalcogenide nanotapers using picosecond pulses,” Opt. Lett. 37, 4639–4641 (2012). [CrossRef]

] and have provided a detailed investigation of their linear properties and GVD characteristics [21

21. S. Shabahang, G. Tao, J. J. Kaufman, and A. F. Abouraddy, “Dispersion characterization of chalcogenide bulk glass, composite fibers, and robust nanotapers,” J. Opt. Soc. Am. B 30, 2498–2506 (2013). [CrossRef]

].

2. NONLINEAR CHARACTERIZATION OF BULK CHALCOGENIDES

We start by characterizing the nonlinear characteristics of the ChGs of interest in bulk form. The ChGs used in fabricating the fibers and nanotapers investigated here were prepared from commercial glass (Amorphous Materials, Inc.) [43

43. G. Tao, S. Shabahang, E.-H. Banaei, J. J. Kaufman, and A. F. Abouraddy, “Multimaterial preform coextrusion for robust chalcogenide optical fibers and tapers,” Opt. Lett. 37, 2751–2753 (2012). [CrossRef]

]. We produced 1-cm-diameter disk-shaped samples of three glasses: As2Se1.5S1.5, As2Se3, and As2S. 1.5-mm-thick samples were used to measure the spectral transmittance and nonlinear refractive index n2, while 2.5-cm-long cylindrical rods were used to measure the GVD parameter β2 and the linear refractive index n (at λ=1.55μm). The sample facets were all polished to submicrometer surface roughness. Table 1 summarizes the measured linear characteristics (see also Refs. [21

21. S. Shabahang, G. Tao, J. J. Kaufman, and A. F. Abouraddy, “Dispersion characterization of chalcogenide bulk glass, composite fibers, and robust nanotapers,” J. Opt. Soc. Am. B 30, 2498–2506 (2013). [CrossRef]

,43

43. G. Tao, S. Shabahang, E.-H. Banaei, J. J. Kaufman, and A. F. Abouraddy, “Multimaterial preform coextrusion for robust chalcogenide optical fibers and tapers,” Opt. Lett. 37, 2751–2753 (2012). [CrossRef]

]).

Table 1. Linear Characterization of Bulk ChGs Used in Fiber Fabricationa

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The nonlinear refractive indices n2 of the ChGs were measured at λ=1.55μm by the standard Z-scan technique [45

45. M. Sheik-Bahae, A. A. Said, T. H. Wei, D. J. Hagan, and E. W. Van Stryland, “Sensitive measurement of optical nonlinearities using a single beam,” IEEE J. Quantum Electron. 26, 760–769 (1990). [CrossRef]

] using 100fs (FWHM) pulses generated by an optical parametric generator/amplifier (OPA/OPG, TOPAS-C Light Conversion Ltd.); see Appendix A for details. The closed- and open-aperture Z-scan data (along with fitted curves) are shown in Figs. 1(a) and 1(b), respectively, for the As2Se1.5S1.5 sample. After fitting the data, n2 was found for As2Se3 to be (5.2±1.0)×1014cm2/W, for As2Se1.5S1.5 to be (4.6±0.9)×1014cm2/W, and for As2S3 to be (1.6±0.9)×1014cm2/W. We note that the measurements for the As2Se1.5S1.5 sample indicate three-photon absorption (3PA) in the open-aperture signal with a 3PA coefficient of (5.5±2.5)×102cm3/GW2 [Fig. 1(b)]. This measurement is consistent with the result 8.7×102cm3/GW2 calculated from a two-parabolic-band model (a more complete analysis is provided in [46

46. P. D. Olszak, C. M. Cirloganu, S. Webster, L. A. Padilha, T. R. Ensley, H. Hu, G. Nootz, D. J. Hagan, and E. W. Van Stryland, CREOL, Orlando, Florida, 32816 USA, are preparing a manuscript to be called “Three-photon absorption in direct-gap semiconductors.”

]) assuming the bandgap is 1.74eV, which is based on the linear transmittance spectrum of this ChG composition [21

21. S. Shabahang, G. Tao, J. J. Kaufman, and A. F. Abouraddy, “Dispersion characterization of chalcogenide bulk glass, composite fibers, and robust nanotapers,” J. Opt. Soc. Am. B 30, 2498–2506 (2013). [CrossRef]

]. For the other two samples, no nonlinear absorption was observed for the irradiances used and two-photon absorption is not expected at this wavelength.

Fig. 1. (a) Closed-aperture and (b) open-aperture (three-photon absorption here) Z-scan results for a As2Se1.5S1.5 sample at different incident femtosecond-pulse energies (see text and Appendix A for details). The dots are the measured values and the solid lines are the fitted curves. The inset in (b) is a photograph of the As2Se1.5S1.5 sample.

3. NONLINEAR CHARACTERIZATION OF ROBUST COMPOSITE CHG NANOTAPERS

A. Nanotaper Samples

Fig. 2. (a) Scanning electron microscope (SEM) micrograph of the cross section of the robust, composite ChG fiber used to prepare the nanotaper samples. P: PES polymer jacket. (b) A higher-magnification SEM micrograph of the fiber cross section highlighting the ChG core/cladding structure. (c) A typical robust nanotaper with minimum core diameter at the axial midpoint of dcmin=500nm maintained over 20% of its 50-mm length.

Nanotapers are produced using a home-built tapering setup [48

48. S. Shabahang, J. J. Kaufman, D. S. Deng, and A. F. Abouraddy, “Observation of the Plateau–Rayleigh capillary instability in multi-material optical fibers,” Appl. Phys. Lett. 99, 161909 (2011). [CrossRef]

]. Since the polymer (PES) is thermally compatible with the ChGs, the composite fiber may be tapered without removing the polymer jacket. The axial profile of the tapers can be controlled by adjusting the tapering speed, length, and temperature [21

21. S. Shabahang, G. Tao, J. J. Kaufman, and A. F. Abouraddy, “Dispersion characterization of chalcogenide bulk glass, composite fibers, and robust nanotapers,” J. Opt. Soc. Am. B 30, 2498–2506 (2013). [CrossRef]

]. Utilizing this methodology, we have produced tapers with minimum core diameters at the taper axial midpoint dcmin ranging from a few micrometers to <100nm, and lengths ranging from a few millimeters to tens of centimeters (while avoiding potential fluid-instability-driven breakup mechanisms [48

48. S. Shabahang, J. J. Kaufman, D. S. Deng, and A. F. Abouraddy, “Observation of the Plateau–Rayleigh capillary instability in multi-material optical fibers,” Appl. Phys. Lett. 99, 161909 (2011). [CrossRef]

,49

49. J. J. Kaufman, G. Tao, S. Shabahang, E.-H. Banaei, D. S. Deng, X. Liang, S. G. Johnson, Y. Fink, and A. F. Abouraddy, “Structured spheres generated by an in-fibre fluid instability,” Nature 487, 463–467 (2012). [CrossRef]

]).

We plot the measured longitudinal profiles of the four nanotaper samples used in our measurements in Fig. 3(a). Other relevant parameters of the nanotaper Samples 1 through 4, such as the total length L, the minimum core diameter at the nanotaper axial midpoint dcmin, and the optical transmission using CW laser light at λ=1.55μm (raw transmission, which includes 21.6% Fresnel reflection at each facet) are listed in Table 2. Although the short transition regions in the samples between the tapered and nontapered sections increase the optical losses, they nevertheless reduce the energy transfer from the cladding modes back into the core modes at the end of the tapered sections [51

51. G. Brambilla, F. Xu, P. Horak, Y. Jung, F. Koizumi, N. P. Sessions, E. Koukharenko, X. Feng, G. S. Murugan, J. S. Wilkinson, and D. J. Richardson, “Optical fiber nanowires and microwires: fabrication and applications,” Adv. Opt. Photon. 1, 107–161 (2009). [CrossRef]

].

Fig. 3. (a) Longitudinal core diameter dc profiles of the four samples used in the experiments; z is taken along the nanotaper axis. (dcmin, L) of Samples through 4 are: (580 nm, 42 mm), (400 nm, 25 mm), (375 nm, 46 mm), and (250 nm, 68 mm), respectively. See also Table 2. (b) The total GVD parameter β2 as a function dc calculated at λ=1.55 and 2 μm. (c) The blue curve (left vertical axis) shows the diameter of the fundamental mode dm at λ=1.55μm, which indicates the optical-mode confinement when compared to dc, and the green curve (right vertical axis) is the nonlinear coefficient γ of the fiber as a function of core diameter dc, calculated for the fundamental mode at λ=1.55μm. The inset shows the region of the curve of γ that is encircled (dashed circle) and highlights the non-monotonic relationship between dm and dc at submicrometer core diameters (250nm<dc<1μm). The two-dashed vertical lines in the inset correspond to dc=600nm (where dm/dc exceeds 1) and dc=450nm (where confinement by the polymer jacket starts to dominate). See text for details.

Table 2. Nanotaper Sample Parametersa

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To set the stage for our nonlinear optical characterization experiments, we plot two optical parameters that change with core diameter dc, and thus vary axially along the nanotaper: total GVD at λ=1.55μm and 2 μm [Fig. 3(b)], and the nonlinear parameter γ=(2πn2/λAeff) [52

52. G. P. Agrawal, Nonlinear Fiber Optics (Academic, 2001).

] at λ=1.55μm [Fig. 3(c)]; here Aeff is the effective core area (see Ref. [52

52. G. P. Agrawal, Nonlinear Fiber Optics (Academic, 2001).

]). The total GVD is the sum of the measured material GVD and the calculated waveguide GVD [21

21. S. Shabahang, G. Tao, J. J. Kaufman, and A. F. Abouraddy, “Dispersion characterization of chalcogenide bulk glass, composite fibers, and robust nanotapers,” J. Opt. Soc. Am. B 30, 2498–2506 (2013). [CrossRef]

]. Note that the GVD changes rapidly with dc at the nanoscale due to strong field confinement effects (resulting from the large core/cladding index contrast) [21

21. S. Shabahang, G. Tao, J. J. Kaufman, and A. F. Abouraddy, “Dispersion characterization of chalcogenide bulk glass, composite fibers, and robust nanotapers,” J. Opt. Soc. Am. B 30, 2498–2506 (2013). [CrossRef]

]. We plot β2 also at λ=2μm to highlight the change in β2 with wavelength and also in light of the current availability of high-power Tm-doped fiber lasers [53

53. G. Imeshev and M. E. Fermann, “230-kW peak power femtosecond pulses from a high power tunable source based on amplification in Tm-doped fiber,” Opt. Express 13, 7424–7431 (2005). [CrossRef]

56

56. F. Haxsen, D. Wandt, U. Morgner, J. Neumann, and D. Kracht, “Pulse characteristics of a passively mode-locked thulium fiber laser with positive and negative cavity dispersion,” Opt. Express 18, 18981–18988 (2010). [CrossRef]

], which are potentially useful as pumps for SCG.

B. Nonlinear Characterization Methodology

We now proceed to describe the results of the nonlinear characterization experiments carried out on our nanotapers. Two classes of experiments were performed. In the first, we obtained an independent measurement of n2 from observations of spectral broadening resulting from SPM using low-peak-power picosecond and femtosecond laser pulses. In the second class of experiments, we used higher peak-power femtosecond pulses to observe SCG extending over one octave of bandwidth. We then employed a computational model that utilizes parameters extracted from our linear and nonlinear measurements to validate the spectral broadening observed.

Both classes of experiments (SPM and SCG) were carried out using the setup shown in Fig 4. We used fiber-coupled lasers at λ=1.55μm and the collimated beam was coupled in and then out of the 10-μm-diameter core of the nontapered ends of the nanotaper samples [Fig. 2] using a pair of aspheric lenses having 6.2-mm focal length. An infrared camera (7290A Micron-Viewer) was used to monitor the beam profile and optimize the coupling into the core. The output spectrum was measured with 0.1-nm spectral resolution using two optical spectrum analyzers (OSAs) to cover the 1–2 μm spectral range: Advantest Q8381A (up to λ=1.7μm) and Yokogawa AQ6375 (beyond λ=1.7μm). Two laser sources at λ=1.55μm were used. The first is a femtosecond passively mode-locked fiber laser (Calmar, FPL-M2CFF) producing pulses with a 400-fs FWHM pulsewidth at a 20-MHz repetition rate and a 1.7-mW average power (corresponding to a maximum peak power of 212 W). The second is a passively mode-locked erbium-doped fiber laser (PriTel, PFL-10000) producing picosecond pulses with a 10-ps FWHM pulses at 5 MHz repetition rate and 4.5 mW average power (corresponding to a maximum peak power of 90 W).

Fig. 4. Experimental setup for SPM and SCG characterization. L, aspheric lens; FC, fiber collimator; FM, flip mirror; SMF, single-mode fiber. Two identical lenses L are used at the nanotaper sample input and output for coupling.

C. Self-Phase Modulation Measurements

The goal of this experiment is to estimate n2 of the ChGs in the nanotaper form-factor and to confirm and complement the values obtained in bulk ChG using Z-scan measurements. We extract n2 from the total nonlinear phase shift [52

52. G. P. Agrawal, Nonlinear Fiber Optics (Academic, 2001).

] of an optical pulse traversing the nanotapers as estimated from the spectral broadening.

First, coupling at the input 4.5 mW average-power picosecond pulses (90 W peak power) to nanotaper Sample 4 [Fig. 5(a)] and 1.7 mW average-power femtosecond pulses (212 W peak-power) to nanotaper Sample 2 [Fig. 5(b)] both led to the development of three distinct spectral peaks. Varying the input power using a variable attenuator leads to a gradual evolution of the output spectrum from the single-peaked input to the three-peaked output spectra shown in Fig. 5.

Fig. 5. SPM characterization results obtained in two different experiments: (a) picosecond pulses launched into nanotaper Sample 4 and (b) femtosecond pulses launched into nanotaper Sample 2. The power levels are the input peak powers for each sample without corrections for Fresnel reflection from the sample facets.

To extract an estimate of n2 from these spectral broadening experiments we compared the spectra with simulated spectra obtained by solving the generalized nonlinear Schrödinger equation (GNLSE) using the symmetrized split‐step method [52

52. G. P. Agrawal, Nonlinear Fiber Optics (Academic, 2001).

] taking into account the axial variation in the optical mode, GVD, and γ as given in Fig. 3; see Appendix B for details. In estimating the power coupled into the fundamental mode at the nanotaper waist, where most of the nonlinear phase accumulates, several factors must be accounted for: input coupling efficiency, Fresnel reflection at the facets, linear fiber loss, tapering loss, scattering, and potentially nonlinear absorption. The measured input and output power levels after accounting for Fresnel reflection-place upper and lower bounds on the power at the nanotaper waist. For each input power level, we carried out the simulations while scanning the resultant power at the nanotaper waist between these two bounds and also the values for n2 to optimize the matching between the measured and simulated spectra. The simulated spectra thus obtained are in good agreement with the experimental results in both the picosecond and femtosecond pulse regimes [Figs. 6(a) and 6(b)]. The n2 values obtained from the picosecond and femtosecond SPM measurements are (3.75±1.45)×1014cm2/W and (3.15±0.85)×1014cm2/W, respectively. The values are slightly lower than the n2 value obtained in bulk As2Se1.5S1.5 given above that was measured via the Z-scan technique. This result is expected since the n2 values obtained from the nanotapers combine the impact of both the core and the cladding ChGs (the cladding has lower n2 than the core).

Fig. 6. SPM simulation results for the two different experiments shown in Fig. 5: (a) picosecond pulses launched into Sample 4 and (b) femtosecond pulses launched into Sample 2. The power levels are the estimated peak powers in the tapered section of each sample after correcting for Fresnel reflections at the sample facets and nanotaper losses (see text for details).

D. Supercontinuum Generation Measurements

Experiments were performed to demonstrate the potential of such robust multimaterial nanotapers for efficient SCG using femtosecond pulses. At relatively low power levels (peak power 100W, corresponding to 40 pJ of energy per pulse), we achieve hundreds of nanometers of spectral broadening in these nanotapers [Fig. 7(a)]. Moreover, the agreement between the measured supercontinua and their simulated counterparts [Fig. 7(b)] that were obtained using the measured sample parameters further confirm the validity and consistency of our computational model [Appendix B].

Fig. 7. (a) Measured and (b) simulated supercontinua for Sample 1, Sample 3, and Sample 4 when femtosecond pulses are launched. The input pulse parameters are as follows: 4.5 mW average power, 20 MHz repetition rate, and 400 fs FWHM pulsewidth. The peak power levels in the simulated spectra are estimated at 125, 100, and 106 W for Sample 1, Sample 3, and Sample 4, respectively.

The femtosecond laser was coupled to Sample 1, Sample 3, and Sample 4; the measured output spectra are shown in Fig. 7(a). The supercontinuum produced by Sample 1 spans 1.3–1.75 μm, while that produced by Sample 3 was slightly broader to cover 1.2–1.9 μm, which we attribute mainly to its lower GVD compared to Sample 1 [see Figs. 3(a) and 3(b)]. The broadest spectrum was achieved by Sample 4, which has the longest tapered section and the smallest dcmin, resulting in one octave of supercontinuum bandwidth, 1–2 μm. Some slow variations in power and drift occurring over time scales of the order of a few minutes were observed in the spectra during the measurements. We hypothesize that these fluctuations originate from a combination of thermal effects and multimodal interactions in the samples. In each case, after adjusting the input lens to slightly modify the coupling, the spectrum was immediately retrieved. We expect that further optimization of the nanotaper profiles will lead to enhanced spectral stability.

4. CONCLUSION

In conclusion, we have presented the results of nonlinear characterization of ChG bulk samples (via the Z-scan technique) and of robust composite ChG nanotapers (via spectral broadening resulting from SPM). These nanotapers were prepared from step-index ChG fibers produced from the bulk ChGs we investigated. The fibers have a unique structure that makes them particularly apt for nonlinear MIR applications. Specifically, a thick built-in polymer jacket is provided to the ChG core/cladding, which vastly improves the fiber mechanical properties over conventional bare ChG fibers. The thermal compatibility of the polymer and the ChGs allowed us to prepare robust nanotapers without removing the polymer. Using picosecond and femtosecond pulses at λ=1.55μm launched into the nanotaper samples, we find good agreement between the nonlinear refractive index estimated from SPM observations and those estimated from Z-scan measurements of the bulk ChGs. The large core-to-cladding index contrast leads to strong mode confinement, enhances the optical nonlinearities in the nanotapers, and enables dispersion engineering leading to a full octave 1–2 μm SCG using low-peak-power femtosecond pulses. The results were compared to a computational model that makes use of optical parameters extracted from the samples, and good agreement between the measurements and the simulations was obtained. Our results indicate that such novel robust, multimaterial ChG nanotapers offer a useful platform for MIR SCG and infrared nonlinear fiber optics in general.

APPENDIX A: Z-SCAN MEASUREMENT

The nonlinear refractive indices n2 of the ChGs we make use of here were measured at λ=1.55μm by the Z-scan technique [45

45. M. Sheik-Bahae, A. A. Said, T. H. Wei, D. J. Hagan, and E. W. Van Stryland, “Sensitive measurement of optical nonlinearities using a single beam,” IEEE J. Quantum Electron. 26, 760–769 (1990). [CrossRef]

] using 100fs (FWHM) pulses generated by an OPA/OPG (Light Conversion Ltd. model TOPAS-C) pumped by a regenerative Ti:sapphire amplifier (Clark-MXR CPA-2010) operating at a 1 kHz repetition rate, delivering 1mJ pulses at 780 nm. To verify the setup accuracy and calibrate the input-beam spot size and pulse duration, open-aperture Z-scans were performed on the bulk semiconductor GaAs [58

58. W. C. Hurlbut, Y.-S. Lee, K. L. Vodopyanov, P. S. Kuo, and M. M. Fejer, “Multiphoton absorption and nonlinear refraction of GaAs in the mid-infrared,” Opt. Lett. 32, 668–670 (2007). [CrossRef]

], for which the two-photon absorption spectrum is theoretically calculated and experimentally verified (14 cm/GW) [59

59. M. Sheik-Bahae, D. C. Hutchings, D. J. Hagan, and E. W. Van Stryland, “Dispersion of bound electronic nonlinear refraction in solids,” IEEE J. Quantum Electron. 27, 1296–1309 (1991). [CrossRef]

]. Closed-aperture Z-scan measurements are also calibrated against fused silica with documented n2 values (n2quartz=2.6×1016cm2/W) [60

60. D. Milam, “Review and assessment of measured values of the nonlinear refractive-index coefficient of fused silica,” Appl. Opt. 37, 546–550 (1998). [CrossRef]

]. Due to sample inhomogeneity, surface irregularities, or nonparallel facets, a linear transmittance change of 20% was observed using a small spot size and weak probe beam scanning across the sample; therefore, initial Z-scan traces showed a large background. To obtain the Z-scan background, a low-energy Z-scan (0.3–0.5 nJ) was performed, restricted by the sensitivity of the Ge detectors, prior to higher energy Z-scans. All Z-scan curves at higher energy levels are then obtained by dividing the normalized raw data with this low-energy scan to cancel the background. Due to relatively large n2 values of the samples, a small closed-aperture signal, estimated to be about 4%–5% from peak to valley, was already induced but buried in the background signal. Given the fact that only n2 and no higher order nonlinearity is involved in nonlinear refraction, the energy of the “background-free” Z-scan curves [shown in Fig. 1(a)] is estimated by subtracting the actual energy from the background scan energy.

APPENDIX B: SIMULATIONS

Simulations of the nonlinear propagation dynamics of ultrafast pulses along nanotapers with axially varying diameter were carried out using the generalized nonlinear Schrödinger equation (GNLSE) [24

24. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78, 1135–1184 (2006). [CrossRef]

] integrated via the symmetrized split-step method [52

52. G. P. Agrawal, Nonlinear Fiber Optics (Academic, 2001).

]. In our simulations, we included in the GNLSE terms that account for the following effects: (1) wavelength-dependent linear loss, (2) GVD, (3) SPM, (4) Raman response, and (5) self-steepening. Some of these effects depend on the core diameter dc and, hence, vary along the nanotaper axis z. A linearly polarized transform-limited Gaussian pulse (discretized on a lattice of 215 points) is launched in the fundamental mode. To ensure convergence we adaptively changed the axial step size to restrict the nonlinear SPM phase shift to 0.001 rad/step [44

44. S. Shabahang, M. P. Marquez, G. Tao, M. U. Piracha, D. Nguyen, P. J. Delfyett, and A. F. Abouraddy, “Octave-spanning infrared supercontinuum generation in robust chalcogenide nanotapers using picosecond pulses,” Opt. Lett. 37, 4639–4641 (2012). [CrossRef]

]. The parameters used in the simulations were chosen as follows.
  • (1) Linear spectral losses were determined by FTIR transmission measurements [21

    21. S. Shabahang, G. Tao, J. J. Kaufman, and A. F. Abouraddy, “Dispersion characterization of chalcogenide bulk glass, composite fibers, and robust nanotapers,” J. Opt. Soc. Am. B 30, 2498–2506 (2013). [CrossRef]

    ].
  • (2) The GVD used was the sum of the material GVD for the core ChG (measured in bulk [21

    21. S. Shabahang, G. Tao, J. J. Kaufman, and A. F. Abouraddy, “Dispersion characterization of chalcogenide bulk glass, composite fibers, and robust nanotapers,” J. Opt. Soc. Am. B 30, 2498–2506 (2013). [CrossRef]

    ]) and the waveguide GVD calculated in COMSOL using the refractive indices in Table 1 and the axial profiles of the nanotaper samples shown in Fig. 3(a). We assumed that the material GVD varied linearly between λ=1.55 and 2 μm; higher-order dispersion terms were neglected.
  • (3) The SPM term depends on mode confinement (related to dc and the core-to-cladding index contrast) and the nonlinear indices n2 for the core and cladding ChGs. We calculated the axially varying fundamental mode field distribution along the nanotaper using COMSOL (at λ=1.55μm) and used the bulk n2 values reported above. At the input, γ=0.23W1m1 where dc=10μm, which subsequently increases to γ=4.41, 11.72, and 19.95W1m1 at dc=2μm, 1 μm, and 250 nm, respectively.
  • (4) The Raman response function and Raman strength coefficient for As2Se1.5S1.5 are taken to be an average of those for As2Se3 [61

    61. J. Hu, C. R. Menyuk, L. B. Shaw, J. S. Sanghera, and I. D. Aggarwal, “Maximizing the bandwidth of supercontinuum generation in As2Se3 chalcogenide fibers,” Opt. Express 18, 6722–6739 (2010). [CrossRef]

    ] and As2S3 [62

    62. C. Xiong, E. Magi, F. Luan, A. Tuniz, S. Dekker, J. S. Sanghera, L. B. Shaw, I. D. Aggarwal, and B. J. Eggleton, “Characterization of picosecond pulse nonlinear propagation in chalcogenide As2S3 fiber,” Appl. Opt. 48, 5467–5474 (2009). [CrossRef]

    ].
  • (5) The self-steepening is modeled to first order using an optical-shock time constant τs=(1/ωo) [24

    24. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78, 1135–1184 (2006). [CrossRef]

    ], where ωo is the optical frequency corresponding to the central wavelength.

The integration of the parametric four-wave-mixing gain over the length of nanotapers was found not to play a significant role in the spectral broadening, so four-wave mixing was neglected in our calculations.

ACKNOWLEDGMENTS

We thank J. J. Kaufman, O. M. Piracha, D. Nguyen, L. Shah, and M. C. Richardson for assistance and loan of equipment. We are particularly grateful to E. W. Van Stryland and D. J. Hagan for granting us access to their laboratory and to the Z-scan measurement setup, and for valuable comments on the manuscript. This work was supported by the U.S. National Science Foundation (ECCS-1002295).

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46.

P. D. Olszak, C. M. Cirloganu, S. Webster, L. A. Padilha, T. R. Ensley, H. Hu, G. Nootz, D. J. Hagan, and E. W. Van Stryland, CREOL, Orlando, Florida, 32816 USA, are preparing a manuscript to be called “Three-photon absorption in direct-gap semiconductors.”

47.

G. Tao, A. M. Stolyarov, and A. F. Abouraddy, “Multimaterial fibers,” Int. J. Appl. Glass Sci. 3, 349–368 (2012). [CrossRef]

48.

S. Shabahang, J. J. Kaufman, D. S. Deng, and A. F. Abouraddy, “Observation of the Plateau–Rayleigh capillary instability in multi-material optical fibers,” Appl. Phys. Lett. 99, 161909 (2011). [CrossRef]

49.

J. J. Kaufman, G. Tao, S. Shabahang, E.-H. Banaei, D. S. Deng, X. Liang, S. G. Johnson, Y. Fink, and A. F. Abouraddy, “Structured spheres generated by an in-fibre fluid instability,” Nature 487, 463–467 (2012). [CrossRef]

50.

J. J. Kaufman, G. Tao, S. Shabahang, D. S. Deng, Y. Fink, and A. F. Abouraddy, “Thermal drawing of high-density macroscopic arrays of well-ordered sub-5-nm-diameter nanowires,” Nano Lett. 11, 4768–4773 (2011). [CrossRef]

51.

G. Brambilla, F. Xu, P. Horak, Y. Jung, F. Koizumi, N. P. Sessions, E. Koukharenko, X. Feng, G. S. Murugan, J. S. Wilkinson, and D. J. Richardson, “Optical fiber nanowires and microwires: fabrication and applications,” Adv. Opt. Photon. 1, 107–161 (2009). [CrossRef]

52.

G. P. Agrawal, Nonlinear Fiber Optics (Academic, 2001).

53.

G. Imeshev and M. E. Fermann, “230-kW peak power femtosecond pulses from a high power tunable source based on amplification in Tm-doped fiber,” Opt. Express 13, 7424–7431 (2005). [CrossRef]

54.

M. A. Solodyankin, E. D. Obraztsova, A. S. Lobach, A. I. Chernov, A. V. Tausenev, V. I. Konov, and E. M. Dianov, “Mode-locked 1.93 μm thulium fiber laser with a carbon nanotube absorber,” Opt. Lett. 33, 1336–1338 (2008). [CrossRef]

55.

K. Kieu and F. W. Wise, “Soliton thulium-doped fiber laser with carbon nanotube saturable absorber,” IEEE Photon. Technol. Lett. 21, 128–130 (2009). [CrossRef]

56.

F. Haxsen, D. Wandt, U. Morgner, J. Neumann, and D. Kracht, “Pulse characteristics of a passively mode-locked thulium fiber laser with positive and negative cavity dispersion,” Opt. Express 18, 18981–18988 (2010). [CrossRef]

57.

M. Yaman, H. E. Kondakci, and M. Bayindir, “Large and dynamical tuning of a chalcogenide Fabry–Perot cavity mode by temperature modulation,” Opt. Express 18, 3168–3173 (2010). [CrossRef]

58.

W. C. Hurlbut, Y.-S. Lee, K. L. Vodopyanov, P. S. Kuo, and M. M. Fejer, “Multiphoton absorption and nonlinear refraction of GaAs in the mid-infrared,” Opt. Lett. 32, 668–670 (2007). [CrossRef]

59.

M. Sheik-Bahae, D. C. Hutchings, D. J. Hagan, and E. W. Van Stryland, “Dispersion of bound electronic nonlinear refraction in solids,” IEEE J. Quantum Electron. 27, 1296–1309 (1991). [CrossRef]

60.

D. Milam, “Review and assessment of measured values of the nonlinear refractive-index coefficient of fused silica,” Appl. Opt. 37, 546–550 (1998). [CrossRef]

61.

J. Hu, C. R. Menyuk, L. B. Shaw, J. S. Sanghera, and I. D. Aggarwal, “Maximizing the bandwidth of supercontinuum generation in As2Se3 chalcogenide fibers,” Opt. Express 18, 6722–6739 (2010). [CrossRef]

62.

C. Xiong, E. Magi, F. Luan, A. Tuniz, S. Dekker, J. S. Sanghera, L. B. Shaw, I. D. Aggarwal, and B. J. Eggleton, “Characterization of picosecond pulse nonlinear propagation in chalcogenide As2S3 fiber,” Appl. Opt. 48, 5467–5474 (2009). [CrossRef]

OCIS Codes
(190.4370) Nonlinear optics : Nonlinear optics, fibers
(320.6629) Ultrafast optics : Supercontinuum generation

ToC Category:
Ultrafast Optics

History
Original Manuscript: October 3, 2013
Manuscript Accepted: November 19, 2013
Published: February 10, 2014

Virtual Issues
Vol. 9, Iss. 5 Virtual Journal for Biomedical Optics
March 4, 2014 Spotlight on Optics

Citation
Soroush Shabahang, Guangming Tao, Michael P. Marquez, Honghua Hu, Trenton R. Ensley, Peter J. Delfyett, and Ayman F. Abouraddy, "Nonlinear characterization of robust multimaterial chalcogenide nanotapers for infrared supercontinuum generation," J. Opt. Soc. Am. B 31, 450-457 (2014)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=josab-31-3-450


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  46. P. D. Olszak, C. M. Cirloganu, S. Webster, L. A. Padilha, T. R. Ensley, H. Hu, G. Nootz, D. J. Hagan, and E. W. Van Stryland, CREOL, Orlando, Florida, 32816 USA, are preparing a manuscript to be called “Three-photon absorption in direct-gap semiconductors.”
  47. G. Tao, A. M. Stolyarov, and A. F. Abouraddy, “Multimaterial fibers,” Int. J. Appl. Glass Sci. 3, 349–368 (2012). [CrossRef]
  48. S. Shabahang, J. J. Kaufman, D. S. Deng, and A. F. Abouraddy, “Observation of the Plateau–Rayleigh capillary instability in multi-material optical fibers,” Appl. Phys. Lett. 99, 161909 (2011). [CrossRef]
  49. J. J. Kaufman, G. Tao, S. Shabahang, E.-H. Banaei, D. S. Deng, X. Liang, S. G. Johnson, Y. Fink, and A. F. Abouraddy, “Structured spheres generated by an in-fibre fluid instability,” Nature 487, 463–467 (2012). [CrossRef]
  50. J. J. Kaufman, G. Tao, S. Shabahang, D. S. Deng, Y. Fink, and A. F. Abouraddy, “Thermal drawing of high-density macroscopic arrays of well-ordered sub-5-nm-diameter nanowires,” Nano Lett. 11, 4768–4773 (2011). [CrossRef]
  51. G. Brambilla, F. Xu, P. Horak, Y. Jung, F. Koizumi, N. P. Sessions, E. Koukharenko, X. Feng, G. S. Murugan, J. S. Wilkinson, and D. J. Richardson, “Optical fiber nanowires and microwires: fabrication and applications,” Adv. Opt. Photon. 1, 107–161 (2009). [CrossRef]
  52. G. P. Agrawal, Nonlinear Fiber Optics (Academic, 2001).
  53. G. Imeshev and M. E. Fermann, “230-kW peak power femtosecond pulses from a high power tunable source based on amplification in Tm-doped fiber,” Opt. Express 13, 7424–7431 (2005). [CrossRef]
  54. M. A. Solodyankin, E. D. Obraztsova, A. S. Lobach, A. I. Chernov, A. V. Tausenev, V. I. Konov, and E. M. Dianov, “Mode-locked 1.93 μm thulium fiber laser with a carbon nanotube absorber,” Opt. Lett. 33, 1336–1338 (2008). [CrossRef]
  55. K. Kieu and F. W. Wise, “Soliton thulium-doped fiber laser with carbon nanotube saturable absorber,” IEEE Photon. Technol. Lett. 21, 128–130 (2009). [CrossRef]
  56. F. Haxsen, D. Wandt, U. Morgner, J. Neumann, and D. Kracht, “Pulse characteristics of a passively mode-locked thulium fiber laser with positive and negative cavity dispersion,” Opt. Express 18, 18981–18988 (2010). [CrossRef]
  57. M. Yaman, H. E. Kondakci, and M. Bayindir, “Large and dynamical tuning of a chalcogenide Fabry–Perot cavity mode by temperature modulation,” Opt. Express 18, 3168–3173 (2010). [CrossRef]
  58. W. C. Hurlbut, Y.-S. Lee, K. L. Vodopyanov, P. S. Kuo, and M. M. Fejer, “Multiphoton absorption and nonlinear refraction of GaAs in the mid-infrared,” Opt. Lett. 32, 668–670 (2007). [CrossRef]
  59. M. Sheik-Bahae, D. C. Hutchings, D. J. Hagan, and E. W. Van Stryland, “Dispersion of bound electronic nonlinear refraction in solids,” IEEE J. Quantum Electron. 27, 1296–1309 (1991). [CrossRef]
  60. D. Milam, “Review and assessment of measured values of the nonlinear refractive-index coefficient of fused silica,” Appl. Opt. 37, 546–550 (1998). [CrossRef]
  61. J. Hu, C. R. Menyuk, L. B. Shaw, J. S. Sanghera, and I. D. Aggarwal, “Maximizing the bandwidth of supercontinuum generation in As2Se3 chalcogenide fibers,” Opt. Express 18, 6722–6739 (2010). [CrossRef]
  62. C. Xiong, E. Magi, F. Luan, A. Tuniz, S. Dekker, J. S. Sanghera, L. B. Shaw, I. D. Aggarwal, and B. J. Eggleton, “Characterization of picosecond pulse nonlinear propagation in chalcogenide As2S3 fiber,” Appl. Opt. 48, 5467–5474 (2009). [CrossRef]

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