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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 3 — Jan. 31, 2011
  • pp: 1991–1996
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Fabrication of low loss dispersion engineered chalcogenide photonic crystals

Marcel Spurny, Liam O’Faolain, Douglas A. P. Bulla, Barry Luther-Davies, and Thomas F. Krauss  »View Author Affiliations


Optics Express, Vol. 19, Issue 3, pp. 1991-1996 (2011)
http://dx.doi.org/10.1364/OE.19.001991


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Abstract

We demonstrate low loss photonic crystal waveguides in chalcogenide (Ge33As12Se55) glasses. The measured losses are as low as 21dB/cm. We experimentally determine the refractive index of the thin film chalcogenide glass to be n = 2.6 and demonstrate that dispersion engineering can be performed up to a group index of ng = 40 in this relatively low refractive index contrast system.

© 2011 OSA

Introduction

Slow light is an important phenomenon for enhancing nonlinear operations, however, it has only been realized so far in high contrast silicon waveguides using various techniques [8

8. S. A. Schulz, L. O’Faolain, D. M. Beggs, T. P. White, A. Melloni, and T. F. Krauss, “Dispersion engineered slow light in photonic crystals: a comparison,” J. Opt. 12(10), 104004 (2010). [CrossRef]

]. It is not obvious that the same techniques can be applied to lower refractive index systems. We highlight the issue in Fig. 1
Fig. 1 Band diagram for the fundamental mode of a W1 photonic crystal waveguide for silicon (solid curve, n = 3.48) and AMTIR-1 (Ge33As12Se55, n = 2.6, dashed curve). The light cone is outlined as the light grey area which is delimited with the straight dashed line. The dark grey areas outline the useful frequency range that is accessible below the light line and above the cut-off frequency. The corresponding accessible k-range lies between the crossing point of the dispersion curve of the respective material (denoted by the vertical solid lines) and the band edge at k = π/a. It is obvious that the operating range for the lower index contrast chalcogenide system is much smaller than that for the higher contrast silicon system.
, which displays the dispersion curves of the even mode of a photonic crystal W1 waveguide for materials with a refractive index of n≈2.6 (e.g. chalcogenide, dashed), and n≈3.48 (i.e. silicon, solid) with respect to the light line (straight dashed). It is clear that the useful k-space between the light line and the cut-off point for AMTIR-1 is much smaller than that for silicon. The corresponding wavelength ranges are Δλ≈87nm for silicon and Δλ≈28nm for chalcogenide, which clearly limits the performance of the chalcogenides. Firstly, the operation is closer to the band-edge, which can increase linear losses, and secondly, the window for dispersion engineering is much smaller. Despite these considerable limitations, we demonstrate here that dispersion engineering methods can indeed be applied very successfully.

Fabrication and loss measurements

To avoid sagging of the under-etched access taper, a new design for the access waveguides was used. We widened the access waveguides from 3μm to 6μm and added a 50μm long taper region. Figure 3(a)
Fig. 3 (a) Sketch of the taper which allows performing the HF undercut without an additional masking step. The 6μm wide waveguide is tapered down to the waveguide width of 909nm within 50μm. The supporting, not-etched pedestal can be seen. (b) Cleaved facet of the access waveguide after underetching also showing the remaining silica which acts as a supporting pedestal under the access waveguide.
shows a sketch of this taper layout and Fig. 3(b) shows a corresponding cleaved facet. The remaining 2μm wide silica under the waveguide acts as a supporting pedestal. The advantage of this new design is that no etch mask is required for the HF step, thus reducing the number of processing steps and the exposure of the chemically sensitive chalcogenides to resist developer and remover.

To determine the refractive index of the AMTIR-1 layer experimentally, we fabricated a set of 80μm long photonic crystal waveguides with a range of lattice constants, whilst keeping the air hole fill-factor constant. The measured spectra were then matched to 3D MPB simulations, using the refractive index as a fitting parameter. The best match was achieved for a refractive index of n = 2.6 which agrees reasonably well with the literature bulk value of n = 2.54 [9

9. M. Bass, ed., Handbook of Optics II, 2nd ed. (McGraw-Hill, 1994).

]. We then fabricated W1s with a lattice constant of a = 525nm and lengths varying from 95μm to 1095μm with a typical hole diameter of d = 315nm (r/a = 0.3). The high quality of the fabrication process could be assessed from the sharpness of the transitions near cut-off as well as from cut-back measurements which determined the propagation loss (see Fig. 4
Fig. 4 a) Transmission measurements of the chalcogenide W1 photonic crystal waveguides for the determination of the refractive index. b) Losses extracted from cut-back measurements.
). The transmission was found to drop by 35dB over a range of 3 nm near the band-edge suggesting our samples were of high quality (Fig. 4(a)). Cut-back measurements confirmed this and showed the loss, determined from plots of transmission versus length (Fig. 4(b)), was 21dB/cm. Given the relative immaturity of chalcogenide photonic crystals, this compares well with losses of 12dB/cm obtained for a silicon membrane W1 waveguide fabricated with the same lithography system [10

10. T. P. White, L. O’Faolain, J. Li, L. C. Andreani, and T. F. Krauss, “Silica-embedded silicon photonic crystal waveguides,” Opt. Express 16(21), 17076–17081 (2008). [CrossRef] [PubMed]

].

Dispersion engineering in low-index photonic crystal waveguides and experiments

We used the method based on shifting rows of holes to engineer the dispersion curve to produce a slow-light regime as was pioneered in silicon [11

11. J. Li, T. P. White, L. O’Faolain, A. Gomez-Iglesias, and T. F. Krauss, “Systematic design of flat band slow light in photonic crystal waveguides,” Opt. Express 16(9), 6227–6232 (2008). [CrossRef] [PubMed]

] using the group-index-bandwidth-product (GIBP) ng(Δω/ω) as a figure of merit (Fig. 5(a)
Fig. 5 (a) Map of 3D calculations for the group-index-bandwidth-product (as a function of s1 and s2). The color map displays the calculated GIBP whilst the contours map the achievable group index. (b) Calculated group index curves for modified W1 modes for certain values for s1 and s2. The red areas indicate the constant (ng ± 10%) group index regions.
). As expected for a lower refractive index material, the GIBP reaches a somewhat lower value of about 0.25, compared to 0.3 that can be achieved in silicon. Another difference is that the highest achievable group-index is around ng≈40 rather than >100. Both of these observations highlight the reduced degrees of freedom given by the lower refractive index contrast, which also agrees with the smaller operating window observed in Fig. 1.

The GIBP map was used to confirm the experimental design and Fig. 5(b) shows the group-index curves including the four different designs that were used for the fabrication of chalcogenide slow light samples. Figure 6
Fig. 6 Measured group indices (blue) and calculated group indices (green) for different lattice shifts. (a) s1/a = −0.1 s2/a = 0.02, (b) s1/a = −0.1 s2/a = 0.03, (c) s1/a = −0.1 s2/a = 0.05, (d) s1/a = −0.1 s2/a = 0.06.
shows the measured group-index curves for four different designs with a target ng of 20, 30, 35 and 40. The measurements were carried out using Fourier transform spectral interferometry [12

12. A. Gomez-Iglesias, D. O’Brien, L. O’Faolain, A. Miller, and T. F. Krauss, “Direct measurement of the group index of photonic crystal waveguides via Fourier transform spectral interferometry,” Appl. Phys. Lett. 90(26), 261107 (2007). [CrossRef]

]. It is clear that despite the lower refractive index contrast, sizeable group index values can be achieved, e.g. ng≈40 over a 5 nm bandwidth (Fig. 6d), thus highlighting the suitability of the method.

Conclusion

Chalcogenide photonic crystals are a favorable platform for nonlinear optics due to their high nonlinear figure of merit. Due to their lower refractive index and corresponding weaker confinement, it was not obvious whether the same dispersion engineering techniques previously explored in silicon can be used, and whether similar low losses can be achieved. To investigate these issues, we have fabricated dispersion engineered chalcogenide photonic crystal waveguides and demonstrated losses as low as 21dB/cm. In addition, we have shown that the dispersion engineering toolkit can be applied to the chalcogenide system and have demonstrated slow light waveguides with a group index of ng≈40. Given the lower phase index nφ of the waveguide mode, this corresponds to a slowdown factor (S = ng/nφ) of S≈20, which is considerable and highlights the potential of the system for nonlinear applications. To our knowledge, this is the first demonstration of systematic dispersion engineering in relatively low refractive index photonic crystal waveguides, in particular in chalcogenides. Furthermore, we have shown the benefits of using vapour phase HF etching for the fabrication of photonic crystal membranes.

Acknowledgements

The authors would like to thank T. P. White for the helpful discussions on dispersion engineering. Marcel Spurny was supported by the EU FP6 program SPLASH. Support of the Australian Research Council through its Centre of Excellence Program is gratefully acknowledged.

References and links

1.

T. K. Liang, and H. K. Tsang, “Optical Limiting and Raman Amplification in Silicon Waveguides”, in Optical Fiber Communication Conference and Exposition and The National Fiber Optics Engineers Conference, Technical Digest (CD) (Optical Society of America, 2005), paper JWA15.

2.

M. Dinu, F. Quochi, and H. Garcia, “Third-order nonlinearities in silicon at telecom wavelengths,” Appl. Phys. Lett. 82(18), 2954–2956 (2003). [CrossRef]

3.

V. Mizrahi, K. W. Delong, G. I. Stegeman, M. A. Saifi, and M. J. Andrejco, “Two-photon absorption as a limitation to all-optical switching,” Opt. Lett. 14(20), 1140–1142 (1989). [CrossRef] [PubMed]

4.

D. Lezal, J. Pedlikova, and J. Zavadil, “Chalcogenide Glasses for optical and photonics applications,” Chalcogenide Lett. 1, 11–15 (2004).

5.

K. A. Cerqua-Richardson, J. M. McKinley, B. Lawrence, S. Joshi, and A. Villeneuve, “Comparison of nonlinear optical properties of sulfide glasses in bulk and thin film form,” Opt. Mater. 10(2), 155–159 (1998). [CrossRef]

6.

Y. Ruan, M. Kim, Y. Lee, B. Luther-Davies, and A. Rode, “Fabrication of high-Q chalcogenide photonic crystal resonators by e-beam lithography,” Appl. Phys. Lett. 90(7), 071102 (2007). [CrossRef]

7.

K. Suzuki, Y. Hamachi, and T. Baba, “Fabrication and characterization of chalcogenide glass photonic crystal waveguides,” Opt. Express 17(25), 22393–22400 (2009). [CrossRef]

8.

S. A. Schulz, L. O’Faolain, D. M. Beggs, T. P. White, A. Melloni, and T. F. Krauss, “Dispersion engineered slow light in photonic crystals: a comparison,” J. Opt. 12(10), 104004 (2010). [CrossRef]

9.

M. Bass, ed., Handbook of Optics II, 2nd ed. (McGraw-Hill, 1994).

10.

T. P. White, L. O’Faolain, J. Li, L. C. Andreani, and T. F. Krauss, “Silica-embedded silicon photonic crystal waveguides,” Opt. Express 16(21), 17076–17081 (2008). [CrossRef] [PubMed]

11.

J. Li, T. P. White, L. O’Faolain, A. Gomez-Iglesias, and T. F. Krauss, “Systematic design of flat band slow light in photonic crystal waveguides,” Opt. Express 16(9), 6227–6232 (2008). [CrossRef] [PubMed]

12.

A. Gomez-Iglesias, D. O’Brien, L. O’Faolain, A. Miller, and T. F. Krauss, “Direct measurement of the group index of photonic crystal waveguides via Fourier transform spectral interferometry,” Appl. Phys. Lett. 90(26), 261107 (2007). [CrossRef]

OCIS Codes
(190.4360) Nonlinear optics : Nonlinear optics, devices
(220.4000) Optical design and fabrication : Microstructure fabrication
(130.5296) Integrated optics : Photonic crystal waveguides
(230.5298) Optical devices : Photonic crystals

ToC Category:
Photonic Crystals

History
Original Manuscript: December 3, 2010
Revised Manuscript: January 13, 2011
Manuscript Accepted: January 13, 2011
Published: January 19, 2011

Citation
Marcel Spurny, Liam O’Faolain, Douglas A. P. Bulla, Barry Luther-Davies, and Thomas F. Krauss, "Fabrication of low loss dispersion engineered chalcogenide photonic crystals," Opt. Express 19, 1991-1996 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-3-1991


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References

  1. T. K. Liang, and H. K. Tsang, “Optical Limiting and Raman Amplification in Silicon Waveguides”, in Optical Fiber Communication Conference and Exposition and The National Fiber Optics Engineers Conference, Technical Digest (CD) (Optical Society of America, 2005), paper JWA15.
  2. M. Dinu, F. Quochi, and H. Garcia, “Third-order nonlinearities in silicon at telecom wavelengths,” Appl. Phys. Lett. 82(18), 2954–2956 (2003). [CrossRef]
  3. V. Mizrahi, K. W. Delong, G. I. Stegeman, M. A. Saifi, and M. J. Andrejco, “Two-photon absorption as a limitation to all-optical switching,” Opt. Lett. 14(20), 1140–1142 (1989). [CrossRef] [PubMed]
  4. D. Lezal, J. Pedlikova, and J. Zavadil, “Chalcogenide Glasses for optical and photonics applications,” Chalcogenide Lett. 1, 11–15 (2004).
  5. K. A. Cerqua-Richardson, J. M. McKinley, B. Lawrence, S. Joshi, and A. Villeneuve, “Comparison of nonlinear optical properties of sulfide glasses in bulk and thin film form,” Opt. Mater. 10(2), 155–159 (1998). [CrossRef]
  6. Y. Ruan, M. Kim, Y. Lee, B. Luther-Davies, and A. Rode, “Fabrication of high-Q chalcogenide photonic crystal resonators by e-beam lithography,” Appl. Phys. Lett. 90(7), 071102 (2007). [CrossRef]
  7. K. Suzuki, Y. Hamachi, and T. Baba, “Fabrication and characterization of chalcogenide glass photonic crystal waveguides,” Opt. Express 17(25), 22393–22400 (2009). [CrossRef]
  8. S. A. Schulz, L. O’Faolain, D. M. Beggs, T. P. White, A. Melloni, and T. F. Krauss, “Dispersion engineered slow light in photonic crystals: a comparison,” J. Opt. 12(10), 104004 (2010). [CrossRef]
  9. M. Bass, ed., Handbook of Optics II, 2nd ed. (McGraw-Hill, 1994).
  10. T. P. White, L. O’Faolain, J. Li, L. C. Andreani, and T. F. Krauss, “Silica-embedded silicon photonic crystal waveguides,” Opt. Express 16(21), 17076–17081 (2008). [CrossRef] [PubMed]
  11. J. Li, T. P. White, L. O’Faolain, A. Gomez-Iglesias, and T. F. Krauss, “Systematic design of flat band slow light in photonic crystal waveguides,” Opt. Express 16(9), 6227–6232 (2008). [CrossRef] [PubMed]
  12. A. Gomez-Iglesias, D. O’Brien, L. O’Faolain, A. Miller, and T. F. Krauss, “Direct measurement of the group index of photonic crystal waveguides via Fourier transform spectral interferometry,” Appl. Phys. Lett. 90(26), 261107 (2007). [CrossRef]

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