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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 22 — Oct. 29, 2007
  • pp: 14566–14572
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Low-loss high-index-contrast planar waveguides with graded-index cladding layers

Juejun Hu, Ning-Ning Feng, Nathan Carlie, Laeticia Petit, Jianfei Wang, Anu Agarwal, Kathleen Richardson, and Lionel Kimerling  »View Author Affiliations


Optics Express, Vol. 15, Issue 22, pp. 14566-14572 (2007)
http://dx.doi.org/10.1364/OE.15.014566


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Abstract

We experimentally demonstrate, for the first time, propagation loss reduction via graded-index (GRIN) cladding layers in high-index-contrast (HIC) glass waveguides. We show that scattering loss arising from sidewall roughness can be significantly reduced without compromising the high-index-contrast condition, by inserting thin GRIN cladding layers with refractive indices intermediate between the core and topmost cover of a strip waveguide. Loss as low as 1.5 dB/cm is achieved in small core (1.6 μm × 0.35 μm), high-index-contrast (Δn = 1.37) arsenic-based sulfide strip waveguides. This GRIN cladding design is generally applicable to HIC waveguide systems such as Si/SiO2.

© 2007 Optical Society of America

1. Introduction

High-index-contrast (HIC) strip waveguides (typically with a core-cladding index difference Δn > 1) comprise a key component in integrated microphotonics. Compared to their low-index-contrast (LIC) counterparts, these waveguides allow tight bending structures and small device footprint with minimized inter-guide cross talk, both essential features to compact planar photonic integration. Furthermore, the tight confinement and hence small modal volume in HIC waveguides is also critical to the operation of a number of photonic devices, including on-chip light sources [1

01. C. A. Barrios and M. Lipson, “Electrically driven silicon resonant light emitting device based on slot-waveguide,” Opt. Express 13, 10092 (2005). [CrossRef] [PubMed]

] and nonlinear optical devices [2

02. Q. Xu, V. R. Almeida, and M. Lipson, “Demonstration of high Raman gain in a submicrometer-size silicon-on-insulator waveguide,” Opt. Lett. 30, 35–37 (2005). [CrossRef] [PubMed]

]. Despite these apparent advantages, the high loss in HIC waveguides arising from sidewall roughness scattering has been a major limiting factor in HIC waveguide device development. Sidewall roughness scattering is particularly severe for HIC waveguides, since the scattering loss scales with index difference [3

03. T. Barwicz and H. Haus, “Three-dimensional analysis of scattering losses due to sidewall roughness in microphotonic waveguides,” J. Lightwave Technol. 23, 2719–2732 (2005). [CrossRef]

]. LIC waveguides or rib waveguides suffer much less sidewall roughness scattering and typically exhibit lower propagation loss [4

04. M. Webster, R. Pafchek, G. Sukumaran, and T. Koch, “Low-loss quasi-planar ridge waveguides formed on thin silicon-on-insulator,” Appl. Phys. Lett. 87, 231108–231110 (2005). [CrossRef]

] but they lack the aforementioned HIC-device advantages. Thus to make HIC devices viable, an effective solution is to develop surface smoothing techniques to remove sidewall roughness. A number of methods have been demonstrated in different material systems for waveguide surface roughness reduction, including oxidation smoothing [5

05. D. Sparacin, S. Spector, and L. Kimerling, “Silicon Waveguide Sidewall Smoothing by Wet Chemical Oxidation,” J. Lightwave Technol. 23, 2455–2461 (2005). [CrossRef]

], post-fabrication wet etching [6

06. K. Lee, D. Lim, L. Kimerling, J. Shin, and F. Cerrina, “Fabrication of ultralow-loss Si/SiO2 waveguides by roughness reduction,” Opt. Lett. 26, 1888–1890 (2001). [CrossRef]

] and hydrogen annealing [7

07. M. Wu and M. Lee, “Thermal annealing in hydrogen for 3-D profile transformation on silicon-on-insulator and sidewall roughness reduction,” J. Microelectromech. Syst. 15, 338–343 (2006). [CrossRef]

] in Si/SiO2 systems, and reflow in polymer [8

08. C. Chao and L. Guo, “Reduction of surface scattering loss in polymer microrings using thermal-reflow technique,” IEEE Photon. Technol. Lett. 16, 1498–1500 (2004). [CrossRef]

] and glass materials [9

09. J. Hu, V. Tarasov, N. Carlie, R. Sun, L. Petit, A. Agarwal, K. Richardson, and L. Kimerling, “Low-loss integrated planar chalcogenide waveguides for chemical sensing,” Proc. SPIE 6444, 64440N (2007). [CrossRef]

]. However, some of these methods involve high temperature processing and hence pose integration challenges with other planar photonic and electronic devices. To maintain the advantage of planar integration over a wide range of material systems, an efficient loss reduction technique that is universally applicable to all HIC material systems is highly desirable.

Table 1. Range of refractive indices of some glassy alloys for potential GRIN cladding applications.

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2. Graded-index cladding waveguide modal analysis

According to volume current theory [3

03. T. Barwicz and H. Haus, “Three-dimensional analysis of scattering losses due to sidewall roughness in microphotonic waveguides,” J. Lightwave Technol. 23, 2719–2732 (2005). [CrossRef]

], the scattering loss from waveguide sidewalls is proportional to the equivalent polarization current densities on rough surfaces:

Jrough(r)=jωε0δn2(r)Eg(r)
(1)

Here we present a more universal approach using a graded index cladding design. Both index contrast δn [2

02. Q. Xu, V. R. Almeida, and M. Lipson, “Demonstration of high Raman gain in a submicrometer-size silicon-on-insulator waveguide,” Opt. Lett. 30, 35–37 (2005). [CrossRef] [PubMed]

] and surface field amplitude Eg can be reduced simultaneously simply by sandwiching thin layers of intermediate refractive indices between the high-index waveguide core and the low-index topmost cover. Figure 1(a) shows the cross-sectional schematic of an As42S58 (n = 2.37) strip waveguide coated with Ge17Sb12S71 (n = 2.06)/SiO2 (n = 1.46) double GRIN cladding layers. The As42S58 waveguide core sitting on SiO2 under cladding has a width of 0.75 μm and a height of 0.35 μm. Both Ge17Sb12S71 and oxide GRIN cladding layers have a thickness of 35 nm each. A cross-sectional view of the field along x-axis at the waveguide center of the quasi-TE mode is shown in Fig. 1(b), together with the field distribution in the absence of GRIN cladding layers for comparison, both computed using a full-vectorial mode solver with perfectly matched layer (PML) boundary [16

16. N. Feng, G. Zhou, C. Xu, and W. Huang, “Computation of full-vector modes for bending waveguide using cylindrical perfectly matched layers,” IEEE J. Lightwave Technol. 20, 1976–1980 (2002). [CrossRef]

]. It is evident that GRIN cladding layers decrease the field amplitude at the core/cladding interface. In an ideal case the post-etch deposited oxide layer should have negligible surface roughness, and a theoretical analysis using volume current theory taking into account the different radiation efficiency of current sources on GRIN layer interfaces has predicted a quasi-TE mode loss reduction of 55% for the As42S58 waveguide structure shown in Fig. 1(a), compared to waveguides without GRIN cladding layers. It should be noted though, that if the GRIN cladding layers have rough surfaces, the desired effect of loss reduction may be partially cancelled out.

Fig. 1. (a). Cross-sectional schematic of an As42S58 waveguide with Ge17Sb12S71/SiO2 double GRIN cladding layers; (b) cross-sectional view of the field along x-axis of the quasi-TE mode in the same structure with (red) and without (black) GRIN cladding layers, showing field intensity decrease at the interfaces.

Fig. 2. (a). Bending loss of quasi-TE mode as a function of bending radius in waveguides with and without GRIN cladding at 1550 nm wavelength, indicating that GRIN design also improves waveguide bending performance besides loss reduction; (b) measured insertion loss data of the 0.75 μm As42S58 waveguides with and without GRIN cladding layers, the slopes of the fitted lines represent measured waveguide loss.

3. Waveguide fabrication and characterization

3.1 Waveguide fabrication

Bulk chalcogenide glasses are prepared using a traditional chalcogenide melt-quenching technique. From this bulk high-quality thin films are deposited onto 3 μm-thick oxide-coated Si wafers (Silicon Quest International Inc.) using thermal evaporation. Details of the bulk sample preparation and film deposition process may be found elsewhere [17

17. L. Petit, N. Carlie, F. Adamietz, M. Couzi, V. Rodriguez, and K. C. Richardson, “Correlation between physical, optical and structural properties of sulfide glasses in the system Ge-Sb-S,” Mater. Chem. Phys. 97, 64–70 (2006). [CrossRef]

, 18

18. J. Hu, V. Tarasov, N. Carlie, L. Petit, A. Agarwal, K. Richardson, and L. Kimerling, “Fabrication and Testing of Planar Chalcogenide Waveguide Integrated Microfluidic Sensor,” Opt. Express 15, 2307 (2007). [CrossRef] [PubMed]

]. The refractive index of the as-evaporated films at 1550 nm wavelength is determined using a Metricon 2010 prism coupler. In this study, the waveguide core is made of thermally evaporated As42S58 (n = 2.37). Thermally evaporated Ge17Sb12S71 film (n = 2.06) and sputtered silicon dioxide (n = 1.46) are chosen as the GRIN layers. As42S58 waveguides with a core height of 350 nm and three different width, 0.75 μm, 1.2 μm and 1.6 μm are fabricated using lift-off technique on a 500 nm CMOS line. The lift-off process has been described in detail elsewhere [10

10. J. Hu, V. Tarasov, N. Carlie, N. Feng, L. Petit, A. Agarwal, K. Richardson, and L. Kimerling, “Si-CMOS-compatible lift-off fabrication of low-loss planar chalcogenide waveguides,” Opt. Express 15, 11798 (2007). [CrossRef] [PubMed]

]. The GRIN layers are subsequently deposited on patterned waveguides to form conformal coatings over the As42S58 core. It is worth pointing out that argon gas is intentionally introduced into deposition chamber during Ge17Sb12S71 GRIN cladding evaporation in order to maintain the evaporated gas phase in a viscous flow regime and thus leads to uniform step coverage. Both Ge17Sb12S71 and oxide GRIN cladding layers have a thickness of 35 nm each.

Figure 3(a) shows a top view SEM image of a bent section of a 0.75 μm wide GRIN cladding waveguide, indicating excellent pattern fidelity from the lift-off process, and a cleaved waveguide facet in Fig. 3(b) shows the GRIN cladding layers over the As42S58 core.

Fig. 3. (a). Top-view SEM image of a 0.75 μm wide As42S58 waveguide bending section showing excellent pattern fidelity from lift-off; (b) Tilted view of a cleaved facet of a As42S58 waveguide coated with Ge17Sb12S71 and oxide double GRIN layers.

3.2 Waveguide characterization

Waveguide transmission loss measurements are performed on a Newport AutoAlign workstation in combination with a JD SU SWS tunable laser. Lens-tip fibers are used to couple light from the laser into and out of the waveguides. Reproducible coupling between waveguides and fibers is achieved via an automatic alignment system with a spatial resolution better than 50 nm. Optical loss in the waveguides is measured by a cutback method using paper-clip waveguide patterns. Loss measurements on As42S58 waveguides without GRIN cladding layers are repeated twice consecutively to make sure no oxidation effects interfere with the measurement. Each loss number reported in this paper is averaged over > 25 waveguides. Roughness of the multiple interfaces between core and GRIN cladding layers is measured using a Digital Instruments Nanoscope IIIa Atomic Force Microscope (AFM), yielding RMS roughness values of (10 ± 2) nm, (10 ± 2) nm and (8 ± 2) nm for As42S58-Ge17Sb12S71 interface, Ge17Sb12S71-oxide interface and oxide surface, respectively. The scans are performed parallel to the direction of the waveguides using the tapping mode.

4. Results and discussion

Measured transmission losses at 1550 nm wavelength, in As42S58 waveguides without GRIN cladding layers, As42S58 waveguides coated with a single Ge17Sb12S71 GRIN layer, and waveguides coated with Ge17Sb12S71/SiO2 bilayers, are shown in Table 2. Figure 2(b) shows the insertion loss of quasi-TE mode for the 0.75 μm wide waveguides. The experimental error arising from non-uniformities is estimated to be ±10% of the measured waveguide loss.

Table 2. Measured optical transmission losses of As42S58 strip waveguides without GRIN cladding layers (denoted as “No GRIN”), As42S58 waveguides coated with a single Ge17Sb12S71 layer (denoted as “1 GRIN”) and waveguides coated with Ge17Sb12S71/SiO2 bilayers (denoted as “2 GRIN”) at 1550 nm for three different waveguide widths.

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5. Conclusion

We have experimentally demonstrated, for the first time, waveguide loss reduction using a graded-index cladding design. By conformally coating as-fabricated As42S58 waveguides with thin layers of glasses with lower refractive indices, the index and optical field discontinuity at waveguide core-cladding interfaces is minimized, which leads to significantly reduced transmission losses. Loss value as low as 1.5 dB/cm is obtained in small core (1.6 μm × 0.35 μm), high-index-contrast (Δn = 1.37) As42S58 strip waveguides with a double-layer GRIN structure. As our modal analysis has demonstrated, such GRIN designs provide a non-material-specific solution for reducing waveguide loss in high-index-contrast waveguides without compromising the requirements of tight optical confinement and compact optical guiding structures. By combining the GRIN cladding design with other post-fabrication surface smoothing techniques, HIC waveguides with much lower loss figures can be produced. Such low-loss structures can be applied in a number of technical fields such as intra-chip interconnect, biochemical sensing, optical delay lines and planar photonic device processing.

Acknowledgments

Funding support is provided by the Department Of Energy under award number DE-SC52-06NA27341. The authors would like to thank Dr. Jurgen Michel and Dr. Tymon Barwicz at MIT for helpful discussions. The authors also acknowledge the Microsystems Technology Laboratories at MIT for fabrication facilities and the Center for Materials Science and Engineering at MIT for characterization facilities.

Disclaimer

References and links

01.

C. A. Barrios and M. Lipson, “Electrically driven silicon resonant light emitting device based on slot-waveguide,” Opt. Express 13, 10092 (2005). [CrossRef] [PubMed]

02.

Q. Xu, V. R. Almeida, and M. Lipson, “Demonstration of high Raman gain in a submicrometer-size silicon-on-insulator waveguide,” Opt. Lett. 30, 35–37 (2005). [CrossRef] [PubMed]

03.

T. Barwicz and H. Haus, “Three-dimensional analysis of scattering losses due to sidewall roughness in microphotonic waveguides,” J. Lightwave Technol. 23, 2719–2732 (2005). [CrossRef]

04.

M. Webster, R. Pafchek, G. Sukumaran, and T. Koch, “Low-loss quasi-planar ridge waveguides formed on thin silicon-on-insulator,” Appl. Phys. Lett. 87, 231108–231110 (2005). [CrossRef]

05.

D. Sparacin, S. Spector, and L. Kimerling, “Silicon Waveguide Sidewall Smoothing by Wet Chemical Oxidation,” J. Lightwave Technol. 23, 2455–2461 (2005). [CrossRef]

06.

K. Lee, D. Lim, L. Kimerling, J. Shin, and F. Cerrina, “Fabrication of ultralow-loss Si/SiO2 waveguides by roughness reduction,” Opt. Lett. 26, 1888–1890 (2001). [CrossRef]

07.

M. Wu and M. Lee, “Thermal annealing in hydrogen for 3-D profile transformation on silicon-on-insulator and sidewall roughness reduction,” J. Microelectromech. Syst. 15, 338–343 (2006). [CrossRef]

08.

C. Chao and L. Guo, “Reduction of surface scattering loss in polymer microrings using thermal-reflow technique,” IEEE Photon. Technol. Lett. 16, 1498–1500 (2004). [CrossRef]

09.

J. Hu, V. Tarasov, N. Carlie, R. Sun, L. Petit, A. Agarwal, K. Richardson, and L. Kimerling, “Low-loss integrated planar chalcogenide waveguides for chemical sensing,” Proc. SPIE 6444, 64440N (2007). [CrossRef]

10.

J. Hu, V. Tarasov, N. Carlie, N. Feng, L. Petit, A. Agarwal, K. Richardson, and L. Kimerling, “Si-CMOS-compatible lift-off fabrication of low-loss planar chalcogenide waveguides,” Opt. Express 15, 11798 (2007). [CrossRef] [PubMed]

11.

M. Richardson, L. Shah, J. Tawney, A. Zoubir, C. Rivera, C. Lopez, and K. Richardson, “Photo-induced structural changes in glass,” Glass Sci. Technol. 75, 121–130 (2002).

12.

P. Lucas, D. Le Coq, C. Juncker, J. Collier, D. Boesewetter, C. Boussard-Pledel, B. Bureau, and M. Riley, “Evaluation of toxic agent effects on lung cells by fiber evanescent wave spectroscopy,” Appl. Spectrosc. 59, 1–9 (2005). [CrossRef] [PubMed]

13.

M. Asobe, H. Itoh, T. Miyazawa, and T. Kanamori, “Efficient and ultrafast all-optical switching using high An, small core chalcogenide glass fibre,” Electron. Lett. 29, 1966–1968 (1993). [CrossRef]

14.

J. Hu, L. Petit, X. Sun, A. Agarwal, N. Carlie, T. Anderson, J. Choi, J. Viens, M. Richardson, K. Richardson, and L. Kimerling, “Studies on Structural, Electrical and Optical Properties of Cu-doped As-Se-Te Chalcogenide Glasses,” J. Appl. Phys. 101, 063520–063528 (2007). [CrossRef]

15.

D. Sparacin, R. Sun, A. Agarwal, M. Beals, J. Michel, L. Kimerling, T. Conway, A. Pomerene, D. Carothers, M. Grove, D. Gill, M. Rasras, S. Patel, and A. White, “Low-Loss Amorphous Silicon Channel Waveguides for Integrated Photonics,” in Proceedings of 3rd IEEE International Conference on Group IV Photonics, pp. 255–257.

16.

N. Feng, G. Zhou, C. Xu, and W. Huang, “Computation of full-vector modes for bending waveguide using cylindrical perfectly matched layers,” IEEE J. Lightwave Technol. 20, 1976–1980 (2002). [CrossRef]

17.

L. Petit, N. Carlie, F. Adamietz, M. Couzi, V. Rodriguez, and K. C. Richardson, “Correlation between physical, optical and structural properties of sulfide glasses in the system Ge-Sb-S,” Mater. Chem. Phys. 97, 64–70 (2006). [CrossRef]

18.

J. Hu, V. Tarasov, N. Carlie, L. Petit, A. Agarwal, K. Richardson, and L. Kimerling, “Fabrication and Testing of Planar Chalcogenide Waveguide Integrated Microfluidic Sensor,” Opt. Express 15, 2307 (2007). [CrossRef] [PubMed]

OCIS Codes
(130.2790) Integrated optics : Guided waves
(130.3120) Integrated optics : Integrated optics devices
(160.2750) Materials : Glass and other amorphous materials
(230.7380) Optical devices : Waveguides, channeled
(240.5770) Optics at surfaces : Roughness
(310.1860) Thin films : Deposition and fabrication

ToC Category:
Integrated Optics

History
Original Manuscript: August 27, 2007
Revised Manuscript: October 16, 2007
Manuscript Accepted: October 18, 2007
Published: October 19, 2007

Citation
Juejun Hu, Ning-Ning Feng, Nathan Carlie, Laeticia Petit, Jianfei Wang, Anu Agarwal, Kathleen Richardson, and Lionel Kimerling, "Low-loss high-index-contrast planar waveguides with graded-index cladding layers," Opt. Express 15, 14566-14572 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-22-14566


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References

  1. C. A. Barrios and M. Lipson, "Electrically driven silicon resonant light emitting device based on slot-waveguide," Opt. Express 13, 10092 (2005). [CrossRef] [PubMed]
  2. Q. Xu, V. R. Almeida, and M. Lipson, "Demonstration of high Raman gain in a submicrometer-size silicon-on-insulator waveguide," Opt. Lett. 30, 35-37 (2005). [CrossRef] [PubMed]
  3. T. Barwicz and H. Haus, "Three-dimensional analysis of scattering losses due to sidewall roughness in microphotonic waveguides," J. Lightwave Technol. 23, 2719-2732 (2005). [CrossRef]
  4. M. Webster, R. Pafchek, G. Sukumaran, and T. Koch, "Low-loss quasi-planar ridge waveguides formed on thin silicon-on-insulator," Appl. Phys. Lett. 87, 231108-231110 (2005). [CrossRef]
  5. D. Sparacin, S. Spector, and L. Kimerling, "Silicon Waveguide Sidewall Smoothing by Wet Chemical Oxidation," J. Lightwave Technol. 23, 2455-2461 (2005). [CrossRef]
  6. K. Lee, D. Lim, L. Kimerling, J. Shin, and F. Cerrina, "Fabrication of ultralow-loss Si/SiO2 waveguides by roughness reduction," Opt. Lett. 26, 1888-1890 (2001). [CrossRef]
  7. M. Wu and M. Lee, "Thermal annealing in hydrogen for 3-D profile transformation on silicon-on-insulator and sidewall roughness reduction," J. Microelectromech. Syst. 15, 338-343 (2006). [CrossRef]
  8. C. Chao and L. Guo, "Reduction of surface scattering loss in polymer microrings using thermal-reflow technique," IEEE Photon. Technol. Lett. 16, 1498-1500 (2004). [CrossRef]
  9. J. Hu, V. Tarasov, N. Carlie, R. Sun, L. Petit, A. Agarwal, K. Richardson, and L. Kimerling, "Low-loss integrated planar chalcogenide waveguides for chemical sensing," Proc. SPIE 6444, 64440N (2007). [CrossRef]
  10. J. Hu, V. Tarasov, N. Carlie, N. Feng, L. Petit, A. Agarwal, K. Richardson, and L. Kimerling, "Si-CMOS-compatible lift-off fabrication of low-loss planar chalcogenide waveguides," Opt. Express 15, 11798 (2007). [CrossRef] [PubMed]
  11. M. Richardson, L. Shah, J. Tawney, A. Zoubir, C. Rivera, C. Lopez, and K. Richardson, "Photo-induced structural changes in glass," Glass Sci. Technol. 75, 121-130 (2002).
  12. P. Lucas, D. Le Coq, C. Juncker, J. Collier, D. Boesewetter, C. Boussard-Pledel, B. Bureau, M. Riley, "Evaluation of toxic agent effects on lung cells by fiber evanescent wave spectroscopy," Appl. Spectrosc. 59, 1-9 (2005). [CrossRef] [PubMed]
  13. M. Asobe, H. Itoh, T. Miyazawa, and T. Kanamori, "Efficient and ultrafast all-optical switching using high Δn, small core chalcogenide glass fibre," Electron. Lett. 29, 1966-1968 (1993). [CrossRef]
  14. J. Hu, L. Petit, X. Sun, A. Agarwal, N. Carlie, T. Anderson, J. Choi, J. Viens, M. Richardson, K. Richardson, and L. Kimerling, "Studies on Structural, Electrical and Optical Properties of Cu-doped As-Se-Te Chalcogenide Glasses," J. Appl. Phys. 101, 063520-063528 (2007). [CrossRef]
  15. D. Sparacin, R. Sun, A. Agarwal, M. Beals, J. Michel, L. Kimerling, T. Conway, A. Pomerene, D. Carothers, M. Grove, D. Gill, M. Rasras, S. Patel, A. White, "Low-Loss Amorphous Silicon Channel Waveguides for Integrated Photonics," in Proceedings of 3rd IEEE International Conference on Group IV Photonics, pp. 255-257.
  16. N. Feng, G. Zhou, C. Xu, and W. Huang, "Computation of full-vector modes for bending waveguide using cylindrical perfectly matched layers," IEEE J. Lightwave Technol. 20, 1976-1980 (2002). [CrossRef]
  17. L. Petit, N. Carlie, F. Adamietz, M. Couzi, V. Rodriguez, and K. C. Richardson, "Correlation between physical, optical and structural properties of sulfide glasses in the system Ge-Sb-S," Mater. Chem. Phys. 97, 64-70 (2006). [CrossRef]
  18. J. Hu, V. Tarasov, N. Carlie, L. Petit, A. Agarwal, K. Richardson, and L. Kimerling, "Fabrication and Testing of Planar Chalcogenide Waveguide Integrated Microfluidic Sensor," Opt. Express 15, 2307 (2007). [CrossRef] [PubMed]

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