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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 24 — Nov. 24, 2008
  • pp: 19512–19519
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Diamond waveguides fabricated by reactive ion etching

Mark P. Hiscocks, Kumaravelu Ganesan, Brant C. Gibson, Shane T. Huntington, François Ladouceur, and Steven Prawer  »View Author Affiliations


Optics Express, Vol. 16, Issue 24, pp. 19512-19519 (2008)
http://dx.doi.org/10.1364/OE.16.019512


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Abstract

We demonstrate for the first time the feasibility of all-diamond integrated optic devices over large areas using a combination of photolithography, reactive ion etching (RIE) and focused ion beam (FIB) techniques. We confirm the viability of this scalable process by demonstrating guidance in a two-moded ridge waveguide in type 1b single crystal diamond. This opens the door to the fabrication of a diamond-based optical chip integrating functional elements such as X-crossings, Y-junctions, evanescent couplers, Bragg reflectors/couplers and various interferometers.

© 2008 Optical Society of America

1. Introduction

Diamond has a number of significant optical material properties that make it desirable for a range of applications. For guided wave optics, these properties include low absorption and scattering over a broad spectrum, together with a high refractive index. For integrated optoelectronics, diamond’s high thermal conductivity, high density and large Young’s modulus are also of benefit [1

1. G. E. Harlow, The nature of diamonds (Cambridge: The Press Syndicate of the University of Cambridge, 1998).

]. These attributes make diamond an excellent material for applications involving high power and/or high frequencies [2

2. D. Twitchen, “High purity diamond growth” Quantum Diamond Workshop, July 18th Victoria (2007).

]. Furthermore, the ability of diamond to produce single photons, when a lattice impurity is optically pumped [3

3. A. Beveratos, S. Kühn, R. Briouri, T. Gacoin, J-P. Poizat, and P. Grangier, “Room temperature stable single-photon source,” Eur. Phys. J. D 18, 191–196 (2002). [CrossRef]

], leads to applications in quantum key distribution (QKD) and linear optics quantum computing (LOQC) [4

4. C. H. Bennett and G. Brassard, “Quantum cryptography: Public key distribution and coin tossing,” Proceedings of the IEEE International Conference on Computer Systems and Signal Processing, (Institute of Electrical and Electronics Engineers, Bangalore, 1984), pp. 175–179.

,5

5. E. Knill, R. Laflamme, and G. J. Milburn, “A scheme for efficient quantum computation with linear optics,” Nature 409, 46–52 (2001). [CrossRef] [PubMed]

].

There has been much work done on the dry etching of diamond in recent years with work focussing mostly on reactive ion etching (RIE) [6

6. J. W. Baldwin, M. K. Zalalutdinov, T. Feygelson, B. B. Pate, J. E. Butler, and B. H. Houston, “Nanocrystalline diamond resonator array for RF signal processing,” Diamond Relat. Mater. 15, 2061–2067 (2006). [CrossRef]

13

13. M. P. Hiscocks, C. J. Kaalund, F. Ladouceur, S. T. Huntington, B. C. Gibson, S. Trpkovski, D. Simpson, E. Ampem-Lassen, S. Prawer, and J. E. Butler, “Reactive ion etching of waveguide structures in diamond,” Diamond Relat. Mater. 17, 1831–1834 (2008). [CrossRef]

] or inductively coupled plasma (ICP) [14

14. M. Karlsson and F. Nikolajeff, “Diamond micro-optics: microlenses and antireflection structured surfaces for the infrared spectral region,” Opt. Express 11, 502–507 (2003). [CrossRef] [PubMed]

18

18. D. S. Hwang, T. Saito, and N. Fujimori, “New etching process for device fabrication using diamond,” Diamond Relat. Mater. 13, 2207–2210 (2004). [CrossRef]

] to produce a wide variety of structures in both polycrystalline and single crystal diamond. A multi-mode diamond waveguide was previously demonstrated in single crystal diamond [19

19. P. Olivero, S. Rubanov, P. Reichart, B. C. Gibson, S. T. Huntington, J. Rabeau, A. D. Greentree, J. Salzman, D. Moore, D. N. Jamieson, and S. Prawer, “Ion-beam-assisted lift-off technique for three-dimensional micromachining of freestanding single-crystal diamond,” Adv. Mater. 17, 2427–2430 (2005). [CrossRef]

], although its length was inherently limited to <100 µm by the focused ion beam (FIB) fabrication process which is consequently not scalable.

2. Ridge waveguides in diamond

Diamond has a refractive index of approximately 2.39 for λ=532 nm which means that for a square core waveguide (surrounded by silica or air) to be single-moded the cross-section must have dimensions of roughly 200 nm×200 nm. Waveguides of these dimensions cannot be easily butt-coupled to standard optical fibre and are more difficult to fabricate. Ideally the development of diamond doping to give small (~0.01) changes in refractive index would allow larger waveguides of low index contrast to be fabricated in similar manner to that of silica waveguides. For the present, ridge waveguides permit guides in diamond with high contrast to be microns wide whilst maintaining single-mode operation. Shown in Fig. 1 is a ridge waveguide in diamond surrounded by air. The modes of this waveguide were simulated using the FIMMWAVE FMM real solver [20

20. FIMMWAVE, Photon Design, http://www.photond.com.

]. The waveguide has only one TE mode despite the ridge being 3.5 µm wide and having a height of 1.5 µm. The slab thickness beneath the ridge is 2 µm. These ridge dimensions are readily achievable by photolithography and RIE. The airgap shown beneath the waveguide can be created by ion implantation forming a sacrificial graphitic layer beneath the waveguide which can be removed later (see Section 3).

Fig. 1. Single TE mode of diamond ridge waveguide

It can be shown that the substrate leakage loss depends exponentially on the gap thickness [21

21. F. Ladouceur and J. D. Love, Silica-Based Buried Channel Waveguides and Devices (Chapman & Hall, 1995), Chap. 9.

] through the functional dependence Le-wd where w=β2k2nair2, β is the propagation constant, k=2π/λ, and d is the thickness of the airgap.

Figure 2 illustrates the substrate loss dependence of the fundamental mode of the single mode ridge waveguide as calculated using FIMMWAVE [20] together with the analytical approach sketched above. As can be seen the substrate loss is tolerable even for airgaps as thin as 100 nm and drops off exponentially as the thickness increases. Given the airgap thickness produced by ion implantation is around 200–300 nm (which could be increased by multiple implants if necessary) the substrate loss for diamond waveguides fabricated by this method is expected to be negligible.

Fig. 2. Plot of substrate loss for varying airgap thickness in diamond ridge waveguide structure. a. Analytical treatment. b. Simulated using FIMMWAVE.

3. Fabrication method

The samples used are type 1b high pressure high temperature (HPHT) single crystal diamonds purchased from Sumitomo. The samples are approximately 3.5 mm×3.5 mm×1.5 mm in dimension and appear yellow due to their nitrogen concentration (nominally 10–100 ppm).

After implantation, the sample is annealed at 1100 °C which results in a sharp, well defined interface between the graphitic region and undamaged diamond, while the region between the surface and the graphitic end of range region exhibits a much lower damage density.

Fig. 3. TRIM Monte Carlo simulation of the damage density profile induced by 2 MeV He +ions.

The waveguide structures are then etched into the diamond using the fabrication process described more thoroughly in [13

13. M. P. Hiscocks, C. J. Kaalund, F. Ladouceur, S. T. Huntington, B. C. Gibson, S. Trpkovski, D. Simpson, E. Ampem-Lassen, S. Prawer, and J. E. Butler, “Reactive ion etching of waveguide structures in diamond,” Diamond Relat. Mater. 17, 1831–1834 (2008). [CrossRef]

]. A silica mask deposited by PECVD is patterned using photolithography and RIE. The mask pattern is transferred into the diamond through further RIE in a predominantly oxygen plasma and the mask is removed by an HF (hydrofluoric acid) dip. Figure 4 shows an optical microscope reflection image of the surface of the diamond sample once it has gone through the RIE process. High magnification SEM images of similar ridges may be seen in [13

13. M. P. Hiscocks, C. J. Kaalund, F. Ladouceur, S. T. Huntington, B. C. Gibson, S. Trpkovski, D. Simpson, E. Ampem-Lassen, S. Prawer, and J. E. Butler, “Reactive ion etching of waveguide structures in diamond,” Diamond Relat. Mater. 17, 1831–1834 (2008). [CrossRef]

].

Fig. 4. Optical microscope reflection image of the surface of reactive ion etched sample. 7 ridge waveguide structures can be seen running horizontally across the sample. The waveguides range in length from 1 mm up to 2.7 mm. RIE depth is 1.5 µm.

Finally the sample is processed using the Focused Ion Beam (FIB) with 30 keV Ga+ ions to make 45° cuts into the waveguides to act as total internal reflection mirrors as well as drilling holes through the upper diamond layer to gain access to the 3.5 µm deep layer of graphite for subsequent etching. A 50 nm gold layer was sputter deposited prior to FIB milling to prevent any ion damage to the surface as well as to prevent charging during milling. Figure 5 shows a scanning electron microscope (SEM) image of one of the 45° cuts at one end of the waveguide.

Fig. 5. SEM image of total internal reflection mirror cut into one end of the waveguide.

An electrochemical etch is used to etch the graphite from beneath the waveguides leaving an airgap to provide vertical confinement for the waveguiding structure. A graphite rod and a copper rod were used as electrodes and diluted boric acid was used as electrolyte. Figures 6(a) and (b) show a transmission and reflection optical microscope image of the electrochemically etched sample, respectively. The images clearly show a series of holes that have been milled into the sample with the FIB in order to allow a free flow of acid to remove the graphitic region beneath the waveguide. A diamond waveguide is also shown with a total internal reflection mirror at each end.

Fig. 6. Optical microscope images of the waveguide with graphite etched from beneath structure. (a) depicts a transmission image and (b) shows a reflection image. Both image widths are ~1mm.

4. Results and discussion

The waveguide fabricated has a ridge width of approximately 4.5 µm, roughly 1 µm wider than that of the single-mode waveguide simulated, due to the usage of pressure-contact as opposed to vacuum-contact photolithography. This wider ridge results in a second TE mode for the structure which is shown in Fig. 7 as fabricated with small airgap above the rest of the diamond.

Fig. 7. Intensity plot for second order mode of fabricated ridge waveguide.

In order to optically characterize the structures, an input waveguide coupling and output light-microscope collection system was utilized [19

19. P. Olivero, S. Rubanov, P. Reichart, B. C. Gibson, S. T. Huntington, J. Rabeau, A. D. Greentree, J. Salzman, D. Moore, D. N. Jamieson, and S. Prawer, “Ion-beam-assisted lift-off technique for three-dimensional micromachining of freestanding single-crystal diamond,” Adv. Mater. 17, 2427–2430 (2005). [CrossRef]

]. Light was coupled into and out of the waveguide via reflection off total internal reflection (TIR) mirrors inclined at 45° with respect to the sample surface. A schematic representation of the setup can be observed in Fig. 8 (a). The mirrors exhibit TIR due to the difference in the refractive index of diamond with respect to air, with the critical angle being ≈25°. When using a high power microscope objective such as 50X, the field of view of the coupling/collection setup was limited to around 100 µm. Hence, in order to validate whether optical guidance could be achieved in RIE-diamond structures, two 45° mirrors were milled either end of a waveguide with a length of around 80 µm.

Fig. 8. (a) Schematic of the input waveguide coupling and output light-collection system. (b) and (c) show 80 µm section of waveguide with light coupling in through mirror in bottom right and the multimode output from the mirror in top left corner. (d) shows a close up of mode pattern on output mirror, the variation in the waveguide output modal pattern due to differing input launch conditions can be seen online (Media 1).

5. Conclusion

This paper discusses how a single-mode waveguide can be formed in single crystal diamond and models the substrate loss associated with this method. A diamond waveguide is fabricated which supports two modes and demonstrates the first case of optical guidance in diamond waveguides which have been fabricated using a scalable technique (photolithography and RIE). Ion implantation, and subsequent chemical etching, has been successfully used to provide vertical optical confinement within the diamond ridge waveguide structures. The techniques presented in this paper have the potential to produce large scale diamond waveguides and integrated optical devices in diamond. Future work will focus on the further characterization of these waveguides and the fabrication of other integrated optic structures in diamond.

Acknowledgments

The authors thank Photon Design support for their assistance with FIMMWAVE simulations. This project is proudly supported by the International Science Linkages programme established under the Australian Government’s innovation statement Backing Australia’s Ability. The authors wish to also acknowledge the Victorian Government’s Science, Technology & Innovation Infrastructure Grants Program for the funding of this project.

References and links

1.

G. E. Harlow, The nature of diamonds (Cambridge: The Press Syndicate of the University of Cambridge, 1998).

2.

D. Twitchen, “High purity diamond growth” Quantum Diamond Workshop, July 18th Victoria (2007).

3.

A. Beveratos, S. Kühn, R. Briouri, T. Gacoin, J-P. Poizat, and P. Grangier, “Room temperature stable single-photon source,” Eur. Phys. J. D 18, 191–196 (2002). [CrossRef]

4.

C. H. Bennett and G. Brassard, “Quantum cryptography: Public key distribution and coin tossing,” Proceedings of the IEEE International Conference on Computer Systems and Signal Processing, (Institute of Electrical and Electronics Engineers, Bangalore, 1984), pp. 175–179.

5.

E. Knill, R. Laflamme, and G. J. Milburn, “A scheme for efficient quantum computation with linear optics,” Nature 409, 46–52 (2001). [CrossRef] [PubMed]

6.

J. W. Baldwin, M. K. Zalalutdinov, T. Feygelson, B. B. Pate, J. E. Butler, and B. H. Houston, “Nanocrystalline diamond resonator array for RF signal processing,” Diamond Relat. Mater. 15, 2061–2067 (2006). [CrossRef]

7.

C. F. Wang, Y-S. Choi, J. C. Lee, E. L. Hu, J. Yang, and J. E. Butler, “Observation of whispering gallery modes in nanocrystalline diamond microdisks,” Appl. Phys. Lett. 90, 081110–1 (2007). [CrossRef]

8.

J. W. Baldwin, M. Zalalutdinov, T. Feygelson, J. E. Butler, and B. H. Houston, “Fabrication of short-wavelength photonic crystals in wide-band-gap nanocrystalline diamond films,” J. Vac. Sci. Technol. B 24, 50–54 (2006). [CrossRef]

9.

P. W. Leech, G. K. Reeves, and A. S. Holland, “Patterning and reactive ion etching of diamond films using light coupling masks,” in: Mater. Res. Soc. Symp. Proc. 820, R5.2.1–R5.2.6 (2004).

10.

K. A. Lister, B. G. Casey, P. S. Dobson, S. Thoms, D. S. Macintyre, C. D. W. Wilkinson, and J. M. R. Weaver, “Pattern transfer of a 23 nm-period grating and sub-15 nm dots into CVD diamond,” Microelectron. Eng. 73–74, 319–322 (2004). [CrossRef]

11.

Y. Ando, Y. Nishibayashi, K. Kobashi, T. Hirao, and K. Oura, “Smooth and high-rate reactive ion etching of diamond,” Diamond Relat. Mater. 11, 824–827 (2002). [CrossRef]

12.

Y. Ando, Y. Nishibayashi, and A. Sawabe, “‘Nano-rods of single crystalline diamond,” Diamond Relat. Mater. 13, 633–637 (2004). [CrossRef]

13.

M. P. Hiscocks, C. J. Kaalund, F. Ladouceur, S. T. Huntington, B. C. Gibson, S. Trpkovski, D. Simpson, E. Ampem-Lassen, S. Prawer, and J. E. Butler, “Reactive ion etching of waveguide structures in diamond,” Diamond Relat. Mater. 17, 1831–1834 (2008). [CrossRef]

14.

M. Karlsson and F. Nikolajeff, “Diamond micro-optics: microlenses and antireflection structured surfaces for the infrared spectral region,” Opt. Express 11, 502–507 (2003). [CrossRef] [PubMed]

15.

J. Enlund, J. Isberg, M. Karlsson, F. Nikolajeff, J. Olsson, and D. J. Twitchen, “Anisotropic dry etching of boron doped single crystal CVD diamond,” Carbon 43, 1839–1842 (2005). [CrossRef]

16.

C. L. Lee, E. Gu, and M. D. Dawson, “Micro-cylindrical and micro-ring lenses in CVD diamond,” Diamond Relat. Mater. 16, 944–948 (2007). [CrossRef]

17.

C. L. Lee, H. W. Choi, E. Gu, M.D. Dawson, and H. Murphy, “Fabrication and characterization of diamond micro-optics,” Diamond Relat. Mater. 15, 725–728 (2006). [CrossRef]

18.

D. S. Hwang, T. Saito, and N. Fujimori, “New etching process for device fabrication using diamond,” Diamond Relat. Mater. 13, 2207–2210 (2004). [CrossRef]

19.

P. Olivero, S. Rubanov, P. Reichart, B. C. Gibson, S. T. Huntington, J. Rabeau, A. D. Greentree, J. Salzman, D. Moore, D. N. Jamieson, and S. Prawer, “Ion-beam-assisted lift-off technique for three-dimensional micromachining of freestanding single-crystal diamond,” Adv. Mater. 17, 2427–2430 (2005). [CrossRef]

20.

FIMMWAVE, Photon Design, http://www.photond.com.

21.

F. Ladouceur and J. D. Love, Silica-Based Buried Channel Waveguides and Devices (Chapman & Hall, 1995), Chap. 9.

22.

M. B. H. Breese, D. N. Jamieson, and P. J. C. King, Material Analysis Using a Nuclear Microprobe, (John Wiley and Sons Inc. New York, 1996).

OCIS Codes
(110.5220) Imaging systems : Photolithography
(220.1920) Optical design and fabrication : Diamond machining
(230.7370) Optical devices : Waveguides
(270.5565) Quantum optics : Quantum communications

ToC Category:
Integrated Optics

History
Original Manuscript: September 23, 2008
Revised Manuscript: October 21, 2008
Manuscript Accepted: October 23, 2008
Published: November 10, 2008

Citation
Mark P. Hiscocks, Kumaravelu Ganesan, Brant C. Gibson, Shane T. Huntington, François Ladouceur, and Steven Prawer, "Diamond waveguides fabricated by reactive ion etching," Opt. Express 16, 19512-19519 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-24-19512


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References

  1. G. E. Harlow, The nature of diamonds (Cambridge: The Press Syndicate of the University of Cambridge, 1998).
  2. D. Twitchen, "High purity diamond growth" Quantum Diamond Workshop, July 18th Victoria (2007).
  3. A. Beveratos, S. Kühn, R. Briouri, T. Gacoin, J-P. Poizat, and P. Grangier, "Room temperature stable singlephoton source," Eur. Phys. J. D 18, 191-196 (2002). [CrossRef]
  4. C. H. Bennett and G. Brassard, "Quantum cryptography: Public key distribution and coin tossing," Proceedings of the IEEE International Conference on Computer Systems and Signal Processing, (Institute of Electrical and Electronics Engineers, Bangalore, 1984), pp. 175-179.
  5. E. Knill, R. Laflamme, and G. J. Milburn, "A scheme for efficient quantum computation with linear optics," Nature 409, 46-52 (2001). [CrossRef] [PubMed]
  6. J. W. Baldwin, M. K. Zalalutdinov, T. Feygelson, B. B. Pate, J. E. Butler, and B. H. Houston, "Nanocrystalline diamond resonator array for RF signal processing," Diamond Relat. Mater. 15,2061-2067 (2006). [CrossRef]
  7. C. F. Wang, Y-S. Choi, J. C. Lee, E. L. Hu, J. Yang, and J. E. Butler, "Observation of whispering gallery modes in nanocrystalline diamond microdisks," Appl. Phys. Lett. 90,081110-1 (2007). [CrossRef]
  8. J. W. Baldwin, M. Zalalutdinov, T. Feygelson, J. E. Butler, and B. H. Houston, "Fabrication of short-wavelength photonic crystals in wide-band-gap nanocrystalline diamond films," J. Vac. Sci. Technol. B 24,50 -54 (2006). [CrossRef]
  9. P. W. Leech, G. K. Reeves, and A. S. Holland, "Patterning and reactive ion etching of diamond films using light coupling masks," in: Mater. Res. Soc. Symp. Proc. 820, R5.2.1-R5.2.6 (2004).
  10. K. A. Lister, B. G. Casey, P. S. Dobson, S. Thoms, D. S. Macintyre, C. D. W. Wilkinson, and J. M. R. Weaver, "Pattern transfer of a 23 nm-period grating and sub-15 nm dots into CVD diamond," Microelectron. Eng. 73-74,319-322 (2004). [CrossRef]
  11. Y. Ando, Y. Nishibayashi, K. Kobashi, T. Hirao, and K. Oura, "Smooth and high-rate reactive ion etching of diamond," Diamond Relat. Mater. 11,824-827 (2002). [CrossRef]
  12. <jrn>. Y. Ando, Y. Nishibayashi, and A. Sawabe, "‘Nano-rods of single crystalline diamond," Diamond Relat. Mater. 13,633-637 (2004).</jrn> [CrossRef]
  13. M. P. Hiscocks, C. J. Kaalund, F. Ladouceur, S. T. Huntington, B. C. Gibson, S. Trpkovski, D. Simpson, E. Ampem-Lassen, S. Prawer, and J. E. Butler, "Reactive ion etching of waveguide structures in diamond," Diamond Relat. Mater. 17,1831-1834 (2008). [CrossRef]
  14. M. Karlsson and F. Nikolajeff, "Diamond micro-optics: microlenses and antireflection structured surfaces for the infrared spectral region," Opt. Express 11,502-507 (2003). [CrossRef] [PubMed]
  15. J. Enlund, J. Isberg, M. Karlsson, F. Nikolajeff, J. Olsson, and D. J. Twitchen, "Anisotropic dry etching of boron doped single crystal CVD diamond," Carbon 43,1839-1842 (2005). [CrossRef]
  16. C. L. Lee, E. Gu, and M. D. Dawson, "Micro-cylindrical and micro-ring lenses in CVD diamond," Diamond Relat. Mater. 16,944-948 (2007). [CrossRef]
  17. C. L. Lee, H. W. Choi, E. Gu, M.D. Dawson, and H. Murphy, "Fabrication and characterization of diamond micro-optics," Diamond Relat. Mater. 15,725-728 (2006). [CrossRef]
  18. D. S. Hwang, T. Saito, and N. Fujimori, "New etching process for device fabrication using diamond," Diamond Relat. Mater. 13,2207-2210 (2004). [CrossRef]
  19. P. Olivero, S. Rubanov, P. Reichart, B. C. Gibson, S. T. Huntington, J. Rabeau, A. D. Greentree, J. Salzman, D. Moore, D. N. Jamieson, and S. Prawer, "Ion-beam-assisted lift-off technique for three-dimensional micromachining of freestanding single-crystal diamond," Adv. Mater. 17,2427-2430 (2005). [CrossRef]
  20. FIMMWAVE, Photon Design, http://www.photond.com.
  21. F. Ladouceur and J. D. Love, Silica-Based Buried Channel Waveguides and Devices (Chapman & Hall, 1995), Chap. 9.
  22. M. B. H. Breese, D. N. Jamieson, and P. J. C. King, Material Analysis Using a Nuclear Microprobe, (John Wiley and Sons Inc. New York, 1996).

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