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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 24 — Nov. 22, 2010
  • pp: 24516–24521
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Continuous wave Nd:YAG channel waveguide laser produced by focused proton beam writing

Yicun Yao, Yang Tan, Ningning Dong, Feng Chen, and Andrew A. Bettiol  »View Author Affiliations


Optics Express, Vol. 18, Issue 24, pp. 24516-24521 (2010)
http://dx.doi.org/10.1364/OE.18.024516


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Abstract

We report on mirrorless continuous wave laser oscillation at 1064 nm from a 808 nm pumped Nd:YAG optical channel waveguide fabricated by 1 MeV focused proton beam writing. Pump power threshold has been found to be 94 mW with a laser slope efficiency of 40%. A maximum output power at 1064 nm for the waveguide laser is 63 mW at absorbed pump power of 247 mW.

© 2010 OSA

1. Introduction

Proton Beam Writing (PBW) is an advanced technique that is used for the inscription of three-dimensional (3D) structures in diverse materials [7

7. F. Watt, M. B. H. Breese, A. A. Bettiol, and J. A. van Kan, “Proton beam writing,” Mater. Today 10(6), 20–29 (2007). [CrossRef]

]. PBW is based on the controlled scanning of a focused proton beam within the material to be micro-structured so that local modifications can be achieved in a precise manner at both micro- and submicron scales [8

8. J. A. van Kan, A. A. Bettiol, and F. Watt, “Three-dimensional nanolithography using proton beam writing,” Nucl. Instrum. Methods Phys. Res. B 181, 49 (2001).

]. Because of the energy and momentum mismatch between protons and electrons in the target material, the protons will maintain a straight pathway and cause little proximity effect as energy obtained by secondary electrons will be low [9

9. H. J. Whitlow, M. L. Ng, V. Auželyté, I. Maximov, L. Montelius, J. A. van Kan, A. A. Bettiol, and F. Watt, “Lithography of high spatial density biosensor structures with sub-100 nm spacing by MeV proton beam writing with minimal proximity effect,” Nanotechnology 15(1), 223–226 (2004). [CrossRef]

]. This makes PBW an efficient technology to fabricate 3D high aspect ratio nanostructures in various materials [10

10. T. C. Sum, A. A. Bettiol, C. Florea, and F. Watt, “Proton-beam writing of poly-methylmethacrylate buried channel waveguides,” J. Lightwave Technol. 24(10), 3803–3809 (2006). [CrossRef]

]. When compared to normal ion implantation (that requires the additional use of masks), PBW shows the intriguing advantage of direct fabrication due to the use of focused ion beams of micro- (and even sub-micron) size, realizing maskless implantation of energetic protons [10

10. T. C. Sum, A. A. Bettiol, C. Florea, and F. Watt, “Proton-beam writing of poly-methylmethacrylate buried channel waveguides,” J. Lightwave Technol. 24(10), 3803–3809 (2006). [CrossRef]

]. In recent years PBW has also shown its advantages for optical waveguide fabrication [11

11. K. Ansari, J. A. van Kan, A. A. Bettiol, and F. Watt, “Fabrication of high aspect ratio 100 nm metallic stamps for nanoimprint lithography using proton beam writing,” Appl. Phys. Lett. 85(3), 476–478 (2004). [CrossRef]

,12

12. A. A. Bettiol, S. Venugopal Rao, T. C. Sum, J. A. van Kan, and F. Watt, “Fabrication of optical waveguides using proton beam writing,” J. Cryst. Growth 288(1), 209–212 (2006). [CrossRef]

]. As of yet, successful examples of PBW waveguides include a few organics materials (e.g., PMMA, SU-8) glasses and crystals [13

13. A. A. Bettiol, T. C. Sum, F. C. Cheong, C. H. Sow, S. Venugopal Rao, J. A. van Kan, E. J. Teo, K. Ansari, and F. Watt, “A progress review of proton beam writing applications in microphotonics,” Nucl. Instrum. Methods Phys. Res. B 231(1-4), 364–371 (2005). [CrossRef]

17

17. A. A. Bettiol, S. Venugopal Rao, E. J. Teo, J. A. van Kan, and F. Watt, “Fabrication of buried channel waveguides in photosensitive glass using proton beam writing,” Appl. Phys. Lett. 88(17), 171106 (2006). [CrossRef]

].

Neodymium doped yttrium aluminum garnet (hereafter abbreviated to Nd:YAG) is one of the most important gain media for solid state lasers owing to its outstanding fluorescence, mechanical and thermal properties. Waveguide lasers have been already fabricated in this material (in both single crystals and polycrystalline ceramics) by using a few techniques, such as ion implantation, femtosecond laser inscription, femtosecond laser ablation, film deposition, and waveguide lasers have been generated in some of these samples [18

18. M. Domenech, G. V. Vázquez, E. Cantelar, and G. Lifante, “Continuous-wave laser action at λ=1064.3 nm in proton- and carbon- implanted Nd:YAG waveguides,” Appl. Phys. Lett. 83(20), 4110–4112 (2003). [CrossRef]

23

23. G. A. Torchia, A. Rodenas, A. Benayas, E. Cantelar, L. Roso, and D. Jaque, “Highly efficient laser action in femtosecond-written Nd:yttrium aluminum garnet ceramic waveguides,” Appl. Phys. Lett. 92(11), 111103 (2008). [CrossRef]

]. Despite the fact that PBW has been recently demonstrated to be a powerful technique for the fabrication of high quality channel waveguides in Nd:YAG with 3D control [16

16. A. Benayas, D. Jaque, Y. Yao, F. Chen, A. A. Bettiol, A. Rodenas, and A. K. Kar, “Micro-structuring of Nd:YAG crystals by proton beam writing,” Opt. Lett. (to be published). [PubMed]

], the suitability of the obtained waveguides as integrated laser gain medium has not, to the best of our knowledge, been reported up to now. In this work, we report on the first demonstration of continuous wave stable laser operation at 1064 nm at room temperature from a 808 nm pumped Nd:YAG channel waveguide fabricated by PBW.

2. Experiments in details

The optically polished Nd:YAG (doped by 1 at. % Nd3+ ions) crystal used in this work was cut into dimensions of 10(x)×4.8(y)×1.5(z) mm3. The PBW process was carried out by using the facilities at the Center for Ion Beam Applications, National University of Singapore [24

24. F. Watt, J. A. van Kan, I. Rajta, A. A. Bettiol, T. F. Choo, M. B. H. Breese, and T. Osipowicz, “The National University of Singapore high energy ion nano-probe facility: Performance tests,” Nucl. Instrum. Methods Phys. Res. B 210, 14–20 (2003). [CrossRef]

]. The proton beam was at energy of 1 MeV and focused down to a beam with diameter of 1μm. During the process, the sample was mounted on a motorized stage (Exfo inchworm stage, moving linearly at different speeds), and the proton beam was magnetically scanned over a distance of 4μm in a perpendicular direction to the proton pathway on the x-y face, reaching different writing fluences of 1×1015, 5×1015, 1×1016, and 2×1016 cm−2. The formed channel waveguides were along the 4.8-mm axis of the wafer. Figure 1
Fig. 1 The defects per atom (DPA) (blue solid line) and H+ range (red dashed line) profiles of proton beam at energy of 1MeV and fluence of 1×1016 cm−2 based on SRIM calculation.
shows the calculated defect (defect per atom, DPA) and H+ concentration profiles caused by the 1 MeV proton beam in Nd:YAG, as obtained with the SRIM 2010 code [25

25. J. F. Ziegler, computer code, SRIM http://www.srim.org.

]. As one can see, the projected average ion range (R p) of the 1 MeV protons in Nd:YAG crystal is ~9.8 μm. The proton-induced damage is very low even at the barrier region (maximally of 0.012), which suggests minor modification on the original lattices of Nd:YAG. In addition, the profile of the focused proton beam is approximately Gaussian, and at 1 MeV it will have a lateral straggling of ~0.7 μm, which results in channel waveguides with a transverse width of ~4.7 μm at the cross section.

The confocal microphotoluminescence (μPL) spectra from both the waveguide and bulk were obtained by using a fiber-coupled confocal microscope (Olympus BX-41) as described elsewhere [21

21. J. Siebenmorgen, K. Petermann, G. Huber, K. Rademaker, S. Nolte, and A. Tünnermann, “Femtosecond laser written stress-induced Nd:Y3Al5O12 (Nd:YAG) channel waveguide laser,” Appl. Phys. B 97(2), 251–255 (2009). [CrossRef]

]. The 10-mW cw radiation at 488 nm from an argon laser was focused at sample’s surface by using a 100 × objective (numerical aperture N.A. = 0.95), exciting the Nd3+ ions from their fundamental state (4I9/2) up to the excited state (2G3/2). The subsequent 4F3/24I11/2 emissions were collected by using the same microscope objective and, after passing through a confocal aperture, analyzed by a CCD camera attached to a fiber-coupled spectrometer.

The laser operation experiment was performed by using end-face pumping system. A 808 nm light beam generated from a tunable Ti:sapphire cw laser (Coherent 110) was focused by a convex lens (focus length of 25mm) into the input face of the waveguide laser. The laser cavity (plane-plane geometry) was a mirror-less cavity so that optical feed-back was directly provided by the Fresnel reflections caused by the two end-faces of the Nd:YAG crystal. Thus, since the refractive index of Nd:YAG is close to 1.8, both end-faces act as output couplers with an effective transmittance close to 0.92. The waveguide laser beam generated at ~1064 nm was collected and collimated by using a 20× microscope objective. The laser radiation at 1064 nm was discriminated from the non-absorbed pump radiation by using a dichroic mirror/filter (with transmittance of 90% at 1064 nm and reflectivity of >99% at 808 nm) from the 808-nm beam. The output laser beam was imaged by an infrared CCD camera and analyzed by a spectrometer. The laser power (from both end-faces of the crystal) was monitored by a powermeter.

3. Results and discussion

Figure 3
Fig. 3 Comparison of the room temperature micro-luminescence emission spectra correlated to Nd3+ ions at 4F3/24I11/2 transition obtained from the channel waveguide (dashed line) and the bulk (solid line).
shows the comparison of the 4F3/24I11/2 room-temperature μ-PL emission spectra (intensity vs. wavelength) of Nd3+ ions obtained from the waveguide and the bulk. As it can be observed, the fluorescence intensity and shape obtained from the waveguide are nearly the same as that obtained from the bulk (reflecting that both pumping efficiency and 4F3/2 quantum yield of Nd3+ ions were not strongly modified by the PBW procedure). This is in good agreement with the defect calculation (only ~1% DPA) obtained by SRIM code (Fig. 1). Data included in Fig. 3 shows that the outstanding luminescence features of Nd3+ ions are well preserved in at waveguide’s volume. This, in turn, makes the PBW Nd:YAG waveguides promising candidates for highly efficient laser light generation. From a fundamental point of view, the observed almost unaffected fluorescence efficiency reveals that the waveguide region is not accompanied by a large density of lattice defects and/or luminescence quenching centers, which is ubiquitous in Nd:YAG waveguides produced by normal ion implantation; as an example ion-induced defects leads to a overall Nd3+ luminescence reduction close to 25% in proton implanted Nd:YAG ceramic waveguides [27

27. Y. Tan and F. Chen, “Proton-implanted optical channel waveguides in Nd:YAG laser ceramics,” J. Phys. D 43(7), 075105 (2010). [CrossRef]

].

Figure 4(a)
Fig. 4 (a) Laser oscillation spectra from the waveguide produced by 1 MeV PBW at fluence of 1×1016 cm−2, showing a keen-edged peak at 1064.2 nm with a FWHM of 0.75nm. The mode image of the waveguide laser is shown as inset. (b) The measured output laser power as a function of the absorbed pump power (balls) from the waveguide. The green solid line shows the linear fit of the experimental data.
depicts the laser emission spectra around 1064 nm from the 1 MeV PBW Nd:YAG waveguide at fluence of 1×1016 cm−2 as the absorbed pump power is above the threshold. The center wavelength of the laser spectrum is 1064.2 nm, which indeed corresponds to the strongest emission line within the 4F3/24I11/2 transition of Nd3+ ions. The FWHM of the emission line is ~0.75 nm, clearly denoting the presence of laser oscillation. The inset shows the obtained image of the waveguide laser mode (near-field intensity distribution). The waveguide mainly shows a clear single mode character, being this outstanding feature of relevance in many practical applications.

Considering the widths of the pump beam (~16 μm) at 808 nm and the laser mode (~3.4 μm) the 1064 nm, the pump coupling efficiency is estimated to be ~40%. The absorption efficiency of the waveguide was measured to be 0.9. Figure 4(b) shows the output laser power (at 1064.2 nm) as a function of the absorbed pump power (at 808 nm) generated by the Nd:YAG waveguide at a writing fluence of 1×1016 cm−2. The laser power in Fig. 4(b) accounts for the laser radiations coming out from both faces of the crystal. From the linear fit of the experimental data, we have determined that the power threshold for the laser oscillation is 94 mW and that a slope coefficient of 40% can be achieved from the present waveguide laser system. This laser slope efficiency can be compared to those previously reported from other Nd:YAG mirrorless waveguide lasers. The maximum 1064-nm laser power achieved is 63 mW for the maximum absorbed pump power of 247 mW, leading to an optical conversion efficiency of 25.5%. As an example it has been found that is significantly smaller than that obtained from Ultrafast Laser Inscribed (ULI) waveguides (60%). One possible reason for this difference is the larger propagation losses of the PBW waveguides (~4 dB/cm, mainly due to the nuclear damage induced by the proton beams) in respect to those of ULI waveguides (below 1dB/cm) [16

16. A. Benayas, D. Jaque, Y. Yao, F. Chen, A. A. Bettiol, A. Rodenas, and A. K. Kar, “Micro-structuring of Nd:YAG crystals by proton beam writing,” Opt. Lett. (to be published). [PubMed]

,23

23. G. A. Torchia, A. Rodenas, A. Benayas, E. Cantelar, L. Roso, and D. Jaque, “Highly efficient laser action in femtosecond-written Nd:yttrium aluminum garnet ceramic waveguides,” Appl. Phys. Lett. 92(11), 111103 (2008). [CrossRef]

]. In addition, the two end-faces may not be perfectly perpendicular to the channels, which could bring out dramatic degrade of the performance of the Nd:YAG waveguide laser system. Nevertheless, by reducing the propagation loss of the waveguides by the thermal annealing and avoiding the misalignment of the channels, waveguide lasers with better performance may be expected. It is also important to remark that the PBW waveguide lasers demonstrated here show a superior mode quality (highly symmetric) in respect to ULI mirrorless waveguide lasers in Nd:YAG crystals [23].

4. Summary

Acknowledgments

The work is supported by the National Natural Science Foundation of China (No. 10925524), the Program for New Century Excellent Talents for Universities, China (No. NCET-08-0331) and the 973 Project (No. 2010CB832906) of China. The authors thank D. Jaque and A. Benayas for measurement of confocal μPL spectra and helpful discussions, and Q.M. Lu for polishing the samples.

References and links

1.

J. I. Mackenzie, “Dielectric Solid-State Planar Waveguide Lasers: A Review,” IEEE J. Sel. Top. Quantum Electron. 13(3), 626–637 (2007). [CrossRef]

2.

G. Lifante, Integrated Photonics: Fundamentals (John Wiley & Sons Ltd, West Sussex, 2003).

3.

G. I. Stegeman and C. T. Seaton, “Nonlinear integrated optics,” J. Appl. Phys. 58(12), R57 (1985). [CrossRef]

4.

C. J. M. Smith, H. Benisty, S. Olivier, M. Rattier, C. Weisbuch, T. F. Krauss, R. M. De La Rue, R. Houdré, and U. Oesterle, “Low-loss channel waveguides with two-dimensional photonic crystal boundaries,” Appl. Phys. Lett. 77(18), 2813–2815 (2000). [CrossRef]

5.

F. Chen, X. L. Wang, and K. M. Wang, “Development of ion-implanted optical waveguides in optical materials: A review,” Opt. Mater. 29(11), 1523–1542 (2007). [CrossRef]

6.

E. J. Murphy, Integrated optical circuits and components: Design and applications (Marcel Dekker, New York, 1999).

7.

F. Watt, M. B. H. Breese, A. A. Bettiol, and J. A. van Kan, “Proton beam writing,” Mater. Today 10(6), 20–29 (2007). [CrossRef]

8.

J. A. van Kan, A. A. Bettiol, and F. Watt, “Three-dimensional nanolithography using proton beam writing,” Nucl. Instrum. Methods Phys. Res. B 181, 49 (2001).

9.

H. J. Whitlow, M. L. Ng, V. Auželyté, I. Maximov, L. Montelius, J. A. van Kan, A. A. Bettiol, and F. Watt, “Lithography of high spatial density biosensor structures with sub-100 nm spacing by MeV proton beam writing with minimal proximity effect,” Nanotechnology 15(1), 223–226 (2004). [CrossRef]

10.

T. C. Sum, A. A. Bettiol, C. Florea, and F. Watt, “Proton-beam writing of poly-methylmethacrylate buried channel waveguides,” J. Lightwave Technol. 24(10), 3803–3809 (2006). [CrossRef]

11.

K. Ansari, J. A. van Kan, A. A. Bettiol, and F. Watt, “Fabrication of high aspect ratio 100 nm metallic stamps for nanoimprint lithography using proton beam writing,” Appl. Phys. Lett. 85(3), 476–478 (2004). [CrossRef]

12.

A. A. Bettiol, S. Venugopal Rao, T. C. Sum, J. A. van Kan, and F. Watt, “Fabrication of optical waveguides using proton beam writing,” J. Cryst. Growth 288(1), 209–212 (2006). [CrossRef]

13.

A. A. Bettiol, T. C. Sum, F. C. Cheong, C. H. Sow, S. Venugopal Rao, J. A. van Kan, E. J. Teo, K. Ansari, and F. Watt, “A progress review of proton beam writing applications in microphotonics,” Nucl. Instrum. Methods Phys. Res. B 231(1-4), 364–371 (2005). [CrossRef]

14.

T. C. Sum, A. A. Bettiol, H. L. Seng, I. Rajta, J. A. van Kan, and F. Watt, “Proton beam writing of passive waveguides in PMMA,” Nucl. Instrum. Methods Phys. Res. B 210, 266–271 (2003). [CrossRef]

15.

T. C. Sum, A. A. Bettiol, J. A. van Kan, F. Watt, E. Y. B. Pun, and K. K. Tung, “Proton beam writing of low-loss polymer optical waveguides,” Appl. Phys. Lett. 83(9), 1707–1709 (2003). [CrossRef]

16.

A. Benayas, D. Jaque, Y. Yao, F. Chen, A. A. Bettiol, A. Rodenas, and A. K. Kar, “Micro-structuring of Nd:YAG crystals by proton beam writing,” Opt. Lett. (to be published). [PubMed]

17.

A. A. Bettiol, S. Venugopal Rao, E. J. Teo, J. A. van Kan, and F. Watt, “Fabrication of buried channel waveguides in photosensitive glass using proton beam writing,” Appl. Phys. Lett. 88(17), 171106 (2006). [CrossRef]

18.

M. Domenech, G. V. Vázquez, E. Cantelar, and G. Lifante, “Continuous-wave laser action at λ=1064.3 nm in proton- and carbon- implanted Nd:YAG waveguides,” Appl. Phys. Lett. 83(20), 4110–4112 (2003). [CrossRef]

19.

F. Chen, Y. Tan, and D. Jaque, “Ion-implanted optical channel waveguides in neodymium-doped yttrium aluminum garnet transparent ceramics for integrated laser generation,” Opt. Lett. 34(1), 28–30 (2009). [CrossRef]

20.

G. A. Torchia, P. F. Meilán, A. Rodenas, D. Jaque, C. Mendez, and L. Roso, “Femtosecond laser written surface waveguides fabricated in Nd:YAG ceramics,” Opt. Express 15(20), 13266–13271 (2007). [CrossRef] [PubMed]

21.

J. Siebenmorgen, K. Petermann, G. Huber, K. Rademaker, S. Nolte, and A. Tünnermann, “Femtosecond laser written stress-induced Nd:Y3Al5O12 (Nd:YAG) channel waveguide laser,” Appl. Phys. B 97(2), 251–255 (2009). [CrossRef]

22.

A. Ródenas, G. A. Torchia, G. Lifante, E. Cantelar, J. Lamela, F. Jaque, L. Roso, and D. Jaque, “Refractive index change mechanisms in femtosecond laser written ceramic Nd:YAG waveguides: micro- spectroscopy experiments and beam propagation calculations,” Appl. Phys. B 95(1), 85–96 (2009). [CrossRef]

23.

G. A. Torchia, A. Rodenas, A. Benayas, E. Cantelar, L. Roso, and D. Jaque, “Highly efficient laser action in femtosecond-written Nd:yttrium aluminum garnet ceramic waveguides,” Appl. Phys. Lett. 92(11), 111103 (2008). [CrossRef]

24.

F. Watt, J. A. van Kan, I. Rajta, A. A. Bettiol, T. F. Choo, M. B. H. Breese, and T. Osipowicz, “The National University of Singapore high energy ion nano-probe facility: Performance tests,” Nucl. Instrum. Methods Phys. Res. B 210, 14–20 (2003). [CrossRef]

25.

J. F. Ziegler, computer code, SRIM http://www.srim.org.

26.

D. Yevick and W. Bardyszewski, “Correspondence of variational finite-difference (relaxation) and imaginary-distance propagation methods for modal analysis,” Opt. Lett. 17(5), 329–330 (1992). [CrossRef] [PubMed]

27.

Y. Tan and F. Chen, “Proton-implanted optical channel waveguides in Nd:YAG laser ceramics,” J. Phys. D 43(7), 075105 (2010). [CrossRef]

OCIS Codes
(140.3570) Lasers and laser optics : Lasers, single-mode
(160.3380) Materials : Laser materials
(230.7380) Optical devices : Waveguides, channeled

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: September 24, 2010
Revised Manuscript: November 1, 2010
Manuscript Accepted: November 2, 2010
Published: November 9, 2010

Citation
Yicun Yao, Yang Tan, Ningning Dong, Feng Chen, and Andrew A. Bettiol, "Continuous wave Nd:YAG channel waveguide laser produced by focused proton beam writing," Opt. Express 18, 24516-24521 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-24-24516


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References

  1. J. I. Mackenzie, “Dielectric Solid-State Planar Waveguide Lasers: A Review,” IEEE J. Sel. Top. Quantum Electron. 13(3), 626–637 (2007). [CrossRef]
  2. G. Lifante, Integrated Photonics: Fundamentals (John Wiley & Sons Ltd, West Sussex, 2003).
  3. G. I. Stegeman and C. T. Seaton, “Nonlinear integrated optics,” J. Appl. Phys. 58(12), R57 (1985). [CrossRef]
  4. C. J. M. Smith, H. Benisty, S. Olivier, M. Rattier, C. Weisbuch, T. F. Krauss, R. M. De La Rue, R. Houdré, and U. Oesterle, “Low-loss channel waveguides with two-dimensional photonic crystal boundaries,” Appl. Phys. Lett. 77(18), 2813–2815 (2000). [CrossRef]
  5. F. Chen, X. L. Wang, and K. M. Wang, “Development of ion-implanted optical waveguides in optical materials: A review,” Opt. Mater. 29(11), 1523–1542 (2007). [CrossRef]
  6. E. J. Murphy, Integrated optical circuits and components: Design and applications (Marcel Dekker, New York, 1999).
  7. F. Watt, M. B. H. Breese, A. A. Bettiol, and J. A. van Kan, “Proton beam writing,” Mater. Today 10(6), 20–29 (2007). [CrossRef]
  8. J. A. van Kan, A. A. Bettiol, and F. Watt, “Three-dimensional nanolithography using proton beam writing,” Nucl. Instrum. Methods Phys. Res. B 181, 49 (2001).
  9. H. J. Whitlow, M. L. Ng, V. Auželyté, I. Maximov, L. Montelius, J. A. van Kan, A. A. Bettiol, and F. Watt, “Lithography of high spatial density biosensor structures with sub-100 nm spacing by MeV proton beam writing with minimal proximity effect,” Nanotechnology 15(1), 223–226 (2004). [CrossRef]
  10. T. C. Sum, A. A. Bettiol, C. Florea, and F. Watt, “Proton-beam writing of poly-methylmethacrylate buried channel waveguides,” J. Lightwave Technol. 24(10), 3803–3809 (2006). [CrossRef]
  11. K. Ansari, J. A. van Kan, A. A. Bettiol, and F. Watt, “Fabrication of high aspect ratio 100 nm metallic stamps for nanoimprint lithography using proton beam writing,” Appl. Phys. Lett. 85(3), 476–478 (2004). [CrossRef]
  12. A. A. Bettiol, S. Venugopal Rao, T. C. Sum, J. A. van Kan, and F. Watt, “Fabrication of optical waveguides using proton beam writing,” J. Cryst. Growth 288(1), 209–212 (2006). [CrossRef]
  13. A. A. Bettiol, T. C. Sum, F. C. Cheong, C. H. Sow, S. Venugopal Rao, J. A. van Kan, E. J. Teo, K. Ansari, and F. Watt, “A progress review of proton beam writing applications in microphotonics,” Nucl. Instrum. Methods Phys. Res. B 231(1-4), 364–371 (2005). [CrossRef]
  14. T. C. Sum, A. A. Bettiol, H. L. Seng, I. Rajta, J. A. van Kan, and F. Watt, “Proton beam writing of passive waveguides in PMMA,” Nucl. Instrum. Methods Phys. Res. B 210, 266–271 (2003). [CrossRef]
  15. T. C. Sum, A. A. Bettiol, J. A. van Kan, F. Watt, E. Y. B. Pun, and K. K. Tung, “Proton beam writing of low-loss polymer optical waveguides,” Appl. Phys. Lett. 83(9), 1707–1709 (2003). [CrossRef]
  16. A. Benayas, D. Jaque, Y. Yao, F. Chen, A. A. Bettiol, A. Rodenas, and A. K. Kar, “Micro-structuring of Nd:YAG crystals by proton beam writing,” Opt. Lett. (to be published). [PubMed]
  17. A. A. Bettiol, S. Venugopal Rao, E. J. Teo, J. A. van Kan, and F. Watt, “Fabrication of buried channel waveguides in photosensitive glass using proton beam writing,” Appl. Phys. Lett. 88(17), 171106 (2006). [CrossRef]
  18. M. Domenech, G. V. Vázquez, E. Cantelar, and G. Lifante, “Continuous-wave laser action at λ=1064.3 nm in proton- and carbon- implanted Nd:YAG waveguides,” Appl. Phys. Lett. 83(20), 4110–4112 (2003). [CrossRef]
  19. F. Chen, Y. Tan, and D. Jaque, “Ion-implanted optical channel waveguides in neodymium-doped yttrium aluminum garnet transparent ceramics for integrated laser generation,” Opt. Lett. 34(1), 28–30 (2009). [CrossRef]
  20. G. A. Torchia, P. F. Meilán, A. Rodenas, D. Jaque, C. Mendez, and L. Roso, “Femtosecond laser written surface waveguides fabricated in Nd:YAG ceramics,” Opt. Express 15(20), 13266–13271 (2007). [CrossRef] [PubMed]
  21. J. Siebenmorgen, K. Petermann, G. Huber, K. Rademaker, S. Nolte, and A. Tünnermann, “Femtosecond laser written stress-induced Nd:Y3Al5O12 (Nd:YAG) channel waveguide laser,” Appl. Phys. B 97(2), 251–255 (2009). [CrossRef]
  22. A. Ródenas, G. A. Torchia, G. Lifante, E. Cantelar, J. Lamela, F. Jaque, L. Roso, and D. Jaque, “Refractive index change mechanisms in femtosecond laser written ceramic Nd:YAG waveguides: micro- spectroscopy experiments and beam propagation calculations,” Appl. Phys. B 95(1), 85–96 (2009). [CrossRef]
  23. G. A. Torchia, A. Rodenas, A. Benayas, E. Cantelar, L. Roso, and D. Jaque, “Highly efficient laser action in femtosecond-written Nd:yttrium aluminum garnet ceramic waveguides,” Appl. Phys. Lett. 92(11), 111103 (2008). [CrossRef]
  24. F. Watt, J. A. van Kan, I. Rajta, A. A. Bettiol, T. F. Choo, M. B. H. Breese, and T. Osipowicz, “The National University of Singapore high energy ion nano-probe facility: Performance tests,” Nucl. Instrum. Methods Phys. Res. B 210, 14–20 (2003). [CrossRef]
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