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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 4 — Feb. 19, 2007
  • pp: 1817–1822
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Direct laser writing of three-dimensional photonic crystal lattices within a PbS quantum-dot-doped polymer material

Michael James Ventura, Craig Bullen, and Min Gu  »View Author Affiliations


Optics Express, Vol. 15, Issue 4, pp. 1817-1822 (2007)
http://dx.doi.org/10.1364/OE.15.001817


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Abstract

We report on the synthesis of a homogenous PbS quantum-dot-doped polymer material of thickness up to 100 micrometers. It is shown that high quality micro-void channels of submicrometer diameters can be directly fabricated into this nanocomposite by using an ultrafast femtosecond laser beam. Periodically stacked channels in the form of a three-dimensional photonic crystal woodpile lattices reveals a main stop gaps as well as higher-order gaps that overlaps the near-infrared emission wavelength range of PbS quantum dots. These partial stop gaps are well defined in an angular range from zero to 15 degrees in the stacking direction.

© 2007 Optical Society of America

1. Introduction

Direct writing of micro-structures in a thick solid medium has proven to be a powerful method for many photonic applications including optical data storage and photonic crystals [1–6

1. H. Misawa and S. Juodkazis, eds., 3D laser microfabrication, Principles and applications (Wiley-Vch Verlag, Weinheim, 2006). [CrossRef]

]. Micro-explosions induced by an ultrafast femtosecond laser beam focused by a high numerical-aperture (NA) objective has facilitated the generation of micro-voids in glass [7

7. E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. H. Her, J. P. Callan, and E. Mazur, “Three-dimensional optical storage inside transparent materials,” Opt. Lett. 21,2023–2025 (1996). [CrossRef] [PubMed]

], nonlinear crystals [8

8. G. Zhou and M. Gu, “Anisotropic properties of ultrafast laser-driven microexplosions in lithium niobate crystal,” App. Phys. Lett. 87,1–3 (2005). [CrossRef]

] and polymer materials [2

2. D. Day and M. Gu, “Formation of voids in a doped polymethylmethacrylate polymer,” App. Phys Lett. 80,2404–2406 (2002). [CrossRef]

, 9

9. K. Yamasaki, S. Juodkazis, M. Watanabe, H. B. Sun, S. Matsuo, and H. Misawa, “Recording by microexplosion and two-photon reading of three-dimensional optical memory in polymethylmethacrylate films,” App. Phys. Lett. 76,1000–1002 (2000). [CrossRef]

]. In particular, the low threshold power for generating voids in polymer materials allows for the fabrication of micro-void channels of a high degree of perfection [3

3. M. J. Ventura, M. Straub, and M. Gu, “Void channel microstructures in resin solids as an efficient way to infrared photonic crystals,” App. Phys Lett. 82,1649–1651 (2003). [CrossRef]

]. This has provided a method for creating three-dimensional (3D) woodpile photonic crystal lattices with multiple-order stop gaps in the near-infrared wavelength region [4

4. M. Straub, M. Ventura, and M. Gu, “Multiple higher-order stop gaps in infrared polymer photonic crystals,” Phys. Rev. Lett. 91,043901 (2003). [CrossRef] [PubMed]

]. Another advantage of using polymer is the linear and nonlinear optical property of the photonic crystals can be engineered if semiconductor nanocrystals or quantum dots (QD) whose size determines their absorption and emission properties are doped in polymers.

2. Nanocomposite synthesis

PbS QDs were chosen in our experiments because they have a large band gap in the near-infrared wavelength range which matches the stop gaps of the 3D photonic crystals fabricated in NOA63 [3

3. M. J. Ventura, M. Straub, and M. Gu, “Void channel microstructures in resin solids as an efficient way to infrared photonic crystals,” App. Phys Lett. 82,1649–1651 (2003). [CrossRef]

]. Further, it has been found that doping PbS QDs in the NOA63 resuts in less aggregation than PbSe QDs due to a cleaner chemical process. The route that was taken to achieve a homogeneous PbS QD-doped polymer involved the synthesis of semiconductor nanocrystals in a non-aqueous solution followed by surface chemistry modification. A wet chemical method based on Hines and Scholes routine [16

16. M. A. Hines and G. D. Scholes, “Colloidal PbS nanocrystals with size-tunable near-infrared emission: Observation of post-synthesis self-narrowing of the particle size distribution,” Adv. Mat. 15,1844–1849 (2003). [CrossRef]

] was used to synthesis PbS QDs. QDs were then dispersed in tetrachloroethylene (TCE) a solvent chosen for its low absorption at infrared wavelengths and finally evaluated using transmission electron microscopy (TEM) and infrared absorption microscopy. Varying synthetic conditions of lead and sulphur ratios allowed for particle size tuning, alternatively size tuning was achieved by varying the reaction time. Highly spherical QDs of mean size 4.5 nm (TEM insert Fig. 1) were obtained one minute after injection and showed a strong peak in absorption at 1.4 μm (Fig. 1). Extended reaction times of twenty minutes resulted in an equilibrium of the mean QD size of 3 nm.

Fig. 1. Infrared transmission absorption spectra of PbS (solid black line), Mercaptopropyl-trimethoxysilane (MPS) caped PbS QDs (dashed red line) and the nanocomposite material of doped NOA63 and capped PbS QDs (dotted blue line). Insert, TEM image of 4.5 μm PbS QDs, showing their uniform shape and size.

Purified PbS QDs were doped into the commercial optical adhesive NOA63 obtained from Norland Products. This ultra-violet sensitive photo-polymerizable resin has a refractive index of 1.542 upon solidification and is transparent at infrared wavelengths. Direct doping of QDs into this material caused substantial and rapid aggregation of the QDs, and an inhomogeneous composite. Varying the dispersion solvent, QD concentration, temperature of the resin, and mixing method did little to improve the quality of the composites. To overcome this problem the surface chemistry of the QDs was altered by treatment with Mercaptopropyl-trimethoxysilane (MPS) followed by ultra-sonication [17

17. L. Bakueva, I. Gorelikov, S. Musikhin, X. S. Zhao, E. H. Sargent, and E. Kumacheva, “PbS quantum dots with stable efficient luminescence in the near-IR spectral range,” Adv. Mat. 16,926–929 (2004). [CrossRef]

]. In this way, homogenous nanocomposites were obtained both before and after photo-polymerization. Figure 1 illustrates that the absorption spectra pre (solid black line) and post (dashed red line) capping are unchanged suggesting that capping does not change the individual nano-crystal dimensions.

The MPS stabilized PbS QDs were then introduced by volume to the host matrix and stirred vigorously to form a homogeneous, transparent material. Samples were then sandwiching between two glass cover slips with a spacer of approximately 100 μm, which is measured by micrometer vernire caliper, and exposed to a broad UV light source for one hour. Upon solidification all the photo-initiators and TCE were removed leaving a transparent homogeneously dispersed nanocomposite. It was noted however that an upper limit existed at which aggregation of QDs prevented the synthesis of a homogeneous material. Infrared absorption microscopy of a sample doped at a low density of 0.05 parts per million (PPM) shows a peak coinciding with the QD absorption band at 1.4 μm (Fig. 1, dotted blue line) and also highlights that at low concentrations the contribution of the QD absorption in the nanocomposite is far outweighed by the inherent polymer absorptions of the matrix visible beyond 1.6 μm.

3. Characterization

Fig. 2. (a). Refractive index as a function of doping levels (parts per million) of PbS nano-crystals to NOA63. Microscope transmission images of (b) un-doped NOA63 (50 μm scale bar), (c) nanocomposite with a PPM of 0.05, (d) nanocomposite with a PPM of 0.6 and (d) nanocomposite with a PPM greater than 1 PPM.

4. Micro-void channel woodpile photonic crystals

To explore the feasibility of fabricating complex micro devices in the homogeneous nanocomposite samples, we adopted the micro-explosion method using a femtosecond laser beam [3

3. M. J. Ventura, M. Straub, and M. Gu, “Void channel microstructures in resin solids as an efficient way to infrared photonic crystals,” App. Phys Lett. 82,1649–1651 (2003). [CrossRef]

]. This single step, direct writing process involved the focusing of femtosecond pulsed laser light at a wavelength of 570 nm using a 100× 1.4 high NA inside the nanocomposite in the experimental setup reported elsewhere [3

3. M. J. Ventura, M. Straub, and M. Gu, “Void channel microstructures in resin solids as an efficient way to infrared photonic crystals,” App. Phys Lett. 82,1649–1651 (2003). [CrossRef]

, 4

4. M. Straub, M. Ventura, and M. Gu, “Multiple higher-order stop gaps in infrared polymer photonic crystals,” Phys. Rev. Lett. 91,043901 (2003). [CrossRef] [PubMed]

].

3D woodpile photonic crystals were fabricated with an in-plane spacing (δx) of 1.4 μm and a layer-spacing (δz) of 1.5 μm [Fig. 3(a)]. We confirmed that up to 15 periods can be fabricated in the sample of a thickness of 100 μm which we have prepared. An example of six periods in the stacking direction (Γ-X′) with a total depth of 36 μm is shown Fig. 3. It should be noted that beyond this depth the effect of refractive index mismatch [1

1. H. Misawa and S. Juodkazis, eds., 3D laser microfabrication, Principles and applications (Wiley-Vch Verlag, Weinheim, 2006). [CrossRef]

, 18

18. D. Day and M. Gu, “Effects of refractive-index mismatch on three-dimensional optical data-storage density in a two-photon bleaching polymer,” Appl. Opt. 37,6299–6304 (1998). [CrossRef]

] significantly distorts the 3D structure. Fabrication conditions were fixed to a power of 19 mW and a scanning speed 1000 μm/sec. No visible difference between individual channels was noted between the doped and un-doped materials below densities of 0.5 PPM. Infrared transmission measurements in the stacking direction showed pronounced main and higher-order photonic bandgaps in both samples at approximately 4.5 μm and 3.2 μm respectively [Fig. 3(a)]. A red shift of 125 nm is noted for the doped sample and is attributed to the increased refractive index. Iterative eigen-solving of the photonic bands in the Γ-X′ direction [4

4. M. Straub, M. Ventura, and M. Gu, “Multiple higher-order stop gaps in infrared polymer photonic crystals,” Phys. Rev. Lett. 91,043901 (2003). [CrossRef] [PubMed]

] was preformed with fixed lattice parameters as mentioned while varying the refractive index and the main and first higher-order gap positions were plotted [Fig. 3(b)]. Due to the compression of materials on formation of individual void channels the effective refractive index of the un-doped sample was calculated to be 1.67 [Fig. 3(b)], solid black line), comparable with the previously reported structures [4

4. M. Straub, M. Ventura, and M. Gu, “Multiple higher-order stop gaps in infrared polymer photonic crystals,” Phys. Rev. Lett. 91,043901 (2003). [CrossRef] [PubMed]

]. A fit with a refractive index of 1.73 was attained for the doped structure (Fig. 3(b) dotted red line), showing that indeed the nanocomposite does affect photonic bandgap positions through the increase refractive indices.

Fig. 3. (a). Infrared transmission measurements in the stacking direction of an un-doped (solid black line) and doped (dotted red line) woodpile photonic crystal lattice. A shift of the doped lattice bandgaps to longer wavelengths fits band calculations (b) where white areas denote the band positions as a function of wavelength and refractive index. (c) A series of woodpile lattices with reducing in-plane layer spacing for a fixed δz/δx of 1.1.

The angular dependence of these partial stop gaps is revealed in Fig. 4, showing the transmission plot of a optimized structure over an angle range spanning from perpendicular to the stacking direction at zero degrees (Γ-X′) towards 15 degrees in the Γ-W’ crystal direction. The first and second-order gaps at 1.95 μm and 1.48 μm respectively can be seen to maintain a suppression of transmission of approximately 50 % (blue bands) over an angle range of 10 degrees; beyond this angle both gaps become weaker.

Fig. 4. Angularly resolved infrared transmission measurement of a nanocomposite woodpile photonic crystal over the angle range zero degrees (Γ-X′) towards 15 degrees in the Γ-W′ crystal direction. The lattice is transparent (green) at wavelength outside gaps. First-order and second-order gaps show a suppression of transmission of approximately 50 % (blue) over this range and are wavelength invariant.

5. Conclusion

In conclusion, we have synthesized a homogeneous thick nanocomposite material consisting of PbS QDs and a photo-polymerisable resin. Experiments have revealed that the bulk refractive index of the nanocomposite could be increased from 1.542 in the un-doped state to 1.55 after doping. High-quality photonic crystals of woodpile lattices with substantial higher-order bandgaps at telecommunication wavelengths have been realized using the micro-fabrication technique in this nanocomposite. It should be pointed out that compared with the PbSe-QD-doped resin in which two-photon polymerization is applicable [19

19. J. Li, B. Jia, G. Zhou, and M. Gu, “Fabrication of three-dimensional woodpile photonic crystals in a PbSe quantum dot composite material,” Opt. Express 14,10740–10745 (2006). [CrossRef] [PubMed]

], the surface quality of the 3D photonic crystals produced in the PbS-QD-doped 3D structures is better due to less aggregation, which is evident from the appearance of higher-order stop gaps shown Figs. 3 and 4.

Acknowledgments

This work was produced with the assistance of the Australian Research Council under the ARC Centres of Excellence Program. CUDOS (the Centre for Ultrahigh-bandwidth devices for Optical Systems) is an ARC Centre of Excellence.

References and links

1.

H. Misawa and S. Juodkazis, eds., 3D laser microfabrication, Principles and applications (Wiley-Vch Verlag, Weinheim, 2006). [CrossRef]

2.

D. Day and M. Gu, “Formation of voids in a doped polymethylmethacrylate polymer,” App. Phys Lett. 80,2404–2406 (2002). [CrossRef]

3.

M. J. Ventura, M. Straub, and M. Gu, “Void channel microstructures in resin solids as an efficient way to infrared photonic crystals,” App. Phys Lett. 82,1649–1651 (2003). [CrossRef]

4.

M. Straub, M. Ventura, and M. Gu, “Multiple higher-order stop gaps in infrared polymer photonic crystals,” Phys. Rev. Lett. 91,043901 (2003). [CrossRef] [PubMed]

5.

E. G. Gamaly, S. Juodkazis, K. Nishimura, H. Misawa, and B. Luther-Davies, “Laser-matter interaction in the bulk of a transparent solid: Confined microexplosion and void formation,” Phys. Rev. B 73,214101 (2006). [CrossRef]

6.

S. Juodkazis, K. Nishimura, S. Tanaka, H. Misawa, E. G. Gamaly, B. Luther-Davies, L. Hallo, P. Nicolai, and V. T. Tikhonchuk, “Laser-induced microexplosion confined in the bulk of a sapphire crystal: Evidence of multimegabar pressures,” Phys. Rev. Lett. 96,166101 (2006). [CrossRef] [PubMed]

7.

E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. H. Her, J. P. Callan, and E. Mazur, “Three-dimensional optical storage inside transparent materials,” Opt. Lett. 21,2023–2025 (1996). [CrossRef] [PubMed]

8.

G. Zhou and M. Gu, “Anisotropic properties of ultrafast laser-driven microexplosions in lithium niobate crystal,” App. Phys. Lett. 87,1–3 (2005). [CrossRef]

9.

K. Yamasaki, S. Juodkazis, M. Watanabe, H. B. Sun, S. Matsuo, and H. Misawa, “Recording by microexplosion and two-photon reading of three-dimensional optical memory in polymethylmethacrylate films,” App. Phys. Lett. 76,1000–1002 (2000). [CrossRef]

10.

K. Wundke, J. Auxier, A. Schülzgen, N. Peyghambarian, and N. F. Borrelli, “Room-temperature gain at 1.3 μm in PbS-doped glasses,” App. Phys. Lett. 75,3060–3062 (1999). [CrossRef]

11.

V. Sukhovatkin, S. Musikhin, I. Gorelikov, S. Cauchi, L. Bakueva, E. Kumacheva, and E. H. Sargent, “Room-temperature amplified spontaneous emission at 1300 nm in solution-processed PbS quantum-dot films,” Opt. Lett. 30,171–173 (2005). [CrossRef] [PubMed]

12.

L. Bakueva, S. Musikhin, M. A. Hines, T. W. F. Chang, M. Tzolov, G. D. Scholes, and E. H. Sargent, “Size-tunable infrared (1000–1600 nm) electroluminescence from PbS quantum-dot nanocrystals in a semiconducting polymer,” App. Phys. Lett. 82,2895–2897 (2003). [CrossRef]

13.

S. Hoogland, V. Sukhovatkin, I. Howard, S. Cauchi, L. Levina, and E. H. Sargent, “A solution-processed 1.53 μm quantum dot laser with temperature-invariant emission wavelength,” Opt. Express 14,3273–3281 (2006). [CrossRef] [PubMed]

14.

S. A. McDonald, G. Konstantatos, S. G. Zhang, P. W. Cyr, E. J. D. Klem, L. Levina, and E. H. Sargent, “Solution-processed PbS quantum dot infrared photodetectors and photovoltaics,” Nature Mat. 4,138–142 (2005). [CrossRef]

15.

L. Pang, Y. M. Shen, K. Tetz, and Y. Fainman, “PMMA quantum dots composites fabricated via use of pre-polymerization,” Opt. Express 13,44–49 (2005). [CrossRef] [PubMed]

16.

M. A. Hines and G. D. Scholes, “Colloidal PbS nanocrystals with size-tunable near-infrared emission: Observation of post-synthesis self-narrowing of the particle size distribution,” Adv. Mat. 15,1844–1849 (2003). [CrossRef]

17.

L. Bakueva, I. Gorelikov, S. Musikhin, X. S. Zhao, E. H. Sargent, and E. Kumacheva, “PbS quantum dots with stable efficient luminescence in the near-IR spectral range,” Adv. Mat. 16,926–929 (2004). [CrossRef]

18.

D. Day and M. Gu, “Effects of refractive-index mismatch on three-dimensional optical data-storage density in a two-photon bleaching polymer,” Appl. Opt. 37,6299–6304 (1998). [CrossRef]

19.

J. Li, B. Jia, G. Zhou, and M. Gu, “Fabrication of three-dimensional woodpile photonic crystals in a PbSe quantum dot composite material,” Opt. Express 14,10740–10745 (2006). [CrossRef] [PubMed]

OCIS Codes
(160.5470) Materials : Polymers
(220.4000) Optical design and fabrication : Microstructure fabrication

ToC Category:
Photonic Crystals

History
Original Manuscript: December 4, 2006
Revised Manuscript: January 31, 2007
Manuscript Accepted: February 8, 2007
Published: February 19, 2007

Citation
Michael J. Ventura, Craig Bullen, and Min Gu, "Direct laser writing of three-dimensional photonic crystal lattices within a PbS quantum-dot-doped polymer material," Opt. Express 15, 1817-1822 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-4-1817


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References

  1. H. Misawa and S. Juodkazis, eds., 3D laser microfabrication, Principles and applications (Wiley-Vch Verlag, Weinheim, 2006). [CrossRef]
  2. D. Day and M. Gu, "Formation of voids in a doped polymethylmethacrylate polymer," Appl. Phys Lett. 80, 2404-2406 (2002). [CrossRef]
  3. M. J. Ventura, M. Straub, and M. Gu, "Void channel microstructures in resin solids as an efficient way to infrared photonic crystals," Appl. Phys Lett. 82, 1649-1651 (2003). [CrossRef]
  4. M. Straub, M. Ventura, and M. Gu, "Multiple higher-order stop gaps in infrared polymer photonic crystals," Phys. Rev. Lett. 91, 043901 (2003). [CrossRef] [PubMed]
  5. E. G. Gamaly, S. Juodkazis, K. Nishimura, H. Misawa, and B. Luther-Davies, "Laser-matter interaction in the bulk of a transparent solid: Confined microexplosion and void formation," Phys. Rev. B 73, 214101 (2006). [CrossRef]
  6. S. Juodkazis, K. Nishimura, S. Tanaka, H. Misawa, E. G. Gamaly, B. Luther-Davies, L. Hallo, P. Nicolai, and V. T. Tikhonchuk, "Laser-induced microexplosion confined in the bulk of a sapphire crystal: Evidence of multimegabar pressures," Phys. Rev. Lett. 96, 166101 (2006). [CrossRef] [PubMed]
  7. E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. H. Her, J. P. Callan, and E. Mazur, "Three-dimensional optical storage inside transparent materials," Opt. Lett. 21, 2023-2025 (1996). [CrossRef] [PubMed]
  8. G. Zhou, and M. Gu, "Anisotropic properties of ultrafast laser-driven microexplosions in lithium niobate crystal," Appl. Phys. Lett. 87, 1-3 (2005). [CrossRef]
  9. K. Yamasaki, S. Juodkazis, M. Watanabe, H. B. Sun, S. Matsuo, and H. Misawa, "Recording by microexplosion and two-photon reading of three-dimensional optical memory in polymethylmethacrylate films," Appl. Phys. Lett. 76, 1000-1002 (2000). [CrossRef]
  10. K. Wundke, J. Auxier, A. Schülzgen, N. Peyghambarian, and N. F. Borrelli, "Room-temperature gain at 1.3 µm in PbS-doped glasses," Appl. Phys. Lett. 75, 3060-3062 (1999). [CrossRef]
  11. V. Sukhovatkin, S. Musikhin, I. Gorelikov, S. Cauchi, L. Bakueva, E. Kumacheva, and E. H. Sargent, "Room-temperature amplified spontaneous emission at 1300 nm in solution-processed PbS quantum-dot films," Opt. Lett. 30, 171-173 (2005). [CrossRef] [PubMed]
  12. L. Bakueva, S. Musikhin, M. A. Hines, T. W. F. Chang, M. Tzolov, G. D. Scholes, and E. H. Sargent, "Size-tunable infrared (1000-1600 nm) electroluminescence from PbS quantum-dot nanocrystals in a semiconducting polymer," Appl. Phys. Lett. 82, 2895-2897 (2003). [CrossRef]
  13. S. Hoogland, V. Sukhovatkin, I. Howard, S. Cauchi, L. Levina, and E. H. Sargent, "A solution-processed 1.53 µm quantum dot laser with temperature-invariant emission wavelength," Opt. Express 14, 3273-3281 (2006). [CrossRef] [PubMed]
  14. S. A. McDonald, G. Konstantatos, S. G. Zhang, P. W. Cyr, E. J. D. Klem, L. Levina, and E. H. Sargent, "Solution-processed PbS quantum dot infrared photodetectors and photovoltaics," Nat. Mater 4, 138-142 (2005). [CrossRef]
  15. L. Pang, Y. M. Shen, K. Tetz, and Y. Fainman, "PMMA quantum dots composites fabricated via use of pre-polymerization," Opt. Express 13, 44-49 (2005). [CrossRef] [PubMed]
  16. M. A. Hines, and G. D. Scholes, "Colloidal PbS nanocrystals with size-tunable near-infrared emission: Observation of post-synthesis self-narrowing of the particle size distribution," Adv. Mater. 15, 1844-1849 (2003). [CrossRef]
  17. L. Bakueva, I. Gorelikov, S. Musikhin, X. S. Zhao, E. H. Sargent, and E. Kumacheva, "PbS quantum dots with stable efficient luminescence in the near-IR spectral range," Adv. Mater. 16, 926-929 (2004). [CrossRef]
  18. D. Day, and M. Gu, "Effects of refractive-index mismatch on three-dimensional optical data-storage density in a two-photon bleaching polymer," Appl. Opt. 37, 6299-6304 (1998). [CrossRef]
  19. J. Li, B. Jia, G. Zhou, and M. Gu, "Fabrication of three-dimensional woodpile photonic crystals in a PbSe quantum dot composite material," Opt. Express 14, 10740-10745 (2006). [CrossRef] [PubMed]

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