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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 9 — May. 5, 2014
  • pp: 11090–11098
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Graphene oxide-based waveguide polariser: From thin film to quasi-bulk

W. H. Lim, Y. K. Yap, W. Y. Chong, C. H. Pua, N. M. Huang, R. M. De La Rue, and H. Ahmad  »View Author Affiliations


Optics Express, Vol. 22, Issue 9, pp. 11090-11098 (2014)
http://dx.doi.org/10.1364/OE.22.011090


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Abstract

Abstract: We have demonstrated a broadband waveguide polariser with high extinction ratio on a polymer optical waveguide coated with graphene oxide via the drop-casting method. The highest extinction ratio of nearly 40 dB is measured at 1590 nm, with a variation of 4.5 dB across a wavelength range from 1530 nm to 1630 nm, a ratio that is (to our knowledge) the highest reported for graphene-based waveguide polarisers to date. This result is achieved with a graphene oxide coating length along the propagation direction of only 1.3 mm and a bulk film thickness of 2.0 µm. The underlying principles of the strongly polarisation dependent propagation loss demonstrated have been studied and are attributed to the anisotropic complex dielectric function of graphene oxide bulk film.

© 2014 Optical Society of America

1. Introduction

Polarisation control is one of the essential functions required in the realization of photonic integrated circuits (PICs) [1

1. D. C. Hutchings and B. M. Holmes, “A waveguide polarization toolset design based on mode beating,” IEEE Photon. 3, 450 (2011).

]. Polarisation control is also required in fibre optical communication systems that exploit polarisation diversity to increase capacity [2

2. K. Kikuchi and S. Tsukamoto, “Evaluation of sensitivity of the digital coherent receiver,” J. Lightwave Technol. 26(13), 1817–1822 (2008). [CrossRef]

]. In addition, most high index-contrast waveguides currently used as platforms for PIC fabrication exhibit intrinsic birefringence, which limits the sensitivity and coherence, as well as the bandwidth, of the functional circuits required in applications such as optical sensing and optical signal processing [1

1. D. C. Hutchings and B. M. Holmes, “A waveguide polarization toolset design based on mode beating,” IEEE Photon. 3, 450 (2011).

, 3

3. S. M. Ohja, C. Cureton, T. Bricheno, S. Day, D. Moule, A. J. Bell, and J. Taylor, “Simple method of fabricating polarisation-insensitive and very low crosstalk AWG grating devices,” Electron. Lett. 34(1), 78–79 (1998). [CrossRef]

]. Devices may be needed that can change the polarisation state of the light, both actively and passively. Two approaches have been proposed and demonstrated in an effort to alleviate these limitations. The first is to control the birefringence of the waveguide structure, either by making it birefringence-free or by using polarisation compensation [3

3. S. M. Ohja, C. Cureton, T. Bricheno, S. Day, D. Moule, A. J. Bell, and J. Taylor, “Simple method of fabricating polarisation-insensitive and very low crosstalk AWG grating devices,” Electron. Lett. 34(1), 78–79 (1998). [CrossRef]

5

5. J. J. He, E. S. Koteles, B. Lamontagne, L. Erickson, A. Delage, and M. Davies, “Integrated polarization compensator for WDM waveguide demultiplexers,” IEEE Photon. Technol. Lett. 11(2), 224–226 (1999). [CrossRef]

] – while the second approach is to place an integrated polariser at the input of the PIC, in order to allow only one state of polarised light to propagate in the circuit [6

6. A. Morand, C. Sanchez-Perez, P. Benech, S. Tedjini, and D. Bose, “Integrated optical waveguide polarizer on glass with a birefringent polymer overlay,” IEEE Photon. Technol. Lett. 10(11), 1599–1601 (1998). [CrossRef]

8

8. D. Dai, Z. Wang, N. Julian, and J. E. Bowers, “Compact broadband polarizer based on shallowly-etched silicon-on-insulator ridge optical waveguides,” Opt. Express 18(26), 27404–27415 (2010). [CrossRef] [PubMed]

].

Coincidentally, it has recently been shown that GO typically exhibits a strongly anisotropic complex dielectric function [15

15. G. Eda, A. Nathan, P. Wöbkenberg, F. Colleaux, K. Ghaffarzadeh, T. D. Anthopoulos, and M. Chhowalla, “Graphene oxide gate dielectric for graphene-based monolithic field effect transistors,” Appl. Phys. Lett. 102(13), 133108 (2013). [CrossRef]

]. This property was demonstrated in FET-type electronic devices. At optical frequencies, such dielectric anisotropy may be expected to lead to differences in the propagation loss for different polarisation states, which can be used to provide the function of polarisation selection in an optical waveguide. It is therefore logical for these properties of GO to be explored for applications such as waveguide polarisers. In the present paper, a simple and cost-effective fabrication method has been developed. We demonstrate the deposition of GO multi-layers directly on to a polymer optical waveguide using the drop-casting method. High-concentration, chemically exfoliated, GO solution is drop-cast on to the polymer waveguide using a micropipette, in order to create a waveguide section with a micrometre-scale thickness GO-coating. We then exploit the polarisation selection capability of GO to demonstrate a TM-pass polymer waveguide-based polariser with a large extinction-ratio. A large polarising effect is observed in the GO-coated polymer waveguides. A maximum extinction ratio value of 40 dB, which is (we believe) the highest reported for graphene-based waveguide polarisers to date, was achieved in the 1550 nm wavelength band. It is worth mentioning that this extinction ratio value has been achieved with a GO coating length of only 1.3 mm along the propagation direction.

2. Device fabrication and characterization

A CR-39 polymer sheet with a thickness of 0.5 mm and a refractive index of 1.486, measured at 1550 nm with a Sairon Technology SPA-4000 Prism Coupler, was used as the substrate for the polymer waveguide. The optical characteristics of CR-39 resemble those of crown glass and it is commonly used in the manufacture of plastic lenses. SU-8 polymer with a refractive index of 1.569 measured at 1550 nm in a Sairon Technology SPA-4000 Prism Coupler was spin-coated onto the CR-39 sheet and patterned using the contact photolithography technique, forming the core of a channel waveguide structure. The measured height of the channel waveguide was 5.0 ( ± 0.1) μm and its width ranged between 10 and 15 μm over several different samples, with waveguide width variation of less then 0.2 μm over the length of a given waveguide section.

The GO coated samples were then diced and polished into 1 cm long waveguide channels. The fibre butt-coupling technique was used to measure the insertion loss of both the TE- and TM-polarised modes of the GO-coated waveguide channels. Guided light polarisation in the launch fibre was controlled using a fibre polarisation controller, and the polarisation state of the GO polariser output was measured in free space using a polarimeter (Thorlabs PAX 5710) before the output was fibre-coupled, in order to measure the insertion loss. Although the SU-8 waveguide core cross-section is large enough for several characteristic modes to be available for both (quasi-)TE- and (quasi-)TM-modes, the alignment of the set-up guaranteed that, to a good approximation, only the fundamental guided mode for each polarisation was launched. In addition, extra care in the handling of the launch fibre was required, since the linear polarisation state could easily be scrambled by small movements or vibration. The polarisation-dependent loss of the uncoated polymer waveguide was measured first - and was found to be lower than 0.5 dB, limited by the performance of the measurement setup. For measurements across the broad frequency range, fibres with different cut-off wavelengths were used, in order to maintain the launching polarisation state and single waveguide-mode excitation conditions.

3. Physical properties

4. Simulation and modelling

For the simulation of the guided-light propagation in the GO-coated polymer waveguide, our choice of values for the real and imaginary parts of the anisotropic complex refractive index of the GO multi-layer is consistent with values published in the literature. For the component of the optical electric-field that is normal to the GO planes, we have assumed that the imaginary part of the refractive index may be neglected. This choice is justified by the fact that electron transport in this direction will be substantially blocked, on the atomic scale, by the barriers between nearest-neighbour graphene layers formed by layers of oxygen atoms – as well as atoms of any other element, e.g. nitrogen, that may be present. The real part of the refractive index has been set to 2, corresponding to a value of 4 for the real part of the dielectric function. This choice of n = 2 is in reasonable conformity with the values (~1.9) given by Vaupel and Stobel [19

19. M. Vaupel and U. Stoberl, “Appication note: graphene and graphene oxide,” http://www.nanofilm.de/sales-support/downloads/application-notes/applicationnote_graphene.pdf.

] and is an intermediate value by comparison with refractive index values that would be obtained from the low-frequency (‘quasi-dc) values for the (relative) dielectric function that are given by Loh et al [20

20. K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nat. Chem. 2(12), 1015–1024 (2010). [CrossRef] [PubMed]

].

For the optical electric field component that is parallel to the GO layers – and therefore to the atomic sheets of graphene – the situation is substantially different. Because of the very sub-wavelength scale of the individual GO layers, an effective (average) medium approach is appropriate. For the real part of the refractive index, we have chosen to use the same value as for the perpendicular direction, but the choice for the imaginary part reflects the fact that the graphene atomic sheets, even though they are fully (or almost fully) oxidised, are still capable of significant levels of conduction. Choice of an optical-frequency conductance value of σ = 2700 S.m−1 implies a significant absorption coefficient for TE-polarised light. This value for the conductance is two orders of magnitude lower than that given by the usual Kubo expression for pure graphene (~1.8 x 105 S.m−1) – and it is also less than the value of 9.0 x 104 S.m−1 that applies for THz frequencies in reduced graphene oxide (RGO) [21

21. J. T. Hong, K. M. Lee, B. H. Son, S. J. Park, D. J. Park, J.-Y. Park, S. Lee, and Y. H. Ahn, “Terahertz conductivity of reduced graphene oxide films,” Opt. Express 21(6), 7633–7640 (2013). [CrossRef] [PubMed]

]. With these choices for the refractive indices, the effective refractive index of the guided mode is obtained via simulation. By assuming a uniform coating of GO film, the optical propagation loss of the TE- and TM-polarised modes can be calculated using the Beer-Lambert law. Close agreement between simulation and experiment is obtained, as shown in Fig. 3
Fig. 3 Polarising effect of GO-coated waveguides. a) Polar plot of the GO waveguide polariser measured at 1550 nm. The insertion loss is lowest when the incident light is highly TM-polarised, indicating a TM-pass waveguide polariser. b) Insertion loss of TE- and TM-polarised light of waveguide polarisers coated with different number of GO solution drops corresponding to different GO film thickness. Both values are measurement averages in the 1530 to 1630 nm wavelength range. c) Top view of GO coated waveguide channel with TM- and TE- polarised light (650 nm) propagating through the waveguide. There was no observable scattering in the case of TE-polarised light, while the observed scattering in the case of TM-polarised light propagation was highly p-polarised. d) Broadband response of GO waveguide polariser, where an extinction ratio of more than 20 dB is measured from 1250 – 1640 nm.
. Our specific choices for the complex refractive indices should be taken as being indicative, rather than absolute. They are physically reasonable and are sufficiently accurate for identification of the characteristic behaviour that is found in our experimental measurements.

5. Optical properties and performance

Addition of more GO solution drops did not change the size of the drop-casting area and – therefore – it should increase the number of GO layers that is observed as the film thickness increases. The addition of further drops of the GO solution beyond a certain point (typically three solution drops) does not measurably increase the propagation loss of the TM-mode, since the additional layers of GO coating are progressively further from the waveguide-GO interface and therefore interact less strongly with the light propagating in the channel waveguide.

The polarisation extinction ratio for wavelengths ranging from 650 nm to 1640 nm has been measured and is shown in Fig. 3(d). It shows a gradual decrease in value towards shorter wavelengths. The extinction ratios at wavelengths of 1590 nm, 1310 nm, 980 nm and 650 nm were found to be 40.0 dB, 25.0 dB, 16.0 dB and 8.5 dB, respectively. Though steps have been taken to ensure only fundamental mode propagation in the GO waveguide polariser, we believe that partial excitation of higher order modes of the waveguide at shorter wavelengths is quite probable and that their presence would reduce the effective strength of the interaction between the polymer waveguide and GO film, resulting in a reduced extinction ratio between the TE- and TM-polarised modes. Nevertheless, an extinction ratio of more than 20 dB has been measured over the entire wavelength range from 1250 nm to 1640 nm. The extinction ratio for wavelengths longer than 1640 nm was not measured, due to the unavailability of a suitable laser source.

The ability to introduce GO coating using the drop-casting method provides a simple and effective means for waveguide polariser fabrication. The performance of the GO waveguide polariser can be improved using the coat-and-etch method demonstrated by Kim et. al. [12

12. J. T. Kim and C.-G. Choi, “Graphene-based polymer waveguide polarizer,” Opt. Express 20(4), 3556–3562 (2012). [CrossRef] [PubMed]

], or (for example) a laser ablated waveguide [22

22. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef] [PubMed]

], where the GO film will only be coated on top of the channel waveguide. In addition, this method also enables the flexibility of selective-areas drop-casting on an optical circuit intended for the polarisation function, without the need for a physical mask during deposition or mechanical transfer of the graphene layers. With the use of an automated micropipette positioning and dispensing process, accurately localized deposition of GO coatings could be achieved in volume production.

6. Conclusions

A broadband waveguide polariser with high extinction ratio has been demonstrated in a polymer waveguide coated with GO film deposited using the drop-casting method. Drop-casting of GO solution results in a substantially ordered GO layer stack, with its thickness increasing as the number of GO droplets applied increases. The polarisation effect of the GO waveguide polariser has been shown to be a result of the anisotropic complex dielectric function of GO film. The extinction ratio of the GO polariser is dependent on the GO film thickness. The average extinction ratio is 38 dB between 1530 nm and 1630 nm, with the highest extinction ratio of 40dB measured at 1590 nm and the lowest value of 35.5 dB measured at 1530 nm. The extinction ratio is achieved with only ~1.3 mm of GO coating length along the propagation direction. The short interaction length required to produce a high extinction ratio over a broad fibre-telecom wavelength band will provide a solution for the integrated waveguide polarisers required in applications that include optical Lab-on-Chip and photonic integrated circuits.

Acknowledgment

This work was supported by University of Malaya High Impact Research Grant (UM.C/625/1/HIR/MOHE/SCI/29), and UM Grant (RU002/2013) and (BK026-2011B).

References and links

1.

D. C. Hutchings and B. M. Holmes, “A waveguide polarization toolset design based on mode beating,” IEEE Photon. 3, 450 (2011).

2.

K. Kikuchi and S. Tsukamoto, “Evaluation of sensitivity of the digital coherent receiver,” J. Lightwave Technol. 26(13), 1817–1822 (2008). [CrossRef]

3.

S. M. Ohja, C. Cureton, T. Bricheno, S. Day, D. Moule, A. J. Bell, and J. Taylor, “Simple method of fabricating polarisation-insensitive and very low crosstalk AWG grating devices,” Electron. Lett. 34(1), 78–79 (1998). [CrossRef]

4.

C. K. Nadler, E. K. Wildermuth, M. Lanker, W. Hunziker, and H. Melchior, “Polarization insensitive, low-loss, low-crosstalk wavelength multiplexer modules,” IEEE J. Sel. Top. Quant 5(5), 1407–1412 (1999). [CrossRef]

5.

J. J. He, E. S. Koteles, B. Lamontagne, L. Erickson, A. Delage, and M. Davies, “Integrated polarization compensator for WDM waveguide demultiplexers,” IEEE Photon. Technol. Lett. 11(2), 224–226 (1999). [CrossRef]

6.

A. Morand, C. Sanchez-Perez, P. Benech, S. Tedjini, and D. Bose, “Integrated optical waveguide polarizer on glass with a birefringent polymer overlay,” IEEE Photon. Technol. Lett. 10(11), 1599–1601 (1998). [CrossRef]

7.

H. Lin, J. Ning, and G. Fan, “A waveguide polarizer based on Si-coated Ti:LiNbO3 planar structure,” Chin. Opt. Lett. 2, 89 (2004).

8.

D. Dai, Z. Wang, N. Julian, and J. E. Bowers, “Compact broadband polarizer based on shallowly-etched silicon-on-insulator ridge optical waveguides,” Opt. Express 18(26), 27404–27415 (2010). [CrossRef] [PubMed]

9.

J. R. Feth and C. L. Chang, “Metal-clad fiber-optic cutoff polarizer,” Opt. Lett. 11(6), 386–388 (1986). [CrossRef] [PubMed]

10.

Q. Bao, H. Zhang, B. Wang, Z. Ni, C. H. Y. X. Lim, W. Wang, D. Y. Tang, and K. P. Loh, “Broadband graphene polarizer,” Nat. Photonics 5(7), 411–415 (2011). [CrossRef]

11.

J. T. Kim and S.-Y. Choi, “Graphene-based plasmonic waveguides for photonic integrated circuits,” Opt. Express 19(24), 24557–24562 (2011). [CrossRef] [PubMed]

12.

J. T. Kim and C.-G. Choi, “Graphene-based polymer waveguide polarizer,” Opt. Express 20(4), 3556–3562 (2012). [CrossRef] [PubMed]

13.

P. Sun, R. Ma, K. Wang, M. Zhong, J. Wei, D. Wu, T. Sasaki, and H. Zhu, “Suppression of the coffee-ring effect by self-assembling graphene oxide and monolayer titania,” Nanotechnology 24(7), 075601 (2013). [CrossRef] [PubMed]

14.

D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner, G. H. B. Dommett, G. Evmenenko, S. T. Nguyen, and R. S. Ruoff, “Preparation and characterization of graphene oxide paper,” Nature 448(7152), 457–460 (2007). [CrossRef] [PubMed]

15.

G. Eda, A. Nathan, P. Wöbkenberg, F. Colleaux, K. Ghaffarzadeh, T. D. Anthopoulos, and M. Chhowalla, “Graphene oxide gate dielectric for graphene-based monolithic field effect transistors,” Appl. Phys. Lett. 102(13), 133108 (2013). [CrossRef]

16.

N. M. Huang, H. N. Lim, C. H. Chia, M. A. Yarmo, and M. R. Muhamad, “Simple room-temperature preparation of high-yield large-area graphene oxide,” Int. J. Nanomedicine 6, 3443–3448 (2011). [CrossRef] [PubMed]

17.

A. Buchsteiner, A. Lerf, and J. Pieper, “Water dynamics in graphite oxide investigated with neutron scattering,” J. Phys. Chem. B 110(45), 22328–22338 (2006). [CrossRef] [PubMed]

18.

A. Lerf, A. Buchsteiner, J. Pieper, S. Schöttl, I. Dekany, T. Szabo, and H. P. Boehm, “Hydration behavior and dynamics of water molecules in graphite oxide,” J. Phys. Chem. Solids 67(5-6), 1106–1110 (2006). [CrossRef]

19.

M. Vaupel and U. Stoberl, “Appication note: graphene and graphene oxide,” http://www.nanofilm.de/sales-support/downloads/application-notes/applicationnote_graphene.pdf.

20.

K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nat. Chem. 2(12), 1015–1024 (2010). [CrossRef] [PubMed]

21.

J. T. Hong, K. M. Lee, B. H. Son, S. J. Park, D. J. Park, J.-Y. Park, S. Lee, and Y. H. Ahn, “Terahertz conductivity of reduced graphene oxide films,” Opt. Express 21(6), 7633–7640 (2013). [CrossRef] [PubMed]

22.

K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef] [PubMed]

OCIS Codes
(130.0130) Integrated optics : Integrated optics
(230.0230) Optical devices : Optical devices
(130.5440) Integrated optics : Polarization-selective devices

ToC Category:
Integrated Optics

History
Original Manuscript: February 5, 2014
Revised Manuscript: March 27, 2014
Manuscript Accepted: March 30, 2014
Published: May 1, 2014

Citation
W. H. Lim, Y. K. Yap, W. Y. Chong, C. H. Pua, N. M. Huang, R. M. De La Rue, and H. Ahmad, "Graphene oxide-based waveguide polariser: From thin film to quasi-bulk," Opt. Express 22, 11090-11098 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-9-11090


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References

  1. D. C. Hutchings, B. M. Holmes, “A waveguide polarization toolset design based on mode beating,” IEEE Photon. 3, 450 (2011).
  2. K. Kikuchi, S. Tsukamoto, “Evaluation of sensitivity of the digital coherent receiver,” J. Lightwave Technol. 26(13), 1817–1822 (2008). [CrossRef]
  3. S. M. Ohja, C. Cureton, T. Bricheno, S. Day, D. Moule, A. J. Bell, J. Taylor, “Simple method of fabricating polarisation-insensitive and very low crosstalk AWG grating devices,” Electron. Lett. 34(1), 78–79 (1998). [CrossRef]
  4. C. K. Nadler, E. K. Wildermuth, M. Lanker, W. Hunziker, H. Melchior, “Polarization insensitive, low-loss, low-crosstalk wavelength multiplexer modules,” IEEE J. Sel. Top. Quant 5(5), 1407–1412 (1999). [CrossRef]
  5. J. J. He, E. S. Koteles, B. Lamontagne, L. Erickson, A. Delage, M. Davies, “Integrated polarization compensator for WDM waveguide demultiplexers,” IEEE Photon. Technol. Lett. 11(2), 224–226 (1999). [CrossRef]
  6. A. Morand, C. Sanchez-Perez, P. Benech, S. Tedjini, D. Bose, “Integrated optical waveguide polarizer on glass with a birefringent polymer overlay,” IEEE Photon. Technol. Lett. 10(11), 1599–1601 (1998). [CrossRef]
  7. H. Lin, J. Ning, G. Fan, “A waveguide polarizer based on Si-coated Ti:LiNbO3 planar structure,” Chin. Opt. Lett. 2, 89 (2004).
  8. D. Dai, Z. Wang, N. Julian, J. E. Bowers, “Compact broadband polarizer based on shallowly-etched silicon-on-insulator ridge optical waveguides,” Opt. Express 18(26), 27404–27415 (2010). [CrossRef] [PubMed]
  9. J. R. Feth, C. L. Chang, “Metal-clad fiber-optic cutoff polarizer,” Opt. Lett. 11(6), 386–388 (1986). [CrossRef] [PubMed]
  10. Q. Bao, H. Zhang, B. Wang, Z. Ni, C. H. Y. X. Lim, W. Wang, D. Y. Tang, K. P. Loh, “Broadband graphene polarizer,” Nat. Photonics 5(7), 411–415 (2011). [CrossRef]
  11. J. T. Kim, S.-Y. Choi, “Graphene-based plasmonic waveguides for photonic integrated circuits,” Opt. Express 19(24), 24557–24562 (2011). [CrossRef] [PubMed]
  12. J. T. Kim, C.-G. Choi, “Graphene-based polymer waveguide polarizer,” Opt. Express 20(4), 3556–3562 (2012). [CrossRef] [PubMed]
  13. P. Sun, R. Ma, K. Wang, M. Zhong, J. Wei, D. Wu, T. Sasaki, H. Zhu, “Suppression of the coffee-ring effect by self-assembling graphene oxide and monolayer titania,” Nanotechnology 24(7), 075601 (2013). [CrossRef] [PubMed]
  14. D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner, G. H. B. Dommett, G. Evmenenko, S. T. Nguyen, R. S. Ruoff, “Preparation and characterization of graphene oxide paper,” Nature 448(7152), 457–460 (2007). [CrossRef] [PubMed]
  15. G. Eda, A. Nathan, P. Wöbkenberg, F. Colleaux, K. Ghaffarzadeh, T. D. Anthopoulos, M. Chhowalla, “Graphene oxide gate dielectric for graphene-based monolithic field effect transistors,” Appl. Phys. Lett. 102(13), 133108 (2013). [CrossRef]
  16. N. M. Huang, H. N. Lim, C. H. Chia, M. A. Yarmo, M. R. Muhamad, “Simple room-temperature preparation of high-yield large-area graphene oxide,” Int. J. Nanomedicine 6, 3443–3448 (2011). [CrossRef] [PubMed]
  17. A. Buchsteiner, A. Lerf, J. Pieper, “Water dynamics in graphite oxide investigated with neutron scattering,” J. Phys. Chem. B 110(45), 22328–22338 (2006). [CrossRef] [PubMed]
  18. A. Lerf, A. Buchsteiner, J. Pieper, S. Schöttl, I. Dekany, T. Szabo, H. P. Boehm, “Hydration behavior and dynamics of water molecules in graphite oxide,” J. Phys. Chem. Solids 67(5-6), 1106–1110 (2006). [CrossRef]
  19. M. Vaupel, U. Stoberl, “Appication note: graphene and graphene oxide,” http://www.nanofilm.de/sales-support/downloads/application-notes/applicationnote_graphene.pdf .
  20. K. P. Loh, Q. Bao, G. Eda, M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nat. Chem. 2(12), 1015–1024 (2010). [CrossRef] [PubMed]
  21. J. T. Hong, K. M. Lee, B. H. Son, S. J. Park, D. J. Park, J.-Y. Park, S. Lee, Y. H. Ahn, “Terahertz conductivity of reduced graphene oxide films,” Opt. Express 21(6), 7633–7640 (2013). [CrossRef] [PubMed]
  22. K. M. Davis, K. Miura, N. Sugimoto, K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef] [PubMed]

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