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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 20 — Sep. 27, 2010
  • pp: 20894–20899
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Tunable photonic crystal based on capillary attraction and repulsion

Chia-Tsung Chan and J. Andrew Yeh  »View Author Affiliations


Optics Express, Vol. 18, Issue 20, pp. 20894-20899 (2010)
http://dx.doi.org/10.1364/OE.18.020894


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Abstract

A tunable photonic crystal (PhC) based on the capillary action of liquid is demonstrated in this work. The porous silicon-based photonic crystal (PSiPhC) features periodic porosity and is fabricated by electrochemical etching on 6” silicon wafer followed by hydrophobic modification on the silicon surface. The capillary action is achieved by varying the mixture ratio of liquids with high and low surface tension, yielding either capillary attraction or capillary repulsion in the nanoscale voids of the PSiPhC. By delivering the liquid mixture into and out of the voids of the PSiPhC, the reflective color of the PSiPhC can be dynamically tuned.

© 2010 OSA

1. Introduction

Tunable photonic crystals (PhCs) are PhCs with photonic band gap (PBG) tunability [1

1. K. Busch and S. John, “Liquid-crystal photonic-band-gap materials: the tunable electromagnetic vacuum,” Phys. Rev. Lett. 83(5), 967–970 (1999). [CrossRef]

]. Tunability in the visible range permits color changes in the PhCs and can potentially lead to applications in imaging, displays, and lighting, among others. During the past decade, reported driving methods for tunable PhCs in the visible range include mechanical deformation [2

2. K. Yoshino, Y. Kawagishi, M. Ozaki, and A. Kose, “Mechanical tuning of the optical properties of plastic opal as a photonic crystal,” Jpn. J. Appl. Phys. 38(Part 2, No. 7A), L786–L788 (1999). [CrossRef]

], magnetically induced particle motion [3

3. X. Xu, G. Friedman, K. D. Humfeld, S. A. Majetich, and S. A. Asher, “Synthesis and utilization of monodisperse superparamagnetic colloidal particles for magnetically controllable photonic crystals,” Chem. Mater. 14(3), 1249–1256 (2002). [CrossRef]

], chemical swelling [4

4. H. Fudouzi and Y. Xia, “Colloidal crystals with tunable colors and their use as photonic papers,” Langmuir 19(23), 9653–9660 (2003). [CrossRef]

], the electro-optic effect of ferroelectrics [5

5. B. Li, J. Zhou, L. Li, X. J. Wang, X. H. Liu, and J. Zi, “Ferroelectric inverse opals with electrically tunable photonic band gap,” Appl. Phys. Lett. 83(23), 4704–4706 (2003). [CrossRef]

], temperature controlled dopant solubility in liquid crystal [6

6. Y. Huang, Y. Zhou, C. Doyle, and S. T. Wu, “Tuning the photonic band gap in cholesteric liquid crystals by temperature-dependent dopant solubility,” Opt. Express 14(3), 1236–1242 (2006). [CrossRef] [PubMed]

], thermally induced phase transition of liquid crystal [7

7. K. Yoshino, Y. Shimoda, Y. Kawagishi, K. Nakayama, and M. Ozaki, “Temperature tuning of the stop band in transmission spectra of liquid-crystal infiltrated synthetic opal as tunable photonic crystal,” Appl. Phys. Lett. 75(7), 932–934 (1999). [CrossRef]

], electrically controlled birefringence of liquid crystal [8

8. Y. Shimoda, M. Ozaki, and K. Yoshino, “Electric field tuning of a stop band in a reflection spectrum of synthetic opal infiltrated with nematic liquid crystal,” Appl. Phys. Lett. 79(22), 3627–3629 (2001). [CrossRef]

], and liquid mixture imbibitions [9

9. J. Li, W. Huang, and Y. Han, “Tunable photonic crystals by mixed liquids,” Colloids Surf. A Physicochem. Eng. Asp. 279(1-3), 213–217 (2006). [CrossRef]

]. Table 1

Table 1. Methods for Tunable PhCs at Visible Range (An asterisk * denotes the active media in PhC.)

table-icon
View This Table
summarizes approaches mentioned above that adjust the lattice constant a, refractive index n, or both a and n for tunable PhCs in the visible range.

In this work, we proposed a tunable PhC method based on the capillary action of liquid. As shown in Fig. 1
Fig. 1 Schematic illustration of PhC with hydrophobic voids immersed in binary mixture; the capillary action is achieved by varying the mixture ratio of liquids with high and low surface tension.
, a PhC with hydrophobic voids is immersed in a binary liquid mixture. The surface tension of the liquid mixture can be modulated by varying the mixture ratio of liquids with high and low surface tension. The PhC is tuned using reciprocal capillary action that drives liquid in and out of the voids of the PhC by modulating the surface tension of the liquid mixture.

Porous silicon-based photonic crystal (PSiPhC) is used in this application as the higher-index medium. Porous silicon with parametric pore sizes and porosity (i.e. volume densities of the pores) can be fabricated by electrochemical etching [10

10. V. Lehmann, R. Stengl, and A. Luigart, “On the morphology and the electrochemical formation mechanism of mesoporous silicon,” Mater. Sci. Eng. B 69–70, 11–22 (2000). [CrossRef]

]. The periodic refractive index structure of the PSiPhC can be achieved in a one step etching process [11

11. C. Mazzoleni and L. Pavesi, “Application to optical components of dielectric porous silicon multilayers,” Appl. Phys. Lett. 67(20), 2983–2985 (1995). [CrossRef]

]. The nanoscale voids of the porous silicon also benefit the capillary action for the scale effect of capillary pressure. The lower-index medium is constituted of an ethanol-water mixture and vapor in the voids of the PSiPhC. The surface tension of the ethanol-water mixture can be adjusted from 22.0 dyne/cm to 72.0 dyne/cm over the entire concentration range at room temperature [12

12. G. Vhquez, E. Alvarez, and J. M. Navaza, “Surface Tension of Alcohol + Water from 20 to 50 °C,” J. Chem. Eng. Data 40(3), 611–614 (1995). [CrossRef]

]. In addition, the combination of silicon and liquid/vapor has a strong contrast of refractive index over the visible spectrum.

To clarify the significance of the liquid mixture in the PSiPhC, we compare the method of liquid mixture imbibitions and our work. The former has an adjustable refractive index of the liquid mixture, and the liquid mixture is permanently stayed in the PhC voids [9

9. J. Li, W. Huang, and Y. Han, “Tunable photonic crystals by mixed liquids,” Colloids Surf. A Physicochem. Eng. Asp. 279(1-3), 213–217 (2006). [CrossRef]

]. In contrast, the liquid mixture in our work has adjustable surface tension while offering no significant change in refractive index. The refractive index change of the PSiPhC primarily relies on the fill ratio of the liquid in the voids.

2. Sample preparation

The PSiPhC was fabricated on a six-inch 0.004 Ω-cm n-type (111) silicon wafer where an area of about three inches at the center defined by a polypropylene (PP) mask permits electric current to flow through the silicon surfaces. The electrochemical etching process was carried out in 1:2.33 mixtures of 49% hydrogen fluoride (HF) and 95% ethanol at 10 °C. Periodic structures were fabricated by alternating the current density between 100 mA/cm2 and 40 mA/cm2, which resulted in alternating high and low porosity structure in the PSiPhC. The PSiPhC was cleaned by an ethanol rinse and air dry. The voids' surfaces of the PSiPhC were deposited with a heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane (FDTS) self-assembled monolayer (SAM) via molecular vapor deposition [13

13. Z. H. Yang, C. Y. Chiu, J. T. Yang, and J. A. Yeh, “Investigation and Application of an artificially hybrid-structured surface with ultrahydrophobic and anti-sticking character,” J. Micromech. Microeng. 19, 085022–085033 (2009). [CrossRef]

,14

14. Y. X. Zhuang, O. Hansen, T. Knieling, C. Wang, P. Rombach, W. Lang, W. Benecke, M. Kehlenbeck, and J. Koblitz, “Vapor-phase self-assembled monolayers for anti-stiction applications in MEMS,” J. MEMS 16, 1451–1460 (2007).

]. During the deposition process, the PSiPhC was pretreated using oxygen plasma for 10 minutes. Then, the deposition reaction of 15 minutes was carried out. In the reaction, FDTS and de-ionized water acted as a precursor and a catalytic agent respectively. The hydrophobic FDTS SAM is conformal and uniform, reducing fluidic friction in the voids of the PSiPhC and making the voids initially hydrophobic.

The nanostructures of the PSiPhC were observed using a scanning electron microscope (SEM), as shown in Fig. 2
Fig. 2 The periodic structure of the PSiPhC revealed by an SEM cross-section image
. The SEM micrograph of the cross section reveals five and a half periods of alternating porosity. The PSiPhC is estimated to be 520 nm in thickness. Reflectivity of the PSiPhC over visible spectra (i.e. 400 nm to 700 nm) was measured at normal incidence and at an incident angle of 45° (see Fig. 3
Fig. 3 Reflectivity spectra of the PSiPhC at normal incidence 0° and at an incident angle of 45°
). The maximum reflection peak at normal incidence, denoted as peak A, approximates 80% at a center wavelength of 470 nm. At an incident angle of 45°, the peak A shifts to a shorter wavelength of about 400 nm. Unlike an ideal quarter wave reflector, more than one peak was observed in the reflection spectra because the low porosity layers are intentionally thinner to facilitate fluidic motion.

3. Experiments

Contact angle and reflectivity measurements for the ethanol-water mixtures on top of the PSiPhC were carried out before the color-changing experiment. The ethanol-water mixtures are composed of 99.5% ethanol and de-ionized water with different mixture ratios. As shown in Fig. 4
Fig. 4 Contact angles of ethanol-water mixtures on top of the PSiPhC
, the contact angle measurements reveal the affinity between the ethanol-water mixtures and the FTDS coated PSiPhC surface. The contact angle reduced from 105° to 30° with the increase ethanol concentration of the mixture from 0% to 99.5%. This contact angle result demonstrates the significant change of affinity between the mixture and the PSiPhC surface. As shown in Fig. 5
Fig. 5 Colorless droplets of 0%, 50% and 99.5% ethanol mixtures on top of the PSiPhC
, colorless droplets of 0% (i.e. pure de-ionized water), 50%, and 99.5% ethanol mixtures on top of the PSiPhC were inspected from the top and a 45° angle. The 0% ethanol droplet has the color of the underneath PSiPhC unaltered as shown in the top view and the 45° view. The unaltered color implies no infiltrated liquid in the voids of the PSiPhC. Contrastingly, the PSiPhC surface under the 50% and 99.5% ethanol droplets shows remarkable color changes from cyan blue toward green (as shown in the top view) and from purple toward cyan blue (as shown in the 45° view). A significant increase of the refractive index of the PSiPhC voids can explain the color change toward longer wavelength and indicates that the PSiPhC is infiltrated with liquid. Finally, quantitative reflectivity measurements for the color changes were carried out with individual ethanol-water droplets sandwiched by transparent glasses and the PSiPhC. As shown in Fig. 6
Fig. 6 Reflectivity spectra of the PSiPhC under ethanol-water mixtures
, the center wavelength of peak A shifts from 470 nm to 530 nm with the increase of ethanol concentration, which is consistent with the observed color changes in Fig. 5.

To demonstrate the color-changing experiment, the PSiPhC was initially immersed in a 0% ethanol mixture (i.e. pure de-ionized water). Then the ethanol mixture was gradually exchanged with 99.5% ethanol to obtain specific ethanol concentrations from 0% to 90%. In each step, the drained ethanol mixture and the newly added 99.5% ethanol were measured and precisely controlled prior the exchanging procedure. The time duration between each step was minimized to abate the influence of liquid evaporation. After the concentration of the ethanol mixture in the trough reached 90%, the ethanol concentration was reduced in a similar way by gradually exchanging the ethanol mixture with de-ionized water until the ethanol concentrations dropped to 2.5%.

4. Experimental results

The color evolution of the PSiPhC in the experiment is shown in Fig. 7
Fig. 7 Color evolution of the PSiPhC immersed in the ethanol-water mixture; the cyan-blue color of the PSiPhC turned to green with the increase of the ethanol concentration from 0% to 90% and returned to cyan blue with the decrease of the ethanol concentration from 90% to 2.5%.
. The irregular line patterns on the PSiPhC result from the effect of non-uniform doping of silicon after electrochemical etching. At first, the color of the PSiPhC under 0% ethanol was unaltered cyan blue, which indicates no infiltrated liquid in the voids of the PSiPhC. After gradually increasing the ethanol concentration to 90%, the color of the PSiPhC evolved to green. This color evolution indicates that the voids of the PSiPhC were gradually being infiltrated with ethanol mixture. After gradually decreasing the ethanol concentration to 2.5%, the color of the PSiPhC returned to unaltered cyan blue. Consequently, the color evolution proves the capillary attraction and repulsion of the liquid mixture in the voids of the PSiPhC.

Finally we carried out in situ reflectivity measurement and examined the reversibility of the tunable PhC. The center wavelength of peak A as function of the varying ethanol concentrations is shown in Fig. 8
Fig. 8 Hysteresis of the capillary-driven tuning process for the tunable PSiPhC.
. Remarkably, a significant hysteresis effect reveals the different energy barriers during the capillary-driven tuning process. This hysteresis imply that the liquid exfiltration requires more energy to restore a higher energy state (i.e. the liquid stayed on top of the vapor filled PSiPhC), whereas the liquid infiltration requires less energy to penetrate the vapor voids in the PSiPhC with liquid [15

15. C. Ishino, K. Okumura, and D. Quéré, “Wetting transitions on rough surfaces,” Europhys. Lett. 68(3), 419–425 (2004). [CrossRef]

].

5. Conclusion

In this work, the nanoscale voids of a PSiPhC were hydrophobically modified with FTDS SAM. The capillary attraction and repulsion in the nanoscale voids of the PSiPhC were achieved by modulating the surface tension of the liquid mixture. The color of the PSiPhC was tuned by capillary-driven liquid infiltration and exfiltration. Consequently, capillary action is proven to be an applicable driving method for dynamically tuning the PBG of PhCs.

References and links

1.

K. Busch and S. John, “Liquid-crystal photonic-band-gap materials: the tunable electromagnetic vacuum,” Phys. Rev. Lett. 83(5), 967–970 (1999). [CrossRef]

2.

K. Yoshino, Y. Kawagishi, M. Ozaki, and A. Kose, “Mechanical tuning of the optical properties of plastic opal as a photonic crystal,” Jpn. J. Appl. Phys. 38(Part 2, No. 7A), L786–L788 (1999). [CrossRef]

3.

X. Xu, G. Friedman, K. D. Humfeld, S. A. Majetich, and S. A. Asher, “Synthesis and utilization of monodisperse superparamagnetic colloidal particles for magnetically controllable photonic crystals,” Chem. Mater. 14(3), 1249–1256 (2002). [CrossRef]

4.

H. Fudouzi and Y. Xia, “Colloidal crystals with tunable colors and their use as photonic papers,” Langmuir 19(23), 9653–9660 (2003). [CrossRef]

5.

B. Li, J. Zhou, L. Li, X. J. Wang, X. H. Liu, and J. Zi, “Ferroelectric inverse opals with electrically tunable photonic band gap,” Appl. Phys. Lett. 83(23), 4704–4706 (2003). [CrossRef]

6.

Y. Huang, Y. Zhou, C. Doyle, and S. T. Wu, “Tuning the photonic band gap in cholesteric liquid crystals by temperature-dependent dopant solubility,” Opt. Express 14(3), 1236–1242 (2006). [CrossRef] [PubMed]

7.

K. Yoshino, Y. Shimoda, Y. Kawagishi, K. Nakayama, and M. Ozaki, “Temperature tuning of the stop band in transmission spectra of liquid-crystal infiltrated synthetic opal as tunable photonic crystal,” Appl. Phys. Lett. 75(7), 932–934 (1999). [CrossRef]

8.

Y. Shimoda, M. Ozaki, and K. Yoshino, “Electric field tuning of a stop band in a reflection spectrum of synthetic opal infiltrated with nematic liquid crystal,” Appl. Phys. Lett. 79(22), 3627–3629 (2001). [CrossRef]

9.

J. Li, W. Huang, and Y. Han, “Tunable photonic crystals by mixed liquids,” Colloids Surf. A Physicochem. Eng. Asp. 279(1-3), 213–217 (2006). [CrossRef]

10.

V. Lehmann, R. Stengl, and A. Luigart, “On the morphology and the electrochemical formation mechanism of mesoporous silicon,” Mater. Sci. Eng. B 69–70, 11–22 (2000). [CrossRef]

11.

C. Mazzoleni and L. Pavesi, “Application to optical components of dielectric porous silicon multilayers,” Appl. Phys. Lett. 67(20), 2983–2985 (1995). [CrossRef]

12.

G. Vhquez, E. Alvarez, and J. M. Navaza, “Surface Tension of Alcohol + Water from 20 to 50 °C,” J. Chem. Eng. Data 40(3), 611–614 (1995). [CrossRef]

13.

Z. H. Yang, C. Y. Chiu, J. T. Yang, and J. A. Yeh, “Investigation and Application of an artificially hybrid-structured surface with ultrahydrophobic and anti-sticking character,” J. Micromech. Microeng. 19, 085022–085033 (2009). [CrossRef]

14.

Y. X. Zhuang, O. Hansen, T. Knieling, C. Wang, P. Rombach, W. Lang, W. Benecke, M. Kehlenbeck, and J. Koblitz, “Vapor-phase self-assembled monolayers for anti-stiction applications in MEMS,” J. MEMS 16, 1451–1460 (2007).

15.

C. Ishino, K. Okumura, and D. Quéré, “Wetting transitions on rough surfaces,” Europhys. Lett. 68(3), 419–425 (2004). [CrossRef]

OCIS Codes
(000.0000) General : General
(000.2700) General : General science

ToC Category:
Photonic Crystals

History
Original Manuscript: June 25, 2010
Revised Manuscript: September 3, 2010
Manuscript Accepted: September 12, 2010
Published: September 17, 2010

Citation
Chia-Tsung Chan and J. Andrew Yeh, "Tunable photonic crystal based on
capillary attraction and repulsion," Opt. Express 18, 20894-20899 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-20-20894


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References

  1. K. Busch and S. John, “Liquid-crystal photonic-band-gap materials: the tunable electromagnetic vacuum,” Phys. Rev. Lett. 83(5), 967–970 (1999). [CrossRef]
  2. K. Yoshino, Y. Kawagishi, M. Ozaki, and A. Kose, “Mechanical tuning of the optical properties of plastic opal as a photonic crystal,” Jpn. J. Appl. Phys. 38(Part 2, No. 7A), L786–L788 (1999). [CrossRef]
  3. X. Xu, G. Friedman, K. D. Humfeld, S. A. Majetich, and S. A. Asher, “Synthesis and utilization of monodisperse superparamagnetic colloidal particles for magnetically controllable photonic crystals,” Chem. Mater. 14(3), 1249–1256 (2002). [CrossRef]
  4. H. Fudouzi and Y. Xia, “Colloidal crystals with tunable colors and their use as photonic papers,” Langmuir 19(23), 9653–9660 (2003). [CrossRef]
  5. B. Li, J. Zhou, L. Li, X. J. Wang, X. H. Liu, and J. Zi, “Ferroelectric inverse opals with electrically tunable photonic band gap,” Appl. Phys. Lett. 83(23), 4704–4706 (2003). [CrossRef]
  6. Y. Huang, Y. Zhou, C. Doyle, and S. T. Wu, “Tuning the photonic band gap in cholesteric liquid crystals by temperature-dependent dopant solubility,” Opt. Express 14(3), 1236–1242 (2006). [CrossRef] [PubMed]
  7. K. Yoshino, Y. Shimoda, Y. Kawagishi, K. Nakayama, and M. Ozaki, “Temperature tuning of the stop band in transmission spectra of liquid-crystal infiltrated synthetic opal as tunable photonic crystal,” Appl. Phys. Lett. 75(7), 932–934 (1999). [CrossRef]
  8. Y. Shimoda, M. Ozaki, and K. Yoshino, “Electric field tuning of a stop band in a reflection spectrum of synthetic opal infiltrated with nematic liquid crystal,” Appl. Phys. Lett. 79(22), 3627–3629 (2001). [CrossRef]
  9. J. Li, W. Huang, and Y. Han, “Tunable photonic crystals by mixed liquids,” Colloids Surf. A Physicochem. Eng. Asp. 279(1-3), 213–217 (2006). [CrossRef]
  10. V. Lehmann, R. Stengl, and A. Luigart, “On the morphology and the electrochemical formation mechanism of mesoporous silicon,” Mater. Sci. Eng. B 69–70, 11–22 (2000). [CrossRef]
  11. C. Mazzoleni and L. Pavesi, “Application to optical components of dielectric porous silicon multilayers,” Appl. Phys. Lett. 67(20), 2983–2985 (1995). [CrossRef]
  12. G. Vhquez, E. Alvarez, and J. M. Navaza, “Surface Tension of Alcohol + Water from 20 to 50 °C,” J. Chem. Eng. Data 40(3), 611–614 (1995). [CrossRef]
  13. Z. H. Yang, C. Y. Chiu, J. T. Yang, and J. A. Yeh, “Investigation and Application of an artificially hybrid-structured surface with ultrahydrophobic and anti-sticking character,” J. Micromech. Microeng. 19, 085022–085033 (2009). [CrossRef]
  14. Y. X. Zhuang, O. Hansen, T. Knieling, C. Wang, P. Rombach, W. Lang, W. Benecke, M. Kehlenbeck, and J. Koblitz, “Vapor-phase self-assembled monolayers for anti-stiction applications in MEMS,” J. MEMS 16, 1451–1460 (2007).
  15. C. Ishino, K. Okumura, and D. Quéré, “Wetting transitions on rough surfaces,” Europhys. Lett. 68(3), 419–425 (2004). [CrossRef]

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