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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 21 — Oct. 17, 2007
  • pp: 14078–14085
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Electrically switchable and optically rewritable reflective Fresnel zone plate in dye-doped cholesteric liquid crystals

Ko-Ting Cheng, Cheng-Kai Liu, Chi-Lun Ting, and Andy Ying-Guey Fuh  »View Author Affiliations


Optics Express, Vol. 15, Issue 21, pp. 14078-14085 (2007)
http://dx.doi.org/10.1364/OE.15.014078


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Abstract

This work demonstrates a reflective Fresnel zone plate based on dye-doped cholesteric liquid crystals (DDCLC) using the photo-induced realignment technique. Illumination of a DDCLC film with a laser beam through a Fresnel-zone-plate mask yields a reflective lens with binary-amplitude structures - planar and focal conic textures, which reflect and scatter probed light, respectively. The formed lens persists without any external disturbance, and its focusing efficiency, analyzed using circularly polarized light, is ~ 23.7%, which almost equals the measured diffraction efficiency of the used Fresnel-zone-plate mask (~ 25.6%). The lens is thermally erasable, rewritable and switchable between focusing and defocusing states, upon application of a voltage.

© 2007 Optical Society of America

1. Introduction

2. Experiments

The materials used herein were right-handed cholesteric liquid crystals (CLC), prepared by mixing 64 wt% nematic liquid crystal (E7, Merck) with 36 wt% chiral agent (CB15, Merck). The measured reflection band was between 615 and 665 nm. The pitch length of the used CLC is approximately 380 nm. The chiral dopant impurities reduce the clearing temperature from ~ 61°C (clearing temperature of pure E7) to ~ 30.7°C. The dye adopted in this experiment was an azo dye, methyl red (MR, Aldrich), whose absorption band in the trans-state spans 440 to 550 nm and peaks at about 530 nm. The mixing ratio of MR to CLC was 2:98 by weight. Each empty cell was fabricated by combining two indium tin oxide (ITO)-coated glass slides, separated by two 11 μm-thick spacers, each of which was coated with an alignment film of poly(vinyl alcohol) (PVA) and rubbed in the direction R. Finally, the homogeneously mixed compound was injected into an empty cell to produce a DDCLC in planar texture. The edges of the DDCLC cells were sealed with epoxy. Notably, the cell gap should be larger than ~ ten pitch lengths in order to achieve a complete Bragg reflection from a cholesteric planar texture film [14

14. Q. Hong, T. X. Wu, and S. T. Wu, “Optical wave propagation in a cholesteric liquid crystal using the finite element method,” Liq. Cryst. 30, 367–375 (2003). [CrossRef]

]. The reason why we employed an 11 μm-thick cell in the present work is the consideration of having a complete Bragg reflection and a good contrast ratio of light intensity reflected from the planar texture regions to that scattered from the focal conic texture regions.

Fig. 1. Schematic fabrication of a DDCLC reflective Fresnel zone plate.
Fig. 2. Experimental setup for (a) recording, and (b) analyzing a DDCLC reflective Fresnel zone plate.

Figures 2(a) and 2(b) present the experimental setup for fabricating and analyzing a reflective Fresnel zone plate, respectively. In Fig. 2(a), linearly polarized green light with an intensity of 100 mW/cm2 (E G, from a diode-pumped solid state laser, DPSS laser, λG = 532 nm), expanded and collimated to a beam diameter of 1 cm through two convex lenses, was used to pump the DDCLC cell through the mask for 10 minutes. The polarization of the pumped beam, E G, relative to the PVA-rubbing direction, R, could be set arbitrarily, since the absorption in all polarization directions is always equal in a planar DDCLC cell. Here, E G and R were set to be parallel. Finally, a reflective Fresnel zone plate was formed. Right-hand circularly polarized red light with an intensity of 1.2 mW/cm2 (E R, from a He-Ne laser, λR = 632.8 nm), whose wavelength was in range of the reflection band of the used DDCLC, was adopted to analyze the characteristics of the zone plate. The red-light beam, expanded by two convex lenses, made a small angle of ~ 1° with the normal of the DDCLC cell, to enable the focused image to be observed easily on the screen and photographed.

3. Results and discussion

Fig. 3. Images of (a) Fresnel-zone-plate mask (b) formed reflective Fresnel zone plate observed under crossed-polarizer optical microscope. AT and AO represent transparent and opaque regions. AP and AF are the planar and focal conic texture regions, respectively. P and A are, respectively, the transmissive axes of the polarizer and the analyzer.

Fig. 4. Focusing patterns of the fabricated reflective Fresnel zone plate using right-hand circularly polarized red light. The distances between lens and screen are (a) 20 cm; (b) 30 cm; (c) 40 cm; (d) 50 cm; (e) 60 cm. The Fresnel-zone-plate mask has a focal length ~ 40 cm at a wavelength of 632.8 nm..

Figure 5 plots the measured focusing efficiency of a reflective Fresnel zone plate under the applied AC (1 kHz) voltages. The inset (a) in Fig. 5 presents the focusing pattern obtained without an application of an AC voltage. When a voltage is applied, the diffraction efficiency remains almost unchanged initially, and then decreases sharply at voltages just above the threshold of ~ 3 V, at which the cell transits from planar to focal conic textures, and finally saturates at a diffraction efficiency of ~ 2% at voltages that markedly exceeds the threshold. The inset (b) in Fig. 5 displays an image recorded at an applied voltage of ~ 10 V. The focusing effect disappears in inset (b). Additionally, when a high voltage of 50 V is applied and abruptly switched off, the focused image quickly returns to that presented in inset (a). The behavior that is shown in Fig. 5 is reasonable, and is understood as follows. Applying an AC voltage below the threshold cannot orient the LCs in the DDCLC sample. The LCs of the odd (even) zones remain in their original states with planar (focal conic) textures. However, when the applied voltage exceeds the threshold voltage, the planar texture in the odd zones should be transferred to foal conic textures. Therefore, the textures of both the odd and the even zones in a DDCLC sample are focal conic, the focusing effect disappears. Finally, the diffraction efficiency saturated at ~ 2% is associated with the scattering of light from the focal-conic-texture cell. Accordingly, the reflective Fresnel zone plate is clearly electrically switchable. The measured rise- and fall-times (10–90%) for the reflective Fresnel zone plate are, respectively, 620 ms and 5 ms with the sample being applied with an AC voltage (50 V, 1 kHz).

Fig. 5. Measured focusing efficiency of reflective Fresnel zone plate in DDCLC as a function of applied AC (1 kHz) voltage. Inset (a) and (b) present focusing patterns of lens without and with applied voltage (10 V), respectively. Notably, the focusing pattern of the lens returns to (a) after a higher voltage of 50 V is applied and rapidly switched off.

The sample is thermally treated to confirm that the fabricated lens is erasable and rewritable. The fabricated lens was heated to ~ 80°C and the cell was kept at this temperature for 10 minutes to erase the randomly adsorbed dyes. The experimental results that are shown in Fig. 6 reveal that the reflective Fresnel zone plate was thermally erasable and optically rewritable. Figures 6(a) and 6(b) depict images, obtained under an optical microscope, of the fabricated reflective Fresnel zone plate before and after thermal treatment, respectively. Thermal disturbance can cause desorption of adsorbed MR [17

17. A. Y.-G. Fuh, K. T. Cheng, and C. R. Lee, “Biphotonic recording effect of polarization gratings based on dye-doped liquid crystal films,” Liq. Cryst. 34, 389–393 (2007). [CrossRef]

]. Hence, the reflective Fresnel zone plate recovers to its initial planar texture throughout the cell when the temperature returns below the clearing temperature of the DDCLC. Additionally, the thermally treated sample is rewritable using the setup presented in Fig. 2(a). Figure 6(c) depicts the rewritten reflective Fresnel zone plate. The measured focusing efficiency is ~ 23.1%, which is close to that before erasure (~ 23.7%). Therefore, the reflective Fresnel zone plate after thermal erasure treatment is optically rewritable.

Fig. 6. Images of reflective Fresnel zone plate observed under a crossed-polarizer optical microscope (a) before, and (b) after thermal treatment. (c) Image of rewritten reflective Fresnel zone plate. Additionally, P and A are transmissive axes of polarizer and analyzer, respectively.

Notably, the demonstrated reflective Fresnel zone plate in DDCLC can act as a transflective lens if the polarization of the probe beam is linear, elliptical or un-polarized, rather than circularly polarized. Figures 7(a) and 7(b) depict, respectively, the reflective and transmissive focusing patterns at the primary focal point with the formed Fresnel zone plate probed using a linearly polarized red light. The polarizations of the reflective and transmissive focusing light are right- and left-hand circular, respectively. Restated, such a device can function as a polarization-beam-splitter lens. Additionally, the lifetime of the lens is very long. The reflective Fresnel zone plate is persistent, and its focusing efficiency remains unchanged over two months without any external disturbance, such as optical, thermal or electrical, because of the memory effect of MR-adsorption and the bistable states of planar and focal conic textures.

Fig. 7. (a). Reflective and (b) transmissive focusing patterns of the fabricated Fresnel zone plate using a linearly polarized red light.

4. Conclusion

In conclusion, this investigation successfully demonstrated a Fresnel zone plate based on a dye-doped cholesteric liquid crystal with binary structures that comprise planar and focal conic textures. It can be operated as a reflective (transflective) lens, if the incident beam is circularly polarized (linearly polarized). The focusing efficiency of the reflective Fresnel zone plate is close to that of the used Fresnel-zone-plate mask. The formed Fresnel zone plates are electrically switchable, thermally erasable and rewritable. The lifetime of the fabricated lens is long. The lens is persistent without any external disturbance. In additions, the direction of the reflective beam can be slightly turned by simply rotating the lens. Therefore, it has potential for real applications. For an example, it can be used in the maskless photolithographic process. The reflective wavelength can be set exactly to match the chosen photoresist materials.

Acknowledgment

The authors would like to thank the National Science Council (NSC) of the Republic of China (Taiwan) for financially supporting this research under Grant No. NSC 95-2112-M-006-022-MY3.

References and links

1.

L. Mingtao, J. Wang, L. Zhuang, and S. Y. Chou, “Fabrication of circular optical structures with a 20 nm minimum feature size using nanoimprint lithography,” Appl. Phys. Lett. 76, 673–675 (2000). [CrossRef]

2.

J. Canning, K. Sommer, S. Huntington, and A. Carter, “Silica-based fiber Fresnel lens,” Opt. Commun. 199, 375–381 (2001). [CrossRef]

3.

M. Ye and S. Sato, “Optical properties of liquid crystal lens of any size,” Jpn. J. Appl. Phys. 41, L571–L573 (2002). [CrossRef]

4.

S. H. Chung, S. W. Choi, Y. J. Kim, H. J. Ahn, and H. K. Baik, “Liquid crystal lens for compensation of spherical aberration in multilayer optical data storage,” Jpn. J. Appl. Phys. 43, 1152–1157 (2006). [CrossRef]

5.

Y. S. Hwang, T. H. Yoon, and C. Kim, “Design and fabrication of variable focusing lens array using liquid crystal for integral photography,” Jpn. J. Appl. Phys. 42, 6434–6438 (2003). [CrossRef]

6.

H. Ren, Y.-H. Fan, and S. T. Wu, “Tunable Fresnel lens using nanoscale polymer-dispersed liquid crystals,” Appl. Phys. Lett. 83, 1515–1517 (2003). [CrossRef]

7.

Y. H. Fan, H. Ren, and S. T. Wu, “Switchable Fresnel lens using polymer-stabilized liquid crystals,” Opt. Express 11, 3080–3086 (2003). [CrossRef] [PubMed]

8.

D. W. Kim, C. J. Yu, H. R. Kim, S. J. Kim, and S. D. Lee, “Polarization-insensitive liquid crystal Fresnel lens of dynamic focusing in an orthogonal binary configuration,” Appl. Phys. Lett. 88, 203505–203507 (2006). [CrossRef]

9.

T. H. Lin, Y. Huang, A. Y.-G. Fuh, and S. T. Wu, “Polarization controllable Fresnel lens using dye-doped liquid crystals,” Opt. Express 14, 2359–2364 (2006). [CrossRef] [PubMed]

10.

L. C. Lin, H. C. Jau, T. H. Lin, and A. Y.-G. Fuh, “Highly efficient and polarization-independent Fresnel lens based on dye-doped liquid crystal,” Opt. Express 15, 2900–2906 (2007). [CrossRef] [PubMed]

11.

C. R. Lee, T. L. Fu, K. T. Cheng, T. S. Mo, and A. Y.-G. Fuh, “Surface-assisted photo-alignment in dye-doped liquid crystal films,” Phys. Rev. E 69, 031704 (2004). [CrossRef]

12.

K. Rastani, A. Marrakchi, S. F. Habiby, W. M. Hubbard, H. Gilchrist, and R. E. Nahory, “Binary phase Fresnel lenses for generation of two-dimensional beam arrays,” Appl. Opt. 30, 1347–1354 (1991). [CrossRef] [PubMed]

13.

P. G. de Gennes and J. Prost, The Physics of Liquid Crystal (Oxford University Press, New York, 1993), Chapt. 6.

14.

Q. Hong, T. X. Wu, and S. T. Wu, “Optical wave propagation in a cholesteric liquid crystal using the finite element method,” Liq. Cryst. 30, 367–375 (2003). [CrossRef]

15.

F. Simoni and O. Francescangeli, “Effects of light on molecular orientation of liquid crystals,” J. Phys. Condens. Matter 11, R439–R487 (1999). [CrossRef]

16.

G. H. Heilmeier and L. A. Zanoni, “Guest-Host interactions in nematic liquid crystals: A new electro-optic effect,” Appl. Phys. Lett. 13, 91–92 (1968). [CrossRef]

17.

A. Y.-G. Fuh, K. T. Cheng, and C. R. Lee, “Biphotonic recording effect of polarization gratings based on dye-doped liquid crystal films,” Liq. Cryst. 34, 389–393 (2007). [CrossRef]

OCIS Codes
(050.1970) Diffraction and gratings : Diffractive optics
(160.3710) Materials : Liquid crystals
(220.3620) Optical design and fabrication : Lens system design
(230.0230) Optical devices : Optical devices

ToC Category:
Diffraction and Gratings

History
Original Manuscript: September 4, 2007
Revised Manuscript: October 4, 2007
Manuscript Accepted: October 10, 2007
Published: October 11, 2007

Citation
Ko-Ting Cheng, Cheng-Kai Liu, Chi-Lun Ting, and Andy Y. Fuh, "Electrically switchable and optically rewritable reflective Fresnel zone plate in dye-doped cholesteric liquid crystals," Opt. Express 15, 14078-14085 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-21-14078


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References

  1. L. Mingtao, J. Wang, L. Zhuang, and S. Y. Chou, "Fabrication of circular optical structures with a 20 nm minimum feature size using nanoimprint lithography," Appl. Phys. Lett. 76, 673-675 (2000). [CrossRef]
  2. J. Canning, K. Sommer, S. Huntington, and A. Carter, "Silica-based fiber Fresnel lens," Opt. Commun. 199, 375-381 (2001). [CrossRef]
  3. M. Ye and S. Sato, "Optical properties of liquid crystal lens of any size," Jpn. J. Appl. Phys. 41, L571-L573 (2002). [CrossRef]
  4. S. H. Chung, S. W. Choi, Y. J. Kim, H. J. Ahn and H. K. Baik, "Liquid crystal lens for compensation of spherical aberration in multilayer optical data storage," Jpn. J. Appl. Phys. 43, 1152-1157 (2006). [CrossRef]
  5. Y. S. Hwang, T. H. Yoon and C. Kim, "Design and fabrication of variable focusing lens array using liquid crystal for integral photography," Jpn. J. Appl. Phys. 42, 6434-6438 (2003). [CrossRef]
  6. H. Ren, Y.-H. Fan, and S. T. Wu, "Tunable Fresnel lens using nanoscale polymer-dispersed liquid crystals," Appl. Phys. Lett. 83, 1515-1517 (2003). [CrossRef]
  7. Y. H. Fan, H. Ren, and S. T. Wu, "Switchable Fresnel lens using polymer-stabilized liquid crystals," Opt. Express 11, 3080-3086 (2003). [CrossRef] [PubMed]
  8. D. W. Kim, C. J. Yu, H. R. Kim, S. J. Kim, and S. D. Lee, "Polarization-insensitive liquid crystal Fresnel lens of dynamic focusing in an orthogonal binary configuration," Appl. Phys. Lett. 88, 203505-203507 (2006). [CrossRef]
  9. T. H. Lin, Y. Huang, A. Y.-G. Fuh and S. T. Wu, "Polarization controllable Fresnel lens using dye-doped liquid crystals," Opt. Express 14, 2359-2364 (2006). [CrossRef] [PubMed]
  10. L. C. Lin, H. C. Jau, T. H. Lin and A. Y.-G. Fuh, "Highly efficient and polarization-independent Fresnel lens based on dye-doped liquid crystal," Opt. Express 15, 2900-2906 (2007). [CrossRef] [PubMed]
  11. C. R. Lee, T. L. Fu, K. T. Cheng, T. S. Mo and A. Y.-G. Fuh, "Surface-assisted photo-alignment in dye-doped liquid crystal films," Phys. Rev. E 69, 031704 (2004). [CrossRef]
  12. K. Rastani, A. Marrakchi, S. F. Habiby, W. M. Hubbard, H. Gilchrist and R. E. Nahory, "Binary phase Fresnel lenses for generation of two-dimensional beam arrays," Appl. Opt. 30, 1347-1354 (1991). [CrossRef] [PubMed]
  13. P. G. de Gennes and J. Prost, The Physics of Liquid Crystal (Oxford University Press, New York, 1993), Chap. 6.
  14. Q. Hong, T. X. Wu and S. T. Wu, "Optical wave propagation in a cholesteric liquid crystal using the finite element method," Liq. Cryst. 30, 367-375 (2003). [CrossRef]
  15. F. Simoni and O. Francescangeli, "Effects of light on molecular orientation of liquid crystals," J. Phys. Condens. Matter 11, R439-R487 (1999). [CrossRef]
  16. G. H. Heilmeier and L. A. Zanoni, "Guest-Host interactions in nematic liquid crystals: A new electro-optic effect," Appl. Phys. Lett. 13, 91-92 (1968). [CrossRef]
  17. A. Y.-G. Fuh, K. T. Cheng and C. R. Lee, "Biphotonic recording effect of polarization gratings based on dye-doped liquid crystal films," Liq. Cryst. 34, 389-393 (2007). [CrossRef]

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