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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 25 — Dec. 7, 2009
  • pp: 22386–22392
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All-optical and polarization-independent spatial filter based on a vertically-aligned polymer-stabilized liquid crystal film with a photoconductive layer

Chia-Yi Huang, Jia-Ming Ma, Tin-Shan Mo, Kuo-Ching Lo, Kuang-Yao Lo, and Chia-Rong Lee  »View Author Affiliations


Optics Express, Vol. 17, Issue 25, pp. 22386-22392 (2009)
http://dx.doi.org/10.1364/OE.17.022386


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Abstract

An all-optical and polarization-independent spatial filter was developed in a vertically-aligned (VA) polymer-stabilized liquid crystal (PSLC) film with a photoconductive (PC) layer. This spatial filter is based on the effect of light on the conductivity of PC layer: high (low)-intensity light makes the conductivity of the PC layer high (low), resulting in a low (high) threshold voltage of the PC-coated VA PSLC cell. Experimental results indicate that this spatial filter is a high-pass filter with low optical-power consumption (about 1.11 mW/cm2) in an optical Fourier transform system. The high-pass characteristic was confirmed by simulation. Accordingly, the all-optical and polarization-independent spatial filter can be used to enhance the edges of images.

© 2009 OSA

1. Introduction

2. Sample preparation and experimental setup

Figure 1(a)
Fig. 1 (a) Configuration of PC-coated VA PSLC cell; (b) Side-view of optical Fourier transform system (A: aperture, L1 and L2: transforming and inverse transforming lenses with same focal length, Ii 0, Ii 1 and Ii 3: intensities of incident zeroth-, first- and third-order diffracted beams, V: external dc-voltage, f: focal length, Σt and Σi: transform and image planes).
depicts the configuration of the PC-coated VA PSLC cell. An empty cell was fabricated using two indium-tin-oxide (ITO) glass substrates, which were separated by two 15-μm-thick plastic spacers. On the top substrate of the cell was deposited [3-(trimethoxysilyl)propyl]octadecyl- dimethylammonium chloride (DMOAP) as the vertically aligning layer, as shown in Fig. 1(a). The bottom substrate of the cell was coated with a 0.25-μm-thick PC layer of a mixture of 0.05 wt % C60 and poly(N-vinyl carbazole) (PVK from Aldrich Corp.): the C60 was used to make the PVK sensitive to visible light [7

7. J. Qian, C. Xu, S. Qian, and W. Peng, “Optical characteristic of PVK/C60 films fabricated by physical jet deposition,” Chem. Phys. Lett. 257(5-6), 563–568 ( 1996). [CrossRef]

,9

9. M. Kaczmarek, A. Dyadyusha, S. Slussarenko, and I. C. Khoo, “The role of surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers,” J. Appl. Phys. 96(5), 2616–2623 ( 2004). [CrossRef]

]. Then, DMOAP was deposited on the PC layer as the vertically aligning layer, as displayed in Fig. 1(a). The empty cell was filled with a mixture of LCs (Merck MLC-6882, Δε = −3.0), 5 wt % monomer (RM257 from Merck Corp.) and 0.5 wt % photoinitiator (BME from Aldrich Corp.), and then sealed with polymer gel. The filled cell was polymerized under UV light with an intensity of ~10 mW/cm2 and an irradiation period of 10 minutes, forming the PC-coated VA PSLC cell, as presented in Fig. 1(a).

Figure 1(b) shows the side-view of the Fourier transform configuration (a 4-f system) to study the ability of image processing by the all-optical and polarization-independent spatial filter. An unpolarized and pre-expanded green laser beam (λ = 514.5 nm) passed through an aperture (A) with a diameter of 0.4 cm which controlled the light of the incident beam. Then, the beam was diffracted into a set of the diffracted beams (with specified spatial frequencies) through a one-dimensional (1D) black-white grating (as an object) with a spacing (Λ) of 50 μm or a two-dimensional (2D) black-white grating of circular apertures with diameters (ϕ) of 25 μm, as shown in Fig. 1(b). In Fraunhofer diffraction, each diffracted beam behind an object has an order (mth order, m = 0, 1, 2, 3,…) that is proportional to its distance from the optical axis. In this experiment, the beams diffracted from the 1D black-white grating had only zeroth order and odd orders (m = 0, 1, 3, 5…), and the diffracted beams from the 2D black-white grating were of all orders (m = 0, 1, 2, 3…) [10

10. E. Hecht, Optics (Addison Wesley, San Francisco, 2002), Chap. 11.

]. Lens 1 (L1) was placed one focal length (f) from the grating as a transform lens, focusing the Fraunhofer diffracted beams at the transfer plane (∑t) where the PC-coated VA PSLC cell was placed. As presented in Fig. 1(b), the intensity of each incident diffracted beam is Iim, and decreased as the order increased, as in typical Fraunhofer diffraction. The PC-coated VA PSLC cell acts as the filter, to enable the passing of frequencies to be controlled by the application of an external dc-voltage (V). Finally, lens 2 (L2), which was placed one focal length from the PC-coated VA PSLC cell, inversely transformed the transmitted diffracted beams into the reconstructed image of the grating on the image plane (∑i), where image was recorded by a charge-coupled device (CCD) camera (SONY Model SSC-DC50A), which was placed at ∑i.

3. Results and discussion

Figure 2
Fig. 2 The external dc-voltage dependent transmittance of the PC-coated VA PSLC cell at the incident diffracted intensities Ii 0 = 1 mW/cm2, Ii 1 = 0.4 mW/cm2, and Ii 3 = 0.04 mW/cm2.
plots the external dc-voltage dependent transmittance of the PC-coated VA PSLC cell; the incident zeroth-, first- and third-order intensities were Ii 0 = 1 mW/cm2, Ii 1 = 0.4 mW/cm2, and Ii 3 = 0.04 mW/cm2, respectively. The transmittance of the PC-coated VA PSLC cell is defined as the ratio of the intensity of the transmitted mth-order diffracted beam to that of the transmitted zeroth-order diffracted beam at no applied external dc-voltage. To describe the experimental results, the PC-coated VA PSLC cell is defined as a bright (dark) state if the transmittance of the incident diffracted beam is above (below) or equal to 0.9 (0.1).

When the external voltage is below 5 V, the zeroth-, first- and third-order diffracted beams exhibit the maximum transmittance ~1 and therefore the PC-coated VA PSLC cell is in the bright state to the three incident diffracted beams. As the external voltage increases from 5 V to 25 V, the transmittance(s) of the zeroth- (first- and third-) order diffracted beam(s) decreases to 0.1 (remain at least 0.9). Therefore, the PC-coated VA PSLC cell is in the dark state to the zeroth-order diffracted beam, but in the bright state to the first- and third-order diffracted beams. When the external voltage increases from 25 V to 40 V, the transmittance of the first- (third-) order diffracted beam declines to 0.1 (remains at least 0.9). Hence, the PC-coated VA PSLC cell is in the dark state to the zeroth- and first-order diffracted beams, but in the bright state to the third-order diffracted beam. When the external voltage increases over 40 V, the transmittance of the third-order diffracted beam starts to decrease markedly. As the external voltage increases over 65 V, the zeroth-, first- and third-order diffracted beams exhibit the minimum transmittance ~0.05 and therefore the PC-coated VA PSLC cell is in the dark state to the three incident diffracted beams.

These results show that increasing the external voltage can filter the incident diffracted beams from the zeroth order to the third order, indicating that the all-optical and polarization-independent spatial filter is a high-pass filter. The mechanism to control electrically the passing of the mth-order incident diffracted beam (m = 0, 1, 3) will be discussed in Section 4. The decrease in transmittance of the mth-order incident diffracted beam (m = 0, 1, 3) is attributed to the reason that the mth-order incident diffracted beam is scattered by a multi-domain structure in the VA PSLC film [11

11. S. T. Wu, and D. K. Yang, Reflective Liquid Crystal Displays (John Wiley & Sons Press, New York, 1993), Chap. 3.

].

4. Mechanism of electrical control of spatial filter

The PC layer was used herein to control electrically the VA PSLC film. Therefore, the voltage-dependent transmittance of the VA PSLC film must firstly be known [11

11. S. T. Wu, and D. K. Yang, Reflective Liquid Crystal Displays (John Wiley & Sons Press, New York, 1993), Chap. 3.

]. At zero voltage, light passes through this homogenous film because it encounters a uniform refractive index. When the applied voltage is increased above a threshold, light is scattered by the VA PSLC film with a multi-domain structure because it encounters different refractive indices in different domains. This scattering extent increases with the applied voltage. After the applied voltage is high enough, the scattering of light reaches the maximum extent.

The scattering of light incident on a VA PSLC film satisfies VPSLC>VC, where VC is the critical voltage of the Freedericksz transition of the PSLC film, which depends on the morphology of polymer bundles [12

12. R. Q. Ma and D. K. Yang, “Freedericksz transition in polymer-stablized nematic liquid crystals,” Phys. Rev. E 61(2), 1567–1573 ( 2000). [CrossRef]

]. Substituting Eq. (1) into this scattering condition yields
V>(1+pσPCdark+α(Icosθ)β)VC,
(3)
wherep=dPCσPSLC/dPSLC. Equation (3) states that if incident light is to be scattered by the PC-coated VA PSLC cell, then the external dc-voltage must exceed the threshold voltage (V th) of the PC-coated VA PSLC cell,
Vth(I)=(1+pσPCdark+α(Icosθ)β)VC.
(4)
Equation (4) shows that V th is inversely proportional to I: inputting light with a high (low) intensity gives rise to a low (high) threshold voltage in the PC-coated VA PSLC cell. Because the intensity of the incident diffracted beam in a typical Fraunhofer diffraction declines as the spatial order increase, the threshold voltage of the PC-coated VA PSLC cell to the incident diffracted beam increases with its spatial order, as in Eq. (4). Therefore, the incident Fraunhofer diffracted beams from a low order to a high one “experience” increasing threshold voltages in the PC-coated VA PSLC cell. In the OFT process, the increasing dc-voltage exceeds these threshold voltages of the PC-coated VA PSLC cell, causing the incident Fraunhofer diffracted beams to be scattered from a low order to a high one, which is consistent with the result of Fig. 2. Briefly, the mechanism of the electrical control of the all-optical and polarization-independent spatial filter is based on the effect of light on the conductivity of the PC layer: high (low)-intensity light makes the conductivity of the PC layer high (low), resulting in a low (high) threshold voltage of the PC-coated VA PSLC cell.

5. Applications of high-pass spatial filter

To show the high-pass character of the all-optical and polarization-independent spatial filter, the reconstructed images of the 1D grating were recorded by a CCD camera, which was placed on ∑i, as shown Fig. 1(b). Figure 3
Fig. 3 Reconstructed and simulated images of 1D black-white grating, consistent with the results in Fig. 2. Left: all pass (zeroth-ninth orders); middle: zeroth order filtered; right: zeroth and first orders filtered.
shows the reconstructed and simulated images of the 1D black-white grating, which correlates with the operated voltages and the incident intensities of Fig. 2. The simulation involved an inverse Fourier transform [10

10. E. Hecht, Optics (Addison Wesley, San Francisco, 2002), Chap. 11.

]. The left image in Fig. 3 presents that all orders of diffracted beams, from zero to nine, transmitted through the PC-coated VA PSLC cell at V = 0 V; the middle image displays filtering of the zeroth-order diffracted beam at V = 25 V, and the right image depicts the filtering of zeroth- and first-order diffracted beams at V = 40 V. Figure 3 indicates that the reconstructed images are consistent with the simulated ones, and that the PC-coated VA PSLC cell can be applied in the edge enhancement of 1D images.

The edge enhancement of a 2D image using an all-optical and polarization-independent spatial filter was also studied. 2D edge-enhanced images were projected on white paper (as a screen), which was placed 10 cm behind ∑i to display the image directly. Figure 4
Fig. 4 Reconstructed images of 2D black-white grating, operated with applied external dc-voltage, at incident diffracted intensities Ii 0 = 1 mW/cm2, Ii 1 = 0.1 mW/cm2, and Ii 2 = 0.01 mW/cm2. Left: all pass (zeroth-third orders); left-of-center: zeroth order filtered; right-of-center: zeroth and first orders filtered; right: zeroth, first and second orders filtered.
depicts the reconstructed images of the 2D black-white pattern, which were obtained by applying an external dc-voltage. The incident diffracted intensities were Ii 0 = 1 mW/cm2, Ii 1 = 0.1 mW/cm2, and Ii 2 = 0.01 mW/cm2. In Fig. 4, the left-most image shows all orders of diffracted beams, from zero to three, which pass through the PC-coated VA PSLC cell at an applied external dc-voltage of V = 0 V; the image just to the left of center displays the blocking of the zeroth-order diffracted beam at V = 25 V; the image just to the right of center presents the blocking of the zeroth- and first-order beams at V = 40 V, and the right-most image displays the blocking of the zeroth-, first- and second-order diffracted beams at V = 65 V. These results reveal that the PC-coated VA PSLC cell can also employed in the edge enhancement of 2D images, and the total intensity of the incident beams, Ii 0 + Ii 1 + Ii 2, is 1.11 mW/cm2. Therefore, the all-optical and polarization-independent spatial filter consumes little optical power in the OFT system.

The results in Figs. 3 and 4 show that the all-optical and polarization-independent spatial filter is the high-pass filter at the appropriate voltage and intensity, and so the OFT system can enhance the edges of images because the high-order transmitted diffracted beams carry precise information about the original images, and especially their edges. Figure 5(a)
Fig. 5 (a) Images of rectangle and Chinese character at external dc-voltage V = 0 V. (b) Edge-enhanced images of those at external dc-voltage V = 40 V. Incident diffracted intensities are Ii 0 = 1 mW/cm2, Ii 1 = 0.1 mW/cm2, and Ii 2 = 0.01 mW/cm2.
displays the images of rectangle and Chinese character at V = 0 V. Figure 5(b) shows the edge-enhanced images of rectangle and Chinese character at V = 40 V; the incident diffracted intensities are Ii 0 = 1 mW/cm2, Ii 1 = 0.1 mW/cm2, and Ii 2 = 0.01 mW/cm2. The experimental results show that the PC-coated VA PSLC cell effectively enhances the edges of the images because the all-optical and polarization-independent spatial filter is a high-pass filter.

6. Conclusion

This study utilized a PC-coated VA PSLC cell to develop an all-optical and polarization-independent spatial filter with the features of high pass, electrical control, and low optical-power consumption (of the order of 1.11 mW/cm2) for use in an OFT system. This spatial filter is based on the effect of light on the conductivity of a PC layer. This effect in the OFT system causes low- (high-) order incident Fraunhofer diffracted beams to “experience” a high- (low-) threshold voltage in the PC-coated PSLC cell. A high-pass spatial filter can enhance the edges of the images in an OFT system. This high-pass feature was simulated using an inverse Fourier transform, and the results were consistent with the experimental results.

Acknowledgments

This work was financially supported by the National Science Council of Taiwan (Project No. NSC 97-2112-M-006-013-MY3) and the Advanced Optoelectronic Technology Center, National Cheng Kung University, under projects from the Ministry of Education.

References and links

1.

C. Egami, Y. Suzuki, T. Uemori, O. Sugihara, and N. Okamoto, “Self-adaptive spatial filtering by use of azo chromophores doped in low glass-transition-temperature polymers,” Opt. Lett. 22(18), 1424–1426 ( 1997). [CrossRef]

2.

C. S. Yelleswarapu, P. Wu, S. R. Kothapalli, D. V. G. L. N. Rao, B. R. Kimball, S. S. S. Sai, R. Gowrishankar, and S. Sivaramakrishnan, “All-optical spatial filtering with power limiting materials,” Opt. Express 14(4), 1451–1457 ( 2006). [CrossRef] [PubMed]

3.

T. H. Lin and A. Y. Fuh, “Polarization controllable spatial filter based on azo-dye-doped liquid-crystal film,” Opt. Lett. 30(11), 1390–1392 ( 2005). [CrossRef] [PubMed]

4.

H. C. Yeh, J. D. Wang, K. C. Lo, C. R. Lee, T. S. Mo, and S. Y. Huang, “Optically controllable transflective spatial filter with high- and low-pass or notch- and band-pass functions based on a dye-doped cholesteric liquid crystal film,” Appl. Phys. Lett. 92(1), 011121 ( 2008). [CrossRef]

5.

M. Y. Shih, A. Shishido, and I. C. Khoo, “All-optical image processing by means of a photosensitive nonlinear liquid-crystal film: edge enhancement and image addition-subtraction,” Opt. Lett. 26(15), 1140–1142 ( 2001). [CrossRef] [PubMed]

6.

A. Y. G. Fuh and T. H. Lin, “Electrically switchable spatial filter based on polymer-dispersed liquid crystal film,” J. Appl. Phys. 96(10), 5402–5404 ( 2004). [CrossRef]

7.

J. Qian, C. Xu, S. Qian, and W. Peng, “Optical characteristic of PVK/C60 films fabricated by physical jet deposition,” Chem. Phys. Lett. 257(5-6), 563–568 ( 1996). [CrossRef]

8.

F. L. Vladimirov, A. N. Chaika, I. E. Morichev, N. I. Pletneva, A. F. Naumov, and M. Yu. Loktev, “Modulation characteristics of optically controllable transparencies based on a photoconductor-liquid-crystal structure,” J. Opt. Technol. 67, 712–716 ( 2000). [CrossRef]

9.

M. Kaczmarek, A. Dyadyusha, S. Slussarenko, and I. C. Khoo, “The role of surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers,” J. Appl. Phys. 96(5), 2616–2623 ( 2004). [CrossRef]

10.

E. Hecht, Optics (Addison Wesley, San Francisco, 2002), Chap. 11.

11.

S. T. Wu, and D. K. Yang, Reflective Liquid Crystal Displays (John Wiley & Sons Press, New York, 1993), Chap. 3.

12.

R. Q. Ma and D. K. Yang, “Freedericksz transition in polymer-stablized nematic liquid crystals,” Phys. Rev. E 61(2), 1567–1573 ( 2000). [CrossRef]

OCIS Codes
(070.6110) Fourier optics and signal processing : Spatial filtering
(100.1160) Image processing : Analog optical image processing
(160.5140) Materials : Photoconductive materials
(160.5470) Materials : Polymers
(230.3720) Optical devices : Liquid-crystal devices

ToC Category:
Fourier Optics and Signal Processing

History
Original Manuscript: October 1, 2009
Revised Manuscript: November 3, 2009
Manuscript Accepted: November 17, 2009
Published: November 23, 2009

Citation
Chia-Yi Huang, Jia-Ming Ma, Tin-Shan Mo, Kuo-Ching Lo, Kuang-Yao Lo, and Chia-Rong Lee, "All-optical and polarization-independent spatial filter based on a vertically-aligned polymer-stabilized liquid crystal film with a photoconductive layer," Opt. Express 17, 22386-22392 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-25-22386


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References

  1. C. Egami, Y. Suzuki, T. Uemori, O. Sugihara, and N. Okamoto, “Self-adaptive spatial filtering by use of azo chromophores doped in low glass-transition-temperature polymers,” Opt. Lett. 22(18), 1424–1426 (1997). [CrossRef]
  2. C. S. Yelleswarapu, P. Wu, S. R. Kothapalli, D. V. G. L. N. Rao, B. R. Kimball, S. S. S. Sai, R. Gowrishankar, and S. Sivaramakrishnan, “All-optical spatial filtering with power limiting materials,” Opt. Express 14(4), 1451–1457 (2006). [CrossRef] [PubMed]
  3. T. H. Lin and A. Y. Fuh, “Polarization controllable spatial filter based on azo-dye-doped liquid-crystal film,” Opt. Lett. 30(11), 1390–1392 (2005). [CrossRef] [PubMed]
  4. H. C. Yeh, J. D. Wang, K. C. Lo, C. R. Lee, T. S. Mo, and S. Y. Huang, “Optically controllable transflective spatial filter with high- and low-pass or notch- and band-pass functions based on a dye-doped cholesteric liquid crystal film,” Appl. Phys. Lett. 92(1), 011121 (2008). [CrossRef]
  5. M. Y. Shih, A. Shishido, and I. C. Khoo, “All-optical image processing by means of a photosensitive nonlinear liquid-crystal film: edge enhancement and image addition-subtraction,” Opt. Lett. 26(15), 1140–1142 (2001). [CrossRef] [PubMed]
  6. A. Y. G. Fuh and T. H. Lin, “Electrically switchable spatial filter based on polymer-dispersed liquid crystal film,” J. Appl. Phys. 96(10), 5402–5404 (2004). [CrossRef]
  7. J. Qian, C. Xu, S. Qian, and W. Peng, “Optical characteristic of PVK/C60 films fabricated by physical jet deposition,” Chem. Phys. Lett. 257(5-6), 563–568 (1996). [CrossRef]
  8. F. L. Vladimirov, A. N. Chaika, I. E. Morichev, N. I. Pletneva, A. F. Naumov, and M. Yu. Loktev, “Modulation characteristics of optically controllable transparencies based on a photoconductor-liquid-crystal structure,” J. Opt. Technol. 67, 712–716 (2000). [CrossRef]
  9. M. Kaczmarek, A. Dyadyusha, S. Slussarenko, and I. C. Khoo, “The role of surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers,” J. Appl. Phys. 96(5), 2616–2623 (2004). [CrossRef]
  10. E. Hecht, Optics (Addison Wesley, San Francisco, 2002), Chap. 11.
  11. S. T. Wu, and D. K. Yang, Reflective Liquid Crystal Displays (John Wiley & Sons Press, New York, 1993), Chap. 3.
  12. R. Q. Ma and D. K. Yang, “Freedericksz transition in polymer-stablized nematic liquid crystals,” Phys. Rev. E 61(2), 1567–1573 (2000). [CrossRef]

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