Volume 10 Issue 4 - August 28, 2009 PDF
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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
H.-C. Yeh1, J.-D. Wang1, K.-C. Lo1, Chia-Rong Lee1,*, and T-S. Mo2

1Institute of Electro-Optical Science and Engineering, National Cheng Kung University 
2Department of Electronic Engineering, Kun Shan University of Technology
crlee@mail.ncku.edu.tw

APPLIED PHYSICS LETTERS Vol.92, p.011121 (2008)

 
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The optical information processing using optical Fourier transform (OFT) technique has continually attracted much attention in the re-cent two decades since it has many potential applications in character recognition, edge enhancement, image addition-subtraction, image correlation, medical image processing, and controllable spatial filter-ing. This study utilized the photoisomerization-induced change of the red-shift in reflection spectrum of the dye-doped cholesteric liquid cr-ystal (DDCLC) film to develop an optically controllable transflective spatial filter. Different spatial distributions of diffraction patterns via the grating object can be controlled for filtering by selecting various intensities of incident beam; in turn, various high- (low-) pass, or notch- (band-) pass transmitted (reflected) images via the OFT processing could be acquired.

An DDCLC compound was prepared by mixing two types of right-handed chiral agents, R1011 and CB15 (both from Merck), a nematic LC, ZLI 2806 (n=1.4746, Δn=0.0437 at 25˚C, from Merck) and an azo dye, 4-Methoxyazobenzene (from Aldrich). The mixing ratio of ZLI 2806: CB15: R1011: azo dye in the DDCLC mixture was 24.8: 2.4: 2.1: 1. The concentration of the azo dye in the DDCLC mixture was 3.3wt%. Two indium-tin-oxide (ITO) glass slides separated by two 12-μm-thick plastic spacers were used to fabricate an empty cell. Both glass slides were pre-coated with a homogeneously-aligned polyvinyl alcohol (PVA) film and then pre-rubbed in a unique direction. The homogeneously mixed DDCLC compound was then injected into the empty cell to generate a planar DDCLC cell.

Before performing the investigation of the optically-tunable transflective spatial filter in the fabricated DDCLC cell, the affection of the green beam irradiation on the reflection spectrum of the cell needed to be pre-clarified. Curves (a), (b), (c), (d) and (e) in Fig. 1 present the spectral features of the DDCLC film under irradiation of one right-circularly polarized green laser beam (λG=514.5nm) with various intensities of IG =0, 8, 20, 50 and 61mW/mm2, respectively, for 1 minute. The central wavelength of the reflection band of the cell in dark was intentionally pre-adjusted at approximately 498 nm, such that λG was just located at the right edge of the reflection band so that the green beam was seldom reflected (curve (a) in Fig. 1). Curves (b)–(e) in Fig. 1 display that the reflection band red-shifts after the green beam irradiated the cell, and the red-shift of the reflection band increased with increasing IG. As is well known, an azo dye possesses two possible isomeric structures, trans- and cis-isomers, which shapes are, respectively, elongated and bended. The azo dye is generally stable in trans-state in dark and tends to change
Fig. 1 Spectral features of the dye-doped cholesteric liquid crystal (DDCLC) film under irradiation of one right-circularly polarized green laser beam (λG=514.5nm) with various intensities of IG = 0 (a), 8 (b), 20 (c), 50 (d) and 61 (e) mW/mm2 for 1 minute. The inset shows the absorption spectrum of the azo dye in an auxiliary DDCLC cell, in which the reflection band was pre-adjusted at the red region (610–655 nm).
structurally to cis-isomers under the stimulus of light with an appropriate wavelength (generally in green or UV region), resulting in the increase of the helical pitch of the CLC structure and, in turn, the increase of the red-shift of the reflection band. Higher pumped intensity would result in more cis-isomers, and thus more red-shift of reflection band, as shown in Fig. 1. The inset in Fig. 1 shows the absorption spectrum of the azo dye in an auxiliary DDCLC cell with same constituents as those described in Section 2, in which the reflection band is pre-adjusted in the red region (610–655nm, not shown) in order to avoid the light reflection in the green-blue region. According to the experimental result shown in the inset, the decay of the cell transmission in the green-blue region, shown in Fig. 1, was due to the strong absorption of the azo dye in this band. Notably, the experimental results showed that the reflection band in the DDCLC cell seldom red-shifts if the intensity of the green beam does not exceed 2mW/mm2 (not shown).

Fig. 2 Experimental setup for the investigation of the optically tunable spatial filter in an DDCLC cell. The cell was placed on the transform plane (∑t) with its surface normal at an angle of 45˚ to the incident beam (Ei). L1: transform lens; L2 and L3: inverse-transform lenses for transmitted and reflected diffraction patterns; ∑it and ∑ir: image planes for the transmitted and reflected images of the object.
Figure 2 displays the experimental setup for the study of the optically-tunable transflective spatial filter in the DDCLC cell. One incident right-circularly polarized laser beam (Ei, 514.5nm) impinged on a black-white grating (object) with a spacing of Λ=100μm and then diffracted into a new set of plane waves, each corresponding to a diffracted beam with a given order or spatial frequency. Lens L1 served as a transform lens, generating the Fraunhofer diffraction pattern via the grating on the transform plane Σt, where the DDCLC cell was placed with its surface normal at an angle of 45˚ to the incident beam. The spatial distribution of the incident diffraction pattern via the grating could be divided into two complementary transmitted and reflected diffraction patterns by the photo-stimulated DDCLC cell. Lenses L2 and L3 could, respectively, inversely transform the transmitted and reflected diffraction patterns onto the transmitted and reflected image planes, ∑it and ∑ir, to reconstruct the transmitted and reflected images of the grating. A charge-coupled device (CCD) camera (SONY Model SSC-DC50A), linked to a computer, was placed on ∑it or ∑ir to record the transmitted or reflected image of the grating. The amplitude distribution of the object field through the black-white grating could be represented as a periodic step function in mathematics. The relative intensity of the mth-order to the zeroth-order diffracted beam could be calculated to be proportional to the function of sinc2(mπ/2) following Fourier transformation, where m=0, 1, 2, 3,…. The even order diffracted beams (m=2, 4, 6,...) were vanished. The intensity of the remaining odd order (m=1, 3, 5,…) decreased as m increased, such that a low order diffracted beam could respond to a significant increase of red-shift in the reflection band of the DDCLC film at the corresponding diffracted spot.

Figure 3 presents the variation of the measured transmittance and reflectance of the incident zeroth-(0th-), first- (1st-) and third- (3rd-) order diffracted beams with the measured intensity of the incident 0th- or 1st-order diffracted beam via the grating, when these incident diffracted beams transmitted through and reflected from the DDCLC film, respectively. The horizontal axis in the bottom (top) of Fig. 3 represents the measured intensity of the incident 0th- (1st-) order diffracted beam, Ii0 (Ii1) for short. Both Ii0 and Ii1 could be increased by increasing the intensity of the incident beam of Ei. When Ii0 was adjusted at 1.5, 20, 50, and 61mW/mm2, the corresponding Ii1 was measured approximately 0.4, 8, 20, and 24mW/mm2, respectively. For incident mth-order diffracted beam, the transmittance (reflectance) was defined as the ratio of the intensity of the incident mth-order diffracted beam transmitted through (reflected from) the DDCLC cell to that of the incident mth-order diffracted beam. As it should be that the variations of the transmittance and reflectance with Ii0 (or Ii1) for each diffracted beam were complementary, as presented in Fig. 3. Since the measured intensity of any incident high order diffracted beam (m≥3) was too weak (≤2mW/mm2) to influence the reflection spectrum of the cell at the range of Ii0=1.5~61mW/mm2, the transmittances (reflectances) of these high order diffracted beams maintained their individual maximum (minimum (~zero)) at their corresponding diffracted spots, as shown in Fig. 3 (for the 3rd-order diffracted beam only). However, the 0th- and 1st-order diffracted beams could vary differently in transmittance and reflectance with increasing Ii0 (or Ii1). In Fig. 3, the measured transmittance (reflectance) for the 0th- or 1st-order diffracted beam was located at its maximum (minimum (~zero)) at a weak intensity of Ii0=1.5mW/mm2 or Ii1=0.4mW/mm2. At the first interval, the transmittance (reflectance) of the 0th-order diffracted beam decreased to its minimum (increased to its maximum) with increasing Ii0 from 1.5 to 20 mW/mm2, and the 1st-order diffracted beam sustained nearly its maximum transmittance and minimum reflectance with increasing Ii1 from 0.4 to 8mW/mm2. This experimental result showed that the cell could be used as a high-pass filter for transmission (all diffracted orders transmitted through the cell except for the 0th-order) and a low-pass filter for reflection (the 0th-order diffracted beam reflected) at Ii0=20mW/mm2 or Ii1=8mW/mm2. The significant decrease (increase) of the transmittance (reflectance) of the 0th-order diffracted beam with increasing Ii0 at this interval was attributed to the reason that the reflection band of the cell increasingly red-shifted with increasing Ii0 until it covered λG at the corresponding irradiated spot of the cell at Ii0=IG=20mW/mm2, as shown by the curve (c) of Fig. 1. The red-shift of the reflection band was small under the irradiation of the incident 1st-order diffracted beam with Ii1=IG=8mW/mm2 (as shown by curve (b) of Fig. 1) at the corresponding irradiated spot of the cell, so that the transmittance (reflectance) of the 1st-order diffracted beam almost remained at its maximum (minimum).
Fig. 4 Reconstructive transmitted (reflected) images of the grating on ∑it (∑ir) at Ii0=1.5 (a), 20 (b), 50 (c), and 61 (d) mW/mm2.
Fig. 3 Variations of the measured transmittance and reflectance of the incident zeroth- (0th-), first- (1st-) and third- (3rd) order diffracted beams via the grating with the measured intensity of the incident 0th- or 1st-order diffracted beam (Ii0 or Ii1).


At the second interval, the 0th-order diffracted beam sustained its minimum transmittance and maximum reflectance with increasing Ii0 from 20 to 50mW/mm2, and the transmittance (reflectance) of the 1st-order diffracted beam decreased (increased) from its maximum to minimum (minimum to maximum) with increasing Ii1 from 8 to 20mW/mm2. This experimental result indicated that the cell could be regarded as a high-pass filter for transmission (all diffracted orders transmitted except for the 0th- and 1st-order) and a low-pass filter for reflection (Both the 0th- and 1st-order diffracted beams reflected) at Ii0=50mW/mm2 or Ii1=20mW/mm2. The significant decrease (increase) of the transmittance (reflectance) of the 1st-order diffracted beam with increasing Ii1 at this interval was because the red-shift of the reflection band of the cell increased with increasing Ii1 until the reflection band covered λG at the corresponding diffracted spot of the cell at Ii1=IG=20mW/mm2 (as shown by curve (c) of Fig. 1). The maintenance of the lowest transmittance and highest reflectance of the 0th-order diffracted beam with increasing Ii0 from 20 to 50 mW/mm2 was because λG was always covered in the reflection band of the cell at this intensity range (curves (c) and (d) of Fig. 1). At the third interval, the transmittance (reflectance) of the 0th-order diffracted beam increased (decreased) from its minimum to maximum (maximum to minimum) with increasing Ii0 from 50 to 61mW/mm2, and the 1st-order diffracted beams sustained its lowest transmittance and highest reflecta nce with increasing Ii1 from 20 to 24mW/mm2. This experimental result implied that the cell could be regarded as a notch-pass filter for transmission (all diffracted orders transmitted except for the 1st-order) and a band-pass filter for reflection (the 1st-order diffracted beam reflected) at Ii0=61mW/mm2 or Ii1=24mW/mm2. The significant increase (decrease) of the transmittance (reflectance) of the 0th-order diffracted beam with increasing Ii0 at this interval was because the red-shift of the reflection band increased with increasing Ii0 until λG was totally out of the reflection band at the corresponding diffracted spot of the cell at Ii0=IG=61mW/mm2 (as shown by curve (e) of Fig. 1). The maintenance of the lowest transmittance and highest reflectance of the 1st-order diffracted beam with increasing Ii1 from 20 to 24mW/mm2 was because λG always remained within the reflection band at this intensity range. To sum up the analyses above, the DDCLC cell could be used as an optically-tunable high- and low-pass or notch- and band-pass transflective spatial filter, in which various compensate spatial distributions of transmitted and reflected diffraction patterns could be controlled to transmit through and reflect from the DDCLC cell, respectively, by selecting various intensity of the incident beam. In turn, the obtained reconstructive transmitted and reflected images on ∑it and ∑ir could be changed.

Figures 4(a), 4(b), 4(c) and 4(d) present the reconstructive transmitted (reflected) images of the grating on ∑it (∑ir) at Ii0=1.5, 20, 50, and 61mW/mm2, respectively. The experimental results shown in Fig. 4 could be analyzed by contrasting with those in Fig. 3. All diffracted orders could transmit through the cell at Ii0=1.5mW/mm2, such that a complete grating image (the observed spacing=100μm) with clear boundary edges on ∑it but nothing on ∑ir could be obtained, as shown in Fig. 4(a). At Ii0=20mW/mm2, the 0th-order diffracted beam was reflected from, while other orders (m≥1) transmitted completely through, the cell, such that the formed images on ∑ir and ∑it were, respectively, an uniformly grey image and a blurred and grey grating image (the observed spacing=50μm), as shown in Fig. 4(b). At Ii0=50mW/mm2, both the 0th- and 1st-order diffracted beams were reflected from, while other orders (m≥3) transmitted completely through, the cell. Therefore, a grating image (the observed spacing=100 μm) with slightly blurred boundary edges on ∑ir and a gloomy but clear grating image (the observed spacing=50μm) on ∑it could be obtained, as shown in Fig. 4(c). At Ii0=61mW/mm2, the 1st-order diffracted beam was reflected from, while other orders (m=0 and m≥3) transmitted completely through, the cell; therefore, a bright and clear grating image (the observed spacing=50μm) on ∑ir and a grating image (the observed spacing=25μm) with unclear boundary edges on ∑it could be obtained, as shown in Fig. 4(d). Moreover, a simulation based on Fourier analysis was performed, and the results (not shown herein) were highly consistent with the experimental results shown in Fig. 4.
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