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2/February 2006/J. Opt. Soc. Am. A. 267 ..... author B. Yao's email address is [email protected]opt.ac.cn. ... effect to optical processing in partially coherent light,” Opt.

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Optical image processing using the photoinduced anisotropy of pyrrylfulgide Neimule Menke, Baoli Yao, Yingli Wang, Yuan Zheng, Ming Lei, Liyong Ren, and Guofu Chen State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710068, China

Yi Chen, Meigong Fan, and Tiankai Li Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, China Received March 11, 2005; accepted July 8, 2005 A synthesized photochromic compound—pyrrylfulgide—is prepared as a thin film doped in a polymethylmethacrylate (PMMA) matrix. Under irradiation by UV light, the film converts from the bleached state into a colored state that has a maximum absorption at 635 nm and is thermally stable at room temperature. When the colored state is irradiated by a linearly polarized 650 nm laser, the film returns to the bleached state; photoinduced anisotropy is produced during this process. Application of optical image processing methods using the photoinduced anisotropy of the pyrrylfulgide/PMMA film is described. Examples in non-Fourier optical image processing, such as contrast reversal and image subtraction and summation, as well as in Fourier optical image processing, such as low-pass filtering and edge enhancement, are presented. © 2006 Optical Society of America OCIS codes: 160.4890, 100.1160, 100.1930, 260.1440.

1. INTRODUCTION Optical signal processing can be performed analogically or digitally. Over the past few years optical digital methods have been developed to perform operations in optical sensor networks, optical packet switching networks, and in substation applications using fiber Fabry–Perot interferometers (FFPI), associated signal conditioning units, etc.1 Analogical signal processing includes Fourier and non-Fourier methods. Among them is that based on photoanisotropy developed by Jonathan and May (nonFourier optical signal processing method) and Joseph et al. (Fourier optical signal processing method). Jonathan and May2,3 described applications of treated photographic plates (silver halide emulsions) in optical configurations that performed operations such as contrast reversal and image addition and subtraction. There are other photoanisotropic materials (dyed gelatin, dyed plastic) used for performing optical processing operations mentioned before by Calixto et al.4,5 Using the nonlinear photoanisotropy of materials (bacteriorhodopsin and treated photographic plates), the implementation of Fourier transform operations such as edge enhancement, bandpass filtering, and pattern recognition were established by Joseph et al.,6 Korchemskaya and Stepanchikov,7 and Henriot and May.8 Fulgides are well known as thermally irreversible organic photochromic compounds. Many studies on the photochromic properties have been made in order to apply them for photon-controlled memories and switches.9–12 However, the photoinduced anisotropy of fulgides and its possible applications have only rarely been investigated. We find that there exists photoinduced anisotropy in fulgide-doped polymeric films and have demonstrated 1084-7529/06/020267-5/$15.00

that this property can be explored in polarization pattern hide and display.13,14 In this paper, we present a more detailed description of the photoinduced anisotropic properties of a pyrrylfulgide/polymethylmethacrylate (PMMA) film, followed by examples of applications in image processing like contrast reversal, subtraction and summation of two signals, low-pass filtering, and edge enhancement. Fourier optical image processing is realized by using the nonlinear dependence of photoanisotropy on the excitation intensity.

2. PROPERTIES OF MATERIAL Pyrrylfulgide has been synthesized by the Stobbe condensation routine.12 The target compound of 3 mg was dissolved in a 0.1 ml 10% (by weight) PMMA-cyclohexanone solution. The solution was then coated on a 1 mm thick K9 glass plate with a spin coater and dried in air. The thickness of the film was about 10 ␮m. The photochromic and photoinduced anisotropic properties of fulgides are due to the reversible photoisomerization reaction that occurs between one of the colorless E-forms (bleached state) and the C-form (colored state). These are the two spectrally separated forms, whose molecular formula and absorption spectra are shown in Fig. 1 and Fig. 2, respectively. Considering the absorption maxima of both forms, we chose UV light and a laser diode emitting at 650 nm as eraser and exciting source, respectively. The extent of the photoinduced dichroism was determined by measuring the absorption spectra of the film with a UV-VIS-IR spectrophotometer (UV-3101PC, Shimadzu Inc., Japan). This instrument was used for testing light polarized parallel and perpendicular to the © 2006 Optical Society of America

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polarization direction of the exciting beam after the film was excited by the linearly polarized 650 nm laser for an optimal exposure. Then the difference in the optical density ⌬D共␭兲 = D⬜ − D储 = log共T储 / T⬜兲 was obtained. The birefringence was calculated according to the Kramers– Kronig relation15: ⌬n共␭兲 = n⬜ − n储 =

ln 10 2␲ d 2

p.v.





0

⌬D共␭⬘兲 1 − ␭⬘2/␭2

d␭⬘ ,

共1兲

where n储 and n⬜ are the refractive indices of the film along the photoinduced extraordinary and ordinary axes, respectively; d is the thickness of the film; and p.v. denotes the Cauchy principal value of the integral. The measured ⌬D and calculated ⌬n are plotted in Fig. 3. The maximal photoinduced dichroism is ⬇0.1 at 625 nm and the maximal photoinduced birefringence is 8.3⫻ 10−4 at 710 nm. From Fig. 3 it can be seen that the dichroism is more pronounced in the red region of the spectrum than in the blue–green. Note that the sign of the dichroism changes when going from the blue–green to the red side of the spectrum. When the sample is observed between two crossed polarizers [the axis of one polarizer P is at 45° with respect to the optical axis] by white light, the excited area exhibits a pink color. A slight deviation of analyzer A from the orthogonal position in the clockwise direction gives the sample a light green hue and in the counterclockwise direction a light red hue. To explain these observations we have drawn the scheme shown in Fig. 4. Lr and Lb-g represent the long axes of the elliptically polarized red and blue-green light, respectively, after the anisotropy material. If analyzer A is at the orthogonal position 共A0 ⬜ P兲, the dichroism is larger for red light and thus produces pink. When A is rotated clockwise 共A1 ⬜ Lr兲, the red light will be weakened and the sample becomes green.

Fig. 1. Molecular formula of the pyrrylfulgide and the photochromic reaction. Left, E-form; right, C-form.

Fig. 2.

Absorption spectra of the pyrrylfulgide/PMMA film.

Fig. 3. Dichroism and birefringence spectra of the pyrrylfulgide/ PMMA film.

Fig. 4. The different angular displacements of red and blue– green light. Lr and Lb-g represent the long axes of the elliptically polarized red and blue-green light, respectively.

When A is rotated counterclockwise 共A2 ⬜ Lb-g兲, the bluegreen light will be weakened and the sample becomes red. This phenomenon is so weak that we could just observe it by eye, but could not document it by a CCD camera. If the phenomenon were distinct, we could use it to realize spatial multicolor coding. We find that there is an optimal exposure of the excitation for obtaining the maximum of the photoinduced anisotropy. Figure 5 shows the curve of the probing beam transmittance versus the exposure of the exciting beam. Here, a laser diode (650 nm, 100 mW/ cm2, horizontally polarized) was used as exciting beam and a He–Ne laser ( 633 nm, 0.1 mW/ cm2) was used as probing beam. The investigated film was placed between two orthogonal polarizers (P and A) along the pathway of the probing beam. The axis of polarizer P was at 45° with respect to the polarization of the exciting beam. In Fig. 5 the transmittance of the probing beam through the P–film–A system increases linearly at the beginning and reaches a maximum at ⬇15 J / cm2, and then it decreases with increasing exposure. This effect can be explained by the mechanism of polarization-saturated absorption of molecules. The anisotropically absorbing molecules of pyrrylfulgide are immobilized randomly in the PMMA polymeric matrix, which shows isotropical characteristics in its initial state. When the sample is irradiated by linearly polarized light, photoselection of molecules takes place. The molecules whose long axes are parallel to the polarization of the light absorb strongly, whereas those with perpendicular orientation to the polarization of the light have low ab-

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sorption. As a result a macroscopic optical dichroism is induced. At the beginning of the excitation, the parallel molecules are abundant, so the photoanisotropy increases with exposure. With increasing exposure, more and more parallel molecules are exhausted and finally reach a state of saturated absorption, where a maximal photoanisotropy is obtained. However, when the exposure increases, some molecules with other nonparallel orientations, and even perpendicular orientation, will begin absorbing the light, which results in a decrease in the photoinduced dichroism. At room temperature the photoinduced anisotropy of the material can persist for 24 h in darkness. It can be erased by irradiation with perpendicularly polarized light or circularly polarized light. After the recorded signal has been erased completely, the photoinduced anisotropy can be produced anew. This can be repeated several hundred times, i.e., the material seems to have no fatigue.12

Fig. 5. Dependence of the photoinduced anisotropy on the excitation exposure.

Fig. 6. Contrast reversal of optical image. The pyrrylfulgide/ PMMA film is placed between two polarizers: (a) Positive image; the two polarizers are orthogonal. (b) Negative image; the analyzer is slightly rotated from the crossed position.

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3. NONFOURIER OPTICAL IMAGE PROCESSING A. Contrast Reversal of Optical Image The demonstration of contrast reversal is performed as follows. The polarized 650 nm laser diode beam is expanded to illuminate a transparency. Through an optical lens, the image of the transparency is projected onto the pyrrylfulgide/PMMA film. The colored sample is exposed for 200 s. This writing process results in the excited part of the film becoming anisotropic while the nonexposed parts (background) remain optically isotropic. A red LED is used as a reading beam and the sample is observed between a polarizer P and an analyzer A. For crossed polarizers 共P ⬜ A0兲, as we know from Section 2, behind the P–film–A system we can see just the excited parts of the sample, called here the positive image. When the analyzer A is rotated from its crossed position A0 to A1, the transmitted light from the excited parts becomes weaker, while the background will slowly brighten. At some point (the long axis of the elliptically polarized light ⬜A1) the background will be brighter than the excited parts of the sample, thus producing a negative image. Examples of such contrast reversal experiments are shown in Fig. 6. B. Optical Summation and Subtraction Optical summation and subtraction are demonstrated as follows. The exciting beam is the linearly polarized 650 nm laser diode, which is expanded to illuminate a transparency (the object) that is imaged by an optical lens onto the pyrrylfulgide/PMMA film. For simplification we use two slits, A and B, with different orientations as objects. First the laser is adjusted to 0° polarization and the image of a perpendicular slit (slit A) is recorded into the sample. Then the laser is rotated to 45° polarization, and in the same way the image of a slit tilted 45° (slit B) is recorded in the same area of the sample. The excited sample is observed between two orthogonal polarizers while the polarization directions of the polarizer P are varied (the analyzer A always remains perpendicular to the polarizer P). Since consecutive irradiation by two differently polarized beams is the same as simultaneous irradiation by two incoherent beams, A and B, of differing linear polarization, the transmitted light intensity of the P–film–A system is given by the following expression5: I ⬀ 关b2 sin 2共␪ − ␣兲 − a2 sin 2␣兴2

共2兲

Here a2 and b2 are the exposures of the beams A and B, respectively; ␪ is the angle between the polarizations of

Table 1. Different Conditions for the Transmission Images of the P–Film–A System Corresponding to Different Polarizations of the Polarizer P

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Fig. 7.

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Experimental demonstration of optical summation and subtraction (for details see Table 1).

the two beams (in our experiment ␪ = 45°); and ␣ is the angle between the polarizations of the polarizer P and the beam A. It can be seen that for ␣ = ␪, the image of slit A will be reconstructed with an irradiance proportional to a4 sin2 2␣, having its maximum at ␣ = 45°. To reconstruct slit B the angle ␣ should be zero, and the irradiance is proportional to b4 sin2 2␪, which analogously becomes maximum for ␪ = 45°. Two interesting things occur when ␣ attains the values ␪ / 2 or ␪ / 2 − ␲ / 4. In the first case the result is a subtraction of the signals with 共b2 − a2兲2 sin2 ␪; in the second case we achieve a summation of the signals with 共b2 + a2兲2 cos2 ␪. The best condition for these operations is ␪ = 45°. An overview of interesting results that can be realized by adjusting ␣ is given in Table 1. Experimental demonstrations are shown in Fig. 7.

Fig. 8. Experimental configuration for Fourier optical image processing.

4. FOURIER OPTICAL IMAGE PROCESSING

Fig. 9. Experimental results of Fourier optical image processing. (a) Low-pass filtering, (b) edge enhancement.

Optical Fourier transformation is a powerful tool in optical computing and processing systems. Recently photoanisotropic materials have shown great promise as nonlinear materials for optical computing and processing. Figure 8 shows the schematic setup of an optical Fourier processing system employing photoinduced anisotropy of fulgide films. Lens L1 performs the Fourier transformation of the object information (O) at the position of the fulgide film, and lens L2 performs the inverse Fourier transform at the plane of the CCD camera to yield the processed image. A laser diode at a wavelength of 650 nm is used as the exciting and testing beam. An attenuator is used to control the intensity of the beam: For excitation of the film it is set for high intensity, for probing it is set for low intensity. The polarization of the analyzer A is 90°. First P is rotated to 45° and the fulgide film is irradiated for several minutes. Then we rotate P to 0° and observe the result with the CCD camera. Now we exploit the fact that according to the findings of Section 2 there is an optimal exposure range of 15 J / cm2. For most arbitrarily chosen objects the low-frequency signals are stronger than the high-frequency signals. So when the film is excited at the low-frequency areas at optimal exposure, the anisotropy will be maximal in these areas, whereas the high-frequency areas are excited less. Hence the latter exhibit only a weak anisotropy. As a consequence the transmission of the low-frequency signals through the P–fulgide–A system will be larger than that of the high-frequency signals. By this technique we obviously realize low-pass filtering. Figure 9(a) gives an example of the result we obtained. On the other hand, if the film is excited with optimal exposure at areas of high spatial frequency, the anisotropy is maximal there, whereas the low-frequency areas are excited above optimal expo-

sure, resulting in reduced anisotropy. This results in edge enhancement as demonstrated in Fig. 9(b).

5. CONCLUSIONS Under exposure of linearly polarized red light, the C-form molecules of pyrrlfulgide are transferred into the E-form. As a result the molecules whose long axes are parallel to the polarization of the excitation light absorb strongly, whereas those with perpendicular orientation to the polarization of the light have low absorption, resulting in a macroscopic optical dichroism. At room temperature the anisotropy can persist for 24 hours in darkness and can also be erased by irradiation with perpendicularly polarized light or circularly polarized light. From the measured photoinduced dichroism spectrum, the birefringence spectrum can be calculated according to the Kramers–Kronig relation. The maximal photoinduced dichroism is ⬇0.1 at 625 nm and the maximal photoinduced birefringence is 8.3⫻ 10−4 at 710 nm. The optimal exposure is about 15 J / cm2. The feasibility of performing optical image processing by exploiting the photoinduced anisotropy of the pyrrylfulgide/PMMA film has been demonstrated. The operations that could be performed include contrast reversal, summation and subtraction of two images, low-pass filtering, and edge enhancement.

ACKNOWLEDGMENTS This research is supported by the Natural Science Foundations of China under grant 60337020, 60278026 and the Knowledge Innovation Project of the Chinese Acad-

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emy of Sciences under grant 40001043. Corresponding author B. Yao’s email address is [email protected]

Vol. 23, No. 2 / February 2006 / J. Opt. Soc. Am. A 8. 9.

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