1. Introduction

Fulgide is a kind of organic photochromic material, which is famous of thermally irreversible photochromism, that is expected to have applications in rewritable optical memories and photofunctional switches [1]. Matsui et al. [2] prepared a recording disk using 5-dimethylaminoindolyfulgide and found that readout of pits was possible at a long wavelength inside the absorption band of the colored form. Up to 10⁵ readouts were obtained with only 20% decrease of initial absorption. Fan et al. [3] also prepared a recording disk with pyrrylfulgide by spin-coating it with PMMA or by vacuum evaporation. Over 500 write-erase cycles were possible without an observable change of disk performance. Belfield et al. [4] investigated the femtosecond laser-induced two-photon photochromism of a fulgide and proved that it can be used in two-photon holographic recording. Liao et al. [5] also demonstrated that multi-layer three-dimensional data storage based on the laser-induced two-photon photochromism was possible in fulgides. In our previous work we have reported usage of pyrrylfulgide in parallel optical data storage [6]. We also found that photochromism in fulgides was accompanied by photoinduced anisotropy. This property could be used in image storage by polarization holography [7]. In this paper, we study the photoinduced anisotropic properties of the pyrrylfulgide. The photoinduced anisotropy originates from the molecular alignment of the photochromic reaction under the excitation of linearly polarized light [8]. The photoinduced anisotropy includes dichroism and birefringence. The first means that the absorbance depends on the polarization of light, the latter results from the photoinduced optical axis which gives rise to different refractive indices in two orthogonal directions [9]. Based on the photoinduced anisotropy, polarization discrimination of images recorded in some media, e.g., silver-chloride emulsion [10], dyed plastic [11] and bacteriorhodopsin [12–13], had been investigated. Here we demonstrated that this property could be used for polarization patterns hide and display.

2. Results and discussion

The preparation of pyrrylfulgide was described in Ref. [3]. It was doped into a PMMA matrix by preparing a solution with cyclohexanone. The film was obtained by spreading several droplets of the pyrrylfulgide-PMMA solution on optical glass (ϕ25 mm×1.5 mm) and drying in air. The doped concentration in the sample is about 10% (w/w, pyrrylfulgide/PMMA) and the thickness of the film is about 10 µm. The absorption peak of the colored form of the sample is at a wavelength of 626 nm and an optical density OD=2.44. The bleached form has its absorption maximum at 373 nm.

A Linearly polarized laser diode (650 nm, 40 mW) was used as excitation source. Non-polarized He-Ne laser (633 nm, 4 mW) together with a polarizer was used for detection. Excitation and detection were performed simultaneously. Vertical and horizontal polarizations were separately used as excitation beam, while the detection beam had always horizontal polarization. Laser power was measured by a digital power meter (11A Photometer/Radiometer, United Detector Technology Inc. USA, sensitivity 0.01 nW). Figure 1 shows the transmittance curves versus excitation time and presents the indication of dichroism. The transmittance under horizontal excitation (T_‖) is higher than that under vertical excitation (T⊥). The interpretation is that during the transit of the fulgide molecules from the C-form to the E-form a significant number of molecules have been aligned in the direction of the polarization of the excitation beam, which results in an induced optical axis in the film.

 

Fig. 1. Transmittance (at 633 nm) versus excitation (at 650 nm) time of the pyrrylfulgide/PMMA film for different excitation polarizations. T_‖: excitation and detection beams have same polarizations; T⊥: excitation and detection beams have orthogonal polarizations.

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To measure the photoinduced birefringence, the film was placed between two orthogonal polarizers P and A. Before excitation the detection beam cannot pass through the P-fulgide-A system. Since excitation of the sample with a linearly polarized beam will induce an optical axis in the sample, the polarization state of the detection beam will be changed. In particular, if the polarization of the excitation beam is inclined at 45° with respect to P, the linearly polarized detection beam becomes elliptically polarized after passing through the film. Therefore some intensity are transmitted through the analyzer A and can be detected. The transmittance obeys the Malus law, i.e., T=cos ² (π/2+f), where f is the photoinduced birefringence in the film. Figure 2 shows the transmittance of the P-fulgide-A system versus excitation time for an excitation intensity of 122 mW/cm² and a detection intensity of 0.6 mW/cm². From this figure it can be seen that there exists an optimal exposure for maximum photoinduced anisotropy (here about 5 J/cm²).

It should be noticed that the linearly polarized detection beam becomes elliptically polarized due to the photoinduced birefringence, while the long ellipse axis is deflected from the axis of polarizer P due to the different transmittance for the light polarized parallel and orthogonally to the optical axis induced by the excitation beam (i.e., the photoinduced dichroism). Figure 2 reflects both dichroism and birefringence induced in the film, whereas Fig. 1 reflects only the photoinduced dichroism.

 

Fig. 2. Transmittance (at 633 nm) of the P-fulgide-A system versus the excitation (at 650 nm) time. The cross polarization angle between the excitation beam and the polarizer P is 45⁰.

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Fig. 3. Experimental setup for writing and reading polarization spot patterns

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Fig. 4. Polarizations of the light spots recorded on the film

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Based on the above photochromic and photoinduced anisotropic properties, we proposed an application as polarization pattern display. For demonstration, a non-polarized He-Ne laser with a polarizer was used as the writing beam. A matrix of light spots with different polarizations were recorded on the film under the microscope as shown in Fig. 3. Polarizer P_e was used to adjust the polarization of He-Ne laser. Polarizers P and A were used to read the polarization spot patterns out. The polarizations of the light spots recorded on the film are shown in Fig. 4. The exposure time for each spot was 0.5 s. The diameter of the spot was about 20 µm, the interval distance between two spots was 80 µm.

The polarization patterns embedded in the matrix cannot be recognized by directly inspecting the matrix of light spots, i.e., without applying two crossed polarizers. Figure 5(a) shows the observed pattern by just using non-polarized light for readout. A homogeneous 4×4 matrix of light spots is seen. But when the film is placed between two orthogonal polarizers P and A, different patterns can be observed when rotating the polarizers. If the polarizer P is 0° or 90° (polarizer A is 90⁰ or 0⁰), only the light spots polarized at 45° can pass through the P-fulgide- A system. This results in the readout pattern shown in Fig. 5(b). If the polarization of P is 45° or 135° (polarizer A is 135⁰ or 45⁰), only the vertical polarization light spots can pass through the P-fulgide-A system. Therefore the readout pattern is like Fig. 5(c). By this way two polarization patterns can be hidden in the matrix and recorded in the pyrrylfulgide/PMMA film based on its photochromic and photoinduced anisotropic properties, and only readout with two orthogonal polarizers at certain polarization angles can pick up the patterns separately. This property is possibly applied in camouflage technology.

 

Fig. 5. Readout patterns from the film without polarizers or by applying two orthogonal polarizers (P and A) at different polarization angles. (a) without polarizers; (b) the polarization of P is 0° or 90° ; (c) the polarization of P is 45° or 135°.

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3. Conclusions

Pyrrylfulgide/PMMA film is a photochromic medium with photoinduced anisotropy. Excitation with linearly polarized light induces a bleaching process transferring molecule from the colored form to the bleached form. Thereby an optical axis is induced in the film, which is oriented in the polarization direction of the excitation beam. Both photoinduced dichroism and birefringence were observed. The transmittance curves versus exposure time were measured and the optimal exposure was found to be at about 5 J/cm². Two sets of polarization spot-patterns were recorded in the pyrrylfulgide/PMMA film by respectively using two beams of different linear polarization (cross angle 45⁰). The patterns embedded in the 4×4 spots matrix can only be revealed by using two orthogonal polarizers at certain polarization angles. Otherwise a homogeneous matrix of light spots is observed. This property may be applied in camouflage technology.