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The advantage of the circular polarization of light in updatable holographic responses in an azo-carbazole monolithic dye dispersed acrylate matrix has been investigated. Compared to linear polarization holography, four times higher diffraction efficiency was measured without the depression of the response time. The rise time of the holographic recording becomes faster with the increase in the cycled recording time, and no change in diffraction efficiency was observed in the cycled updatable holographic recording using a circularly polarized light. On the other hand, a significant depression in the diffraction efficiency and a slower rise time were observed in a cycled updatable hologram recording using a linearly polarized light. The advantage of the orthogonal circular polarization of light in updatable holographic responses is discussed on the basis of the difference in the photo-oriented state of the azo-carbazole molecules under the orthogonal circular polarization holographic recording and the parallel linear polarization state.
© 2017 Optical Society of America
1. Introduction
The holographic technique invented by Gabor in 1948 [1] has been widely developed by Leith and Upatnieks [2–4] as well as by Denisyuk and associates [5, 6] as a means of displaying full-parallax three-dimensional (3D) images. A hologram commonly records the amplitude as well as the phase of light from the objective in an interference pattern. Therefore, the reproduced hologram is a real 3D image. Holograms have been widely used in the field of art, security, and 3D display devices. A conventional hologram is recorded in silver halides and photopolymers, which are non-rewritable. Updatable or rewritable holographic display systems have widely been desired in the field of amusement and commercial game machines. Recently, updatable 3D holographic displays have been demonstrated using photorefractive materials [7–10] and photochromic materials [11, 12], and they are promising candidates for updatable holographic devices.
We investigated the updatable holographic capability of an azo-carbazole (ACzE) monolithic dye dispersed in poly(methyl methacrylate) (PMMA) [11–14]: the capability of ACzE for use in updatable holographic devices has been demonstrated [11, 12], the relationship between the photo-orientation and holographic properties has been clarified [13], and the molecular design of the ACzE molecule for updatable full-color holograms has been achieved [14]. ACzE has unique properties for holographic recording of high diffraction efficiency with a fast recording time and prolonged image reading when it is excited at an isosbestic point of absorption for the trans and cis forms.
Azobenzene has unique properties of photo-orientation through photo-isomerization between the trans and cis forms [15]. The effect of circular polarization on holographic recording and reading in azobenzene molecules has been widely investigated [16–25]. Chiral structures can be formed in the matrix, including azobenzene molecules [26–28].
In our previous studies [11–14], a holographic grating was recorded using parallel linear polarization beams and read using a linear polarization beam with the same polarization as the recording beams. The polarization of the recording and reading beams significantly affects the holographic grating of the monolithic ACzE molecules in the acrylate matrix. However, the effect of circular polarization on the holographic properties in ACzE/PMMA has not been investigated yet.
In this report, we investigated updatable holographic diffraction properties in ACzE/PMMA using circularly polarized light and discussed the advantage of the circular polarization of light on the recording/erasing/over-recording updatable holographic properties of ACzE/PMMA compared to those using linearly polarized light.
2. Experimental sections
2.1 Materials and sample preparation
3-[(4-Nitrophenyl)azo]-9H-carbazole-9-ethanol (NACzE) was used as a representative ACzE in this study. 30 wt% NACzE and 70 wt% PMMA were dissolved in tetrahydrofuran (THF) for 24 h and then cast on a glass plate. The ratio of 30/70 by wt. is one of better known ratios for NACzE/PMMA [29, 30]. The cast film was dried at 70 °C for 24 h, and the dried film was melt-pressed between two glass plates with 80-μm Teflon spacers at 250 °C.
2.2 Measurements
The nondegenerate four-wave mixing (NFWM) method was employed to determine the diffraction response. Schematic diagram for NDWM is shown in Fig. 1. The recording laser source is a diode-pumped solid-state (DPSS) Samba laser at 532 nm (25 mW, Cobolt AB, Sweden), and the reading (probe) laser source is a He-Ne laser at 632.8 nm (10 mW, Lasos Lasertechnik GmbH, Germany). The probe beam intensity was controlled using a variable neutral density filter. The intensity of the probe beam was weaker than that of the recording beams. The angle between interference beams was fixed at 15 °. Circular polarization was obtained inserting a quarter-wave plate as shown in Fig. 1.
Fig. 1 Schematic diagram for NFWM. Quarter-wave plate is inserted for circularly polarized measurements.
The rise time (τr) for the grating formation and the fall time (τf) for the disappearance of the grating were evaluated from the time profile of the diffraction efficiency (η) using the Kohlrausch-Williams-Watts (KWW) stretched exponential function of
where η0 is the maximum diffraction efficiency and β is the dispersion parameter from a single exponential (0 < β ≤ 1).The sensitivity S is defined by
where I is the intensity of the laser power. In the present case, I = 0.53 J cm−2 at 532 nm.3. Results and discussion
Figure 2 shows the time growth and the decay of the holographic diffraction from the grating recorded using orthogonal circularly polarized beams and using linearly polarized beams in NACzE/PMMA. In Fig. 2(a), the highest diffraction efficiency (η) of 57% was observed with the rise time (τr) of 8.6 s for the sample recorded with the orthogonal circularly polarized beams and read with the right-hand circularly polarized beam. A middle diffraction efficiency of 28.7% was achieved with the rise time of 7.9 s for the sample recorded with orthogonal circularly polarized beams and read with a linearly p-polarized beam, and the lowest diffraction efficiency of 13.4% was obtained with the rise time of 9.7 s for the sample recorded and read with the linearly p-polarized beam. The rise time in every case ranges from 7.9 to 9.7 s, with little difference. These results are summarized in Table 1, as is the calculated sensitivity for the holographic recording. Recording and reading with orthogonal circularly polarized beams enhances the diffraction efficiency and sensitivity. These differences in the diffraction efficiency are explained as follows. Figure 2(b) represents the normalized decay profile of the diffraction efficiency after stopping the laser illumination. The decay of the diffraction efficiency means that the recorded holographic grating thermally dissipates as time passes. A decay time (τd) of 440 - 471 s was measured for the recording with orthogonal circular polarization and one of 82.4 s for the recording with parallel linear polarization. For linear polarization, the dependence of the decay time on sample temperature was discussed in our previous report: higher temperature led to faster decay time [13].
Fig. 2 Time growth and decay of holographic diffraction from the grating recorded with orthogonal circularly polarized beams (RCP and LCP) and with linearly polarized beams (LP) for NACzE/PMMA.
Table 1. Diffraction efficiency, rise time and decay time for diffraction response, and resulting sensitivity for various polarization of recording and reading.
Table 2 shows the polarization configuration of the recording beams and the polarization distribution in the position inside the grating. In the case of the recording condition with parallel linearly polarized beams, the bright region is located at 0 and 2π, the dark region at π, and the transient regions between them at π/2 and 3π/2. On the other hand, in the case of the recording condition with orthogonal circularly polarized beams, the position at 0 and 2π is the p-polarization state, that at π is the s-polarized state, and the transient regions between them are at π/2 and 3π/2.
Table 2. Polarization configuration of recording beams and polarization distribution in the position inside grating
ACzE molecules have two types of conformation, trans and cis forms, which are intramolecularly switchable by illumination and heating. The wavelength of light that induces the conformational change depends on the azo-molecules. Another characteristic of azo-molecules is the photo-orientation of the molecule through trans-cis-trans conformational changes. These conformational changes induce a large optical anisotropy in the bright region formed by the interference pattern of parallel linearly polarized beams but no conformational change in the dark region, as expected from the polarization configuration shown in Table 2. The amplitude of the modulated refractive index is determined by the optical anisotropy between the bright and dark regions. The interference of the orthogonal circularly polarized beams induces twice the amplitude of the modulated refractive index because of the alternating p-polarization and s-polarization along the grating vector direction, as predicted from the polarization configuration shown in Table 2. Furthermore, as discussed below, the intensity of the beam diffracted by a circularly polarized beam is twice that of a beam diffracted by a linearly polarized beam.
The Jones matrix [31, 32] is used for describing the polarization difference. The Jones vector E is described as
where . The Jones vector can describe the polarization state of the beam. The output beam Eout is described with the Jones matrix T and the input beam Ein,The Jones matrix formed by the interference of the orthogonal circularly polarized beams Tocp [33, 34] is
where, , Δn is the photoinduced birefringence, d is the film thickness, λ is the wavelength of the reading beam, x is the position, and Λ is the grating spacing.The diffracted beam intensities for E+1 and E-1 read by the circularly polarized beam Ein (Eq. (7)) are written as Eqs. (8) and (9), respectively.
The diffracted beams for intensities E+1 and E-1 read by the linearly polarized beam Ein (Eq. (10)) are written as Eqs. (11) and (12), respectively, The intensity of the diffracted beam read by the circularly polarized beam is twice as high as that read by the linearly polarized one, as described in Eqs. (8) and (11).The speed of the grating formation and disappearance for the cycled recording and erasing in the updatable hologram is an important factor for practical usage. Figure 3 shows five cycles of grating formation and disappearance monitored during 50 s of recording followed by 50 s erasing in NACzE/PMMA. In the erasing period, one of the two beams is turned off, and the other beam is used as an erasing beam. The response profile with time for recording using the orthogonal circularly polarized beams is shown in Fig. 3(a), and that for the linearly polarized beams is shown in Fig. 3(b). The response time for the grating formation by recording is evaluated from the rise time (τr) in the growth profile of the diffraction efficiency and that of the grating disappearance by erasing is from the fall time (τf) in the decay profile of the diffraction efficiency. Tables 3(a), (b) and (c) summarize the evaluated results of the maximum diffraction efficiency (η) in the growth profile, the minimum (residual) diffraction efficiency (ηr) in decay profile, the rise time for recording, and the fall time for erasing in each event with both polarization cases. In the case of recording using orthogonal circularly polarized beams, the right-hand circularly polarized (RCP) beam and left-hand circularly polarized (LCP) beam were employed as reading beams. With recording using orthogonal circularly polarized beams, the rise time for the grating formation becomes faster, the fall time becomes slower and the related residual diffraction efficiency increases with the increase in the number of events without significant variation in the maximum diffraction intensity. On the other hand, in the case of linearly polarized beams, the rise time becomes slower and the maximum diffraction efficiency decreases with the increase in the number of cycled events.
Fig. 3 Five cycled diffraction responses in NACzE/PMMA as a function of time: recording: 50 s, erasing: 50 s. a) Recording beams: interfered orthogonal circularly polarized beams (RCP and LCP), reading beam: RCP, and erasing beam: RCP. b) Recording beams: interfered linearly polarized beams (LP), reading beam: LP, and erasing beam: LP.
Table 3. Maximum diffraction efficiency (η), residual diffraction efficiency (ηr), rise time (τr), fall time (τf), and resulting sensitivity for cycled recording.
The effect of erasing for a long period of time on the next recording event is studied. Figure 4 shows two cycled grating formations and disappearances monitored by the diffraction response for 50 s recording followed by 450 s erasing in NACzE/PMMA. The response profile with time for the orthogonal circularly polarized beams is shown in Fig. 4(a), and that for the linearly polarized beams is shown in Fig. 4(b). Tables 3(d) and (e) summarize the evaluated results of the maximum diffraction efficiency in growth profile, the minimum (residual) diffraction efficiency in decay profile, the rise time for recording, and the fall time for erasing in each event for both polarization cases. As shown in Fig. 4(a) and Table 3(d), for the case of orthogonal circular polarization, the 2nd diffraction efficiency is almost the same as the 1st one, and the rise time for the grating formation becomes faster for the 2nd recording. On the other hand, as shown in Fig. 4(b) and Table 3(e), a significant depression in the diffraction efficiency and a longerrise time are observed in the 2nd recording in the case of linear polarization. These results imply that the usage of orthogonal circular polarization is preferred for the recording and erasing in the updatable hologram in NACzE/PMMA. It must be considered why the holographic recording using orthogonal circularly polarized beams leads to a faster rise time for the next recording while keeping almost the same maximum diffraction efficiency, which is the opposite phenomenon to that observed using linearly polarized beams.
Fig. 4 Two cycled diffraction responses in NACzE/PMMA as a function of time: recording: 50 s, erasing: 450 s. a) Recording beams: interfered orthogonal circularly polarized beams (RCP and LCP), reading beam: RCP, and erasing beam: RCP. b) Recording beams: interfered linearly polarized beams (LP), reading beam: LP, and erasing beam: LP.
Figure 5 shows schematic pictures of the morphological change in the assembled NACzE molecules during recording. As shown in Fig. 5(a), during the recording by the orthogonally circularly polarized interference beams, the alignment of the NACzE molecules change with the direction of the polarization of light along the grating vector. During the period of erasure by a single beam, the aligned NACzE molecules are randomized by the RCP and LCP. The randomized state after the erasure is almost the same as that before the 1st recording, so the 2nd holographic recording maintains the same diffraction efficiency as the 1st one. Furthermore, in the 1st recording process, the cis-trans conformational change causes extra free volume in the matrix [35]. This extra free volume enhances the molecular photo-orientation and leads to a faster rise time of the holographic diffraction for 2nd- or higher-cycle recording. As shown in Fig. 5(b), in the erasure process by the linearly polarized single beam, the NACzE molecules in the entire area are perpendicularly aligned in the direction of the polarization of light. This aligned state is quite different from the initial state before recording or that during recording by the linearly polarized interference beams.
Fig. 5 Schematic pictures of morphological change in assembled azo-carbazole molecules during recording. (a) Orthogonal circular polarization recording and (b) parallel linear polarization recording.
4. Conclusion
The advantages of holographic recording and reading with circularly polarized beams are investigated for updatable holographic diffraction responses in NACzE/PMMA. The rise time for the grating formation on the cycled recording and erasing with orthogonally circularly polarized beams is accelerated with the increase in the number of cycles without the reduction of the maximum diffraction efficiency close to 80%, while that using linearly polarized beams is slowed down, together with a significant reduction in the maximum diffraction efficiency. The RCP or LCP beam randomizes the aligned NACzE molecules in the erasing process, and the expansion of free volume due to the trans-cis-trans conformational change leads to a faster rise time for 2nd- and higher-cycled grating formations.
Funding
Strategic Promotion of Innovative Research and Development (S-Innovation) program, Japan Science and Technology Agency.
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