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Abstract
Biphotonic holographic recording through the competition of linearly polarized laser beams with wavelengths of 532 nm and 632.8 nm was investigated in an azobenzene liquid crystal film. With the irradiation of 532 nm excitation light as a switch, holographic gratings were able to be turned on and off. It was found that diffraction behaviors of the recorded gratings were strongly dependent on not only the polarization angle between the recording light and excitation light, but also the sample temperature. Formation processes and diffraction properties of biphotonic holographic gratings were discussed based on the photoisomerization of azobenzene groups. Besides, biphotonic polarization holographic recording was also achieved by means of adjusting the polarization states of recording light and the diffraction characteristics were analyzed in terms of Jones matrices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Holography is a preferred technique utilized to record and reconstruct perfect wavefronts of objects, and has been applied in many fields, such as information processing [1], displays [2,3], large-capacity memories [4], and measurement of multidimensional objects [5,6]. Due to the increase demand on data storage, holographic storage has become one of the most extensively studied mass storage technologies in recent years [7,8]. For example, a novel type of plasmonic metasurfaces was proposed and experimentally demonstrated, which could achieve broadband multiplane holography with large information capacity [9]. In addition, a silver nanoparticle doped poly(2-hydroxyethyl methacrylate-co-methacrylic acid) recording medium was synthesized for reversibly recording 3D holograms [10]. During the data reading process, the data is restored instantaneously and illuminated by a single reference beam at the same time, which makes it possible for holographic data storage systems to have a rapid data transfer rate. On the other hand, with the development of scientific researches in the last few decades, polarization holography has emerged as a new branch of holographic technology [11,12]. It is an attractive technique for the unique capacities of recording the intensity, phase and polarization state of a wave simultaneously [13]. Because of the great importance of holography, holographic materials attract considerable interest. Among various types of materials, azobenzene liquid crystals materials have recently attached a lot of attention owing to their promising applications in dynamic holographic display [14], optical storage [15], and optical devices [16]. Optical storage in azobenzene liquid crystals is believed to take place due to the statistical reorientation of azobenzene chromophores [17,18]. This photoisomerization process can lead to the photo-induced anisotropy of azobenzene chromophores, which produces intermolecular torques between azobenzene chromophores and liquid crystal molecules. Such procedures may align the molecules in the sample perpendicular to the polarization of incident light [19,20].
However, azobenzene liquid crystals are usually focused on the applications in resonant regions [21–25] and large absorption in resonant regions can lead to the enhancement of photo-induced thermal effect. Large photo-induced thermal effect is not beneficial to practical applications. For dynamic holographic display applications, when the recording intensity of resonant light is above a critical level, diffraction effect may diminish in a few seconds [26]. Moreover, in terms of energy-absorbing based optical instruments located in the absorption band, they normally employ reverse saturated absorption [27] to reduce the transmittance at high input intensities. Therefore, biphotonic holographic recording becomes a good candidate for practical applications. Under the illumination of one linearly polarized green light with the simultaneous irradiation of an interference pattern created by two linearly polarized red lights, the biphotonic gratings can be recorded. With further researches, electrically switchable and thermally erasable biphotonic holographic gratings [28] as well as biphotonic laser-induced ripple structures have been investigated in dye-doped liquid crystal films [29–31]. In this paper, two kinds of holographic gratings were recorded in an azobenzene liquid crystal film by means of the competition between the recording light and excitation light. With the irradiation of an excitation beam as a switch, the recorded gratings could be turned on and off. The recording processes are contributed to the participation of two photons, which is known as the biphotonic effect. Formation mechanisms and diffraction properties of the recorded gratings were analyzed based on the photoisomerization of azobenzene groups and Jones matrices.
2. Experiments
The sample in the experiment was a supermolecule material by ionic self-assembly [32] of poly ionic liquid (PIL, Sigma-Aldrich) and azobenzene dye. The charged polymer poly (1-butyl-vinylpyridinium bromide) PIL was selected as the main chain segment, and the methyl orange dye (MO, Sigma-Aldrich) was selected as the building unit. For preparation of ionic self-assembly complex, 2 mg/ml PIL aqueous solution was added dropwisely to MO aqueous solution in a 1:1 molar charge ratio. The precipitated complex was filtrated and washed several times with doubly distilled water, then dried in vacuum at 60°C for 12 hours. The complexes powder melt at about 180°C and high orientational order of complexes appeared during the cooling stage. The thickness of resultant films was about 5 μm, measured by a Dektak profilometer. The absorption spectrums of the complex film and the chemical structure of the compound are shown in Fig. 1.
Fig. 1 Absorption spectrums of the azobenzene liquid crystal film before and after the irradiation of 532 nm light. Inset: chemical structure of the compound.
Figure 2 presents the experimental setup in this work. A linearly polarized Gaussian beam with a wavelength of 632.8 nm from a He-Ne laser was applied as recording light (~250 mW/cm2). The incident light was divided into two beams (W1 and W2) with the same intensity through a beam splitter and interfered at the surface of the sample with an spatial intersection angle of 6°. The incident bisector of two recording beams was normal to the surface of the film. A linearly polarized 532 nm beam from a CW Nd:YAG laser was employed as excitation light (~200 mW/cm2). The polarization states of recording light and excitation light were manipulated by 632.8 nm as well as 532 nm half-wave plates respectively, and measured by a Free-Space Polarimeter (THORLABS, PAX5710VIS-T). Another s-polarized 632.8 nm beam from He-Ne laser, shown in Fig. 2, was employed as probe light whose intensity was reduced with an attenuator. All measurements were performed at room temperature.
Fig. 2 Experimental setup for biphotonic holographic recording. W1 and W2 are linearly polarized recording waves. A, attenuator; P, polarizer; BS, beam splitter; M, mirror; Q1, 632.8 nm half-wave plate; Q2, 532 nm half-wave plate.
3. Results and discussion
During holographic recording, polarization states of W1, W2 and the 532 nm beam were all set to the s-direction. The intensity of probe light was controlled in order not to influence the recorded grating. It was found that no diffraction light existed when the recording light was turned on without excitation light. As the green light was switched on, the first diffraction order started to appear and a biphotonic holographic grating was achieved. The diffraction pattern, polarization states as well as diffraction efficiency (DE) of the recorded grating were measured and the details were presented in Fig. 3. There was only one diffraction order and the diffraction beams were all s-polarized. After the excitation light was turned on, the efficiency of the diffractive signal underwent an upsurge first and tended to slow down gradually until reaching a stable value (~10.5%), which was marked with arrows and dotted lines. This rising process can be divided into two stages, a fast process and a slow process. The fast process is caused by the movement of azobenzene dyes as well as the side chains of macromolecules. When exposed to recording light and excitation light, azobenzene groups transform from a trans-longitudinal to a cis-excited state and back to the trans state. The absorbed energy of azobenzene groups is proportional to the angle between the molecular axis and the polarization of incident light. Such an angular dependence tends to align the molecules in the minimal-energy configuration, which is perpendicular to the polarization direction. With the photo-induced reorientation of azobenzene dyes and side chains of macromolecules, the arrangement of the main chain of liquid crystal molecules is driven, resulting in the slow process. These two processes can be described by a biexponential equation [33]:
where A and B are preexponential factors, Ka and Kb represent rate constants of fast and slow processes respectively and t is the recording time. ηoff is DE which is defined as ηoff = It/I0, where It is the intensity of the diffraction light and I0 is the intensity of the light passing through the sample before recording. When the recording and excitation beams were turned off, DE was kept within a certain extent with a very small decline (less than 1%). The recorded grating could last for a few months, which indicates that the sample possesses long-term optical storage characteristics. The main reason is that azobenzene groups were “frozen” in new molecular orientation directions and formed a stable recording condition because of the arrangement properties of liquid crystals. Moreover, the stable stored information can be erased in two ways. First, the sample is heated, in which thermally randomizing the molecular orientations occurs. Second, the grating can be erased in optical method. Because of a new uniform re-ordering of azobenzene molecules under the action of the circularly polarized light field, a circularly polarized beam can be used to randomize the orientation of the azobenzene groups. This writing-erasing cycle could be repeated over 100 times on the same spot of the film without fatigue.Fig. 3 Real time behavior of the first-order diffraction efficiency during a biphotonic holographic grating was recorded at room temperature. Inset: diffraction patterns of the recorded holographic grating and polarization states of diffraction light.
After holographic recording was achieved with both s-polarized 532 nm and 632.8 nm light, we modulated the half-wave plate in the path of excitation light to control its polarization state, while 632.8 nm recording light was kept s-polarized. The angle α between the s-direction and the polarization direction of green light was manipulated from 0° to 90° at equal angle-intervals (10°) without changing the intensity of light. Results and details are shown in Fig. 4. In the range of 0°<α<20°, DE decreased slowly. With α continuing to increase, DE underwent a rapid decline. When α = 90°, DE reached the minimum value which was about 0.9%. One reasonable explanation is that when the sample was irradiated by laser beams, photoisomerization effects of azobenzene molecules exerted intermolecular torques to align liquid crystals perpendicular to the polarization states of incident beams [34]. When polarization states of recording light and excitation light were parallel, two kinds of intermolecular torques were in the same direction, resulting in the maximum DE. During the process of varying the polarization state of excitation light to the p-direction, intermolecular torques effects caused by recording light and excitation light gradually cancelled each other out, leading to a corresponding decrease in DE.
Fig. 4 Dependence of DE on the polarization state of 532 nm excitation light. Inset: diagrams of intermolecular torques when the polarization directions of recording light and excitation light are parallel (a), intersecting (b), and orthogonal (c) respectively.
In addition, the thermal influence on biphotonic holographic recording was investigated as well. Based on the experiment above, polarization states of recording light and excitation light were all set to s-polarized for the maximum DE. The experimental temperature range was set between 20°C and 60°C with equal temperature-intervals (10°C). Figure 5 presents real time behaviors of the first-order DE at different temperatures. With the sample temperature rising, the efficiency of the first diffraction order increased gradually from 9.49% to 11.97%. This temperature dependence can be understood as a competition effect. During recording processes, two opposite effects, irregular thermal movement of molecules and photo-induced reorientation of azobenzene groups, compete with each other. Within the experimental temperature range, an increase of the sample temperature weakens intermolecular effects and contributes to the rearrangement of azobenzene groups caused by incident light, resulting in a rise of DE. It can be noticed that the DE curve is not stable at 60°C. The reason is that the environment temperature was much lower than 60°C. The heat exchange velocity in this case is rapid and makes the sample temperature unstable, which makes DE fluctuate.
Fig. 5 Typical time evolution showing the biphotonic holographic recording at different temperatures. Inset: diffraction patterns of biphotonic holographic gratings.
With the purpose of investigating the formation mechanism of biphotonic holographic gratings, two experiments on diffraction rings were proposed. A single 532 nm Gaussian beam was first applied to irradiate the sample. The pattern of the transmitted light was projected onto a screen after the sample. Results and details are presented in Fig. 6. With the continuous increase of light intensity, one diffraction ring started to appear at the intensity of 0.15 W/cm2. When the intensity reached 0.45 W/cm2 and 0.7 W/cm2, the second- and the third-order diffraction ring could be detected respectively. The nonlinear optical diffraction rings are contributed to the photoisomerization of azobenzene dyes. In this experiment, the laser beam has a Gaussian intensity profile, hence the photo-induced nonlinear refractive index variation will be larger at the beam axis and diminish radially towards the beam edge. Different parts of the beam will interfere with each other and cause diffraction rings. According to the theory of spectral broadening effect caused by self-phase modulation [34], the phase shift Δ𝜙 and the number of diffraction rings N have the following forms:
where Δn is refractive-index change in the sample, λ is the wavelength of the laser beam, d is the thickness of the sample and z is the transverse position in the beam.Fig. 6 The number of diffraction rings in the azobenzene side-chain liquid crystal film as a function of laser intensity under CW Nd:YAG laser irradiation. Inset: the mechanism of photo-induced reorientation and cis(right)-trans(left) isomerization of azobenzene groups.
Then, a single 632.8 nm beam was employed. Because the wavelength of red light was not located at the absorption spectra of the sample, there was no photo-induced reorientation of azobenzene groups and no diffraction rings could be observed as shown in Fig. 7(a). Therefore, for this sample, the red light was known as off-resonant light. A 532 nm beam was applied to illuminate the sample at the same point and the first-order diffraction ring of red light gradually appeared, which demonstrated that the function of excitation light was a switch to activate azobenzene groups in order to give the photo-reactivity at the wavelength of 632.8 nm. The details are depicted in Fig. 7(b) and 7(c). Two processes contribute to this phenomenon. One is the adsorption effect induced by green light through the trans–cis isomerization and the other is the reverse effect of dye adsorption induced by red light through the cis–trans back isomerization. Most of the absorption bands of azobenzene polymers lie in the blue–green region of shorter wavelengths. Activation of green light transformed the azobenzene dyes from trans- to cis-isomers, causing the absorption band to shift to red light area with a longer wavelength. The low absorption of red light returned the cis-isomers to the trans-isomers. During the process of holographic recording, two red beams interfered at the surface of the film. The incident interference light showed constant polarization states and modulated intensity. Thus, a periodic distribution of alternative trans- and cis-isomers can be formed in response to the low- and high-intensity fringes of the interference pattern, bringing about a periodic distribution of photo-induced refractive index. Consequently, a biphotonic holographic grating was generated. According to TBC theory [35], the intensities of transmitted beams, I1(d) and I2(d), can be expressed by:
where ζd is the coupling strength, γ is the absorption coefficient, d is the thickness of the sample, and z0 represents I1(0)/I2(0) (z0 = 1 in this experiment). With Eq. (4) and (5), the absorption coefficient of recording beams based on green light can be calaclulated.Fig. 7 Diffraction rings of red light in the azobenzene side-chain liquid crystal film without (a) and with (b) the irradiation of a 532 nm beam (c).
Furthermore, a biphotonic polarization holographic grating was able to be recorded with the same experimental setup. The half-wave plate in the path of W1 was modulated to make W1 p-polarized. Probe light and W2 were both s-polarized. Experimental results showed that only the first diffraction order could be observed and the DE was about 4%. The polarization state of diffraction light was p-polarized. This polarization modulation property was able to be calculated with Jones matrices and such phase transmission matrix was developed from our previous work [36]. The coordinate system is established as shown in Fig. 8. The phase transmission matrix is:
where Δφoff = 2πΔnoffd/λp, with Δnoff being the photo-induced birefringence, d being the thickness of the film, δoff being the phase difference between two recording waves and λp being the wavelength of probe light. The Jones vector of probe light (along Z direction) R can be described by:where, θ is the angle between X-Z plane and the vibration plane of the probe light. Through Taylor expansion of the phase transmission matrix Toff, the ± first-order diffracted light has the form:From Eq. (7) and Eq. (8), it can be obtained that when probe light is s-polarized, E ± 1 off is p-polarized. Similarly, we changed the polarization state of probe light and the experimental results were all in consistent with theoretical discussion. In this case, the intensity of interference fields kept unchanged, while polarization states were periodically distributed in space. Azobenzene groups were excited to cis-isomers by green light and back to trans-isomers with red light. This repeated photoisomerizations in interference regions caused the spatial distribution of polarization states in the interference area. This process was accompanied by the reorientation of crystal liquids, resulting in a corresponding distribution of birefringence index in the film. Thus, a polarization holographic grating began to form.
Compared with the holographic grating recorded first, DE of the polarization holographic grating was much lower. The key point is attributed to the polarization dependence discussed above. During polarization holographic recording, it was not possible to make the polarization direction of excitation light parallel to two recording beams simultaneously, which caused the mutual inhibition of intermolecular torques, leading to the low DE of the biphotonic polarization grating. On the other hand, the distinction of DE between two kinds of biphotonic gratings is different from the resonant condition. According to Ref [36], DE of polarization holographic gratings is twice that of holographic gratings in the resonant region. It is reasonable to believe that the formation mechanisms of holographic recording in two conditions are different. In terms of the resonant condition, azobenzene groups absorb resonant interference light and reorient to a stable arrangement. The intensity of the resonant interference light is much higher than the excitation light. Therefore, this recording process is mainly attributed to both photo-induced thermal and photoisomerization effects [37]. For the biphotonic condition, azobenzene groups are excited by green light all through the recording process. The reverse effect of red interference light causes repeated photoisomerizations. Because of the low absorption of recording light, biphotonic gratings are formed through the photoisomerization effect rather than photo-induced thermal effect.
4. Conclusion
In summary, biphotonic holographic recording was achieved in an azobenzene liquid crystal film by means of the competition between the recording light and excitation light. The recorded gratings were able to be switched on and off with the manipulation of the 532 nm beam. The main mechanism is the recording of the off-resonant interference pattern based on the activation of excitation light, which enables the formation of periodical polymer structures across the azobenzene liquid crystal film through photoisomerization processes. It was found that diffraction characteristics of the recorded gratings were associated with polarization angles and sample temperatures. When the polarization angle between recording light and excitation light increased, DE underwent a sharp decline. Moreover, with the sample temperature rising within the experimental range, DE gradually increased. Intermolecular torques as well as competition effects played leading roles in the properties of polarization- and temperature-dependence respectively. Besides, a biphotonic polarization holographic grating was able to be recorded by two orthogonally linearly polarized red beams with excitation light. Polarization states of diffraction light were discussed theoretically with Jones matrices. The DE was much lower than that of biphotonic holographic gratings, which were different from the resonant condition. This distinction was attributed to the photoisomerization and photo-induced thermal effects. On account of the controllability and low absorption, switchable biphotonic holographic recording possesses a wide range of applications.
Funding
National Natural Science Foundation of China (NSFC) (11574211).
Acknowledgments
The authors would like to thank the State Key Laboratory of Advanced Optical Communication Systems and Networks for supporting this research.
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