1. Introduction

Holography has revolutionized optical information storage and information display by enabling the capture and reproduction of three-dimensional images [1,2]. Extensive research has been conducted to explore various aspects of holography, resulting in advancements in techniques and materials that enhance its capabilities and broaden its applications [3–6]. Few applications include, security labels (bank notes), 3D displays, waveguides, beam shaping, optical testing, bio-sensing, optical processing etc.

Holography involves a ‘writing step’, where a light beam containing the data to be recorded (object beam/ signal beam) is superposed with a reference beam and exposed onto a suitable material. After recording, the material sample (now hologram) is irradiated with a reading beam (often with one similar to the reference beam) to retrieve the information back, known as the ‘reading step’. Based on the polarization state of the light beams used during the writing step, holograms can be classified as ‘intensity holograms’ and ‘polarization holograms’. Intensity holograms are ones that are recorded by a coherent superposition of two light beams of identical polarization state. The generated light intensity modulations (interference fringes) are stored as refractive index modulation, transmittance modulation, or surface-relief pattern on the irradiated sample. Hence, they are said to utilize the phase and amplitude degrees-of-freedom to record/reconstruct information. A polarization hologram employs two light beams of orthogonal polarization state, which generates/subtends a modulation in polarization across the sample. These polarization modulations are stored as ‘birefringence modulations’ and/or as ‘surface-relief patterns’ on the irradiated sample and are said to utilize the polarization degree-of-freedom in addition to the amplitude and phase [7–9].

Now, during the reading step, the behaviour of an ‘intensity hologram’ differs based on the recording material. When a non-birefringent material (e.g. silver halide) is used, the holographic reconstructions are unaffected by the polarization of the reading beam. But, in the case of a photo-birefringent material (e.g. azopolymers), the reconstructions depend on the polarization state of the reading beam. Since polarization holograms can be recorded only on a photo-birefringent material, their reconstructions are always polarization-dependent. Thus, based on the independence or dependence of the holographic reconstruction on the polarization state of reading beams, holograms can be further classified as ‘scalar holograms’ and ‘vector holograms’, respectively. In other words, vector holograms allow different information to be recorded (by changing the polarization state of the writing beams) and reconstructed (by changing the polarization state of the reading beam), and is termed as ‘polarization multiplexing’[10,11]. Hence, vector holograms are known to possess significant advantages compared to scalar holograms. From the above discussion, it can be noted that a hologram classified as an ‘intensity hologram’ by virtue of its recording configuration can behave as a ‘scalar hologram’ or a ‘vector hologram’ during reconstruction. But a ‘polarization hologram’ always behaves as a ‘vector hologram’ during reconstruction. In this research, we focus on investigating the ‘vector holographic’ properties of both ‘intensity holograms’ and ‘polarization holograms’ recorded on azo-derived copolymer film.

Generally speaking, vector holograms demand one of the following properties in recording media, (i) photo-induced birefringence, in the case of bulk materials and films, and (ii) patterning nano (subwavelength) structures, in the case of metamaterials [12]. This leaves us with a limited choice of materials or a complex nanofabrication process. In spite of these limitations, significant progress has been made in the vectorial reconstruction capabilities of recorded intensity and polarization holograms. Considering the diverse abilities and performances exhibited by different materials [13,14], the ones being tested and reported for vector holography can be classified as, a)Photopolymer, b) Azo-based polymer, c)Azobenzene liquid crystals, and d)Metamaterials [15–22]. Each of these materials has their own advantages and disadvantages in the context of polarization holography which is discussed below.

Photopolymer materials based on PQ/PMMA (Phenanthrenequinone (PQ) doped poly(methyl methacrylate) (PMMA)) have been extensively studied for recording polarization holograms. Diffraction efficiency values reported initially were low (7-40%), but improved later through the addition of dopants such as gold nanoparticles and THMFA to achieve better efficiency (47%), photosensitivity, and birefringence [23–25]. In addition, graphene oxide has been incorporated into the photopolymer to enhance the sensitivity and diffraction efficiency; however, the reported diffraction efficiency remained notably low [26]. Moreover, the addition of eight methacryl polyhedral oligomeric silsesquioxane (Ma-POSS) in PQ/PMMA led to a significant improvement in diffraction efficiency (upto 75%) and sensitivity [16]. Subsequently, the introduction of triethanolamine (TEA) and the co-monomer acrylamide (AA) in PQ/PMMA enhanced the diffraction efficiency (upto 65%) [27]. These materials are well suited for write once-read many (WROM) types of optical storage but are not suited for dynamic applications due to their non-rewritability nature.

Azo-derivative polymers are re-writable, among which azobenzene-based polymers have demonstrated consistent performance in recording polarization holograms [28,29]. However, polarization holograms recorded in these polymers tend to be thin due to their high optical absorption and limited thickness. The gratings formed in these thin film polymers are primarily attributed to either surface relief or photo-induced birefringence [30–34]. Thin polarization holograms lack angular selectivity, reducing the data storage capacity. To address this limitation, the preference is for thick holograms. However, thick azopolymer holograms often exhibit limited diffraction efficiency and stability [35]. Dilution of the azo-content in polymer has been explored to improve diffraction efficiency [36]. Other approaches using statistical copolymers, terpolymers, and di-block copolymers have shown some improvements, but the increased weight resulted in prolonged recording times of hundreds of milliseconds [37–40]. Thick films composed of an azomethacrylic BC and a PMMA homopolymer have been studied to enhance storage capacity and response time. Still, the reported diffraction efficiency was notably low (of the order of 10$^{-5}$) [41]. Achieving higher diffraction efficiency, shorter writing times, and longer stability remains a challenge till date in azo-derivative compounds in the context of recording polarization holograms.

Liquid-crystalline (LC) azobenzene polymers have been extensively investigated due to their dynamic birefringent capabilities [19,42]. In the case of thin azo-benzene side-chain LC polymers, the diffraction efficiency was notably low (upto 31%) however, in thick LC polymer films, a substantial enhancement in diffraction efficiency was observed (up to 100%) [43–45]. It is worth noting that these thick films exhibited a significant drawback in the form of prolonged response times, lasting several minutes. To achieve a further improvement in response time, extending up to a few seconds, LC material doped with Disperse Red 1 was utilized. However, in this scenario, the diffraction efficiency was notably low [46,47]. Achieving high diffraction efficiency and a fast response time simultaneously in all-optical read/write cycles still remains a challenge.

In the past decade, with the advancement of nanofabrication technologies, there has been a surge in metamaterials-based holography. These materials have garnered extensive research interest in their potential to design high-efficiency meta-holograms, which exhibit enhanced diffraction properties [48–50]. The sub-wavelength structures in metamaterials can affect (modulate) the polarization of light beams, which led to the successful demonstration of vector holograms using meta-materials [51]. Despite their promising applications, the practical use of metamaterials still faces several challenges, including but not limited to narrowband operation, energy dissipation, cross-talk, scalability, and complex/costly fabrication process [52].

In this paper, we investigate the vector holographic properties of azo-carbazole copolymer films for the first time. Fundamental material characteristics such as diffraction efficiency, stability, recording time (sensitivity), and retention time have been analyzed, and the results show significant potential for vector holographic applications. This paper is organized as follows in section 2, we present comprehensive details on the sample preparation process. In the subsequent results section (sec. 3), the measured optical properties of the material, including photo-induced birefringence, diffraction efficiency, and retention time, are presented, which ensures that the suggested material satisfies the requirement for vector holography. Following this, a detailed study of the polarization properties of the materials as a vector hologram is analyzed using hologram recording and reconstruction experiments.

2. Materials and sample film preparation

The polymer film prepared consists of three components. First is an azobenzene-functionalized copolymer, poly(CACzE-MMA), consisting of a 3-[(4-cyanophenyl)azo]-9H-carbazole-9-ethanol (CACzE) and a methyl methacrylate (MMA). Second, it contains free CACzE as a separate component to accelerate the diffraction response. Lastly, the film uses diphenyl phthalate (DPP) as a plasticizer. The details on the synthesis and material characterization studies of poly(CACzE-MMA) and CACzE can be found in our earlier papers [53,54]. DPP was purchased from a commercial source (Tokyo Kasei Co.). The chemical structure of the three components poly(CACzE-MMA), CACzE, and DPP are shown in Fig. 1. Irradiation with light at a wavelength of 532 nm induces a trans-cis and cis-trans photoisomerization in the molecules. When the irradiation is turned-off, the molecules settle down to the more stable trans-state and orient themselves in a direction perpendicular to the polarization direction of the irradiation. This molecular orientation induces birefringence (a distribution of birefringence pattern in accordance with the irradiating beam pattern, to be specific) on the material. The recorded birefringence (refractive index) pattern lasts until further irradiation or heating, and the hologram is said to be recorded. Poly(CACzE-MMA) assists in the long-term preservation of the recorded refractive index pattern in the sample. In addition, DPP is used to improve the transparency, flexibility, and durability of the sample.

 

Fig. 1. Chemical structure of the materials used to make the polymer film.

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The process of preparing the film involves a well-defined sequence of four distinct steps. In the first step, the three components, poly (CACzE-MMA), CACzE, and DPP, are mixed in a specific ratio of 45/15/5 wt%. This mixture, carefully prepared, plays a key role in the achievement of the desired properties and characteristics of the resulting film. In the second step, the mixture obtained in the first step is dissolved in tetrahydrofuran (THF)and is gently stirred for 48 hours. In the third step, the sample is heated on a hot plate at 70°C for 48 hours until the mixture dries completely. This step is most important as it aims to remove solvents, moisture, and other volatile elements that might be present in the sample. The molten sample is then carefully placed between two glass plates of size 3 cm $\times$ 3 cm, with a polyimide spacer measuring 35 $\mu m$ (since penetration depth is around 25$\mu m$) in thickness carefully inserted between them. The polyimide spacer determines the thickness of the sample film. In the final step, the sample was pressed between the two glass substrates at 140 - 180°C. The glass plates, along with the spacer, act as precision molds, shaping the molten sample into a thin film with the desired thickness and dimensions. The 2D thickness profile of the sample film also depends on the uniformity/flatness of the glass plate, which was not experimentally measured. But results obtained were repeatable on multiple samples, showing that the flatness of the glass plates were good enough to not influence the results.

3. Results and discussion

3.1 Calculation of birefringence

Photo-induced birefringence plays a crucial role in influencing the polarization-dependent behaviour of materials and hence measuring this optical parameter is important to determine the potential of the material to record vector holograms. The optical arrangement for measuring birefringence in the polymer film is shown in Fig. 2(a). A red laser beam of wavelength 633nm (probe beam) of spot size 2 mm and intensity 59$\mu W/cm^2$ is allowed to pass through the sample film kept between two polarizers, P2 and P3. The intensity of the beam is measured after it exits the polarizer P3. The orientation of the two polarizers, P2 (45°) and P3 (-45°), are set orthogonal to each other. This arrangement results in the red laser being completely absorbed in P3. Hence, the detector will register a zero intensity (or close to zero) from the red laser. Now, the sample film is irradiated with a green laser of 532 nm wavelength with a spot size of 5 mm and intensity of 44$mW/cm^2$ on the same spot illuminated by a red laser on the sample film. To ensure the input beam from the green laser is linearly polarized, the polarizer P1 is used. When the green laser is turned on, it induces birefringence in the sample film, which in turn affects the polarization state of the red beam as it passes through the sample film. Due to this change in polarization state, some components of the red beam will be able to pass through the polarizer P3, making the detector register a non-zero intensity. The amount of light passing is proportional to the birefringence induced in the sample film. From the recorded intensity, the value of induced birefringence in the polymer film can be calculated using Eq. (1):

 

Fig. 2. (a)Optical setup for measuring birefringence (P1, P2, P3: Polarizer, and BE: Beam expander), (b)plot of the variation of induced birefringence measured with time.

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3.2 Diffraction efficiency of intensity and polarization holographic grating

The optical arrangement used to record ‘intensity’ and ‘polarization’ holographic grating is shown in Fig. 3(a). A laser beam with a wavelength of 532nm is used to record the holograms since this wavelength is close to the isosbestic point (529nm) for this polymer film [54]. The laser beam is split into two and combined again at an angle to create a light interference pattern (intensity or polarization), which is recorded in the sample film as a holographic grating. For an intensity hologram, both the signal (object) and reference beams are set to have the same linear or circular polarization states. On the other hand, for recording a polarization hologram, the signal and reference beams are set to orthogonal linear or circular polarized. In both cases, the signal and reference beam are set to be collimated plane beams with a spot size of 5 mm throughout their entire path length in the optical setup. We also confirm the accuracy in collimation using a shear plate. Following the above two observations we assume the light beams to have negligible divergence. Half-wave plates (HWP2 & HWP3) are used for recording with linearly polarized light, which are then replaced with quarter-wave plates (QWP1 & QWP2) for recording with circularly polarized light. Each recording exposure lasts for 5 seconds, with a total input intensity of 88 $mw/cm^2$. The signal beam interferes with the reference beam at a 20°angle, such as the signal beam comes in perpendicular to the sample, and the reference beam makes a 20°angle with the signal beam.

 

Fig. 3. (a) Optical setup for the recording of intensity and polarization holographic gratings (SH: shutter, BE: Beam expander, HWP: Half-wave-plate, QWP: Quarter-wave plate, M: Mirror and PBS: Polarizing beam splitter). (b) Grating pattern captured by phase-contrast microscope from a recorded (b1-b3) intensity hologram (P+P) at the polarizing angle of 0°(b1), 90°(b2), 180°(b3), and (b4-b6) polarization hologram (P+S) at 0°(b4), 45°(b5), and 90°(b6). (c) Optical setup for the reconstruction of the recorded holographic gratings (S: stopper, D1, D2: Detectors, M: Mirror, HWP: Half-wave-plate, QWP: Quarter-wave plate, PBS: Polarizing beam splitter). (d) Plot of the variation of normalized diffraction efficiency with the angle of rotation of sample film.

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The recorded grating pattern (birefringence modulations) was visualized using a polarization-varying phase contrast microscope (OLYMPUS BX53M), and the results are shown in Fig. 3(b). Fig. 3(b1-b3) are images of an intensity hologram(recorded using linearly polarized light with ‘P+P’ writing configuration) captured at different polarization orientations of light illuminating the sample. Similarly, Fig. 3 (b4 -b6) corresponds to images of a polarization hologram(recorded with ‘P+S’ configuration) captured at different polarization orientations of microscopic illumination. From Fig. 3 (b), it can be observed that, in the case of an intensity hologram (Fig. 3 (b1-b3)), the modulation pattern disappears completely when the polarizer is rotated by 90°(Fig. 3 (b2)) and after further 90°rotation of polarizer the modulation pattern will reappear without any shift(Fig. 3 (b3)). In the case of polarization hologram (Fig. 3 (b4 and b6)), the modulation pattern will not disappear entirely even when the polarizer is rotated (Fig. 3 (b2)). When the polarizer is rotated by 90°, (from the initial position), then a shift in the hologram will appear(dark areas will become bright and vice-versa, as seen with the help of vertical guide-lines) as shown in Fig. 3 (b6). These results agree with the individual characteristics of an intensity and polarization grating. The reason for no pattern seen in Fig. 3(b2) and for a weak pattern seen in Fig. 3(b5) is explained later with the support of the schematic shown in Fig. 4. Moreover, in order to record a thick grating, is necessary to satisfy the condition $\Lambda ^2 \ll d\lambda$, where $\Lambda$ represents the grating spacing, $d$ (20-25$\mu$m) is the sample thickness, and $\lambda$ denotes the wavelength of the laser beam [56]. From the pattern seen in Fig. 3 (b), the grating spacing ($\Lambda$) was measured to be 1.5 $\mu$m for both the intensity and polarization recorded configurations. This specific value of $\Lambda$ signifies that the recorded grating falls within the Bragg regime and qualifies to be a thick hologram. Consequently, during reconstruction, only one diffracted and one zero-order beam will be observed.

 

Fig. 4. (a) Plot of the variation of diffraction efficiency with time for different hologram writing and reading configurations for an intensity hologram. Schematic of molecular orientations after recording a grating with (b) P+P polarized light and (c) RCP+RCP polarized light. (d) Plot of the variation of diffraction efficiency with time for different hologram writing and reading configurations for a polarization hologram. Molecular orientations after recording a grating with (e) P+S polarized light and (f) RCP+LCP polarized light.

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The optical arrangement shown in Fig. 3(c) is used to reconstruct the recorded holographic grating and evaluate the diffraction efficiency. The reconstruction process involves employing a red laser (633nm) as the reading beam (coming from the opposite direction of the signal beam) because the polymer film shows negligible absorption at this wavelength (absorption coefficient, $\alpha <100 cm^{-1}$) [56]. The polarization of the reading beam is set to align with the recording setup through the use of a half-wave plate (HWP4) and a quarter-wave plate (QWP3). The diffracted and non-diffracted intensities are recorded by detectors D1 and D2, respectively. Using the recorded intensity values, the diffraction efficiency (DE) is calculated for various writing and reading configurations, as given by Eq. (2):

Figure 4(a) shows the temporal evolution of diffraction efficiency for a recorded ‘intensity hologram’ (recording beams have the same polarization state), for different writing and reading beam configurations. The measurements are made after the writing beams were turned off to understand the drop-off in diffraction efficiency when writing beams are not present. Here, ‘$\text {P+P}/\text {P}$’ represents a recording & reconstruction configuration where both the writing beams are ‘P’ polarized, and the reading beam is also ‘P’ polarized. The graph clearly illustrates that the maximum diffraction efficiency, of approximately 58%, is obtained when both the writing and reading beams are linearly polarized and possess the same polarization state i.e., ‘$\text {P+P}/\text {P}$’. In contrast, when a circularly polarized writing beam is employed i.e., ‘$\text {RCP+RCP}/\text {RCP}$’, the diffraction efficiency decreases to 25%. Again, when the reading beam alone is changed to linear polarization i.e., ‘$\text {RCP+RCP}/\text {P}$’ then the diffraction efficiency further reduces to 20%. The observed difference in diffraction efficiencies can be explained using the schematic of molecular orientation as depicted in Fig. 4(b-c) (The vertical axis in the plots is in the direction of the cell thickness and we neglect the absorption of the recording beams). Before irradiation (recording), the molecules in the sample film are randomly oriented in the trans-state. When irradiated, the uniform polarization state of the writing beams results in a modulation of intensity pattern characterized by consecutive bright and dark regions (intensity modulation) across the sample film. The molecules in the brighter region undergo cis-trans isomerization, while the ones in the darker region experience no change and remain in the (randomly oriented) trans-state. The cis-trans isomerization cycle continues until the irradiation is present. When the irradiation (writing beam) is turned off, the molecules relax into a trans-state, where they are aligned perpendicular to the polarization direction of the light present during writing. When linearly polarized writing beams are employed (Fig. 4(b)), the molecules align primarily perpendicular to the polarization direction of the beam in the bright region. Due to this molecular alignment, the brighter regions acquire birefringence, resulting in a relative refractive index difference between the bright and dark regions. It is this difference that is being manifested as a phase contrast image shown in Fig. 3(b1,b3). From Fig. 4(b), it can also be observed that the orientation of birefringence in the bright regions is uniform throughout. When this birefringence orientation is exactly orthogonal to the polarization of the illuminating beam, then the difference in refractive index difference between the birefringent (bright) and non-birefringent (dark) regions disappears. This is the reason why the phase contrast image shown in Fig. 3(b2) reveals no variations in the refractive index for that particular direction of polarized illumination. Now, in the case of circularly polarized writing beams, the alignment of the molecules becomes less ordered. This is because circularly polarized light does not establish a preferred alignment direction for the molecules, as shown in Fig. 4(c), resulting in a more random orientation. As a result, the overall change in the refractive index weakens, leading to a lower diffraction efficiency in the case of circularly polarized writing beams. Thus, ‘intensity holograms’ recorded in the azo-copolymer sample are sensitive to the polarization state of the reading beam and hence, these intensity holograms can be considered as a vector hologram.

Figure 4(d) illustrates the temporal evolution of diffraction efficiency in the case of recording ‘polarization holograms’ (recording beams are orthogonally polarized). The results reveal that the maximum diffraction efficiency achieved reaches high values upto 85% when employing orthogonal circular beams for both writing and reading processes i.e., ‘$\text {RCP+RCP}/\text {RCP}$’. Conversely, when adopting a linearly polarized reading beam, the diffraction efficiency drops to 65%. In the case of linearly polarized writing & reading beams i.e., ‘$\text {P+S}/\text {P}$’, the maximum observed diffraction efficiency is 50%. Here again, molecular orientation plays a crucial role in determining diffraction efficiency and polarization sensitivity. During the recording of a polarization hologram, an ‘intensity modulation’ pattern is not generated as the beams are orthogonally polarized. However, two superimposing orthogonally polarized beams do create a ‘polarization modulation’ across the sample film, as shown in Fig. 4(e, f). Consequently, the molecules also respond to this modulation, resulting in a modulation in molecular alignment, as shown in Fig. 4(e, f). This modulation in alignment exposes itself as a modulation in birefringence across the sample film. Hence, a polarized beam, when passing through this birefringent sample, experiences a modulation in the refractive index for that particular direction of polarization. This refractive index modulation gets translated to a grating, and the beam gets effectively diffracted. Two different cases can be considered here. First, when both the writing beams are linearly polarized, as shown in Fig. 4(e). Here the resultant polarization modulation varies from linear to circular across the sample film. The molecules in areas where the resultant polarization direction is linear, align themselves perpendicular to the polarization direction. On the contrary, the molecules in the areas where the resultant polarization is circular are less aligned compared to the adjacent linear areas. This difference in the degree of molecular orientation (and hence birefringence) results in a refractive index modulation which determines the diffraction efficiency. Since the sample is birefringent throughout, the refractive index variation (low/high) depends on the polarization direction of the illuminating beam. This is the reason we see a shift in the phase contrast pattern (low becomes high) when the illuminating beam’s polarization is rotated (Fig. 3(b4,b5)). Moreover, since there is a non-uniform distribution of polarization, the illuminating beam can never be completely orthogonal to the sample’s birefringence. This is the reason why the phase contrast seen in Fig. 3(b5) is weak but does not completely disappear. Second, when both the writing beams are orthogonally and circularly polarized, the resulting polarization state modulation is linear only (no circular component) but with a gradually changing orientation angle (Fig. 4(f)). In this case, all molecules throughout the sample film (irrespective of their location) align themselves in some direction dictated by the direction polarized irradiation as shown in Fig. 4(f). This results in a higher degree of birefringence modulation and hence a larger refractive index modulation, which eventually leads to a very high diffraction efficiency for this particular writing configuration. To conclude, an ‘intensity hologram’ recorded on an azo-copolymer uses the difference in refractive index generated by the birefringent and non-birefringent regions induced (recorded) in the sample. Whereas a ‘polarization hologram’ utilizes the refractive index modulations generated by the difference in orientations of induced birefringence. Both intensity and polarization hologram are sensitive to the polarization state of the reading beam, whereas a polarization hologram recorded using an orthogonal circular polarized beam offers the highest diffraction efficiency.

We now examine, the rewritable property of the polymer film using read/write cycle experiments. The recorded grating within the film can be erased through two distinct methods: (i) reheating the sample and (ii) irradiating the sample with a single beam ($\lambda$ = 532nm ). The latter approach is employed herein to explore the rewritability. Initially, a polarization hologram (S+P) is recorded onto the film over a duration of five seconds, with a laser beam intensity of $88 mW/cm^2$. Subsequently, one of the beams (signal) is obstructed, allowing only the reference beam (P - polarized of intensity $44 mW/cm^2$) to pass through the sample for 20 seconds, which erases the previously recorded hologram. A reading beam (P - polarized) of wavelength 633nm is passed through the irradiated area of the sample throughout the writing and erasing cycles. This allows for continuous monitoring of diffraction efficiency, which directly corresponds to the writing and erasure of the hologram. The outcome of iterative writing and erasing (6 cycles) in the case of polarization holograms (recorded using S+P) is illustrated in Fig. 5 (a). Next, the same procedure is repeated for an intensity hologram (P+P) recording, with a writing duration of 5 seconds and an erasing duration of 30 seconds. The cumulative result of consecutive writing and erasing of multiple intensity holograms is depicted in Fig. 5 (b). From Fig. 5 (a,b), it can also be understood that a polarization hologram gets erased completely, whereas an intensity hologram accumulates a residual grating pattern with each cycle. The above results demonstrate the polymer film’s potential for deployment as a rewritable material. The retention time (stability) of the recorded vector holograms is contingent upon the duration during which the molecular trans-states persist in their respective alignments. Experimental observations reveal that, under ambient conditions (298 K), the diffraction efficiency of the recorded holograms declines from 85% to 25% within a 30-day period. This decline suggests that, over time, the molecules gradually reorient themselves into a randomized configuration. But when the samples were stored at a lower temperature of 276 K we could observe a significant improvement in the retention time. The diffraction efficiency only reduced from 85% to 77% over the period of 30 days. Further extending the period to 200 days, the diffraction efficiency was found to drop down upto 50%. It is to be noted that the samples were stored in a refrigerator which is dark and hence the effect of storage in ambient room light, but in a lower temperature is not known. This observation indicates that the sample is suitable for long-term storage, provided they are stored at lower temperatures and in dark regions. All the recording and reading experiments are performed in the dark room and at room temperature (298 K), and the samples are stored at lower temperatures (276 K) after the experiments.

 

Fig. 5. Plot of the variation of normalized diffraction efficiency with time for (a)Polarization holograms recorded with writing/reading beam of polarization S+P/P and erasing beam of polarization P for six cycles of writing and erasing, (b) Intensity holograms recorded with writing/reading beam of polarization P+P/P and erasing beam of polarization P for five cycles of writing and erasing.

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3.3 Polarization properties of the vector hologram

In this section, the polarization properties of diffracted beams are analyzed for various combinations of polarization states of signal (object) and reference beams. The results are presented in the form of ‘polar plots’, which provide an individual as well as comparative perspective, on the polarization response of different vector holograms. We follow the standard convention and use the term ‘signal beam’ instead of the term ‘object beam’, for the discussion on polar plots. A polar plot is a popular method used in singular optics to represent the polarization state of a light beam, where the measured intensity of the polarized beam, after passing through a rotating polarizer, is plotted for each angle of rotation. We use the optical setup shown in Fig. 3(a) for the measurement of the polarization of the signal and reference beam by placing a polarizer after HWP/QWP (which will later be removed before recording). The polarization state of the reading and diffracted beam has been measured by using the optical setup shown in Fig. 3(c), but by adding a polarizer after HWP4/QWP3 (for reading beam) and before the detector D1 (for diffracted beam). These plots consist of concentric circles, where the radius indicates the normalized intensity of the measured (diffracted) light beam. The angles correspond to the degree of rotation of a polarizer through which the beams pass. The plotted points represent the intensity measured through the polarizer for each angle of rotation of the polarizer. In our experiment, an ‘$\infty$’ shaped pattern in the polar plot corresponds to the ‘P’ polarized beam, and an ‘$\infty$’ shaped pattern corresponds to its orthogonal state, the ‘S’ polarized beam. Similarly, the circularly polarized light assumes a ‘$\bigcirc$’ shape on the polar plot, while an elliptically polarized beam assumes a ‘$0$’ or ‘$0$’ shape, based on the ellipticity. Polar plots in Fig. 6 (a-d) showcase the polarization properties observed in the case of an intensity holographic grating. The first configuration shown in case (a) demonstrates that when a hologram is recorded with identical linearly polarized writing beams (‘P’ in this case), the intensity of the diffracted beam depends on the reading beam’s polarization. If the reading beam shares the same polarization as the reference beam (‘P’ in this case), the diffracted beam will exhibit the same polarization ‘P’ with maximum intensity. However, if the reading beam possesses an orthogonal polarization state (‘S’ in this case), no diffracted order will be observed, as shown in case (b). In all instances, the diffracted beam will only exhibit the polarization state of the identical writing beams (‘P’ in this case).

 

Fig. 6. The observed polarization of the diffracted light beam for different writing and reading configurations (signal+Reference/Reading) (a) P+P/P, (b) P+P/S, (c) RCP+RCP/RCP, and (d) RCP+RCP/LCP in case of intensity hologram. The polarization state of the output diffracted beam for each of these cases is depicted in the last column.

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When identical circularly polarized writing beams are utilized (‘RCP’ in this case), as depicted in cases (c) and (d), the diffracted beam will never reach zero intensity regardless of the reading beam’s polarization. If the reading beam has the same polarization state as the writing beams (‘RCP’ in this case), then the diffracted beam will exhibit an unequal contribution from the x and y components of the electric field, resulting in elliptical polarization, as illustrated in case (c). There will be a switch in the major and minor axes of this elliptical polarization if the reading beam is orthogonal (‘LCP’ in this case) to the reference beam, as demonstrated in (d). Ideally, in the paraxial approximation, the diffracted beam should exhibit circular polarization. However, here we see an elliptical polarization for the following reasons, (i) the angle between the signal and reference beams is 20°, which violates the paraxial approximation, and (ii) the signal and reference beams themselves are not perfectly circularly polarized (due to imperfections in waveplates). To summarize, the diffracted beam of an intensity hologram, recorded using a circularly polarized beam will never reach zero. But it will reach zero when the hologram is recorded using identical linearly polarized beams, and read with a beam that is orthogonally linearly polarized to both the signal and reference as shown in Fig. 6(b).

Figure 7 (a-d) showcases the polarization properties observed in the case of recording a ‘polarization hologram’. In the first case (a), signal and writing reference beams have orthogonal linear polarization states, denoted as ‘P’ and ‘S’, respectively. If the reading beam shares the same polarization state as the reference beam, then the diffracted beam will exhibit the same polarization state as the signal beam (Fig. 7 (b)). Conversely, if the reading beam is orthogonally polarized as that of the reference beam, the diffracted beam will adopt the orthogonal polarization state of the signal beam(Fig. 7 (a)). The above results show that the polarization of the diffracted beam depends on the polarization state of the reading beam. Next, we consider the case of circular polarization holograms, where the signal and reference beams have orthogonal circular polarization states (‘RCP’ and ‘LCP’) respectively, as depicted in cases (c) and (d). When the reading beam possesses the same polarization state as the signal beam (case (c)), the diffracted beam will be elliptically polarized with the same handedness as the reference beam (which is the orthogonal polarization state of the signal beam). On the other hand, when the reading beam has an orthogonal polarization state to the signal beam (case (d)), there will be no diffracted beam (or a very weak elliptically polarized beam with the same handedness as the signal beam). Based on this finding, it can be concluded that in a polarization hologram, the diffracted beam can exhibit the same or orthogonal polarization state as the signal beam, depending on the polarization of the reading beam. The diffracted beam’s intensity will never reach zero in a linearly polarized recording configuration, but, it will do so in the case of a circularly polarized recording configuration(Fig. 7(d)). In other words, to cut-out/filter a particular signal information, an ‘intensity hologram’ recorded using linear polarized beams (Fig. 6(b)) or a ‘polarization hologram’ recorded using circularly polarized beams (Fig. 7(d)) can be employed. These properties greatly aid the polarization multiplexing capabilities in a vector hologram.

 

Fig. 7. The observed polarization of the diffracted light beam for different writing and reading configurations (signal+Reference/Reading) (a) P+S/P, (b) P+S/S, (c) RCP+LCP/RCP, and (d) RCP+LCP/LCP in case of polarization hologram. The polarization state of the output diffracted beam for each of these cases is depicted in the last column.

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4. Conclusions

In this study, we have investigated the performance of azo-carbazole polymer film as a vector hologram material for the first time. Experimental measurements reveal that this material possesses a larger degree of optical birefringence, exhibits a higher diffraction efficiency up to 85%, rewritable property and a retention time longer than 200 days. Two types of vector holograms namely, ‘intensity hologram’ and ‘polarization hologram’ were examined, and their individual merits were explored. A molecular orientation scheme to explain the dependence of diffraction efficiency on the polarization state of writing beams is also presented. Finally, the polarization properties of the diffracted beam in different writing and reading configurations of vector holograms have been investigated. All the above results confirm that the material qualifies to perform as a vector holography recording material. It can be concluded that the reported azo-carbazole copolymer-based composite film presents itself as a strong candidate for realizing vector holography and has the potential to act as the main driving force toward advancements in holographic data storage, 3D and dynamic displays, polarization control, etc.