Ultrafast holographic recording in bulk phenanthrenequinone dispersed poly (methyl methacrylate) photopolymers is experimentally examined under nanosecond pulsed exposure. A modified interference optical system is set to investigate the dark enhancement effect and real-time diffraction grating strength. Single transmission diffraction grating is recorded in a 6 nanosecond pulse exposure. Grating enhancement formation with different pulse quantity, repetition rate and spatial frequency are also measured. Diffraction efficiency is enhanced by increasing the pulse number as well as the single-pulse energy. The grating strength of 0.58 within 1.8 μs cumulative exposure time is obtained. Moreover, holographic reciprocity failure occurring in the ultrafast holographic storage is analyzed. This paper presents a practical support for PQ/PMMA photopolymers in applications of transient information holographic storage.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Over the last few decades, the development of volume holographic storage technology has reached remarkable achievements [1–4]. Photopolymer has become a potential holographic recording material. Among the photopolymer materials, phenathrenequinone (PQ) doped poly (methyl methacrylate) (PMMA) has been extensively studied due to its neglectable shrinkage, excellent stability and high storage density [5–7]. Applicability and photochemical process of PQ/PMMA photopolymer is already demonstrated [8–10]. Meanwhile, the preparation and holographic characteristics with continuous wave (CW) laser exposure have been thoroughly examined [8-9,11]. However, there are few studies on holographic characteristics of thick PQ/PMMA polymer with pulsed exposures . Presently, with the rapid development of science and technology, there are substantial transient information in our lives. Pulsed holographic recording has become a feasible method to record such transient information. Studies on ultrafast holographic data storage, photorefractive polymers and PVA/AA photopolymers under nanosecond exposure, have been demonstrated [13–15]. In their studies, ultrafast pulsed holographic recording exhibited excellent holographic performances. It is indicated that ultrafast holographic recording has become one of the main application directions of photopolymer materials. Holographic performances of bulk PQ/PMMA photopolymers with high density storage ability were researched under pulsed exposure.
Transient information means some transient phenomenon, such as holographic diagnosis of blowout of particles during blasting, 60-120 or even higher frame rate video and the recordings of morphologies by synchronous satellite. The information mentioned above needs a fast recording time (<1s). However, the holographic recording in PQ/PMMA photopolymers under CW exposure needs over 100 s, which is unable to meet the needs of today's high speed storage. By recording information under pulsed exposure, the response time can shorten from hundred seconds to several microseconds. This is far beyond the limits of the human eye. However, there are many phenomena in military, research and life that a human eye cannot be distinguished. By recording holograms in microseconds even nanoseconds, people can see the indistinguishable phenomena such as the information mentioned above. It can be an available method to record transient information which is useful in military field and our life.
In this paper, we proposed a modified interference optical system to examine the holographic properties of PQ/PMMA photopolymers with pulsed exposures. A grating is recorded using a 6 nanoseconds pulsed exposure. Additionally, dark enhancement effect of the gratings under different pulse quantities, repetition rates and spatial frequencies was also investigated. Samples of 2 mm and 3 mm thickness are compared under pulsed exposure. Finally, a dynamic process of multi-pulse exposure is analyzed and discussed.
2. Materials and methods
A typical thermal polymerization method is used to prepare PQ / PMMA photopolymer [16–18]. The samples are consisted of poly (methyl methacrylate) (PMMA) (Tianjin chemicals, China) host matrix, phenanthrenequinone (PQ) (Sigma-Aldrich) photosensitizer and initiator azo-di-iso-butyro-nitrile (AIBN) (Tianjin chemicals, China). The AIBN was used to enhance the formation of PMMA host matrix. In our fabricating process, PQ (0.1 wt %) and AIBN (0.05 wt %) powders were dissolved in MMA solvent. The mixture is pre-polymerized at 60 °C for 2-2.5h for the sake of eliminating the nitrogen produced by thermal decomposition in AIBN. By using this method, approximately 1.0wt% PQ molecules were dissolved into the PMMA host matrix. This solution was then initiated at 85 °C for 15 min and then solidified at 60 °C for 72 h. After thermal polymerization, samples with diameter of 6 cm and thickness in millimeters (2-3mm) were prepared. Figure 1(a) shows the absorption spectrum of PQ/PMMA polymers. In the experiment, a 532 nm wavelength pulsed laser is chosen to avoid excessive holographic scattering, which has already been demonstrated in Ref . A modified holographic interference optical system is set up to record unslanted transmission gratings, as shown in Fig. 1(b). In this system, a pulsed laser of 532nm wavelength is applied to record the holographic grating. After recording, CW laser of the same wavelength is used to reconstruct the grating. Angle between the reference beam and the object beam is changed from 10° to 80° that enables recording holographic gratings with the spatial frequency from 653 to 3702 lines/mm. Based on this operation, the holographic properties of PQ/PMMA with ultrafast pulsed exposure are measured and analyzed.
For assessing holographic properties of the material, the grating strength plays a crucial role in describing holographic storage performance, which is firstly investigated. In the experiment, the first-order diffracted intensity, Id, and the transmitted intensity, It, of the probe beam were measured. The grating strength M(t) represents the square root of diffraction efficiency (DE), expressed as,20], we calculate the refractive index modulation , as shown in Eq. (3)21], expressed as,
3. Results and discussion
3.1 Single pulse exposure
Dark diffusion enhancement process (DDEP), after a single pulse exposure (SPE), was used to characterize the holographic performance. This was experimentally achieved. With the dark diffusion time passing by, grating strength gradually increases and finally becomes stable. The DDEP is a very important holographic characteristic of PQ/PMMA material. The essence of the reaction is diffusion effect of PQ molecules in the material [22–24]. As shown in Fig. 2(a), during the dark diffusion, PQ molecules diffuse from the unexposed regions to the exposed areas to compensate the reduction of chemical potential during exposure. The DDEP comes to the end when the chemical potential of two areas is dynamically balanced. Diffusion of PQ molecules results in grating enhancement . The grating strength and the DE come to be stable when diffusion ends. The intersection angle was set to 30° in this section.
The DDEP of holograms after SPE was examined. The grating strength improved by 0.8% after 15 minutes with a single pulse of 20 mJ/cm2, as shown in Fig. 2(a). Meanwhile, Samples with different thickness were exposed with SPE and single-pulse energy increased from 10 mJ/cm2 to 20 mJ/cm2.The results of the saturated diffraction efficiency (SDE) are shown in Fig. 2(b). According to experimental results, the SDE rises with increment of the single-pulse energy. Higher energy leads to excitation of more molecules; i.e. more PQ molecules diffuse. Consequently, with increment of energy of single pulse, holographic recording with SPE can be achieved. The SPE measurement provides a practical support for ultrafast holographic recording in PQ/PMMA materials. Comparing the samples of different thickness, the sample of 2 mm exhibits a better holographic performance, the holographic scattering and the consumption of photosensitizer are occurred in the photopolymer during the exposure process. In low thickness samples, the effect of holographic scattering is very small, but the content of photosensitizer is not sufficient. In high thickness, the holographic scattering is serious, but the photosensitizer is sufficient. Both these two factors avoid the diffraction efficiency. Therefore, the limit thickness of the sample is between 1 and 2mm.
3.2 Short-time pulse exposure
After examining the grating strength with SPE, a further research on the DDEP of PQ/PMMA under short-time pulse exposure (StPE) (pulse number< 100 times) was investigated. The sample with 2 mm thickness was selected to investigate in this section.
Figure 3(a) depicts the grating strength with various pulse quantities. An increase in the pulse number gradually, increase the exposure energy. The increase in the pulse number causes an enhancement on grating strength. Therefore, it is indicated that the photo-polymerization process of PQ/PMMA materials is an energy accumulation process. The DE gradually increase with increasing exposure energy in a certain range. We also examined the influence on pulse repetition rate, as shown in Fig. 3(b). Here, the number of pulses was controlled (equal to 60 times) and then exposed the sample with different pulse repetition rates. The single-pulse exposure energy was set to 20mJ/cm2. From the result we can see, the dark enhancement increases with the expansion of time interval between adjacent pulses. This is due to the light absorption efficiency of photosensitizer, which increases when the diffusion time is prolonged with longer pulse time intervals. This, eventually leads to an increased saturated grating strength.
The spatial frequency is another main factor that affects the property of PQ/PMMA materials. Different incident angles can determine the spatial frequency of gratings. Single-pulse energy was also set to 20 mJ/cm2, while repetition rate was 1 Hz and exposure time was 30s. The incident angle was increased from 10° to 80° while measuring the SDE at each 10 degrees as shown in Fig. 4(a). The corresponding spatial frequency was obtained between 653 to 3702 lines/mm.
In Fig. 4(a), when the incident angle is less than 40°, the SDE of the sample does not decreased. However, the SDE is clearly declined when the angle is greater than 40°. This is because higher incident angles make denser grating fringes. If the material cannot distinguish the grating fringe, the grating strength decays rapidly. Also, the coherent optical path inside the material rises with the increase of incident angle, rapid enhancement of material absorption of beams causes a sharp decline in DE. The DE within the incident angle of 40° was not affected; i.e. the limit spatial resolution of PQ/PMMA material is 2417 lines/mm, which meets the basic requirement of holographic data storage.
Figure 4(b) describes the dark enhancement process with StPE. The single-pulse exposure energy was set to 20 mJ/cm2 and the repetition rate was 10 Hz. The experimental results show that the dark enhancement process has been carrying out for a long time after the StPE. Thus, the photo-induced polymerization process of PQ/PMMA materials and the diffusion process of PQ are separated from each other under StPE. The grating enhancement with no exposure is actually caused by the diffusion of PQ molecules.
3.3 Multi pulse exposure
In this section, grating formation process of PQ/PMMA material under multi pulse exposure (MPE) is investigated. In the experiment, the pulse number was increased from 20 to 1000 with the repetition rate of 10Hz. A synchronous measurement was implemented at every 20 pulses, as shown in Fig. 5.
According to the curves, with the increasing number of single-pulse exposure, the grating strength gradually increases, followed by a slight decrease, eventually stabilized. Firstly, the main mechanism of grating strength increasing in PQ/PMMA photo-photo-polymerization process is depicted in Eq. (6)-(8). A Refractive index grating is formed due to the consumption of PQ molecules in bright areas. Then, there is a attenuation of grating strength. There are two main reasons for the attenuation. On the one hand, because the pulse number is gradually increased, the exposure energy is also enhanced. The PQ molecules in the exposure area are consumed, while the PQ molecules in the dark area cannot have enough time to diffuse from the dark zone to the bright zone which causes the lack of PQ molecules in the bright area. Therefore, the grating process becomes weakened resulted in the decrease of grating strength. On the other hand, when the grating formation reaches its maximum, the holographic grating scattering caused by the MPEs also weakens the grating’s intensity. Finally, the grating strength becomes stable when the diffusion of PQ molecules comes to a balance. Thickness of samples deeply influence their holographic performances. Thin samples have a faster response time due to smaller holographic scattering. Thick samples have a higher grating strength because of sufficient photosensitizer. Samples of the corresponding thickness can be selected with different needs.
Table 1 depicts the measurement of holographic performance parameters. Compared to experiment result with CW exposure in , a decline in DE occurs under MPEs. Some reports have confirmed the reciprocity failure in photorefractive polymers under pulsed exposure [25–28]. However, photochemical mechanisms between photorefractive polymers and photopolymers are different. In the case of PQ/PMMA materials, the photochemical reaction requires a sustained period. If the pulse width and photochemical reaction time  do not match, the holographic reciprocity failure will occur. Holographic reciprocity law means that the exposure energy and the pulse duration can be proportional to reciprocity with the same exposure flux. There is a delay in the exposure to initiate photosensitive molecules, which change from the ground state to the excited state. When the pulse duration is too fast and the photosensitive molecules do not have enough time to respond, the reciprocity law failure happens.
The photochemical reaction process in PQ/PMMA polymers is described in Fig. 6 [8,9]. There are two main photo-polymerization processes in PQ/PMMA photopolymers. During exposure the PQ molecules play two significant roles inside the sample. The first is that PQ as photosensitizers can absorb photons and initiate the photochemical reactions. The other is PQ molecules as primary reactants can attach to the polymer matrix and change the modulation depth. However, the polymerization of PQ with matrix is the dominated photochemical process in pulsed exposure, and the contribution of chain polymerization of PMMA can be neglected according to Ref . The simplified mechanism of photo-polymerization in PQ/PMMA photopolymers can be depicted in Eq. (6)-(8).
Under CW exposure, according to Eq. (6), a PQ molecule is firstly converted to the singlet 1PQ* by absorbing a photon, then part of 1PQ* turns into triplet 3PQ*, where Ra is the photon absorption rate and Rb is the intersystem transition rate. Therefore, PQ molecules transform into a stabilized triplet excited state. The 3PQ*s trigger a reaction with the surrounding PMMA polymers which leads to the formation of two radicals: macroradical (R•) and semi-quinone radical (HPQ•). The final photoproduct PQ-PMMA is formed by interactions of HPQ• and R•, where Ri is the initial rate constant and Rt is the termination rate constant. The PMMA is already formed during the thermal polymerization, and no MMA polymerization happens in the process of exposure. The formation of grating is determined by three main factors; one is due to the generation of PQ-PMMA photo-product, which gives a dominated contribution to the DE, the second is the concentration distribution difference of residual PQ molecules between dark zones and bright zones of the interference pattern, and thirdly, the uneven distribution of corresponding unreactive 1PQ*s reduces their barriers to the formation of gratings. With the increase of reacting time, the distributions of the PQ and 1PQ* molecules become homogenous; i.e. the grating strength comes to be stable.
As for pulsed exposure, the pulsed laser can produce higher density photons in interference region in nanosecond order of magnitude. Compared to CW exposure, since the shortening of exposure time in the polymer composite, the photon absorption efficiency of PQ molecules will decrease; i.e. both the yield of 3PQ*s and the PQ molecule concentration modulation will decrease. Meanwhile, the light-induced reaction between 3PQ* and PMMA will also be attenuated due to the ultrafast pulse exposure. Finally, it causes a decline in concentration difference of PQ molecules, the amount of 1PQ*s and the output of photo-products; i.e. the reduction of DE occurs.
Although the grating strength of PQ/PMMA materials will decrease under the pulsed exposure, some solutions can also be proposed to shorten the gap with CW exposure. Increase the amount of exposure and single pulse energy can induce more PQ molecules to absorb photons. A longer pulse width of laser will also improve the efficiency of light-induced reaction.
Figure 7 depicts the AFM topography of PQ/PMMA photopolymers before and after exposure. The matrix of PQ/PMMA is PMMA, which is also called “organic glass”. It has a high temperature resistance. This material can remain solid at 200°C. By comparing the AFM characterization of samples before and after exposure, the surface morphology of the sample does not cause thermal damage due to multiple pulse exposure, which indicates that the PQ/PMMA is a suitable organic material recording under high intensity pulsed exposure. Scratches are due to polishing and traces of the base.
For practical application, reconstruction of hologram images in the sample is presented. In the recording stage, the static particle field with hundred μm diameters was loaded into the interference optical system as shown in Fig. 1(b). Each hologram was exposed for 300 ns with the energy of 5 J/cm2. Five images were consecutively recorded by the angle multiplexing technique in 1.5 μs. Each image was recorded in 3 × 3 mm square of PQ/PMMA samples with 700 × 700 pixels. We intercepted each image with 150 × 150 pixels to make a clearer observation of the particle field with hundred μm diameters, as shown in Fig. 8. This can be a support for application of ultrafast holographic memory.
In this paper, we examined the ultrafast holographic storage in PQ/PMMA materials with nanosecond exposure. Gratings can be recorded with single pulse exposure. Dark enhancement effect of gratings under different pulse quantity, repetition rate and spatial frequency was measured in detail. The limit spatial resolution of PQ/PMMA material is 2417 lines/mm in pulsed exposure. The response time of grating formation 0.80 μs and the photosensitivity 4.2 × 10−5 was obtained in PQ/PMMA polymers under multi pulse exposure. The holographic reciprocity failure was happened under pulsed exposure which caused a decline in grating strength. All measurements and analyses provides a basis for a further research on recording transient information in bulk PQ/PMMA photopolymers.
National Basic Research Program of China (2013CB328702); the National Natural Science Foundation of China (11374074).
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