Hologram is regarded as a key platform for large-volume data storage and information encryption. Diversity of plasmonic nanostructures makes it being a kind of vibrant hologram memory media. However, recording of amplitude, phase and polarization of light is restricted by difficulty to obtain anisotropic morphology of metal particles. Photocatalysis approach allows wide size distribution of Ag plasmonic nanoparticles after a long growth time on titania but suffers from the disadvantage that the shape of plasmonic nanostructures is mostly isotropic, which weakens optical vector sensitivity and information stability. Herein, Ag nanocubes exhibiting high polarization response ability are deposited on orderly mesoporous titania via UV photocatalysis. Recording efficiency of hologram by orthogonally linearly polarized lights is enhanced and the memorized information can be resistant to UV-erasure, both benefiting from the distal resonance of Ag nanocubes. This work delivers a guideline for long-term data storage and high-efficiency display devices.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Super-exponential-growing amount of data that would outpace the existing storage capacity creates an urgent need for information storage techniques [1,2]. Under this circumstance, optical memory is prominent among current various storage modes due to the merits of high storage density, long-term stability and low energy consumption [3,4]. People have made tremendous efforts to address the challenge of optical diffraction limitation in the past decades, such as two photon excitation  and far-field super-resolution recording [6,7]. Unlike the methods mentioned above, hologram memory enhances storage performance depending on low technical requirements and unique “page-wise” mode, which is a promising candidate for next-generation information storage devices [8–10]. However, implementing large capacity based on multi-dimensional utilization of light and stable memory media has been a long-term, yet elusive goal. Therefore, it is imperative to explore advanced optical storage materials. Photochromic materials exhibit excellent photo-response performance with adjustable spectral range, which has been applied widely in optical display, multiplexing holography, volatile/nonvolatile memory devices [11–16]. Nevertheless, the response of photochromic materials to vector element of light such as polarization is still demand to meet for developing multifunctional devices.
Plasmonic metal nanoparticles are characterized by strong interaction between resonant electrons and photons, which realizes manipulation of wavelength, phase and polarization for incident light at nanoscale [17,18]. Alternatively, benefitting from the hybridization with semiconductor, the excited hot electrons in the metal can flow across heterogeneous interface and participate to the resultant reaction [19–21]. One significant paradigm in this regard is the combination of nano-Ag and TiO2, which exhibits multicolor photochromism and excellent high-density optical storage such as holographic memory [22–25]. Theoretically, the polarization response ability of metal-semiconductor system is strongly dependent on metal nanoparticles shape [26–28]. In particular, Ag nanocubes (Ag NCs) have attracted much interest in the burgeoning direction of nanostructures fabrication due to the electromagnetic field enhancement and higher-order surface plasmon resonance modes [29–31]. It was investigated that anisotropic metal nanoparticles (NPs) in direct contact with semiconductor can improve photoelectric properties of the composite system . The growth of Ag NPs by UV photocatalysis creates a direct contact with semiconductor but confronts a bottleneck that the shape of Ag NPs is mostly spherical, which impedes to boosting optical phase modulation ability. The growth of anisotropic shape Ag NPs can be realized by introducing auxiliary laser and electron acceptor during photocatalytic reduction. However, it brings complexity for the preparation of nanocomposite system . As a neglected factor, modulating physical or chemical property of the semiconductor substrate may be an effective way to tackle the issue of poor recording efficiency for polarized light.
Herein, orderly mesoporous TiO2 substrates are fabricated by evaporation-induced self-assembly (EISA) method, providing a foundation for Ag NC growth. The populations of Ag NCs are strongly dependent on the orderliness of TiO2 pores. Physical mechanism may be deduced from distal resonance of Ag NCs and the following electrons transfer through Ag/TiO2 interface and external oxygen capture. The precisely constructed nanocomposite system is harnessed in enhancement of vector-hologram memory.
2.1 Preparation of initial solution and Ag/TiO2 films
TiO2 initial solution was synthesized by the method of Crepaldi et al.  with slight modifications. In short, the surfactant of Pluronic F127 (0.67 g) was mixed with anhydrous ethanol (18.43 mL) and was stirred at 45 °C for 30 min, followed by addition of TiCl4 (1.93 mL, 99.9% metal basis, Macklin) in solution. After stirring 1 h, deionized water (2.2 mL) was slowly added into the ethanol/F127/TiCl4 mixed solution and stirring was continued for 1 h.
Orderly mesoporous TiO2 films were prepared on glass substrates by a dip-coating technique, then aged at room temperature (300 K) and relative humidity of 80% for 2 h by evaporation-induced self-assembly, and finally annealed at 400 °C, 450 °C and 500 °C in air (concentration ratio of oxygen is ∼21%) for 30 min, respectively. The obtained TiO2 film were then immersed in aqueous solution of silver nitrate (AgNO3, 0.05 M, 50 mL) which was mixed with ethanol (1 mL) as a hole scavenger, followed by irradiation of a UV lamp (∼365 nm, 1 mW/cm2) for 15 min. The Ag NPs deposited TiO2 film was removed from the AgNO3 solution, and then washed with ultra-pure water, followed by blowing dry with nitrogen, finally stored in darkness. The whole preparation process is shown in Figs. 1(a)–1(d).
2.2 Optical setup
Optical setup for holographic recording is shown in Fig. 2(a). Diffraction gratings were recorded with coherent lights from a blue-violet laser (405 nm, TOPTICA Photonics). A half-wave plate was used to adjust the polarization state of recording beams. The intersectional angle between the two coherent beams was fixed at 10°. The power density of the interference beams was the same and equal to 10 mW/0.07 cm2. A red laser (671 nm, Changchun New industries Optoelectronics Tech. Co. Ltd.) generating s-polarized light, was used as a probe source to monitor the holographic grating dynamics. The power density of the 671 nm laser was set as 1 mW/0.07 cm2 to avoid the oxidation of Ag NPs caused by long-term irradiation at non-resonant wavelength. The UV laser (360 nm, Changchun New industries Optoelectronics Tech. Co. Ltd.) generated erasing beam to investigate UV-resistant performance of holographic grating. The power density of the 360 nm laser beam was set as 20 mW/cm2. The first-order diffracted signal was registered on a photo-diode interfaced with a computer. Diffraction efficiency of holographic gratings, taking Fresnel losses into account, can be defined as the ratio between intensities of the first-order diffracted beam and the incident beam after passing through the sample. Besides, one of the writing beams acting as object beam was expanded by a beam expander after spatial filter, collimated to pass through a mask, and focused onto the center of the Ag/TiO2 nanocomposite film. The other beam was focused on the same spot as reference beam. Holographic images were reconstructed by red laser and were collected by CMOS video camera. For photo-induced birefringence experiment, the pumping-beam power density was held at 10 mW/0.07 cm2. The probe-beam power density was held at 1 mW/0.07 cm2. The polarization angle between the pumping and probe beams was 45°. Two laser beams were nearly parallel incident to the sample. Probe-beam transmittance was registered on a photodiode faced with a computer, as shown in Fig. 2(b). The environmental temperature is ∼300 K, and the relative humidity is 40%.
3. Results and discussions
3.1 Film characterization and photochemical reaction
The surface morphology of orderly mesoporous TiO2 films at different annealing temperatures is observed by Scanning Electron Microscope (SEM), as shown in Figs. 3(a)–3(c). Titania films annealed at 400 °C, 450 °C and 500 °C are named as T-400, T-450 and T-500, respectively. The three samples all exhibit nano-channel arrays. The statistical distributions of TiO2 pore size are calculated and analyzed by Image pro plus 6.0, as shown in Figs. 3(d)–3(f). Each sample has two size distribution bands. However, the band distance and the band shape are quite different. For T-400, the size distribution band is flat, varying from 5 nm to 20 nm. The nearly equivalent population of pores with the smaller and the larger sizes may result from incomplete calcination of the pore-forming agent (F127). However, T-500 has an obvious bipolar size distribution. The distance between the two band centers is ∼13 nm, which may be deduced in pores collapse and join due to excessive heat-treatment. From the cumulative percentage curves, inflexion points of the curves are observed obviously in T-400 and T-500, which means that the pore size distribution is not uniform. Quite differently, T-450 presents nearly normal distribution of pore size centered at 12 nm resulting from suitable annealing temperature. The highly uniform size distribution of TiO2 pores provides excellent deposition environment for Ag NPs. The surface morphology of deposited Ag NPs on T-400, T-450 and T-500 after UV photocatalysis is present in Figs. 3(g)–3(i), respectively. The resultant Ag/TiO2 nanocomposite films are named as S-400, S-450 and S-500 accordingly. Loading density of Ag NCs for S-450 is calculated to be 2×108/cm2, which is much higher than those of the other two samples (3×106/cm2 for S-400 and 2×105/cm2 for S-500). XRD measurements for T-400, T-450 and T-500 were carried out. However, no obvious characteristic peaks of TiO2 crystal were observed, indicating the weak crystallinity of the three TiO2 samples. Thus, the orderliness of TiO2 pores may play a major role in the formation of Ag NCs, while the crystal structure of oxide has little effect on the plasmon photochemical reaction.
The possible photochemical growth process is shown in Fig. 4(a). Ag+ ions quickly nucleate at the pore edge after capturing photo-generated electrons in the initial stage of UV irradiation. The uniformity of the pores determines the orderly arrangement of the original Ag NPs. Prolonging illumination time, the orderly-distributed Ag NPs begin to gather, forming a NC structure based on the Ostwald Ripening . A large amount of Ag NCs with size uniformity can grow on the regular oxide substrate. For S-450, the Ag NCs with the size varying from 75 nm to 95 nm occupied a considerable volume fraction of 75%, as shown in Fig. 4(b). Besides, a rather low amount of chlorine element on TiO2 surface was tested by SEM-EDAX energy spectrum and X-ray photoelectron spectroscopy, indicating that the reaction between Ag+ and Cl− can be ignored in the UV photocatalysis reduction.
The UV-Vis absorption spectra of S-400, S-450 and S-500 are shown in Fig. 4(c). The two resonant absorption peaks at ∼400 nm and 510 nm for S-450 correspond to distal and proximal modes, at top and bottom sides, respectively . S-400 presents one broad absorption band centered at 470 nm, while S-500 exhibit a wider spectrum band from 400 nm to 650 nm. In combination with the SEM analysis, the dual-absorption band of S-450 arises from the presence of Ag NCs, which exhibits the anisotropic shape. In addition, it can be observed that the films of S-400, S-450 and S-500 are all dark brown. The film color is gradually deepened from S-400 to S-500 which is consistent with their absorbance.
Metal NPs, exhibiting localized surface plasmon resonance (LSPR), have strong interaction with photons. The NCs can be oxidized at the resonant wavelength, resulting in releasing of hot electrons, and then Ag+ ions are generated. The oxidation process is named as “photo-dissolution” [35,36]. The Ag+ ion concentration near the resonant Ag NPs is higher than that at the non-resonant region. Under ambient conditions of the relative humidity greater than 40%, Ag+ ion migration easily occur with the help of the adsorbed water on the surface of TiO2 [37,38]. Concentration gradient force of Ag+ ions contributes to the diffusion of Ag+ ions. Based on photo-dissolution of Ag NCs, absorption coefficient and refractive index are both changed, which can be applied in optical information storage. Here, in-situ absorption spectra and SEM are utilized to investigate the morphology change of Ag NCs. As shown in Fig. 5(a), absorbance increases when prolonging the blue-violet irradiation time. Besides, two initial absorption bands tend to be merged, indicating the structural characteristic of Ag NCs is weakened. Absorbance at 405 nm versus irradiation time is inserted and fit as an approximate exponential growth:2):
Similarly, the data of absorbance versus irradiation time for S-400 and S-500 were also obtained (not shown here). ΔAS-400=0.107, ΔAS-450=0.289 and ΔAS-500=0.028 are calculated according to Eqs. (1) and (2). The maximum absorbance variation of S-450 is much higher (3-9 times) than those of the other samples under the same irradiation time, which may be related to the largest amount of the Ag NCs with amplitude modulation ability.
Two possible photochemical processes may be produced under the blue-violet excitation. One is the photo-dissolution of ultra-small sized Ag NPs (∼5 nm) which may exist on the surface or inside TiO2 mesoporous film. The transferable Ag+ ions tend to be gathered around non-resonant silver particles, i.e. the parasitic growth at bottom of Ag NCs. The other is originated from distal resonance mode of Ag NCs. The edge of top side of Ag NC is easier to be excited. On one hand, the electrons transfer through the Ag/air interface; on the other hand, the electrons at the top of Ag NCs can also transfer through the Ag/TiO2 interface when the distance between the top of Ag NCs and the interface of Ag/TiO2 is close to the hot carrier mean free path . Figure 5(b) schematically illustrates the routes of distal hot electrons transfer and Ag+ ion migration. Accordingly, photoinduced anisotropic deformation of Ag NCs is observed directly by in-situ SEM, as shown in Figs. 5(c)–5(e). The profiles of top and bottom parts of the Ag NCs are obviously different from that before the 405 nm irradiation. It has been investigated that the Ag NCs with size of ∼100 nm on TiO2 can be excited by ∼420 nm light due to distal resonance mode . In our case, absorption spectrum and near-field map for Ag NCs on the TiO2 surface were calculated by means of finite-difference time-domain (FDTD) method. The calculation model is that single Ag NC is located on the TiO2 (thickness of 120 nm) surface supported by glass substrate. It should be noted that the size of Ag NCs in actual sample has a distribution range, which is different from that of single Ag NC in simulation model. The simulated resonant absorption peaks are distributed in 400-500 nm for TiO2 loaded with Ag NCs with sizes of 40-140 nm. It is similar to that of the actual absorption spectrum despite a little red shift. In addition, the optical near field is found to be localized at the top of the Ag NCs with side length of 90 nm depositing on mesoporous TiO2 under the blue-violet irradiation, as shown in Fig. 5(f). The simulation results of FDTD confirm that the distal resonance mode for the Ag NCs is reasonable.
Meanwhile, the phase modulation ability of Ag NCs is proved by the photoinduced birefringence experiment. The transmitting light intensity increases exponentially versus excitation time of pumping light for all the three samples, as shown in Fig. 6. The increasement of transmittance for S-450 is also the highest. The value of the change of effective refractive index is calculated to be 0.022 with unit of radian according to Eq. (3).
3.2 UV-resistant memory and vector-hologram reconstruction
Based on the previous investigation , the spatial structure of oxides matrices may limit the growth of Ag NPs. The plasmonic particles are usually formed as the spherical shape inside the mesoporous TiO2. After visible light irradiation, the transition of spherical to non-spherical Ag NPs is inevitable . However, the electrons generated from TiO2 under UV irradiation can flow back to the metal side, which causes a second reduction of Ag+ ions and the resultant recovery of Ag shape. Such a cycled photochemical process affects long-term and stable information storage. Whereas photo-dissolution of Ag NCs is quite different from the case mentioned above. The distal electrons transfer and Ag+ ions migration may result in irreversible silver deformation.
For clarification, the UV-intervened s-p polarization holographic performance of S-400, S-450 and S-500 are investigated. Figure 7(a) shows diffraction efficiency of s-p holographic gratings as a function of time in three cycles of Vis/UV irradiation. S-500 exhibits nearly invariable value near to zero for little amount of resonant Ag particles produce holographic fringes, which are also weakened gradually by the UV damage. For S-400, the first writing process produces obvious increase of diffraction efficiency. However, saturation value of diffractive intensity by the coherent blue-violet light decreases gradually in the following writing-erasing periods. The non-uniform pore distribution of T-400 causes that the distribution of Ag NCs is disturbed. The growth characteristic of holographic gratings in S-400 is also destroyed under the alternating irradiation of 405 nm and 360 nm lights. Quite differently, the resident diffraction efficiency of S-450 is accumulated with increasing the writing-erasure cycle times. A considerable amount of Ag NCs plays a key role in the UV-resistant information storage. Our previous work reveals Ag+ ions migration in a long-term process of visible light excitation . After the initial photo-dissolution, Ag+ ions or Ag2O remain near the edge of top side of Ag NCs. The first UV-erasing process results in the transition from the dissolved Ag+ ions to Ag. However, after several alternated Vis/UV irradiations, the Ag+ ions are far away from the initial Ag NCs, resulting in an irreversible photochemical process. The process can also be proved by differential absorption spectra of S-450 after alternate irradiation of 405 nm and 360 nm lights. The change of LSPR spectrum shape by the blue-violet light is maintained even under UV excitation. The ability to resist UV damage is further verified by the holographic storage under UV direct interference. As shown in Fig. 7(b), the diffractive signal of S-450 still keeps a sustained increase trend under the co-action of the coherent 405 nm lights and the 360 nm light while those of the other two samples are almost suppressed completely. For Ag NC structure, the hot electrons after distal resonance excitation are far from the interface between Ag and TiO2. The hot electrons are easier to be captured by the environmental oxygen. The edge part of Ag NCs is dissolved preferentially, which can also realize the anisotropic photo-dissolution even the interfacial transfer is seriously blocked. Moreover, reconstruction efficiency of sample can be further improved by increasing the power of writing light in a certain range, as shown in Fig. 7(c). Therefore, Ag NCs loaded on orderly mesoporous TiO2 can enhance the recording efficiency of polarization hologram and the ability of resisting UV-erasure.
Based on the aforementioned discussions and photoinduced birefringence result, the Ag NC has strong phase modulation capability that provides possibilities for vector hologram storage. The hologram with star pattern is stored and reconstructed successfully in s-p recording mode for the three samples, as shown in Figs. 8(a)–8(c). Obviously, S-450 presents the hologram with maximum brightness, followed by S-400 and S-500. This observation indicates that the brightness of diffractive image is enhanced by depositing more Ag NCs on TiO2, which is reasonable considering the previous results. In addition, polarization-sensitive materials have the effect of phase modulation of probe light. Adjusting and controlling the polarization state of probe light has great prospects in the field of information optics. Brightness of the reconstruction light field can be controlled by varying polarization state. Furthermore, it provides a new dimension for optical data storage or information encryption, contributing to explore high-density optical memory materials.
Commonly, the matching degree between the LSPR absorption of Ag NPs and the recording wavelength determines the exposure sensitivity and writing rate in hologram storage. Therefore, at the determined writing wavelength, it is necessary to design appropriate plasmon resonance absorption characteristics to enhance the diffraction efficiency. It should be pointed out that the distal resonance wavelength of Ag NCs is near 400 nm, which can realize Blu-ray hologram memory even without fabricating the ultra-small sized Ag NPs as the previous work . In fact, the transfer of electrons for Ag NCs on the surface of TiO2 is less efficient than that from the Ag NPs embedded in TiO2. To address the issue, construction of layer by layer arrangement for metal/semiconductor may be another feasible way. Besides, careful control of nitrogen/oxygen level in annealing environment for TiO2 will also be beneficial to the optimization of the performance of nanocomposite system and the improvement of its application ability. More detailed investigations are still needed.
Orderly mesoporous TiO2 are obtained via EISA combined with suitable annealing temperature. Based on the excellent growth environment, Ag NCs are fabricated by bottom-up ultraviolet photocatalysis approach. Further investigations revealed that Ag NCs have strong optical phase modulation ability and can be applied in UV-resistant vector-holographic memory devices, benefiting from the distal resonance of Ag NCs and the following electrons transfer through Ag/TiO2 interface and external oxygen capture. It is also shown that the brightness of the diffractive image is enhanced by introducing a large amount of Ag NCs. Precisely constructed nanocomposite system delivers a promising tactic for advanced optical storage and polarization display devices.
National Natural Science Foundation of China (11974073, 51732003); Overseas Expertise Introduction Project for Discipline Innovation (B13013); Fundamental Research Funds for the Central Universities (2412017FZ011, JGPY201906); Natural Science Foundation of Jilin Province (20180101218JC).
The authors declare that there are no conflicts of interest related to this article.
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