Efficient optical phase modulation is essential for information processing. The polarization response of metallic nanoparticles plays a key role in the formation of vector hologram. Commonly, Ag nanoparticles reduced by incoherent UV lamp tend to grow in isotropy, which is unsuitable for the formation of polarization hologram and the resultant multicolor holograms recording-readout in polarization channels. Here, the Ag nanoparticles storage with high polarization-sensitivity and anti-radiation interference are realized via bi-photonic irradiation from UV and visible lasers with participation of electron-acceptor. For the first time, the orthogonal polarization storage efficiency of the system is higher than that of the parallel mode. Color-tunable holographic reconstruction by polarization-multiplexing is achieved. This work provides a significant research strategy for stable, high-density data storage and advanced display.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Ultra-high-density data storage is essential for the current information processing [1–3]. Holographic storage with “page-wise”  mode breaks the limitation of traditional “bit” recording  and presents abilities in large capacity and fast transmission-rate, being regarded as one of the effective ways to deal with mass data explosion. Commonly, holograms are recorded in suitable storage medium in terms of interference fringes induced by the coherent irradiation with object and reference lights [4,6,7]. Thus, the response ability of recording media to stimulating-light field will play a key role in the formation of vector hologram.
Plasmonic nanoparticles (NPs) or surface nanostructures are considered as one of the most promising platforms on polarization information processing . Wavelength- and polarization-dependent optical information can be recorded as form of metasurface by electron/ion lithography [8–10] or plasmonic nanorods in polymer matrices by ultra-high energy laser induced thermal melting of the metallic nanoparticles . However, more efficient photo-energy transformation mode needs to be developed, such as photo-quantum excitation . Recently, Ag NPs interfaced with TiO2 was found to present photochromism  and holographic storage based on oxidization reaction . Under the photonic excitation of visible wavelength, Ag NPs responding to the wavelength can absorb photons, resulting in localized surface plasmon resonance (LSPR) [15,16]. Hot electrons from LSPR can jump across the Schottky barrier with the help of built-in electronic field of TiO2 , and transfer to oxygen or adsorbed H2O via TiO2 [18,19]. The separation of electrons from the resonant Ag NPs results in oxidation of Ag NPs and production of Ag+ ions . Thus, absorption coefficient and refractive index of the system can be changed according to exciting light patterns. In this principle, Ag/TiO2 can record hologram. Storage capacity will be further increased by selecting recording lights with different wavelengths, i.e. wavelength multiplexing . It was found that polarized excitation can induce oriented LSPR. Anisotropic dissolution of Ag NPs was investigated accordingly . Based on such property, polarization-multiplexing in the plasmonic media was also realized [23,24]. Recently, the holographic storage efficiency was enhanced by the orthogonal polarization recording mode . It gives an important physical insight that refractive index modulation ability can be enhanced if it changes from the original anisotropic plasmonic nanostructures. To obtain Ag/TiO2 nanocomposite system, incoherent UV lamp was commonly used to excite TiO2 to generate electrons for reduction of Ag+ ions and growth of Ag NPs [26,27]. However, the fabricated media show isotropic optical property resulting in a rather low recording efficiency of polarization holography . Besides, the recorded hologram shows poor stability under UV exposure that results in the recombination of Ag+ ions and photo-generated electrons from TiO2. It is urgent to explore an effective method to obtain the plasmonic nanocomposite system with natural anisotropy characteristics and UV resistant ability, which is an important way to develop high-efficient and stable phase holography.
In this paper, polarization-sensitive Ag NPs with anisotropic absorption in nanoporous TiO2 films are obtained by the simultaneous irradiation of UV and visible lasers. Introduction of silicotungstic acid (SA) as electron acceptor contributes to anti x-ray radiation. For the first time, orthogonal linearly-polarized gratings (s-p) present higher diffraction efficiency than that in parallel (s-s). Based on the property, stable and high-density holograms are reconstructed in tunable-color with linearly-polarized lights.
2.1 Ag/TiO2 film preparation
Nanoporous TiO2 film was obtained onto glass by dip-coating from the titania sol followed by thermal annealing at 500°C which was reported on our previous work . The TiO2 film was immersed in silicotungstic acid (0.2 mol/L) which was used as an electron-acceptor for 2 hours. The SA-adsorbed TiO2 film was then immersed into the aqueous solution of 0.1 M silver nitrate (AgNO3), followed by the co-irradiation of continuous UV (360 nm, 1 mW/cm2) and visible (532 nm, 4.3 mW/cm2) lasers (both s- polarized, Changchun New Industries Optoelectronics Tech. Co. Ltd.). The fabricated sample is denoted SSA. The experimental process is shown in Fig. 1.
2.2 Optical setup
Diffraction gratings were recorded with two coherent beams from a red (671 nm, s-polarized, 5 mW) laser. Another red laser with rather low power (639 nm, 0.5 mW) was used as a probe source to monitor the holographic grating dynamics. The first-order diffracted signal was registered on a photodiode interfaced with a computer. Diffraction efficiency of holographic gratings, taking Fresnel losses into account, can be calculated accordingly . Then, two sets of holograms were recorded at the same point of the sample in parallel and orthogonal linear-polarization configurations, respectively. Half-wave plate was used to modulate the polarization state of object beam to s or p state. Green (532 nm) and yellow (589 nm) continuous lasers were used to reconstruct the stored holograms independently, which were collected by a CMOS video camera interfaced with a computer. The sketch map for optical setup is shown in Fig. 2.
3. Results and discussions
3.1 Polarization-sensitive LSPR
Differential absorption spectra of the laser-fabricated SSA films in the UV-Vis-NIR region (350 nm-900 nm) irradiated with s- and p-polarized red lights (671 nm, both 0.5 mW) are shown in Figs. 3(a) and 3(b). Absorbance around 671 nm decreases gradually versus exposure time in the s-polarized irradiation modes. While the “hole-burning” in the red region by p-polarized irradiation changes dramatically versus irradiation time. As the Ag NPs locate inside the hole of the nanoporous TiO2 film, direct observation of the morphology change of Ag NPs is rather difficult. The photochromism behaviour of the Ag/TiO2 film may help us to reveal the nature of the photochemical reaction between the nanocomposite system and the polarization light field. The difference of “hole-burning” under the two orthogonal polarization modes may be ascribed to the different degrees of oxidative dissolution of Ag NPs which resonate with the excitation light. Commonly, smaller sized spherical Ag NPs resonate with the light at shorter wavelength. In addition, it was also proved that the Ag+ ions in the adsorbed water on the surface of TiO2 can move to nonresonant Ag NPs and recombine with electrons, resulting in the redeposition of Ag NPs. Taking these effects into account, the absorbance enhancements at ∼550 nm under the two polarized irradiations (s and p) result from the population increase of the smaller sized nonresonant Ag NPs. As shown in Fig. 3(c), the differential absorbance at 671 nm decreases in an exponential fashion versus excitation time, which can be described by Eq. (1):
These observations can be explained by the formation of Ag NPs interfaced with TiO2 matrices under co-action of polarized UV and visible lights. It was noticed that the photochromic reaction of Ag/TiO2 films can occur not only in air but also in water [30,31]. Hence, in the aqueous solution of AgNO3, the plasmonic photo-chemical reduction and dissolution reaction may both be realized. As shown in Fig. 4(a), the photo-generated electrons from TiO2 can be combined with Ag+ ions to form Ag NPs under UV excitation. Meanwhile, the polarized visible beam plays a role in the plasmonic NP formation. In our previous investigations, Anisotropic Photo-Dissolution (APD) of Ag NPs has been proposed . In the direction of the visible light polarization, the Ag NPs tend to be dissolved as long as they grow reaching the resonant size. Thus, the reduction reaction from Ag+ to Ag can only occur freely in the direction perpendicular to the visible polarized light, resulting in the gradual formation of rod-like plasmonic nanostructures. In addition, Ag+ ions dissolved from the resonant Ag NPs under visible irradiation can combine randomly with non-resonant Ag NPs to form the Ag NPs with different orientations, which breaks normal growth regulation that Ag NPs are reduced by UV light alone. Moreover, with the assistance of polyoxometalate as electron acceptor, the visible light induced APD is accelerated, meanwhile anti-UV reduction is enhanced. Hence, the anisotropic characteristics of nanoparticles can be further improved. SEM image of the surface of SSA is shows in Fig. 4(b). Ag NPs with different morphology were observed on the surface of TiO2, including irregular Ag NPs such as oriented-directionally rod-like Ag nanostructures. Top-viewed SEM image of Ag NPs reduced by incoherent UV lamp on TiO2 is also shows in Fig. 4(c). In contrast, Ag NPs with near spheroidal morphology were observed on the surface of TiO2.
3.2 Polarization holographic recording
The difference of “hole-burning” under s- and p-polarized irradiations, and the growth of anisotropic plasmonic nanostructures make it possible to achieve highly efficient polarization holographic storage. Here, three holographic gratings with the polarization configurations of s-s, p-p and s-p were recorded separately by two coherent polarized red beams (671 nm) in three different points of the sample. Circularly-polarized red laser beam (639 nm) was chosen to probe source to monitor kinetics of holographic gratings with different polarization configurations, which is shown in Fig. 5. All the curves present standard exponential growth, which can be fitted by Eq. (2):
Obviously, diffraction efficiency of holographic storage recorded in orthogonal mode is much higher than that in parallel. Thus polarization-multiplexed hologram storage can be carried out. We checked the diffraction light from the (s-p) grating in the Ag/TiO2 film which has a strong orthogonal component of polarization state to the zero order one, and the diffraction light from the (s-s) grating which remains the original polarization state. The results are consistent with the previous report . As is known to all, the light intensity in the overlapping region of two s-polarized coherent beams presents periodic sinusoidal variation, which results in the periodic alternate distribution of Ag and Ag2O for the Ag/TiO2 system. Under s-p recording mode, the periodic variation of electric field vectors of incident lights results in periodic distribution of morphology of Ag NPs. Thus optical phase modulation ability of Ag NPs in different places is different. Polarization gratings by are formed. Compared with (s-s) gratings consisted of Ag/Ag2O alternated distribution, (s-p) gratings present more erasable as the morphology change of Ag NPs is easier. Therefore, the polarization holographic experiment was carried out as (s-s) gratings recording was followed by the (s-p) grating recording.
The + 1 and −1 order diffractive signals of the 671 nm probe beam were monitored simultaneously by two polarizer-filtered photodiodes in s- and p-polarized diaphanous directions, respectively. As shown in Fig. 6(a), in the first 450 s, only the (s-s) grating was recorded. After that, one of the writing beams was changed from s- to p-polarization state by a half-wave plate of 671 nm, overlapping the original excited region with the (s-p) grating recording. Diffraction efficiency from (s-s) gratings decreases gradually and that from (s-p) one increases. At 890 s, diffractive intensities of the two kinds of gratings reach the same value. As the amplitude grating of Ag/Ag2O alternated distribution has been formed before (s-p) recording, the amount of Ag NPs is decreased greatly. The efficiency and response rate for the (s-p) grating are both inhibited accordingly compared with those described in Fig. 5.
3.3 Color-tunable display of stable holograms
Very recently, we found polyoxometalate-modified Ag/TiO2 films present UV-resisted ability . Further, the anti-X-ray interfering ability is also investigated, as shown in Fig. 7. The diffraction efficiency of SSA present continuous increase with time even under the interference of X-ray. However, the diffraction efficiency of Ag/TiO2 film without SA decreases obviously after 100 s. The environmental stability of the plasmonic hologram storage is improved greatly, safeguarding high-efficient hologram reconstruction.
Here, we divide color information of the mixed holograms via polarization channels. The red (671 nm) laser beam was used as writing source, as shown in Fig. 6(b). The s-polarized object beam with a “star” shape was overlapped with s-polarized reference beam at the same point of the SSA sample for 450 s, forming the first hologram. And then the p-polarized object beam with a “pagoda” shape was overlapped with the reference beam to form the second hologram for 440 s. The equivalent diffraction intensities for the two holograms were obtained. Figure 8 shows the color reconstruction of the mixed holograms with green and yellow laser beams (532 nm s-polarized and 589 nm p-polarized, both 1 mW). “star” and “pagoda” can appear in the same position by adjusting the incident angles of the reconstruction beams. First, in s-polarized diaphanous direction of the polarizer, the s-polarized green light remains polarization state after passing through the (s-s) recorded hologram while the polarization state of the yellow light with changes from p to s after passing through the (s-p) recorded hologram. Then the combined holographic images of green “star” and yellow “pagoda” is obtained. When the diaphanous direction of the polarizer is set to p, the color of reconstruction image is completely opposite to the previous one. Here, the mix image is the combination of yellow “star” and green “pagoda”. Finally, for the polarizer diaphanous direction of 45°, the two-color mixed images can be observed. The enhancements of polarization sensitivity and holographic storage capability are not only suitable for red region, but also for blue region in which similar results can be obtained. The property is very important for full-band high-density data storage.
In fact, the multiplexing-storage mechanism at one point is quite different from that at two different locations. The multiplexing recording efficiency at certain wavelength is related to the electron transition ability across Schottky barrier. The metallic particle size distribution is also limited by the size of titania nanopore. The height of Schottky barrier can be controlled better when modulating the pore spacing, the morphology of the plasmonic nanoparticles, the contact area between metal and semiconductor. More detailed investigation will be carried out.
TiO2 nanoporous films were loaded with anisotropic Ag NPs by the co-action of UV and visible lasers with introduction of SA as electron acceptor. The polarization-selective “hole-burning” is observed under visible light irradiation with different polarization states. The diffraction efficiency in orthogonal polarization mode is found to be higher than that in the parallel one for the first time. Mixed polarization holograms were recorded at red region under parallel (s-s) and orthogonal (s-p) polarization states. Colored-holographic reconstruction was also achieved in diffraction of the green and yellow lights with orthogonal linear polarization states.
National Natural Science Foundation of China (10974027, 31271442, 51372036, 51732003, 61007006); The 111 Project (B13013); Fundamental Research Funds for the Central Universities (2412017FZ011); Natural Science Foundation of Jilin Province of China (20180101218JC).
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