Use of two-photon absorption in a photorefractive crystal for three-dimensional optical memory

Strategy for Simple Control of High Performance PQ/PMMA Holographic Media

Holographic data storage technology is a cost-effective solution for long-term archival data storage. However, the development of suitable holographic recording materials remains a challenge. Among these materials, phenanthraquinone-doped poly(methyl methacrylate) (PQ/PMMA) stands out due to its low cost and controllable thickness. Nevertheless, its limited photosensitivity and diffraction efficiency hinder its widespread application. In order to solve these problems, we put forward a kind of convenient and simple, low cost strategy, by adding plasticizer N,N-dimethylformamide (DMF) for preparation of DMF-PQ/PMMA photopolymer, avoid the use of complex compounds. The addition of DMF not only influences the thermal polymerization stage but also forms weak interactions with PQ during the photoreaction process, thereby enhancing the holographic performance of DMF-PQ/PMMA. Consequently, we achieved a remarkable 9.1-fold increase in photosensitivity (from ∼0.35 to 3.18 cm J-1), improved diffraction efficiency by 20% (from 65% to 80%), and reduced volume shrinkage by a factor of 8 (from 0.4% to 0.05%). Furthermore, utilizing a collinear holographic storage system with multiplexing shift at a scale of 5 μm resulted in an impressively low minimum bit error rate (BER) of only 0.36% (with an average BER of 1.4%), highlighting the fast processing capability and potential for low BER applications in holographic information storage using DMF-PQ/PMMA.

Deep-trap persistent materials for future rewriteable optical information storage

Deep-trap persistent luminescent (PersL) materials with enriched traps, which allow signals to quickly write-in and read-out with low-energy consumption, are one of the most promising materials for information storage. In this review, considering the demand for optical information storage, we provide comprehensive insights into the data storage mechanism of PersL materials. Particularly, we focus on various "trap-state tuning" strategies involving doping to design new deep-trap persistent phosphors with controlled carrier trapping-de-trapping for non-volatile and high-capacity information storage. Subsequently, various recent significant strategies, including wavelength-multiplexing, intensity-multiplexing, mechanical-multiplexing, and three-dimensional and multidimensional trap-multiplexing technologies for improving the information storage capacity of PersL phosphors are highlighted. Finally, the challenges and opportunities regarding optical information storage by PersL materials are discussed. We hope that this review will provide new insights for the future development of PersL materials in the field of optical data storage.

Multiphoton absorption enhancement by graphene-gold nanostructure

We present a hybrid graphene-gold nanoantenna designed to enhance multiphoton absorption signals in molecules. The enhancement process involves two key steps: Firstly, the graphene component of the antenna supports molecular absorption in the mid-infrared and terahertz bands. By applying gate voltage, one can adjust the spectral positions of its resonances and select the desired absorption order, determining the number of photons absorbed in a single transition event. Secondly, gold nanorods with carefully tailored geometrical parameters enhance fluorescent single-photon emission. As a proof of concept, we adjust the geometry parameters of the hybrid antenna to the ATTO 700 dye molecule, taking into account its spectrally resolved emission characteristics. We predict a significant local enhancement of the fluorescence signal indicating the highly nonlinear process of N-photon absorption to exceed 5 orders of magnitude for N = 2 and 13 orders of magnitude for higher nonlinearity orders. Our proposed nanoantenna offers a promising platform for the tunable enhancement of highly nonlinear light-matter interactions.

A 3D nanoscale optical disk memory with petabit capacity

High-capacity storage technologies are needed to meet our ever-growing data demands1,2. However, data centres based on major storage technologies such as semiconductor flash devices and hard disk drives have high energy burdens, high operation costs and short lifespans2,3. Optical data storage (ODS) presents a promising solution for cost-effective long-term archival data storage. Nonetheless, ODS has been limited by its low capacity and the challenge of increasing its areal density4,5. Here, to address these issues, we increase the capacity of ODS to the petabit level by extending the planar recording architecture to three dimensions with hundreds of layers, meanwhile breaking the optical diffraction limit barrier of the recorded spots. We develop an optical recording medium based on a photoresist film doped with aggregation-induced emission dye, which can be optically stimulated by femtosecond laser beams. This film is highly transparent and uniform, and the aggregation-induced emission phenomenon provides the storage mechanism. It can also be inhibited by another deactivating beam, resulting in a recording spot with a super-resolution scale. This technology makes it possible to achieve exabit-level storage by stacking nanoscale disks into arrays, which is essential in big data centres with limited space.

Expanding the toolbox of photon upconversion for emerging frontier applications

Photon upconversion in lanthanide-based materials has recently shown compelling advantages in a wide range of fields due to their exceptional anti-Stokes luminescence performances and physicochemical properties. In particular, the latest breakthroughs in the optical manipulation of photon upconversion, such as the precise tuning of switchable emission profiles and lifetimes, open up new opportunities for diverse frontier applications from biological imaging to therapy, nanophotonics and three-dimensional displays. A summary and discussion on the recent progress can provide new insights into the fundamental understanding of luminescence mechanisms and also help to inspire new upconversion concepts and promote their frontier applications. Herein, we present a review on the state-of-the-art progress of lanthanide-based upconversion materials, focusing on the newly emerging approaches to the smart control of upconversion in aspects of light intensity, colors, and lifetimes, as well as new concepts. The emerging scientific and technological discoveries based on the well-designed upconversion materials are highlighted and discussed, along with the challenges and future perspectives. This review will contribute to the understanding of the fundamental research of photon upconversion and further promote the development of new classes of efficient upconversion materials towards diversities of frontier applications in the future.

Reversible Mn valence state switching in submicron α-Al2O3:Mn by soft X-rays and blue light - a potential pathway towards multilevel optical data storage

The generation of Mn⁴⁺ in α-Al₂O₃:Mn³⁺ by soft X-ray exposure is demonstrated with a large dynamic range of the X-ray generated Mn⁴⁺ luminescence signal, indicating the potential use of α-Al₂O₃:Mn³⁺ for multilevel optical data storage. Samples with a range of Mn concentrations (0.05, 0.1, 0.2, 0.4, 0.6 and 1.2 atom%) were prepared via a facile combustion method and the sample with 0.4 atom% was found to display the highest luminescence intensity. The stored information can be read out via the R-lines (²E → ⁴A₂) under ∼470 nm (⁴A₂ → ⁴T₂), or ∼630 nm (⁴A₂ → ²T₁) excitation with the latter being preferred since photobleaching is minimized. Interestingly, the Mn⁴⁺ valence state can be fully switched back to Mn³⁺ by blue light exposure (e.g., 462 nm laser diode). The stored information could be repeatedly written and erased, showing no significant deterioration over five consecutive cycles, with less than 5% uncertainty.

Ultra-high capacity for three-dimensional optical data storage inside transparent fluorescent tape

In this Letter, we show an ultralarge capacity for three-dimensional optical data storage inside transparent fluorescent tape using the two-photon absorption photo-bleaching method. We can obtain transparent fluorescent tape by means of the simple dip method. We successfully demonstrate recording and reading of six layers of binary data bits with lateral separation of 2 µm and longitudinal layer separation of 3 µm. Thus, this result leads to a storage density of approximately ${80}\;{{\rm Gbits/cm}^3}$80Gbits/cm³. Therefore, we can realize authentic ultrahigh capacity optical data storage using long transparent fluorescent tape in the future, like magnetic tape, and fundamentally solve the data explosion disaster.

Polarization sensitive microstructures fabricated on lithium niobate surfaces by using femtosecond laser pulses

Polarization sensitive microstructures with different morphologies were induced by irradiating dual lithium niobate crystals with femtosecond laser pulses. An upper lithium niobate crystal served as a mask plate to tailor light field, which led to the formation of crater and arc-shaped structures on the surface of a lower lithium niobate crystal. In single-shot irradiation, the orientation and morphology of resultant microstructures can be tailored by controlling the focusing position, because focus splitting took place when a focused laser light propagated through dual lithium niobate crystals. In scanning, the width and morphology of laser scan lines can be governed using various combinations of focusing position and scanning direction. Furthermore, large-area micro/nanostructures with different topography features were successfully fabricated on the crystal surface and their absorption spectra indicated that the absorptance in the visible wavelength range was strongly dependent on fabricated micro/nanostructures. This new type of structured lithium niobate surfaces can be potentially applied in optical and photonic devices.

Giant refractive-index modulation by two-photon reduction of fluorescent graphene oxides for multimode optical recording

Graphene oxides (GOs) have emerged as precursors offering the potential of a cost-effective and large-scale production of graphene-based materials. Despite that their intrinsic fluorescence property has already brought interest of researchers for optical applications, to date, refractive-index modulation as one of the fundamental aspects of optical properties of GOs has received less attention. Here we reported on a giant refractive-index modulation on the order of 10⁻² to 10⁻¹, accompanied by a fluorescence intensity change, through the two-photon reduction of GOs. These features enabled a mechanism for multimode optical recording with the fluorescence contrast and the hologram-encoded refractive-index modulation in GO-dispersed polymers for security-enhanced high-capacity information technologies. Our results show that GO-polymer composites may provide a new material platform enabling flexible micro-/nano-photonic information devices.

Superresolution-focal-volume induced 3.0 Tbytes/disk capacity by focusing a radially polarized beam

This paper reports on the study of the superresolution volume of the focal spot by focusing a radially polarized beam. This feature is achieved by increasing the inner radius of a high NA annular objective to break the diffraction-limited volume of a focal spot. The application of this finding into two-photon induced three-dimensional optical data storage leads to an enhanced photoreduction threshold effect in recording. As such, multilayer subdiffraction optical recording is experimentally demonstrated with an equivalent capacity of 3.0 Tbytes/disk.

Two-photon-induced polarization-multiplexed and multilevel storage in photoisomeric copolymer film

We present a two-photon-induced polarization-multiplexed and multilevel data storage method with a bisazobenzene copolymer film. A polarization-adjustable femtosecond pulsed laser is used as writing beam to induce anisotropy, and the recorded information is retrieved by a CCD sensor from the film with corresponding polarized illumination. It is found that the optical axis of bisazobenzene molecules can be reoriented under two-photon excitation by the polarized femtosecond laser via a photoisomerization process. Polarization-multiplexed and multilevel storage is demonstrated by using this method. The capability to combine both advantages of these distinct techniques makes it a novel approach to obtain higher optical data density.

Confocal reflection readout thresholds in two-photon-induced optical recording

Confocal reflection readout thresholds in two-photon-induced optical recording in photoisomerization polymer are studied both theoretically and experimentally. A threshold of the axial response from a planar reflector with a refractive-index change of the order of 10(-2) is revealed. However, the threshold is reduced to 0.006 when strong forward scattering caused by the recorded bits leads to multiple reflection between the bit and the rare surface, which enhances the image contrast and reduces the readout threshold. The quality of the reconstructed bit image is strongly dependent on the refractive-index mismatch at the sample rare interface as well as the distance between the recorded position and the rare surface.

Nanoparticle-Based Photorefractive Polymers

Photorefractivity has attracted intense attention owing to its ability to spatially modulate the refractive index under non-uniform light illumination. In particular, photorefractive polymers are appealing materials as they enable the high non-linear performance that underpins many areas of photonics. The incorporation of nanoparticles into photorefractive polymers shows an enormous potential owing to the broad spectroscopic tuning range and the high photogeneration efficiency, which are inaccessible to traditional photorefractive materials. This article reviews the recent developments in the field of nanoparticle-doped photorefractive polymers. The merit and functionality of these hybrid materials are summarized and future challenges are discussed. The application of nanoparticle-doped photorefractive polymers under two-photon excitation is also described, which facilitates a promising new area of high-density optical data storage, the third-generation of optical data storage.

Rewritable polarization-encoded multilayer data storage in 2,5-dimethyl-4-(p-nitrophenylazo)anisole doped polymer

We report a rewritable polarization-encoded multilayer data storage method with a polymer film doped with the azo dye DMNPAA (2,5-dimethyl-4-(p-nitrophenylazo)anisole). It is found that under two-photon excitation by a linearly polarized femtosecond laser beam at wavelength 780 nm the optical axis of DMNPAA molecules can be oriented to the perpendicular direction of the beam via a trans-cis-trans isomerization process. As a result, multilayer polarization-encoded optical data storage is demonstrated by recording two letters of a bit spacing of 4 microm in the same region of a given layer. It is shown that erasing and rewriting a particular layer is possible.

Multiphoton excited fabricated nano and micro patterned extracellular matrix proteins direct cellular morphology

We use multiphoton excited (MPE) photochemistry to fabricate patterned extracellular matrices (ECM) and to investigate the morphology of human dermal fibroblasts adhered to the resulting photocrosslinked linear structures of fibronectin (FN), fibrinogen (FG), and bovine serum albumin (BSA). These proteins were chosen to systematically investigate the roles of topography and ECM biochemistry on cell spreading, as fibroblasts bind directly to both FN and FG at RGD sites through known integrins, whereas BSA provides no comparable ECM cues for cell binding. MPE crosslinked patterns are created from parallel linear structures 600 nm in width, 200 microm in length, and spaced by either 10 or 40 microm. Immunofluorescence staining of FN and FG was used to assay the functionality of crosslinked proteins. The metrics of orientation, elongation, and cell perimeter were used to quantitate the resulting cellular behavior on the crosslinked protein patterns. These parameters all reflect statistical differences for cells on BSA, relative to the similar statistical behavior on fibronectin and fibrinogen. Cells on the BSA patterns are constrained by physical guidance and orientation between linear structures. In contrast, cells adhered on both FN and FG had a greater propensity to spread across adjacent structures, indicating the importance of cell matrix interactions. Focal adhesion staining of cells adhered to the protein structures revealed similar trends. These findings are consistent with our hypothesis that these crosslinked matrix protein structures are expected to direct cell adhesion and spreading and that the topography and ECM cues lead to different forms of guidance.

Light propagation in a multilayered medium for three-dimensional optical memory

We report on the optimization of film thicknesses in a multilayered medium to increase readout signal intensity. The multilayered medium consists of a stack of photosensitive and transparent films, arranged alternately. The thicknesses of the photosensitive and transparent films in the multilayered medium were optimized for a reflection confocal system to read out data by analyzing the propagation of light focused into a multilayered medium.

Measurement of normal and anomalous diffusion of dyes within protein structures fabricated via multiphoton excited cross-linking

We demonstrate microscale spatial and chemical control of diffusion within protein matrixes created through the use of nonlinear multiphoton excited photochemistry. The mobility of fluorescent dyes of different mass and composition within controlled cross-linked environments has been measured using two-photon excited fluorescence recovery after photobleaching (FRAP). The diffusion times for several rhodamine and sulforhodamine dyes within these fabricated structures were found to be approximately 3-4 orders of magnitude slower than in free solution. The precise diffusion times can be tuned by varying the laser exposure during the fabrication of the matrix, and the diffusion can be correlated with the mesh size determined by TEM and Flory-Rehner analysis. We find that the hydrophobic Texas Red dyes (sulforhodamines) exhibit diffusion that is highly anomalous, indicative of a strong interaction with the hydrophobic cross-linked protein matrix. These results suggests the use of these cross-linked protein matrixes as ideal model systems in which to systematically study anomalous diffusion. Finally, the diffusion can be tuned within a multilayered protein matrix, and this in conjunction with slow diffusion also suggests the use of these structures in controlled release applications.

Properties of crosslinked protein matrices for tissue engineering applications synthesized by multiphoton excitation

We demonstrate the fabrication of model scaffolds and extracellular matrices using multiphoton excited photochemistry. This method is three-dimensional in nature and has excellent biocompatibility. Crosslinked matrices were fabricated from the proteins fibrinogen, fibronectin, and concanavalin A using two-photon rose bengal photoactivation and the relatives rates were determined. Immunofluorescence labeling of fibrinogen and fibronectin indicated retention of bioactivity following the multiphoton crosslinking process. Using the fluorescence recovery after photobleaching method, we measured the lateral mobility of fluorescent dyes of different mass and chemistry in order to model the behavior of therapeutic agents and bioactive molecules and found diffusion coefficients within these fabricated structures to be on the order of 10(-9)-10(-10) cm(2)/s, or approximately three to four orders of magnitude slower than in free solution. The precise diffusion coefficients can be smoothly tuned by varying the laser exposure during the fabrication of the matrix, which results in both an increase in crosslink density as well as protein concentration in the matrix. Terminal crosslink density is achieved at integrated high exposure dose and the relative fabrication rates were determined for these proteins. For all the proteins, the range of diffusion coefficients between the threshold for fabrication and the terminal limit is correlated with the change in matrix mesh size as determined by Flory-Rehner swelling analysis. Both normal Fickian as well as hindered anomalous diffusion is observed depending on specific molecular interactions of the tracer dyes and protein host. (c) 2004 Wiley Periodicals, Inc. J Biomed Mater Res 71A: 359-368, 2004.

Compact confocal readout system for three-dimensional memories using a laser-feedback semiconductor laser

We present a compact confocal readout system for three-dimensional optical memories that uses the thresholding property of a semiconductor laser for feedback light. The system has higher axial resolution than a conventional confocal system with a pinhole. We demonstrate readout results for data recorded in two recording layers with the developed system.

Active aberration correction for the writing of three-dimensional optical memory devices

We describe an active optical system that both measures and corrects the aberrations introduced when writing three-dimensional bit-oriented optical memory by a two-photon absorption process. The system uses a ferroelectric liquid-crystal spatial light modulator (FLCSLM) configured as an arbitrary wave-front generator that is reconfigurable at speeds as great as 2.5 kHz. A method of aberration measurement by the FLCSLM wave-front generator is described. The same device is also used to correct the induced aberrations by preshaping the wave fronts with the conjugate phase aberration as well as to scan the focal spot in three dimensions. Experimental results show the correction of both on- and off-axis aberrations, allowing the writing of data at depths as great as 1 mm inside a LiNbO3 crystal.

Three-dimensional observation of internal defects in semiconductor crystals by use of two-photon excitation

We present a method of three-dimensional observation of internal defects in semiconductor crystals for blue lasers by use of two-photon excitation. We excite photoluminescence by using a two-photon process. Since semiconductor materials have intrinsically high absorption in the short-wavelength region, the excitation light of photoluminescence is largely absorbed by the crystals. It is difficult to observe defects in deep regions. Two-photon excitation can overcome this limitation because near-infrared light can be used instead of blue light for excitation of photoluminescence, and the near-infrared light is absorbed at only the focused point. The excitation light can penetrate into the deep regions of the crystal. We succeeded in observing defects in a ZnSe crystal ~200 microm below the crystal surface. This is, as far as we know, the first demonstration of the observation of three-dimensional structures of defects in semiconductor crystals by the use of two-photon excitation.

Three-dimensional coherent transfer function for reflection confocal microscopy in the presence of refractive-index mismatch

The three-dimensional (3-D) coherent transfer function for reflection confocal microscopy of high-numerical-aperture objectives is derived and calculated in the presence of refractive-index mismatch when a laser beam is focused into a medium of refractive index different from its immersion medium. This aberrated coherent transfer function is then used to estimate the readout efficiency of 3-D data bits recorded in a thick medium. It is shown that the readout efficiency of confocal microscopy for 3-D bit data storage is decreased with the focal depth of an objective in a recording medium. However, a high readout efficiency can be maintained if the tube length of a reading objective is linearly altered to compensate for the spherical aberration caused by the refractive-index mismatch.

Three-dimensional imaging analysis of confocal and conventional polarization microscopes by use of Mie scattering theory

We present a three-dimensional imaging analysis of confocal and conventional polarization microscopes by using the extended Mie scattering theory. In the analysis, we calculate the images of a Mie particle whose diameter is comparable with the wavelength of confocal and conventional microscopes. It was found that, when we observe a Mie particle, polarization confocal microscopy is not affected by the polarization distortion that is due to focusing with high-numerical-aperture lenses and does not produce pseudopeaks in the images in comparison with conventional polarization microscopy. The three-dimensional resolution of the polarization microscope and the verification of the proposed analysis method are also discussed.

Polarization-multiplexed optical memory with urethane-urea copolymers

We present a polarization-multiplexed optical memory with urethane-urea copolymers. The side chains of the urethane-urea copolymers induce cis-trans isomerization by illumination of blue or green light, and they align perpendicular to the linear polarization of the illuminated light, thus producing optical anisotropy. We found that the material showed selective anisotropy for the particular direction that was perpendicular to that of the recording beam polarization. By use of the anisotropic property three different data pages were multiplexed at the same spot of the medium. Erasure of the recorded bit data is also demonstrated.

Use of two-photon excitation for erasable-rewritable three-dimensional bit optical data storage in a photorefractive polymer

We report what is believed to be the first use of a photorefractive polymer in erasable-rewritable three-dimensional bit optical data storage under two-photon excitation. We successfully demonstrate writing, erasing, and rewriting of multilayered information in a photorefractive polymer consisting of 2,5-dimethyl-4-(p-nitrophenylazo)anisole, 2,4,7-trinitro-9-fluorenone, 9-ethylcarbazole, and poly(N-vinylcarbazole). A three-dimensional bit density of 5 Gbits/cm(3) is achieved by two-photon absorption under pulsed beam illumination at an infrared wavelength of 800 nm in the recording process. Complete erasing of the recording information is achieved by use of ultraviolet illumination.

Use of continuous-wave illumination for two-photon three-dimensional optical bit data storage in a photobleaching polymer

We show that a continuous-wave laser beam at an infrared wavelength of 800 nm can be used for two-photon three-dimensional bit data storage in a photobleaching polymer. We successfully demonstrate recording and reading of six layers of data bits with a transverse bit separation of 4.3microm and an axial layer separation of 20microm . This result leads to a three-dimensional bit density of approximately 3Gbits/cm(3) .

Reflection confocal microscope readout system for three-dimensional photochromic optical data storage

We propose to use a reflection confocal microscope (RCM) as a readout system for digital data stored in a three-dimensional (3D) photochromic optical memory. We describe the merits and the difficulties for 3D optical memory that are associated with the use of such a RCM.It is shown by means of 3D Fourier space analysis that successful reading of 3D data can be obtained by selection of the appropriate parameter for the RCM.The system parameters include the numerical aperture of the objective lens and the wavelength of the light used for analysis (reading). Experimental results of multilayer recording and reflection confocal reading in photochromic-molecule-doped poly(methyl methacrylate) are presented. Good-contrast images are obtained.

Reflection-type confocal readout for multilayered optical memory

We present a multilayered optical memory for use in reading data with a confocal reflection microscope system. We use a recording medium in which photosensitive thin films and nonphotosensitive transparent films are stacked alternately. Since the photosensitive films are thinner than the depth of focus of the recording beam, the spatial frequency distribution of the recorded bit data is extended in the axial direction. The extended distribution overlaps the coherent optical transfer function of the reflection-type confocal microscope. Urethane-urea copolymer film is used as a photosensitive material. The recording and reading of two layers are demonstrated.