Nondestructive readout of a three-dimensional photochromic optical memory with a near-infrared differential phase-contrast microscope

Design and Study of a Photo-Switchable Polymeric System in the Presence of ZnS Nanoparticles under the Influence of UV Light Irradiation

Recent progress in the field of photosensitive materials has prompted a need to develop efficient methods to synthesize materials with basic intermolecular architectural designs and novel properties. Accordingly, in this work we design and study a photoactive polymer as a photo-switchable polymeric system in the presence and absence of ZnS nanoparticles (average size < 10 nm) at 5 wt.%. The influence of UV light irradiation on its properties were also studied. The photoactive block copolymer was obtained from styrene (S) and methyl methacrylate (MMA) as monomers and 1-(2-hydroxyethyl)-3,3-dimethylindoline-6-nitrobenzopyran (SP) was grafted to the block copolymer backbone as a photochromic agent. Furthermore, the incorporation of ZnS (NPs) as photo-optical switch component into the system enhances the purple colored photo-emission, with the open form of the spiropyran derivative (merocyanine, MC). The ZnS stabilize the isomeric equilibrium in the MC interconversion of the photochromic agent. The photo-switchable properties of the PS-b-PMMA-SP in the presence of ZnS (NPs) were examined using UV-VIS spectroscopy, Photoluminescence (PL) spectroscopy, optical fluorescence and scanning electronic microscopy (SEM-EDX.). The observed changes in the absorbance, fluorescence and morphology of the system were associated to the reversible interconversion of the two states of the photochromic agent which regulates the radiative deactivation of the luminescent ZnS NPs component. After UV irradiation the photoactive polymer becomes purple in color. Therefore, these basic studies can lead to the development of innovative functional and nanostructured materials with photosensitive character as photosensitive molecular switches.

The influence of gold nanoparticles on the two photon absorption of photochromic molecular systems

In this study, we investigate the influence of gold nanoparticles on the nonlinear optical properties of the dihydroazulene/vinylheptafulvene photo- and thermochromic system.

Sub-Ångstrom electric field measurements on a universal detector in a scanning transmission electron microscope

Scanning transmission electron microscopy (STEM) excels in accessing atomic-scale structure and chemistry. Enhancing our ability to directly image the functionalities of local features in materials has become one of the most important topics in the future development of STEM. Recently, differential phase contrast (DPC) imaging has been utilized to map the internal electric and magnetic fields in materials from nanoscale features such as p-n junctions, skyrmions, and even from individual atoms. Here, we use an ultra-low noise SCMOS detector in as the diffraction plane camera to collect four-dimensional (4D) datasets. The high angular resolution, efficient high-SNR acquisition, and modifiability of the camera allow it to function as a universal detector, where STEM imaging configurations, such as DPC, bright field, annular bright field, and annular dark field can all be reconstructed from a single 4D dataset. By examining a distorted perovskite, DyScO3, which possesses projected lattice spacings as small as 0.83 Å, we demonstrate DPC spatial resolution almost reaching the information limit of a 100 keV electron beam. In addition, the perovskite has ordered O-coordinations with alternating octahedral tilts, which can be quantitatively measured with single degree accuracy by taking advantage of DPC's sensitivity to light atoms. The results, acquired on a standard Ronchigram camera as opposed to a specialized DPC detector, open up new opportunities to understand and design functional materials and devices that involve lattice and charge coupling at nano- and atomic-scales.

Fourier ptychographic microscopy using an infrared-emitting hemispherical digital condenser

Fourier ptychographic microscopy is demonstrated in the near-infrared spectral range using a computer-controlled hemispherical digital condenser comprising multiple 940 nm wavelength light emitting diodes. This technique was used to image periodic patterned samples (photonic crystals). Experimental and simulated results using a phase retrieval algorithm were found to be in excellent correspondence. We show that for samples with a single period in each direction, the resolution of the obtained high-resolution near-infrared images is limited by the Rayleigh criteria.

Efficient photochromic transformation of a new fluorenyl diarylethene: one- and two-photon absorption spectroscopy

Efficient reversible phototransformation of a new diarylethene-fluorene derivative, 1,2-bis(5-(9,9-didecyl-7-nitro-9H-fluoren-2-yl)-2-methylthiophen-3-yl)cyclopent-1-ene (1), was demonstrated in organic media under low-intensity laser excitation. Linear photophysical characterization of 1 was performed at room temperature in solvents of different polarity and viscosity. Significantly, close to unity quantum yield for the cyclization reaction of 1 was shown in nonpolar solutions. The lifetimes of the excited states of the open (OF) and closed (CF) forms of 1 were measured by a femtosecond transient absorption technique, and corresponding values of ∼0.7 and ∼0.9 ps were shown in dichloromethane (DCM), respectively. Degenerate two-photon absorption (2PA) spectra of the OF and CF of 1 were obtained over a broad spectral range by the open aperture Z-scan method under 1 kHz femtosecond excitation. The values of 2PA cross sections of the OF in DCM (∼50-70 GM) were found to increase up to 1 order of magnitude (∼600 GM) after cyclization to the CF. The nature of cyclization and cylcoreversion processes were investigated by quantum chemistry with employment of DFT-based methods implemented in the Gaussian'09 program. The potential of 1 for application in optical data storage was shown using poly(methyl methacrylate)-doped films and two-photon fluorescence microscopy readout.

Fabrication of multilayered photochromic memory media using pressure-sensitive adhesives

We fabricated a multilayered medium by a laminating process with pressure-sensitive adhesives. It was possible to reduce the thickness variations of both photosensitive layers and transparent layers by applying laminating films. This method is easy to use to pile up many layers for a multilayered medium. We fabricated twenty recording layers and demonstrated the capability to record with significant reading results. The recorded data in each layer were read out without crosstalk. We evaluated the signal-to-noise ratio and crosstalk between neighboring layers. It was found that the signal-to-noise ratio of a multilayered medium was higher than 50 dB.

High-resolution optical storage by use of minute spheres

We have developed a new optical storage system that uses minute spheres arranged upon a surface-relief grating. By using dye-doped minute spheres as recording bits we can limit a sensitive region within a sphere's diameter. Using a reflection-type confocal optical system, we read out shape signals from minute spheres at high resolution. The shape signals from minute spheres are utilized as clock signals in recording and readout.

Theoretical study of signal-to-noise ratio on near-field photochromic memory with fluorescence readout

The signal-to-noise ratio (SNR) on the fluorescence readout of a near-field photochromic memory was theoretically studied. Under various conditions the shot-noise-limited SNR was analyzed. SNR by bright spot recording (BSR) that was better than that by dark spot recording (DSR) was obtained under the condition of low writing power or wide bandwidth. Under the condition of bandwidth W = 1 MHz and P(write) = 10(-8) W only BSR can attain sufficiently high SNR, and the SNR was greater by as much as 30 dB than that of DSR. It was concluded that BSR is a promising method for high-density near-field photochromic memory with a fluorescence readout.

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.

Numerical simulations of scattering in a two-photon optical data storage system

Scattering effects in a two-photon optical data storage system are numerically studied. Surface scattering analysis with a scalar, beam propagation model is performed. We analyze the problem by modeling scattering from randomly varying surfaces and also by Fourier surface decomposition. Scattering induced by propagation through multiple pages of randomly recorded data marks is also studied with a hybrid finite-difference-time-domain/angular-spectrum model. Both surface and bulk scattering are shown to influence the spatial properties of the optical beam. Results and some possible implications are presented.