Three-dimensional Optical Memory Research Articles

Helical plasma filaments from the self-channeling of intense femtosecond laser pulses in optical fibers.

We experimentally and numerically study the ignition of helical-shaped plasma filaments in standard optical fibers. Femtosecond pulses with megawatt peak power with proper off-axis and tilted coupling in the fiber core produce plasma skew rays. These last for distances as long as 1000 wavelengths thanks to a combination of linear waveguiding and the self-channeling effect. Peculiar is the case of graded-index multimode fibers; here the spatial self-imaging places constraints on the helix pitch. These results may find applications for fabricating fibers with helical-shaped core micro-structuration as well as for designing laser components and three-dimensional optical memories.

Multitarget imaging through scattering media beyond the 3D optical memory effect.

A robust method for efficient spatial separation optical imaging through scattering media regardless of the three-dimensional (3D) optical memory effect is proposed. In this method, the problems of imaging dealiasing, decomposition, and separation of speckle patterns are solved by employing independent component analysis. Multitarget imaging behind a scattering layer with diverse spatial positions is observed experimentally, for the first time, to the best of our knowledge. In this work, we demonstrate that, by knowing the number of targets and keeping each subtargets' size in the optical memory effect range while isolating them beyond this range without overlap in the axial direction, speckle dealiasing and multitarget imaging are solved effectively. The strategy provides a potentially useful means for incoherent imaging through scattering media in a wide class of fields such as optical microscopy, biomedical imaging, and astronomical imaging.

Taking Two-Photon Excitation to Exceptional Path-Lengths in Photonic Crystal Fiber

The well-known, defining feature of two-photon excitation (TPE) is the tight, three-dimensional confinement of excitation at the intense focus of a laser beam. The extremely small excitation volume, on the order of 1 μm3 (1 femtoliter), is the basis of far-reaching applications of TPE in fluorescence imaging, photodynamic therapy, nanofabrication, and three-dimensional optical memory. Paradoxically, the difficulty of detecting photochemical events in such a small volume is a barrier to the development of the two-photon-activated molecular systems that are essential to the realization of such applications. We show, using two-photon-excited fluorescence to directly visualize the excitation path, that confinement of both laser beam and sample solution within the 20 μm hollow core of a photonic crystal fiber permits TPE to be sustained over an extraordinary path-length of more than 10 cm, presenting a new experimental paradigm for ultrasensitive studies of two-photon-induced processes in solution.

Directed evolution of bacteriorhodopsin for applications in bioelectronics

In nature, biological systems gradually evolve through complex, algorithmic processes involving mutation and differential selection. Evolution has optimized biological macromolecules for a variety of functions to provide a comparative advantage. However, nature does not optimize molecules for use in human-made devices, as it would gain no survival advantage in such cooperation. Recent advancements in genetic engineering, most notably directed evolution, have allowed for the stepwise manipulation of the properties of living organisms, promoting the expansion of protein-based devices in nanotechnology. In this review, we highlight the use of directed evolution to optimize photoactive proteins, with an emphasis on bacteriorhodopsin (BR), for device applications. BR, a highly stable light-activated proton pump, has shown great promise in three-dimensional optical memories, real-time holographic processors and artificial retinas.

Low energy-density recording with a high-repetition-rate laser beam in gold-nanorod-embedded discs

In this paper, we report on the low energy-density recording with a high-repetition-rate femtosecond pulsed beam in homogenous gold-nanorod-dispersed discs by using low numerical aperture (NA) micro-optics. By focusing a femtosecond pulsed beam at a repetition rate of 82 MHz using a low NA DVD optical head, the spatially-stretched energy density introduces a temperature rising of the polymer matrix. This temperature rising facilitates the surface melting of gold nanorods, which leads to over one-order-of-magnitude reduction in the energy-density threshold for recording, compared with that by focusing single pulses through a high NA objective. Applying this finding, we demonstrate the dual-layer recording in gold-nanorod-dispersed discs with an equivalent capacity of 69 GB. Our results demonstrate the potential of ultra-high density three-dimensional optical memory with a low-cost and DVD-compatible apparatus.

Three-Dimensional Optical Bit Memory in Sm(DBM)<sub>3</sub>Phen-Doped and Un-Doped Poly(methyl methacrylate)

The feasibility of three-dimensional optical bit memory is demonstrated by using the change of fluorescence and refractive index in Sm(DBM)3Phen-doped and un-doped Poly(methyl methacrylate). After a femtosecond pulsed laser irradiation, a refractive-index bit and a fluorescent bit can be formed at the same position inside the bulk sample. Multilayer patterns recorded by tightly focusing the pulsed laser beam were read out by a reflection-type fluorescent confocal microscope, which can detect the reflection signal and also the fluorescent signal of the stored bits. The signal-to-noise ratio via the two retrieval modes was compared as a function of recording depth. The stored bits were retrieved with a high signal-to-noise ratio in the absence of any crosstalk and the detection of the fluorescent signal enables retrieval of the stored bits with a higher S/N ratio.

Ce(DBM) 3Phen doped poly(methyl methacrylate) for three-dimensional multilayered optical memory by femtosecond laser

The formation of submicrometer voids within Ce(DBM) 3Phen doped poly(methyl methacrylate)(PMMA) was reported under multiphoton absorption excited by an infrared laser beam. The absorption spectra, photoluminescence and electron spin resonance (ESR) spectra before and after femtosecond laser irradiation were discussed. An ultrashort-pulsed laser beam with a pulse width of 200 femtosecond at a wavelength of 800 nm was focused into doped PMMA. The large changes in refractive index and the fluorescence associated with a void allowed conventional optical microscopy and reflection-type confocal microscopy to be used as detection methods. Voids could be arranged in a three-dimensional multilayered structure for high-density optical data storage. The separation of adjacent bits and layers were 4 and 16 μm, respectively.

Three-Dimensional Two-Photon Bit-Recording With a Compact Fiber Laser

We presented a three-dimensional optical memory using a newly developed compact fiber laser. The fiber laser can produce pulse with high peak power of 5.3 kW. This allows two-photon absorption recording for next generation optical recording media possible for consumer application. We demonstrated two-photon absorption recording on a photochromic materials; diarylethene and achieved multilayer recording using the developed fiber laser.

Multilayered optical bit memory with a high signal-to-noise ratio in fluorescent polymethylmethacrylate

We report on the three-dimensional optical memory utilizing a photoluminescence (PL) change in polymethylmethacrylate. Irradiation with a femtosecond pulsed laser (800 nm, 1 kHz, 100 fs) induced a strong PL spectrum in the visible range, which may result from the photogeneration of emissive radicals. Multilayered patterns were recorded inside the bulk sample by tightly focusing a pulsed laser beam. The pattern images were read out by a reflection-type fluorescent confocal microscope which detected the blue-green emission at 410–510 nm. The stored bits were retrieved with a high signal-to-noise ratio in the absence of any cross-talk.

Synthesis and study of photosensitive chromone derivatives for recording media of archival three-dimensional optical memory

The synthesis and photochemical study of 3-acetyl-2(2’-furyl)chromone derivatives are described, which are of interest for the creation of photo-induced irreversible luminescent media for one-time superdense recording on a multilayer carrier. New thiophene-containing 3-acetyl2(2’-furyl)chromone derivatives are prepared with examples of their chemical modification and their photochemical properties studied, including spectral data for the individual substances and those incorporated in polymeric matrices. These show prospects as light-sensitive components of recording media for archival three-dimensional optical memory.

Compact and High-Power Mode-Locked Fiber Laser for Three-Dimensional Optical Memory

We developed a compact and high-power mode-locked fiber laser for three-dimensional optical memory. Fiber lasers have the potential to be compact and stable light sources that can replace bulk solid-state lasers. To generate high-power pulses, we used stretched-pulse mode locking. The average power and pulse width of the output pulse from the fiber laser that we developed were 109 mW and 2.1 ps, respectively. The dispersion of the output pulse was compensated with an external single-mode fiber of 2.5 m length. The pulse was compressed from 2.1 ps to 93 fs by dispersion compensation. The pulse emitted from this fiber laser has an energy sufficient to generate two photon recording effectively, so the fiber laser we have developed is possible to use as a light source of three-dimensional optical memory.

Two-photon energy transfer enhanced three-dimensional optical memory in quantum-dot and azo-dye doped polymers

A three-dimensional bit-by-bit optical memory system in quantum-dot and azo-dye doped polymers is demonstrated in this paper. It is shown that two-photon absorption can be used to excite resonance energy transfer from luminescent CdS quantum dots to azo-dye molecules. As a result, the isomerization of azo-dye molecules leads to a pronounced refractive index change. This two-photon-excited energy transfer process in the quantum-dot and azo-dye polymer not only allows for high efficient multilayer information recording into the volume of thick samples but also offers a refractive index window for readout.

Effect of PbO on precipitation of laser-induced gold nanoparticles inside silicate glasses

We report on three-dimensional precipitation of Au nanoparticles in gold ions-doped silicate glasses by a femtosecond laser irradiation and further annealing. Experimental results show that PbO addition plays the double roles of inhibiting hole-trapped centers generation and promoting formation and growth of gold nanoparticles. Additionally, glass containing PbO shows an increased non-linear absorption after femtosecond laser irradiation and annealing. The observed phenomena are significant for applications such as fabrications of three-dimensional multi-colored images inside transparent materials and three-dimensional optical memory, and integrated micro-optical switches. (c) 2007 Elsevier B.V. All rights reserved.

Femtosecond laser-induced microfeatures in glasses and their applications

In the past decades, femtosecond laser has been widely used to study the interaction between light and matter, and dynamic processes of various reactions. In this article, we review femtosecond laser-induced microfeatures in glasses and their promising applications in micro-optics and optoelectronics. Various highly localized microstructures, e.g. color center defects, refractive index change, micro-void and micro-crack, have been observed in the glasses after the femtosecond laser irradiation. The mechanisms of the observed phenomena are discussed. We also demonstrate the fabrication of various micro-optical components, e.g. three-dimensional optical memory, optical waveguide, micro-grating, micro-lens and fiber attenuator, by using the femtosecond laser-induced structures.

Design and Fabrication of Thin-Film Diarylethene Recording Layer and Its Reflective Reproduction for Super-Multilayered Optical Memories

We studied the feasibility of two-photon absorption recording and reflective reproduction for three-dimensional (3D) optical memories by comparison between bulk and super-multilayered media. The basic design of such a super-multilayered structure for achieving a high reflectance from a pit and for reducing layer thickness was described. To evaluate basic optical characteristics, two types of sample with a 100% diarylethene recording layer were fabricated. It was experimentally confirmed that reflective reproduction can be possible in the super-multilayered structure, whereas reflective signals were not obtained in the bulk structure as indicated by the results of theoretical analysis. Therefore, 3D recording and reproducing in a super-multilayered medium would be promising for a future high-capacity memory system.

Laser-matter interaction in the bulk of a transparent solid: Confined microexplosion and void formation

We present here the experimental and theoretical studies of a single femtosecond laser pulse interaction inside a bulk of transparent media (sapphire, glass, polymer). This interaction leads to the drastic transformations in a solid resulting in a void formation inside a dielectric. The laser pulse energy is absorbed within a volume of approximately $0.15\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}{\mathrm{m}}^{3}$ creating a pressure and temperature comparable to that in the core of a strong multi-kilo-tons explosion. The material within this volume is rapidly atomized, ionized, and converted into a tiny super-hot dense cloud of expanding plasma that generates strong shock and rarefaction waves which result in the formation of a void, whose diameter is $\ensuremath{\sim}200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ (for a $100\phantom{\rule{0.3em}{0ex}}\mathrm{nJ}$ pulse in sapphire). The way this structure forms can be understood from high-temperature plasma hydrodynamics. We demonstrate that unique states of matter characterized by temperatures $\ensuremath{\sim}{10}^{5}\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, heating rates up to the ${10}^{18}\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{s}$, and pressures more than 100 times the strength of any material were created using a standard table-top laser in well-controlled laboratory conditions. We discuss the properties of the laser-affected solid and possible routes of laser-affected material transformation to the final state long after the pulse end. These studies will find application for the design of new materials and three-dimensional optical memory devices, and for formation of photonic band-gap crystals.

Photopolymerizable recording media for three-dimensional holographic optical memory

The development of investigations in recent years on photopolymerizable recording holographic media for using as photosensitive layers of holographic optical disks, which satisfy the conditions for the application in archive optical memory is analyzed. It is shown that the main emphasis in the development of photopolymerizable holographic recording media is put on the improvement of their component composition. Formulations that permit joint radical and cationic photopolymerization were designed for thermally stable holographic recording. The holographic characteristics achieved for these media ensure the fabrication of single-layer optical disks 150 mm in diameter with the information capacity of 200 GB instead of 5 GB for modern optical disks. Ways of improving photopolymerizable holographic recording media for the manufacture of optical disks with an information capacity of more than 1 Tb cm−3 are considered.

Femtosecond laser induced PL change in Sm-doped sodium borate glass and 3D optical memory

We report on a femtosecond laser-induced photoluminescence (PL) change in Sm-doped sodium-borate glass and its potential application to three-dimensional optical memory. Irradiation with a femtosecond pulsed laser (800 nm, 1 kHz, 100 fs) induced a new PL peak near 682 nm, resulting from the photoreduction of Sm ions from Sm 3+ to Sm 2+. This makes it possible to readout the stored data by detecting the new PL peak as a signal. Multilayer patterns of various dot sizes and interlayer spacings were fabricated by femtosecond pulsed laser. These patterns were read out by a fluorescent confocal microscope which detected the emission at 682 nm as a signal. Here we discuss the feasibility of 3D optical memory by use of PL change in Sm-doped sodium borate glass.

Three-Dimensional Optical Recording in tert-Butoxycarbonyl-Protected Quinizarin Methacrylate Polymers

We report photoacid-induced nonerasable recording in tert-butoxycarbonyl-protected quinizarin methacrylate polymers for three-dimensional optical memory. 1 m/s-speed recording was achieved using an 800 nm femtosecond laser at an 80 MHz-repetition rate, without a postexposure bake after laser irradiation. Three-layer recording indicates that no optical damage during recording enables modulation of the Compact Disc type memory. The linewidth of recorded marks of 0.4 µm indicates a 49 Gbits/cm2 areal density when the numerical aperture is 1.35.

Optimization of Multilayered Media Structure for Three-Dimensional Optical Memory

We analyzed the optimum structure of recording media for a reflection confocal readout system of three-dimensional optical memory. In this analysis, we discuss the overlapping regions of the coherent transfer function of the reflection confocal readout system and the spatial frequency distribution of recorded bits. The intensity of the readout signal is proportional to the multiplication of the three-dimensional distribution of the coherent transfer function of the reflection confocal system and the spatial frequency distribution of recorded bits. We optimized film thickness to increase the signal from recorded bits. It was found that the readout signal increased twentyfold by optimization of film thickness. In addition, we optimized the relative refractive index of recording media by light propagation analysis of multilayered media. In the case of optimal film thickness, the ratio of refractive indices between recording layers and transparent layers should be 1.01 for obtaining maximum reflection.