Solving the problem of UV radiation in holographic data storage technology



    From time immemorial, a person who possesses some information has tried to keep it. The reason for this could be the desire to re-use this information or the desire to pass it on to the next generations. In any case, to save information, you need a “container” where it will be stored. The first such carriers of information were rocks, on which ancient people depicted various events from their lives (hunting, life, observations of the surrounding world, etc.). Now we have stepped far forward from the pictures on the rocks. Optical disks, HDD, SDD, flash memory and other media have become absolutely ordinary things for us. However, what can you say about the use of holographic technologies for storing information? This non-standard method is not new, but only recently have scientists been able to solve a fundamental problem which prevented this technology from migrating from theory to practice. What is the problem, how was it solved and should we wait for the data storage revolution? We will try to answer these and other questions today. Go.

    Brief history of HVD


    HVD (or Holographic Versatile Disc) is a holographic multipurpose disk. Back in 1963, Peter van Heerden, a scientist at Polaroid, proposed a method for “bulk storage” of data. Since then, many companies have led their development in this area.


    HVD and DVD

    In 2000, Sony announced the development of UDO (Ultra Density Optical - High Density Optical Media), which could store about 30 GB. Already in 2007, the format was updated to UDO2 with a storage capacity of up to 60 GB.

    In 2006, New Medium Enterprises introduced a new and revolutionary format to the public - HD VMD. After 2 years, the first sales began.


    VMD player from New Medium Enterprises

    However, as we know, holographic carriers of the shelves of electronics stores are not overwhelmed until now. And there are reasons for that a little later.

    VMD technology principle



    Schematic illustration of the layers of the disk and the impact on them of laser beams (Source: en.wikipedia.org) :

    • 1 - green laser read / write (532 nm);
    • 2 - red positioning / index laser (650 nm);
    • 3 - hologram (data);
    • 4 - polycarbonate layer;
    • 5 - photopolymer (photopolimeric) data layer;
    • 6 - the dividing layer (Distans layers);
    • 7 - a layer reflecting green color (Dichroic layer);
    • 8 - aluminum layer reflecting red light; 9 - transparent base; P - recesses (pitas)

    The first thing that catches your eye is the presence of two lasers - green and red. Information is recorded in the form of a holographic image due to the connection of these two rays (due to their coherence). The green beam is the reference and contains no data. The red beam passes through an optical modulator that changes the characteristics of the beam. When two beams intersect, a holographic image is formed in the interference zone. As a result, we can store data in a three-dimensional form, and not in a two-dimensional one, as is the case with conventional optical media.



    * A rough example : Imagine that there is a room that needs to be filled with boxes. If you arrange them on the floor area, then the boxes will fit much less. And if you use the entire space from floor to ceiling, then you can place many more boxes.

    New from China



    Northeastern Pedagogical University

    As we have already understood, there are many people who want to create their own HVD. Each of the companies researching this technology contributes something new to its overall development. Scientists from Northeast Normal University (Northeast Pedagogical University) did not want to stand by. They managed to develop a new type of film that will serve as a repository of holographic images (that is, data). Such a medium will have high data density, excellent read / write speed and will be able to survive drastic environmental changes. In addition, they also managed to cope with the problem of the fragility of holographic carriers due to exposure to ultraviolet radiation.

    The process of making a prototype


    The main substance used in research was octahedrite, one of the polymorphic modifications of TiO 2 (titanium oxide). The glass base is coated with a mixture of nanoparticles of titanium oxide (0.4 mol / l) and the block copolymer PEO20-PP070-PPO20 (20 g / l). Next, the base is lowered into a mixture of water and ethanol (in equal shares), obtained by the sol-gel method. The removal rate was 2 cm / s, which was necessary to create a uniform, transparent and smooth titanium oxide film. Next, to remove polymers, the film was subjected to calcification at 450 ° C for 1 hour.

    The next step was the immersion of a nanoporous titanium oxide film in a solution of phosphotungstic acid (concentration - 0.016 mol / l) for 5 hours. Such a long process is necessary so that the acid molecules are successfully absorbed by the surface of the film.


    Schematic representation of the process of creating a prototype nanoporous film.

    The resulting film was immersed in a mixture of 0.01 M silver nitrate (AgNO 3 ) —49 ml and ethanol — 1 ml. The silver nanopores were deposited on a film (titanium oxide with phospholophramic acid molecules) during UV irradiation at room temperature for 20 minutes. The final step was washing the film in deionized water and irradiating the UV for 5 minutes to reduce the residual Ag + ions.

    During ultraviolet irradiation, the sample became brown-gray due to the localized absorption of the surface plasmon resonance of deposited silver nanopores. The optical properties and morphology of the nanostructures of the sample were obtained using a UV-2600 UV-spectroscope and a plant-based electron microscope.

    Photoelectrochemical experiment


    This experiment was carried out at room temperature using a potentiostat (a device for automatic control of the electrode potential and support of its predetermined value). For the experiment, the standard three-electrode configuration was used:

    • tin oxide based glass - working electrode;
    • Ag / AgCl (silver chloride electrode) - reference electrode;
    • platinum black - counter electrode.

    The illumination was provided by a Hayashi LA-410 xenon lamp with a light intensity of 20 mW / cm 2 . The measurements were carried out in an electrolyte of 0.5 M Na 2 SO 4 (sodium sulfate) with a pH level of 5.8.

    Holographic recording process


    The diffraction grating was recorded using a coherent s-polarized laser beam (532 nm, 714 mW / cm 2 ). The intersection angle of the recording beams was set at 10 degrees. The power density of the writing rays was the same and was 57 mW / cm 2 . A red laser generating 671 nm s-polarized light was used to monitor the dynamics of the holographic grating. The power density of the 671 nm laser was set to 7 mW / cm 2to minimize the damaging effect of reading radiation, which leads to photochemical reactions. The first-order diffracted signal was recorded on a photodiode interfaced with a computer. The diffraction efficiency of holographic gratings can be defined as the ratio of the intensities of a diffraction beam of the first order and the incident beam after passing through the sample.


    Appearance of the system.

    In addition, one of the writing beams was expanded after the spatial filter, collimated to pass through the mask and focused in the center of the Ag / PW 12 / TiO 2 nanocomposite film.. Another beam was sent to the same place as the reference. Reconstructed holographic images were collected using a CMOS camcorder. A red laser (671 nm) was used as a test for reading a holographic image.


    A simplified diagram of the optical setting of a holographic recording, where:

    M - mirror is a mirror;
    BS - beam splitter;
    F - lens;
    BE - beam expande - beam expander;
    PD - photodiode - photodiode;
    Sample - sample;
    Mask - mask.


    Test results



    In the hands of the researcher the same sample that was used in testing

    Below are graphs and snapshots of test results with a description. For a more detailed study I recommend to get acquainted with the report of the research group, which you can find on the link or on this link (PDF document) .

    Film morphology and ultraviolet absorption spectra



    Image No. 3

    The images (a) and (c) show surface and transverse SEM images of a film of titanium oxide with phospholopramic acid molecules (PW 12 ). And on images (b) and (d) there is a film of titanium oxide without additional components. The thickness of each of the samples is 620 nm. A sample with PW12 shows a significantly smaller distribution of silver nanopores (about 14.7 nm) than a sample without PW 12 (about 21.2 nm). Such a difference may be due to the inhibition of aggregation of silver nanopores under the influence of UV radiation.

    Using acceptors, plasmonic silver nanopores (less than 30 nm) occupy approximately 98% of the volume fraction, which is a very good indicator for achieving a high level of efficiency and speed of photochromism (e) . And more widely distributed silver nanopores (from 4 to 52 nm) were obtained by direct contact of this metal with a film of titanium oxide (f) .

    In addition, the concentration of nanopores in the Ag / PW 12 / TiO 2 film is 7.94 Å ~ 109 / cm 2 , which is lower than that of the Ag / TiO 2 film (~ 9.42 Å ~ 109 / cm 2 ).


    Image # 4:
    (a) Diagram showing the reduction of silver nanopores on PW 12 filmsTiO 2 and TiO 2 due to UV exposure.
    (b) Spectroscopy of an Ag / PW 12 / TiO 2 film and Ag / TiO 2 films on a glass substrate (base) in the UV range.


    Among other things, the use of PW 12 provides additional electron transport channels in the photocatalytic deposition and transfer of electrons (a) . Photogenerated electrons from TiO 2 are distributed, and some of them can be transferred to PW 12 with UV excitation, which effectively slows down the deposition of silver nanopores. Delay effect tested in UV absorption spectra of an Ag / PW 12 / TiO sample2 was ~ 0.95, and for the Ag / TiO2 sample, approximately ~ 1.38 (b) .


    Image No. 5:
    (a) Linear sweep of voltammograms of electrons PW 12 / TiO 2 and electrons TiO 2 (scanning at a speed of 10 mV / s). The inset (lower right corner) shows the test results in the dark.
    (b) Electron transition process in an Ag / PW 12 / TiO 2 film when exposed to UV radiation.


    Modulation of reversible photochromism



    Image No. 6: Differential absorption of Ag / PW 12 / TiO 2 (a) and Ag / TiO 2
    (b) films alternately irradiated with green light (532 nm, 57 mW / cm 2 ) and UV radiation (360 nm, 71 mW / cm 2 ). Changes in the absorption of Ag / PW 12 / TiO 2 © and Ag / TiO 2 (d) with alternate exposure to green and UV radiation.



    Image No. 7: First-order diffraction efficiency of holographic gratings in an Ag / PW 12 / TiO 2 film (a) and in an Ag / TiO 2 film (b) when exposed to (s + s) green beam (record) and UV beam (erasing) for four cycles.

    The findings of researchers


    The use of holographic technology allows you to write and read millions of bytes at a time, which is much faster than using optical and magnetic media. Also an important advantage of this technology is the recording of data in three-dimensional form, which allows you to store more data on the carrier, the actual size of which does not increase. It is a kind of the most efficient use of space.

    The biggest problem is the detrimental effect of UV radiation, which tritely erases data from the media. However, the researchers managed to cope with this difficulty. It was for this purpose that such materials as silver and titanium oxide were used. With the help of a laser, silver particles were converted into silver cations with a positive charge due to additional electrons.

    One of the researchers, Shencheng Fu , says the following about this:
    We found that UV radiation can erase data because it causes electrons to transition from a semiconductor film to metallic nanoparticles, causing the same photon conversion as the laser. The introduction of electrons that “attract” molecules has led some electrons to pass from semiconductor into these molecules, reducing the erasable properties of UV radiation and creating an environmentally sustainable high-density data storage medium.
    The importance of nanopores lies in their ability to allow nanoparticles, electron attracting molecules, and semiconductor to interact with each other. And the incredibly small size of the molecules that attract electrons allows them to attach inside the pores, without affecting their structure. The final dimensions of the film were only 620 nanometers in thickness.

    The test results showed that data can be recorded on a new film even with constant exposure to UV radiation. And the use of molecules attracting electrons, forms a lot of transfer paths of electrons, which improved the response of the material to laser beams. And this, respectively, means an increase in the speed of data recording. As for the speed of reading data, according to scientists, it is about 1 GB / s.

    Shenchen Fu words:
    The use of noble metals, such as silver, in the field of optical storage is usually considered as a medium with a slow response. We have demonstrated that the use of electron fluxes increases the speed of the optical response of particles, while maintaining the other qualities of these particles that are useful for storing data.
    The next step in the study of this technology will be testing environmental stability outside the laboratory, so to speak under the open sky.

    Epilogue


    Researchers believe in their innovation. And for good reason, because they managed to cope with a problem that has existed for many years - the detrimental effect of UV radiation. What is the point in a holographic information carrier, if it can be used only in the dark, roughly speaking. Despite their success, scientists say that to use their workings, it will be necessary to create devices for reading new types of media. And it will take a lot of time and even more effort.

    In any case, as we already know perfectly well, any research (especially so successful) has a tremendous positive effect on the development of technology as such. Wait in the near future futuristic holographic media, the size of a plate of gum, of course not worth it. However, as we remember, even 15 years ago no one would have believed that in the future there will be flash drives, the volume of which will be measured not by megabytes, but by terabytes.

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