Information Encryption and 3D Optical Storage in Glass

By Samudrapom DamReviewed by Susha Cheriyedath, M.Sc.May 27 2022

A team of researchers recently published a paper in the journal ACS Energy Letters that demonstrated a new entirely photostimulation-induced reversible photochromic glass for three-dimensional (3D) optical storage applications.

Study: Entirely Reversible Photochromic Glass with High Coloration and Luminescence Contrast for 3D Optical Storage. Image Credit: Gorodenkoff/Shutterstock.com

Background

The significant growth of digital information in the modern era has necessitated the development of high-density storage media and optical storage technologies. Although several new optical storage technologies, such as multiwavelength optical storage, holographic optical storage, super-resolution optical storage, and near-field optical storage, have been developed, these technologies require sophisticated manipulation and expensive equipment, which restricted their use in many practical applications.

Photochromism has gained significant attention as an effective optical storage technology. The phenomenon is described as the photostimulation-induced reversible color change of materials. Currently, photochromic ceramics and thin films are used extensively for optical storage applications. However, these two-dimensional (2D) photochromic storage media lack high storage capacity as the optical information is only recorded on their surface.

The integration of multiple dimensions such as luminescence, phase, wavelength, and space into a single optical carrier can potentially increase the storage capacity. Glass, a versatile material with good transparency, is considered a suitable alternative for multidimensional optical storage applications. Thus, a high optical storage capacity can be achieved in photochromic glass in 3D bulk dimensions.

Lanthanide (Ln) ion-doped luminescent glasses have demonstrated significant potential for applications in the fields of nonlinear optics, energy storage, 3D displays, solid-state lasers, and optical communication. Recently, the luminescence in Ln-ion doped glasses was modulated successfully. For instance, the optical storage application and luminescence modification were achieved through the transformation of the valence state from samarium (III) ion (Sm³⁺) to Sm²⁺ when an Sm³⁺ -doped glass was illuminated using an 800 nm femtosecond laser.

Double-model rewritable optical storage can be realized by integrating luminescence modification and photochromism into a bulk glass. Thus, photochromic glasses with luminescence modification can potentially act as an alternative optical storage media.

For instance, a rare earth (RE) ion-doped photochromic tungsten phosphate glass demonstrated a 3D optical storage capacity and information encryption based on the reversible transformation of valence state from tungsten ion (W⁶⁺) to W⁵⁺. However, the glass was bleached using thermal stimulation, which restricted its applications. Thus, a completely photostimulation-induced reversible photochromic glass could be an attractive option for 3D optical storage applications as it does not require thermal treatment.

The Study

In this study, researchers synthesized an entirely photostimulation-induced reversible photochromic germanium borate glass doped with (RE-Ag) using the conventional melt-quenching method. The reversible photochromism mechanisms of the synthesized RE-Ag germanium borate glass were thoroughly evaluated under alternating simulation between a 690 nm laser light and 365 nm ultraviolet (UV) light by determining the decomposition and formation of Ag nanoparticles in the RE-Ag germanium borate glass host.

Dysprosium (III) oxide, europium (III) fluoride, AgCl, cuprous oxide, strontium carbonate, zinc oxide, boron trioxide, aluminum oxide, silicon dioxide, and germanium dioxide were used as raw materials to prepare Ag- and RE ions/dysprosium ion (Dy³⁺) or europium ion (Eu³⁺) ion-doped germanium borate glasses.

Initially, the raw materials were weighed, mixed, and ground using an agate mortar. Then, the obtained mixtures were melted at 1300 °C for 30 min in an alumina crucible under an ambient atmosphere.

Subsequently, the glass melt was transferred to a stainless-steel container preheated at 350 °C and left undisturbed for 20 min. The as-prepared bulk glasses were then heat-treated for one h at 600 °C to crystallize AgCl nanocrystals. The rectangular glass was finally obtained by grinding and polishing.

An X-ray diffractometer, transmission electron microscope with energy dispersive X-ray spectrometer, X-ray photoelectron spectroscope, infrared spectrometer, paramagnetic resonance spectrometer, Hitachi F-7000 spectrophotometer using a xenon (Xe) lamp as the light source, and an Edinburgh FLS 980 instrument were used to characterize the synthesized glass.

Observations

Researchers successfully synthesized an entirely photostimulation-induced reversible photochromic RE-Ag germanium borate glass. The germanium borate glass possessed a high defect tolerance and a less rigid environment, which was beneficial for the AgCl nanocrystal precipitation from borosilicate glasses. The photostimulation-induced reversible color change was attributed to the decomposition and precipitation of Ag microcrystals in the glass host.

Intricate optical information patterns were erased and written in the photostimulated glass by alternating a 690 nm laser and 365 nm UV light. An optical information readout was realized after the UV light irradiation by exciting the glass using an excitation light from a Xe lamp.

Both luminescence modulation and photochromic behavior properties demonstrated high repeatability and fatigue resistance. The change of the Ag element valence state was primarily responsible for the photochromic mechanism. The Ag nanoparticle precipitation due to UV light irradiation induced darkening of the glass.

In the Eu³⁺-doped germanium borate (Eu-Ag) glass, the reversible downshifted luminescence modification was attained due to decolorization and photochromism. The reversible photochromism/coloration contrast from transparent to black and luminescence modulation was 91% and 93%, respectively, when the glass was alternatively irradiated by 365 nm UV light and 690 nm laser light for 30 s.

Taken together, the findings of this study demonstrated that the synthesized photostimulation-induced AgCl and RE-ion doped photochromic glass possesses a significant potential for information encryption and storage applications.

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