Photoluminescent Microbit Inscripion Inside Dielectric Crystals by Ultrashort Laser Pulses for Archival Applications

At a Glance

Metadata	Details
Publication Date	2023-06-24
Journal	Micromachines
Authors	S. I. Kudryashov, П. А. Данилов, Nikita Smirnov, Evgeny V. Kuzmin, Alexey Rupasov
Institutions	P.N. Lebedev Physical Institute of the Russian Academy of Sciences
Citations	3
Analysis	Full AI Review Included

Executive Summary

This research demonstrates the feasibility of creating high-density, three-dimensional (3D) archival optical memory using photoluminescent (PL) microbits inscribed inside bulk dielectric crystals (LiF, CaF₂, and natural diamond) via ultrashort laser pulses.

Core Achievement: Successful inscription and 3D confocal read-out of PL microbit arrays using 525 nm, 0.2 ps laser pulses focused by a 0.65 NA objective in a sub-filamentation regime.
Storage Density Potential: Preliminary evaluation in LiF yields an optical storage density of 25 Gbits/cm³ for a simple cubic lattice, comparable to advanced 5D optical storage technologies.
Spatial Resolution: Minimal resolvable lateral separation was achieved at 1.5 µm (LiF), and longitudinal (interlayer) separation was measured at 11 µm (using 1 µm vertical resolution optics).
Mechanism: Inscription relies on the non-linear photoexcitation and aggregation of mobile Frenkel pairs (vacancies and interstitials) to form stable color centers (F-centers in fluorides, NV-centers in diamond).
Thermal Robustness Contrast: Diamond NV centers exhibit superior thermal stability, remaining stable up to 1200 °C, making them ideal for ultra-archival memory, while LiF centers anneal at moderate temperatures (200-300 °C).
Value Proposition: Paves the way for novel optomechanical memory storage platforms offering high capacity, low power consumption, and robustness against environmental factors (radiation, humidity).

Technical Specifications

Parameter	Value	Unit	Context
Laser Wavelength	525	nm	Second harmonic of Yb-crystal laser
Pulsewidth (FWHM)	0.2	ps	Writing pulse duration
Repetition Rate	80	MHz	Base repetition rate
Max Output Pulse Energy (E)	50	nJ	TEM₀₀ mode
Focusing Objective NA	0.65	N/A	Micro-objective used for inscription
Inscribed Spot Diameter (1/e)	≈1	µm	Lateral spot size
Peak Laser Fluence	<6	J/cm²	Inscription condition
Peak Laser Intensity	<30	TW/cm²	Inscription condition
Inscription Depth (Fluorides)	100	µm	Below surface
Inscription Depth (Diamond)	120	µm	Below surface
Inscription Exposure Range	10⁷ - 10⁹	pulses/microbit	Total exposure per spot
Minimal Lateral Separation (LiF)	1.5	µm	Robust optical read-out
Minimal Longitudinal Separation (LiF)	11	µm	Resolvable interlayer separation (1 µm resolution)
Optical Storage Density (LiF, Preliminary)	25	Gbits/cm³	Simple cubic lattice assumption
LiF Color Center Peak (F₂)	670	nm	Photoluminescence peak
Diamond Color Center Peak (NV^-)	637	nm	Zero-Phonon Line (ZPL)
Diamond Thermal Stability	Up to 1200	°C	NV centers remain stable
LiF Thermal Stability	200-300	°C	Color centers anneal (delete signal)
NV^- PL Yield Power Slope	5.5 ± 0.2	N/A	Highly non-linear dependence on pulse energy

Key Methodologies

The experiment utilized Direct Laser Inscription (DLI) followed by 3D-scanning confocal microscopy for characterization and read-out.

Material Preparation: Undoped LiF and CaF₂ slabs (5 mm thick) and IaA-type natural diamond bricks (2 mm thick) were used, selected for transparency at 525 nm.
Laser Inscription Setup:
- A TEMA Yb-crystal laser (525 nm, 0.2 ps, 80 MHz) was used, operating in a sub-filamentation regime (pulse energy E up to 50 nJ).
- Pulses were focused inside the bulk material (100-120 µm depth) using a 0.65 NA micro-objective, creating an ≈1 µm spot.
- Samples were mounted on a computer-driven 3D motorized micropositioning stage for precise spatial control.
Microbit Array Writing: Linear and square arrays of microbits were inscribed by varying pulse energy (2.5 nJ to 27 nJ) and exposure (10⁷ to 10⁹ pulses/microbit) at controlled transverse (1-5 µm) and longitudinal (1-28 µm) spacings.
Characterization (Read-Out):
- A Confotec MR520 3D-scanning confocal photoluminescence/Raman microscope was used for visualization.
- PL imaging was performed using a 532 nm continuous-wave pump laser and high-NA objectives (1.45 NA for high resolution).
- PL spectra were acquired to identify the specific color centers generated (e.g., F₂/F₃ in LiF, NV⁰/NV^- in diamond).
Thermal Stability Testing:
- LiF samples were annealed in a temperature-controlled mount (25-300 °C) for 30 minutes of stationary heating.
- Diamond samples were annealed in an evacuated oven (25-1200 °C) for 1 hour to test NV center stability.

Commercial Applications

This technology is highly relevant to industries requiring ultra-robust, high-density, and long-term data storage, particularly leveraging the stability of diamond NV centers.

Archival Data Storage: Development of next-generation 3D optical memory disks (e.g., 120 mm diameter, 10 mm thickness) capable of storing several Tbits per disk, offering superior longevity and robustness compared to current CD/DVD/Blu-ray standards.
Radiation-Hardened Memory: Utilizing diamond’s extreme stability for data storage in harsh environments, such as aerospace, nuclear facilities, or deep-sea applications, where traditional magnetic or semiconductor memory fails.
Diamond Tracing and Authentication: High-resolution, bulk inscription of unique PL microbits (NV centers) for robust anti-counterfeiting, tracing, and protecting trademarks of high-quality natural and synthetic diamonds.
Optomechanical Memory Platforms: Enabling novel memory architectures where data is accessed via precise micromechanical positioning combined with non-destructive confocal PL read-out.
Quantum Information Science (Indirect): The precise, localized creation of NV centers in diamond, a key platform for quantum computing and sensing, provides a method for scalable fabrication of quantum registers.

View Original Abstract

Inscription of embedded photoluminescent microbits inside micromechanically positioned bulk natural diamond, LiF and CaF2 crystals was performed in sub-filamentation (geometrical focusing) regime by 525 nm 0.2 ps laser pulses focused by 0.65 NA micro-objective as a function of pulse energy, exposure and inter-layer separation. The resulting microbits were visualized by 3D-scanning confocal Raman/photoluminescence microscopy as conglomerates of photo-induced quasi-molecular color centers and tested regarding their spatial resolution and thermal stability via high-temperature annealing. Minimal lateral and longitudinal microbit separations, enabling their robust optical read-out through micromechanical positioning, were measured in the most promising crystalline material, LiF, as 1.5 and 13 microns, respectively, to be improved regarding information storage capacity by more elaborate focusing systems. These findings pave a way to novel optomechanical memory storage platforms, utilizing ultrashort-pulse laser inscription of photoluminescent microbits as carriers of archival memory.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi14071300

References

2020 - Sub-nanometre resolution in single-molecule photoluminescence imaging [Crossref]
2015 - Upconversion for photovoltaics-a review of materials, devices and concepts for performance enhancement [Crossref]
2019 - Symmetry-wise nanopatterning and plasmonic excitation of gold nanostructures by structured femtosecond laser pulses [Crossref]
2020 - Light-emitting nanophotonic designs enabled by ultrafast laser processing of halide perovskites [Crossref]
2007 - Intraband and interband optical deformation potentials in femtosecond-laser-excited α− Te [Crossref]
2023 - Attosecond magnetization dynamics in non-magnetic materials driven by intense femtosecond lasers [Crossref]
2004 - Optics at critical intensity: Applications to nanomorphing [Crossref]
2017 - Direct laser printing of chiral plasmonic nanojets by vortex beams [Crossref]
2011 - Anatomy of a femtosecond laser processed silica waveguide [Crossref]