02-17 photonic glasses

[Image above] (a) SEM image of a chalcogenide glass microdisk resonator, with 45 μm diameter, coupled to an access waveguide. (b) Magnified view of the waveguide-resonator coupling region. (c) SEM image of the transverse electric mode grating used to couple light to the waveguide from an external fiber. Reproduced from Zhang et al., Optics Express under the terms of the Open Access Publishing Agreement.

The International Year of Glass may be over, but the Age of Glass is just beginning!

In homage to IYOG, an international group of researchers, including ACerS Distinguished Life Member Kathleen Richardson and ACerS Fellow John Ballato, published an open-access review paper describing the past, present, and future of glass in the exciting field of photonics.

The paper focuses on optical glasses and how they enable the Age of Glass in the area of information and data transmission. The paper opens with a succinct history of glass and its optical vein. The authors impressively cover four millennia of glass application history in a few paragraphs.

While acknowledging there are many applications for functional glass, this review narrows its focus to photonic applications.

The paper highlights four categories of novel optical glasses, including luminescent glasses, photochromic/photosensitive glasses, magneto-optical glasses, and optical glass waveguides.

Before delving into these novel glasses, the paper begins with the history of optical glasses and a tutorial on glass formation, fabrication, and characterization. Though brief, the tutorial is an excellent and comprehensive summary of key glass principles, including

  • Glass formation theory,
  • Nucleation and growth theory,
  • Common fabrication methods, and
  • Characterization (for example, viscosity, thermal expansion, transmittance, refractive index, bond structure, microstructure, and mechanical properties).

Professors may find this section to be a useful resource to supplement materials science and engineering textbooks, which tend to be light on glass science fundamentals.

Each of the four sections comprises an overview of the science and resulting engineering properties. The authors concisely link science to a host of applications, and they close each section with a summary of recent trends and future directions.

Professors, students, researchers, and practitioners will appreciate that each of these sections is a self-contained tutorial on the glass, the science behind it, applications, and future research.

For example, in the section on luminescent glasses, we learn about the rare earth dopants and glass former compositions; rare earth band structure and dipole transitions; and how those features lead to useful behaviors, such as fluorescence and phosphorescence. These principles and properties are familiar to us in applications such as lighting, laser glasses, scintillator glasses, solar cell concentrators, taggants, and data storage devices.

A wide swath of new research focuses on luminescent glasses with enhanced spectroscopic properties, including lasing glasses that emit in the mid-infrared range, chalcogenide glass compositions for the mid-infrared, and glasses that convert near-infrared radiation to visible light to activate release of active agents in biomedical situations.

Each section provides specific topical insights into current and future opportunities for advancements in glass optical materials. More generally, “… as true now as it has been over a hundred years, a deeper understanding of the interconnections between glass composition, processing, structure, and properties across all the states application spaces is critical,” the authors note.

They add, “It also is worth noting that the marriage of glass and light will only grow stronger with time as photonics continues to impact every corner of modern life.”

The article closes with the following cross-cutting suggestions for future research and development.

  1. Machine learning and artificial intelligence to help predict passive and active optical properties of glasses in all forms (e.g., bulk, films, fiber), particularly as it relates to
    – Luminescent and nonlinear optical properties;
    – Empirical or theoretical predictions of crystallization process for any given glass composition, which will enable consideration of novel composite materials that enhance and expand the capabilities of glass or glass-ceramic media; and
    – Glasses with higher refractive index and less dispersion.
    1. Glass process research that permits reduced cost, environmental impact, and scaling of more exotic glasses for high performance applications, e.g., infrared optics and sources, plasmonic and nanocrystalline glass-ceramics.
    2. Pushing the transmission limits of glass, both spectrally (e.g., deeper into the ultraviolet and further into the infrared) and with respect to loss (e.g., lower-than-silica attenuation across broad spectral ranges).
    3. Compositions and fabrication methods that permit nonlinearities that are markedly higher or lower than currently available, including second-order phenomena.
    4. Novel approaches to create microstructures inside glass forms.
    5. Hybrid integration with semiconductors, crystals, and organic materials to realize novel and complex functions, such as optical computing, sensing, monitoring, and data processing.
    6. More complete understanding of photosensitivity and photo-induced properties to make greater use of such glasses.

    Clearly, we are only at the beginning of understanding the opportunities available and what the future impact of photonic glasses could be!

    The open-access paper, published in Progress in Materials Science, is “The past, present, and future of photonic glasses: A review in homage to the United Nations International Year of Glass 2022” (DOI: 10.1016/j.pmatsci.2023.101084).