A contactless dual technique for non-destructive characterization of glass microspheres

by Virginia Greco

A study published in APL Materials provides a method, based on the combination of two measurements techniques, to extract the optical and elastic properties materials in microspheres structures without damaging them, which had proved particularly challenging so far. This research also revealed that both the refractive index and the elastic constants of the examined glass materials are reduced in the microspheres with respect to their bulk counterparts. This is crucial information for integrated optical glass elements.

High quality microspheres made of polymers, crystalline or glassy compounds are widely used in various industrial applications –spanning (bio-)sensing, phononics, photonics, etc.— as standalone structures or as mechanical reinforcement to other compounds and components. In fact, a deeper understanding of their elastic and optical properties is crucial, though performing a quantitative measurement of such properties without modifying or destroying the microspheres is very challenging.

Researchers from the Phononic and Photonic Nanostructures Group led by ICREA Prof. Dr Clivia Sotomayor-Torres at the ICN2 and the group led by Prof. Daniel Navarro Urrios at the Institute of Nanoscience and Nanotechnology of the University of Barcelona (IN²UB), in collaboration with colleagues at the University of La Laguna (Spain), have demonstrated that the combination of two contactless optical techniques allows characterizing the elastic and optical constants of glass microspheres. This research has been recently published in a paper in APL Materials and was selected as Editor’s pick.

The first technique utilized in the study is called Brillouin Light Scattering (BLS). By analysing the scattering of longitudinal acoustic phonons in microspheres of a barium-titanium-silicate glass materials (of diameters ranging from 10 to 60 µm), researchers discovered that the frequency of these phonons in the microspheres was approximately 5% lower than in the bulk material. This finding suggests a reduction in the refractive index or a relaxation of the elastic properties, or perhaps a combination of both.

The second technique, called optomechanical coupling (OC), involves exciting specific vibration modes –i.e., the optical whispering gallery modes— of the glass microspheres and transducing some of them, which allowed the researchers to extract the elastic constants of the glass microspheres. The study revealed that both the refractive index and the elastic constants of the material are reduced in the microspheres with respect to their bulk material counterparts. The authors attributed these results to an effective decrease in density caused by the fabrication process.

The significance of this research lies in the non-destructive nature of the characterisation methods. The combination of the two techniques proposed by the authors of this work will enable the characterization of a wide range of glass materials in different microsphere geometries. These findings hold great promise for various applications, such as sensing, magnetometry, optomechanical oscillators, and design of composite materials, where precise knowledge of elastic and optical properties is crucial. The ability to evaluate these properties below the millimeter scale without causing damage is a major breakthrough and paves the way for further exploration and application of glass microspheres in diverse fields.

Reference article:

Jeremie Maire, Tomasz Necio, Emigdio Chávez-Ángel, Martín F. Colombano, Juliana Jaramillo-Fernández, Clivia M. Sotomayor-Torres, Nestor E. Capuj, and Daniel Navarro-Urrios, Contactless characterization of the elastic properties of glass microspheres. APL Mater 11, 041128 (2023). DOI: 10.1063/5.0146969