The development of techniques for measuring material properties under high pressure is important in fields such as planetary science and condensed matter physics. Previously, a method for measuring changes in the refractive index of materials inside anvil cells which create high-pressure environments was proposed as a simple yet highly sensitive technique, based on color changes in gold nanoparticles that become colored through localized surface plasmon resonance. However, challenges have emerged due to the soft nature of gold nanoparticles. They deform significantly above a certain pressure, causing unexpected color changes.
A research group led by Assistant Professor Hiromasa Niinomi of the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, along with Professor Yuki Kimura of the Institute of Low Temperature Science, Hokkaido University; Professor Hiroki Nada of the Department of Mechanical and Physical Engineering, Faculty of Engineering, Tottori University; Associate Professor Tetsuya Hama of the Komaba Institute for Science, the Graduate School of Arts and Sciences, the University of Tokyo; and Associate Professor Kazuhiro Gotoh of the Graduate School of Science and Technology, Niigata University, focused on a phenomenon called Mie-void resonance, in which sub-micrometer voids formed in dielectric materials become colored. They proposed a new method for measuring changes in refractive index based on color changes in Mie voids fabricated on the surface of a hard anvil. The research was published online in The Journal of Physical Chemistry C.
(Top left) Scanning electron microscope (SEM) image and optical microscope image of a gallium arsenide (GaAs) substrate with formed Mie voids. The image shows the substrate when the refractive index (n) of the surrounding medium is 1 and 1.38. The color of the Mie void resonance is observed to change depending on the refractive index.
(Bottom) Photograph of the 4H-SiC anvil developed in this research, scanning electron microscope (SEM) images and optical microscopy of Mie voids fabricated on the anvil by focused ion-beam (FIB) lithography. The bird's-eye SEM image was obtained at a tilt angle of 52°.
Adapted with permission from Arslan et al., ACS Photonics 2025, 12, 7, 3950-3958. Copyright 2025 American Chemical Society.
Provided by Tohoku University
The research group focused on Mie-void resonance, a phenomenon that has attracted attention in the field of metaphotonics in recent years. Like localized surface plasmon resonance, the color of Mie-void resonance responds sensitively to the refractive index of the surrounding material, with the spectrum shifting toward longer wavelengths as the refractive index increases. Meanwhile, dielectric materials are commonly used for anvils. The group therefore reasoned that using Mie voids fabricated on the surface of a hard anvil could overcome the challenges faced by methods that use localized surface plasmon resonance.
Mie voids were fabricated by focused ion beam (FIB) processing on the surface of a silicon carbide anvil, which is widely used and has a relatively high refractive index. An anvil cell was constructed using this, and experiments were conducted with ultrapure water as the sample. The experiments were performed at room temperature (22.5℃), starting from a state where high-pressure ice VII — crystallized by pressurizing water — filled the sample chamber. The pressure was gradually reduced while observing the phase transition from high-pressure ice VII to high-pressure ice VI and the melting of high-pressure ice VI into water, and the reflection spectra of the Mie voids were acquired. As the pressure decreased, the color of the Mie voids changed from red to green, and it was confirmed that the reflection spectrum shifted toward shorter wavelengths.
It was also confirmed that deformation or strain of the Mie voids had almost no effect on this spectral shift toward shorter wavelengths. Furthermore, it was found that diamond, commonly used for anvils, showed equivalent sensitivity. This demonstrated that this method has greater robustness for high pressure and is an alternative to methods using localized surface plasmon resonance.
Refractive index detection using Mie voids is, in principle, capable of detecting changes in the refractive index of extremely small volumes of material that enter the voids. Therefore, this method can measure highly localized changes in the refractive index of materials confined within an anvil cell. Such changes are otherwise difficult to detect using conventional methods based on interferometry or Brillouin scattering.
In previous research, the group discovered homoimmiscible water, an unknown type of water that macroscopically separates from bulk water at the interface between ice and water; the ice is crystallized from ultrapure water under pressure applied by an anvil cell and high-pressure ice. Understanding the properties of homoimmiscible water is extremely important for understanding the most essential liquid for life. However, because homoimmiscible water forms locally at the interface between water and ice with a thickness of only a few micrometers, measuring its refractive index has been difficult with conventional methods. By attaching homoimmiscible water to Mie voids, it may be possible to measure the refractive index, one of the physical properties of homoimmiscible water.
These findings are expected to bring new degrees of freedom to high-pressure property measurements. Furthermore, they contribute to the advancement of scientific fields where high-pressure properties are key, such as planetary science and high-pressure materials science.
Journal Information
Publication: The Journal of Physical Chemistry C
Title: Mie Voids for High-Pressure Refractive Index Sensing
DOI: 10.1021/acs.jpcc.5c05941
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