New Applied Developments Born from Basic Research on Optical Properties of Materials
We investigate the interaction between light and matter by shining light on a material and observing its "response." This, in a nutshell, describes the research in "optical properties of materials physics," my area of expertise. The term "response" covers a wide range of phenomena. This includes investigating nonlinear optical phenomena, where the wavelength of light changes due to the presence of a material, and luminescence phenomena. We also manipulate the properties of materials (such as metallic, insulating, or superconducting states) using light. Moreover, the field of optical properties of materials physics deals with a wide variety of materials for measurement, ranging from semiconductors to superconductors and soft matter. Essentially, any substance that falls into the category of "physical properties" (so-called "things") can be measured and controlled using light. I myself have conducted research on various materials, including semiconductor quantum nanostructures, organic superconductors, magnetic materials, and polymer materials.
In this broad research field, my work has always been driven by an interest in how the microscopic structure of a material affects its optical response. In our recent research, we discovered that by skillfully utilizing the symmetry of a semiconductor's microscopic crystal structure, we can determine the unknown polarization state of light with extreme accuracy using nonlinear optical response [1]. Figure 1 schematically shows that by simultaneously irradiating a zinc telluride semiconductor crystal with light 1 (red) of an unknown polarization state and light 2 (blue) of a known polarization state, and then analyzing the polarization state of the newly generated light 3 (light blue) resulting from frequency mixing due to the nonlinear optical effect, we can accurately determine the unknown polarization state of light 1. This research, which stems from understanding the nonlinear interaction between light and matter within a semiconductor crystal in conjunction with its crystal structure symmetry, is considered a fundamental and significant achievement in the physics of optical properties of materials.
Figure 1: Schematic diagram of a new polarization measurement method using nonlinear optical response. By irradiating a semiconductor crystal with light 1 (red) and light 2 (blue) and analyzing the polarization state of light 3 (light blue) generated by the nonlinear optical effect, the polarization state of light 1 can be determined.
Now, while the research described above may seem modest at first glance, it actually leads to important applied research. High-performance polarization optical elements exist in the visible light frequency range, but it is extremely difficult to create effective polarizers for light in the mid-infrared and far-infrared frequency ranges (so-called terahertz light), which have lower frequencies. By using our idea—employing terahertz light as light 1 and visible light for light 2 and light 3—we can transfer the polarization information of the terahertz light to that of the visible light, which can then be read out accurately in a short measurement time. This has enabled us to succeed in developing a terahertz polarization measurement device that can perform measurements at a speed unparalleled anywhere in the world [2].
In industry, terahertz light is attracting attention as a new light source for non-destructive testing because it can penetrate materials like paper and plastic that are opaque to visible light. Furthermore, when combined with terahertz polarization measurement technology, it allows for the rapid, non-destructive inspection of refractive index anisotropy inside materials. The technology born from our basic research is garnering significant attention for its potential application in these new non-destructive testing techniques. Perhaps this immediate connection between the pursuit of fundamental physics and its applications is one of the great attractions of research in the physics of optical properties of materials.
[References]
[1] K. Oguchi, N. Yasumatsu, and S. Watanabe, J. Opt. Soc. Am. B 31, 3170 (2014).
[2] N. Yasumatsu, A. Kasatani, K. Oguchi, and S. Watanabe, Appl. Phys. Express 7, 092401 (2014).