Applied Physics

Kazuhiko Oka

Creating New Measurement Devices Using Polarization ? from Inspecting Semiconductors to the Detection of Planets outside the Solar System.

Kazuhiko Oka , Associate Professor,

Faculty of Engineering / Graduate School of Engineering (School of Engineering, Department of Applied Science and Engineering, Applied Physics and Engineering Course)

High school : Niigata Prefectural Takada High School

Academic background : Masters from the University of Tokyo

Research areas
optical engineering, optical physics
Research keywords
polarization, compact polarimeter, optical vortex

What is the subject of your research?

We learn in high school that light is a wave. However, none of us have ever actually seen the vibration of light. This is because light waves vibrate so rapidly that humans are not able to directly observe it. As a matter of fact, the “invisible waves” of light possess a range of properties that are very different to the waves that we would imagine. For example, Photo 1 shows some words on paper viewed through a crystal known as calcite. You can see that the image is doubled. In addition, if you place a polarizer over this calcite and rotate it, the two images alternatively fade in and out. “Polarization” is a property of waves relating to the difference in the direction of vibration, but it also relates to the creation of a double image. One further thing – this phenomenon does not even fulfill Snell’s law.

Photo 1 Strange light waves: double image made by calcite.

In high school physics we learn only the basic, albeit the ideal, situation regarding light waves, which are characterized as being similar to water and sound waves (this is important in itself). However, if we look at the properties of light waves in non-ideal conditions, we see that they possess some strange but fascinating properties. The purpose of my research is to use these strange properties of light waves to create useful devices. In particular, I have recently developed several devices using polarization.


Research project 1: Creating a super-compact polarimeter

Polarization is invisible, so you probably don’t realize it exists, but it has recently been used in a wide range of areas. Items immediately around you that use polarization include LCD TVs, 3D TVs, mobile phones, CDs/DVDs and fiber optic Internet connections. Furthermore, devices used in factories for precise inspection of semiconductors and solar cells, and devices used in hospitals to examine eye diseases, sometimes use polarization. Devices that use polarization in this way are increasing rapidly around us. In order to apply this technology even more, it is vital to be able to accurately and efficiently measure polarization. One of the themes of my research is the development of new polarimeters to meet this demand.

Up until recently, the measurement of polarization was mainly done by use of a motor that rotates a polarizing element; the polarization is determined from the changes in the brightness of the transmitted light. This principle is based on basic and easily understandable polarization theory, but the use of a motor limits the size and the performance of the measurement system. In contrast, we used the peculiar polarizing properties of calcite to develop an entirely new polarization measurement method that does not require a motor. This method makes it possible, for example, to build a semiconductor inspection device that fits in the palm of the hand, as in Photo 2. Conventional inspection devices were the size of a copier machine, but we were able to reduce the size significantly. In addition, as shown in Photo 3, we are now prototyping a compact polarimeter that is the size of a pencil. In the future, we would like to build even smaller polarimeters so that we may be able to examine the inside of the human body. We know that cancer and myocardial infarction cells have specific polarization attributes, and the technology of the future is expected to lead to new methods regarding their diagnosis and treatment.

Photo 2 Semiconductor inspection device that fits in the palm of the hand. Conventional devices were the size of a copier machine.

Photo 3 Compact polarimeter with the size of a pencil.



Research project 2: Searching for aliens with a helical beam

Another one of our research themes is about “optical vortex”; this is light with a helical wavefront like that shown in Fig. 1. This light possesses several interesting properties. For example, the center of the light beam is completely dark as shown in Fig. 2, and the light has torque to rotate microscopic particles.

Fig. 3 Astronomical instrumentation used for detection of planets orbiting stars other than our Sun.

We are engaged in research into how optical vortices are formed using a special polarization device, and we’re also thinking a lot about various applications of optical vortices. For example, this technology has been combined with an astronomical telescope to search for “exoplanets” orbiting stars other than our Sun. By making use of optical vortex, it is possible to extinguish the light from a bright star so that we can detect the light from faint exoplanets (Fig. 3). We may be able to find life in extrasolar system in this way.

Fig. 1 Helical wavefront of an optical vortex.

Fig. 2 Distribution of optical intensity in cross-section of optical vortex. The center has a zero intensity.