Optical Physics
from Keio's Faculty of Science and Technology Observing and controlling new
physical properties being opened
up by terahertz-wave technology

Investigating physical properties of condensed matters using light and controlling their properties

For a long time, humankind has used light with different frequencies (wavenumbers) to observe objects and change physical properties of things to improve living by harnessing electrical energy converted from light energy. In this context, coming into focus of attention is “terahertz light” that has longer wavelengths than visible light. We asked Associate Professor Shinichi Watanabe, a research specialist in THz spectroscopy about the terahertz-wave technology forefront.

Light is indispensable for examining physical properties of things and for transforming such properties. Research based on light has developed in tandem with the development of technology to the extent that it is now contributing even to elucidating the origin of the universe – one of the greatest mysteries for humankind. Of the various kinds of light as a tool, terahertz light (“tera” signifies 10 raised to the power of 12) strongly comes into the limelight. Focused on studies based on terahertz light, Dr. Watanabe explains its advantages as follows:
“Terahertz light has extremely long wavelength, its frequency being 1/100 to 1/1000 that of visible light. Since light having a high frequency (wavelength is short) like X-ray has greater energy, we can say that terahertz light is light with a low level of energy.
The fact that its energy is extremely lower than that of X-rays means that terahertz light has significantly less adverse impact on the human body. This is why expectations are high for terahertz light as “safe light.”
You can also say that examining physical properties ofthings is investigating their energy structure. For example, when attempting to examine low-energy objects like a superconductor, use of terahertz light is the only way to see it directly. In other words, each type of light has its specialty fields of application.”
Moreover, terahertz light is very useful for penetrating materials, such as clothing, plastic packages and paper that do not transmit visible light. As such, much is expected of terahertz light for application to virtually all industrial fields – security check, semiconductor product inspection, non-destructive testing (on buildings, etc.), medical care and pharmaceutical development, to name a few. The study of terahertz light had remained unexplored until recently partly because the light can be absorbed with water and partly because it had been difficult to generate this light efficiently.
Dr. Watanabe continues: “An intriguing aspect of terahertz light is that its slow vibration allows us to observe its waveform (flying through the air) as is. I’m interested in both seeing physical properties of things and manipulating them. So it’s very exciting to be able to see firsthand what’s going on the moment light strikes an object and how they interact with each other. Due to its extremely low frequency, terahertz light also has properties very similar to radio waves. This makes it possible to produce similar effects – effects that could be obtained when an electric or magnetic field is given – without attaching an electrode to the object concerned. This is truly intriguing.”
By having both the radio wave-like property of transmitting a variety of substances together with the property of travelling straight like visible light, terahertz light covers a wide range of measurement objects. This is the greatest characteristic of terahertz light.

The Watanabe lab currently employs a method known as the “terahertz timedomain spectroscopy.” By mixing longwavelength “terahertz light” and shortwavelength “near-infrared light pulse” in a transparent substance known as the “nonlinear optical crystal,” this method allows us to identify terahertz light waveforms just like an oscilloscope (a waveform measuring device used to observe behavior of electrical signals). He states: “In optical physics, we usually examine an object’s energy structure by exposing an object to light and determining how much the intensity of its reflected light or transmitted light has decreased. In addition to determining changes in light intensity, “terahertz timedomain spectroscopy” also allows us to stagger the timing of near-infrared light pulse irradiation. Thus we can obtain double amount of information – changes in “amplitude” and “phase” of light waves.” Furthermore, by exercising additional ingenuity on this method, the Watanabe lab has succeeded in accurately measuring information relating to “polarization” in addition to amplitude and phase. By rotating at a high speed the semiconductor crystal for the terahertz pulse detection at a prescribed angular velocity, it has become possible to determine the magnitude and direction of terahertz light’s electric field vector simultaneously. Thus, we are now able to accurately analyze the direction of reflected wave vector component. Highly sophisticated calculation had been required to analyze signals resulting from semiconductor crystal rotation. But Mr. Naoya Yasumatsu, then a senior, completed the task after struggling with enormous amount of calculation involved. “This method made it possible to accurately measure the direction of electric field vectors (at a given time) of terahertz-wave pulse light irradiated from two points of different height, leading us to perform highly accurate testing on the contour and surface roughness of metallic objects. As a result, we became able to distinguish unevenness down to the depth of one-thousandth of the wavelength. This achievement was publicized on the American Optical Society’s ‘Optics Letters’ (online bulletin board version).” Highly motivated, Dr. Watanabe said that he would like to expand application of this method even to the short-wave domains of infrared and visible light.

Dr. Watanabe’s research themes also include attempts to transform physical properties of an object by irradiating it with terahertz light with large vibration amplitude.
He continues: “Intrinsically, energy comparable to that of visible light is required to excite (to transfer from the ground state to the high-energy state) electrons in a semiconductor. However, terahertz light can do the same if we make its vibration amplitude extremely larger. What’s more, the great thing about terahertz light is that its waveforms are visible, which allows us to closely observe how the state of electrons changes and at which point of time.
Since the frequency of terahertz light is close to the vibration resonance frequency of lattices that constitute an object, it is relatively easy to cause resonance. By swaying the lattice greatly, we can also expect to cause a structural change to the object. We are thus able to change physical properties of objects as desired, which is expected to lead to diverse applications to new physical sciences. I’m highly motivated to establish innovative methods for controlling physical properties of things.”
We would like to keep our eye on the development of terahertz light – a field of study with great potential.

I was born as the first boy of self-employed parents engaging in electrical work business. As such, word processors and PCs were always around me in my childhood. This family environment naturally led me to take interest in computer programming. In those days, my father left home early in the morning, came back in the evening and spent most of the night at home with me. Since childhood, it had been my dream to choose an occupation by which I can spend time freely like my father.

Given the highly competitive world of researchers, I’m always thinking about how I can survive the competition. In this sense, it’s my policy not to take up themes that are “in fashion.” If you jump at a theme “in fashion,” you will eventually find yourself nowhere in terms of achievements. That said, it’s true that “terahertz light” is “in fashion today.” I know my remark sounds a bit contradictory. Yet, I can say I’m pursuing original research from my own unique approach. On the other hand, if I went too far with my personal likes and focused on themes that are too far from being “in fashion,” I would surely find myself in difficulty to raise vitally needed research funds. This is the point requiring careful “consideration,” which annoys me all the time.
By the way, at the Keio University Department of Physics, “reading original texts and making presentation” (students are required to read original papers in English – from every age and everywhere – and express their comments) is a compulsory subject for seniors. In this class, I join my students to learn how famous researchers of the past opened up their fields of study using their own strategies. To produce new ideas and approaches necessary for promoting research activities, following the footsteps of great figures of the past is the best shortcut. As such, I’m striving daily to produce breakthrough research achievements while learning strategies taken by our great predecessors.

A number of problems remain unsolved, such as the origin of high-temperature superconductivity and mysteries of life, for which a variety of hypotheses have been proposed. Decades later, it is likely that these may become commonplace for humankind. Theories, which support experiments to prove why they are no longer mysteries, will have been established by that time. If these theories are easier to understand, they will be easier for humankind to use, contributing more to society.
I believe that natural phenomena, though apparently complex, can be broken down into simple elements that could be explained by theories. I’d like to become the world’s first to bring such “easy-to-understand theories” to light.
To refine and enhance experimental techniques is the best way to shed light on yet-to-be defined truths. Because experimental results contain a lot of clues to untangle complex natural phenomena, it’s very exciting to use these clues to solve puzzles.
In the past, for example, I created a terahertz light pulse comparable to the then world’s highest-class electric field, and used it for optical control of matters – an attempt no one else had ever challenged. Soon after assuming a post at Keio University, I also acquired a world-leading technique capable of identifying information regarding polarization of light in the terahertz frequency domain, speedily and accurately. I’d like to continue to meet the challenge of yet-to-be-defined physical property problems taking advantage of our world’s top-class techniques.

From junior high school days up to now, I have been an active member of an amateur brass band, playing the tuba.
Many of my favorite pieces of music are marches. Since the tuba is used to beat out monotonous rhythms, people often tell me “It’s boring, isn’t it?” But I’d rather say that many pieces of music, for which I play the tuba, are simple yet moving. It’s fun. I like things simple just as I do when it comes to work.

Teachers and administrative staff are all supportive and spare no effort to promote young and middle-class researchers like myself, for which I’m always grateful. My first impression of Keio upon assuming my post was that everything was well organized and everyone was cooperating with each other and appeared very eager to seek the highest possible achievements both in education and research. Given the superb educational environment, student’s levels of basic scholastic ability and appetite for learning are also very high, which makes me comfortable when learning and doing research with them. Synergistic effects of cooperation between teachers and students characterize Keio’s academic environment, ensuring high quality research.