Light emitted from an object that is heated to a high temperature is called thermal radiation. Sunlight, light emitted from an incandescent light bulb, and the red light from burning charcoal are examples of thermal radiation. Because thermal radiation varies widely in wavelength, it is used in diverse fields. For example, in solar power generation, a solar cell converts the energy of visible and near-infrared radiations into electric energy. On the other hand, many gasses and chemical substances uniquely absorb mid-infrared light of a certain wavelength. Based on this property, highly sensitive sensing of each substance has been actualized via use of the mid-infrared light fraction in thermal radiation. However, in thermal radiation from an ordinary object, lights of all wavelengths are emitted at the same time, resulting in huge energy loss because only light of a certain wavelength is needed. Therefore, we are conducting studies to establish a highly efficient method for producing thermal radiation of a desired wavelength at a desired time point and to deploy the method in various applications.
Firstly, let us see how thermal radiation is produced. When the temperature of an object rises, electrons in the object become active and emit light. The emitted light interacts with electrons in the object again and is absorbed. When such interactions between electrons and light are repeated until a static state is reached, the light emitted from the object becomes thermal radiation. Because the interactions between electrons and light are repeated over a long time at all wavelengths (energy) in an ordinary object, the thermal radiation produced contains lights of diverse wavelengths. On the other hand, we aim to produce thermal radiation of a narrowed wavelength range in our studies by controlling the interactions between electrons and light so that thermal radiation occurs within a specific wavelength range. For actualizing such control, we use semiconductor materials such as gallium arsenide and silicon and an optical nanostructure called a photonic crystal prepared from these materials. Semiconductor materials have a lower density of free electrons compared to metals and have not been utilized in thermal radiation control studies in the past because they do not readily produce thermal radiation even when heated. We discovered that we can create a situation in which thermal radiation is produced only within a specific wavelength range and not in other ranges by using a semiconductor into which a specific layer called a quantum well is introduced. Furthermore, we verified that we can limit the wavelength further and obtain thermal radiation of high intensity by preparing periodic unevenness (photonic crystal) close to the size of the desired light wavelength on the surface of the semiconductor material. We will expand the controllable wavelength ranges so that we can control wavelength at any arbitrary range from visible to infrared lights.
As our studies progressed, I have recently envisioned a possibility of a new control mechanism of thermal radiation–rapid control of thermal radiation. In general, the thermal radiation intensity of a high-temperature object is determined by its temperature, and thus the object needs to be heated or cooled to start or stop its thermal radiation, resulting in very slow responses. I returned to the basics, i.e., “interactions between electrons and light produce thermal radiation” and estimated that it would be possible to quickly start or stop thermal radiation by rapidly generating or extinguishing electrons within the object. Actually when I prepared and tested a light source that can alter electron density in semiconductor quantum wells by changing the voltage applied externally, I observed that thermal radiation intensity changed rapidly depending on the voltage although the temperature of the light source was constant. Such rapid control of thermal radiation is applicable for a sensing technology that uses infrared light of a specific wavelength such as measuring the amount of toxic substances in exhaust gas and CO2 concentration in the atmosphere. This new control may lead to discovery of physical phenomena at high temperatures that have not been observable with conventional technologies, and the possibilities are very interesting.
Our studies have disclosed that thermal radiation of a high-temperature object, which has been believed to result in large energy loss and to respond very slowly, is a profound luminous phenomenon that can be condensed to a narrow wavelength range and controlled rapidly. I will further investigate interesting possibilities of thermal radiation by designing and preparing new semiconductor materials and optical nanostructures and actualizing free control of thermal radiation at an arbitrary wavelength ranging from visible to infrared light. A final goal is to use thermal radiation of a specific narrowed wavelength range for greatly improving the energy conversion efficiency of a solar cell and developing miniaturized environmental and medical sensing systems that consume little energy, and thus contribute to construction of a sustainable society.