研究目的とテーマ

■ Backgroud and objectives

A hundred year ago, people used a printing press and black-and-white films to, respectively, mass-produce documents and take pictures. The first fully-programmable computer was made using a mechanical system. X-ray, hemadynamometer, and electrocardiography entered the stage of medical diagnosis. Telephone system was available, yet wireless communication was about to be established. Nowadays, many of those instruments and systems become highly effective by, some of them totally replaced by, the concepts born out from modern physics, in particular electronics. Wouldn’t be exciting to imagine our life a hundred year from now, and think how we approach to this heart-leaping problem? There are so many approaches to this endless, interesting, yet somehow distressful problem. My approach is to study new behavior of electrons in electromagnetic fields in solids, and propose concepts of new devices and applications which would lead us to a paradigm full of new knowledge and exciting ideas, and consequently to the improvement of our life.

   Modern electron-based devices rely on the motion of charges. When charge of electrons move in matters which are full of atoms, they collide with the atomic network and yield heat. This is one of the fundamental limitations that electron-based devices face nowadays; the faster move electrons the hotter get devices! This means we generate more and more heat as far as we keep improving our life using the technology based on nowadays electronics. This is why people in fundamental research　have started looking into phenomena that do not involve the motion of charges, before we face serious energy crisis in the future.

   Spintronics is one of the candidate directions which people working in condensed matter physics have proposed; it relies on quantized direction, up or down, of electrons and atomic nuclei. The past half-century has been devoted primarily to build a framework in which spins and charge motions both participate. Thanks to this effort, knowledge in spintronics has been materialized into the realization of ultra-high-density magnetic storage and development of integrated non-volatile memory. At present, scientists worldwide are about to start building the second framework in which spin information is input, processed, and output without the charge motion. To this end, I, with my colleagues, am studying the way to manipulate spins in magnetic materials with light, kinds of high-frequency electromagnetic waves of around 10¹⁵ Hz and higher.

   Why use light? Firstly, it is mass-less so that it does not induce motion of a charge center**. Secondly, it propagates very fast through the real space, e.g., c = 3.0 ´ 10⁸ m/sec in the free space so that it can carry lots of information. Thirdly, it has both non-local and local characters depending on its frequency. Forth, since fire became a ground-breaking tool for our ancestors, light (an electromagnetic wave) has been and will be an indispensable partner for human being to exploit our future.

** Momentum of light is ignored for simplicity. 

■    Present research subjects

At the present stage, it is very important to pave the way toward establishing fundamental concepts of optical manipulation of spins. A key is the spin-orbit interaction. In order to understand this interaction, we pursue experimentally (i) how fast we can manipulate spins and (ii) to what extent the power of optical excitation can be reduced to manipulate spins, and (iii) development of new materials to pursue (i) and (ii). Another important subject is establishing fundamental concepts of devices for mutual conversion between light and spins; we study this with (iv) circularly polarized light emitters/detectors and (v) spin-controlled optical waveguides.