Condensed matter physicists are studying ways of exploiting the spin instead of the charge of electrons for developing new optoelectronic devices. However, the interaction between light and electrical spin is weaker than that between light and electrical charge. But members of the spin-photonics team at the Tokyo Institute of Technology (Tokyo Tech) have found through the study of photoexcited precession of magnetization using ultrashort (10-13 sec) weak laser pulses of 1μJ/cm2 or less, that spins in ultra-thin cobalt/palladium (Co/Pd) multilayer films are very susceptible to light; namely, these femtosecond-laser-induced materials are candidates for photosensitive magnets that could be used for a variety of optoelectronics applications.
The spin-photonics team includes KosukeYamamoto, a master's course graduate student at the School of Interdisciplinary Science and Engineering, Yoshitaka Kitamoto, professor at the same school, and Hiro Munekata, professor at the Imaging Science and Engineering Laboratory (ISEL) and the leader of the team.
These findings were soon followed by the demonstration of polarization modulation of light signals in an optical waveguide with the same class of magnets, where this work was carried out by Kazuhiro Nishibayashi, a lecturer at ISEL and member of spin-photonic team, in collaboration with Hitoki Yoneda, professor at the University of Electro-Communications, and Atsushi Kuga, at the researcher of Science and Technology Research Laboratories, Japan Broadcasting Corporation. In this work, Nishibayashi has emphasized the feasibility of the multiplexed transmission of polarization-modulated signals, controlled ultimately by photo-excitation of a class of light-sensitive magnetic layers.
Munekata and Kitamoto started their work by discussing a class of materials in which interaction between electron orbitals and spin is strong, and later, when Yamamoto joined them, they decided to focus on the interface of Co and Pd, where the spin states are strongly sensitive to the slight charge imbalance at the Co/Pd interface. Illumination with a femtosecond laser pulse in the ultrashort time regime enabled them to successfully modulate the charge imbalance and vary the direction of spin ordering instantaneously. This has given rise to clear observations of oscillatory signals due to precession of ordered spins.
It is very common to use a light beam propagating in free space for studying the interactions between light and matter. In an optical waveguide, however, light propagates through strong and weak propagation channels as a consequence of interference induced by light partially reflected at the sidewalls of the waveguide. Therefore, it is not straightforward to engineer the interaction between light and spins with light propagating through an optical waveguide. Experimental results obtained by Nishibayashi and colleagues show that specific points of spin in a magnetic film interact with selected modes of light, which is one of the unique points found in a magnet-waveguide system.
Inducement of periodic motion of magnetization (ordered spins) with weak laser pulses would enable the researchers to control the polarization plane and group velocity of optical digital signals that propagate in the vicinity of magnetization. Not only multiplexing/de-multiplexing, but various applications such as optical memory (random access type) and signal delay lines (buffer memory) may also be developed by combining photosensitive magnetic layers and optical waveguides.
SOURCE: Tokyo Institute of Technology; http://www.titech.ac.jp/english/news/2015/031961.html?utm_source=bulletin&utm_medium=media&utm_campaign=bulletin39