Quantum nature of condensed helium and hydrogen appears clearly because of the low mass of the atoms and the extremely weak forces between them. Our current topics are (1) the study on the nonlinear optics of solid hydrogen and (2) the interfacial friction of physisorbed helium films.

(1) The study on the nonlinear optics of solid hydrogen.
The solid hydrogen is an optical material with both a high density and a narrow line width of spectra because its molecule in solid can rotate and vibrate freely. At present, we have been developing the method of the crystal growth on the solid hydrogen. This is a joint study with Prof. Hakuta and Associate Prof. Katuragawa.

(2) The interfacial friction of physisorbed helium films.

Although the interfacial friction is a very familiar phenomenon, the mechanism on atomic scale is still an open question. We have been studying the interfacial friction of physisorbed helium films at low temperatures. This friction of the inert helium films shows a peculiar feature that the frictional force decreases drastically with decreasing temperature. This temperature dependence can be explained by the pinning of thermally activated excitations of the films. The understanding of the friction on atomic scale is important to open the possibility of controlling the friction.