Study of the origin of the complex beam profile of a hole-coupled free electron laser oscillator
H. Zen and H. Ohgaki, Journal of Optical Society of America A, 38, pp.1656-1661 (2021).

Mode-selective excitation of an infrared-inactive phonon mode in diamond using mid-infrared free electron laser
R. Akasegawa, H. Zen, K. Hachiya, K. Yoshida, T. Goto, T. Sagawa, and H. Ohgaki, Japanese Journal of Applied Physics, 60, 102001 (2021).

Application of mid-infrared free electron laser for structural analysis of biological materials
T. Kawasaki, H. Zen, K. Ozaki, H. Yamada, K. Wakamatsu and S. Ito, Journal of Synchrotron Radiation, 28, https://doi.org/10.1107/S160057752001406X (2021).