An article written by H. Zen, H. Ohgaki, R. Hajima published in Applied Physics Express.
Title; Record high extraction efficiency of free electron laser oscillator
This paper was selected as a Special Paper of the Year 2020.
Highlights of 2020: https://iopscience.iop.org/journal/1882-0786/page/APEX_JJAP_Highlights_2020
Advanced Energy Generation Division, Quantum Radiation Energy Research Section