Eiji TOKUNAGA^1,2,3*, Naoto SATO¹, Jiro KORENAGA¹, Takayuki IMAI³, Shin SATO⁴, and Hiro-o HAMAGUCHI⁴

¹Department of Physics, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
²Research Center for Green Photo-Science and Technology, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
³Infrared Free Electron Laser Research Center, Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
⁴Department of Chemistry, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan

(Received September 19, 2009; Accepted December 15, 2009)

Visible nonlinear band-edge luminescence in ZnSe and CdS bulk crystals was observed upon excitation by a mid-infrared free-electron laser (mid-IR FEL) at approximately 9 μm. The emission intensity is proportional to the 74th and 45th powers of the excitation intensity for ZnSe and CdS, respectively. For ZnSe, the temporal profile of the emission intensity does not follow the profile of the excitation macropulse of the FEL, but sharply rises and decays only at the maximum of the macropulse profile. These features are in marked contrast to those of a previous report, where the emission profile follows that of the macropulse, and the emission intensity scales with the 4th power of the excitation intensity. The experimental observations were reproduced by a numerical simulation based on impact ionization and avalanche ionization by electrons accelerated by the optical electric field of the FEL. The large nonlinearity in the band-edge emission comes from the macropulse temporal structure, which consists of micropulses densely spaced to allow excited carriers to survive when the next micropulse arrives. They work as seed carriers in the next carrier multiplication step.

Key words: free electron laser, mid infrared, semiconductor, upconversion, optical nonlinearity, bandedge luminescence, impact ionization, ponderomotive potential, ZnSe, CdS

^*E-mail address: eiji@rs.kagu.tus.ac.jp