Abstract
Two-photon selective excitation of an infrared (IR)-inactive phonon mode by a mid-infrared tunable free-electron laser (FEL) was accomplished. The mode-selective phonon excitation of IR-active mode by one-photon excitation using FEL (Yoshida et al. (2013) [20]) was extended to the IR-inactive mode excitation. The T2g optical phonon mode in single-crystal diamond at 1,332cm−1 was selected, and the FEL photon energy corresponding to the oscillation wavelength was tuned to the half of the target phonon energy. The successful mode excitation was confirmed by anti-Stokes Raman scattering signal of T2g for the sample at cryogenic temperature, where all other modes were suppressed. The signal was observed at T2g phonon energy with red-shift of the peak to 1,306cm−1, which was accompanied by surface damage induced by intense laser irradiations.
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