We study the generation of a self-chirped optical pulse in a free-electron laser (FEL) oscillator. In a high-gain FEL oscillator, the frequency chirp is induced in the slippage region as a result of superradiant FEL resonance, and this time-frequency correlation evolves continuously into a few-cycle regime, if the optical cavity length is perfectly synchronized to the electron bunch interval. Numerical simulations based on the slowly evolving wave approximation and experimental results are presented.

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We perform a proof-of-principle experiment for a nondestructive method for detecting the elemental and isotopic composition of materials concealed by heavy shields such as iron plates with a thickness of several centimeters. This method uses nuclear resonance fluorescence (NRF) triggered by an energy-tunable laser-Compton scattering (LCS) -ray source. One-dimensional mapping of a lead block hidden behind 1.5-cm-thick iron plates is obtained by measuring an NRF -ray of a lead isotope 208 Pb. We observe a 5512-keV -ray from 208 Pb excited by the quasi-monochromatic LCS -rays with energies up to 5.7 MeV. The edge position of the lead block is consistent with the exact position within the uncertainty.

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