Recent dental studies have reawakend interested in photo-induced ablation for hard tissue such as enamel or dentine. On the other hand, a novel Free Electron Laser (FEL) haveing the broad tunability and the unique short pulse structure has been developed and used for various apprications. Our FEL irradiation experiments show high efficiency for the photo-induced ablation when the laser is tuned to absorption maximam of the target. Using the FEL tuned to phosphoric acid ion(9.4 g m)which is an

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... sorption maximum in infrared region on dentine, we investigate the irradiation effect on dentine. As a result, it was observed that irradiated dentine surface which was amorphous changed to the recrystalized structure. This result indicates that 9.4 g m-FEL irradiation caused selective ablation of phosphoric acid ion and annealin g .