Development of latent images due to transient free carrier electrons by femtosecond laser pulses and its application to grating shape trimming

The effects of prepulse exposure on the threshold energy to encode a micrograting structure on a SiO₂ glass surface by a prepulse-postpulse exposure technique using femtosecond (∼150 fs pulse duration) laser pulse irradiation were examined by measuring the laser intensity diffracted by the microgratings. It was revealed that prepulse exposure reduces the threshold energy of the postpulse from ∼0.4 to ∼0.2 J cm^-2, which enables to develop a latent image of the free carriers generated by the prepulse with the postpulse. This technique was applied to trim the shape of the micrograting pattern with the prepulse pattern. The role of the prepulse was investigated by comparing the dynamics of the photoexcited carriers in two dielectrics (SiO₂ glass and MgO single crystal) that have largely different lifetimes of photoexcited carriers. It was determined that the ultrashort relaxation time of the photoexcited carriers in SiO₂ glass limits the time delay of the postpulse to develop a latent image.