Enhance refractive index modification of fused silica by multiwavelength excitation process using F2 and KrF excimer lasers

A multiwavelength excitation process using F2 and KrF excimer lasers for the high-efficiency and high-speed refractive index modification of fused silica is demonstrated. This process is essentially superior to conventional single-wavelength F2 laser processing. The former process achieves twice the diffraction efficiency of the latter process at the same number of total photons supplied to the sample. This high-speed and high-efficiency modification is realized within ±50 ns of the delay time of each F2 and KrF excimer laser beam irradiation. In addition, the refractive index of the multiwavelength sample was increased to 8.2× 10−3, which is 1.78 times larger than that of the single-wavelength F2 laser irradiation sample at the same irradiation time. This superiority of the multiwavelength excitation process is attributed to a resonance-photoionization-like process based on excited-state absorption in fused silica.