Femtosecond laser ablation of TiO2 films for two-dimensional photonic crystals

Femtosecond laser ablation of TiO2 thin films was studied as a function of laser pulse energy and number of pulses. The ablated holes were characterized by atomic force microscopy and the optimum irradiation conditions were chosen for producing a periodic structure of triangular lattice of air holes in the TiO2 films. A photonic structure with period of 1.5 μm was designed to show photonic band gap in the near-infrared, at the telecommunication wavelengths. The plane wave expansion method was used to compute the photonic band gap of the laser ablated structure. The impact of limited laser processing accuracy on the photonic band gap has been studied as variation of the radius of the holes. The structure was produced by tightly focused femtosecond laser beam in multi-pulses ablation regime, using a Ti:Sapphire CPA laser system with pulse duration of 200 fs and energy per pulse of tens of nanojoule for a focusing optics with numerical aperture NA=0.5. The optimum irradiation conditions were found as following: number of pulse N=15 and laser beam energy E=58 nJ. The resulting periodic structure has an estimated photonic band gap centered at 1.53 μm with a bandwidth of about 42 nm.