Optical nonlinearities and defect generation in a-Si:H thin films

The non-linear change of refractive index and absorption coefficient of a-Si:H films has been measured through the Z-scan technique using a picosecond (τFWHM = 1 ps) high repetition rate (View the MathML source) laser. The films were prepared by the radiofrequency glow discharge technique. A reversible increase of the optical absorption under high-intensity illumination conditions has been measured, indicating an increase of dangling bond states concentration up to Nr ≈ 1019 cm−3. In order to explain the experimental results, a theoretical model has been developed which takes into account the change of absorption, due to light-induced metastable changes, during a Z-scan measurement. Non-linear changes of the refractive index as large as Δn = −0.07 have been detected.