OPTICAL NONLINEARITIES AND DEFECT GENERATION IN A-SI-H THIN-FILMS

The non-linear change of refractive index and absorption coefficient o f a-Si:H films has been measured through the Z-scan technique using a picosecond (tau(FWHM) = 1 ps) high repetition rate (f(rep) = 3.8-9.5 M Hz) laser. The films were prepared by the radiofrequency glow discharg e technique. A reversible increase of the optical absorption under hig h-intensity illumination conditions has been measured, indicating an i ncrease of dangling bond states concentration up to N-r approximate to 10(19) cm(-3). In order to explain the experimental results, a theore tical model has been developed which takes into account the change of absorption, due to light-induced metastable changes, during a Z-scan m easurement. Non-linear changes of the refractive index as large as Del ta n = -.0.07 have been detected.