Low power optical limiting in ClAl-Phthalocyanine due to self defocusing and self phase modulation effects

Thermally induced optical nonlinearity of chloroaluminiumphthalocyanine (ClAlPc) in ethanol is studied in the regime of low laser powers using a cw He–Ne laser as the source of excitation. The optical response is characterized by measuring the intensity dependent refractive index (n2) of the medium using the z scan technique. The value of n2 is found to be 18 × 10−12 m2/W, which is reasonably large. The predominant mechanism of nonlinearity is the thermally induced refractive index variation. Self diffraction of the laser beam arising due to self phase modulation induced by the sample is investigated both experimentally and theoretically. Aperture limited optical limiting is performed by making use of self diffraction. Parameters of optical limiting such as the threshold value can be engineered by modifying the parameters of the set up.