Effect of conjugation length on nonlinear optical parameters of anthraquinone dyes investigated using He–Ne laser operating in CW mode

• Nonlinear optical parameters of the dyes depend on the conjugation length.
• Dyes are characterized with self-defocusing type of nonlinearity.
• Multiple self-diffraction rings were observed due thermal lensing.

We report the studies on third-order optical nonlinearity and optical limiting of anthraquinone dyes. Z-scan technique was employed to evaluate the nonlinear parameters such as nonlinear absorption coefficient βeff and nonlinear index of refraction n2. Continuous wave He–Ne laser was used as the source of excitation. The estimated values of βeff, n2 and χ(3) are of the order of 10−3 cm/W, 10−5 esu and 10−7 esu respectively. The presence of donor and acceptor groups in the structure results in increase in conjugation length. This resulted in the enhancement of nonlinear optical parameters values of the dye. Multiple diffraction rings were observed when the samples were exposed to laser beam due to thermal lensing. Dyes exhibited good optical limiting behavior under the experimental conditions. The results indicate that the dyes investigated here are materialise as candidates for photonics device applications such as optical power limiters.