The rich nonlinear optical responses in a squaraine derivative: Nonlinear absorption and transient nonlinear refraction dynamics

The abundant nonlinear optical responses of a squaraine derivative (2, 4-bis[4-(diethylamino)-2-hydroxyphenyl] squaraine, SQ-640) are explored at visible and near-infrared (NIR) regions. The sign and magnitude of the thirdorder (intensity depend) nonlinear absorption and refraction for SQ-640 are determined under different pluses with Z-scan method. The results of this work show reverse saturable absorption (RSA) of SQ-640 at visible and NIR region and its potential for optical limiting (OL) application. Moreover, SQ-640 shows self-focusing at visible wavelength while shows self-defocusing at NIR wavelengths. The transient refraction dynamics indicate that positive excited-state refraction causes self-focusing at visible region while negative nonlinear refraction at NIR region mainly originates from bound electron response (Kerr effect) and negative excited state refraction. Besides, the nonlinear refraction index of SQ-640 under 532 nm ranges from - 10-19 m2/W at femtosecond pulses to - 10-16 m2/W at nanosecond pulses and the second order hyperpolarizability is about 4 x 10-28 esu at nanosecond case. The Kerr refraction coefficient is about -6.54 x 10-19 m2/W at 700 nm under femtosecond laser pulses. The obtained OL thresholds at 532 nm are - 26.8 mJ/cm2 for picosecond pulses and - 85.6 mJ/cm2 for nanosecond pulses. The enriching information extracted from NLR in SQ-640 may offer an optional perspective to understand the structure-property relationships in these compounds. The results of optical nonlinearities in SQ-640 gained from this study may be of assistance to its better applications in laser protection and optical switching.