We present third-order optical nonlinear absorption in CdSe quantum dots (QDs) with particle sizes in the range of 4.16–5.25？nm which has been evaluated by the -scan technique. At an excitation irradiance of 0.54？GW/cm2 the CdSe QDs exhibit reverse saturation indicating a clear nonlinear behavior. Nonlinearity increases with particle size in CdSe QDs within the range of our investigations which in turn depends on the optical band gap. The optical limiting threshold of the QDs varies from 0.35？GW/cm2 to 0.57？GW/cm2 which makes CdSe QDs a promising candidate for reverse-saturable absorption based devices at high laser intensities such as optical limiters. 1. Introduction Before the advent of lasers, transparent optical materials were assumed to be essentially passive unaffected by the light travelling through them. The high powers of laser beams made it possible to observe that the effect of light on a medium can indeed change its properties such as refractive index or absorption. These are optical nonlinear phenomena. When this happens, the light itself gets affected by this change in a nonlinear way; for example, the nonlinear response of the material can convert the laser light into new colours, both harmonics of the optical frequency and sum and difference frequencies. With the development of optical communication networks, various nonlinear optical (NLO) devices such as optical switches, optical limiters, optical detectors, and optical sensors have attracted considerable attention because of their widespread usage for scientific and industrial purposes. Among all the NLO properties, optical limiting is one of the most promising practical applications, as it can protect the human eye and photosensitive components from damage caused by intense optical radiation [1, 2]. Optical limiting results from irradiance-dependent NLO responses of materials in which the incoming intense light alters the refractive and absorptive properties, resulting in a greatly reduced transmitted intensity. It is important to select suitable materials as optical limiting media by determining the magnitude of their nonlinearity. Organic materials characterized by large NLO responses are of major interest owing to their large NLO susceptibilities, fast response time, architectural flexibility, low cost, and ease of fabrication [3, 4]. In recent years, interest in the synthesis, characterization, and application of colloidal quantum dot (QD) semiconductor materials has grown markedly. QDs of cadmium selenide (CdSe) are by far the most studied system among all the semiconducting
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