Preparation, structure and third-order nonlinear optical properties of Gd-doped TGA@CdSe quantum dots for optical device applications

Water-soluble Gd-doped CdSe quantum dots (QDs) were prepared using a thioglycolic acid (TGA) ligand as a coating agent through a simple method at a low temperature. Different analyses, such as high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), energydispersive X-ray (EDX), and UV-visible absorption spectroscopy, were utilized to investigate the optical and structural characteristics of Gd-doped TGA@CdSe QDs. According to the results, the QDs were prepared in a cubic crystal structure with an average size of ~6.2 nm. The presence of Gd was confirmed by EDX analysis. Absorption analysis shows that Gd-doped TGA@CdSe QDs have an absorption edge at 436 nm. The blue shift of the absorption edge of the QDs compared to bulk CdSe is related to the effects of quantum confinement. In this research, we also report the optical nonlinearities of Gd-doped TGA@CdSe QDs using Zscan method at a wavelength of 532 nm and at different laser powers. The results of the openaperture (OA) and closed-aperture (CA) measurements indicate that the Gd-doped TGA@CdSe QDs exhibit thermally induced excited state absorption (ESA) and self-defocusing effects on the order of 10 −3 (cm/W) and 10 −8 (cm 2 /W), respectively. The imaginary and real parts of the third-order susceptibility (χ (3) ) were calculated based on β and n 2 under different laser powers. The results showed that the nonlinear optical (NLO) response of Gd-doped TGA@CdSe QDs, namely the third-order susceptibility χ (3) , was calculated to be on the order of 10 −6 (esu) and was found to increase with laser power. The obtained Z-scan results confirm that the grown QDs are suitable for photonics and laser-based applications. In addition, the optical limiting (OL) performance of the Gd-doped TGA@CdSe QDs was evaluated under CW laser excitation, where a gradual reduction in normalized transmittance with increasing incident intensity confirms the presence of NLA–induced OL behavior.