Largely Enhanced Optical Nonlinear Response of Heavily Doped Ag:CdTe Nanocrystals around the Excitonic Band Edge

Semiconductor quantum dots (SQDs) have good nonlinear figures of merit (FOMs) but relatively poor nonlinear refraction. The linear and nonlinear optical processes of SQDs in the proximity of metallic nanostructures can be enhanced by the surface plasmon resonance, but the nonlinear FOMs are limited by the enhanced linear absorption. Here, we investigate optical third-order nonlinearity and the corresponding FOMs of the CdTe quantum dots heavily doped with Ag. The excitonic resonant absorption is enhanced and the IS peak is red-shifted and broadened after doping silver. Intriguingly, the nonlinear refraction near the band edge is enhanced more than 35 times, while the nonlinear absorption keeps very small at the crossover of the one-photon saturation absorption and two-photon excitation near the band edge, leading to the desired one- and two-photon FOMs (W and T) for the demands of all-optical waveguide switching. Our observations offer a strategy to prepare doped semiconductor quantum dots with large third-order susceptibility and good nonlinear FOMs and thus show prospective applications in optical information processing, switching, and modulating.