Nonlinear optical properties of semiconductor quantum dots

Abstract

The shift of the lowest quantized state ▿ E 1 in quantum dots (QDS) is connected with the crystallite radius R, determined, e.g. by X-ray techniques. Theory gives ▿ E 1 = CE B ex ( a B π / R) 2 with C = 0.67. C varies from 0.3 to 10 when using it as a fit parameter for samples from different sources indicating some inconsistencies in the determination of R. The splitting ▿ E 1/2 between first and second absorption peak is independent of R but is a function of x for R / a B ⪆ 1 and agrees with the spin-orbit splitting and / or other valence band pecularities. ▿ E 1/2 agrees for R / a B ⪅ with the R dependent separation between first and second quantized conduction band states. From the Stokes shift of absorption and emission and their halfwidths, we determine an upper limit for the Huang-Rhys factor S. It increases with decreasing R up to S ⩽ 2.0. Pump and probe beam (DTS) measurements show bleaching of the absorption structures almost independent of ℏ pump and sample temperature. The width of the DTS signal confirms S values between 1 and 2. One-beam bleaching measurements agree with theory and give values for the saturation intensity around 6 MW / cm 2 depending on ℏ pump and x. Luminescence under high excitation shows an additional high energy peak, which is the result of recombination of an excited two electron-hole pair state in the dot, in agreement with theory. This state can also be seen in DTS measurements as induced absorption. Spectroscopy with self-diffraction from laser-induced gratings at room temperature shows rather broad (≈ 20 nm) efficiency spectra. The nonlinearity is gain the bleaching of absorption, which yields values of some 10 -9 esu if expressed in terms of an effective x (3). This value is much lower than for bulk CdS due to the dilution of the semiconductor in the glass.