Picosecond-resolved nonlinear absorption of spin-processible lead sulfide (PbS) nanocrystals from 1100 to 1600 nm

Solution-synthesized nanocrystals which manifest strong quantum-confinement effects enable size-tunability of spectral properties and strong excitonic effects. Lead sulfide (PbS) nanocrystals are especially interesting for applications in telecommunication because their resonance is tunable to wavelengths from 1.3-1.55 μm and beyond. In other quantum dot systems, optically-induced bleaching of absorption has been shown to lead to a strong nonlinearity in the vicinity of the exciton peak wavelength [1][2-4][5-8]. We report herein results of picosecond-resolved transient absorption in spin-processible solution-synthesized PbS nanocrystals across the wavelength range 1100 nm to 1600 nm. The sample was synthesized using the solution phase organometallic method (hot injection technique), which provides good control over the size of the nanocrystals [9]. The sample consisted of nanocrystals with diameter around 5.2 nm resulting in an exciton peak at 1330 nm. Since the Bohr radius for the bulk PbS is 18 nm, these nanocrystals lay within the strong quantum-confinement regime [10]. Time-resolved absorption was studied using the single-wavelength collinear pump-probe setup. From the observed double-exponential decay trace of the transmission, fast and slow time constants were extracted. The fast component of few 10ís of ps was attributed to Auger recombination. The slow component is on the order of ns. The saturation intensity was also measured in this wavelength range using the Z-Scan technique [11]. The open aperture signals were fit to the intensity-dependent absorption model. The value of the saturation intensity was found to be 0.6 GW/cm ² around the exciton peak.