Gain Control in Semiconductor Quantum Dots via State-Resolved Optical Pumping

Excitonic state-resolved optical pumping experiments were performed on strongly confined semiconductor quantum dots. We demonstrate for the first time that optical gain is dependent upon the initial excitonic state. By prescribing the specific multiexcitonic states which can create, block, and ultimately control optical gain spectra, we recover the theoretically predicted size independence, even in systems which previously showed zero gain. In addition, we show for the first time that stimulated emission in quantum dots can be controlled via specific multiexcitonic interactions.

Figure 1

State-resolved optical pumping of quantum dots. Absorption, emission, and nonlinear absorption spectra (band-edge excitation) of CdSe quantum dots ($R=2.8\mathrm{nm}$) in toluene solution (a). The nonlinear spectra were measured following excitation of the transitions denoted by the arrows. The negative region (red) corresponds to optical gain. Nonlinear absorption spectra as a function of the initial excitonic state. The maximum of the stimulated emission redshifts as excitonic energy is increased (b). Stimulated emission cross sections normalized to the band-edge exciton absorption cross section as a function of exciton concentration ($⟨N⟩$) for the different initial excitonic states (c).

Figure 2

Size-independent gain thresholds obtained under conditions of band-edge excitation. The normalized absorption bleaching ($-\Delta \mathrm{OD}/{\mathrm{OD}}_0$) as a function of the average number of excitations per particle $⟨N⟩$ for one size of dot (a). Gain is achieved when this ratio is greater than one. The upper panel displays the normalized linear absorption, photoluminescence, and stimulated emission spectra. The differential gains and thresholds for three sizes of quantum dots ($R=2.8$, 2.1, and 1.4 nm) (b). The development of optical gain is independent of particle size, recovering the universal behavior predicted by theory.

Figure 3

Gain tailoring via the initial excitonic state. The stimulated emission spectra as a function of $⟨N⟩$ for colloidal CdSe/ZnS ($R=2.8\mathrm{nm}$) in toluene, scaled to the absorption cross section of the band-edge exciton for two initial excitonic states (a),(b). The upper panels show the stimulated emission spectra for each pump at the maximum fluence. Development and spectral tuning of amplified spontaneous emission based upon the initial state (c). State-dependent ASE thresholds (d).