(a) The linear absorption spectrum of CdSe quantum dots dispersed in toluene, and associated PL. (b) The TA spectrum for the same sample 100 fs after low fluence excitation of the transition. The PA at 615 nm is largely responsible interfering with the development of optical gain in these systems. (c) The nonlinear absorption spectrum 1 ps after high fluence excitation of the transition. The negative spectral feature is a direct indication of SE and resides in the region of the PA seen in (b).
The fluence dependent transient dynamics of the optical gain region for CdSe quantum dots [, , same sample as Figs. 1(a)–1(c)] dispersed in toluene. These dynamics are normalized to the sample’s available optical density at 620 nm (i.e., ). Values of correspond to negative signals in the nonlinear spectrum and imply the presence of SE. These dynamics were recorded after resonantly exciting the (a) and (b) transitions, as labeled in Fig. 1(a). Clearly, excitation of the transition forces the system to first overcome the influence of the PA before establishing the gain regime.
[(a)–(d)] The nonlinear spectra as a function of for colloidal CdSe/ZnS quantum dots dispersed in toluene, scaled to the absorption cross section of the band-edge exciton (i.e., , see text). The spectra are obtained 1 ps after excitation of the transition specified in the upper panels. The upper panels show the SE spectra for each pump at the maximum fluence. There is a distinct broadening and redshifting of the nonlinear spectrum as the energy of the exciton is increased.
(a) The occupancy, , and initial state dependence of the nonlinear cross section, , at the maximum of the measured optical gain spectra, , for colloidal CdSe/ZnS quantum dots dispersed in toluene. These correspond to each spectrum acquired in Fig. 3. is reported relative to the absorption cross section of the transition in the linear absorption spectrum, (i.e., ). Positive values of imply negative features in the nonlinear spectrum and the presence of SE. As the excitonic energy is increased the differential gain decreases. (b) The influence of initial excitonic state on the occupation threshold for the development of optical gain for the system of Fig. 3 (closed circles), as well as organically passivated CdSe quantum dots of the same radius (, , open circles). As the energy of the excitons is increased also increases. (c) The influence of initial excitonic state on the maximum measured SE cross-section . decreases as the exciton energy is increased. The SE and occupation thresholds improve for CdSe/ZnS (closed circles) relative to CdSe quantum dots (open circles).
[(a)–(c)] The spectrally resolved normalized absorption bleaching as a function of the average number of excitations per particle, , for three different CdSe quantum dot sizes (, 2.1, and 1.4 nm) dispersed in toluene. Gain is achieved when is greater than one. These spectra were taken 1 ps following resonant excitation of their associated band-edge transitions. The upper panels display the linear absorption , PL, and the SE spectra for each size dispersion. (d) The resulting differential gain and occupation thresholds are almost entirely independent of particle size and demonstrate near-universal behavior at the maximum of the SE spectrum for each particle size.
(a) The negative portion of the nonlinear spectrum of colloidal CdSe quantum dots dispersed in toluene following band-edge excitation as a function of time. The nonlinear absorption spectrum is normalized to an optical density for band-edge absorption of . The optical gain persists for . (b) In the case of CdSe/ZnS quantum dots dispersed in toluene, the measured nonlinear spectrum continues to exhibit gain for more than 200 ps following band-edge excitation.
(a) The measured SE spectra for CdSe/ZnS quantum dots dispersed in toluene 1 ps after excitation of the and transitions. The spectra are normalized to the absorption cross section of the band-edge transition, (i.e., ). The redshifting and broadening of the spectrum with increasing excitonic energy is evident. (b) The influence of initial excitonic state on the maximum of the measured SE spectra, , for colloidal CdSe (, , open circles) and CdSe/ZnS (, , closed circles) quantum dots. The spectra progressively redshift as excitonic energy is increased.
(a) ASE spectra of CdSe/ZnS quantum dots prepared in the form of close packed, drop cast films. With excitation at 600 nm, corresponding to the exciton, the ASE appears at 649 nm. Exciting the system at 500 nm, corresponding to the exciton with contributions from P-type character (see text), redshifts the ASE to 654 nm. The fluences of the 600 and 500 nm pumps for the largest ASE spectra shown are 6.57 and , respectively. (b) The fluence dependence of emission intensity at the ASE wavelength for each pump. The typical threshold behavior for the development of ASE is apparent. (c) As the fluence of the 600 nm pump is increased to , the magnitude of the ASE increases by more than an order of magnitude relative to the spectra seen in (a), largely obscuring contributions from the PL. The PL and ASE intensities (with 7.10 and pumping fluence) are presented on the scale. The full fluence dependence at 649 nm is provided in the inset.
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