Extended Data Fig. 9: PL and optical gain characteristics of wider-bandgap type-(I+II) QDs.

a, PL spectrum of type-(I+II) CdSe QDs with r = 2.3 nm, l = 2.2 nm, h = 1.6 nm, and d = 0.3 nm. The experimental data are shown as open grey circles, and the double Gaussian fit is shown as a black line. The two individual Gaussian bands used in the fit are shown as blue and orange lines. The PL spectrum was measured using cw excitation with a photon energy of 2.76 eV. b, PL dynamics of the same sample collected for two different pump levels: 〈N〉 = 0.08 (black) and 0.38 (red) (the traces are normalized to match the signal intensity at 500 ns). The biexciton dynamics obtained by subtracting these two traces are shown as blue circles. A biexponential fit (blue line) yields a hybrid (direct/indirect) biexciton lifetime of 10.1 ns. The sample was excited using 190 fs, 3.1 eV pulses. c, The TA spectrum of wider bandgap type-(I+II) QDs presented as the excited state absorption coefficient (α) for a pump-probe delay of 10 ps and an excitation level 〈N〉 = 54 (190 fs, 2.4 eV pump pulses). The optical gain regime corresponds to α < 0. d, The TA dynamics yields an optical gain lifetime of 3.1 ns.