In-plane radiative recombination channel of a dark exciton in self-assembled quantum dots

We demonstrate evidence for a radiative recombination channel of dark excitons in self-assembled quantum dots. This channel is due to a light hole admixture in the excitonic ground state. Its presence was experimentally confirmed by a direct observation of the dark exciton photoluminescence from a cleaved edge of the sample. The polarization-resolved measurements revealed that a photon created from the dark exciton recombination is emitted only in the direction perpendicular to the growth axis. Strong correlation between the dark exciton lifetime and the in-plane hole $g$ factor enabled us to show that the radiative recombination is a dominant decay channel of the dark excitons in CdTe/ZnTe quantum dots.

Figure 1

PL of the neutral exciton measured from the cleaved edge of the sample at zero magnetic field under cw excitation. (a) PL spectra measured under different excitation powers related to different average dot repopulation times (the saturation power of this QD corresponds to 2 mW). Relative intensity of the ${\mathrm{X}}_{\mathrm{d}}$ line becomes stronger for the repopulation time comparable to dark exciton lifetime. (b) Polar plot presenting the polarization of the dark and bright exciton emission for $0.6\mu$W excitation power. The ${90}^{\circ }$ and ${270}^{\circ }$ directions are along the QD growth axis. Symbols for angles $\phi$ and $\phi +{180}^{\circ }$ correspond to the same data points.

Figure 2

(a) PL spectra of a single QD for different values of the in-plane magnetic field. (b) PL decay curves of the dark exciton for different magnetic fields, measured without polarization resolution. Solid lines represent the fits of biexponential decays, which are related to the presence of two dark excitons with almost equal emission energies, but different efficiencies of mixing with bright excitons. (c) Inverse lifetime of the shorter-lived dark exciton as a function of the in-plane magnetic field. The solid line represents the fitted curve described by Eq. (3). The zero magnetic field lifetime of the dark exciton for this QD yields ${\tau }_0=32\pm 3$ ns.

Figure 3

Square root of the inverse zero-field dark exciton lifetime versus in-plane hole $g$ factor (each experimental point corresponds to different randomly selected QD). The solid line represents the linear fit predicted by the theory.

Figure 4

PL decay curve of the bright exciton at $B=0$ T. The dashed line represents the PL decay of the bright exciton predicted by the spin-flip model assuming equal initial populations of bright and dark states after an excitation pulse due to independent capture of electrons and holes under nonresonant excitation[7, 17] (the integrals under fast and slow components appear to be different due to the semilog scale of the plot). The zero-field lifetime of the dark exciton for this dot yielded $125\pm 20$ ns. Inset: PL decay of the dark exciton for the same quantum dot at $B=0.5$ T with exponential fit (solid line). For clarity, in both plots the cw background was artificially set to 10.