Unveiling spin-flip processes in a neutral quantum dot using an anisotropic photonic structure
Résumé
We introduce a method to investigate excitonic spin flips in a neutral quantum dot (QD) that is driven
nonresonantly. By inserting the QD in an anisotropic photonic structure, one creates an imbalance between the
radiative decay rates of the two bright excitons. Direct spin flips between the bright excitons as well as indirect
ones (via a dark exciton) mix the level populations and profoundly affect the degree of linear polarization of
the excitonic emission. Measuring this quantity under continuous wave optical excitation yields the spin-flip
rate over a broad range of excitation powers. Additional time-resolved experiments allow disentangling the
contributions of bright-bright and dark-bright spin flips in the low-excitation regime. After providing theoretical
background, we demonstrate the method on a self-assembled InAs QD embedded in a GaAs photonic wire
featuring an elliptical cross section. For low-excitation power and at T = 5 K, bright-bright spin flips are much
slower than dark-bright spin flips, which, in turn, remain much slower than the radiative decays. Upon increasing
the temperature, we observe a superlinear increase in the bright-bright spin-flip rate which completely reverses
the rate hierarchy above T = 50 K. Moreover, polarization measurements reveal a dramatic increase in the
spin-flip rate with the pumping power. Our findings are relevant to spontaneous emission control by anisotropic
photonic structures and to the spectral coherence of QD-based quantum light sources.