Quantum Dot Spectroscopy Using Cavity Quantum Electrodynamics

We show how cavity quantum electrodynamics using a tunable photonic crystal nanocavity in the strong-coupling regime can be used for single quantum dot spectroscopy. From the distinctive avoided crossings observed in the strongly coupled system we can identify the neutral and single positively charged exciton as well as the biexciton transitions. Moreover we are able to investigate the fine structure of those transitions and to identify a novel cavity mediated mixing of bright and dark exciton states, where the hyperfine interactions with lattice nuclei presumably play a key role. These results are enabled by a deterministic coupling scheme which allowed us to achieve unprecedented coupling strengths in excess of $150\mu \mathrm{eV}$.

Figure 1

$X^0$ level scheme. (a) QD eigenstates and the cavity mode one-photon state. (b) As the cavity electric field vector is parallel to the $y$ direction at the QD, coherent coupling $g$ occurs with $|X_y^0,0_c⟩$, giving rise to the polariton states $|p_{\pm }⟩$, while $|X_x^0,0_c⟩$ is not influenced by the cavity mode.

Figure 2

PL color plot when tuning the cavity mode across the $X^{+}$ and $X^0$ transitions. For both lines a clear anticrossing can be observed. The inset shows a typical PL spectrum for the cavity blue detuned from the $X^{+}$ line. For wavelengths on the red side of 934.5 nm the color scale has been offset by a factor of 26 in order to highlight the biexciton emission.

Figure 3

Comparison of $X^0$ dynamics for different emission polarization. (a) Zoomed-in version of Fig. 2 for the polarizer set in order to maximize cavity emission. The inset shows level schemes for two different positions of the cavity mode. In the lower inset the cavity is red-detuned from the $X^0$. As the cavity photon energy is reduced, $|p_{-}⟩$ moves in resonance with the dark states $|D_{a,b}⟩$ as indicated in the upper inset. (b) Here the polarizer is set to 55° with respect to the cavity emission and the signal is plotted in a logarithmic color scale.