Observation of Dressed Excitonic States in a Single Quantum Dot

We report the observation of dressed states of a quantum dot. The optically excited exciton and biexciton states of the quantum dot are coupled by a strong laser field and the resulting spectral signatures are measured using differential transmission of a probe field. We demonstrate that the anisotropic electron-hole exchange interaction induced splitting between the $x$- and $y$-polarized excitonic states can be completely erased by using the ac-Stark effect induced by the coupling field, without causing any appreciable broadening of the spectral lines. We also show that by varying the polarization and strength of a resonant coupling field, we can effectively change the polarization axis of the quantum dot.

Figure 1

(a) Energy scheme of the quantum dot optical transitions coupled by polarized laser fields of Rabi frequency ${\Omega }_p$ (weak probe laser) and ${\Omega }_c$ (strong coupling laser) with the corresponding polarization vectors ${\widehat{u}}_p$ and ${\widehat{u}}_c$. The exciton states $|X⟩$ and $|Y⟩$, split by fine-structure ${\delta }_{xy}$ due to anisotropic exchange interaction, couple through linearly polarized transitions ${\widehat{\pi }}_x$ and ${\widehat{\pi }}_y$ to the biexciton state $|XX⟩$ and the empty quantum dot ground state $|0⟩$. The detuning $\Delta$ of the coupling laser is measured with respect to the energy $E=\frac{1}{2}(E_x+E_y)$ of a neutral exciton without fine-structure splitting. (b) Differential transmission spectra of the neutral-exciton transitions in a single quantum dot probed with ${\widehat{\pi }}_{+45}$-polarized laser at 4.2 K: The two spectra were measured for the same dot with the coupling laser on but far detuned (red line) and in the absence of a coupling laser (black line).

Figure 2

Neutral exciton states in the presence of a strong exciton-biexciton coupling field in gray scale representation (black and white corresponding to a relative absorption of $0.7\times {10}^{-3}$ and 0, respectively). Copolarized probe and coupling in (a) reveal the Autler-Townes doublet and the ac-Stark shift of the neutral exciton $|X⟩$ that is parallel to the coupling laser polarization. The exciton state $|Y⟩$ orthogonal to the coupling laser remains unaffected, as shown for cross-polarized probe and coupling in (b). Experimental data (c) showing that the exchange splitting can be canceled by exclusively shifting one transition (note different abscissa scales in the upper and lower panel). The corresponding calculation is plotted in (d). Parameters: $\hslash {\Omega }_c=45\mu \mathrm{eV}$ in Figs. 2a, 2b and $\hslash {\Omega }_c=120\mu \mathrm{eV}$ in Figs. 2c, 2d; $\hslash {\Omega }_p=5.3\mu \mathrm{eV}$ in all graphs.

Figure 3

The strong laser is coupled equally to the two biexcitonic transitions. The upper panel shows experimental data for (a) copolarized lasers and (b) cross-polarized lasers. The lower panel shows the corresponding simulation with $\hslash {\Omega }_{c,x}=\hslash {\Omega }_{c,y}=40\mu \mathrm{eV}$ and $\hslash {\Omega }_p=5.3\mu \mathrm{eV}$ which follow from experimental settings. The gray scales are the same as in Fig. 2.