Optical Stark Effect in a Quantum Dot: Ultrafast Control of Single Exciton Polarizations

We report the first experimental study of the optical Stark effect in single semiconductor quantum dots (QD). For below band gap excitation, two-color pump-probe spectra show dispersive line shapes caused by a light-induced blueshift of the excitonic resonance. The line shape depends strongly on the excitation field strength and is determined by the pump-induced phase shift of the coherent QD polarization. Transient spectral oscillations can be understood as rotations of the QD polarization phase with negligible population change. Ultrafast control of the QD polarization is demonstrated.

Figure 1

(a) Schematic energy diagram. (b) PL spectrum of a single QD resonance ${\omega }_{\mathrm{QD}}=1.6503\mathrm{eV}$ and differential reflectivity $\Delta R(\omega )/R_0$ for above band gap excitation at $\Delta t=50\mathrm{ps}$ (solid circles). Open circles: $\Delta R(\omega )/R_0$ for below band gap excitation at $\Delta t=-4\mathrm{ps}$ with 2 ps pulses at 1.647 eV ($\sigma =0.8\mathrm{meV}$). The pump power is varied between 0.12 and $0.58\mu \mathrm{W}$. Solid lines: Bloch equation model. (c) Variation of the signal magnitude $\Delta R_m({\omega }_{\mathrm{QD}})/R_0$ with pump power. (d) Phase shift of the QD polarization versus pump power.

Figure 2

(a) Time evolution of $\Delta R_m(\Delta t)/R_0$ for a single QD at ${\omega }_{\mathrm{QD}}=1.6544\mathrm{eV}$. $\Delta R_m(\Delta t>0)$ vanishes and the signal for $\Delta t<0$ decays on a picosecond time scale. (b),(c) Bloch equation simulation of the QD polarization $P_{\mathrm{QD}}$ and $\Delta R$ spectrum (inset) in the weak and strong excitation regime.

Figure 3

Near-field $\Delta R(\omega )/R_0$ spectra (circles) measured with 2 ps pump pulses at different delay times $\Delta t$ (exciton resonance at ${\omega }_{\mathrm{QD}}=1.6575\mathrm{eV}$, ${\omega }_p=1.6531\mathrm{eV}$, $P_p=0.7\mu \mathrm{W}$). Solid lines: Bloch equation simulations assuming $T_2=8\mathrm{ps}$.