Observation of Mollow Triplets with Tunable Interactions in Double Lambda Systems of Individual Hole Spins

Although individual spins in quantum dots have been studied extensively as qubits, their investigation under strong resonant driving in the scope of accessing Mollow physics is still an open question. Here, we have grown high quality positively charged quantum dots embedded in a planar microcavity that enable enhanced light-matter interactions. Under a strong magnetic field in the Voigt configuration, individual positively charged quantum dots provide a double lambda level structure. Using a combination of above-band and resonant excitation, we observe the formation of Mollow triplets on all optical transitions. We find that when the strong resonant drive power is used to tune the Mollow-triplet lines through each other, we observe anticrossings. We also demonstrate that the interaction that gives rise to the anticrossings can be controlled in strength by tuning the polarization of the resonant laser drive. Quantum-optical modeling of our system fully captures the experimentally observed spectra and provides insight on the complicated level structure that results from the strong driving of the double lambda system.

Figure 1

(a) Above-band excitation power dependence of the emission from the investigated quantum dot. Saturation occurs at $1.25\pm 0.05\mu \mathrm{W}$ driving power. (b) Time-resolved QD emission under wetting-layer excitation giving a lifetime of $597\pm 2\mathrm{ps}$. The inset depicts the sample’s structure. (c) Zeeman splitting of the charged quantum dot. The inset shows the four-level structure of the positively charged QD under a high magnetic field in the Voigt configuration, along with the allowed transitions and the driving schemes investigated in this work. (d) Polarization analysis of the PL spectrum at 5 T magnetic field.

Figure 2

(a)–(d) Power-dependent experimental PL spectra for a QD strongly driven with a laser spectrally positioned in-between the two inner transitions ($0\mu \mathrm{eV}$), for four different excitation (detection) angles. The plots are shown in linear color scale and have the eigenenergies of Eq. (1) superposed in dotted red lines. (e)–(h) Calculated spectra from quantum-optical modeling for the same polarization settings as in (a)–(d), respectively.

Figure 3

(a) Power-dependent spectra when the laser is in resonance with the highest energy transition of the four-level system [transition 1–4 or yellow arrow in Fig. 1] at $112\mu \mathrm{eV}$. The color scale is here logarithmic to highlight the ghost branches of the two inner transitions which appear at energies higher than any of the QD states that exist in the lab frame. (b) PL spectrum when pumping the lowest energy transition [transition 2–3 or violet arrow in Fig. 1] at $-112\mu \mathrm{eV}$. The dotted red lines are the eigenenergies calculated from Eq. (1). (c),(d) Modeled spectra with parameters that correspond to the experimental conditions of (a) and (b), respectively.