Control of the Strong Light-Matter Interaction between an Elongated ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ Quantum Dot and a Micropillar Cavity Using External Magnetic Fields

We have studied a strongly coupled quantum dot–micropillar cavity system subject to an external magnetic field. The large diamagnetic response of elongated ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum dots is exploited to demonstrate magneto-optical resonance tuning in the strong coupling regime. Furthermore, the magnetic field provides an additional degree of freedom to in situ manipulate the coupling constant. A transition from strong coupling towards the critical coupling regime is attributed to a reduction of the quantum dot oscillator strength when the magnetic confinement becomes significant with regards to the exciton confinement above 3 T.

Figure 1

(a) Diamagnetic shift of InGaAs QDs with an In content of 60%, 45%, and 30%, respectively, (b) $\mu \mathrm{PL}$ emission spectrum of a single ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ QD at low temperature (10 K), (c) measured continuous wave autocorrelation function of the $X$ transition [cf. panel (b)]. Photon emission from a single quantum emitter is clearly reflected in $g^{(2)}(0)=0.11<0.5$.

Figure 2

Temperature and magneto-optical resonance tuning of coupled ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ QD-micropillar system ($d_c=1.6\mu \mathrm{m}$, $Q=11000$). (a) Temperature dependent set of $\mu \mathrm{PL}$ spectra recorded at $B=0$. The anticrossing behavior of the QD exciton ($X$) and the cavity mode ($C$) is a clear signature of the SC regime. (b) Corresponding energy dispersion of the two emission modes. (c) Resonance tuning of $X$ and $C$ under variation of the magnetic field at $T=28\mathrm{K}$. (d) Corresponding energy dispersion of the two emission modes. In panels (b) and (d) the energy dispersion of the uncoupled modes is indicated by red dashed lines.

Figure 3

(a) Normalized $\mu \mathrm{PL}$ spectra at resonance for magnetic fields between 0 and 5 T. (b) Corresponding vacuum Rabi splitting $\Delta E_R$ and coupling constant $g$ as a function of magnetic field. A strong decrease of $\Delta E_R$ is observed for $B>3\mathrm{T}$. Inset: Scanning electron microscopy image of ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ QDs before overgrowth. (c) Effective oscillator strength of $X$ and magnetic length as a function of the magnetic field $B$. The horizontal black line indicates the lateral extension of the electron and hole wave function ($2\sqrt{⟨{\rho }_{e,h}^2⟩}=15\mathrm{nm}$). Magnetic confinement becomes significant above 3 T when $l_{\mathrm{mag}}<2\sqrt{⟨{\rho }_{e,h}^2⟩}$. The thick solid red line represents the expected oscillator strength when taking the magnetic confinement in a simple geometric approach into account. (d) PL decay time of ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ QDs and oscillator strength determined from SC data as well as from the decay times, respectively, as a function of $B$.