Spectroscopy of the $D_1$ transition of cesium by dressed-state resonance fluorescence from a single (In,Ga)As/GaAs quantum dot

We use a laser-driven single (In,Ga)As quantum dot (QD) in the dressed-state regime of resonance fluorescence $(T=4\text{K})$ to observe the four $D_1$-transition lines of alkali atomic cesium $\left(\mathrm{Cs}\right)$ vapor at room temperature. We tune the frequency of the dressing continuous-wave laser in the vicinity of the bare QD resonance $\sim 335.116\mathrm{THz}$ $(\sim 894.592\mathrm{nm})$ at constant excitation power and thereby controllably tune the center and side channel frequencies of the probe light, i.e., the Mollow triplet. Resonances between individual QD Mollow triplet lines and the atomic hyperfine-split transitions are clearly identified in the $\mathrm{Cs}$ absorption spectrum. Our results show that narrow-band (In,Ga)As QD resonance fluorescence (RF) is suitable to optically address individual transitions of the $D_1$ quadruplet without applying magnetic field or electric field tuning.

Figure 1

(a) Emission spectra of two (In,Ga)As/GaAs QDs under near-resonant optical pumping (laser-${\mathrm{QD}}_1$ detuning $\Delta =+280\mu \mathrm{eV}/67.7$ GHz; $P_0=150\mu \text{W}$). (b) Autocorrelation of the excitonic emission of ${\mathrm{QD}}_1$ in (a); black dashed (bold red [gray]) lines represent fits to the data (deconvoluted with $\Delta {\tau }_{\det }=400$ ps). (c) Strictly resonant emission spectrum $(\Delta =0$; $P_0=150\mu \text{W}$) of ${\mathrm{QD}}_1$. (d) Corresponding high-resolution (HRPL) measurement of the dressed-state RF (Mollow triplet) of ${\mathrm{QD}}_1$, revealing a bare Rabi splitting of ${\Omega }_0=9.9\pm 0.2$ GHz in this case. Inset: Linear dependence of Rabi splitting on square root of power ${\Omega }_0\propto {\left(P_0\right)}^{1/2}$ [8].

Figure 2

(a) Cesium-$D_1$ quadruplet of dipole-allowed transitions transition between states $6{}^2{S}_{1/2}$ and $6{}^2{P}_{1/2}$ [14]. (b) Dependence of dressed-state emission on laser-QD detuning $\Delta ={\nu }_{\mathrm{laser}}-{\nu }_0^{QD}$: The position and Rabi splitting ${\Omega }_{\mathrm{eff}}\left(\Delta \right)$ of lines $R$ and $F/T$ can be tuned by the value of $\Delta$. Inset: Diagram of a dressed two-level state at $\Delta =0$ [8].

Figure 3

(a) Resonance scan between the full RF (Mollow triplet) of $Q{D}_1$ and ${}^{133}\mathrm{Cs}$: Photon signal after passing the vapor. Dips in the absorption spectrum (black dots) denote resonances between the $R/F$ sidebands and the $D_1$ quadruplet. (b) Calculated Doppler-broadened absorption profile of QD RF behind a 7.5-cm-long Cs cell $(T=293\text{K})$: Normalized absorption of $F,R$, and $T$ sidebands (black) and their weighted sum (bold red [gray]). (c) Resonance tuning between $Q{D}_1$ RF and $\mathrm{Cs}-D_1$. Black symbols: Mollow triplet center $\left(R\right)$ and side band $\left(F,T\right)$ frequencies in dependence on laser detuning $\Delta ={\nu }_{\mathrm{laser}}-{\nu }_0^{QD}$. Absorption resonances with $D_1$ transitions ${\nu }_{F{F}^{\prime }}$ can be clearly assigned to corresponding dips in Fig. 3.