Resonant Excitation of a Quantum Dot Strongly Coupled to a Photonic Crystal Nanocavity

We describe the resonant excitation of a single quantum dot that is strongly coupled to a photonic crystal nanocavity. The cavity represents a spectral window for resonantly probing the optical transitions of the quantum dot. We observe narrow absorption lines attributed to the single and biexcition quantum dot transitions and measure antibunched population of the detuned cavity mode [$g^{(2)}(0)=0.19$].

Figure 1

(a) Cross-polarized confocal microscope setup. (b) $E_y$ field in the $L3$-cavity structure. The structure is perturbed at the sites indicated by dashed circles, as described in Ref. 9. (c) Scanning electron micrograph of fabricated structure. (d) Energy levels of the coupled QD-cavity system showing the two polarization states of the single exciton ($X$, $Y$) and the biexciton state ($XX$). The table lists the system parameters derived from the measurements, where $g$, $\kappa$, ${\gamma }^{*}$ are the vacuum Rabi frequency, cavity field decay rate, and dipole dephasing rate, respectively. The dipole decay rate $\gamma$ is estimated from the lifetime of uncoupled QDs. (e) The photoluminescence (PL) shows the QD-cavity anticrossing as the QD is temperature tuned through the cavity. (f) Vacuum Rabi splitting observed in the reflectivity from the strongly coupled QD-cavity system. For comparison, we show the reflectivity of an empty cavity.

Figure 2

QD spectroscopy through cavity mode at 10 K. (a) Intensity on spectrometer when the pump wavelength ${\lambda }_p$ is scanned across the detuned QD-cavity system (the laser has a weak red-detuned side band; power is 12 nW before the objective lens). When ${\lambda }_p$ is resonant with the dot (cavity), the cavity (QD) intensity rises. (b) Spectrum when QD is pumped (the middle peak corresponds to the laser side mode) (c) spectrum when cavity is pumped. In (b) and (c), we model the driving mechanism by a pure dephasing process with ${\gamma }^{*}=0.1g$. The model agrees well with the emission through the cavity (QD) when the QD (cavity) is pumped. (d) The integrated cavity emission as a function of the pump wavelength ${\lambda }_p$ shows the single-exciton absorption resolved to 3 GHz. (e) The integrated QD emission.

Figure 3

(a) Dependence of integrated cavity emission on QD detuning when the QD is resonantly pumped. (b) Pump power dependence: at a pump power of 200 nW, a second line becomes visible at a pump wavelength ${\lambda }_p=919.87\mathrm{nm}$. (The lines are slightly shifted since the temperature was, for technical reasons, raised to 18 K). (c) Integrated cavity emission as ${\lambda }_p$ is scanned. Inset: The power dependence of the two lines suggests exciton ($X$) and biexciton ($XX$) states; in this inset, a background of 40 was subtracted.

Figure 4

(a) Resonant QD excitation at 10 K. For the autocorrelation measurements, the cavity emission is isolated from the pump using a grating filter providing 17 dB of isolation at $\delta =-1.2\mathrm{nm}$ detuning. The laser is operated in continuous wave here so that is has low overlap with the cavity. (b) Autocorrelation histogram of cavity emission when the QD is pumped resonantly with 40-ps pulses (spectrum not shown). (c) Time-resolved resonant pumping of the quantum dot and emission into the cavity mode. A theoretical fit to the cavity emission yields an estimate of the pure dephasing rate.