High guided mode–cavity mode coupling for an efficient extraction of spontaneous emission of a single quantum dot embedded in a photonic crystal nanobeam cavity

We demonstrate efficient extraction of a single InAs quantum dot spontaneous emission through an air-suspended waveguide to which a photonic crystal nanobeam cavity is coupled. First, the quantum dot emission into the cavity mode is enhanced due to a high Purcell effect arising from a large quality factor and a small mode volume of the cavity mode. Second, the cavity photons can be efficiently transferred to the guided mode by optimizing the design of a structure that exhibits a high coupling rate between the two modes. This configuration has lead to the observation of a high extraction efficiency ($79\%$) of the quantum dot emission through the waveguide. Moreover, as a result of the high extraction efficiency to the guided mode, the unwanted microphotoluminescence coupling to the free space modes and detected from the top surface of the sample is strongly suppressed. Indeed, we show that it exhibits a negative peak as a function of the detuning between the cavity mode energy and the quantum dot transition energy.

Figure 1

(a) SEM image of the cavity coupled to an air-suspended waveguide. (b) SEM image of a nanobeam cavity. (c) Schematic of the nanobeam cavity. The guided mode intensity $I_{wg}$ is detected from the side of the sample, whereas the free space mode intensity $I_{fs}$ is detected from the top of the sample.

Figure 2

(a) Representation of the four spontaneous emission channels in which the QD can emit a photon. The number of photons per unit of time in each channel is indicated on the arrows. (b) Schematic of the nanobeam cavity exhibiting the coupling constants ${\kappa }_{wg}$, ${\kappa }_{fs}$, ${\gamma }_{wg}$, and ${\gamma }_{fs}$.

Figure 3

Nanobeam quality factor $Q$ vs number of holes $N_h$ per mirror, obtained by FDTD simulation.

Figure 4

(a) ${\beta }_c\left(0\right)$ factor, (b) $C_{wg}$ factor, and (c) overall extraction efficiency into the guided mode ${\beta }_c\left(0\right)\times C_{wg}$ as function of $N_h$, derived from the FDTD simulation shown in Fig. 3.

Figure 5

(a) and (b) Color plots of the measured spectra under various detuning obtained (a) in the guided configuration and (b) in the top surface detection configuration. (c) $\mu$PL intensity $I_{wg}$ measured from the waveguide (solid circles) vs the cavity mode QD transition detuning. (d) $\mu$PL intensity $I_{fs}$ measured from the top surface of the sample (solid circles) vs the cavity mode QD transition detuning. The fitting curves (solid line) are obtained from the Eqs. (15) and (16).

Figure 6

(a) and (b) Color plots of the measured spectra under various detuning obtained (a) in the guided configuration and (b) in the top surface detection configuration. (c) $\mu$PL intensity $I_{wg}$ measured from the waveguide (solid circles) vs the cavity mode QD transition detuning for three different pumping powers 1 $\mu$W (red curve), 3 $\mu$W (green curve), and 10 $\mu$W (blue curve). (d) $\mu$PL intensity $I_{fs}$ measured from the top surface of the sample (solid circles) vs the cavity mode QD transition detuning. The fitting curves (solid lines) are obtained from Eqs. (15) and (16).

Figure 7

Three-level system describing the QD spontaneous emission under nonresonant pumping from the ground state $G$ into the hot electron-hole pairs level R. The hot pairs can relax towards the studied QD (${\gamma }_{RX}$). Then the confined exciton (X) relaxes radiatively with a decay rate ${\gamma }_X$, the total number of photons emitted per second being $N_P(\delta ,P)$.