Spontaneous Two-Photon Emission from a Single Quantum Dot

Spontaneous two-photon emission from a solid-state single quantum emitter is observed. We investigated photoluminescence from the neutral biexciton in a single semiconductor quantum dot coupled with a high $Q$ photonic crystal nanocavity. When the cavity is resonant to the half energy of the biexciton, the strong vacuum field in the cavity inspires the biexciton to simultaneously emit two photons into the mode, resulting in clear emission enhancement of the mode. Meanwhile, the suppression of other single photon emission from the biexciton was observed, as the two-photon emission process becomes faster than the others at the resonance.

Figure 1

(a) Schematic illustration of the energy diagram of QD, including two single exciton states $|X⟩$, $|Y⟩$ and a biexciton state $|XX⟩$. (b) Energy diagram of the QD-cavity system in the cavity resonance with $|X⟩\to |G⟩$ transition, (c) with $|XX⟩\to |X⟩$ transition and (d) in the two-photon resonance. The weight of the superposition states, which depends on the cavity detuning, is omitted for simplicity.

Figure 2

(Top) Emission spectra under the cavity detuning of ${\delta }_c={\omega }_c-{\omega }_X\sim 300\mu \mathrm{eV}$. (Bottom) Color plot of the measured spectra under various cavity detunings. Whole spectra are normalized to the highest peak in the series of the cavity scanned spectra.

Figure 3

Color plot of the calculated spectra under various cavity detunings. The included cavity photon number is up to 2 for (a) and up to 1 for (b). Each spectrum at different detuning conditions in (a) is normalized to the highest peak among the whole spectra. The same normalization factor as (a) is used for (b).

Figure 4

Integrated intensities of the several emission lines around the two-photon resonance condition. (a) Experimental and (b) calculation results corresponding to Fig. 3. The data are normalized to the sum of the whole emission intensity at each detuning. $X$, $C$, $XX$, and $XXP$ lines are expressed as black square, red ball, blue up triangle, and magenta down triangle, respectively. Solid lines in (b) are from the calculation including the limited photon number up to one.