All-optical tailoring of single-photon spectra in a quantum-dot microcavity system

Semiconductor quantum-dot cavity systems are promising sources for solid-state-based on-demand generation of single photons for quantum communication. Commonly, the spectral characteristics of the emitted single photon are fixed by system properties such as electronic transition energies and spectral properties of the cavity. In the present work we study cavity-enhanced single-photon generation from the quantum-dot biexciton through a partly stimulated nondegenerate two-photon emission. We show that frequency and linewidth of the single photon can be fully controlled by the stimulating laser pulse, ultimately allowing for efficient all-optical spectral shaping of the single photon.

Figure 1

Sketch of emission scheme and spectral control. The external control pulse (blue arrow) triggers the first photon, the second one (yellow arrow) is spontaneously emitted. Panel (a) schematically illustrates the scheme. Panel (b) shows the partly stimulated two-photon transition between biexciton $B$ and ground state $G$ in the energy-level diagram of the quantum-dot cavity system for the horizontal mode. The cavity (yellow vertical braces) enhances the second photon. It is off resonant to the transitions of the biexciton-exciton cascade, indicated for the vertical mode starting from the biexciton (black arrows).

Figure 2

Controlling the single-photon emission frequency. Shown is the numerically computed cw emission spectrum (yellow solid line) for a control Rabi energy ${\mathrm{\Omega }}_0=0.25$ meV in comparison with the analytic result (black dashed line) for a detuning of (a) ${\mathrm{\Delta }}_{\mathrm{L}}=0.0$ meV, (b) ${\mathrm{\Delta }}_{\mathrm{L}}=-0.3$ meV, and (c) ${\mathrm{\Delta }}_{\mathrm{L}}=+0.3$ meV from the two-photon resonance condition (semilogarithmically). The emission line of the single photon is marked by the white arrow and the biexciton-to-exciton emission is marked by the label $B\leftrightarrow X$.

Figure 3

Controlling the single-photon emission linewidth. The computed single-photon spectrum is shown for pulsed scenarios with parameters ${\mathrm{\Omega }}_0=0.75$ meV, $\mathrm{\Delta }t=5$ ps, ${\mathrm{\Delta }}_{\mathrm{L}}=-0.09$ meV (yellow solid line) and ${\mathrm{\Omega }}_0=0.25$ meV, $\mathrm{\Delta }t=15$ ps, ${\mathrm{\Delta }}_{\mathrm{L}}=0$ (black dashed line) together with the cw scenario from Fig. 2 (blue solid line). Note that the pulses have equal pulse areas. The cavity Lorentzian (shaded area) is shown for comparison. The spectra are each normalized to their respective peak maximum.

Figure 4

Spectral shaping of the single photon. Computed emission spectra for a Gaussian pulse with $\mathrm{\Delta }t=15$ ps, containing three frequency components ${\mathrm{\Delta }}_{\mathrm{L}}$ with equal intensities (yellow solid line) and tuned intensities (black dashed line). Details are given in the text. The spectra are normalized to the peak maximum of the yellow curve. The cavity line is shown for comparison (shaded area).