Effi›cient and broadband spontaneous emission control in fiber-like photonic nanowires

Funneling a large fraction of the spontaneous emission (SE) of a quantum emitter into a single optical mode is a powerful strategy for improving the brightness of quantum light sources or developing an efficient spin-photon interface. In the solid state, preferential emission into a single localized mode has been first achieved taking advantage of the Purcell effect that arises in semiconductor optical microcavities.

In the last years, the need to overcome the limited operation bandwidth inherent to a resonant approach has triggered intense research on SE control in waveguide structures. Among the investigated platforms, fiber-like photonic nanowires are particularly appealing, as shown by the recent development of a very bright single-photon source based on a wire with carefully engineered ends [1,2].

Here we focus on the mechanisms governing the SE dynamics of the embedded emitter and consider a photonic nanowire made of GaAs (refractive index n=3.5) and surrounded by air (n=1). It features a circular section (diameter d), and contains spectrally isolated single InAs quantum dots (QD) with a free space emission wavelength around 920 nm.

The large refractive index contrast between the wire and the air cladding has two important consequences: i) The coupling to the 3D continuum of non-guided modes is strongly inhibited, thanks to a pronounced dielectric screening effect. Experimentally, the coupling to these modes can be probed by studying the luminescence decay of QDs embedded in ’small’ wires (d=120 nm), for which the coupling to the guided mode is vanishingly small.

In that case, we measure a slow-down of the SE rate by a factor 16, a value which is comparable to the one obtained in state-of-the-art photonic crystal structures. ii) For larger structures (d=220 nm), the fundamental guided mode is tightly confined in the wire. The emitter is well coupled to this mode, and the SE rate becomes comparable to the one measured on a QD embedded in bulk GaAs.

These experimental results demonstrate the ability of these simple structures to funnel a large fraction (>90%) of the SE into the guided mode [3]. For some applications (e.g. polarization encoded quantum key distribution, generation of indistinguishable photons), it is desirable to control the polarization of the emitted photon.

This control can be efficiently implemented in a wire featuring an elliptical section with a moderate aspect ratio (∼ 2). In that case, calculations show that the local density of optical modes is largely dominated by a single guided mode, with a linear polarization oriented along the major axis of the ellipse.

Polarization-resolved measurements conducted on elliptical GaAs photonic nanowires embedding spectrally isolated InAs QDs fully confirm the predicted performances: the fraction of collected photons with the desired polarization can be as high as 95% [4].