Abstract
Future on-chip quantum photonics requires controllable quantum emitters that can be operated on-demand and with the possibility of in situ control of the photon emission wavelength and its polarization state. Here, we report the first proof-of-principle demonstration of the dynamic real-time control, using radio frequency surface acoustic waves (SAWs), of the optical emission from quantum states induced by:
(i) quantum dots (QDs) embedded in epitaxially grown core-shell GaN/InGaN nanowire (NW) heterostructures in which the emitting QD-like exciton localization centers are induced by indium content fluctuations within the InGaN nanoshell, and
(ii) localized atomic-scale defects in two-dimensional (2D) hexagonal boron nitride (h-BN) flakes mechanically exfoliated from bulk crystals.
In both cases, the luminescent quantum states are identified using spatially, polarization- and time-resolved stroboscopic micro-photoluminescence (μ-PL) spectroscopy. They exhibit narrow and highly linearly polarized emission lines in the μ-PL spectra and a pronounced antibunching signature of single-photon emission in the photon correlation experiments.
For GaN/InGaN NW-QDs, depending on their location within the InGaN nanoshell, nonpolar (m-), semipolar (r-) or polar (c-facet) QDs are discerned, thereby making these NWs the first experimentally demonstrated single nanostructures able to host non-classical light emitters with both high- and low-polarity crystallographic orientations. Owing to their short radiative lifetimes resulting from weak built-in electric field values along the growth axis, the III-nitride QDs grown on alternative low-polarity crystallographic planes are highly beneficial for future high-speed quantum information technologies.
For h-BN defects, the direct visualization of their structure using atomic force microscopy under ambient conditions combined with density functional theory calculations of their band structures and electronic properties made it possible to associate the existence of several single-photon optical transitions with the observed defects, thus shedding light on the origin of quantum emitters in h-BN.
When such NWs or h-BN flakes are perturbed by the propagating SAW, the embedded emitters are periodically strained and their radiative transitions are dynamically modulated by the acousto-mechanical coupling, giving rise to a spectral fine-tuning within ~2 meV bandwidth at the acoustic frequency of ~330 MHz. This outcome is further combined with spectral detection filtering for temporal control of the emitted photons. In this way, both spectral tunability and on-demand emission of single photons are achieved simultaneously.
Moreover, the SAW-triggered acousto-electric effect inflicts changes in the charge population of the emitting quantum state as well as in its optical polarization degree (up to 30%). This is an important advance since, to date, the photon polarization state of III-nitride QDs, in particular, has been either probabilistic or pre determined by the electronic properties of the system.
In addition, by deliberately engineering different periodic waveforms of the driving RF signal, the resulting SAW mediated fine-spectral tuning of the emitter’s response allows to achieve more complex temporal sequence of the emitted photons. In this way, we can use the photon emission or arrival time as a degree of freedom to encode a qubit of information on a photon. This offers various advantages over other encoding schemes, including resilience against decoherence which makes this time-bin encoding better suited for fiber optics applications compared to e.g. polarization encoding.
Altogether, this study opens the door to the use of sound for scalable integration of various types of quantum emitters in nanophotonic and quantum information technologies. The advantage of the acousto-optoelectric over other control schemes is that it allows in-situ manipulation of the optical emission properties over a wide frequency range (up to GHz frequencies).
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