Sketch of the studied system. A QD (green trapezoid) is placed a distance z below a metal mirror. The lateral extension of a QD emitting at 1.2 eV is typically a=20 nm. The plasmon wavelength is λpl=262 nm (figure is not to scale). The field amplitude of the plasmon decays exponentially away from the interface with a change of the electric field over the extension of the QD. The arrow over μ indicates the orientation of the point-dipole moment and the arrows over Λ indicate the orientation of the first-order mesoscopic moment. b, Boundaries of a QD (green frame) with the spatial extension of electron (blue) and hole (red) wavefunctions indicated inside. c, Sketch of a QD placed near a metallic structure. The QD can decay by emitting a photon (γph), by exciting a propagating plasmon (γpl), by coupling to lossy modes in the metal (γls) or by intrinsic non-radiative recombination (γnr; not shown).
Researchers from the Quantum Photonics Group at DTU Fotonik in collaboration with the Niels Bohr Institute, University of Copenhagen surprise the scientific world with the discovery that light emission from solid-state photon emitters, the so-called quantum dots, is fundamentally different than hitherto believed. The new insight may find important applications as a way to improve efficiency of quantum information devices.
Today it is possible to fabricate and tailor highly efficient light sources that emit a single photon at a time, which constitutes the fundamental unit of light. Such emitters are referred to as quantum dots and consist of thousands of atoms. Despite the expectations reflected in this terminology, quantum dots cannot be described as point sources of light, which leads to the surprising conclusion: quantum dots are not dots!
Semiconductor quantum dots (QDs) provide useful means to couple light and matter in applications such as light-harvesting and all-solid-state quantum information processing. This coupling can be increased by placing QDs in nanostructured optical environments such as photonic crystals or metallic nanostructures that enable strong confinement of light and thereby enhance the light–matter interaction. It has thus far been assumed that QDs can be described in the same way as atomic photon emitters—as point sources with wavefunctions whose spatial extent can be disregarded. Here we demonstrate that this description breaks down for QDs near plasmonic nanostructures. We observe an eightfold enhancement of the plasmon excitation rate, depending on QD orientation as a result of their mesoscopic character. Moreover, we show that the interaction can be enhanced or suppressed, determined by the geometry of the plasmonic nanostructure, consistent with a newly developed theory that takes mesoscopic effects into account. This behaviour has no equivalence in atomic systems and offers new opportunities to exploit the unique mesoscopic characteristics of QDs in the development of nanophotonic devices that use the increased light–matter interaction
This new insight was realized by experimentally recording photon emission from quantum dots positioned close to a metallic mirror. Point sources of light have the same properties whether or not they are flipped upside down, and this was expected to be the case for quantum dots as well. However, this fundamental symmetry was found to be violated in the experiments at DTU where a very pronounced dependence of the photon emission on the orientation of the quantum dots was observed.
The experimental findings are in excellent agreement with a new theory of light-matter interaction developed by DTU-researchers in collaboration with Anders S. Sørensen from the Niels Bohr Institute. The theory takes the spatial extent of quantum dots into account.
At the metal mirror surface, highly confined optical surface modes exist; the so-called plasmons. Plasmonics is a very active and promising research field, and the strong confinement of photons, available in plasmonics, may have applications for quantum information science or solar energy harvesting. The strong confinement of plasmons also implies that photon emission from quantum dots can be strongly altered, and that quantum dots can excite plasmons with very large probability. The present work demonstrates that the excitation of plasmons can be even more efficient than previously thought. Thus the fact that quantum dots are extended over areas much larger than atomic dimensions implies that they can interact more efficiently with plasmons.
The work may pave the way for new nanophotonic devices that exploit the spatial extent of quantum dots as a novel resource. The new effect is expected to be important also in other research areas than plasmonics, including photonic crystals, cavity quantum electrodynamics, and light harvesting.
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