Enhanced Coupling of Mesoscopic Quantum Dots to Plasmons
[Left to Right] Mads Lykke Andersen, Peter Lodahl and Søren Stobbe
[This is an invited article based on a recent work by the authors.
-- 2Physics.com Team]
Authors: Mads Lykke Andersen, Søren Stobbe, and Peter Lodahl
Affiliation : Quantum Photonics Group, DTU Fotonik, Technical University of Denmark.
Today it is possible to fabricate and tailor highly efficient solid-state light-sources that emit a single photon at a time. Such solid-state 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!
In collaboration with Anders Søndberg Sørensen from the Niels Bohr Institute at University of Copenhagen, we have in an article in Nature Physics  recently reported on the discovery that light emission from quantum dots is fundamentally different than hitherto believed. The new insight may find important applications as a way to improve the coupling between light and matter, which is a prerequisite for efficient quantum information devices .
In our experiments we recorded the photon emission rate from quantum dots positioned close to a metallic mirror. Using this simple nanostructure it was possible to directly compare the experimental findings to the expectation for a point-dipole source, and a pronounced discrepancy was observed. 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 violated in the experiments at DTU where a very pronounced dependence of the photon emission rate on the orientation of the quantum dots was observed.
Figure 1: Measured decay rates of quantum dots as a function of distance to the silver mirror for the direct (a) and inverted (b) structure at a wavelength of λ=1,030 nm. The dashed curves are the predictions for a point-dipole emitter. The solid curves show the results of a new theory valid for mesoscopic emitters, and are found to match the experimental data very well. The insets show the orientation of the quantum dots relative to the silver mirrors for the direct and inverted structures. [From Ref.  -- Thanks to 'Nature Physics']
The observation that the photon emission rate is dependent on the orientation of the quantum dots relative to the silver mirror is the experimental tell-tale that the point-dipole description breaks down and that the mesoscopic character of the quantum dot leads to modified light-matter interaction. The experimental data are found to be in excellent agreement with a new theory for light-matter interaction that takes the spatial extent of the quantum dots into account, see Figure 1.
The significant breakdown of the point-dipole description observed in our experiments is strongly promoted by the vicinity of the quantum dot to the silver mirror. At the mirror surface highly confined optical surface modes exist; the so-called surface plasmons, see Figure 2. Plasmonics is a very active and promising research field and the strong confinement of light available in plasmonics may have applications for quantum information science and solar energy harvesting . In our experiments the plasmons at the mirror surface give rise to a strongly varying electric field over the spatial extent of the quantum dot, see Figure 2. Quantum dots coupled to surface plasmons have been suggested as a way to achieve very efficient light-matter interaction enabling, e.g., highly efficient single-photon sources . Our 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.
Figure 2 : Sketch of the studied system. A quantum dot (green trapezoid) is placed a distance z below a metal mirror. The lateral extension of a quantum dot 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 leading to a variation of the electric field over the extension of the quantum dot. In describing light-matter interaction for such a system both the point-dipole moment μ as well as a mesoscopic moment Λ must be taken into account. [From Ref.  -- Thanks to 'Nature Physics']
The discovery that light emission can be strongly modified for quantum dots in optical nanostructures may pave the way for new nanophotonic devices that exploit the spatial extent of quantum dots as a novel resource. This may have important implications also in other research areas where quantum dots are applied, including photonic crystals, cavity quantum electrodynamics, and light harvesting.
Figure 3: Artist's impression of the discovery. Quantum dots are made up of thousands of atoms (yellow spheres) embedded in a semiconductor (blue spheres). Due to their mesoscopic dimensions, the point-emitter description is revealed to break down by comparing photon emission from quantum dots with opposite orientations relative to a metallic mirror.
 M. L. Andersen, S. Stobbe, A. S. Sørensen, P. Lodahl, "Strongly-modified Plasmon-matter interaction with mesoscopic quantum emitters", Nature Physics, (published online December 19, 2010). doi:10.1038/nphys1870.
 T.D. Ladd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe & J.L. O’Brien, "Quantum computers", Nature 464, 45–53 (2010). Abstract.
 H.A. Atwater & A. Polman, "Plasmonics for improved photovoltaic devices", Nature Materials, 9, 205–213 (2010). Abstract.
 A.V. Akimov, A. Mukherjee, C.L. Yu, D.E. Chang, A.S. Zibrov, P.R. Hemmer, H. Park & M.D. Lukin, "Generation of single optical plasmons in metallic nanowires coupled to quantum dots", Nature 450, 402–406 (2007). Abstract.