Enhancement of Light-Matter Interaction with Mesoscopic Emitters – University of Copenhagen

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Quantum Photonics > Research > Mesoscopic Emitters

02 November 2011

Enhancement of Light-Matter Interaction with Mesoscopic Emitters

Enhancement of light-matter interaction is of great interest for fundamental science and technology alike, e.g., for cavity quantum electrodynamics, solar cells, photodetectors, etc. There are two possible routes, by either modifying the emitter (the oscillator strength) or the optical field (the projected local density of optical states). However, the experimental signature of enhancement of light-matter interaction is the same as that of detrimental non-radiative processes: fast spontaneous emission rates. We have developed a unique method of employing very simple nanostructures, in order to separate radiative from non-radiative processes. This has led to accurate measurements of the oscillator strength and quantum efficiency of quantum dots [1,2,3] and provided insight into their spin-flip dynamics [4]. We have also shown that large quantum dots can have unexpectedly small oscillator strength [5], contrary to previous results that did not take non-radiative processes into account.

Measured decay rates (triangles) of excitons in quantum dots placed at different distances to a semiconductor-silver interface. The colors indicate the orientation of the quantum dots. The mesoscopic character of quantum dots can lead to enhancement (upper panel) or reduction (lower panel) of the decay rate relative to dipole theory (dashed curves). We find good agreement with our theory including the mesoscopic moment of the quantum dot in the interaction term (solid lines).

In a recent breakthrough, we have found that there is a new way of enhancing the light-matter interaction strength. In a paper published 2011 in Nature Physics [6] we report on the experimental observation that light emission from quantum dots is fundamentally different than hitherto believed, see Fig. 1. We have observed and theoretically explained that due to their mesoscopic character, quantum dots emit photons in plasmon nanostructures with rates that violate the celebrated dipole approximation, which is usually taken for granted in solid-state quantum optics. The deviations from the dipole approximation can in fact be employed to enhance the interaction strength between light and matter thereby potentially improving the efficiency of quantum information devices.

Presently we are developing a general theory of light-matter beyond the dipole approximation and preparing a new generation of experiments employing electrical control of light-matter interaction.

[1] J. Johansen, S. Stobbe, I. S. Nikolaev, T. Lund-Hansen, P. T. Kristensen, J. M. Hvam, W. L. Vos, and P. Lodahl, ”Size dependence of the wavefunction of self-assembled InAs quantum dots from time-resolved optical measurements,” Physical Review B 77, 073303 (2008).

[2] S. Stobbe, J. Johansen, P. T. Kristensen, J. M. Hvam and P. Lodahl, “Frequency dependence of the radiative decay rate of excitons in self-assembled quantum dots: Experiment and theory,” Physical Review B 80, 155307 (2009).

[3] S. Stobbe, J. M. Hvam, and P. Lodahl, “On the interpretation of wave function overlaps in quantum dots,” Physica Status Solidi (b) 248, 855 (2011). (Cover story). 

[4] J. Johansen, B. Julsgaard, S. Stobbe, J. M. Hvam, and P. Lodahl, “Probing long-lived dark excitons in self-assembled quantum dots,” Physical Review B 81, 081304(R) (2010). 

[5] S. Stobbe, T. W. Schlereth, S. Höfling, A. Forchel, J. M. Hvam, and P. Lodahl, “Large quantum dots with small oscillator strength,” Physical Review B 82, 233302 (2010).

[6] M. L. Andersen, S. Stobbe, A. S. Sørensen, and P. Lodahl, “Strongly modified plasmon-matter interaction with mesoscopic quantum emitters,” Nature Physics 7, 215 (2011). (Cover story).