Efficient Coupling of a Quantum Dot to a Photonic Crystal Waveguide – Niels Bohr Institute - University of Copenhagen

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Efficient Coupling of a Quantum Dot to a Photonic Crystal Waveguide

The interest in all-solid state implementations of quantum photonic devices for quantum information technology has increased tremendously in recent years. The basis for many of these devices is the efficient interaction between a single emitter and a single optical mode. Much work has centred on quantum dots embedded in cavities. The efficient coupling between a single quantum dot and the optical cavity mode allows channelling single photons to the cavity mode with near-unity efficiency. However, one serious limitation of this approach is that the bandwidth is rather modest. Furthermore, out coupling in general implies high optical losses, which significantly limits the overall efficiency of the single-photon source.  An alternative technique is to couple a quantum emitter to a propagating mode by implementing a waveguide in a photonic crystal. In this case, the efficient coupling is mediated by the pronounced slow-down of light propagation near the edge of the photonic crystal waveguide mode. Furthermore, due to the 2D photonic bandgap of the structures, coupling of the quantum dot to radiation modes is efficiently inhibited [1]. The combined action of these two effects leads to a very high b-factor of the single-photon source. The b-factor expresses the fraction of photons emitted to the photonic crystal waveguide relative to the total number of photons emitted, and is the figure of merit for a single-photon source.  We will discuss here the experimental realization of a photonic crystal waveguide single-photon source [2]. By time-resolved spectroscopy we extract the b-factor of the device, which approaches 90%. By probing quantum dots at different emission wavelengths we extract an unprecedented large bandwidth of 20 nm, which is a major advantage of the technology compared to the case of a cavity. References [1] B. Julsgaard, J. Johansen, S. Stobbe, T. Stolberg-Rohr, T. Sünner, M. Kamp, A. Forchel, and P. Lodahl, Appl. Phys. Lett. 93, 094102 (2008).[2] T. Lund-Hansen, S. Stobbe, B. Julsgaard, H. Thyrrestrup, T. Sünner, M. Kamp, A. Forchel, and P. Lodahl, Phys. Rev. Lett. 101, 113903 (2008).