Quantum Electrodynamics Experiments in Photonic Crystal Waveguides – University of Copenhagen

Quantum Photonics > Research > Waveguide-QED

01 November 2011

Quantum Electrodynamics Experiments in Photonic Crystal Waveguides

The three dimensional spatial confinement of carriers in semiconductor quantum dots results in the discretization of their energy spectrum. Electrons from high lying energy states in the conduction band get trapped in the potential box defined by the three dimensional spatial confinement, and relax to the valance band of the semiconductor through a discrete jump in energy. This relaxation process is predominantly radiative, resulting in single-photon emission.

Scanning electron microscope (SEM) image of a membranized photonic crystal waveguide.

Controlling the emission dynamics of semiconductor quantum dots is essential in many applications in the field of quantum information science, which require indistinguishable single photons emitted on demand. The rate of spontaneous single-photon emission from an excited emitter is proportional to the local optical density of states. By modifying the photonic environment, the emission rate can be inhibited or enhanced. Photonic crystals have been shown to be a powerful way of designing the photonic environment of the emitter [1]. The spatial modulation of the refractive index, which defines the photonic crystal, results in the opening of so-called photonic band gaps. Inside the photonic band gap light propagation is fully suppressed for a certain range of frequencies.

Our samples consist of a periodic arrangement of air holes etched into a GaAs membrane (See Figure 1). By introducing defects into these photonic structures - like a missing row of holes - the periodicity of the refractive index modulation is broken and light at engineered frequencies within the band gap can propagate along the resulting photonic crystal waveguide.Spectrally and spatially overlapping the semiconductor quantum dots with the propagating waveguide mode enables the emitter to radiatively decay through this channel, while all other modes remain inhibited due to the photonic band gap. The light-matter coupling strength can be additionally increased by designing the waveguide to support slow light. This allows for a broadband enhancement of the spontaneous emission rates [2].

Decay rates of the five quantum dots plotted as a function of the emission wavelength. The extracted
β-factors for four of the quantum dots are shown in
the legend.

The combination of the enhanced coupling to the waveguide mode and inhibition of all other modes leads to a large mode selectivity of the emission. The β-factor, which denotes the fraction of the total emission that is channeled into the target mode, is the parameter used to characterize this selectivity. β-factors approaching 90% have recently been demonstrated in our group [2,3]. Our current experimental efforts are focused on the strong few-photon nonlinearity mediated by a single quantum dot strongly coupled to a one-dimensional waveguide. This research is interesting, not solely for the new physical phenomena being investigated, but also for its practical applications in the field of quantum information science.

[1] B. Julsgaard, J. Johansen, S. Stobbe, T. Stolberg-Rohr, T. Sunner, M. Kamp, A. Forchel, and P. Lodahl, Decay dynamics of quantum dots influenced by the local density of optical states of two-dimensional photonic crystal membranes, 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, Experimental realization of highly efficient broadband coupling of single quantum dots to a photonic crystal waveguide, Phys. Rev. Lett. 101, 113903 (2008).

[3] H. Thyrrestrup, L. Sapienza, and P. Lodahl, Extraction of the β-factor for single quantum dots coupled to a photonic crystal waveguide, Appl. Phys. Lett. 96, 231106 (2010).