Enhancement of Light-Matter Interaction in Semiconductor Nanostructures – University of Copenhagen

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02 November 2011

Enhancement of Light-Matter Interaction in Semiconductor Nanostructures

Enhancement of light-matter interaction is of great interest for fundamental science and technology alike. The efficiency of solar cells, semiconductor lasers, photodetectors, and single-photon sources all rely on a strong light-matter interaction so any possible increase in the light-matter interaction strength would be an asset to such devices.

Illustration of a terrace structure fabricated by photolithography on a GaAs semiconductor wafer with embedded InAs quantum dots (QDs).

Quantum dots are particularly interesting light-sources because they have a high optical quality and can be integrated with semiconductor technology. Furthermore, an increased light-matter interaction strength can lead to completely new physical phenomena. For instance, a quantum dot in a nanocavity can reach the strong coupling regime, in which spontaneous emission becomes reversible and a single electron in the quantum dot is entangled with a single photon in the cavity. Eventually such effects could become highly technologically relevant in quantum information processing devices.

We study the enhancement of light-matter interaction by means of nanostructures as well as by exciton wavefunction engineering. In a series of experiments we have performed time-resolved spectroscopy on quantum dots and used the proximity of a nearby semiconductor-air interface to extract fundamental quantum optical parameters of the quantum dots, such as the quantum efficiency and oscillator strength [1,2].

Measured decay rate from quantum dots placed at different distances to a semiconductor-air interface for six different emission energies. The excellent agreement with theory (solid lines) allows extraction of the frequency dependence of the quantum efficiency, oscillator strength, and electron-hole wavefunction overlap [1,2].

Quantum dots are not two-level systems. Besides the optically active electron states most often considered, the quantum dot decay dynamics is influenced also by optically inactive states, which are inactive due to the angular momentum selection rules of optical transitions. It turns out that the optically active states can become inactive and vice versa if a spin-flip occurs. Using the detailed knowledge about the optical properties of quantum dots obtained from the semiconductor-air interface experiments [1,2] we can accurately extract the spin-flip rate [3].

These are the first measurements of the spin-flip rate, which is a very important parameter for quantum information applications. Furthermore, we have observed an unexpected dependence of the spin-flip rate on the distance to a nearby interface.