Controlling spontaneous emission from quantum dots by modified vacuum fluctuations 

Contacts:

Luca Sapienza, DTU Fotonik, 4525 6877 (lucs@fotonik.dtu.dk)

Peter Lodahl, DTU Fotonik, 4525 3807 (peter.lodahl@fotonik.dtu.dk)

Quantum Photonics Group: http://www.fotonik.dtu.dk/Quantumphotonics

Upper panel: A GaAs/air photonic crystal membrane embedding semiconductor quantum dots. Bottom panel: Examples of the measured decay curves (intensity of emitted light vs time) of semiconductor quantum dot emitters, embedded in photonic crystal membranes with different lattice constant a.

In this project, spontaneous emission of light from semiconductor quantum dots will be investigated. Quantum dots are nanometre-sized ‘artificial atoms’ located inside a semiconductor material that just like ordinary atoms can spontaneously emit photons once at a time (a single-photon source). Spontaneous emission is a quantum optics phenomenon and photon emission occurs due to the presence of vacuum fluctuations that stimulate the decay of the quantum dot. This has remarkable consequences. Thus spontaneous emission is not an immutable property of the quantum dot, but can be controlled by nano-structuring the material surrounding the quantum dot. The nano-structuring of the surrounding medium modifies locally the density of vacuum fluctuations, and thus provides a technique to control spontaneous emission.

In this experimental project, you will measure spontaneous emission from quantum dots embedded in nano-structured photonic media. By modifying the vacuum fluctuations, the decay rate of spontaneous emission will be altered. Various nanophotonic structures are used for controlling spontaneous emission in our group, including interfaces, photonic crystals, and nano-cavities and waveguides.

Photonic crystals are periodic structures composed of materials with different dielectric constants (for example GaAs and air – see left panel of the figure above). The resulting periodic potential of the dielectric permittivity strongly modifies the radiation modes: photonic band gaps – reminiscent of electronic band gaps in semiconductors - can be opened, forbidding the propagation of light in the system for specific energies. In this experiment, spontaneous emission decay curves (examples of decay curves are shown in the right panel of the figure above) will be measured in photonic crystals with different lattice parameters. By varying the lattice parameter, the photonic crystal band-gap is modified and, depending on the wavelength of the emission of the quantum dot with respect to the photonic band-gap, the decay of the emitter will be either speeded up or slowed down. By comparing the measured decay rate we can extract the fundamental light-matter coupling strength determining the interaction of quantum dots with nanophotonic structures.