Colloquia & Guest Speakers
Coupling Light to a Single Nanostructure
Dr. Gerd Leuchs University of Ottawa
Monday, March 7, 2016
Goergen Hall, Room 101
The ability to measure phase, amplitude and the direction of the electric field with sub wavelength resolution, e.g. in a focusing geometry, is essential for testing theoretical predictions, such as the occurrence of a Möbius strip type polarization topology. This has also led to the observation of light fields carrying an angular momentum pointing perpendicular to the wave vector. It is well known that such transverse angular momentum can occur locally within the light field but also the integrated angular momentum may be transverse, which is quite an unfamiliar scenario. Light fields with an overall transverse angular momentum may be referred to as photonic wheels. This enhanced freedom can be used to let a small particle spin with the axis of rotation oriented in an arbitrary direction in space. Transverse angular momentum occurs also in the context of total internal reflection allowing the direction of emission of a radiating dipole to be steered and controlled by the angular momentum of the exciting light beam. If one wants to couple light very efficiently to a sub-wavelength dipole such as a single atom, one has to focus an in-going dipole wave onto the dipole. A deep parabolic mirror may serve as a mode converter. A special type of cylindrical vector beam is required as an input beam to the parabolic mirror. Such light beams show strong correlations between polarization and position within the cross section, with the different degrees of freedom being non-separable For efficient light atom coupling, a single atom is predicted to be able to phase shift a coherent driving beam by up to 180 degrees. In a preliminary experiment we observed 3 degrees, which is on par with the best value obtained in different scenarios in free space so far. Ultimately one should be able to demonstrate the reversibility of spontaneous emission in such a configuration. To do so one has to provide a single photon wave packet shaped in time to resemble the time-reversed version of a spontaneously emitted photon. Such an asymmetric single photon wave packet can be generated efficiently using spontaneous parametric down conversion in a nonlinear whispering gallery mode resonator.
Refreshments will be served.