Photon Bose-Einstein condensation in arbitrary potential landscapes

Bose-Einstein condensation (BEC) is the massive occupation of a single quantum state in a gas of bosons at a finite temperature and is one of the few phenomena demonstrating quantum behaviour at the macroscopic scale. BEC of photons can be relised in quantum confined structures such as optical microcavities, where photons can thermalise to the temperature of the optical medium through repeated absorption and emission cycles. The critical particle number for such a system can be as low as 10 photons at room temperature, making photon BEC experimentally easily accessible.

In an optical microcavity, photons behave like massive 2-dimensional particles confined in a potential defined by the mirror geometry and the optical medium refractive index. Controlling this potential is essential for exploring the physics of the condensation process. In this thesis, we develop, characterise and employ a number of flexible methods which rely on modified dielectric mirrors that have an additional optically absorptive silicon layer between the substrate and the dielectric stack. This layer allows us to locally heat the mirror using laser light. The first method is the permanent nanostructuring, resulting from pore formation inside the dielectric stack layers. The second method involves thermal expansion. Finally, introducing a thermoresponsive polymer into the optical medium allows us to control the refractive index.

We experimentally investigate BEC under non-equilibrium conditions in potentials resembling a Mach-Zehnder interferometer. The switching behaviour of the interferometer in various configurations provides insight into the formation process of the condensate. We show that by adjusting their frequency, BECs naturally try to avoid particle loss and destructive interference in their environment.

As a first step towards spin-glass simulators, an optical analogue of a 0,pi Josephson junction is implemented and characterised. Our experiments show a high degree of control over the junction state. Alternative types of condensate couplings and their possible implementations are investigated as well. In particular, we show that couplings can range from dispersive to dissipative, and show that the loss channel in dissipative couplings may be realised by drilling holes using focused ion beam milling.