Transformations from one state (phase) of matter to another often occur all out of a sudden as parameters undergo a change, for example water freezes suddenly at 0 degree Celsius and boils at 100 degrees Celsius. These sudden changes are referred to as phase transitions, in the specific example of water it is a first order phase transition.

Since the 1990s it is also possible to study phase transitions from classical states of matter to a non-classical state, such as the Bose-Einstein condensation (BEC) of Bosons. These exotic states of matters have been studies extensively ever since, spanning a wide variety of physical systems from ultracold atomic gases to solid-state systems like exciton-polariton condensates, or photon gases in 2010.

Phase transitions between classical phases were studied extensively for centuries, and the influence of various conditions like pressure, volume and temperature on the transition at the phase boundaries has been explored. It is evident that such physical influences were and are being studied for BEC phase transitions as well. One ingredient that influences the phase transition is the physical dimension in which the matter is confined in. While we live in three dimensions, one can change the dimension by confining potentials, which then often gives rise to interesting properties of the phases.

The transition from a (classical) thermal gas to a Bose-Einstein condensate is a second order phase transition, which for the ideal gas is sensitive to the dimensionality of the Bose gas. In many platforms, such as ultracold atomic gases and polariton condensates, the influence of dimensionality on phase transitions is challenging to study as the dimension also influences the energy redistribution between the bosons, and the particles are usually interacting. We overcome this challenge by employing photons which are thermalized by a dye solution trapped in a high finesse microcavity, where the dye solution serves as a dimension-independent heat bath.

We study the BEC phase transition of the photon gas as we cross from a two-dimensional harmonic confinement to a one-dimensional harmonic one. These engineered confinement for the photon gas is realized by using a direct laser writing (DLW) system to fabricate parabolic polymer structures on top of the cavity mirrors. Using varying transversal curvatures, which translate into different trapping frequencies, we can tune the dimensionality of the system. We observe both in the spectral and caloric properties that the second order BEC phase transition present for a two-dimensional system softens when the confinement along one direction gets tighter and tighter, and when we reach the case of a one-dimensional system the phase transition is absent. While systems in different dimensions have been studied in the past, our optical quantum gas allows us to tune the dimension independently of the thermalization mechanism, and to also study cases with an effective dimensional between the cases of a one- and a two-dimensional gas. The versatile structuring method using DLW will allow us to study a variety of potentials, ranging from logarithmic confinement to tunnel-coupled lattice structures to study the physics of open quantum systems.