We measure the polarization of a photon gas in a dye-filled microcavity. The polarization is obtained by a single-shot measurement of the Stokes parameters. We find that the polarization of both the thermal cloud and the Bose-Einstein condensate of photons (phBEC) does not differ from shot to shot. In the case of the phBEC, we find that the polarization correlates with the polarization of the pump pulse. The polarization of the thermal cloud is independent of parameters varied in the experiment and is governed by a hidden anisotropy in the system.

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Slowly chirped two-dimensional photonic crystal cavities are promising devices for creating photonic Bose-Einstein condensates. Before experimentally achieving such a condensate, one first has to thoroughly investigate the electromagnetic eigenmodes in such crystals. However, slowly chirped photonic crystals leading to cavities for light will easily have sizes in the order of tens of micrometers. Therefore simulating the behaviour of light in these crystals is very time consuming. In this thesis we demonstrate a novel and intuitive approach to obtain the envelopes of the electromagnetic modes in these crystals. An enormous advantage of this approach is that it can calculate the energies and the envelopes of these eigenmodes to a high accuracy in a few seconds. We model a chirped photonic crystal using a local density approach; we assign a potential energy for light, extracted from photonic bandstructure calculations, to each unit cell of the crystal. We also obtain an effective mass from the curvature of the photonic band at this energy. With these ingredients we are left with the task of solving the corresponding Schrödinger equation, which is an elegant and far less time-consuming exercise than calculating the envelopes of the electromagnetic modes using finite-difference time-domain simulations. In this thesis it is shown that for one-and two-dimensional quadratically chirped photonic crystals the agreement between the envelopes obtained by these simulations and the analytical ones resulting from this model is larger than 90% for the lowest energy eigenmodes. Moreover, even with small distortions on this quadratic behaviour of the widths of a onedimensional chirped photonic crystal, the corresponding numerically obtained solutions of the Schrödinger equation clearly have an excellent agreement with the simulated mode envelopes.

We measure the dynamics of the thermo-optical nonlinearity of both a mode-gap nanocavity and a delocalized mode in a Ga0.51In0.49P photonic crystal membrane. We model these results in terms of heat transport and thermo-optical response in the material. By step-modulating the optical input power we push the nonlinear resonance to jump between stable branches of its response curve, causing bistable switching. An overshoot of the intensity followed by a relaxation tail is observed upon bistable switching. In this way, the thermal relaxation of both the localized resonance and the delocalized resonance is measured. Significant difference in decay time is observed and related to the optical mode profile of the resonance. We reproduce the observed transient behavior with our thermo-optical model, implementing a non-instantaneous nonlinearity, and taking into account the optical mode profile of the resonance, as experimentally measured.

We demonstrate a nonlinear photomodulation spectroscopy method to image the mode profile of a high-Q photonic crystal resonator (PhCR). This is done by scanning the PhCR surface with a 405 nm pump beam that modulates the refractive index by local thermal tuning, while probing the response of the resonance. We increase resolution by probing at high power, using the thermooptical nonlinear response of the PhCR. Spatial resolution of the thermo-optical effect is typically constrained by the broad thermal profile of the optical pump. Here we increase the resolution and show that we can approach the diffraction limit of the pump light. This is due to free carrier absorption that heats up the PhCR only when there is overlap between the optical pump spot and the optical mode profile. This is supported with a thermo-optical model that reproduces the high-resolution mode mapping. Results reveal that the observed enhanced resolution is reached for surprisingly low carrier density.