Waveguide polaritons are the quasi-particles arising from the strong coupling of quantum well excitons to the photonic mode of a waveguide. These are complimentary to the polaritons observed in semiconductor microcavities which in the two decades following their first observation have been a rich source of interesting physical phenomena such as parametric scattering, condensation, superfluidity and solitons. Whilst waveguide polaritons are a complimentary scheme they do have a number of important advantages over microcavities, firstly the use of total internal reflection to confine the photonic mode in principle affords lower losses and the reduced mode volume increases the coupling to quantum well excitons. Additionally the thin structure more naturally lends the system towards to fabrication of complex polaritonic devices.
The waveguide polariton scheme was first investigated in the late 1980s and early 1990s however the lack of direct access to the dispersion hindered progress. Recently however advancements in photonics have led to the development of integrated grating couplers which are used in this thesis to couple light in and out of the waveguide structure. The relationship between the emission angle from these grating couplers and the internal wavevector is exploited in Chapter 3 to make the first unambiguous observations of waveguide polaritons by a direct observation of the characteristic anti-crossing dispersion indicative of the strong coupling regime.
In the second half of Chapter 3 the design of the waveguide device was improved by adding further quantum wells to increase the Rabi-splitting and reduce the effect of absorption in the tail of the exciton line. It is then shown that the strong coupling regime is preserved in this device up to 100 K.
In Chapter 4 it is shown that the interactions between polaritons inherited from the exciton component leads to an optical nonlinearity which causes the defocusing of high intensity beams travelling through the waveguide. This nonlinearity can be described as a negative nonlinear refractive index which can support the generation of single or pairs of dark-spatial solitons depending on the initial conditions. Finally this nonlinearity is also shown to persist to 100 K suggesting the possibility of future polaritonic devices operating at higher temperatures.
In Chapter 5 it is shown that the curvature of the polariton dispersion in the anti-crossing regions gives rise to a massive group velocity dispersion which causes the dilation of injected pulses as they propagate along the waveguide. At high particle densities within the pulse this group velocity can be balanced against the optical nonlinearity arising from inter-particle interactions to form bright temporal solitons. Finally due to the comparable nonlinear-, diffractive- and dispersive-length scales it is shown that this system support the formation of a hereto unobserved hybrid of a spatially-dark and temporally-bright soliton.
In this thesis waveguide polaritons are reintroduced as a complimentary system to microcavities and the first observations made of their formation and interactions. This thesis lays the foundation for future studies into waveguide polaritons and showcases their nonlinear properties through the study of spatio-temporal solitons.
