This thesis details two investigations of highly nonlinear optical phenomena using few-cycle light pulses. Specifically, self-focusing in air with carrier-envelope phase locked pulses and high-order harmonic generation from successive sources. Producing few-cycle light pulses with a sufficient intensity to observe these phenomena has only become possible with the recent development of mode-locked lasers. By observing the behaviour of atoms in such an intense electromagnetic field, new conclusions can be drawn on the nature of nonlinear phenomena.

Intense electromagnetic fields propagate nonlinearly and one effect that plays an important role in determining the propagation is the Kerr effect. This effect makes it possible for an intense light pulse to undergo self-focusing, in which the light modifies the spatial refractive index of the medium such that the light is brought to a focus. The position of the self-focus is determined by the power of the light pulse relative to a critical power for self-focusing that is a property of the medium. When using a few-cycle pulse, shifting the carrier-envelope phase alters the peak value of the electric field within the pulse envelope that determines the maximum instantaneous power achieved. In this thesis, self-focusing in air with a 6.3 fs, 800 nm pulse was investigated. The critical power of self-focusing was measured to be 18 ± 1 GW. A first-order theory was developed and predicted that altering the carrier-envelope phase would shift the focus by 790 µm. When the experiment was performed, no change in the focus position was observed and a 3σ upper limit to a fit of the data gave a total shift of 180 µm.