This thesis describes the phenomenon of high harmonic generation from atoms irradiated
by intense, ultrashort laser pulses. Particular attention is paid to the benefits
achieved by using multicolour driving fields.
A theoretical description of few-cycle laser pulses is presented, together with their interaction
with free-electrons and the role they play in various nonlinear optical processes.
A number of numerical models are presented to simulate high harmonic generation from
atomic systems. These are used to analyse and explain the temporal structure of the
emitted high-frequency dipole radiation.
Propagation of the macroscopic harmonic response through a gaseous volume is modelled
and the role of phase-matching explained in detail. We consider focussing geometry
in optimising the yield of particular harmonics, together with the effects of free-electrons
within the interaction region.
We discuss means by which multicolour fields may overcome some of the constraints of
single-colour high harmonic generation. Using two delayed pulses of the same frequency
and parallel polarisation we demonstrate significant cut-off extension without increasing
total ionisation throughout the pulse, crucial for maintaining harmonic yield close to the
saturation limit. We also explain the significant yield enhancements observed in recent
experiments using two parallel colours of incommensurate frequency.
Finally, we describe the use of a second, perpendicularly polarised colour in trajectory
selection, allowing for a temporal filtering of harmonic emission. Using an ω + 1.5ω
frequency ratio also allows for a reduction in the periodicity of emitted attosecond pulse
trains, permitting the production of isolated attosecond pulses with longer driving fields.
Furthermore, by controlling the relative phase between the two colours, the ellipticity of
these attosecond pulses may also be controlled.