Ellipticity and polarization effects in harmonic generation in ionizing neon.

We report observations of the ellipticity dependence of harmonic generation in a thin layer of neon gas at an intensity of ${10}^{15}$ W/${\mathrm{cm}}^{2}$ and a wavelength of 7540 \AA{}. We observe a markedly different ellipticity dependence and polarization for harmonic energies below and above the ionization potential of neon (21.56 eV). The ellipticity dependence of the higher harmonics can be understood in terms of free-bound transitions of ionizing electrons within the first cycle of the fundamental subsequent to tunnel ionization, while other mechanisms must be invoked to explain the characteristics of the lower harmonics.