  We investigate frequency up-conversion of low power cw resonant radiation in
Rb vapour as a function of various experimental parameters. We present evidence
that the process of four wave mixing is responsible for unidirectional blue
light generation and that the phase matching conditions along a light-induced
waveguide determine the direction and divergence of the blue light.
Velocity-selective excitation to the 5D level via step-wise and two-photon
processes results in a Doppler-free dependence on the frequency detuning of the
applied laser fields from the respective dipole-allowed transitions. Possible
schemes for ultraviolet generation are discussed.
