Resonantly enhanced parametric four-wave mixing in sodium vapour

The conical and axially propagating UV emissions have been registered in sodium vapour excited with picosecond light pulses. The near-resonant two-photon excitation of 4D sodium level and efficient generation of coherent radiation in 330 nm spectral region (near the frequencies of 3S-4P1/2,3/2 sodium transitions) have been obtained for the pump wavelength tuned between 578.5 and 586 nm. The spectral and angular properties of the emitted radiation are discussed in terms of the phase matched parametric four-wave mixing.