Characterization of Below Threshold Harmonics Generated in Argon by Ultrashort Laser Pulses

This work reports studies on the generation of below threshold harmonics by ultrashort laser pulses in gas nozzles in vacuum. Odd harmonics of the laser, ranging from the 3
^rd
up to the 9
^th
, were generated in the UV and VUV regions, and the frequency conversion occurred in flowing gas. The harmonics were spectrally characterized, and the orders of the nonlinear processes were determined, showing that, for the higher order harmonics, competing processes decrease the energy coupling into the new frequencies. Also, the conversion efficiencies were estimated, showing that a few percent of the laser pulse energy are converted into the harmonics.