Spectral modulation of ultraviolet femtosecond laser pulse by molecular alignment of CO₂, O₂, and N₂

We demonstrate efficient third harmonic generation of a near infrared femtosecond pulse through cascaded frequency doubling and sum-frequency generation processes, where the group velocity mismatching between the involved fundamental and generated second harmonic pulses, before they are sent to frequency mixing, are precompensated with a properly inserted nonlinear crystal. The spectrum of the generated third harmonic pulse with energy of 1.1 mJ is further modulated by using impulsive molecular alignments of CO₂, O₂, and N₂, where significantly broadened spectrum in the ultraviolet spectral region is observed due to the additional cross-focusing effect from the parallel aligned molecules and the consequently enhanced self-phase modulation.