Molecular alignment allows low-order harmonic generation by circular light in a gas

We experimentally investigate odd-order harmonic generation in molecular gases produced by circularly polarized laser fields. While forbidden in isotropic medium, this effect is allowed by symmetry breaking resulting from nonadiabatic laser-induced molecular alignment. The demonstration is provided by generating the third harmonic in CO${}_{2}$ molecules. Attractive properties and challenging potential applications are discussed in the context of higher-order harmonic generation.