Electric-field induced optical nonlinearities in air : how to make gases behave like uniaxial crystals

In Nonlinear Optics Classes, we typically teach that certain nonlinear optical effects like second-harmonic generation (SHG) can only appear in crystals without inversion symmetry. However, this requirement can easily shown to be wrong in the presence of an external static electric field. Using atmospheric air within a rather simple plate capacitor arrangement, we experimentally demonstrated second-harmonic generation of femtosecond pulses. Compared to crystal-based schemes, electric-field induced SHG has the advantage of unlimited scalability and the absence of catastrophic optical damage. Moreover, we also conducted experiments on the extremely faint Kerr-effect in air, using a plate capacitor within a femtosecond laser cavity. Possible applications of both these experiments are discussed.