Laser-induced order–disorder phase transition and second harmonic generation with a threshold in fiber waveguides

A mechanism is proposed of optical second harmonic generation in centrosymmetric media, which has a threshold and is an efficient process. It is assumed that a light-induced phase transition occurs in a system of centers with constant dipole moments interacting via a strong effective Lorentz field and a lattice displacement field, as well as a field representing reemission at the second harmonic frequency. The proposed mechanism accounts for the main results of experimental investigations of second harmonic generation in centrosymmetric fiber waveguides.