Second-harmonic generation in the bulk of a chiral liquid by a focused laser beam

Two mechanisms of coherent second-harmonic generation (SHG) are theoretically investigated to explain the effect observed in an isotropic gyrotropic medium. The first mechanism is based on the nonlocality of the quadratic optical response of the medium together with the existence of a longitudinally polarized field component of a focused pump beam and the admixture of differently polarized higher-order transverse modes in it. The second one is due to the local-type fourth-order optical nonlinearity in combination with the focusing of the beam. Formulas are given for the powers of the second-harmonic waves that are generated in a long medium by a tightly focused laser beam through each SHG mechanism. Dependences of the signal power on the polarization state of the pump beam and on the wave-vector mismatch are considered. (C) 1998 Elsevier Science B.V. All rights reserved.