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

Two mechanisms of coherent second-harmonic generation (SHG) are theoretical ly investigated to explain the effect observed in an isotropic gyrotropic m edium. The first mechanism is based on the nonlocality of the quadratic opt ical response of the medium together with the existence of a longitudinally polarized field component of a focused pump beam and the admixture of diff erently polarized higher-order transverse modes in it. The second one is du e to the local-type fourth-order optical nonlinearity in combination with t he focusing of the beam. Formulas are given for the powers of the second-ha rmonic 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 po larization state of the pump beam and on the wave-vector mismatch are consi dered. (C) 1998 Elsevier Science B.V. All rights reserved.