Doubly forbidden second-harmonic generation from isotropic suspensions: Studies on the purple membrane of Halobacterium halobium

This paper reports the observation of coherent second-harmonic generation in a macroscopically centrosymmetric system, with circularly polarized pump radiation. This measurement of such a doubly forbidden harmonic is accounted for by a theory developed by the authors, which pinpoints a precise interplay between the coherent and incoherent emission mechanisms at work. The angular distribution of the second harmonic in suspensions of bacteriorhodopsin purple membrane identifies coherent evolution of the harmonic within each membrane unit. The dependence on pump polarization is explicable only in terms of incoherent addition of harmonic signals from the different membrane units.