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

In this paper we report results on second-harmonic generation ~SHG! from aqueous suspensions of the purple membrane of Halobacterium halobium. A detailed theory describing this SHG signal is supported by results for the angular distribution of the harmonic emission and validated by experimental measurements with circularly polarized pump radiation—a condition that normally precludes SHG. Hence we identify the precise mechanism for second-harmonic emission in this macroscopically isotropic but microscopically ordered system, resolving confusion over whether the signal should be considered coherent SHG or incoherent hyperRayleigh scattering.