First hyperpolarizability of bacteriorhodopsin, retinal and related molecules revisited

The previously reported β values of BR and retinal based chromophores were very high but subsequent measurements found them to be much less. We have found that the β values of these compounds do not vary so much with experimental conditions as with the method of analysis. Hyper-Rayleigh scattering measurements at 1543 and 1907 nm produce more realistic β values close to the intrinsic (static) hyperpolarizability, β0 which for BR is still very high (275 × 10−30 esu). The optical nonlinearity of BR arises entirely due to the protonated retinal Schiff Base (PRSB) which in its isolated form has the same intrinsic hyperpolarizability as that of the protein.