Quantification of sudden light-induced polarization in bacteriorhodopsin by optical rectification

Upon population of its excited state, the retinal chromophore in the membrane protein bacteriorhodopsin (bR) undergoes a sudden (less than ∼10 fs) change in dipole moment, Δμ, that can be visualized in a direct way by optical rectification of a broadband visible femtosecond light pulse to the infrared but has not been quantified in this way. Here we show that a transparent thick AgGaS₂ crystal delivers infrared radiation with the same spectral profile as bR and is a suitable reference for quantifying conversion efficiency. Using this reference, we estimate the projection of Δμ on the membrane normal at 11 D, corresponding to the displacement of a full charge over approximately half the length of the retinal chromophore. This result may help to evaluate models describing the interplay between the initial polarization change and the subsequent isomerization of the retinal.