  We report on a strong and tunable magnetic optical nonlinear response of
Bacteriorhodopsin (BR) under "off resonance" femtosecond (fs) pulse excitation,
by detecting the polarization map of the noncollinear second harmonic signal of
an oriented BR film, as a function of the input beam power. BR is a
light-driven proton pump with a unique photochemistry initiated by the all
trans retinal chromophore embedded in the protein. An elegant application of
this photonic molecular machine has been recently found in the new area of
optogenetics, where genetic expression of BR in brain cells conferred a light
responsivity to the cells enabling thus specific stimulation of neurons. The
observed strong tunable magnetic nonlinear response of BR might trigger
promising applications in the emerging area of pairing optogenetics and
functional magnetic resonance imaging susceptible to provide an unprecedented
complete functional mapping of neural circuits.
