Bacteriorhodopsin films as efficient material for nonlinear optical devices

Materials with non linear optical properties can be found also among biological molecules presenting an ordered arrangement and non-inversion-symmetric structure in space, or having a chromophore. The latter group, called chromoproteins, is best represented by bacteriorhodopsin (BR), one of the most studied biomolecules exhibiting NLO properties, which is also inexpensive and easy to produce and which can function in extreme environmental conditions. Interest in this protein stems not only from its unique photochemistry as a light-driven proton pump, but also from its potential as an active component of biomolecular device applications. BR is a robust trans-membrane protein found in distinct patches, the purple membrane that is an efficient second harmonic generator due to the retinal´s structure which is enhanced by the protein environment. The periodic spatial arrangement of the retinal chromophores in the purple membranes also contributes to the favourable photonic and NLO properties of this material in the cell membrane of Halobacterium salinarium, a naturally occurring archaeon in salt marshes. Interesting chiral properties of BR have been also observed. We have studied BR from an electrophoretic deposition technique to grow a 15 μm thick oriented film onto a substrate covered by a 60 nm thick ITO film. The resulting BR films, composed by ~2600 purple membrane layers (of 5 nm thickness each) were characterized in terms of homogeneity, optical and electrical properties. We performed second harmonic generation (SHG) measurements in the noncollinear configuration. The noncollinear scheme with two input pump beams, offer very high flexibility in the handling and control the SHG signal which becomes selectively addressed by choosing the appropriate polarization state for the fundamental beams. The noncollinear second harmonic generation from the BR film was obtained using the output of a mode-locked femtosecond Ti:Sapphire laser system tuned at λ=83- - 0 nm (76 MHz repetition rate, 130 fs pulse width, average power of 500 mW) which was split into two beams of about the same intensity. The tightly focused beams were sent to intersect, with an angle of 6° with respect to one another, in the focus region. The sample was placed onto a motorized combined translation and rotation stage which allowed the variation of the incidence angle, α formed by the bisector of the two input beams to the sample surface normal, as well as the z-scan of the sample position within the two beam overlap (fig.1).