Nonlinear Optical Properties of Bacteriorhodopsin: Assignment of Second Order Hyperpolarizabilities of Randomly Oriented Systems Using Two-Photon Spectroscopy
Molecular Crystals and Liquid Crystals Incorporating Nonlinear Optics
We demonstrate that the second order hyperpolarizability of a randomly oriented molecule can be determined directly from two-photon spectroscopic measurements on the low-lying excited state manifold. Equations are derived which allow not only a determination of β, but also a determination of the error associated with the numerical method. We apply our two-photon technique to an analysis of the second order hyperpolarizability of light adapted bacteriorhodopsin. Our analysis of this protein in D2O at ambient temperature yields a value of β (= βxxx + (1/3)[βxyy + 2βyyx + βxzz + 2βzzx]) of (2250 ± 240) × 10−30 cm5/esu for a laser wavelength of 1.06μ (Nd:YAG fundamental). The large second-order nonlinear properties of bacteriorhodopsin are due primarily to the large change in dipole moment associated with excitation into the lowest-lying strongly allowed “1Bu +” π, π* state (Δμ = 13.5 ± 0.8 D). We derive an equation which estimates Ωβδ, the ratio of the number of second harmonic photons generated by the system divided by the number of photons absorbed by the system via two-photon processes. Our analysis indicates that molecular doubters can be optimized by maximizing the oscillator strength of the low-lying charge transfer state (fso), the orientation angle of the transition dipole with the polarization of the laser flux, the sample length and the chromophore concentration. All of the above manipulations will also increase the efficiency of doubling, and thus optimization of these parameters is critical to overall doubling performance.
Copyright © 1990, Taylor & Francis
Taylor & Francis
Birge, Robert R.; Fleitz, P. A.; Lawrence, Albert F.; Masthay, Mark; and Zhang, Chian-Fan, "Nonlinear Optical Properties of Bacteriorhodopsin: Assignment of Second Order Hyperpolarizabilities of Randomly Oriented Systems Using Two-Photon Spectroscopy" (1990). Chemistry Faculty Publications. 81.