Nonlinear Optical Properties of Bacteriorhodopsin: Assignment of Second Order Hyperpolarizabilities of Randomly Oriented Systems Using Two-Photon Spectroscopy

Abstract

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 p, 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 D20 at ambient temperature yields a value of p ( = pxXx + (1/3)[pxYy + 2p, , , + pxu + 2p, , , ] ) of (2250 * 240) x cm5/esu for a laser wavelength of 1 . 0 6~ (Nd:YAG fundamental). The large secondorder nonlinear properties of bacteriorhodopsin are due primarily to the large change in dipole moment associated with excitation into the lowest-lying strongly allowed "'B;+'' T,T* state (AF = 13.5 ? 0.8 D). We derive an equation which estimates a,,, 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 doublers can be optimized by maximizing the oscillator strength of the low-lying charge transfer state (f-), 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.

Bacteriorhodopsin as a Photochromic Retinal Protein for Optical Memories

“…Two-photon excitation of chromophore molecules (see Figure 12) is not surprising, but the cross section for two-photon absorption of BR is astonishingly high compared to other materials. 131 Since the two photons have to coincide at the molecule under very strict conditions, the absorption probability is rather low compared to one-photon absorption. In the literature such processes are often named "biphotonic absorption", because there tends to be confusion with two consecutive one-photon absorption processes.…”

“…Apart from one-photon excitation, two-photon excitation of BR was investigated in detail because two- photon excitation is an attractive method for volume storage of data. Two-photon excitation of chromophore molecules (see Figure ) is not surprising, but the cross section for two-photon absorption of BR is astonishingly high compared to other materials …”

Second-Harmonic Generation in GFP-like Proteins

“…Even at the isoelectric point of proteins, the electric field that is used to induce the noncentrosymmetry by dipolar orientation orients proteins on the basis of the charge distribution over the protein rather than on the basis of the chromophore dipole moment. 25,26 HRS takes advantage of the local deviations (in both time and space) from the average noncentrosymmetry. [27][28][29] Since GFPs are highly fluorescent, the HRS signal at the second-harmonic wavelength has contributions from both incoherent scattering and multiphoton fluorescence.…”

Bacteriorhodopsin: a biological material for information processing

“…Associative memories and neural networks also involve volume-type holograms. Proposals for BR (Birge et al 1990c/) were made on the basis of a setup described by Peak & Psaltis (1987).…”