Strong Wavelength Dependence of Hyperpolarizability in the Near-Infrared Biological Window for Second-Harmonic Generation by Amphiphilic Porphyrins

Abstract

We have applied the Thomas-Kuhn sum rules to model the wavelength dependence of the second-order nonlinear polarizability of an amphiphilic porphyrin chromophore designed for cellular imaging on the basis of the complete analysis of its linear absorption spectrum. We predict huge oscillations for this first hyperpolarizability in the biological transparency window with the second-order response exhibiting three minima and two maxima in the wavelength range between 700 and 900 nm and a second region of enhanced response between 1200 and 1500 nm. We confirmed the predicted values experimentally demonstrating both the validity of our approach and the need for a wavelength scan to find a maximum in the resonance-enhanced signal for cellular imaging. These results suggest a new approach toward achieving spectroscopic selectivity during second-harmonic generation imaging.