Second harmonic generation and the dynamics of optical excitation in small particles

In several studies of second harmonic emission from isotropic suspensions it has been shown that the unusually strong signal detected from some particles, notably purple membrane material, is attributable to optical coherence within the separate particles of the suspension. As such, the emission displays an amalgam of the characteristics associated with full coherence (second harmonic generation) and incoherence (hyper-Rayleigh scattering). The principle of local additivity for the hyperpolarisabilities associated with different optical centres or chromophores within such systems leads to intriguing possibilities for materials strongly pumped by an ultrafast source. The key feature is the relationship between the hyperpolarisabilities of optical centres in their ground and electronic excited states under conditions of resonance with the applied radiation, a result which is derived from a full quantum electrodynamical treatment of the interaction. As a consequence of this relationship, the effective second order susceptibility of each domain or particle proves to be very strongly influenced by the instantaneous degree of optical excitation, and the harmonic signal acquires a temporal signature which faithfully registers the dynamics of optical excitation and decay. Thus, where a significant degree of optical excitation is established in such a system by a primary laser source, studying the characteristics of the second harmonic generated by a probe beam offers the means for direct measurement of the excited state population dynamics. It is shown that the effect is strongly dependent on the nature of the excited state damping, whose correct representation in the expressions for hyperpolarisability is crucial for analysis of the results.