Local coherence and the temporal development of second harmonic emission

In a variety of mesoscopically disordered systems, high levels of optical excitation resulting from pulsed laser irradiation can establish optical coherence within separate particles or locally ordered domains, leading to second harmonic emission whose temporal signature characterizes the decay of the excited state population. Examples of such systems will include colloids, cell and membrane suspensions, and many plastics, glasses and other modern materials. With pulsed excitation of sufficient intensity to elicit the onset of saturation, second harmonic emission on the throughput of a subsequent probe beam exhibits a characteristic decay and recovery. Detailed calculations show that such features arise not only in systems whose optical response involves two discrete levels, but also in systems of considerably greater electronic complexity. Deconvolution of the temporal trace of the harmonic signal serves as an independent means of monitoring the decay of the excited state. The extent of recovery in the harmonic signal also serves to register the extent of local coherence, and hence in many systems the localization of structural order. Finally, the principles introduced in the theory are shown to be applicable to other types of system such as certain photochromic materials.