NONLINEAR-OPTICAL PROPERTIES OF SEMICONDUCTOR NANOCRYSTALS

This review is devoted to the description of recent experimental resul ts concerning the nonlinear optical properties of semiconductor-doped glasses SDGs with particular emphasis on the regime in which the energ y of the incident photon is smaller than the energy gap. A considerabl e theoretical and experimental effort has been devoted in the last 10 years to the fundamental aspects of quantum-confined structures, which have properties somewhat intermediate between the bulk crystals and a toms or molecules. From this point of view, SDGs represent an easily a vailable test system, and optical techniques have been a major diagnos tic tool. Luminescence and absorption spectroscopy were extensively us ed to characterize the electronic states. The experiments aimed at the measurement of the real and imaginary parts of the third-order optica l susceptibility of SDGs below the bandgap are described in some detai l, and the results obtained with different techniques are compared. Be sides the intrinsic fast nonlinearity due to bound electrons, SDGs may present a larger but much slower nonlinearity due to the free carrier s generated by two-photon absorption. This implies that experiments ha ve to be properly designed for separation of the two effects. In this article we stress the importance of a detailed structural characteriza tion of the samples. Knowledge of the volume fraction occupied by the nanocrystals is necessary in order to derive from the experimental dat a the intrinsic nonlinearity and to compare it with the bulk nonlinear ity. We discuss recent experiments in which the dependence of the intr insic nonlinearity on the crystal size is derived by performing, on th e samples, measurements of the real part and imaginary part of the non linear optical susceptibility and measurements of crystal size and vol ume fraction. Structural characterization is of interest also for a be tter understanding of the physical processes underlying the growth of crystallites in SDGs. The average size of nanocrystals can be tailored by controlling the temperature or time of the treatment. The major pr oblem is the size dispersion of the crystallites, which is intrinsic t o the diffusion process. At present, this is the major source of the u ndesired inhomogeneous broadening of the optical transition lines of t he SDGs. Efforts are at present being made to fabricate materials, SDG s included, which embed nanocrystals with a reduced spread of sizes. T he interest in the nonlinear optical properties is due not only to fun damental reasons but also to possible applications for optical devices . Generally speaking, resonant nonlinearities are much larger than non -resonant nonlinearities, but they are not necessarily the most intere sting for applications because materials at resonance absorb the incid ent radiation and also present long response times. The studies below the bandgap seem to indicate that the values of the intrinsic nonlinea rities of nanocrystals in the structures which are at present availabl e are similar to those of the bulk. New and better controlled structur es are now under development and have to be tested from the view-point of optical nonlinearities. In several situations SDGs cannot be model led as an ensemble of freely standing nanocrystals, with the glass mat rix playing the role of an inert support. Phenomena such as trapping a nd darkening, which are very probably connected with electronic states at the glass-semiconductor interface, may play a role in determining the optical response. They might give rise to an extrinsic optical non linearity which can be even larger than the intrinsic nonlinearity. Th e physical processes which are involved in these extrinsic nonlinearit ies are poorly understood and at present being investigated.