Recently, the investigators have demonstrated that multiple quantum well structures consisting of very thin (10A) alternating layers of two crystalline organic semiconductors can be grown by the new ultrahigh vacuum process of organic molecular beam epitaxy (OMBE). X-ray and birefringence data indicate that the structures are highly ordered both in the stacking direction, as well as across the water surface. Thus, the growth is considered to be "quasi-epitaxial" in that this ordering occurs even though the lattices of the two organic materials are incommensurate. Control of the crystalline texture is strongly influenced by the dependence of molecular surface mobility on temperature. For example, the highest degree of crystalline ordering is observed for substrates cooled to less than 100 K. In the proposed work, the investigator will use a laser in conjunction with their OMBE system to locally heat the cooled substrate during growth, thus creating highly defined regions of both ordered and disordered growth. This fine control of both layer order, as well as the already demonstrated ability to grow very thin films, should allow them to understand, and hence control the growth processes to a high degree. This control will be utilized to fabricate several optical devices such as gratings and waveguides. Futhermore, by making structures consisting of very thin (< 10 A), deeply layered (> 100 period) multi-layered stacks, they may be able to break the centrosymmetry characteristic of crystalline semiconductors. They, in turn, should lead to a significant second-order non-linear optical response. Thus, they will grow such structures and probe their linear and non-linear optical properties using the same laser system employed for the laser- assisted growth experiments.
