9314899 Gwinn The dynamical response of quasi-3D electron gases will be probed using linear and nonlinear techniques. These types of electron gases are of fundamental interest since they exhibit metallic behavior (rs~2.5) but at much reduced plasma energies (~4 meV) compared to typical metals. The materials are molecular beam epitaxially grown AlxGa1-xAs superlattice structures which are specially designed to yield a 3D electron gas with widths which are less than several Fermi wavelengths (hence quasi-3D). The electronic ground state of the electron gas will be probed using low temperature capacitive profiling and Shubnikov-de-Haas oscillations. The linear studies, using standard Fourier transform spectroscopic techniques, will expand the understanding of the interplay between the excitation of the electron gas and the spatial variation of its ground state density. Proposed density variations include internal gradients, as in the bi-metallic gas, near surface perturbations, and lateral modulations induced by optical illumination through a grating. The nonlinear studies will be carried out using an intense tunable far infrared free-electron laser source. The experiment will measure the harmonic emission generated by the quasi-3D electron gas for frequencies above as well as below the plasma frequency. Modeling of this emission will yield information about nonlinear dynamic screening in an inhomogeneous electron gas. %%% Layered semiconductors can be grown by molecular beam epitaxy to provide electronic systems which are models for a three dimensional gas of free electrons. Because of its finite lateral dimension such a structure is referred to as a quasi-free dimensional electron gas. Proper growth control can establish an inhomogeneous distribution of electrons. Study of the response of such inhomogeneous quasi-3D electronic distributions to low level and high intensity radiation is proposed. The angular frequency of the radiation is comparable to t he fundamental electronic response frequency of the electron gas (plasma frequency). The expected measurements will likely lead to an improved understanding of the dynamic surface response of free electron materials, and be important for the development of tunable electronic materials that take advantage of the high degree of control over the electron density that can be achieved within these semiconductor structures. ***
