Plasma Oscillations in an Inhomogeneous Electron Gas

9314899 Gwinn将使用线性和非线性技术来探索准3D电子气的动力学响应。这些类型的电子气很有意义，因为它们表现出金属行为(Rs~2.5)，但与典型的金属相比，其等离子体能量(~4 meV)要低得多。这些材料是分子束外延生长的AlxGa1-xAs超晶格结构，它是专门设计来产生宽度小于几个费米波长(因此是准3D)的3D电子气的。电子气的电子基态将使用低温电容剖面图和Shubnikov-de-Haas振荡来探测。使用标准傅里叶变换光谱技术的线性研究将扩大对电子气激发与其基态密度的空间变化之间的相互作用的理解。所提出的密度变化包括双金属气体中的内部梯度、近表面微扰和通过光栅的光学照明所引起的横向调制。非线性研究将使用强度可调的远红外自由电子激光光源进行。该实验将测量准3D电子气在等离子体频率以上和以下频率下产生的谐波发射。对这种发射的模拟将得到关于非均匀电子气中的非线性动态屏蔽的信息。通过分子束外延可以生长%的层状半导体，以提供作为自由电子三维气体模型的电子系统。由于其有限的横向尺寸，这种结构被称为准自由维电子气。适当的生长控制可以建立电子的不均匀分布。研究了这种非均匀准三维电子分布对低能级和高强度辐射的响应。辐射的角频率与电子气的基本电子响应频率(等离子体频率)相当。预期的测量结果可能有助于更好地理解自由电子材料的动态表面响应，并对开发可调谐电子材料具有重要意义，这种材料利用了在这些半导体结构中可以实现的电子密度的高度控制。***

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. ***