Dynamics of the Formation of Composite Bosons from Fermions

半导体中的激子共振经常被用来演示首次在原子中观察到的共振现象，如光子回波、量子拍子、真空拉比分裂和相干场拉比跳跃。对自由载流子连续谱的非共振光激发产生了完全不同的初始条件，更接近于气体等离子体的初始条件，气体等离子体的电子和离子最终重新结合成原子，原子发出光子，返回基态。在半导体中，初始等离子体由自由载流子、电子和空穴组成，它们的动能取决于激发能超过带隙的程度。自由载流子的激子形成研究已经有四十多年的历史了，但人们对此知之甚少。除了少数例外，数据已经被分析，假设随着1s电子/重空穴跃迁的能量发射的每个光子一定来自激子复合。最近的一项理论突破表明，等离子体发射也在1s共振时达到峰值。这一重要发现要求重新评估以前的所有结论。我们建议研究量子阱中激子形成的动力学，并学习如何控制1s基态的布居。该项目的目标是了解从最初的纯费米子系统(由光吸收产生的自由载流子等离子体)到玻色子系统(激子群)的根本转变。如果像原子一样，低密度激子的玻色子性质压倒了其组成部分的费米子性质，那么在比原子系统高得多的温度下，可以在半导体中实现玻色-爱因斯坦凝聚。

The exciton resonance in semiconductors is often used to demonstrate resonant phenomena first observed with atoms, such as photon echoes, quantum beats, vacuum Rabi splitting, and coherent-field Rabi flopping. Nonresonant optical excitation into the free-carrier continuum produces an entirely different initial condition, much closer to that of a gas plasma whose electrons and ions eventually recombine into atoms that emit photons to return to their ground state. In a semiconductor, the initial plasma consists of free carriers, electrons and holes, whose kinetic energies depend upon how much the excitation energy exceeds the band gap. Exciton formation from the free carriers has been studied for more than four decades, yet little is known. With few exceptions data have been analyzed assuming that each photon emitted with the energy of the 1s electron/heavy-hole transition must have come from an exciton recombining. A recent theoretical breakthrough showed that the plasma emission is also peaked at the 1s resonance. This important finding calls for the reevaluation of all previous conclusions. It is proposed to study the dynamics of exciton formation in a quantum well and to learn how to control the population of the 1s ground state. The goal of the project is to understand the fundamental transition from an initially purely fermionic system (free-carrier plasma generated by optical absorption) to a bosonic system (exciton population). If the bosonic nature of low-density excitons dominates over the fermionic nature of its components, as is the case for atoms, then Bose-Einstein condensation could be achieved in semiconductors at much higher temperatures than those of atomic systems.