Exciton-polariton condensates in acoustic periodic potentials.

宏观量子相--由单个波函数描述的物质的基本集体态--一直吸引着人们强烈的研究兴趣。一个著名的例子是原子系统中的玻色-爱因斯坦凝聚体（BEC），其性质已经用磁场和光场进行了研究。通过激光束干涉形成的周期性晶格是用于调制BEC的特别成功的工具。它的应用为量子态的操纵带来了新的概念，对基础研究和高级应用都很有用，因此最近在半导体中观察到的轻物质量子凝聚引起了人们极大的兴趣。在这种情况下，玻色子实体是极化激元-半导体微腔中激子和光子强耦合产生的准粒子。由于它们的质量非常低，冷凝发生在比原子低得多的密度和更高的温度（在开尔文范围内）。尽管处于非平衡状态，它们仍然表现出了扩展的时空相干性、超流性、涡旋性和参量振荡等集体行为。本论文的目的是在可调周期势下研究（Al，Ga）As基微腔中的极化激元凝聚。这种电势将由表面声波（SAW）产生，表面声波特别适合于操纵极化激元凝聚体，因为它们的波长小于凝聚体的相干长度。它们还能够相干地调制激子和光子，从而产生完美的周期势。而且，重要的是，与其他方法不同，SAW电位是可调的。我们已经建立了一个重要的里程碑，实现了微腔样品的生长，并证明了极化激元凝聚的调制SAW是可能的。本项目的目的是扩大这些调查。我们的目标包括：1.提高了SAW对极化激元的调制效率; 2.实现声表面波晶格中极化激元的基础研究3。进行先进的实验，旨在控制凝聚体的相干性和相互作用，4。利用前几点来研究非经典光态的产生和非平衡量子相变。

Macroscopic quantum phases - fundamental collective states of matter described by a single wavefunction - have always attracted an intense research interest. A well-known example are Bose-Einstein condensates (BEC) in atomic systems, whose properties have been studied using magnetic and optical fields. Periodic lattices formed by interference of laser beams are a particularly succesful tool for modulation of BECs. Its use has derived in new concepts for manipulation of quantum states useful for fundamental studies and advanced applications.The recent observation of light-matter quantum condensates in semiconductors has therefore awakened great interest. The bosonic entities in this case are the polaritons - quasi-particles arising from the strong coupling of excitons and photons in a semiconductor microcavity. Due to their very low mass, the condensation occurs at much lower densities and higher temperatures (in the kelvin range) than for atoms. Despite being out-of-equilibrium, they show collective behavior like extended spatial and time coherence, superfluidity, vorticity and parametric oscillation.This proposal aims at the study of polariton condensates in (Al,Ga)As-based microcavities under a tunable periodic potential. This potential will be generated by surface acoustic waves (SAWs), which are particularly well suited to manipulate polariton condensates since their wavelength is smaller than the coherence length of the condensate. They are also capable of coherently modulating both the exciton and the photon creating perfect periodic potentials. And, importantly, and unlike other approaches, the SAW potential is tunable. We have set an important milestone by achieving the growth of the microcavity samples and demonstrating that the modulation of polariton condensates by SAWs is possible. The purpose of this project is to extend these investigations. Our goals include:1. increase the efficiency of modulation of polaritons by SAWs, 2. realize basic studies of polaritons in a SAW lattice 3. perform advanced experiments aimed at the control of the condensate coherence properties and interactions, 4. exploit the former points for the generation of non-classical light states and study of non-equilibrium quantum phase-transitions.