Femtosecond Photon Echose by using Incoherent Light

光子回波是自旋回波的光学模拟，通常用二能级原子系统模型来描述。到目前为止，许多实验都是在这个框架下进行的。凝聚态物质（如固体和液体）的相弛豫时间T_2通常很短，即使在很低的温度下也是如此，并且常常落在皮秒或飞秒区域。随着超短脉冲激光技术的发展，在这种短T_2材料中进行光子回波实验已成为可能。利用时间非相干的纳秒激光脉冲对聚乙烯醇溶液中的甲酚紫进行了光子回波实验，发现在10 K时的回波衰减曲线由两部分组成，一部分是衰减很快的尖峰，另一部分是在10 K时的峰值。另一个是尾部慢慢腐烂的翅膀。这种双重结构与该材料中众所周知的（非）光化学持久性空穴的形状一致。我们认为，回波衰减曲线中的尖峰是由于声子边带跃迁，尾部的翼是由零声子跃迁引起的。空穴中声子边带的出现表明客体分子（甲酚紫）不能看作简单的二能级系统，而应看作多能级系统。我们将多能级系统模型应用于光子回波，并验证了该模型可以解释光子回波的衰减曲线和持续性空穴谱。

Photon echoes are the optical analogue of spin echoes and are usually described by a model of two-level atomic system. Many experiments so far have been explained in this framework. The phase relaxation times T_2 of condensed matter like solid and liquid are generally very short even at very low temperatures and often fall in the picosecond or femtosecond region. By the recent progress in the technology of ultra-short pulse lasers, the photon echo experiments in a material with su ch a short T_2 have become possible. Even without ultra-short pulses, picosecond or femtosecond photon echo experiments are now available by using temporally-incoherent excitation light.We made a photon echo experiment in cresyl violet in polyvinyl alcohol by using temporally- incoherent nanosecond laser pulses, and found that the echo decay curve at 10 K is made of two components, one is a sharp peak which decays very rapidly, and the other is a slowly decaying wing at the tail. This double structure agrees with the shape of the well-known (non)photo- chemical persistent hole in this material. We believe that the sharp peak in the echo decay curve is due to the phonon-sideband transition and the wing at the tail originates from the zero-phonon transition. The appearance of the phonon-sideband in the hole implies that the guest molecule (cresyl violet) cannot be considered as a simple two-level system, and the phonon-dressed molecule should rather be taken as a multi-ievel system. We applied a multi-level system model to photon echoes, and verified that both the photon echo decay-curve and the persistent hole spectrum can be explained by using this model.