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，许多实验都在这个框架内得到了解释。即使在很低的温度下，固体和液体等凝聚体的相松弛时间T_2通常也很短，往往落在皮秒或飞秒范围内。超短脉冲激光技术的最新进展使短T_2材料中的光子回波实验成为可能。在没有超短脉冲的情况下，利用时间非相干激发光可以进行皮秒或飞秒光子回波实验。利用时间非相干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10K时的回波衰减曲线由两个部分组成，一个是衰减非常快的尖峰，另一个是尾部缓慢衰减的峰。这种双层结构与这种材料中众所周知的(非)光化学永久空穴的形状一致。我们认为，回波衰减曲线上的尖峰是由于声子-边带跃迁，尾部的机翼来自于零-声子跃迁。空穴中声子边带的出现意味着客体分子(甲酚紫)不能被看作是一个简单的二能级系统，而声子修饰的分子应该被看作是一个多能级系统。我们将一个多能级系统模型应用于光子回波，验证了该模型可以解释光子回波的衰减曲线和持久空穴谱。

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.