表征非透明高超声速声子晶体时相干损失的提升限制
Lifting restrictions on coherence loss when characterizing non-transparent hypersonic phononic crystals
摘要：
高超音速声子带隙结构限制了其波长与光的波长相当的声学振动，并且已经使用时域或频域光学光谱学进行了研究。脉冲泵浦-探测激光器是用于表征来自普通真空沉积技术的周期性多层堆叠的优选仪器，但是检测机制需要注入的声波在传播期间保持相干性。除了声学布拉格反射镜，使用串联法布里-珀罗干涉仪（TFPI）的频域研究发现了二维和三维声子晶体（PnCs）的色散，即使是高度无序的样品，但PnCs必须是透明的。在这里，我们展示了一种混合技术，用于克服时域和频域方法分别表现出的局限性：我们使用脉冲激光将相干声子注入到非透明PnC中，并在TFPI上获得声学透射谱，其中泵浦出现在自发激发（即非相干）声子旁边。选择一个金属布拉格反射镜进行说明，我们确定的带隙，并与传统的时域光谱比较，发现分辨率的混合方法，以匹配一个国家的最先进的异步光学采样设置。因此，混合泵浦-探测技术保留了已建立的技术的关键性能特征，并且对于无序样品而言，将可能是优选的。
英文摘要：
Hypersonic phononic bandgap structures confine acoustic vibrations whose wavelength is commensurate with that of light, and have been studied using either time- or frequency-domain optical spectroscopy. Pulsed pump-probe lasers are the preferred instruments for characterizing periodic multilayer stacks from common vacuum deposition techniques, but the detection mechanism requires the injected sound wave to maintain coherence during propagation. Beyond acoustic Bragg mirrors, frequency-domain studies using a tandem Fabry-Perot interferometer (TFPI) find dispersions of two- and three-dimensional phononic crystals (PnCs) even for highly disordered samples, but with the caveat that PnCs must be transparent. Here, we demonstrate a hybrid technique for overcoming the limitations that time- and frequency-domain approaches exhibit separately: We inject coherent phonons into a non-transparent PnC using a pulsed laser and acquire the acoustic transmission spectrum on a TFPI, where pumped appear alongside spontaneously excited (i.e. incoherent) phonons. Choosing a metallic Bragg mirror for illustration, we determine the bandgap and compare with conventional time-domain spectroscopy, finding resolution of the hybrid approach to match that of a state-of-the-art asynchronous optical sampling setup. Thus, the hybrid pump-probe technique retains key performance features of the established one and going forward will likely be preferred for disordered samples.
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