Li Zhongming, Aleshire Kyle, Kuno Masaru, Hartland Gregory V
Department of Chemistry and Biochemistry, University of Notre Dame , Notre Dame, Indiana 46556, United States.
J Phys Chem B. 2017 Sep 21;121(37):8838-8846. doi: 10.1021/acs.jpcb.7b06065. Epub 2017 Aug 9.
Infrared (IR) imaging provides chemical-specific information without the need for exogenous labels. Conventional far-field IR imaging techniques are diffraction limited, which means an effective spatial resolution of >5 μm with currently available optics. In this article, we present a novel far-field IR imaging technique based on photothermal heterodyne imaging (IR-PHI). In our version of IR-PHI, an IR pump laser excites the sample, causing a small temperature rise that is detected by a counterpropagating visible probe beam. Images and spectra of several different types of soft matter systems (polystyrene beads, thin polymer films, and single Escherichia coli bacterial cells) are presented to demonstrate the sensitivity and versatility of the technique. Importantly, the spatial resolution in the IR-PHI measurements is determined by the visible probe beam: a spatial resolution of 0.3 μm was achieved with a 0.53 μm probe wavelength and a high numerical aperture focusing objective. This is the highest spatial resolution reported to date for far-field IR imaging. Analysis of the experiments shows that for polymer beads in a dry environment, the magnitude of the IR-PHI signal is determined by the scattering cross section of the nano-object at the probe wavelength. This is in contrast to conventional PHI experiments in a heat-transfer medium, where the signal scales as the absorption cross section. This different scaling can be understood through the optical theorem. Our analysis also shows that both thermal expansion and changes in the refractive index of the material are important and that these two effects, in general, counteract each other.
红外（IR）成像无需外源性标记就能提供化学特异性信息。传统的远场红外成像技术受衍射限制，这意味着使用目前可用的光学器件时有效空间分辨率大于5μm。在本文中，我们提出了一种基于光热外差成像（IR-PHI）的新型远场红外成像技术。在我们的IR-PHI版本中，红外泵浦激光激发样品，导致温度小幅上升，该温度上升由反向传播的可见探测光束检测。展示了几种不同类型软物质系统（聚苯乙烯珠、聚合物薄膜和单个大肠杆菌细菌细胞）的图像和光谱，以证明该技术的灵敏度和通用性。重要的是，IR-PHI测量中的空间分辨率由可见探测光束决定：使用0.53μm探测波长和高数值孔径聚焦物镜实现了0.3μm的空间分辨率。这是迄今为止报道的远场红外成像最高空间分辨率。对实验的分析表明，对于干燥环境中的聚合物珠，IR-PHI信号的大小由纳米物体在探测波长处的散射截面决定。这与在传热介质中的传统PHI实验形成对比，在传统实验中信号与吸收截面成比例。这种不同的比例关系可以通过光学定理来理解。我们的分析还表明，材料的热膨胀和折射率变化都很重要，并且这两种效应通常相互抵消。
