Jahng Junghoon, Potma Eric O, Lee Eun Seong
Center for Nanocharacterization, Korea Research Institute of Standards and Science, Daejeon 34113, South Korea.
Department of Chemistry, University of California, Irvine, CA 92697.
Proc Natl Acad Sci U S A. 2019 Dec 26;116(52):26359-26366. doi: 10.1073/pnas.1913729116. Epub 2019 Dec 11.
When light illuminates the junction formed between a sharp metal tip and a sample, different mechanisms can contribute to the measured photoinduced force simultaneously. Of particular interest are the instantaneous force between the induced dipoles in the tip and in the sample, and the force related to thermal heating of the junction. A key difference between these 2 force mechanisms is their spectral behavior. The magnitude of the thermal response follows a dissipative (absorptive) Lorentzian line shape, which measures the heat exchange between light and matter, while the induced dipole response exhibits a dispersive spectrum and relates to the real part of the material polarizability. Because the 2 interactions are sometimes comparable in magnitude, the origin of the chemical selectivity in nanoscale spectroscopic imaging through force detection is often unclear. Here, we demonstrate theoretically and experimentally how the light illumination gives rise to the 2 kinds of photoinduced forces at the tip-sample junction in the midinfrared. We comprehensively address the origin of the spectroscopic forces by discussing cases where the 2 spectrally dependent forces are entwined. The analysis presented here provides a clear and quantitative interpretation of nanoscale chemical measurements of heterogeneous materials and sheds light on the nature of light-matter coupling in optomechanical force-based spectronanoscopy.
当光照射到尖锐金属尖端与样品之间形成的结时，不同的机制可同时对测量到的光致力产生影响。特别值得关注的是尖端和样品中感应偶极子之间的瞬时光致力，以及与结的热加热相关的力。这两种力机制之间的一个关键区别在于它们的光谱行为。热响应的幅度遵循耗散（吸收）洛伦兹线形，它测量光与物质之间的热交换，而感应偶极子响应呈现色散光谱，并与材料极化率的实部相关。由于这两种相互作用的大小有时相当，通过力检测进行纳米级光谱成像中化学选择性的起源往往不明确。在这里，我们通过理论和实验证明了光照射如何在中红外波段的尖端 - 样品结处产生这两种光致力。我们通过讨论两种光谱相关力相互交织的情况，全面探讨了光谱力的起源。这里给出的分析为异质材料的纳米级化学测量提供了清晰且定量的解释，并揭示了基于光机械力的光谱纳米oscopy中光与物质耦合的本质。
