O'Callahan Brian T, Yan Jun, Menges Fabian, Muller Eric A, Raschke Markus B
Department of Physics, Department of Chemistry, and JILA , University of Colorado at Boulder , Boulder , Colorado 80309 , United States.
Nano Lett. 2018 Sep 12;18(9):5499-5505. doi: 10.1021/acs.nanolett.8b01899. Epub 2018 Aug 14.
Control of photoinduced forces allows nanoparticle manipulation, atom trapping, and fundamental studies of light-matter interactions. Scanning probe microscopy enables the local detection of photoinduced effects with nano-optical imaging and spectroscopy modalities being used for chemical analysis and the study of physical effects. Recently, the development of a novel scanning probe technique has been reported with local chemical sensitivity attributed to the localization and detection of the optical gradient force between a probe tip and sample surface via infrared vibrationally resonant coupling. However, the magnitude and spectral line shape of the observed signals disagree with theoretical predictions of optical gradient forces. Here, we clarify this controversy by resolving and analyzing the interplay of several photoinduced effects between scanning probe tips and infrared resonant materials through spectral and spatial force measurements. Force spectra obtained on IR-active vibrational modes of polymer thin films are symmetric and match the material absorption spectra in contrast to the dispersive spectral line shape expected for the optical gradient force response. Sample thickness dependence shows continuous increase in force signal beyond the thickness where the optical dipole force would saturate. Our results illustrate that photoinduced force interactions between scanning probe tips and infrared-resonant materials are dominated by short-range thermal expansion and possibly long-range thermally induced photoacoustic effects. At the same time, we provide a guideline to detect and discriminate optical gradient forces from other photoinduced effects, which opens a new perspective for the development of new scanning probe modalities exploiting ultrastrong opto-mechanical coupling effects in tip-sample cavities.
对光致力的控制可实现纳米颗粒操纵、原子捕获以及光与物质相互作用的基础研究。扫描探针显微镜能够通过用于化学分析和物理效应研究的纳米光学成像与光谱技术对光致效应进行局部检测。最近，已有报道称开发出了一种新型扫描探针技术，其局部化学灵敏度归因于通过红外振动共振耦合对探针尖端与样品表面之间的光学梯度力进行定位和检测。然而，所观察到的信号的大小和光谱线形与光学梯度力的理论预测不一致。在此，我们通过光谱和空间力测量来解析和分析扫描探针尖端与红外共振材料之间几种光致效应的相互作用，从而澄清这一争议。与光学梯度力响应预期的色散光谱线形不同，在聚合物薄膜的红外活性振动模式上获得的力谱是对称的，并且与材料吸收光谱相匹配。样品厚度依赖性表明，在光学偶极力会饱和的厚度之外，力信号持续增加。我们的结果表明，扫描探针尖端与红外共振材料之间的光致力相互作用主要由短程热膨胀以及可能的长程热致光声效应主导。同时，我们提供了一种从其他光致效应中检测和区分光学梯度力的指导方法，这为开发利用尖端 - 样品腔中超强光机械耦合效应的新型扫描探针模式开辟了新的视角。
