Nanoscale spectroscopic origins of photoinduced tip-sample force in the midinfrared.

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

Nanoscale spectroscopic studies of two different physical origins of the tip-enhanced force: dipole and thermal

When light illuminates the junction formed between a sharp metal tip and a sample, different mechanisms can con-tribute to the measured photo-induced force simultaneously. Of particular interest are the instantaneous force be-tween 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 two force mechanisms is their spectral behaviors. The magnitude of the thermal response follows a dissipative Lorentzian lineshape, 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. Be-cause the two interactions are sometimes comparable in magnitude, the origin of the nanoscale chemical selectivity in the recently developed photo-induced force microscopy (PiFM) is often unclear. Here, we demonstrate theoretically and experimentally how light absorption followed by nanoscale thermal expansion generates a photo-induced force in PiFM. Furthermore, we explain how this thermal force can be distinguished from the induced dipole force by tuning the relaxation time of samples. Our analysis presented here helps the interpretation of nanoscale chemical measure-ments of heterogeneous materials and sheds light on the nature of light-matter coupling in van der Waals materials.Comment: 17 pages, 10 figure

Nanoscale chemical imaging by photoinduced force microscopy.

Correlating spatial chemical information with the morphology of closely packed nanostructures remains a challenge for the scientific community. For example, supramolecular self-assembly, which provides a powerful and low-cost way to create nanoscale patterns and engineered nanostructures, is not easily interrogated in real space via existing nondestructive techniques based on optics or electrons. A novel scanning probe technique called infrared photoinduced force microscopy (IR PiFM) directly measures the photoinduced polarizability of the sample in the near field by detecting the time-integrated force between the tip and the sample. By imaging at multiple IR wavelengths corresponding to absorption peaks of different chemical species, PiFM has demonstrated the ability to spatially map nm-scale patterns of the individual chemical components of two different types of self-assembled block copolymer films. With chemical-specific nanometer-scale imaging, PiFM provides a powerful new analytical method for deepening our understanding of nanomaterials

Photo-induced force microscopy and sepctroscopy

We introduce a novel type of light-matter interaction force microscopy and spectroscopy, called as "Photo-induced force microscopy''. We visualize the optical interplay between tip and sample junction at molecular level in ambient condition. The linear and nonlinear optical responses such as localized permittivity, stimulated Raman and pump-probe are addressed at space--time limit. The ability of control the force at nano/molecular scale in ambient condition leads to a variety of potential application such as single molecule study,nano-printing, nano-architecturing with chemical selectivity