We demonstrate the ability to map photo-induced gradient forces in materials, using a setup akin to atomic force microscopy. This technique allows for the simultaneous characterization of topographical features and optical near-fields in materials, with a high spatio-temporal resolution. We show that the near-field gradient forces can be translated onto electric fields, enabling the mapping of plasmonic hot-spots in gold nanostructures, and the resolution of sub-10 nm features in photocatalytic materials. We further show that the dispersion-sensitive nature of near-field gradient forces can be used to image and distinguish atomically thin layers of 2-D materials, with high contrast.
Nanophotonic systems such as plasmonic and 2-D materials and metamaterials serve as excellent platforms to study and control several optical and chemical phenomena such as spontaneous emission, absorption, Raman scattering and photocatalysis. Techniques such as atomic force microscopy and scanning electron microscopy enable the imaging of nanoscale features, while other techniques such as scanning tunneling microscopy and scanning near-field optical microscopy, enable the near-field optical characterization of nanoscale materials. However, most of these techniques do not allow for simultaneous imaging of topographical features and spectroscopic characterization with high spectral selectivity and temporal resolution. Here, we make use a new imaging technique called photo-induced force microscopy [1,2], which enables imaging and optical characterization of nanoscale materials with very high spatial and temporal resolution. In this technique, a nanoscale tip is brought in the vicinity of the sample, which is optically excited. The photo-induced gradient forces between the tip and the sample can be detected with nanometer-scale spatial resolution, along with topographical information, akin to an atomic force microscope. The photo-induced gradient forces, which are very sensitive to polarization and the distance of the tip from the sample, can be read out and converted to electric fields . As a proof-of-concept demonstration, we image the transverse and longitudinal resonances in gold nanorods and compare their field enhancements to gap plasmons of gold dimers.
 J. Jahng et al. Gradient and scattering forces in photo-induced force microscopy. Phys. Rev. B 90, 155417 (2014).
 F. Huang et. al., Imaging nanoscale electromagnetic near-field distributions using optical forces, Sci. Rep. 5, 10610 (2015).