Significant advances in understanding the basic properties of light-matter interactions have been made these past few decades, due to improved spatial and temporal resolution in experimental measurements. In this context, we have achieved higher spatial resolution, by extending the newly developed multifrequency atomic force microscopy (AFM) [1] technique to the optical domain, on near field scanning optical microscopy (NSOM).

The main challenge faced in NSOM is to achieve high optical contrast, usually solved by detecting optical signals at higher harmonics [2], while eliminating optical background [3], in order to obtain a clear, artifact-free signal. Our theoretical and preliminary experimental results of optical contrast measurements, which are pending publication, have indeed shown an increased resolution with an improved signal to noise ratio by a factor of >40. This is due to a unique lock-in detection scheme, that can only be realized by implementing this multifrequency method. This will not only allow us to measure dynamical responses of nanostructures in very accurate spatial resolution, but will also be the best optical near field microscope, allowing further state of the art capabilities to emerge in other fields, such as imaging and sensing of ultrafast chemical reactions, bio-sensing and metallurgy.

[1] Garcia, R. & Herruzo, E.T. “The emergence of multifrequency force microscopy,” Nat. Nanotechnol. 7, 217–226 (2012).

[2] B. Knoll and F. Keilmann, “Enhanced dielectric contrast in scattering-type scanning near-field optical microscopy,” Opt. Commun. 182, 321–328 (2000).

[3] M. Labardi, S. Patane, and M. Allegrini, “Artifact-free near-field optical imaging by apertureless microscopy,” Appl. Phys. Lett. 77, 621 (2000).