Third-harmonic Generation Microscopy of Metal Nanocones Using Cylindrical Vector Beams

For years, there has been a renewed interest in the unique focusing properties of cylindrical vector beams (CVB), i.e., beams which exhibit rotationally symmetric polarization states, and their novel imaging applications [1]. Particularly, it was found that focused CVBs can be used as versatile orientation probes for single molecules and metal nanostructures [1]. Orientation imaging of metal nanostructures, however, has been mainly restricted to the use of linear optical techniques [2]. Furthermore, the linear response may not have sufficient sensitivity to detect low levels of anisotropy and structural deformations. Recently, we have shown that second-harmonic generation (SHG) with CVBs provides better orientation and defect sensitivity in metal nano-objects than linear optics and conventional polarizations [3]. In this work, we extend the use of focused CVBs to third-harmonic generation (THG) microscopy for metal nano-object characterization. Our samples consist of silver nanocones that were fabricated by ultraviolet-nanoimprint lithography [4]. The strong resemblance between the SHG (Figs. 1a and 1c) and THG (Figs. 1b and 1d) images of the nanocones suggests that orientation and anisotropy of 3D metal nano-objects can also be determined using THG with CVBs. Furthermore, our results provide an evidence of the vectorial aspect and unique utility of THG microscopy with focused unconventional polarizations as discussed previously [5].