Abstract
In scattering-type near-field microscopy (s-SNOM), a metal tip held a few nanometers above a surface and illuminated with infrared light scatters a signal that depends on what lies beneath it. What does that signal actually measure? I will argue that it is a single quantity — the sample's p-polarized reflection coefficient, evaluated at in-plane momenta roughly a hundred times larger than free-space light can supply — and that this one idea organizes most of what near-field microscopy has done over nearly twenty years. The story begins before we had it: in vanadium dioxide, where images of metallic islands growing inside the insulator gave us one number per temperature — the conducting fraction — and where we did not yet ask the tip for a permittivity. The point-dipole picture of the tip gave a first answer, and silicon dioxide films only a few nanometers thick showed where it fails and how the momentum-dependent reflection coefficient repairs it. In passing, I will point out an interesting parallel: the same mathematics, with the s-polarized reflection coefficient in place of the p-polarized one, describes the response of an NV-center probe used in quantum sensing. The same p-polarized coefficient predicts the plasmons of gated graphene, which we found first in spectra and then imaged as interference fringes near sample edges. The fringe method has real limitations: one tip must both launch the plasmon and detect it, an edge must reflect it, and graphene at an edge is not the graphene of the interior. I will show data on each, including an observation I have shown in talks but never published: a blunt tip stops launching plasmons altogether. I will close with the instrument these limitations point to — one probe built to launch, a second built to detect, and only the sample in between — and the measurement it would make clean: a test of plasmon amplification in optically pumped graphene.
Gregory Andreev, Ph.D., is an independent researcher and startup entrepreneur in semiconductor near-field optics. Trained in D. N. Basov's infrared nanoscopy group at UC San Diego, where the work in this talk was done: co-author of the near-field studies of the vanadium dioxide transition (Science 2007), of few-nanometer silicon dioxide films (Phys. Rev. B 2012) and of graphene plasmons (Nano Letters 2011; Nature 2012). Since then, designer of scattering-type near-field instruments that became products at companies in nanoscale instrumentation, holder of patents on that instrumentation, and, more recently, an optical scientist at Meta working on holographic materials for AR glasses.