Novel Concepts in Infrared Nano-Imaging – Competing, Confusing, Controversial, or Complementary?

Over the past decade a number of infrared scanning probe techniques have matured, and new ones emerged, that can extend the spatial resolution of infrared spectroscopy into the nanometer regime. They take advantage of different contrast mechanisms, through the optical or mechanical response of a scanning probe tip under infrared resonant interaction with the sample. Specifically, near-field enhanced light scattering based on a tip-sample coupled polarization makes infrared scattering scanning near-field optical microscopy (IR s-SNOM) an all optical technique, providing the full optical response function of a material, and widely applied for the study of vibrational, phonon, polariton, and other low-energy excitations in molecules, 2D materials, and quantum materials. In addition, associated with the resonant absorption of the sample a thermal expansion, even as small as picometers, gives rise to an opto-mechanical force response of the scanning probe tip in photo-thermal induced resonance (PTIR) or AFM-IR, providing a direct measure of the nanoscale spectral absorption. In addition, albeit controversial, an optical gradient force between the tip and the sample was proposed as the underlying contrast in a method termed photo-induced force microscopy (PiFM). However, as the number of demonstrations and applications grew, showing in part impressive contrast, spatial resolution, and sensitivity, a in part confusing picture emerged with regards to the respective contrast mechanisms, notably in tip-force detection in PiFM and PTIR. In this talk we will compare and quantify the responses from the different techniques, applied to different sample systems and materials excitations, and based on their multimodal implementation in a single instrument. We will discuss possible cross-talk and super-position of the different signal channels and their sensitive dependence on AFM operation. We will conclude with a perspective for the optimization of the different techniques for different applications, their fundamental limits, and a proposal for a more general approach to the exploration of infrared nano-spectroscopy based on optical, opto-thermal, and opto-mechanical principles for the study of molecular, quantum, and bio-systems.