The Nexus of Electron and Photon Microscopy: TERS in the Atomistic Near-Field

Atomically confined photons at the apex of an atomically terminated plasmonic needle allow the atomistic resolution attained in Tip-enhanced Raman spectro-microscopy (TERS) [1,2]. In this limit, TERS carried out at the junction of a scanning tunneling microscope, [3] can be equally well regarded as scanning near-field optical microscopy or near-field scanning tunneling electron microscopy. The two qualifications arise quite generally from the dual field-matter nature of a photon in a dielectric, namely, of Maxwell’s electromagnetic displacement D = E + 4πP. Seeing atoms [4], or seeing atomically resolved sound (acoustic phonons), are examples that highlight the opening of the atomistic world to viewing through optical microscopy [5]. Wiring photons into molecules and tracing intramolecular currents in molecular circuits through atomic contacts made with a spectrum analyzer [1], highlight the paradigm of scanning tunneling electron microscopy. More broadly, the nexus of atomically confined photon and electron heralds pico-science. As illustrated along the 1/time-space axes in Figure 1, the confined electron precipitated nanoscience, leading to quantum electronics with nanometric architecture, while the confined photon (photonics) has provided the speed-up in device operation. The two meet at the focus of a plasmonic lens, with confinement limited by the atomic granularity of matter, on a scale governed by quantum laws of physics. Two examples of pico-electromechanical systems are illustrated in the Figure. The image of the normal mode of a molecule tracks the intramolecular polarization current at 35 THz with 150 pm spatial resolution [2].2 A carbon monoxide molecule wired to a silver tip serves as a single-molecule electrometer [6], as a complete device (sensor, transducer, and sending unit) that operates at 100 THz with 3.4 pm amplitude of motion used to map electrostatic fields with Å-scale spatial resolution. I will present our current understanding of atomically confined photons and touch upon implications regarding outstanding challenges in the description of photons in dielectrics – descriptions that are compelled by experimental observations. The nexus of confined electron and photon reached with a plasmonic lens (gold cone), marks the divide between classical (-C→) and quantum (←Q-) electrophotonics, and the opening of picoscience (light green box). The atomic limit in spatial resolution is marked by the red star, reached in optical microscopy by mapping the intramolecular current carried by a vibrational normal mode [2]. A carbon monoxide terminated silver tip illustrates a single molecule device that operates at 100 THz, on the pico-length scale [6].