Many of the defining optoelectronic properties in low-dimensional materials – e.g. exciton Bohr radii and diffusion lengths, defect sizes and spacings, and Moire lattice periods – are determined by materials physics and processes that occur at the single-digit nm length scale. Their direct investigation and elucidation – crucial for future applications – therefore requires the ability to probe light-matter interactions at a resolution an order of magnitude better than what is generally achievable with existing nano-optical approaches. Here we propose a strategy for achieving single-nm optical resolution by developing a breakthrough capability which we will refer to as Atomic Energy Transfer Scanning nano-Optical Microscopy (AETSOM). The one-nm optical resolution will be attained by the attachment of a lanthanide-doped upconverting nanoparticle (UCNP) at the end of a near-field scanning probe tip. The intended probe is composed of a tapered metal-insulator-metal waveguide fabricated at the end of a glass fiber, enabling the efficient coupling of far-field light to the near-field and vice-versa through the probe tip, over a wide range of wavelengths. Lanthanide-doped UCNPs absorb multiple photons in the NIR and emit at higher energies in the NIR/visible with efficiencies orders of magnitude higher than those of the best 2-photon fluorophores. The robust attachment of the UCNPs to the probe through specific functionalization of the UCNPs will enable illumination/collection to/from single-digit nm volumes. The establishment of this breakthrough single-digit nano-optical capability will provide the ability to perform photon-based characterization and activation over multiple length scales on nearly any sample and in the real environments encountered in most technological applications. The anticipated results will immediately impact numerous fields, from quantum materials to photo-chemistry to energy harvesting to ultrasensitive biomolecular control and detection.
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