Optical (or light) microscopy is arguably one of the most successful techniques for the non-invasive examination of the microscopic world ever created. Robert Hooke coined the term "cells" to describe the substructure of cork he first observed through a microscope in the 17th century. Over the past century a variety of sophisticated methods have been developed that today provide the ability to observe migrating cells, examine the distribution of subcellular structures, map the expression of genes, or form "chemical images" coded according to the molecular structure of the sample. Despite its immense success, optical microscopy is fundamentally limited in its ability to resolve features less than a few hundred nanometers. Specifically, the diffraction limit causes light from points in an object to spread out as it propagates through a lens, thereby blurring images as they are magnified. Near-field microscopy overcomes these limitations by placing an optical probe a few tens of nanometers away from the object and sampling the emitted or scattered light before it experiences diffraction. Boston University researchers are building a versatile near-field microscope providing local and regional users with access to optical resolution on the order of 10 nanometers. The instrument is enabling a range of important research thrusts including (i) studies of protein folding behavior that can shed light on conditions such as Alzheimer's; (ii) the development of new materials for laser sources; (iii) methods for engineering the properties of single atomic thick layers like graphene for next-generation electronics and; (iv) methods for controlling the flow of light on nanometer length scales, important for new optical sensors and communications technologies. The BU project is also engaging women and underrepresented minority undergraduate students in cutting-edge research. Since the ability to "see" objects at the nanoscale can be a powerful motivator for a young mind, the team is working with BU's Upward Bound Math Science 7-week residency college prep program for urban high school students. Every Wednesday in the summer, students perform nanotechnology hands-on experiments and use the new microscope to observe the nano-world.
Optical microscopy is arguably one of the most successful techniques for non-invasive examination of the microscopic world ever created, but is fundamentally limited to length scales of 100 nm or more by the diffraction limit. Near-field microscopy overcomes this by placing a source or probe into the near optical field of a sample to couple the non-propagating or evanescent modes into far-field propagating modes for collection. While there are 5 or 6 companies that sell near-field microscopes, all are limited in capability. Boston University researchers are building a versatile holographic nanoscale optics instrument integrated into an atomic force microscope, combining near-field spectroscopies of elastic scattering, Raman and fluorescence over a wide wavelength range. The instrument operates in both transmission and back-scattering geometries, and includes interferometry for phase-resolved near-field imaging to map 3D field response, providing the flexibility and dexterity that are critical to advance complex research problems. The instrument enables researchers at BU and regional universities to investigate nanoscale optical phenomena in plasmonics, biophysics, and graphene and other two-dimensional (2D) crystal membrane physics. Plasmonic studies are exploring hot spots, local density of states and in particular, phase singularities predicted to occur at the interface between metal and dielectric components. In strain engineered 2D crystals of graphene, MoS2 and hBN, researchers at BU are exploring atomic-scale friction and the exciting possibility of mapping strain-induced pseudo-magnetic fields. In studies of Germanium semiconductor nanomembranes, local optical response can confirm and help engineer nano-devices with direct bandgaps. And in biophysics, long-wavelength tip-enhanced near-field microscopy can provide unprecedented images of intrinsic vibrational modes capable of sub-cellular classification and local presence of important proteins.
