Near-field Scanning Optical Microscopy (NSOM) Motivation: Optical techniques such as imaging, spectroscopy, induction of photocurrents are invaluable tools for the characterization of nanostructures. Unfortunately, the “diffraction limit” to resolution is approximately ½λ, implying that about 300 nm resolution is achievable for red light. In NSOM the resolution is determined by the size of a near-field aperture. LPS in collaboration with the Dennis Drew’s group at the University of Maryland has developed high throughput, polarization-maintaining near-field probes with 20 nm resolution. NSOM Luminescence Imaging: LPS has characterized the “accidental” quantum dots (QDs) that form at the interface of narrow AlGaAs/GaAs/AlGaAs quantum wells (QWs). A combination of photoluminescent (PL) spectral and spatial resolution is required to isolate individual QDs that is not achievable with traditional microscopy. The sharp peaks to the left of the PL spectrum represent the ground-state energy of individual QDs that form on a 7 monolayer thick QW. The energy E8 corresponds to a QD of infinite size with zero lateral confinement energy. The QD energy increases as its size decreases, eventually arriving at E7 which corresponds to a QD of infinite size on a 6 monolayer thick QW. The curve labeled “spatial average” is close what is achievable with traditional “far-field” optics. Click for a schematic of interface QDs or for spatial scans of QDs. Novel Near-field Probes: Traditional “pulled” tips have a practical limit of about 100 nm for typical experiments, but LPS has developed high throughput, polarization-maintaining near-field probes with 20 nm resolution. Using a chemical etching process on elliptical core fiber, short-tapered tips with high throughput have been achieved.
Schematic of the etched near-field probe (left) with a scanning electron microscopic (SEM) images of an actual probe (center) and the probe aperture (right). Applications: Future experiments are planned to use NSOM to characterize “hot spots” on metal nanoparticle aggregates used in metal enhanced fluorescence (MEF) and to measure single molecule fluorescence.
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