UC Santa Barbara

As our ability to engineer, design, and modify nanoscale systems continues to advance, characterization methods must keep pace. Light-matter interactions, in particular optical spectroscopy, provide a wealth of information on the vibrational and electronic structure of matter and can be directly related to physical properties such as phase, chemistry, charge transport, etc. However, the fundamental wave-like nature of light prevents radiation from being focused to arbitrarily small length scales using traditional optics. This is known as the diffraction limit, and is on the order of several hundred nanometers for optical wavelengths (~λ/2). One method of overcoming diffraction is to couple light to nanostructures (e.g., optical antennae) that support resonant oscillations of conduction electrons (plasmons). These charge oscillations generate intense optical fields that are spatially controlled by the antenna size, rather than the radiation wavelength. The set of related techniques utilizing nanoscale optical antennae to interrogate and image surfaces are known as tip-enhanced near-field optical microscopy (TENOM).

This work details the design, construction, and experimental validation of a TENOM instrument, and demonstrates specific applications in near-field spectroscopy and super-resolution chemical imaging. A commercial inverted optical microscope was integrated with a custom-built shear-force atomic force microscope (AFM). The inverted microscope geometry enables high excitation and collection efficiency of light from the antenna apex, while the shear-force AFM ensures the antenna is always positioned at the sample surface, allowing analytes to interact with the locally enhanced optical fields. Experimental validation of the completed TENOM instrument was accomplished using both copper (CuPc) and metal-free phthalocyanine (H2Pc) species. Chemical images of patterned CuPc and H2Pc were obtained with lateral spatial resolutions below 50 nm (<λ/10), unambiguously demonstrating the super-resolution capabilities of the instrument. Multimode imaging of H2Pc was performed with simultaneous collection of spatially correlated fluorescence, Raman, and topographic data. The combination of these measurements allowed nanoscale mapping of the H2Pc aggregation state across a wide range of surface coverages, including isolated molecules, molecular dimers, and continuous films.

Additionally, finite-difference time-domain (FDTD) optical simulations were used to study the fundamental physics of plasmonic optical antennae relevant for near-field spectroscopy. For TENOM applications, tuning the optical properties of support structures with attached plasmonic nanocavities was shown to be critical for either enhancing or quenching local electric field strengths. Support structures with low extinction coefficients were found to produce the largest field enhancements, with the refractive index of the material being further tuned to optimize antenna performance as a function of the specific geometry considered. A quantitative comparison of several antenna designs was carried out, which has not been possible experimentally due to the low reproducibility of nanostructure fabrication procedures and variability in methods of measuring local optical fields. Two promising architectures were identified that both involve focused ion-beam milling a groove near the antenna apex. Methods of tuning the resonance energy of these structures over the full visible spectrum, using different plasmonic metals (Au/Ag) and by varying the groove positions relative to the apex, were also demonstrated.

FDTD simulations were also used to study pairs of plasmonic nanoparticles relevant for surface-enhanced Raman spectroscopy applications. Previous work on this system was shown to significantly overestimate field enhancements due to numerical effects present at nano-gap features. Metal bridging structures were used to halt the field divergence at physically relevant lengths scales, allowing accurate study of experimentally relevant parameters including the fused contact area and presence of a dielectric encapsulation layer. It was found that fused dimer antennae are capable of producing large enhancements at infrared energies, but may be challenging to reproducibly fabricate due to the high sensitivity of the supported plasmon resonances to changes in local morphology.

Advancements across multiple scientific and engineering disciplines are helping push the TENOM technique forward. Improvements in high-intensity broadband laser sources will enable flexible measurement of both the electronic and vibrational structure of materials, and general improvements in nano-manufacturing are expected to reduce the time and cost of producing high-enhancement resonant antennae with well-defined plasmonic structure. The future is bright for TENOM to find use as a versatile optical and physical surface characterization technique.