Optical and electronic interactions at the nanoscale 

The research in Habteyes Group focuses on deeper understanding of fluorophore-metal interactions resolving observables in energy, space and time domains. Semiconductor nanocrystals are used as model fluorophore system because of their strong fluorescence and good photostability. The discrete electronic states of semiconductor nanocrystals allow clear fundamental understanding of the interaction of charge-carrier materials with metal nanoparticles. Depending on the spatial separation and the environment, photo-excitation of a fluorophore in the proximity of a metal surface can be followed by fluorescence enhancement, fluorescence quenching, charge transfer and/or interface charge trapping. Our research aims to investigate these processes using nanospectroscopy, which provides spectral information for a spatial region much smaller than the focal spot of excitation laser source. This few nanometer spatial resolution is achieved by using apertureless near-field scanning optical microscope, which creates excitation source localized to a tip of a sharp probe of atomic force microscope.

                                Experimental set up for correlated near-field imaging and spectroscopy measurements.

The near-field optical microscopy has a unique capability of achieving spatial resolution on the order of 10 nm, overcoming the resolution limit imposed by the diffraction of light. We exploit this unique capability of optical imaging to investigate a fluorophore-metal interaction that inherently takes place when the donor and acceptor materials are in close proximity (less than 30 nm separation). The nature of fluorophore-metal interaction can never be resolved in space with conventional confocal optical microscopy whose resolution is limited to 200 nm to the best. In a case, where hetero-nanoparticles are linked by molecules such DNA, in addition to probing the terminal nanomaterials, the vibration signature of the molecule is mapped using tip-enhanced Raman spectroscopy.

Simultaneously recorded topography, near-field amplitude and phase images of gold nanodisks. The total scan area is 800 nm x 600 nm and the smallest diameter of the disks is ~65 nm. If a confocal microscope were used, the 6 nanodisks seen here would have appeared as one spot in the optical image. Using our nanoscopic imaging approach, not only the nanodisks are observed individually but also the plasmon mode features on each nanodisk is clearly resolved. The double lobes in the amplitude image (middle) and the 180 degree jump in the phase image (right) are characteristics of a dipolar plasmon resonance excited at 633 nm (see our publication for detail).

The research activities in our group includes near-field plasmon mapping, tip-enhanced spectroscopy, single particle fluorescence and scattering measurements, lifetime measurements, synthesis and assembly of semiconductor and plasmonic nanomaterials to create hybrid materials with new functional material properties, molecular coupling of nanoparticles, and design and fabrication of broadband Raman probes as well as sensitive plasmon sensors.

Our lab is housed in the UNM Center for High Technology Materials, specially designed and built for vibration sensitive experiments. Our group is active user of the Center for Integrated Nanotechnologies, a DOE Office of Science User Facility and Nanoscale Science Research Center.