Research Projects

Quantum nature of physical interactions in fabricated or synthesized materials is evident at 1-100 nm lengthscale. Recently, 2D materials such as MoS₂ has enabled optical study of materials with thicknesses even less than 1 nm. Photonics at nanometer scale with light manipulation capabilities of plasmonic structures provides unusual situations, where the spatial extent of light can be even be smaller than the quantum object itself. All of the existing selection rules for optical transitions assume that the spatial extent of light is essentially infinite relative to the size of atoms, molecules, unit cells, etc. Since the dipole interaction induced by light is effectively proportional to the position operator, the states with different parity are coupled, e.g. optical transition occurs between conduction and valence band of semiconductors at the gamma point. With the aid of plasmonic structures, however, we can excite quantum objects with light spot whose size is comparable or even smaller than the quantum object itself. The rapidly diffracting light in proximity of nanoscale plasmonic structures create a large electric field gradient within the size of a quantum emitter, whereby existing selection rules and the associated dynamics may have to be reexamined. In this project, we aim to explore such selection rule breakdown, occurrence of forbidden transitions and alteration of dynamics in semiconductor nanostructures using transmission, Raman and photoluminescence spectroscopies. For plasmonic structures, we propose nano-slit antenna with tunable nano-slit width (w) of 0.5-10 nm and several microns in length, which can support extreme light localization (spot-size) on the scale of w along its width. Such controllable width nano-slit antenna will be fabricated in gold/silver films deposited on flexible polymer substrates using focused ion-beam milling. The width will be controlled by stretching or bending the flexible polymer (e.g. PDMS or PET) substrate. Highly polarization sensitive nano-slits generate momentum uncertainty ~2pi/w along its width, which will facilitate investigation of usually forbidden, non-vertical transitions as long as energy conservation is satisfied. Apart from breakdown of selection rules, our preliminary theoretical modeling and experimental results suggest that extremely small cavity volumes offered by these nano-slits can induce intriguing light-matter interactions like very large Purcell enhancement and vacuum field induced polariton formation, which also will be investigated here using time-resolved spectroscopy techniques. With semiconductor nanostructures deposited on top or below the nano-slits, surface layer of only ~w width can dominate the optical response in transmission, making it an effective platform for probing local physical properties using far-field microscopy with easily available light sources. These nano-slit-semiconductor hybrid nanostructures are also potential candidates for opto-electronic devices.