Optical resonances of semiconductor nanowires [electronic resource]

Semiconductor nanowires are one of the most exciting frontiers of materials research due to their potential applications in a wide range of important fields, including information technology, biomedicine, sustainable energy and artificial intelligence. Embarking on these exciting applications heavily hinges on deep understanding of fundamental properties of the nanowires. For the first time, we experimentally demonstrate the general existence of strong, tunable optical resonances in semiconductor nanowires, and propose a theoretical model, leaky mode resonances (LMRs), that provides an intuitive understanding of the optical resonances. The optical resonances enable to engineer light absorption, scattering and emission of the nanowires for the rational design of high-performance optoelectronic devices, including photodetectors, solar cells, and light emitters. More interestingly, coupled optical resonances in a complex nanowire structure can give rise to many novel optical functionalities that do not exist in stand-alone nanowires, for example, coupled nanowire optical waveguiding. Physically, the optical resonances arise from strong and resonant coupling of light with leaky modes supported by the nanowires. When the light wavelength matches one of the allowed LMRs, the high refractive index wire can capture and trap the light by multiple internal reflections at its boundary and build up strong electromagnetic field inside. As a consequence, the photoresponses of the nanowire at the specific wavelength or wavelength bands, including absorption, scattering and emission, can be dramatically enhanced. By tuning the NW diameter, both the number of allowed LMRs in the nanowire and the spectral position of specific LMRs can be precisely controlled. This size-dependent tunability provides a powerful guidance for the rational design of photonic devices with desired spectral, polarization response features. The technological promise of this approach is illustrated in efficient germanium photodetectors in near infrared regime, silicon solar cells with 250% enhancement in solar absorption efficiency, and multicolored silicon nanostructures. Optical coupling between neighboring nanowires provides extra latitudes to manipulate light at the nanoscale. The essence of the optical coupling lies in the exchange of photons between the nanowires, much like the exchange of electrons between neighboring atoms in molecules. Experimentally, it can be observed by monitoring the light scattering spectra of a bi-nanowire structure that consists of two nanowires with similar diameter and parallel to each other. By taking into account the leaky nature of optical modes in the nanowire resonator, we propose a theoretical model, coupled leaky mode theory (CLMT), to account for the experimental observations and to point towards rational designs of complex nanostructures with desirable light-matter interaction features for nanophotonic applications, such as efficient transfer of optical power at the nanoscale through a chain of coupled nanowires. Overall, these results represent the first systematic studies of optical resonances of semiconductor nanowires. The demonstrated general existence of the LMRs and the coupled LMRs cast new light on semiconductor nanostructures, and open up enormous opportunities to explore novel optical and optoelectronic funtionalities in semiconductor nanostructures for photonics applications.

Keywords: Materials Science and Engineering
Advisor: Brongersma Mark L. primary advisor. Thesis advisor
Cui Yi
Salleo Alberto