Reminder: Ph.D. Oral- Linyou Cao, 1:00pm TODAY, Dec. 16, CISX 101

Optical Resonances in Semiconductor Nanowires

Linyou Cao

Brongersma Group

Department of Materials Science and Engineering

Time: Dec. 16th 1:00pm (Refreshments start at 12:45 pm)                   Location: CISX 101

Abstract:

Semiconductor nanowires constitute one of the most exciting frontiers of materials research because of their potential application in a wide range of important fields, including information technology, biomedicine, sustainable energy, and artificial intelligence.  Embarking on these exciting applications heavily hinges on a deep understanding of the fundamental physical properties of nanowires. For the first time, we experimentally demonstrate the existence of strong, tunable optical resonances in semiconductor nanowires, and propose an intuitive theoretical framework based on leaky mode resonances (LMRs) to understand and engineer the nanowire's optical properties. The optical resonances enable engineering of the nanowire's light absorption, scattering, and emission properties and a rational design of high-performance optoelectronic devices, including photodetectors, solar cells, and light emitters.  I will also show that coupled optical resonances in arrays of nanowire 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 photoresponse of the nanowire at the specific wavelengths can be dramatically enhanced.  By tuning the nanowire 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 by the possibility to realize 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 latitude 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 closely-spaced, parallel nanowires of similar diameter. It will be shown that, unlike the optical coupling of classical microscale resonators, which can be described by conventional coupled mode theory (CMT), the much stronger coupling of nanowire resonators does not strictly follow this model. 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 nanostructure-arrays with desirable light-matter interaction features for nanophotonic applications. One exciting application is the efficient transfer of optical power at the nanoscale through a chain of coupled nanowires.

Overall, these results represent the first systematic studies on the 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 functionalities in semiconductor nanostructures for photonics applications.

Time: Dec. 16th 1:00pm (Refreshments start at 12:45 pm)                   Location: CISX 101