Design and Applications of Nanoscale Light Sources





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Fast and efficient nanoscale light sources are at the heart of on-chip optical communication and computation systems. With the rapid development of advanced fabrication techniques and the use of metal in cavity designs, light confinement, and manipulation at the nanoscale, far below the diffraction limit of light, have become possible. Over the years, various nanoscale lasers and LEDs have been analytically or experimentally demonstrated. From the modulation bandwidth perspective, nanolasers are ultimately limited by gain compression at high injection currents. From the energy efficiency perspective, nanolasers are inefficient due to the required high injection current to reach the lasing threshold. In contrast, nanoLEDs can simultaneously support large modulation bandwidth due to the Purcell effect, and high energy efficiency because they can be operated at low injection currents without the need to reach the lasing threshold. This dissertation is focused on the design and applications of nanoscale light sources towards the realization of nanoLEDs that can support high speed modulation and efficient operation. Firstly, we present an optically pumped version of a shifted-core coaxial nanoLED, with a footprint of merely 1/3 of its emission wavelength in all three dimensions at telecommunication wavelengths. By shifting the metallic core off the center of the coaxial cavity, the effective mode volume can be reduced to 0.0078×(λ0/na)3, resulting in a Purcell factor over 390 and a modulation bandwidth exceeding 60 GHz. Furthermore, this nano-emitter features improved emission directivity, which increases its coupling efficiency to an on-chip waveguide. As this nano-emitter supports only one TEM-like mode over the entire material gain spectrum, the spontaneous emission factor becomes close to unity, which greatly improves its internal quantum efficiency. In order to calculate the Purcell factor precisely, we exhaustively studied the effective modal volume, Veff. We found that for cavities with poor confinement and low quality factors, the choice of a correct field normalization method is crucial to adequately describe the diverging behavior of the cavity’s effective modal volume. Secondly, we present the design of an electrically pumped shifted-core coaxial nanoLED. We design the multiple quantum well III-V gain material to achieve high internal quantum efficiency and an impedance transformer to improve the injection efficiency into the nanoLED. Lastly, we propose a biochemical sensor based on plasmonic nanofocusing phenomenon in a pair of coupled shifted-core coaxial nano-cavities. By placing a fluidic channel between the two cavities in close vicinity to the hotspots created by the coupled modes, the sensitivity of this biochemical sensor can be greatly enhanced. In our simulation, this biochemical sensor shows an ultra-high sensitivity up to 1.5179×104 nm/RIU.



Engineering, Electronics and Electrical