Single-Mode Waveguide Coupling of Optical Antenna-LED for Optical Interconnects


Optical interconnects have displaced electrical wires in data centers and high performance computers due to higher efficiency and bandwidth. The development of on-chip optical interconnects is the next frontier for development, with the promise of reducing on-chip energy consumption. Light emitting diodes have high efficiencies and can achieve low footprint; however, spontaneous emission is typically far too slow to be useful in an interconnect. This can be understood by modeling spontaneous emission as dipole emission with a dipole length on the order of the atomic spacing (~1.3nm) which is much smaller than the radiation wavelength at 1550nm. This becomes a familiar radio frequency (RF) engineering problem: to make the light-emitting diode faster we just need to make it a better antenna.

By placing the radiating dipoles in the feed-gap of an optical antenna we can dramatically enhance the spontaneous emission rate, potentially allowing for greater than 100GHz direction modulation. However, to create a useful on-chip interconnect, we need to couple the output light to a single mode waveguide so we can guide the light across the chip to a photodetector. Additionally, both the device and waveguide coupling scheme need to be compatible with top-down fabrication and electrical injection.

In this dissertation we focus on the cavity backed slot antenna geometry, showing that this device can be efficiently coupled to a single mode waveguide. The first part of this dissertation reviews the theory behind spontaneous emission enhancement, the cavity backed slot antenna, and waveguide coupling.

The waveguide coupled cavity backed slot antenna is then fabricated and measured. Clear evidence of waveguide coupling is demonstrated. The collected data is then compared to theory showing excellent agreement between experiment and finite difference time domain (FDTD) simulations. The estimated experimental waveguide coupling efficiency to a single mode waveguide is ~85.9%.

The efficiency of the device is improved through a novel surface passivation process, leading to a record low surface recombination velocity. Detailed time decay models are presented, including a method to model an arbitrary rate equation. The internal quantum efficiency of a 60nm wide LED ridge is estimated to be 10-20% - with these devices showing >180x increase in photoluminescence.

Finally, the dissertation concludes with a full-link system model and a discussion of how to increase the power from the cavity backed slot antenna-LED and the importance of close integration with CMOS. The modeled end-to-end energy per bit can be less than 1fJ/bit, showing the great potential for on-chip optical interconnects.

Nicolas Andrade
Publication date: 
May 1, 2023
Publication type: 
Ph.D. Dissertation
Nicolas Andrade EECS Department University of California, Berkeley Technical Report No. UCB/EECS-2022-19 May 1, 2022

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