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Optical Antenna Enhanced Spontaneous Emission in Semiconductors

  • Author(s): Messer, Kevin James
  • Advisor(s): Yablonovitch, Eli
  • et al.
Abstract

Optical antennas can be used to dramatically increase the rate that semiconductors spontaneously emit photons. While traditional LEDs are limited in bandwidth due to the "slow" rate of spontaneous emission, antenna-enhanced LEDs have the potential to be a fast, efficient, nanoscale light emitter. Traditionally, lasers have dominated LEDs as the emitter in optical interconnects due to a 200x speed advantage of stimulated emission over spontaneous emission. This paradigm may be reversed by coupling LEDs to optical antennas. In fact, antenna enhanced spontaneous emission can be faster than the fastest stimulated emission.

Spontaneous emission originates from dipole fluctuations within the emitting material. The size of these fluctuations is much less than the wavelength of light emission, which leads to slow spontaneous emission. Coupling the material to an optical antenna corrects the size mismatch and improves the rate of radiation. An optical antenna circuit model is developed to predict the degree to which spontaneous emission can be enhanced. The circuit model presented in this dissertation shows that enhancement over 1000x is possible while still maintaining greater than 50% efficiency.

The circuit model provides insight how to design optical antennas for coupling to dipole sources, for maximum enhancement, and for high efficiency. A method for incorporating the anomalous skin effect, often overlooked in metal optics, is provided. While FDTD/FEM simulations cannot include this effect due to its nonlocal nature, its impact can be examined through the use of the optical antenna circuit model. Analysis of the tradeoff between achieving large spontaneous emission enhancement and maintaining high efficiency leads to an ideal antenna feedgap size of 10nm.

Experimental demonstration of spontaneous emission enhancement from InP coupled to an arch-dipole antenna is presented. Photoluminescence measurements show light emission from antenna-coupled InP over bare InP ridges was enhanced by 120x. The results correspond to a 14x enhancement of spontaneous emission once absorption enhancement is taken into account.

To improve on the previous results, 2D materials from the family of transition metal dichalcogenides are used as the active material. In monolayer form, these materials are direct band gap semiconductors with very low rates of surface recombination. The cavity-backed slot antenna is used to couple to the monolayer semiconductor. The resonant properties of the cavity-backed slot antenna are measured through dark-field scattering and used to match the antenna resonance to the wavelength of light emission. A WSe2 monolayer is etched into ribbons and coupled to the hotspot of a silver cavity-backed slot antenna resulting in an observed spontaneous emission enhancement of 318x.

Finally, an unetched MoS2 monolayer is coupled to an array of gold cavity-backed slot antennas in order to demonstrate both high enhancement and high quantum efficiency. Spatially resolved photoluminescence measurements show efficient emission only in the location of optical antennas. Areas of the MoS2 monolayer coupled to antennas show 7% quantum efficiency corresponding to a >250x increase in efficiency over bare MoS2 monolayers. These results demonstrate the exciting potential for antenna-enhanced LEDs to be both fast and efficient light emitters.

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