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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2016)

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.

    Cover page: Optical Antenna Enhanced Spontaneous Emission in Semiconductors
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2015)
    Atoms and molecules are too small to act as efficient antennas for their own emission wavelengths. By providing an external optical antenna, the balance can be shifted; spontaneous emission could become faster than stimulated emission, which is handicapped by practically achievable pump intensities. In our experiments, InGaAsP nanorods emitting at ∼ 200 THz optical frequency show a spontaneous emission intensity enhancement of 35 × corresponding to a spontaneous emission rate speedup ∼ 115 ×, for antenna gap spacing, d = 40 nm. Classical antenna theory predicts ∼ 2,500 × spontaneous emission speedup at d ∼ 10 nm, proportional to 1/d(2). Unfortunately, at d < 10 nm, antenna efficiency drops below 50%, owing to optical spreading resistance, exacerbated by the anomalous skin effect (electron surface collisions). Quantum dipole oscillations in the emitter excited state produce an optical ac equivalent circuit current, I(o) = qω|x(o)|/d, feeding the antenna-enhanced spontaneous emission, where q|x(o)| is the dipole matrix element. Despite the quantum-mechanical origin of the drive current, antenna theory makes no reference to the Purcell effect nor to local density of states models. Moreover, plasmonic effects are minor at 200 THz, producing only a small shift of antenna resonance frequency.
      Cover page: Optical antenna enhanced spontaneous emission
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2018)
      Optical antennas can enhance the spontaneous emission rate from nanoemitters by orders of magnitude, enabling the possibility of an ultrafast, efficient, nanoscale LED. Semiconductors would be the preferred material for such a device to allow for direct high-speed modulation. However, efficient nanoscale devices are challenging to implement because of high surface recombination typical of most III-V materials. Monolayer transition metal dichalcogenides are an attractive alternative to a III-V emitter due to their intrinsically nanoscale dimensions, direct bandgap, and near-ideal surfaces resulting in high intrinsic quantum yield. In this work, we couple a nanostrip (30 nm × 250 nm) monolayer of WSe2 to a cavity-backed optical slot antenna through a self-aligned fabrication process. Photoluminescence, scattering, and lifetime measurements are used to estimate a radiative spontaneous emission rate enhancement of 318× from WSe2 monolayers coupled to on-resonance antennas. Such a huge increase in the spontaneous emission rate results in an ultrafast radiative recombination rate and a quantum yield in nanopatterned monolayers comparable to unprocessed exfoliated flakes of WSe2.
        Cover page: Ultrafast Spontaneous Emission from a Slot-Antenna Coupled WSe2 Monolayer
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