Michael Eggleston

Optical antenna enhanced spontaneous emission

Significance Since the invention of the laser over 50 y ago, stimulated emission has been stronger and far more important than spontaneous emission, the ordinary light we are accustomed to. Indeed spontaneous emission has been looked down upon as a weak effect. Now a new science of enhanced spontaneous emission is emerging that makes spontaneous emission faster than stimulated emission. This new science depends upon the use of optical antennas to increase the spontaneous emission rate. Antennas emerged at the dawn of radio for concentrating electromagnetic energy to a small volume. Despite the importance of radio antennas, 100 y went by before optical antennas began to be used to help extract optical frequency radiation from very small sources such as dye molecules and quantum dots. 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/d2. 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, Io = qω|xo|/d, feeding the antenna-enhanced spontaneous emission, where q|xo| 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.

Engineering Light Outcoupling in 2D Materials

When light is incident on 2D transition metal dichalcogenides (TMDCs), it engages in multiple reflections within underlying substrates, producing interferences that lead to enhancement or attenuation of the incoming and outgoing strength of light. Here, we report a simple method to engineer the light outcoupling in semiconducting TMDCs by modulating their dielectric surroundings. We show that by modulating the thicknesses of underlying substrates and capping layers, the interference caused by substrate can significantly enhance the light absorption and emission of WSe2, resulting in a ∼11 times increase in Raman signal and a ∼30 times increase in the photoluminescence (PL) intensity of WSe2. On the basis of the interference model, we also propose a strategy to control the photonic and optoelectronic properties of thin-layer WSe2. This work demonstrates the utilization of outcoupling engineering in 2D materials and offers a new route toward the realization of novel optoelectronic devices, such as 2D LEDs and solar cells.

Mass-producible and efficient optical antennas with CMOS-fabricated nanometer-scale gap.

Optical antennas have been widely used for sensitive photodetection, efficient light emission, high resolution imaging, and biochemical sensing because of their ability to capture and focus light energy beyond the diffraction limit. However, widespread application of optical antennas has been limited due to lack of appropriate methods for uniform and large area fabrication of antennas as well as difficulty in achieving an efficient design with small mode volume (gap spacing < 10nm). Here, we present a novel optical antenna design, arch-dipole antenna, with optimal radiation efficiency and small mode volume, 5 nm gap spacing, fabricated by CMOS-compatible deep-UV spacer lithography. We demonstrate strong surface-enhanced Raman spectroscopy (SERS) signal with an enhancement factor exceeding 108 from the arch-dipole antenna array, which is two orders of magnitude stronger than that from the standard dipole antenna array fabricated by e-beam lithography. Since the antenna gap spacing, the critical dimension of the antenna, can be defined by deep-UV lithography, efficient optical antenna arrays with nanometer-scale gap can be mass-produced using current CMOS technology.

Efficient Coupling of an Antenna-Enhanced nanoLED into an Integrated InP Waveguide.

Increasing power consumption in traditional on-chip metal interconnects has made optical links an attractive alternative. However, such a link is currently missing a fast, efficient, nanoscale light-source. Coupling nanoscale optical emitters to optical antennas has been shown to greatly increase their spontaneous emission rate and efficiency. Such a structure would be an ideal emitter for an on-chip optical link. However, there has never been a demonstration of an antenna-enhanced emitter coupled to a low-loss integrated waveguide. In this Letter we demonstrate an optical antenna-enhanced nanoLED coupled to an integrated InP waveguide. The nanoLEDs are comprised of a nanoridge of InGaAsP coupled to a gold antenna that exhibits a 36× enhanced rate of spontaneous emission. Coupling efficiencies as large as 70% are demonstrated into an integrated waveguide. Directional antennas also demonstrate direction emission down one direction of a waveguide with observed front-to-back ratios as high as 3:1.

Efficient Rate Enhancement of Spontaneous Emission in a Semiconductor nanoLED

Optical antennas drastically increase the spontaneous emission rate of a semiconductor LED, yielding 30x improvements in quantum efficiency. Their nanoscale size, high efficiency, and fast emission rate make them good candidates for ultra-low power communication.

Enhancement of photon emission rate in antenna-coupled nanoLEDs

Using a simple antenna model, we show a semiconductor coupled to an optical antenna can significantly increase spontaneous emission rate. Our experimental measurements shows a 20× increase in photoluminescence, which agrees well with the theory.

Optical antenna design for nanophotodiodes

Guidelines for designing an optical antenna for optimizing the performance of a nanophotodiode are proposed. A nanopatch design is simulated with over 70% absorption efficiency using germanium as the absorber.

Optical antenna based nanoLED

An optical antenna based nanoLED design that enhances photoluminescence of a semiconductor emitter by more than 10× is presented. The small mode (0.015 (λ₀/2n)³) and physical (3×10^-4 λ₀³) volumes are attractive for on-chip optical interconnect applications.

Circuit theory of optical antenna shedding light on fundamental limit of rate enhancement

A circuit model of a single-element linear optical antenna is presented. It agrees well with FDTD simulations and predicts spreading resistance will ultimately limit the maximum rate enhancement an efficient antenna can achieve to ~10,000.

Optical antenna enhanced nanoLEDs for on-chip optical interconnects

Since the invention of the laser, stimulated emission has been the de facto king of optical communication. Lasers can be directly modulated at rates as high as 50GHz, much faster than a typical solid state LED that is limited by spontaneous emission to <;1GHz. Unfortunately, lasers have a severe scaling problem; they require large cavities operated at high power to achieve efficient lasing, making on-chip integration a serious challenge. A properly designed LED, on the other hand, can be made arbitrarily small and still operate with high-efficiency. Recent work has shown that the quantum yield and spontaneous emission rate of nanoemitters can be drastically increased by coupling to an optical antenna. In this talk, I will demonstrate that by utilizing proper antenna design, an optical antenna coupled to a semiconductor nanoLED can be created that is faster than a laser while still operating at >50% efficiency. The use of circuit models for antenna design and recent experimental work coupling semiconductor emitters to optical antennas will be discussed.