Kevin Messer

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.

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.

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.

Electrically injected nanoLED with large spontaneous emission enhancement from an optical antenna

We experimentally demonstrate 200x spontaneous emission rate enhancement from an electrically-injected nanoLED coupled to a cavity-backed optical slot antenna. Such a nanoLED device could be used as a fast, efficient, and nanoscale light source for on-chip optical interconnects.

Optical slot antennas for enhancement of WSe 2 spontaneous emission rate

The spontaneous emission rate of light emitters has been shown to have strong dependence on their local electromagnetic environment1. Optical antennas exploit this effect and can be used to greatly increase the spontaneous emission rate of a coupled light emitter. There have been several demonstrations of this effect with promising results using dye molecules and Er3+ ions2,3. It is predicted that spontaneous emission rate enhancements greater than 1000x can be achieved with optical antennas while maintaining greater than 50% optical efficiency4. Demonstration of large spontaneous emission enhancement of semiconductor light emitters could lead to low power, high efficiency, fast light sources useful for short-range optical communications. Transition metal dichalcogenides, such as WSe2, are promising candidates for the light emitter of such a nanoLED device because they are semiconductors that maintain good quantum efficiency even with a nanoscale dimension. In this work we demonstrate an optical slot antenna coupled to a monolayer of WSe2. Photoluminescence measurements show an increase of total light emission >700x when compared to WSe2 that is not coupled to an antenna. We estimate a spontaneous emission rate enhancement of 318x is responsible for this huge increase in light emission.

Optical antenna-enhanced nano-LED for energy efficient optical interconnect

Interconnects accounts for a significant portion of energy consumption in integrated circuits. Optical interconnects, now widely used to link electronic systems such as servers and top of rack switches in data centers, can potentially reduce the energy consumption of electrical interconnects. However, current state-of-the-art optical links consumes about 100s fJ/b to 1 pJ/b, still much too high for on-chip communications [1]. Orders of magnitude improvement in energy efficiency can be achieved by combining (1) ultra-low capacitance optical receivers and (2) optical antenna-enhanced nanoscale light-emitting diodes (LED). By reducing the receiver capacitance to ~ 100 aF [2] and preferably integrating the detector with the first gain stage forming a phototransistor [3][4], the energy consumption of the photoreceiver can be reduced to ~ 100 aJ/b even with 100 photons/bit sensitivity. However, traditional laser source consumes too much power due to the need to bias the laser, usually at several times the threshold current. Light emitting diodes (LEDs), on the other hand, can operate efficiently without threshold. Unfortunately, their modulation speeds are limited by the relatively slow spontaneous emission. Recently, progress has been made using optical antennas to increase the rate of spontaneous emission, opening up the possibility of an efficient, high speed, nanoscale emitter. We have observed 35x enhancement of spontaneous emission rate in optically pumped InGaAsP nano-LEDs with arch-dipole antennas [5]. Recently, using cavity-backed optical slot antennas, electrically injected nano-LEDs with 200x enhancement of spontaneous emission rate have been demonstrated [6]. Even higher enhancement has been observed in nano-LEDs with monolayer two-dimensional semiconductor such as transition metal dichalcogenide, WSe2 [7]. In this talk, we will review the principle and the recent progress in optical antenna-enhanced nano-LEDs.

Waveguide-integrated optical antenna nanoLEDs for on-chip communication

We present on an optical antenna based nanoLED that is fabricated directly on top of an InP waveguide. Waveguide coupling efficiency of 70% and directional emission is achieved with a Yagi-Uda antenna structure. By using an epitaxial lift-off process, we show that this device could be integrated directly onto a Silicon-photonics substrate.