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 = qωjx o j/d, feeding the antenna-enhanced spontaneous emission, where qjx o j 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.A ntennas emerged at the dawn of radio, concentrating electromagnetic energy within a small volume <<λ 3 , enabling nonlinear radio detection. Such coherent detection is essential for radio receivers and has been used since the time of Hertz (1). Conversely, an antenna can efficiently extract radiation from a subwavelength source, such as a small cellphone. 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 (2-10) and quantum dots (11)(12)(13)(14).In optics, spontaneous emission is caused by dipole oscillations in the excited state of atoms, molecules, or quantum dots. The main problem is that a molecule is far too small to act as an efficient antenna for its own electromagnetic radiation. Antenna length, l, makes a huge difference in radiation rate. An ideal antenna would preferably be λ/2, a half-wavelength in size. To the degree that an atomic dipole of length l is smaller than λ/2, the antenna radiation rate Δω is proportional to ω(l/λ) 3 , as given by the Wheeler limit (15). Spontaneous emission from molecular-sized radiators is thus slowed by many orders of magnitude, because radiation wavelengths are much larger than the atoms themselves. Therefore, the key to speeding up spontaneous emission is to couple the radiating molecule to a proper antenna of sufficient size.Since the emergence of lasers in 1960, stimulated emission has been faster than spontaneous emission. Now the opposite is possible. In the right circumstances, antenna-enhanced spontaneous emission could become faster than stimulated emission. Theoretically, very large bandwidth >100 GHz or >1 THz is possible when the light emitter is ...

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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 nearideal surfaces resulting in high intrinsic quantum yield. In this work, we couple a nanostrip (30 nm × 250 nm) monolayer of WSe 2 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 WSe 2 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 WSe 2 .

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