Light-emitting sources and devices permeate every aspect of our lives and are used in lighting, communications, transportation, computing, and medicine. Advances in multifunctional and “smart lighting” would require revolutionary concepts in the control of emission spectra and directionality. Such control might be possible with new schemes and regimes of light–matter interaction paired with developments in light-emitting materials. Here we show that all-dielectric metasurfaces made from III–V semiconductors with embedded emitters have the potential to provide revolutionary lighting concepts and devices, with new functionality that goes far beyond what is available in existing technologies. Specifically, we use Mie-resonant metasurfaces made from semiconductor heterostructures containing epitaxial quantum dots. By controlling the symmetry of the resonant modes, their overlap with the emission spectra, and other structural parameters, we can enhance the brightness by 2 orders of magnitude, as well as reduce its far-field divergence significantly.

Quantum state engineering, the cornerstone of quantum photonic technologies, mainly relies on spontaneous parametric downconversion and four-wave mixing, where one or two pump photons spontaneously decay into a photon pair. Both of these nonlinear effects require momentum conservation for the participating photons, which strongly limits the versatility of the resulting quantum states. Nonlinear metasurfaces have subwavelength thickness and allow the relaxation of this constraint; when combined with resonances, they greatly expand the possibilities of quantum state engineering. Here, we generated entangled photons via spontaneous parametric downconversion in semiconductor metasurfaces with high–quality factor, quasi-bound state in the continuum resonances. By enhancing the quantum vacuum field, our metasurfaces boost the emission of nondegenerate entangled photons within multiple narrow resonance bands and over a wide spectral range. A single resonance or several resonances in the same sample, pumped at multiple wavelengths, can generate multifrequency quantum states, including cluster states. These features reveal metasurfaces as versatile sources of complex states for quantum information.

We report on the room temperature thermal conductivity of AlAs-GaAs superlattices (SLs), in which we systematically vary the period thickness and total thickness between 2 − 24 nm and 20.1 − 2,160 nm, respectively. The thermal conductivity increases with the SL thickness and plateaus at a thickness around 200 nm, showing a clear transition from a quasi-ballistic to a diffusive phonon transport regime. These results demonstrate the existence of classical size effects in SLs, even at the highest interface density samples. We use harmonic Atomistic Green's function calculations to capture incoherence in phonon transport by averaging the calculated transmission over several purely coherent simulations of independent SL with different random mixing at the AlAs-GaAs interfaces. These simulations demonstrate the significant contribution of incoherent phonon transport through the decrease in the transmission and conductance in the SLs as the number of interfaces increases. In spite of this conductance decrease, our simulations show a quasilinear increase in thermal conductivity with the superlattice thickness. This suggests that the observation of a quasilinear increase in thermal conductivity can have important contributions from incoherent phonon transport. Furthermore, this seemingly linear slope in thermal conductivity vs. SL thickness data may actually be non-linear when extended to a larger number of periods, which is a signature of incoherent effects. Indeed, this trend for superlattices with interatomic mixing at the interfaces could easily be interpreted as linear when the number of periods is small. Our results reveal that the change in thermal conductivity with period thickness is dominated by incoherent (particlelike) phonons, whose properties are not dictated by changes in the AlAs or GaAs phonon dispersion relations. This work demonstrates the importance of studying both period and sample thickness dependencies of thermal conductivity to understand the relative contributions of coherent and incoherent phonon transport in the thermal conductivity in SLs.

Perfect absorption of light by an optically thin metasurface is among several remarkable optical functionalities enabled by nanophotonics. This functionality can be introduced into optoelectronic devices by structuring an active semiconductor-based element as a perfectly absorbing all-dielectric metasurface, leading to improved optical properties while simultaneously providing electrical conductivity. However, a delicate combination of geometrical and material parameters is required for perfect absorption, and currently, no general all-dielectric metasurface design fulfills these conditions for a desired semiconductor and operation wavelength. Here, using numerical simulations, we demonstrate that Mie resonators with subwavelength-size interconnecting channels allow this combination of perfect absorption requirements to be satisfied for different wavelengths of operation and different levels of intrinsic material absorption. We reveal the underlying physics and show that interconnecting channels play a critical role in achieving perfect absorption through their effects on the resonant wavelengths and losses for the electric dipole and magnetic dipole modes in Mie resonators. By adjusting only the channel widths, perfect absorption can be achieved for an optically thin GaAs-based metasurface at a desired wavelength of operation in a range from 715 nm to 840 nm, where the intrinsic absorption level in GaAs varies by more than a factor of 2. Optical transmission experiments confirm that these metasurfaces resonantly enhance optical absorption. This work lays out the foundation and guidelines for replacing bulk semiconductors with electrically connected, optically thin, perfectly absorbing metasurfaces in optical detectors.

Ultrafast optical excitation of select materials gives rise to the generation of broadband terahertz (THz) pulses. This effect has enabled the field of THz time-domain spectroscopy and led to the discovery of many physical mechanisms behind THz generation. However, only a few materials possess the required properties to generate THz radiation efficiently. Optical metasurfaces can relax stringent material requirements by shifting the focus onto the engineering of local electromagnetic fields to boost THz generation. Here we demonstrate the generation of THz pulses in a 160 nm thick nanostructured GaAs metasurface. Despite the drastically reduced volume, the metasurface emits THz radiation with efficiency comparable to that of a thick GaAs crystal. We reveal that along with classical second-order volume nonlinearity, an additional mechanism contributes strongly to THz generation in the metasurface, which we attribute to surface nonlinearity. Our results lay the foundation for engineering of semiconductor metasurfaces for efficient and versatile THz radiation emitters.

Despite their wide use in terahertz (THz) research and technology, the application spectra of photoconductive antenna (PCA) THz detectors are severely limited due to the relatively high optical gating power requirement. This originates from poor conversion efficiency of optical gate beam photons to photocurrent in materials with sub-picosecond carrier lifetimes. Here we show that using an ultra-thin (160 nm), perfectly absorbing low-temperature grown GaAs metasurface as the photoconductive channel drastically improves the efficiency of THz PCA detectors. This is achieved through perfect absorption of the gate beam in a significantly reduced photoconductive volume, enabled by the metasurface. This Letter demonstrates that sensitive THz PCA detection is possible using optical gate powers as low as 5 µW—three orders of magnitude lower than gating powers used for conventional PCA detectors. We show that significantly higher optical gate powers are not necessary for optimal operation, as they do not improve the sensitivity to the THz field. This class of efficient PCA THz detectors opens doors for THz applications with low gate power requirements.

Understanding the enhancement of charge carrier generation and their diffusion is imperative for improving the efficiency of optoelectronic devices particularly infrared photodetectors that are less developed than their visible counterpart. Here, using gold nanorods as model plasmonic systems, InAs quantum dots (QDs) embedded in an InGaAs quantum well as an emitter, and GaAs as an active mediator of surface plasmons for enhancing carrier generation and photon emission, the distance dependence of energy transfer and carrier diffusion have been investigated both experimentally and theoretically. Analysis of the QD emission enhancement as a function of distance reveals a Förster radius of 3.85 ± 0.15 nm, a near-field decay length of 4.8 ± 0.1 nm and an effective carrier diffusion length of 64.0 ± 3.0 nm. Theoretical study of the temporal-evolution of the electron-hole occupation number of the excited states of the QDs indicates that the emission enhancement trend is determined by the carrier diffusion and capture rates.Excitons and localized surface plasmons are the two fundamental excitation characteristics of nanoscale materials. The coupling between excitonic and plasmonic materials promises control of photon emission 1-3 and creation of new metamaterial properties 4, 5 that do not exist in nature. Fundamental understanding of exciton-plasmon interaction can lead to development of efficient photovoltaics [6][7][8] , photodetectors 9,10 , photocatalysis 11,12 and other optoelectronic devices. Classic experiments on exciton-plasmon interactions have often used optically transparent spacer materials between the plasmonic metal and excitonic semiconductor materials 1,3,13,14 . Coupling through optically transparent spacers does not allow studying charge transport process. On the other hand, studies on plasmon enhanced near-infrared photo-detectors are focused on coupling metallic two-dimensional-hole-arrays with layered semiconductor materials such as InAs/InGaAs/GaAs dot-in-a-well (DWELL) structures 9,15 . This enhancement mechanism exploits the extraordinary optical transmission effect 16 , where the transmitted field extends to about 1 μm length covering the whole active region 15 , and does not allow fundamental understanding of localized exciton generation, charge carrier diffusion and recombination.In this work, energy transfer and charge carrier diffusion are investigated systematically taking advantage of the tight electric field localization at the interfaces of plasmonic gold nanorods (AuNRs) and semiconductor GaAs that is grown over the InAs/InGaAs DWELL with accurate control of the GaAs thickness. When excitation energy that is above the GaAs band gap is chosen, the localized electric field enhances generation of electron-hole pairs (excitons) in a defined spatial region away from the InAs QDs so that carrier diffusion and capture rates are studied by monitoring the emission intensity of the QDs. The fact that the GaAs thickness can be controlled with sub-nanometer accuracy allows us to study the distanc...

A microscopic many-body theory driven design and optimization supports the experimental demonstration of sub-100 fs pulse duration directly from a semiconductor laser. A passively modelocked vertical external cavity surface emitting laser producing a pulse duration of 95 fs at a central wavelength of 1025 nm is demonstrated. The semiconductor gain and absorber structures used in the experiment are numerically optimized by modelling the pulse formation dynamic of the system. The resulting structure design is described in detail and the physical limitations in terms of pulse duration and power are discussed. Using a ring cavity geometry, a stable colliding pulse modelocking regime with an output power of 90 mW per beam at a repetition rate of 2.2 GHz is demonstrated. The output pulses are thoroughly characterized and are in good agreement with our predictive model.