Controlling Spontaneous Emission from Semiconductor Metasurfaces: From Reconfigurability to Single Photon Sources

The ability to achieve spatiotemporal control of incoherent (thermal and quantum) light emission has been a critical challenge in the field of optics with far ranging applications from remote sensing, holographic displays and quantum information processing. Here we present our results on two light emitting metasurfaces systems: 1) Ultrafast dynamic steering of spontaneous light emission from high-density InAs quantum dots (QDs) embedded inside GaAs resonators under structured optical pumping and 2) Enhancement of single-photon emission from single GaAs local-droplet epitaxial QDs from within AlGaAs Huygens’ metasurfaces.<br/><br/>Spatially varying refractive index profiles are the key to steer incoherent light, since reconfigurable metasurfaces operating as phased-antenna arrays can only steer coherent (laser) light. We discover spatial refractive index profiles beyond human intuition that enable high efficiency (77%) steering of incoherent emission from reconfigurable dielectric metasurfaces over an 80° field of view, using generative models and active learning. We developed a machine learning framework where an active learning agent drives a generative model (variational autoencoder- VAE) to predict a spatially structured optical pump pattern which, when imaged onto the metasurface, generates a highly directional emission of incoherent light. This was achieved by creating a dynamic spatially varying resonant phase gradients across the metasurface using an ultrafast (80fs, 1 Khz rep rate, 3mJ/cm²) 800nm pump which images the pattern from a spatial light modulator (SLM) onto the metasurface. The photoluminescence (beam) steering is initially demonstrated using a periodic blaze (saw-tooth) grating patterns, which created a periodic refractive index profile using Drude free-carrier refraction, resulting in a spatial phase gradient on the metasurface and ultimately directing the momentum of the light emission. We find that AI-generated pump patterns are 10x more efficient in beam steering compared to the initial periodic blaze grating pump pattern designed using human intuition. Our work provides a novel machine learning driven closed-loop platform to understand and discover the physics of incoherent emission steering from metasurfaces with potential applications for augmented and virtual reality displays.<br/><br/>Epitaxially grown III-V QDs are promising sources for single photons with some of the highest metrics in purity, emission efficiency, brightness and indistinguishability reported to date. Here, we explore Al₃₀Ga₇₀As Huygens’ resonators with spectrally overlapping the electric and magnetic dipole modes at the emission wavelength of a GaAs local droplet etching quantum dot for enhancing the emission of single photons. We enhance the emission rate of a single QD by an order of magnitude in comparison to an unstructured layer. We demonstrate anti-bunched single photon emission (g²(0) &lt; 0.2) under continuous wave (515nm) pumping with less than 45nW of pump power. Additionally, we report a 5x life-time reduction of the GaAs QD emission under increased pump fluence from 45 to 400nW. Finally, we demonstrate that the enhancement of the QD emission is independent of the position of the QD within the metasurface which alleviates a major challenge of placing these QDs with resonant nano-cavities. For semiconductor metasurfaces, the control of mode properties can open incredible opportunities in quantum information sciences, for manipulation of single photon qubits, and for enabling photon entanglement schemes and architectures.