Temporal Dynamics of Collective Resonances in Periodic Metasurfaces.

Temporal dynamics of confined optical fields can provide valuable insights into light-matter interactions in complex optical systems, going beyond their frequency-domain description. Here, we present a new experimental approach based on interferometric autocorrelation (IAC) that reveals the dynamics of optical near-fields enhanced by collective resonances in periodic metasurfaces. We focus on probing the resonances known as waveguide-plasmon polaritons, which are supported by plasmonic nanoparticle arrays coupled to a slab waveguide. To probe the resonant near-field enhancement, our IAC measurements make use of enhanced two-photon excited luminescence (TPEL) from semiconductor quantum dots deposited on the nanoparticle arrays. Thanks to the incoherent character of TPEL, the measurements are only sensitive to the fundamental optical fields and therefore can reveal clear signatures of their coherent temporal dynamics. In particular, we show that the excitation of a high-Q collective resonance gives rise to interference fringes at time delays as large as 500 fs, much greater than the incident pulse duration (150 fs). Based on these signatures, the basic characteristics of the resonances can be determined, including their Q factors, which are found to exceed 200. Furthermore, the measurements also reveal temporal beating between two different resonances, providing information on their frequencies and their relative contribution to the field enhancement. Finally, we present an approach to enhance the visibility of the resonances hidden in the IAC curves by converting them into spectrograms, which greatly facilitates the analysis and interpretation of the results. Our findings open up new perspectives on time-resolved studies of collective resonances in metasurfaces and other multiresonant systems.

Direct Patterning of CsPbBr3 Nanocrystals via Electron-Beam Lithography.

Lead-halide perovskite (LHP) nanocrystals have proven themselves as an interesting material platform due to their easy synthesis and compositional versatility, allowing for a tunable band gap, strong absorption, and high photoluminescence quantum yield (PLQY). This tunability and performance make LHP nanocrystals interesting for optoelectronic applications. Patterning active materials like these is a useful way to expand their tunability and applicability as it may allow more intricate designs that can improve efficiencies or increase functionality. Based on a technique for II-VI quantum dots, here we pattern colloidal LHP nanocrystals using electron-beam lithography (EBL). We create patterns of LHP nanocrystals on the order of 100s of nanometers to several microns and use these patterns to form intricate designs. The patterning mechanism is induced by ligand cross-linking, which binds adjacent nanocrystals together. We find that the luminescent properties are somewhat diminished after exposure, but that the structures are nonetheless still emissive. We believe that this is an interesting step toward patterning LHP nanocrystals at the nanoscale for device fabrication.

Integrating Sphere Fourier Microscopy of Highly Directional Emission.

Accurately controlling light emission using nano- and microstructured lenses and antennas is an active field of research. Dielectrics are especially attractive lens materials due to their low optical losses over a broad bandwidth. In this work we measure highly directional light emission from patterned quantum dots (QDs) aligned underneath all-dielectric nanostructured microlenses. The lenses are designed with an evolutionary algorithm and have a theoretical directivity of 160. The fabricated structures demonstrate an experimental full directivity of 61 ± 3, three times higher than what has been estimated before, with a beaming half-angle of 2.6°. This high value compared to previous works is achieved via three mechanisms. First, direct electron beam patterning of QD emitters and alignment markers allowed for more localized emission and better emitter-lens alignment. Second, the lens fabrication was refined to minimize distortions between the designed shape and the final structure. Finally, a new measurement technique was developed that combines integrating sphere microscopy with Fourier microscopy. This enables complete directivity measurements, contrary to other reported values, which are typically only partial directivities or estimates of the full directivity that rely partly on simulations. The experimentally measured values of the complete directivity were higher than predicted by combining simulations with partial directivity measurements. High directivity was obtained from three different materials (cadmium-selenide-based QDs and two lead halide perovskite materials), emitting at 520, 620, and 700 nm, by scaling the lens size according to the emission wavelength.

Observation of Cooperative Purcell Enhancements in Antenna-Cavity Hybrids.

Localizing light to nanoscale volumes through nanoscale resonators that are low loss and precisely tailored in spectrum to properties of matter is crucial for classical and quantum light sources, cavity QED, molecular spectroscopy, and many other applications. To date, two opposite strategies have been identified: to use either plasmonics with deep subwavelength confinement yet high loss and very poor spectral control or instead microcavities with exquisite quality factors yet poor confinement. In this work we realize hybrid plasmonic-photonic resonators that enhance the emission of single quantum dots, profiting from both plasmonic confinement and microcavity quality factors. Our experiments directly demonstrate how cavity and antenna jointly realize large cooperative Purcell enhancements through interferences. These can be controlled to engineer arbitrary Fano lineshapes in the local density of optical states.

Universal direct patterning of colloidal quantum dots by (extreme) ultraviolet and electron beam lithography.

Colloidal quantum dots have found many applications and patterning them on micro- and nanoscale would open a new dimension of tunability for the creation of smaller scale (flexible) electronics or nanophotonic structures. Here we present a simple, general, one-step top-down patterning technique for colloidal quantum dots by means of direct optical or electron beam lithography. We find that both photons and electrons can induce a solubility switch of both PbS and CdSe quantum dot films. The solubility switch can be ascribed to cross-linking of the organic ligands, which we observe from exposure with deep-UV photons (5.5 eV) to extreme-UV photons (91.9 eV), and low-energy (3-70 eV) as well as highly energetic electrons (50 keV). The required doses for patterning are relatively low and feature sizes can be as small as tens of nanometers. The luminescence properties as well as carrier lifetimes remain similar after patterning.

Crystal Orientation and Grain Size: Do They Determine Optoelectronic Properties of MAPbI3 Perovskite?

Growing large, oriented grains of perovskite often leads to efficient devices, but it is unclear if properties of the grains are responsible for the efficiency. Domains observed in SEM are commonly misidentified with crystallographic grains, but SEM images do not provide diffraction information. We study methylammoinium lead iodide (MAPbI3) films fabricated via flash infrared annealing (FIRA) and the conventional antisolvent (AS) method by measuring grain size and orientation using electron back-scattered diffraction (EBSD) and studying how these affect optoelectronic properties such as local photoluminescence (PL), charge carrier lifetimes, and mobilities. We observe a local enhancement and shift of the PL emission at different regions of the FIRA clusters, but we observe no effect of crystal orientation on the optoelectronic properties. Additionally, despite substantial differences in grain size between the two systems, we find similar optoelectronic properties. These findings show that optoelectronic quality is not necessarily related to the orientation and size of crystalline domains.