Control of Quantized Spontaneous Emission from Single GaAs Quantum Dots Embedded in Huygens' Metasurfaces.

Advancements in photonic quantum information systems (QIS) have driven the development of high-brightness, on-demand, and indistinguishable semiconductor epitaxial quantum dots (QDs) as single photon sources. Strain-free, monodisperse, and spatially sparse local-droplet-etched (LDE) QDs have recently been demonstrated as a superior alternative to traditional Stranski-Krastanov QDs. However, integration of LDE QDs into nanophotonic architectures with the ability to scale to many interacting QDs is yet to be demonstrated. We present a potential solution by embedding isolated LDE GaAs QDs within an Al0.4Ga0.6As Huygens' metasurface with spectrally overlapping fundamental electric and magnetic dipolar resonances. We demonstrate for the first time a position- and size-independent, 1 order of magnitude increase in the collection efficiency and emission lifetime control for single-photon emission from LDE QDs embedded within the Huygens' metasurfaces. Our results represent a significant step toward leveraging the advantages of LDE QDs within nanophotonic architectures to meet the scalability demands of photonic QIS.

Orthogonal luminescence lifetime encoding by intermetallic energy transfer in heterometallic rare-earth MOFs.

Lifetime-encoded materials are particularly attractive as optical tags, however examples are rare and hindered in practical application by complex interrogation methods. Here, we demonstrate a design strategy towards multiplexed, lifetime-encoded tags via engineering intermetallic energy transfer in a family of heterometallic rare-earth metal-organic frameworks (MOFs). The MOFs are derived from a combination of a high-energy donor (Eu), a low-energy acceptor (Yb) and an optically inactive ion (Gd) with the 1,2,4,5 tetrakis(4-carboxyphenyl) benzene (TCPB) organic linker. Precise manipulation of the luminescence decay dynamics over a wide microsecond regime is achieved via control over metal distribution in these systems. Demonstration of this platform's relevance as a tag is attained via a dynamic double encoding method that uses the braille alphabet, and by incorporation into photocurable inks patterned on glass and interrogated via digital high-speed imaging. This study reveals true orthogonality in encoding using independently variable lifetime and composition, and highlights the utility of this design strategy, combining facile synthesis and interrogation with complex optical properties.

Fabrication of a Microcavity Prepared by Remote Epitaxy over Monolayer Molybdenum Disulfide.

Advances in epitaxy have enabled the preparation of high-quality material architectures consisting of incommensurate components. Remote epitaxy based on lattice transparency of atomically thin graphene has been intensively studied for cost-effective advanced device manufacturing and heterostructure formation. However, remote epitaxy on nongraphene two-dimensional (2D) materials has rarely been studied even though it has a broad and immediate impact on various disciplines, such as many-body physics and the design of advanced devices. Herein, we report remote epitaxy of ZnO on monolayer MoS2 and the realization of a whispering-gallery-mode (WGM) cavity composed of a single crystalline ZnO nanorod and monolayer MoS2. Cross-sectional transmission electron microscopy and first-principles calculations revealed that the nongraphene 2D material interacted with overgrown and substrate layers and also exhibited lattice transparency. The WGM cavity embedding monolayer MoS2 showed enhanced luminescence of MoS2 and multimodal emission.

Covert MOF-Based Photoluminescent Tags via Tunable Linker Energetics.

Optical anticounterfeiting tags utilize the photoluminescent properties of materials to encode unique patterns, enabling identification and validation of important items and assets. These tags must combine optical complexity with ease of production and authentication to both prevent counterfeiting and to remain practical for widespread use. Metal-organic frameworks (MOFs) based on polynuclear, rare earth clusters are ideal materials platforms for this purpose, combining fine control over structure and composition, with tunable, complex energy transfer mechanisms via both linker and metal components. Here we report the design and synthesis of a set of heterometallic MOFs based on combinations of Eu, Nd, and Yb with the tetratopic linker 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene. The energetics of this linker facilitate the intentional concealment of the visible emissions from Eu while retaining the infrared emissions of Nd and Yb, creating an optical tag with multiple covert elements. Unique to the materials system reported herein, we document the occurrence of a previously not observed 11-metal cluster correlated with the presence of Yb in the MOFs, coexisting with a commonly encountered 9-metal cluster. We demonstrate the utility of these materials as intricate optical tags with both rapid and in-depth screening techniques, utilizing orthogonal identifiers across composition, emission spectra, and emission decay dynamics. This work highlights the important effect of linker selection in controlling the resulting photoluminescent properties in MOFs and opens an avenue for the targeted design of highly complex, multifunctional optical tags.

Manipulation of Exciton Dynamics in Single-Layer WSe2 Using a Toroidal Dielectric Metasurface.

Recent advances in emerging atomically thin transition metal dichalcogenide semiconductors with strong light-matter interactions and tunable optical properties provide novel approaches for realizing new material functionalities. Coupling two-dimensional semiconductors with all-dielectric resonant nanostructures represents an especially attractive opportunity for manipulating optical properties in both the near-field and far-field regimes. Here, by integrating single-layer WSe2 and titanium oxide (TiO2) dielectric metasurfaces with toroidal resonances, we realized robust exciton emission enhancement over 1 order of magnitude at both room and low temperatures. Furthermore, we could control exciton dynamics and annihilation by using temperature to tailor the spectral overlap of excitonic and toroidal resonances, allowing us to selectively enhance the Purcell effect. Our results provide rich physical insight into the strong light-matter interactions in single-layer TMDs coupled with toroidal dielectric metasurfaces, with important implications for optoelectronics and photonics applications.

Recyclable Graphene Sheets as a Growth Template for Crystalline ZnO Nanowires.

Recent advances in nanoscience have opened ways of recycling substrates for nanomaterial growth. Novel materials, such as atomically thin materials, are highly desirable for the recycling substrates. In this work, we report recycling of monolayer graphene as a growth template for synthesis of single crystalline ZnO nanowires. Selective nucleation of ZnO nanowires on graphene was elucidated by scanning electron microscopy and density functional theory calculation. Growth and subsequent separation of ZnO nanowires was repeated up to seven times on the same monolayer graphene film. Raman analyses were also performed to investigate the quality of graphene structure along the recycling processes. The chemical robustness of graphene enables the repetitive ZnO nanowire growth without noticeable degradation of the graphene quality. This work presents a route for graphene as a multifunctional growth template for diverse nanomaterials such as nanocrystals, aligned nanowires, other two-dimensional materials, and semiconductor thin films.

Encoding Multilayer Complexity in Anti-Counterfeiting Heterometallic MOF-Based Optical Tags.

Optical tags provide a way to quickly and unambiguously identify valuable assets. Current tag fluorophore options lack the tunability to allow combined methods of encoding in a single material. Herein we report a design strategy to encode multilayer complexity in a family of heterometallic rare-earth metal-organic frameworks based on highly connected nonanuclear clusters. To impart both intricacy and security, a synergistic approach was implemented resulting in both overt (visible) and covert (near-infrared, NIR) properties, with concomitant multi-emissive spectra and tunable luminescence lifetimes. Tag authentication is validated with a variety of orthogonal detection methodologies. Importantly, the effect induced by subtle compositional changes on intermetallic energy transfer, and thus on the resulting photophysical properties, is demonstrated. This strategy can be widely implemented to create a large library of highly complex, difficult-to-counterfeit optical tags.

Highly Plasmonic Titanium Nitride by Room-Temperature Sputtering.

Titanium nitride (TiN) has recently emerged as an attractive alternative material for plasmonics. However, the typical high-temperature deposition of plasmonic TiN using either sputtering or atomic layer deposition has greatly limited its potential applications and prevented its integration into existing CMOS device architectures. Here, we demonstrate highly plasmonic TiN thin films and nanostructures by a room-temperature, low-power, and bias-free reactive sputtering process. We investigate the optical properties of the TiN films and their dependence on the sputtering conditions and substrate materials. We find that our TiN possesses one of the largest negative values of the real part of the dielectric function as compared to all other plasmonic TiN films reported to date. Two-dimensional periodic arrays of TiN nanodisks are then fabricated, from which we validate that strong plasmonic resonances are supported. Our room-temperature deposition process can allow for fabricating complex plasmonic TiN nanostructures and be integrated into the fabrication of existing CMOS-based photonic devices to enhance their performance and functionalities.

Polarization-pinned emission of a continuous-wave optically pumped nonpolar GaN-based VCSEL using nanoporous distributed Bragg reflectors.

A nonpolar GaN-based vertical-cavity surface-emitting laser (VCSEL) using nanoporous bottom epitaxial distributed Bragg reflector (DBR) is demonstrated at room temperature (RT) under continuous-wave (CW) optical pumping. The porous layers enable the epitaxial growth of lattice-matched high-reflectance DBRs without sacrificing the conductive properties needed for high-performance VCSELs. The 2-λ cavity VCSEL reported here employs a hybrid design with top dielectric DBR and bottom nanoporous DBR. Single longitudinal mode lasing is observed at 462 nm with a threshold power density of ~5 kW/cm2 and a FWHM of ~0.12 nm. The emission polarization was pinned in the a-direction at all measured locations.

High-Temperature Refractory Metasurfaces for Solar Thermophotovoltaic Energy Harvesting.

Solar energy promises a viable solution to meet the ever-increasing power demand by providing a clean, renewable energy alternative to fossil fuels. For solar thermophotovoltaics (STPV), high-temperature absorbers and emitters with strong spectral selectivity are imperative to efficiently couple solar radiation into photovoltaic cells. Here, we demonstrate refractory metasurfaces for STPV with tailored absorptance and emittance characterized by in situ high-temperature measurements, featuring thermal stability up to at least 1200 °C. Our tungsten-based metasurface absorbers have close-to-unity absorption from visible to near-infrared and strongly suppressed emission at longer wavelengths, while our metasurface emitters provide wavelength-selective emission spectrally matched to the band-edge of InGaAsSb photovoltaic cells. The projected overall STPV efficiency is as high as 18% when a fully integrated absorber/emitter metasurface structure is employed, which is comparable to the efficiencies of the best currently available commercial single-junction PV cells and can be further improved to potentially exceed those in mainstream photovoltaic technologies. Our work opens a path forward for high-performance STPV systems based on refractory metasurface structures.

Semiconductor Hyperbolic Metamaterials at the Quantum Limit.

We study semiconductor hyperbolic metamaterials (SHMs) at the quantum limit experimentally using spectroscopic ellipsometry as well as theoretically using a new microscopic theory. The theory is a combination of microscopic density matrix approach for the material response and Green's function approach for the propagating electric field. Our approach predicts absorptivity of the full multilayer system and for the first time allows the prediction of in-plane and out-of-plane dielectric functions for every individual layer constructing the SHM as well as effective dielectric functions that can be used to describe a homogenized SHM.

Compact epsilon-near-zero silicon photonic phase modulators.

In this paper, we analyze a compact silicon photonic phase modulator at 1.55 µm using epsilon-near-zero transparent conducting oxide (TCO) films. The operating principle of the non-resonant phase modulator is field-effect carrier density modulation in a thin TCO film deposited on top of a passive silicon waveguide with a CMOS-compatible fabrication process. We compare phase modulator performance using both indium oxide (In2O3) and cadmium oxide (CdO) TCO materials. Our findings show that practical phase modulation can be achieved only when using high-mobility (i.e. low-loss) epsilon-near-zero materials such as CdO. The CdO-based phase modulator has a figure of merit of 17.1°/dB in a compact 5 µm length. This figure of merit can be increased further through the proper selection of high-mobility TCOs, opening a path for device miniaturization and increased phase shifts.

Viscoelastic optical nonlocality of low-loss epsilon-near-zero nanofilms.

Optical nonlocalities are elusive and hardly observable in traditional plasmonic materials like noble and alkali metals. Here we report experimental observation of viscoelastic nonlocalities in the infrared optical response of epsilon-near-zero nanofilms made of low-loss doped cadmium-oxide. The nonlocality is detectable thanks to the low damping rate of conduction electrons and the virtual absence of interband transitions at infrared wavelengths. We describe the motion of conduction electrons using a hydrodynamic model for a viscoelastic fluid, and find excellent agreement with experimental results. The electrons' elasticity blue-shifts the infrared plasmonic resonance associated with the main epsilon-near-zero mode, and triggers the onset of higher-order resonances due to the excitation of electron-pressure modes above the bulk plasma frequency. We also provide evidence of the existence of nonlocal damping, i.e., viscosity, in the motion of optically-excited conduction electrons using a combination of spectroscopic ellipsometry data and predictions based on the viscoelastic hydrodynamic model.

Spiraling Light with Magnetic Metamaterial Quarter-Wave Turbines.

Miniaturized quarter-wave plate devices empower spin to orbital angular momentum conversion and vector polarization formation, which serve as bridges connecting conventional optical beam and structured light. Enabling the manipulability of additional dimensions as the complex polarization and phase of light, quarter-wave plate devices are essential for exploring a plethora of applications based on orbital angular momentum or vector polarization, such as optical sensing, holography, and communication. Here we propose and demonstrate the magnetic metamaterial quarter-wave turbines at visible wavelength to produce radially and azimuthally polarized vector vortices from circularly polarized incident beam. The magnetic metamaterials function excellently as quarter-wave plates at single wavelength and maintain the quarter-wave phase retardation in broadband, while the turbine blades consist of multiple polar sections, each of which contains homogeneously oriented magnetic metamaterial gratings near azimuthal or radial directions to effectively convert circular polarization to linear polarization and induce phase shift under Pancharatnum-Berry's phase principle. The perspective concept of multiple polar sections of magnetic metamaterials can extend to other analogous designs in the strongly coupled nanostructures to accomplish many types of light phase-polarization manipulation and structured light conversion in the desired manner.

Multifunctional, Tunable Metal-Organic Framework Materials Platform for Bioimaging Applications.

Herein, we describe a novel multifunctional metal-organic framework (MOF) materials platform that displays both porosity and tunable emission properties as a function of the metal identity (Eu, Nd, and tuned compositions of Nd/Yb). Their emission collectively spans the deep red to near-infrared (NIR) spectral region (∼614-1350 nm), which is highly relevant for in vivo bioimaging. These new materials meet important prerequisites as relevant to biological processes: they are minimally toxic to living cells and retain structural integrity in water and phosphate-buffered saline. To assess their viability as optical bioimaging agents, we successfully synthesized the nanoscale Eu analog as a proof-of-concept system in this series. In vitro studies show that it is cell-permeable in individual RAW 264.7 mouse macrophage and HeLa human cervical cancer tissue culture cells. The efficient discrimination between the Eu emission and cell autofluorescence was achieved with hyperspectral confocal fluorescence microscopy, used here for the first time to characterize MOF materials. Importantly, this is the first report that documents the long-term conservation of the intrinsic emission in live cells of a fluorophore-based MOF to date (up to 48 h). This finding, in conjunction with the materials' very low toxicity, validates the biocompatibility in these systems and qualifies them as promising for use in long-term tracking and biodistribution studies.

Solitary Oxygen Dopant Emission from Carbon Nanotubes Modified by Dielectric Metasurfaces.

All-dielectric metasurfaces made from arrays of high index nanoresonators supporting strong magnetic dipole modes have emerged as a low-loss alternative to plasmonic metasurfaces. Here we use oxygen-doped single-walled carbon nanotubes (SWCNTs) as quantum emitters and couple them to silicon metasurfaces to study effects of the magnetic dipole modes of the constituent nanoresonators on the photoluminescence (PL) of individual SWCNTs. We find that when in resonance, the magnetic mode of the silicon nanoresonators can lead to a moderate average PL enhancement of 0.8-4.0 of the SWCNTs, accompanied by an average increase in the radiative decay rate by a factor of 1.5-3.0. More interestingly, single dopant polarization experiments show an anomalous photoluminescence polarization rotation by coupling individual SWCNTs to silicon nanoresonators. Numerical simulations indicate that this is caused by modification of near-field polarization distribution at certain areas in the proximity of the silicon nanoresonators at the excitation wavelength, thus presenting an approach to control emission polarization. These findings indicate silicon nanoresonators as potential building blocks of quantum photonic circuits capable of manipulating PL intensity and polarization of single photon sources.

Directional and monochromatic thermal emitter from epsilon-near-zero conditions in semiconductor hyperbolic metamaterials.

The development of novel thermal sources that control the emission spectrum and the angular emission pattern is of fundamental importance. In this paper, we investigate the thermal emission properties of semiconductor hyperbolic metamaterials (SHMs). Our structure does not require the use of any periodic corrugation to provide monochromatic and directional emission properties. We show that these properties arise because of epsilon-near-zero conditions in SHMs. The thermal emission is dominated by the epsilon-near-zero effect in the doped quantum wells composing the SHM. Furthermore, different properties are observed for s and p polarizations, following the characteristics of the strong anisotropy of hyperbolic metamaterials.

Experimental verification of epsilon-near-zero plasmon polariton modes in degenerately doped semiconductor nanolayers.

We investigate optical polariton modes supported by subwavelength-thick degenerately doped semiconductor nanolayers (e.g. indium tin oxide) on glass in the epsilon-near-zero (ENZ) regime. The dispersions of the radiative (R, on the left of the light line) and non-radiative (NR, on the right of the light line) ENZ polariton modes are experimentally measured and theoretically analyzed through the transfer matrix method and the complex-frequency/real-wavenumber analysis, which are in remarkable agreement. We observe directional near-perfect absorption using the Kretschmann geometry for incidence conditions close to the NR-ENZ polariton mode dispersion. Along with field enhancement, this provides us with an unexplored pathway to enhance nonlinear optical processes and to open up directions for ultrafast, tunable thermal emission.

Metasurface Broadband Solar Absorber.

We demonstrate a broadband, polarization independent, wide-angle absorber based on a metallic metasurface architecture, which accomplishes greater than 90% absorptance in the visible and near-infrared range of the solar spectrum, and exhibits low absorptivity (emissivity) at mid- and far-infrared wavelengths. The complex unit cell of the metasurface solar absorber consists of eight pairs of gold nano-resonators that are separated from a gold ground plane by a thin silicon dioxide spacer. Our experimental measurements reveal high-performance absorption over a wide range of incidence angles for both s- and p-polarizations. We also investigate numerically the frequency-dependent field and current distributions to elucidate how the absorption occurs within the metasurface structure.

Intrinsic polarization control in rectangular GaN nanowire lasers.

We demonstrate intrinsic, linearly polarized lasing from single GaN nanowires using cross-sectional shape control. A two-step top-down fabrication approach was employed to create straight nanowires with controllable rectangular cross-sections. A clear lasing threshold of 444 kW cm(-2) and a narrow spectral line width of 0.16 nm were observed under optical pumping at room temperature, indicating the onset of lasing. The polarization was along the short dimension (y-direction) of the nanowire due to the higher transverse confinement factors for y-polarized transverse modes resulting from the rectangular nanowire cross-section. The results show that cross-sectioned shape control can enable inherent control over the polarization of nanowire lasers without additional environment requirements, such as placement onto lossy substrates.