Scalable and robust beam shaping using apodized fish-bone grating couplers.

Efficient power coupling between on-chip guided and free-space optical modes requires precision spatial mode matching with apodized grating couplers. Yet, grating apodizations are often limited by the minimum feature size of the fabrication approach. This is especially challenging when small feature sizes are required to fabricate gratings at short wavelengths or to achieve weakly scattered light for large-area gratings.

Surface-Normal Free-Space Beam Projection via Slow-Light Standing-Wave Resonance Photonic Gratings.

On-chip grating couplers directly connect photonic circuits to free-space light. The commonly used photonic gratings have been specialized for small areas, specific intensity profiles, and nonvertical beam projection. This falls short of the precise and flexible wavefront control over large beam areas needed to empower emerging integrated miniaturized optical systems that leverage volumetric light-matter interactions, including trapping, cooling, and interrogation of atoms, bio- and chemi- sensing, and complex free-space interconnect.

Integrating planar photonics for multi-beam generation and atomic clock packaging on chip.

The commercialization of atomic technologies requires replacing laboratory-scale laser setups with compact and manufacturable optical platforms. Complex arrangements of free-space beams can be generated on chip through a combination of integrated photonics and metasurface optics. In this work, we combine these two technologies using flip-chip bonding and demonstrate an integrated optical architecture for realizing a compact strontium atomic clock.

Exceptional points in lossy media lead to deep polynomial wave penetration with spatially uniform power loss.

Waves entering a spatially uniform lossy medium typically undergo exponential intensity decay, arising from either the energy loss of the Beer-Lambert-Bouguer transmission law or the evanescent penetration during reflection. Recently, exceptional point singularities in non-Hermitian systems have been linked to unconventional wave propagation. Here, we theoretically propose and experimentally demonstrate exponential decay free wave propagation in a purely lossy medium.

Meta-grating outcouplers for optimized beam shaping in the visible.

Accurate coupling between optical modes at the interface between photonic chips and free space is required for the development of many on-chip devices. This control is critical in quantum technologies where large-diameter beams with designed mode profiles are required. Yet, these designs are often difficult to achieve at shorter wavelengths where fabrication limits the resolution of designed devices.

Probing Electrified Liquid-Solid Interfaces with Scanning Electron Microscopy.

Electrical double layers play a key role in a variety of electrochemical systems. The mean free path of secondary electrons in aqueous solutions is on the order of a nanometer, making them suitable for probing ultrathin electrical double layers at solid-liquid electrolyte interfaces. Employing graphene as an electron-transparent electrode in a two-electrode electrochemical system, we show that the secondary electron yield of the graphene-liquid interface depends on the ionic strength and concentration of the electrolyte and the applied bias at the remote counter electrode.

Nanoscale Mapping of the Double Layer Potential at the Graphene-Electrolyte Interface.

The electrical double layer (EDL) governs the operation of multiple electrochemical devices, determines reaction potentials, and conditions ion transport through cellular membranes in living organisms. The few existing methods of EDL probing have low spatial resolution, usually only providing spatially averaged information. On the other hand, traditional Kelvin probe force microscopy (KPFM) is capable of mapping potential with nanoscale lateral resolution but cannot be used in electrolytes with concentrations higher than several mmol/L.

Metasurface-Integrated Photonic Platform for Versatile Free-Space Beam Projection with Polarization Control.

Densely integrated photonic circuits enable scalable, complex processing of optical signals, including modulation, multiplexing, wavelength conversion, and detection. Directly interfacing such integrated circuits to free-space optical modes will enable novel optical functions, such as chip-scale sensing, interchip free-space interconnect and cooling, trapping, and interrogation of atoms. However, doing this within the limits of planar batch fabrication requires new approaches for bridging the large mode scale mismatch.

Photonic waveguide to free-space Gaussian beam extreme mode converter.

Integration of photonic chips with millimeter-scale atomic, micromechanical, chemical, and biological systems can advance science and enable new miniaturized hybrid devices and technology. Optical interaction via small evanescent volumes restricts performance in applications such as gas spectroscopy, and a general ability to photonically access optical fields in large free-space volumes is desired. However, conventional inverse tapers and grating couplers do not directly scale to create wide, high-quality collimated beams for low-loss diffraction-free propagation over many millimeters in free space, necessitating additional bulky collimating optics and expensive alignment.

From Microparticles to Nanowires and Back: Radical Transformations in Plated Li Metal Morphology Revealed via in Situ Scanning Electron Microscopy.

Li metal is the preferred anode material for all-solid-state Li batteries. However, a stable plating and stripping of Li metal at the anode-solid electrolyte interface remains a significant challenge particularly at practically feasible current densities. This problem usually relates to high and/or inhomogeneous Li-electrode-electrolyte interfacial impedance and formation and growth of high-aspect-ratio dendritic Li deposits at the electrode-electrolyte interface, which eventually shunt the battery.

Interfacial Electrochemistry in Liquids Probed with Photoemission Electron Microscopy.

Studies of the electrified solid-liquid interfaces are crucial for understanding biological and electrochemical systems. Until recently, use of photoemission electron microscopy (PEEM) for such purposes has been hampered by incompatibility of the liquid samples with ultrahigh vacuum environment of the electron optics and detector. Here we demonstrate that the use of ultrathin electron transparent graphene membranes, which can sustain large pressure differentials and act as a working electrode, makes it possible to probe electrochemical reactions in operando in liquid environments with PEEM.

Imaging and Analysis of Encapsulated Objects through Self-Assembled Electron and Optically Transparent Graphene Oxide Membranes.

We demonstrate a technique for facile encapsulation and adhesion of micro- and nano objects on arbitrary substrates, stencils, and micro structured surfaces by ultrathin graphene oxide membranes via a simple drop casting of graphene oxide solution. A self-assembled encapsulating membrane forms during the drying process at the liquid-air and liquid-solid interfaces and consists of a water-permeable quasi-2D network of overlapping graphene oxide flakes. Upon drying and interlocking between the flakes, the encapsulating coating around the object becomes mechanically robust, chemically protective, and yet highly transparent to electrons and photons in a wide energy range, enabling microscopic and spectroscopic access to encapsulated objects.

Graphene Microcapsule Arrays for Combinatorial Electron Microscopy and Spectroscopy in Liquids.

Atomic-scale thickness, molecular impermeability, low atomic number, and mechanical strength make graphene an ideal electron-transparent membrane for material characterization in liquids and gases with scanning electron microscopy and spectroscopy. Here, we present a novel sample platform made of an array of thousands of identical isolated graphene-capped microchannels with high aspect ratio. A combination of a global wide field of view with high resolution local imaging of the array allows for high throughput in situ studies as well as for combinatorial screening of solutions, liquid interfaces, and immersed samples.

Enabling Photoemission Electron Microscopy in Liquids via Graphene-Capped Microchannel Arrays.

Photoelectron emission microscopy (PEEM) is a powerful tool to spectroscopically image dynamic surface processes at the nanoscale, but it is traditionally limited to ultrahigh or moderate vacuum conditions. Here, we develop a novel graphene-capped multichannel array sample platform that extends the capabilities of photoelectron spectromicroscopy to routine liquid and atmospheric pressure studies with standard PEEM setups. Using this platform, we show that graphene has only a minor influence on the electronic structure of water in the first few layers and thus will allow for the examination of minimally perturbed aqueous-phase interfacial dynamics.

Toward Clean Suspended CVD Graphene.

The application of suspended graphene as electron transparent supporting media in electron microscopy, vacuum electronics, and micromechanical devices requires the least destructive and maximally clean transfer from their original growth substrate to the target of interest. Here, we use thermally evaporated anthracene films as the sacrificial layer for graphene transfer onto an arbitrary substrate. We show that clean suspended graphene can be achieved desorbing the anthracene layer at temperatures in the 100 °C to 150 °C range, followed by two sequential annealing steps for the final cleaning, using Pt catalyst and activated carbon.

Photoelectron spectroscopy of wet and gaseous samples through graphene membranes.

Photoelectron spectroscopy (PES) and microscopy are highly important for exploring morphologically and chemically complex liquid-gas, solid-liquid and solid-gas interfaces under realistic conditions, but the very small electron mean free path inside dense media imposes serious experimental challenges. Currently, near ambient pressure PES is conducted using dexterously designed electron energy analyzers coupled with differentially pumped electron lenses which make it possible to conduct PES measurements at a few hPa. This report proposes an alternative ambient pressure approach that can be applied to a broad class of samples and be implemented in conventional PES instruments.

Harmonic modulation of light by means of collinear acousto-optic interaction.

The collinear anisotropic interaction of light with a standing acoustic wave generated in a lithium niobate crystal is discussed. Diffraction may be applied for harmonic modulation of laser light. The analysis of the light modulation is based on the concept of vector diagrams.