Miniaturized spectrometers have significant potential for portable applications such as consumer electronics, health care, and manufacturing. These applications demand low cost and high spectral resolution, and are best enabled by single-shot free-space-coupled spectrometers that also have sufficient spatial resolution. Here, we demonstrate an on-chip spectrometer that can satisfy all of these requirements. Our device uses arrays of photodetectors, each of which has a unique responsivity with rich spectral features. These responsivities are created by complex optical interference in photonic-crystal slabs positioned immediately on top of the photodetector pixels. The spectrometer is completely complementary metal–oxide–semiconductor (CMOS) compatible and can be mass produced at low cost.

We report a new hybrid integration scheme that offers for the first time a nanowire-on-lead approach, which enables independent electrical addressability, is scalable, and has superior spatial resolution in vertical nanowire arrays. The fabrication of these nanowire arrays is demonstrated to be scalable down to submicrometer site-to-site spacing and can be combined with standard integrated circuit fabrication technologies. We utilize these arrays to perform electrophysiological recordings from mouse and rat primary neurons and human induced pluripotent stem cell (hiPSC)-derived neurons, which revealed high signal-to-noise ratios and sensitivity to subthreshold postsynaptic potentials (PSPs). We measured electrical activity from rodent neurons from 8 days in vitro (DIV) to 14 DIV and from hiPSC-derived neurons at 6 weeks in vitro post culture with signal amplitudes up to 99 mV. Overall, our platform paves the way for longitudinal electrophysiological experiments on synaptic activity in human iPSC based disease models of neuronal networks, critical for understanding the mechanisms of neurological diseases and for developing drugs to treat them.

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), hightemperature 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 ºC. 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 employing a fully integrated absorber/emitter metasurface structure, much higher than those achievable by stand-alone PV cells. Our work opens a path forward for high-performance STPV systems based on refractory metasurface structures.3 TEXT Photovoltaics (PV) 1 directly convert sunlight to electricity using semiconductor PV cells, and have been the most prevalent solar energy-harvesting technology. Despite the development over the past few decades, the efficiency of state-of-the-art, single-junction PV cells is still far below the fundamental limit predicted by Shockley and Queisser 2 , which is dictated mainly by energy losses due to below-bandgap photons and hot-carrier thermalization, owing to the broad distribution of the solar spectrum. To minimize these losses, numerous novel PV device concepts have been proposed and realized 3-7 . While they indeed improve the PV efficiency to some extent, they all suffer from their own respective problems, including high manufacturing cost, complex device fabrication processes, as well as material instability and degradation. Solar thermophotovoltaics (STPV) 8, 9 represent a promising alternative to traditional photovoltaics for solar energy harvesting, where an absorber/emitter intermediate structure first absorbs the incoming sunlight, heats up, and then emits thermal photons towards the PV cell to excite charge carriers for power generation. An ideal STPV system has a solar-to-electric energy conversion efficiency much higher than that of a stand-alone PV cell, as a carefully designed STPV intermediate structure can fully capture the incident sunlight and convert it into narrowband thermal emission right above the bandgap of the PV cell 10 . It has been theoretically shown that the STPV efficiency could significantly surpass the aforementioned Shockley-Queisser limit, reaching 85% and 54% under fully concentrated and unconcentrated solar radiation, respectively 11 .Recently, several proof-of-concept STPV experiments have been reported employing various absorber/emitter intermediate structures [12][13][14][15][16] , including multi-walled carbon nanotubes, photonic crystals (PhCs), and two-d...

Toroidal moment is an electromagnetic excitation that lies outside the familiar picture of electric and magnetic multipoles. It has recently been a topic of intense research in the fields of nanophotonics and metamaterials due to its weakly radiating nature and its ability to confine electromagnetic energy. Among extensive studies on toroidal moments and their applications, high quality factor (Q) toroidal resonances have been experimentally realized only in a very limited set of geometries and wavelengths. In this study, we demonstrate that a metasurface consisting of arrays of hollow dielectric cuboids supports a high Q-factor resonances at near-infrared and visible wavelengths due to the destructive interference between toroidal dipoles and magnetic quadrupoles. Using silicon as the high index dielectric, an experimental Q-factor of 728 is realized at a wavelength of 1505 nm, which is one of the highest values reported in the near-infrared using a dielectric metasurface. Importantly, our resonator geometry enables very efficient coupling of the toroidal resonance to the environment. This makes our metasurface design useful for refractometric sensing, where we measure a sensitivity of 161 nm per refractive index unit with a line width of 2.01 nm, efficiently distinguishing an index change of less than 0.02. We also find that a metasurface made of a relatively low-index dielectric, titanium dioxide (n < 2.4), is also capable of supporting the same toroidal mode with an observed Q-factor of 160 at visible wavelengths. With the versatility and robustness that dielectric metasurfaces provide, toroidal resonances are expected to be a powerful tool for investigating strong light–matter interaction and nonlinear phenomena at the nanoscale.

Thermophotovoltaics (TPV) is the process by which photons radiated from a thermal emitter are converted into electrical power via a photovoltaic cell. Selective thermal emitters that can survive at temperatures at or above ∼1000°C have the potential to greatly improve the efficiency of TPV energy conversion by restricting the emission of photons with energies below the photovoltaic (PV) cell bandgap energy. In this work, we demonstrated TPV energy conversion using a high-temperature selective emitter, dielectric filter, and 0.6 eV In 0.68 Ga 0.32 As photovoltaic cell. We fabricated a passivated platinum and alumina frequency-selective surface by conventional stepper lithography. To our knowledge, this is the first demonstration of TPV energy conversion using a metamaterial emitter. The emitter was heated to >1000°C, and converted electrical power was measured. After accounting for geometry, we demonstrated a thermal-to-electrical power conversion efficiency of 24.1 0.9% at 1055°C. We separately modeled our system consisting of a selective emitter, dielectric filter, and PV cell and found agreement with our measured efficiency and power to within 1%. Our results indicate that high-efficiency TPV generators are possible and are candidates for remote power generation, combined heat and power, and heat-scavenging applications.

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.

We demonstrate the active tuning of all-dielectric metasurfaces exhibiting high-quality factor (high-Q) resonances. The active control is provided by embedding the asymmetric silicon meta-atoms with liquid crystals, which allows the relative index of refraction to be controlled through heating. It is found that high quality factor resonances (Q = 270 ± 30) can be tuned over more than three resonance widths. Our results demonstrate the feasibility of using all-dielectric metasurfaces to construct tunable narrow-band filters.There is rapidly growing interest in all-dielectric metasurfaces 1 for optical applications. Examples include flat lenses 2,3 , beam converters 4-6 and holograms 7-9 . Dielectric metasurfaces are two-dimensional arrangements of dielectric nano-resonators which exhibit low absorption in the visible and infra-red spectral range. These low losses allow one to obtain resonances with a high qualityfactor (high-Q) in comparison with their plasmonic counterparts. Importantly, the interference of electric and magnetic Mie-type resonant modes of the same strength allows for fundamentally new effects to be realized, such as unidirectional scattering 10 and unity transmission in the Huygens regime 11-13 . Furthermore, the Fano interference of bright and dark modes in the dielectric resonators 14-16 provides a pathway to obtaining narrowband resonances with quality-factors of up to several hundred 17 . Such quality factors open up new applications for dielectric metasurfaces, such as high-sensitivity sensors 14,15 and narrow-band filters 18 .To date, all narrow-band dielectric metasurfaces have been based on static designs, defined through the choice of geometry. Such static design does not offer the flexibility required for many scientific and industrial applications, including tunable narrow band filters. Therefore implementing dynamic control over the response of the metasurface is essential. There have been various schemes proposed for tuning dielectric metasurfaces which focus on the tuning of the refractive index, either of the dielectric resonators or of the surrounding environment. These include the electrical 19 or thermal 20 tuning of semiconductor metasurfaces, as well as tuning of the embedding medium 21 , using for example liquid crystals (LCs) 22,23 . To date however, the tuning has only been demonstrated with relatively broad resonances. That is, only a low figure of merit (overall spectral shift divided by the resonance width is less than 1) for the tunability has been achieved, so the spectral shifts do not generally produce a dramatic change in transmission over the tuning range. Here, we demonstrate the control of a high-Q (Q = 270 ± 30) dielectric metasurface by changing the properties of the medium surrounding the meta-atoms. We utilize nano-resonators with an asymmetric geometry, which allows for weak coupling into their low-radiative multipoles 17 and therefore produce high-Q transmission features. To enable the tuning of these high-Q resonances we embeded the metasurface with nemat...

Optical antireflection has long been pursued for a wide range of applications, but existing approaches encounter issues in the performance, bandwidth, and structure complexity, particularly in the long-wavelength infrared regime. Here we present the demonstration of bilayer metasurfaces that accomplish dual- and broadband optical antireflection in the terahertz and mid-infrared spectral ranges. By simply tailoring the structural geometry and dimensions, we show that subwavelength metal/dielectric structures enable dramatic reduction of Fresnel reflection and significant enhancement of transmission at a substrate surface, operating either at two discrete narrow bands or over a broad bandwidth up to 28%. We also use a semianalytical interference model to interpret the obtained results, in which we find that the dispersion of the constituent structures plays a critical role in achieving the observed broadband optical antireflection.