In conventional solid-state photovoltaics, electron-hole pairs are created by light absorption in a semiconductor and separated by the electric field spaning a micrometre-thick depletion region. The maximum voltage these devices can produce is equal to the semiconductor electronic bandgap. Here, we report the discovery of a fundamentally different mechanism for photovoltaic charge separation, which operates over a distance of 1-2 nm and produces voltages that are significantly higher than the bandgap. The separation happens at previously unobserved nanoscale steps of the electrostatic potential that naturally occur at ferroelectric domain walls in the complex oxide BiFeO(3). Electric-field control over domain structure allows the photovoltaic effect to be reversed in polarity or turned off. This new degree of control, and the high voltages produced, may find application in optoelectronic devices.

We report a photovoltaic effect in ferroelectric BiFeO3 thin films. The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages ∼0.8–0.9 V and external quantum efficiencies up to ∼10% when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface.

Supporting Information: Experimental Details and Spectral Analysis NaCl (99.999% trace metals basis), KCl (99.999% trace metals basis), NaNO 3 (99.995% trace metals basis), NH 4 Cl (99.998% trace metals basis), Na 2 SO 4 (99.99% trace metals basis), and (NH 4 ) 2 SO 4(99.999% trace metals basis) were purchased from Sigma-Aldrich. NaCl, KCl and Na 2 SO 4 were baked at 500 o C before use. For the aqueous solutions, proper amount of salts were dissolved in ultra-pure distilled water (with resistivity of 18.3 MΩ/cm, Thermo Fisher-EASYpure). Hydrophobic PTFE syringe filter (VWR) was then used to filter out residue particles from solutions. All glassware was soaked in concentrated H 2 SO 4 mixed with NoCromix, and then rinsed thoroughly with ultra-pure water.The details of our PS-SFVS setup have been described elsewhere. 1 We used collinear geometry with an incident angle of 45 o for the green (532nm, 20ps) and IR (3000-3800 cm -1 , 20ps) beams with input energies of ~300 µJ/pulse and ~120 µJ/pulse, respectively. The two beams overlapped in time and space at the sample surface, one focused to an area of 0.2 mm 2 and the other to 0.1 mm 2 . The SF output was collected after proper filtering by a gated detection system. The signal is proportional to χ eff

It is possible to harvest energy from Earth's thermal infrared emission into outer space. We calculate the thermodynamic limit for the amount of power available, and as a case study, we plot how this limit varies daily and seasonally in a location in Oklahoma. We discuss two possible ways to make such an emissive energy harvester (EEH): A thermal EEH (analogous to solar thermal power generation) and an optoelectronic EEH (analogous to photovoltaic power generation). For the latter, we propose using an infraredfrequency rectifying antenna, and we discuss its operating principles, efficiency limits, system design considerations, and possible technological implementations.long-wave infrared | rectenna W henever energy flows from hotter to colder, there is an opportunity to harvest renewable energy. For example, solar energy and biofuels rely on the energy flow from the Sun to the Earth, and wind power and hydroelectricity rely on the energy flow from hotter to colder areas on Earth. However, there is one massive energy flow that has been neglected: The roughly 10 17 W of infrared thermal radiation that Earth continuously emits into cold outer space. The technology does not yet exist to siphon renewable energy out of this flow, but we will argue that it is possible to make a device that does exactly that. We call such a device an emissive energy harvester (EEH).In general terms, we propose a device that has a large emissivity in the long-wave infrared (LWIR) "atmospheric window" at 8-13 μm, where the atmosphere is mostly transparent, and small emissivity at other wavelengths, where the atmosphere is mostly opaque. It would sit outdoors with its emissive surface pointing upward, emitting thermal radiation toward the sky, but receiving far less radiation back (1, 2). This imbalance between incoming and outgoing radiation can be converted into an imbalance in electron motion, i.e., into useful electrical power. With a perfectly transparent atmosphere, an EEH would be a kind of heat engine harnessing the temperature difference between Earth's surface at ∼275-300 K and outer space at 3 K. However, because the atmosphere is not perfectly transparent, EEH power generation will be affected by weather and atmospheric conditions-and stopped altogether by thick, low clouds. On the other hand, because the Sun emits negligible LWIR compared with the atmosphere, an EEH can operate during both day and night. The effects of sunlight are discussed in more detail below.One possible design of an EEH, shown in Fig. 1A, is a heat engine running between the ambient temperature and a cold panel, where the latter maintains its temperature by radiative cooling (1, 2). We will argue below that this is not the most promising EEH design, but it is a simple example that illustrates the principle. Fig. 2A shows the energy flows involved in EEH operation. There are three relevant temperatures, T hot > T cold > T sky , corresponding to the hot reservoir temperature, the cold panel temperature, and the radiation brightness temperature of the sky, r...

A metasurface lens (meta-lens) bends light using nanostructures on a flat surface. Macroscopic meta-lenses (mm-to cm-scale diameter) have been quite difficult to simulate and optimize, due to the large area, the lack of periodicity, and the billions of adjustable parameters. We describe a method for designing a large-area meta-lens that allows not only prediction of the efficiency and far-field, but also optimization of the shape and position of each individual nanostructure, with a computational cost that is almost independent of the lens size. As examples, we design three large NA = 0.94 meta-lenses: One with 79% predicted efficiency for yellow light, one with dichroic properties, and one broadband lens. All have a minimum feature size of 100nm.

Recently a new class of optical interference coatings was introduced which comprises ultra-thin, highly absorbing dielectric layers on metal substrates. We show that these lossy coatings can be augmented by an additional transparent subwavelength layer. We fabricated a sample comprising a gold substrate, an ultra-thin film of germanium with a thickness gradient, and several alumina films. The experimental reflectivity spectra showed that the additional alumina layer increases the color range that can be obtained, in agreement with calculations. More generally, this transparent layer can be used to enhance optical absorption, protect against erosion, or as a transparent electrode for optoelectronic devices. V

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A new approach to MR trabecular bone characterization is presented. This method probes the diffusion of spins through internal magnetic field gradients due to the susceptibility contrast between the bone and water (or marrow) phases. The resulting spin magnetization decay encodes properties of the underlying structure. This method, termed decay due to diffusion in the internal field (DDIF), is well established as a probe of pore size and structure. In the present work its application is shown for in vitro experiments on excised bovine tibiae samples. A comparison with pulsed field gradient (PFG) measurement of restricted diffusion shows a strong correlation of DDIF with the surface-to-volume ratio (SVR) of bones. Calculation of the internal magnetic field within the bone structure also supports this interpretation. These NMR measurements compare well with the image analysis from microscopic computed tomography (CT). The SVR is not accessible in the clinically standard densitometry measurements, and provides vital information on bone strength and therefore on its fracture risk. The DDIF and PFG methods derive this information from a straightforward pulse sequence that does not employ either high ap- Osteoporosis is a disorder of the skeleton in which bone strength is abnormally weak and susceptible to fractures from minor trauma. Therapeutic treatment of osteoporosis is under intense development. Current diagnostics of osteoporosis using dual X-ray measurement of bone density do not entirely predict fracture risk, because the internal bone structure, apart from the bone density, contributes significantly to the mechanical strength and thus fracture risk (1,2). Such bone structure is routinely characterized by microscopic computed tomography (CT) with resolution down to 10 m for small samples. However, it is not available for in vivo examination due to the high radiation dose. Recent efforts to achieve high-resolution 3D images of the trabecular architecture using magnetic resonance imaging (MRI) are very promising. However, it is difficult to drastically improve its resolution far beyond the current levels (ϳ100 m) in clinical implementation, primarily due to the clinically allowed MRI scan time.This article describes a different approach to the characterization of bone architecture compared to the highresolution imaging approach. An NMR technique is used that is well-established in inorganic porous media to obtain statistical properties of the trabecular structure. This technique, referred to as decay from diffusion in an internal field (DDIF), can obtain pore-structure characteristics (such as the pore-size distribution (3)) at a resolution of about 1 m. In vitro DDIF data on bovine trabecular bone samples show a clear correlation with bone strength. This trend correlates well with measurements of the surface-tovolume ratio (SVR) using a pulsed field gradient (PFG), demonstrating the DDIF is also sensitive to the SVR of bones. This interpretation is further understood via theoretical calculations of the interna...