No abstract

For optimal image quality in susceptibility-weighted imaging and accurate quantification of susceptibility, it is necessary to isolate the local field generated by local magnetic sources (such as iron) from the background field that arises from imperfect shimming and variations in magnetic susceptibility of surrounding tissues (including air). Previous background removal techniques have limited effectiveness depending on the accuracy of model assumptions or information input. In this article, we report an observation that the magnetic field for a dipole outside a given region of interest (ROI) is approximately orthogonal to the magnetic field of a dipole inside the ROI. Accordingly, we propose a nonparametric background field removal technique based on projection onto dipole fields (PDF). In this PDF technique, the background field inside an ROI is decomposed into a field originating from dipoles outside the ROI using the projection theorem in Hilbert space. This novel PDF background removal technique was validated on a numerical simulation and a phantom experiment and was applied in human brain imaging, demonstrating substantial improvement in background field removal compared with the commonly used high-pass filtering method.

Titanium nitride (TiN) is a plasmonic material having optical properties resembling gold. Unlike gold, however, TiN is complementary metal oxide semiconductor-compatible, mechanically strong, and thermally stable at higher temperatures. Additionally, TiN exhibits low-index surfaces with surface energies that are lower than those of the noble metals which facilitates the growth of smooth, ultrathin crystalline films. Such films are crucial in constructing low-loss, high-performance plasmonic and metamaterial devices including hyperbolic metamaterials (HMMs). HMMs have been shown to exhibit exotic optical properties, including extremely high broadband photonic densities of states (PDOS), which are useful in quantum plasmonic applications. However, the extent to which the exotic properties of HMMs can be realized has been seriously limited by fabrication constraints and material properties. Here, we address these issues by realizing an epitaxial superlattice as an HMM. The superlattice consists of ultrasmooth layers as thin as 5 nm and exhibits sharp interfaces which are essential for highquality HMM devices. Our study reveals that such a TiN-based superlattice HMM provides a higher PDOS enhancement than goldor silver-based HMMs.refractory plasmonics | metal nitrides | ceramics M etamaterials are artificially created materials with subwavelength building blocks and unconventional electromagnetic properties that enable devices with unique functionalities (1, 2). For example, highly anisotropic metamaterials that consist of deeply subwavelength dielectric-metallic multilayers can effectively act as a material that is metallic in one or two directions and dielectric in the other (3, 4). In such metamaterials, light encounters extreme anisotropy, resulting in a hyperbolic dispersion relation (therefore these materials are referred to as "hyperbolic metamaterials," HMMs), which causes dramatic changes in the light's behavior (5-7). HMMs enable many exotic devices for subwavelength-resolution imaging (5, 8-10), ultracompact resonators (11), highly sensitive sensors (12), and could lead to breakthrough quantum technologies (7, 13). The recent discovery of the enhancement of the photonic density of states (PDOS) within a broad bandwidth in HMMs could revolutionize PDOS engineering (14-17), enabling light sources with dramatically increased photon extraction and ultimately leading to nonresonant single-photon sources (6). These HMMs can be combined with wide-spectrum, room-temperature quantum emitters [such as quantum dots and nitrogen-vacancy color centers in diamonds (18)] to provide greatly enhanced spontaneous emission rates (19). Additionally, HMMs can transform an isotropic spontaneous emission profile into a directional one, leading to new types of light sources (20). Rather than obeying Planck's law, an extreme PDOS enhancement enables nearfield thermal radiation arising from the HMM to be significantly enhanced compared with the near-field thermal radiation from a dielectric (21). Moreover, the thermal conductivity...

Light-emitting diodes (LEDs) based on lead halide perovskites demonstrate outstanding optoelectronic properties and are strong competitors for display and lighting applications. While previous halide perovskite LEDs are mainly produced via solution processing, here an all-vacuum processing method is employed to construct CsPbBr 3 LEDs because vacuum processing exhibits high reliability and easy integration with existing OLED facilities for mass production. The high-throughput combinatorial strategies are further adopted to study perovskite composition, annealing temperature, and functional layer thickness, thus significantly speeding up the optimization process. The best rigid device shows a current efficiency (CE) of 4.8 cd A −1 (EQE of 1.45%) at 2358 cd m −2 , and best flexible device shows a CE of 4.16 cd A −1 (EQE of 1.37%) at 2012 cd m −2 with good bending tolerance. Moreover, by choosing NiO x as the hole-injection layer, the CE is improved to 10.15 cd A −1 and EQE is improved to a record of 3.26% for perovskite LEDs produced by vacuum deposition. The time efficient combinatorial approaches can also be applied to optimize other perovskite LEDs.