Metasurfaces offer a unique platform to precisely control optical wavefronts and enable the realization of flat lenses, or metalenses, which have the potential to substantially reduce the size and ...complexity of imaging systems and to realize new imaging modalities. However, it is a major challenge to create achromatic metalenses that produce a single focal length over a broad wavelength range because of the difficulty in simultaneously engineering phase profiles at distinct wavelengths on a single metasurface. For practical applications, there is a further challenge to create broadband achromatic metalenses that work in the transmission mode for incident light waves with any arbitrary polarization state. We developed a design methodology and created libraries of meta-units-building blocks of metasurfaces-with complex cross-sectional geometries to provide diverse phase dispersions (phase as a function of wavelength), which is crucial for creating broadband achromatic metalenses. We elucidated the fundamental limitations of achromatic metalens performance by deriving mathematical equations that govern the tradeoffs between phase dispersion and achievable lens parameters, including the lens diameter, numerical aperture (NA), and bandwidth of achromatic operation. We experimentally demonstrated several dielectric achromatic metalenses reaching the fundamental limitations. These metalenses work in the transmission mode with polarization-independent focusing efficiencies up to 50% and continuously provide a near-constant focal length over
= 1200-1650 nm. These unprecedented properties represent a major advance compared to the state of the art and a major step toward practical implementations of metalenses.
Efficient and compact high-power high-frequency passive energy storage components are required for miniaturization of power converters and remain a challenging obstacle in power electronics. In this ...paper, multiple energy storage mechanisms are analyzed on an order-of-magnitude basis to identify potential alternatives for conventional passive components, especially magnetics, which have frequency-dependent power losses. The high energy density of mechanical storage methods provides an attractive alternative to the widely used LC resonance. Piezoelectric transduction is explored, and the performance of a piezoelectric resonator is compared to that of an LC resonator in terms of efficiency and power handling capabilities in a resonant switched-capacitor-type circuit. The analysis provides a basis for exploring potential passive energy storage component technologies and comparing their performance limits with those of electromagnetic passive components. The analysis shows that both electromagnetic and mechanical resonance, in the ideal scenarios, can offer much better performance than do passive component technologies in use today. A prototype resonator is built with off-the-shelf C0G capacitors to confirm the low loss and high power predicted by the model. The resulting resonator has an effective resistance of 2.76 <inline-formula><tex-math notation="LaTeX">\text{m}{\Omega }</tex-math></inline-formula> at 2.70 MHz in 1 <inline-formula><tex-math notation="LaTeX">{\text{cm}^{3}}</tex-math></inline-formula> volume and can handle 7.42 kW within the rated temperature.
Abstract
Metasurfaces are optically thin metamaterials that promise complete control of the wavefront of light but are primarily used to control only the phase of light. Here, we present an approach, ...simple in concept and in practice, that uses meta-atoms with a varying degree of form birefringence and rotation angles to create high-efficiency dielectric metasurfaces that control both the optical amplitude and phase at one or two frequencies. This opens up applications in computer-generated holography, allowing faithful reproduction of both the phase and amplitude of a target holographic scene without the iterative algorithms required in phase-only holography. We demonstrate all-dielectric metasurface holograms with independent and complete control of the amplitude and phase at up to two optical frequencies simultaneously to generate two- and three-dimensional holographic objects. We show that phase-amplitude metasurfaces enable a few features not attainable in phase-only holography; these include creating artifact-free two-dimensional holographic images, encoding phase and amplitude profiles separately at the object plane, encoding intensity profiles at the metasurface and object planes separately, and controlling the surface textures of three-dimensional holographic objects.
Research on two-dimensional designer optical structures, or metasurfaces, has mainly focused on controlling the wavefronts of light propagating in free space. Here, we show that gradient metasurface ...structures consisting of phased arrays of plasmonic or dielectric nanoantennas can be used to control guided waves via strong optical scattering at subwavelength intervals. Based on this design principle, we experimentally demonstrate waveguide mode converters, polarization rotators and waveguide devices supporting asymmetric optical power transmission. We also demonstrate all-dielectric on-chip polarization rotators based on phased arrays of Mie resonators with negligible insertion losses. Our gradient metasurfaces can enable small-footprint, broadband and low-loss photonic integrated devices.
The range and efficiency of a wireless power transfer (WPT) system is limited by the quality factor of the resonant coils. Conventional resonant coils are made from solid or Litz wire. At megahertz ...frequencies solid wire is not utilized well due to skin effect, and Litz wire is very lossy due to proximity effect. We present a multilayer self-resonant structure as a low-cost method for creating high-<inline-formula><tex-math notation="LaTeX">Q</tex-math></inline-formula> coils. This structure uses thin foil layers that are separated by a dielectric material in order to form an LC resonator, while also forcing equal current sharing between conductors. The self-resonant structure makes it feasible to achieve advantages similar to Litz wire, but at multi-megahertz frequencies where effective Litz wire is not commercially available. These structures are made with foil layers much thinner than a skin depth, which can make handling these thin layers a challenge. To solve this problem, we also present a modified self-resonant structure in which the layered conductors are made with flex-PCB substrates with no vias. The PCB substrates provide a relatively inexpensive way to handle thin conductive layers, and the modified self-resonant structure ensures that the poor dielectric properties of the PCB substrates do not impact the quality factor of the structure. A prototype of the modified self-resonant structure has a quality factor of 1183 at 7.09 MHz, despite only being 6.6 cm in diameter, which is more than 6.5x larger than other coils presented in the literature with a similar diameter. An experimental WPT setup utilizing two self-resonant structures achieves 94<inline-formula><tex-math notation="LaTeX">\%</tex-math></inline-formula> efficiency at a distance of 5.0 cm, which is more than twice the distance as similarly sized conventional coils can achieve while maintaining the same efficiency.
Piezoelectric energy harvesters generate electrical power from ambient mechanical vibrations, making these vibrations a viable energy source for powering wireless sensor and identifier nodes. In ...order to harvest an appreciable amount of power, piezoelectric devices are typically inserted into high-Q mechanical resonant structures that significantly limit their harvesting bandwidth. The dynamic active energy harvesting method has been proposed as a way to widen the bandwidth of resonant piezoelectric energy harvesters; however, an autonomous design has not yet been demonstrated. This paper demonstrates the first autonomous implementation of this method. This was accomplished through the use of a resonant inverter topology in combination with a low-power analog control circuit design that reduces the computational demand of the microcontroller. Experimental results using the Mide Volture V20w piezoelectric device show that the harvested power is up to twice that of the adaptive rectifier method. These results include previously ignored loss mechanisms such as control losses, gating losses, and phase detection losses, making this system the first autonomous energy harvesting system of its kind.
The modification of geometry and interactions in two-dimensional magnetic nanosystems has enabled a range of studies addressing the magnetic order1–6, collective low-energy dynamics7,8 and emergent ...magnetic properties5, 9,10 in, for example, artificial spin-ice structures. The common denominator of all these investigations is the use of Ising-like mesospins as building blocks, in the form of elongated magnetic islands. Here, we introduce a new approach: single interaction modifiers, using slave mesospins in the form of discs, within which the mesospin is free to rotate in the disc plane11. We show that by placing these on the vertices of square artificial spin-ice arrays and varying their diameter, it is possible to tailor the strength and the ratio of the interaction energies. We demonstrate the existence of degenerate ice-rule-obeying states in square artificial spin-ice structures, enabling the exploration of thermal dynamics in a spin-liquid manifold. Furthermore, we even observe the emergence of flux lattices on larger length scales, when the energy landscape of the vertices is reversed. The work highlights the potential of a design strategy for two-dimensional magnetic nano-architectures, through which mixed dimensionality of mesospins can be used to promote thermally emergent mesoscale magnetic states.
Patterning materials efficiently at the smallest length scales is a longstanding challenge in nanotechnology. Electron-beam lithography (EBL) is the primary method for patterning arbitrary features, ...but EBL has not reliably provided sub-4 nm patterns. The few competing techniques that have achieved this resolution are orders of magnitude slower than EBL. In this work, we employed an aberration-corrected scanning transmission electron microscope for lithography to achieve unprecedented resolution. Here we show aberration-corrected EBL at the one nanometer length scale using poly(methyl methacrylate) (PMMA) and have produced both the smallest isolated feature in any conventional resist (1.7 ± 0.5 nm) and the highest density patterns in PMMA (10.7 nm pitch for negative-tone and 17.5 nm pitch for positive-tone PMMA). We also demonstrate pattern transfer from the resist to semiconductor and metallic materials at the sub-5 nm scale. These results indicate that polymer-based nanofabrication can achieve feature sizes comparable to the Kuhn length of PMMA and ten times smaller than its radius of gyration. Use of aberration-corrected EBL will increase the resolution, speed, and complexity in nanomaterial fabrication.
Purpose
Based on social exchange theory and the substitutes for leadership theory, this paper aims to investigate whether an organization’s high-commitment HRM strategy can substitute for the effect ...of servant leadership in promoting employees’ affective commitment, psychological empowerment and intent to remain with the organization.
Design/methodology/approach
This study’s hypotheses were tested with moderation and mediation analyses conducted on a sample of 172 Chinese employees.
Findings
The results show significant negative interaction effects between high-commitment HRM systems and servant leadership, such that high levels of one will reduce the positive effect of the other on affective commitment and psychological empowerment. Further, the effects of high-commitment HRM systems and servant leadership on turnover intentions are mediated through affective commitment and psychological empowerment. Finally, support was found for a mediated moderation model where the negative interaction effect between high-commitment HRM systems and servant leadership on turnover intentions is mediated through affective commitment.
Practical implications
The results of this study can help practitioners identify alternative means to influence employees’ positive attitudes and work motivation when implementing high-commitment HRM systems is not feasible for the organization.
Originality/value
This study contributes to the leadership literature by providing evidence supporting the substitutes for leadership theory and describing the specific conditions under which this theory is valid, as well as contributing to the HRM literature by examining the dynamic interaction of HRM and leadership.
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