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Thermophotovoltaics (TPVs) convert predominantly infrared wavelength light to electricity via the photovoltaic effect, and can enable approaches to energy storage 1 , 2 and conversion 3 – 9 that use higher temperature heat sources than the turbines that are ubiquitous in electricity production today. Since the first demonstration of 29% efficient TPVs (Fig. 1a ) using an integrated back surface reflector and a tungsten emitter at 2,000 °C (ref. 10 ), TPV fabrication and performance have improved 11 , 12 . However, despite predictions that TPV efficiencies can exceed 50% (refs. 11 , 13 , 14 ), the demonstrated efficiencies are still only as high as 32%, albeit at much lower temperatures below 1,300 °C (refs. 13 – 15 ). Here we report the fabrication and measurement of TPV cells with efficiencies of more than 40% and experimentally demonstrate the efficiency of high-bandgap tandem TPV cells. The TPV cells are two-junction devices comprising III–V materials with bandgaps between 1.0 and 1.4 eV that are optimized for emitter temperatures of 1,900–2,400 °C. The cells exploit the concept of band-edge spectral filtering to obtain high efficiency, using highly reflective back surface reflectors to reject unusable sub-bandgap radiation back to the emitter. A 1.4/1.2 eV device reached a maximum efficiency of (41.1 ± 1)% operating at a power density of 2.39 W cm –2 and an emitter temperature of 2,400 °C. A 1.2/1.0 eV device reached a maximum efficiency of (39.3 ± 1)% operating at a power density of 1.8 W cm –2 and an emitter temperature of 2,127 °C. These cells can be integrated into a TPV system for thermal energy grid storage to enable dispatchable renewable energy. This creates a pathway for thermal energy grid storage to reach sufficiently high efficiency and sufficiently low cost to enable decarbonization of the electricity grid. Two-junction TPV cells with efficiencies of more than 40% are reported, using an emitter with a temperature between 1,900 and 2,400 °C, for integration into a TPV system for thermal energy grid storage.

We discuss an inverse method to compute a freeform two-reflector two-target system. The optical path length constitutes an integral component and can be expressed in terms of position coordinates at the first target. The system is expressed in terms of a generating function, closed with energy balance and requires a sophisticated least-squares solver to compute the shapes of the reflectors. In a numerical example, we illustrate the algorithm’s capabilities to tackle even the most intricate light distributions.

We proposed a dual-channel narrow band filter consisting of top and bottom-distributed Bragg reflectors (DBRs) and a dielectric interlayer inserted with a metasurface. Through the design of the metasurface, the two channels of the filter are guaranteed to exhibit high-quality factors with transmittance beyond 90% and full width at half maximum (FWHM) less than 10 nm. We demonstrate that the central wavelengths of each dual-channel filter can be controlled with a total of 50 nm shifts by only changing the width of the metasurface. Compared with the traditional dual-channel filter, our design is easier to fabricate and more convenient to tune the central wavelength, which is promising for ultracompact optical devices.

High-end imaging lenses have tended to be based on bulk optical components. Advances in fabrication techniques have enabled the development of ultrathin, lightweight, and planar lenses (metalenses) that have unprecedented functionalities. These metalenses have the potential to replace or complement their conventional bulk counterparts. Khorasaninejad and Capasso review the evolution of metalenses, summarizing achievements and applications and identifying future challenges and opportunities. Metalenses can have numerous applications, ranging from cellphone camera modules, to wearable displays for augmented and virtual reality and machine vision, to bio-imaging and endoscopy. Science , this issue p. eaam8100 Recent progress in metasurface designs fueled by advanced-fabrication techniques has led to the realization of ultrathin, lightweight, and flat lenses (metalenses) with unprecedented functionalities. Owing to straightforward fabrication, generally requiring a single-step lithography, and the possibility of vertical integration, these planar lenses can potentially replace or complement their conventional refractive and diffractive counterparts, leading to further miniaturization of high-performance optical devices and systems. Here we provide a brief overview of the evolution of metalenses, with an emphasis on the visible and near-infrared spectrum, and summarize their important features: diffraction-limited focusing, high-quality imaging, and multifunctionalities. We discuss impending challenges, including aberration correction, and also examine current issues and solutions. We conclude by providing an outlook of this technology platform and identifying promising directions for future research.

Performance of a solar reflector depends upon the topology of the reflector unit. Curved reflector certainly performs better but it has high-cost, high manufacturing complexity and sometimes even requires non-indigenous technology for its production. Besides, the flat plate solar reflecting unit (FSRU) offers low cost, are widely available and are indigenous in nature. The present work attempts to give a simple methodology to test different combinations of a large-scale FSRU under azimuthal sun alignment. The obtained result is validated with corresponding CAD model and previously performed experimental results. The FSRU is a funnel-type solar reflector in the form of an inverted frustum of a square pyramid. The path followed by the in the unit is classified in different categories, based on their interaction with reflectors. The results show that there is a good agreement between path of rays observed during experiment and path of rays predicted by a CAD software. The maximum deviation ( δ max ) in path for optical performance is 2.1%. On other hand the maximum uncertainty (χ) in path is 2.96% considering ± 1° error of manufacturing of FSRU. Thermal efficiency of FSRU is 69% considering reflectivity of 97% for the reflectors. The final output of the present study predicts that (a) amount of errors ( δ max and χ) are under control for large size of FSRU and (b) azimuthal alignment improves the FSRU's thermal performance.

Whenever a new device enables our senses to access an uncharted sensible world, our experience needs to be widened to be able to embrace it. Many animals don't recognize themselves in mirrors. Thus, mirrors found in ancient Egyptian tombs bear witness to a device that involved a widening of human experience. While by then reflectors were commonly believed to hold the spirit of their beholder, they also provided the motivating force for use of geometry as a logical framework, rather than the form of the outer world. Euclid of Alexandria conceived of geometric constructions and their rules as the connecting link between visual world and hypothetical-deductive reasoning. Yet, he didn't have a clue about receivers. In our view the problem of linking received information in an image format to a mathematical space cannot be solved once and for all, but rather needs to be posed and understood afresh once in a while. All the more so in an information and telecommunication era, when the techniques of acquisition and rendering of visual information have been extended well beyond the domain of optical instruments, and the language of mathematics has advanced to a different level of proficiency.

This review paper combs through reports that have enhanced antenna gain for ultra-wideband (UWB) frequencies using frequency-selective surface (FSS) techniques. The FSS techniques found across the research landscape were mapped onto a taxonomy in order to determine the most effective method for improving antenna gain. Additionally, this study looked into the motivation behind using FSS as a reflector in UWB frequencies to obtain directional radiation. The FSS suits multiple applications due to its exceptional ability to minimize power loss in undesired transmission areas in the antenna, as well as to hinder the interference that may occur from undesirable and wasted radiation. An efficient way to obtain constant gain over a wide range of frequencies is also elaborated in this paper. Essentially, this paper offers viable prescription to enhance antenna gain for UWB applications. Methods: A comprehensive study was performed using several imminent keywords, such as “high gain using FSS”, “gain enhancement using FSS”, “high gain UWB antennas”, and “gain enhancement of UWB antennas”, in different modifications to retrieve all related articles from three primary engines: Web of Science (WoS), IEEE Xplore, and Science Direct. Results: The 41 papers identified after a comprehensive literature review were classified into two categories. The FSS single- and multi-layer reflectors were reported in 25 and 16 papers, respectively. New direction: An effective method is proposed for FSS miniaturization and for obtaining constant gain over UWB frequencies while maintaining the return loss at −10 dB. Conclusion: The use of FSS is indeed effective and viable for gain enhancement in UWB antennas. This systematic review unravels a vast range of opportunities for researchers to bridge the identified gaps.

In this paper, we propose a new metasurface that is able to reflect a known incoming electromagnetic wave into an arbitrary direction, with perfect power efficiency. This seemingly simple task, which we hereafter call perfect anomalous reflection, is actually highly nontrivial because of the differing wave impedances and complex interference between the incident and reflected waves. Heretofore, proposed metasurfaces that achieve perfect anomalous reflection require complicated, deeply subwavelength and/or multilayer element structures, which allow them to couple to and from leaky and/or evanescent waves. In contrast, we demonstrate that using a bipartite Huygens’ metasurface (BHM)—a passive and lossless metasurface with only two cells per period—perfect anomalous reflection can be achieved over a wide angular and frequency range. Through simulations and experiments at 24 GHz, we show that a properly designed BHM can anomalously reflect an incident electromagnetic wave fromθi=50°toθr=−22.5°, with perfect power efficiency to within experimental precision.