Passive solar vapor generation represents a promising and environmentally benign method of water purification/desalination. However, conventional solar steam generation techniques usually rely on costly and cumbersome optical concentration systems and have relatively low efficiency due to bulk heating of the entire liquid volume. Here, an efficient strategy using extremely low‐cost materials, i.e., carbon black (powder), hydrophilic porous paper, and expanded polystyrene foam is reported. Due to the excellent thermal insulation between the surface liquid and the bulk volume of the water and the suppressed radiative and convective losses from the absorber surface to the adjacent heated vapor, a record thermal efficiency of ≈88% is obtained under 1 sun without concentration, corresponding to the evaporation rate of 1.28 kg (m2 h)−1. When scaled up to a 100 cm2 array in a portable solar water still system and placed in an outdoor environment, the freshwater generation rate is 2.4 times of that of a leading commercial product. By simultaneously addressing both the need for high‐efficiency operation as well as production cost limitations, this system can provide an approach for individuals to purify water for personal needs, which is particularly suitable for undeveloped regions with limited/no access to electricity.

Abstract100% efficiency is the ultimate goal for all energy harvesting and conversion applications. However, no energy conversion process is reported to reach this ideal limit before. Here, an example with near perfect energy conversion efficiency in the process of solar vapor generation below room temperature is reported. Remarkably, when the operational temperature of the system is below that of the surroundings (i.e., under low density solar illumination), the total vapor generation rate is higher than the upper limit that can be produced by the input solar energy because of extra energy taken from the warmer environment. Experimental results are provided to validate this intriguing strategy under 1 sun illumination. The best measured rate is ≈2.20 kg m−2 h−1 under 1 sun illumination, well beyond its corresponding upper limit of 1.68 kg m−2 h−1 and is even faster than the one reported by other systems under 2 sun illumination.

nanoporous lithography methods, [18][19][20][21] etc. However, these techniques are still expensive and complicated for the fabrication of high quality SERS substrates over large areas, thus resulting in high prices for commercial SERS substrates. Furthermore, most commercial SERS substrates can only work for individual excitation wavelengths, i.e., one particular product works at one or two excitation wavelengths only. [22][23][24][25][26][27][28] When one wants to identify anonymous trace molecules or mixed samples, multiple excitation wavelengths will be required. [29][30][31] In this case, different substrates have to be used for different wavelength excitation, which consumes more biological/chemical materials, substrates, and measurement time. This is an obvious disadvantage for conventional SERS substrates. On the contrary, the SERS EF is proportional to the product of the fi eld intensity enhancements at both excitation and Raman scattering wavelengths. It was predicted that the maximum SERS enhancement can be achieved when localized surface plasmon resonance is located between the excitation and Raman scattering wavelengths. [ 32 ] To realize higher EF, double-resonance SERS substrates were proposed to realize strong enhancements for excitation and Raman scattered signals simultaneously using expensive e-beam lithography processes. [ 23 ] Due to the narrowband absorption spectra for both resonant bands, the enhanced SERS signal is still limited within narrow spectral regions. To address this problem, broadband resonant nanostructures are highly desired. For instance, a relatively broadband 1D metal-dielectric-metal metasurface (i.e., ≈70% optical absorption from 420 to 550 nm) was fabricated using e-beam lithography to realize uniform enhancement for SERS sensing. [ 24 ] However, the top-down lithography technique imposed a signifi cant fabrication cost barrier for large-scale practical applications. In addition, 1D grating structures are polarization dependent which can only work for given polarization states (usually transverse magnetic polarization). To overcome these limitations, here we report an ultrabroadband super absorbing metasurface substrate that can enhance the SERS signal for excitation wavelengths in a broad spectral region using lithography-free processes. [ 33 ] Most frequently used excitation wavelengths for SERS (e.g., from 450 to 1100 nm [23][24][25][26][27][28]34 ] are all covered due to the broadband light trapping and fi eld concentration within deep subwavelength Most reported surface-enhanced Raman spectroscopy (SERS) substrates can work for individual excitation wavelengths only. Therefore, different substrates have to be used for different excitation wavelengths, which consumes more biological/chemical materials, substrates, and measurement time. Here, an ultrabroadband super absorbing metasurface that can work as a universal substrate for low cost and high performance SERS sensing is reported. Due to broadband light trapping and localized fi eld enhancement, this structure can...

Optical field can be concentrated into deep-subwavelength volumes and realize significant localized-field enhancement (so called "hot spot") using metallic nanostructures. It is generally believed that smaller gaps between metallic nanopatterns will result in stronger localized field due to optically driven free electrons coupled across the gap. However, it is challenging to squeeze light into extreme dimensions with high efficiencies mainly due to the conventional optical diffraction limit. Here we report a metamaterial super absorber structure with sub-5-nanometer gaps fabricated using atomic layer deposition processes that can trap light efficiently within these extreme volumes. Light trapping efficiencies up to 81% are experimentally demonstrated at midinfrared wavelengths. Importantly, the strong localized field supported in these nanogap super absorbing metamaterial patterns can significantly enhance light-matter interaction at the nanoscale, which will enable the development of novel on-chip energy harvesting/conversion, and surface enhanced spectroscopy techniques for bio/chemical sensing. By coating these structures with chemical/biological molecules, we successfully demonstrated that the fingerprints of molecules in the mid-infrared absorption spectroscopy is enhanced significantly with the enhancement factor up to 10 6~1 0 7 , representing a record for surface enhanced infrared absorption spectroscopy.Due to the diffraction limit of conventional optics, coupling and confinement of light into deepsubwavelength volume is usually very challenging, resulting in difficulties in exploring the lightmatter interaction within these ultra-thin (one-dimensional, 1D) or ultra-small dimensions (two-or three-dimensional, 2D or 3D). The unprecedented ability of metallic nanostructures with nanometric gaps to concentrate light has attracted significant research interest in recent years. [1,2] It has been reported that the optical field can be concentrated into deep-subwavelength volumes and realize This article is protected by copyright. All rights reserved. significant localized-field enhancement using a variety of nanoantenna structures, [3] showing promise for the development of enhanced nonlinear optics, [4] surface photocatalysis [5,6] and

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