Solar-energy conversion usually takes one of two forms: the 'quantum' approach, which uses the large per-photon energy of solar radiation to excite electrons, as in photovoltaic cells, or the 'thermal' approach, which uses concentrated sunlight as a thermal-energy source to indirectly produce electricity using a heat engine. Here we present a new concept for solar electricity generation, photon-enhanced thermionic emission, which combines quantum and thermal mechanisms into a single physical process. The device is based on thermionic emission of photoexcited electrons from a semiconductor cathode at high temperature. Temperature-dependent photoemission-yield measurements from GaN show strong evidence for photon-enhanced thermionic emission, and calculated efficiencies for idealized devices can exceed the theoretical limits of single-junction photovoltaic cells. The proposed solar converter would operate at temperatures exceeding 200 degrees C, enabling its waste heat to be used to power a secondary thermal engine, boosting theoretical combined conversion efficiencies above 50%.

http://absimage.aps.org/image/MAR10/MWS_MAR10-2009-006451.pdf

Photon Enhanced Thermionic Emission for Solar Concentrator Systems

Producing affordable freshwater has been considered as a great societal challenge, and most conventional desalination technologies are usually accompanied with large energy consumption and thus struggle with the trade-off between water and energy, i.e., the water-energy nexus. In recent decades, the fast development of state-of-the-art photothermal materials has injected new vitality into the field of freshwater production, which can effectively harness abundant and clean solar energy via the photothermal effect to fulfill the blue dream of low-energy water purification/harvesting, so as to reconcile the water-energy nexus. Driven by the opportunities offered by photothermal materials, tremendous effort has been made to exploit diverse photothermal-assisted water purification/harvesting technologies. At this stage, it is imperative and important to review the recent progress and shed light on the future trend in this multidisciplinary field. Here, a brief introduction of the fundamental mechanism and design principle of photothermal materials is presented, and the emerging photothermal applications such as photothermal-assisted water evaporation, photothermal-assisted membrane distillation, photothermal-assisted crude oil cleanup, photothermal-enhanced photocatalysis, and photothermal-assisted water harvesting from air are summarized. Finally, the unsolved challenges and future perspectives in this field are emphasized. It is envisioned that this work will help arouse future research efforts to boost the development of solar-driven low-energy water purification/harvesting.

/pdf/harnessing-solar-driven-photothermal-effect-toward-the-water-38vhngvh2u.pdf

Harnessing Solar-Driven Photothermal Effect toward the Water-Energy Nexus.

Using broadband solar energy for producing clean water can potentially and effectively solve the water pollution and shortage crisis. With the rapid development of material science and nanotechnology, solar steam generation (SSG), based on photothermal nanomaterials, is expanding by leaps and bounds. This review comprehensively covers the state-of-the-art designs and applications of SSG. In this review, the photothermal effect, water supply, and thermal management are proposed as the three keys for the high-efficiency SSG system. Various kinds of photothermal materials with strong optical absorption covering the broad solar spectrum are classified and discussed with examples. The rational design of the water supply and steam escape system enabling the SSG to proceed smoothly is reviewed. Current efforts to minimize the heat loss by rational thermal management are also presented. As follows, typical applications, such as desalination of seawater, purification of wastewater, photothermal steam sterilization, as well as related applications including light-driven thermoelectric system and photo-heat-catalyst are overviewed. At the end of this review, the remaining challenges, as well as opportunities to be seized, are raised for consideration.

Solar steam generation based on the photothermal effect: from designs to applications, and beyond

Recent advances in nanofabrication technologies have spurred many breakthroughs in the field of photonic metamaterials that provide efficient ways of manipulating light-matter interaction at subwavelength scales. As one of the most important applications, photonic metamaterials can be used to implement novel optical absorbers. First the morphology engineering of various photonic metamaterial absorbers is discussed, which is highly associated with impendence matching conditions and resonance modes of the absorbers, thus directly determines their absorption efficiency, operational bandwidth, incident angle, and polarization dependence. Then, the recent achievements of various interdisciplinary applications based on photonic metamaterial absorbers, including structural color generation, ultrasensitive optical sensing, solar steam generation, and highly responsive photodetection, are reviewed. This report is expected to provide an overview and vision for the future development of photonic metamaterial absorbers and their applications in novel nanophotonic systems.

Photonic Metamaterial Absorbers: Morphology Engineering and Interdisciplinary Applications.

Carbon-based absorbers for solar evaporation: Steam generation and beyond

Thermoplasmonic and Photothermal Metamaterials for Solar Energy Applications

The coupling of the surface plasmon near-field into the sensing medium is key to the sensitivity of surface plasmon-based sensing devices. A low-index dielectric is necessary for the sensing medium to support a highly-penetrating surface plasmon evanescent field that extends well into the dielectric medium. The air-like refractive index, n, of an aerogel substrate provides another dimension for ultralow-index plasmonic devices. In this paper, we experimentally observed an angular surface plasmon resonance dip at 74° with the ultralow-index aerogel substrate, as was expected from theory. We also demonstrated the comparatively high-sensitivity surface plasmon resonance wavelength, λ, while the change in Δλ/Δn with different substrates was studied in detail. A 740 nm-period metal grating was imprinted on aerogel (n = 1.08) and polydimethylsiloxane (PDMS; n = 1.4) substrates. The ultraviolet–visible–near-infrared spectra were observed in the reflection mode on the grating, resulting in sensitivities of 740.2 and 655.9 nm/RIU for the aerogel and PDMS substrates, respectively. Numerical simulations were performed to understand the near-field of the surface plasmon, which demonstrated resonances well correlated with the experimentally observed results. The near-field due to excitation of the surface plasmon polaritons is observed to be more confined and to penetrate deeper into the sensing medium when a low-index substrate is used.

/pdf/air-like-plasmonics-with-ultralow-refractive-index-silica-4o2af96744.pdf

Air-like plasmonics with ultralow-refractive-index silica aerogels.

Surface plasmon-assisted fluorescence and imaging from Rhodamine 6G dye molecules in polymethyl methacrylate (PMMA) thin films deposited on gold-coated corrugated gratings of various periodicities fabricated by laser interference lithography were investigated. The fluorescence enhancement depends significantly on the periodicity of the corrugated gratings for the green light excitation centered at 548 nm with maximum enhancements at 500 and 1000 nm grating periods. A dielectric PMMA spacer layer placed between the fluorophore layer and the metal surface resulted in a further three-fold enhancement of fluorescence. The period dependent enhancement is caused by the efficient coupling of the excitation radiation to the gold gratings due to the resonant excitation of propagating surface plasmons, which assist the excitation of the fluorophores. Fluorescence imaging shows that the emitted signal is stronger from the top of the corrugated grating surface rather than from that of the valleys. It was ascertained, through surface plasmon resonant dispersion measurements and electromagnetic computations, that enhanced absorption of excitation light through propagating plasmon resonances is the prime contributor to the enhanced fluorescence.

Surface enhanced fluorescence and imaging with plasmon near-fields in gold corrugated gratings

In this study, a plasmonically active substrate is developed with the aim of controlling the perfect absorption and manipulating its optical properties for application in SERS (in NIR regime) and colorimetry. Based on modified fabrication method of anodized aluminum oxide (AAO), the cost-effective self-aggregation technique is presented to fabricate unique topography of bone-fire-like funnel-shaped collapsed and vertically aligned nanowire structures. The length of the nanowire and the modification of surface topography induced by capillary force inside the nanowire are set to structural parameters, and the effect of their changes is closely studied. After deposition of 40 nm gold (Au) film on numerous AAO nanowire structures with different wire lengths and unique topography, the localized surface plasmon resonance excitation is generated, and also its application on reflection and SERS spectra have been shown quantitatively. The length of the wire and surface topography modification are identified as suitable parameters to tune the reflection/absorption (from 90%) as well as colorimetric effect. Finally, an optimized wire length of Au-coated AAO substrate in SERS sensing application with 3.92 × 105 order of enhancement of rhodamine 6G (R6G) Raman signal is demonstrated.

Controlling the Multiscale Topography of Anodized Aluminum Oxide Nanowire Structures for Surface-Enhanced Raman Scattering and Perfect Absorbers

Enhancing the Efficiency of GaSb Photovoltaic Cell Using Thin-Film Multiscale Haze and Radiative Cooling

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