Strong Coupling of Plasmon and Nanocavity Modes for Dual-Band, Near-Perfect Absorbers and Ultrathin Photovoltaics

absorption of light. It is, however, possible to engineer artificial materials that are extremely thin and can absorb nearly 100% of the incident light. The most common approach to achieving near perfect absorption (NPA) of light consists of "blocking" the possibility of transmission by, for example, using a reflective surface. Under these circumstances, the amount of light absorbed is controlled by the reflectance of the material, since, in these cases, the absorption is given by A = 1 − | r | 2 . Clearly, high absorption can be obtained when there is vanishing reflectance (r = 0), which takes place under conditions of optical impedance matching. [4] In the following section, we discuss physical interpretations and mechanisms that have been employed for achieving near complete absorption of light using metallic nanostructures, including metasurfaces, as the absorptive element. This is followed by an overview of the applications of near-perfect absorption in fields such as chemical sensing, optoelectronics and photocatalysis. This review ends with an outlook. We note that near-perfect absorption can be achieved with a wide range of materials, for which excellent review papers have been previously published. [5][6][7] The Physics of Perfect Absorption Thin Film Perfect Absorbers: InterferenceAs an introduction, we first review simple thin film approaches to NPA since these play a fundamental role in understanding the underlying physics. Optical thin-film coatings are essential in optical systems today and the simplest example of a thin-film absorber consists of a stack of an optically thin metal film, a thin dielectric film and a highly reflective metallic film. Thin film coatings are widely used as anti-and high-reflection coatings, beamsplitters, and wavelength filters on lenses, windows, displays, [8] and absorbers for efficiency enhancements in photovoltaic cells, [9] as well light emitting diodes (LED) and photodetectors. [10] Their characteristics, design, and fabrication have been studied for over a century and are well-known. [4] In the next section, we will further discuss thin film absorbers where one layer is textured on the nanoscale, but thin-film systems are particularly useful as optical absorbers exhibiting distinct advantages over nanostructured optical metasurfaces. While textured surfaces require an additional level of processing, potentially requiring complex, top-down nanofabrication techniques, [11][12][13] Near-complete absorption of light has the potential to underpin advances in photodetection, advanced chemistry, coloration of materials, and energy. This review paper reports recent progress on the development of metasurfaces and thin film structures that produce strong absorption bands in the visible and longer wavelength regions of the electromagnetic spectrum, due in part to the excitation of plasmonic resonances. Proof-of-concept demonstrations are discussed for applications of these in chemical sensing, the generation of structural color, the creation of optoelectronic devices, and p...

Section: Applicationsmentioning

confidence: 75%

Plasmonic Near‐Complete Optical Absorption and Its Applications

Wesemann

Panchenko

et al. 2019

“…As already stated, a part of light trapping improvement in the semiconductor‐based devices is associated with light scattering. The introduction of a reflective mirror and a spacer below the plasmonic structure (if designed properly) could maximize the optical path length of the scattered light . This will, in principle, make a “10 nm limit” for efficient ultrathin solar cells possible .…”

Section: Light Trapping Schemesmentioning

confidence: 99%

Semiconductor Thin Film Based Metasurfaces and Metamaterials for Photovoltaic and Photoelectrochemical Water Splitting Applications

Ghobadi

Özbay

2019

throughput and large-scale compatibility. The photoactive material is generally composed of single or multiple semiconductor layers responsible for harvesting solar energy. This harvested solar irradiation can be used to generate electricity using photovoltaic (PV) devices, or it can be converted to chemical energy in the form of hydrogen which is the case for photoelectrochemical water splitting (PEC-WS). In both PV and PEC-WS applications, the ultimate goal is to establish an efficient photon capturing and electron collecting scheme to generate a huge number of photocarriers (including electron-hole pairs) with rational carrier dynamics (efficient charge generation, separation, and collection). This means that the device should be optically thick enough to generate high carrier's density and electrically thin enough to collect photogenerated carrier before they recombine. When light impinges a semiconductor surface, a part of the light is reflected back due to the mismatch between the air and the semiconductor layer refractive index. The rest will propagate within the bulk until it is fully absorbed by the semiconductor slab. The optical penetration depth (LPD) is defined as the depth at which the incident power falls to 1/e (≈37%) of its input value. This parameter differs from one semiconductor to another and it depends on the extinction coefficient (κ) of the In both photovoltaic (PV) and photoelectrochemical water splitting (PEC-WS) solar conversion devices, the ultimate aim is to design highly efficient, low cost, and large-scale compatible cells. To achieve this goal, the main step is the efficient coupling of light into active layer. This can be obtained in bulkysemiconductor-based designs where the active layer thickness is larger than light penetration depth. However, most low-bandgap semiconductors have a carrier diffusion length much smaller than the light penetration depth. Thus, photogenerated electron-hole pairs will recombine within the semiconductor bulk. Therefore, an efficient design should fully harvest light in dimensions in the order of the carriers' diffusion length to maximize their collection probability. For this aim, in recent years, many studies based on metasurfaces and metamaterials were conducted to obtain broadband and near-unity light absorption in subwavelength ultrathin semiconductor thicknesses. This review summarizes these strategies in five main categories: light trapping based on i) strong interference in planar multilayer cavities, ii) metal nanounits, iii) dielectric units, iv) designed semiconductor units, and v) trapping scaffolds. The review highlights recent studies in which an ultrathin active layer has been coupled to the above-mentioned trapping schemes to maximize the cell optical performance.

“…A black body that absorbs broadband omnidirectional radiation is an ideal absorber, which has applications in energy harvesting, photodetectors, photothermal energy generation, and concealment . Film‐based absorbers, such as multiple pairs of stacked metal/dielectric films or multilayer‐stacked films, are suitable for the large‐area fabrication.…”

Section: Performance and Fabrication Characteristics Of The State‐of‐mentioning

confidence: 99%

“…However, the time‐consuming chemical vapor deposition preparation process (>6 coating processes) prevents device integration and industrial applications. Meanwhile, ultrathin metasurface absorbers have been implemented using gratings, nanoholes, nanowires, nanoparticles, nanopatches, tapered hyperbolic arrays, and nanocylinders . Although some of the abovementioned absorbers achieve omnidirectional, broadband performance, despite their sophisticated designs, most of these devices only work in a narrow bandwidth range and require complicated fabrication procedures.…”

Section: Performance and Fabrication Characteristics Of The State‐of‐mentioning

confidence: 99%

Large‐Area, Ultrathin Metasurface Exhibiting Strong Unpolarized Ultrabroadband Absorption

Yan

Jiang

et al. 2019

despite their sophisticated designs, most of these devices only work in a narrow bandwidth range and require complicated fabrication procedures. These issues impede large-area applications for energy harvesting and photothermal energy generation. A large-area, wide angular tolerance, broadband absorber with ultrathin metallic nanoparticles has been recently proposed. [38] However, the suitability of this device for photothermal energy generation applications remains unclear because the stability of these metallic nanostructures at high temperatures is usually low.Due to the outstanding properties, such as compatibility with the complementary metal oxide semiconductor (CMOS) processes, high temperature resistance, large loss in the visible range, and high reflectivity in the infrared (IR) range, [39] titanium nitride (TiN) has emerged as an ideal material for the preparation of near-perfect absorbers for the high-temperature solar/thermophotovoltaics applications. [39,40] A 5-cycle TiN/silicon dioxide (SiO 2 ) multilayer-based absorber has been reported with an average solar absorptivity of 0.68 [1] and a near-perfect absorption, which requires further enhancement. The high average absorptivity of ≈95% by a ringlike TiN layer using a metamaterial absorber was achieved for the entire visible range. [41] However, the bandwidth of these devices is limited in the visible range. For ultrabroadband light absorption, 3D-truncated TiN nanopillars have been demonstrated for the visible and NIR spectral regions with an average absorptivity of 0.94. [42] This TiN nanopillar-based device, which comprises silicon nanopillars with a large thickness (>1 µm), typically requires expensive, time-consuming fabrication processes, and its thermal emissivity is not verified. Therefore, there are few experimental reports on the large-area ultrathin TiN-based metasurfaces for the strong ultrabroadband light absorption with low thermal emissivity.In this paper, we demonstrate wide-angle ultrabroadband strong light absorption that covers almost the whole solar spectrum (250-2250 nm) by the TiN-based metasurface with an ultrathin thickness (330 nm) on a quartz substrate. The metasurface incorporates dielectric cylinder arrays, a TiN layer, and SiN x layers. This metasurface fundamentally differs from the previously reported absorbers. This device, which uses a symmetrical SiO 2 grating, is independent of incident polarization. Specifically, the carefully designed metasurface exhibits a prominent absorption of both polarizations over wide angles. The outstanding performances of this absorber are realized by Large-area, ultrathin devices with strong ultrabroadband absorption and high angular tolerance are in demand for applications such as ultraviolet protection, energy harvesting, photodetectors, and thermal imaging technology. Although there have been considerable efforts to design and fabricate these devices, the simultaneous realization of all desired properties has not been achieved. A 330 nm thick metasurface with an area of 3 cm 2 is e...