© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com REVIEW Micro- and Nanostructured Surfaces for Selective Solar Absorption Iryna E. Khodasevych,* Liping Wang, Arnan Mitchell, and Gary Rosengarten DOI: 10.1002/adom.201500063 1. Introduction Applications utilizing solar energy are being actively developed due to increasing demand for environmentally friendly and sustainable energy solutions. The sun is a source of tremen- dous energy spanning from ultraviolet to infrared wavelengths, with the maximum energy in the visible range, corresponding to a blackbody temperature of 5780 K. [1] It is estimated that the amount of solar energy reaching Earth's surface is enough to Efficient absorption of solar radiation is desired for the renewable energy sector, such as solar thermophotovoltaics and solar thermal applications. In order to minimize thermal re-radiation, wavelength-selective devices are required. Absorbers with structured surfaces are attractive because they derive their electromagnetic properties to a greater extent from their geometry and to a lesser extent from the intrinsic properties of the constituent mate- rials. Thus, they offer greater flexibility in the design and control of absorber features and can be tailored to suit requirements. This article reviews various classes of patterned structures: photonic crystals, metal–dielectric–metal slab arrays, metamaterials, and nanostructures operating in the visible and infrared wavelength ranges. Operation requirements, design principles and under- lying physical phenomena, material and temperature considerations, as well as fabrication methods are discussed. Recent progress in achieving various desirable absorber features, such as broadband and multiband operation, polarization and angle independence, flexibility, and tunability is presented. Suggestions are also given regarding future research directions. Dr. I. E. Khodasevych, Prof. A. Mitchell School of Electrical and Computer Engineering ARC Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS) RMIT University GPO Box 2476, Melbourne, VIC 3001, Australia E-mail: iryna.khodasevych@rmit.edu.au Dr. L. Wang School for Engineering of Matter Transport and Energy Arizona State University Tempe, Arizona 85287, USA Prof. G. Rosengarten School of Aerospace Mechanical and Manufacturing Engineering RMIT University GPO Box 2476, Melbourne, VIC 3001, Australia satisfy world's current and future energy demands. [2] Also, this supply is practi- cally endless. Solar energy has been used for heating and cooling of air and water in domestic as well as industrial applica- tions, electricity generation and chemical reactions. However, there are challenges toward efficiently extracting this energy and converting it into other, usable forms. One of the better-known means of harnessing solar energy is photovoltaics ( Figure 1a), where solar radiation is con- verted into electricity with the help of various semiconducting materials, exhib- iting the photovoltaic effect and referred to as solar or photovoltaic cells. [3–5] Expo- sure of the solar cell to light triggers the generation of electron–hole pairs, car- rying an electric current, thus the system is dependent on the band structure of the corresponding material. Only a narrow spectral range of light waves with suitable energies are used in the conversion process, while the majority of the solar spectrum is left unutilized, resulting in a low photon-to-electron conversion efficiency. The light incident on a photovoltaic cell, however, is not required to originate from the sun. In the field of thermo- photovoltaics (TPV) (Figure 1b), a thermal emitter heated to a high temperature serves as a source of radiation. [6,7] Emitters are typically heated to temperatures in the range of thousands of degrees Celsius, so that they emit most of the radiation in the infrared, where the corresponding energy bands of semi- conductors, used in photovoltaic cells, are located. Solar TPVs (Figure 1c) use solar energy to heat the emitter and take advan- tage of concentrators that focus the radiation collected over a larger area onto a much smaller photovoltaic system, thus reducing the material costs. Such systems allow the conversion of energy stored in wide solar spectrum into the narrow range of frequencies suitable for photovoltaic cell operation, thus increasing the overall efficiency. [8] Solar thermal systems (Figure 1d) use solar radiation to create heat, which can be used directly for applications including industrial processes or absorption cooling. [9] Such systems can also be harnessed for electricity generation through the use of turbines. Ideally, solar thermal receivers will absorb strongly in the visible and NIR regions to extract the optimal energy from solar radiation, but will have suppressed emis- sivity in the mid to far infrared so that, when the solar radiation is converted into heat, this heat is trapped within the absorber Adv. Optical Mater. 2015, DOI: 10.1002/adom.201500063 www.MaterialsViews.com www.advopticalmat.de