Solar energy is the utmost abundantly available, clean, pollution-free, and economically viable energy source on the Earth, and the sun is the ultimate source of this solar energy. The sun will supply an unlimited amount of energy to the Earth for an unlimited time due to the abundance of hydrogen in its core. This abundance of hydrogen inside the sun's core fuses together to produce energy known as solar energy. The sun is continuously supplying solar energy without any expectation therefore, everyone who is working in this field wants to use this freely available energy (Aybar, 2006; Govorov and Richardson, 2007; Modi and Nayi, 2020). Solar energy attracts the scientific community because it is economically viable, free from pollution, and available throughout the year. Due to its inherent features, it is believed that solar energy is an alternative source of energy (Atwater and Polman, 2010; Liu et al., 2011; Pathak et al., 2014). There are a variety of technologies available throughout the globe that are being used to harness sunlight. These technologies include solar photovoltaics, solar stills, solar cookers, and solar heat collectors. In solar photovoltaics, solar energy is converted into electrical energy by using a solar cell device. A solar still is a kind of technology that is used to obtain fresh water from seawater/brackish water (Duffie and Beckman, 1991; Shukla and Modi, 2017). A solar cooker is used for cooking food by solar radiation, and a solar heat collector is used to collect the sun's radiation for distilling and heating water for kitchen and general home purposes (Riffat and Cuce, 2011; Cuce and Cuce, 2013; Modi and Modi, 2019). Several efforts have been made in the last three to four decades to design, simulate, and fabricate a variety of technologies to harness sunlight. Researchers are still working on these technologies to optimize the various parameters for better output efficiency (Park, 2015; Pathak et al., 2017).

There have been several upgrades to optimize the parameters of the said technologies for better output. The upgrades have been in terms of the miniaturization of technologies. Miniaturization means shifting the technologies from a larger scale to the nanoscale domain. In the nanoscale domain, the optical and electrical responses of materials change dramatically. These nanoscale responses of materials are utilized by research communities and industries. The key objective of all of the above-mentioned technologies is to trap more sunlight by absorption. For example, (1) in a solar cell, the higher the photon absorption inside the active material, the more electricity will be generated, and (2) in a solar still, the efficiency of the still can be enhanced by heating/vaporization of water by sunlight. At present, there is sufficient available literature discussing the optimal device design, choice of materials, and material structures for achieving better efficiency (Duffie and Beckman, 1991; Riffat and Cuce, 2011; Shukla and Modi, 2017; Modi and Modi, 2019; Modi and Nayi, 2020). However, in order to resolve the energy problem, researchers are still trying to develop innovative technologies and devices that reduce the use of conventional sources of energy. Since the Earth receives nearly 3.85 million EJ (Exa-Joule) of solar energy every year, we are motivated to think about this freely available energy and how it could beneficial for society. In order to improve the efficiency of the above-said technologies, many researchers have used the concept of plasmonic metal nanostructures in different ways for different purposes. With this nascent technology, one can increase the light-trapping capability for one's own purpose. For example, metal nanoparticles can be used in a solar still as nanofluids and will improve the output efficiency. The metal nanoparticles present in these nanofluids absorb the sunlight and produce heat. Meanwhile, in a solar cell, metal nanoparticles have been scattered onto the surface of active materials (materials that are responsible for producing current) with different concentrations and coverage factors. Under the influence of solar radiation, these metal nanoparticles improve the photon absorption capability of solar cell material (Ji et al., 2015; Pathak et al., 2016; Sharshir et al., 2017; Modi et al., 2019, 2020).

Plasmonics is a very recent and new branch of nanophotonics that has been developed over the last two decades due to its potential application in every field of science and technology. It focuses on phenomena related to the plasmon, which is a quantum of plasma oscillation like a photon, phonon, exciton, or magnon. The plasmon that generally appears in metal nanostructures is the collective oscillation of electrons, which oscillate at a certain frequency known as the plasmon frequency (Kelly et al., 2003; Le et al., 2005; Pathak and Sharma, 2016; Wang et al., 2017). When the metal nanostructures are irradiated with an incident electromagnetic field in a range of frequency matching with metal's plasmon frequency, a resonance known as surface plasmon resonance (SPR) is established (Esteban et al., 2012; Pandey et al., 2016, 2017). The main feature of SPR is that it can be tuned in a desired range of the solar spectrum by choosing different types of metal nanostructures of different size and shape. The plasmon resonances exhibited by metal nanostructures are visualized in terms of scattering, absorption, and extinction spectrum. The main thrust of this work is to study the extinction spectrum of gold metal nanostructures in two different cases that cover the regime of the solar spectrum.

Simulation of the optical response of gold nanostructures was performed by using the metal nanoparticle boundary element method (MNPBEM) Matlab-based toolbox. This is a well-known toolbox, and its details can be found in the literature (Hohenester and Trügler, 2012). The main purpose of this toolbox is to solve Maxwell's equations for different types of metal nanostructures surrounded by homogeneous dielectric environments. The MNPBEM toolbox has the advantage over the other techniques that it takes less computation time to produce an output. It discretizes the boundary between the different dielectric environments instead of the whole volume, as in the case of the DDA (Discrete Dipole Approximation) and FDTD (Finite Difference Time Domain) methods. In this article, isolated and coupled gold nanospheres are taken into account, and their optical properties are simulated in terms of the extinction spectrum. The embedding media is water, which has a constant refractive index (1.32), and the dielectric constant of gold has been taken from the literature (Palik, 1985; Draine and Flatau, 2012; Pathak et al., 2019). Here, our interest is in calculating the optical cross-sections of metal nanostructure when it is irradiated with light using the Boundary Element Method. The optical cross-sections are expressed in terms of the Poynting vector, the details of which are taken from the literature (Bohren and Huffman, 1998; Jackson, 1999). The optical cross-sections in terms of a field are expressed as

where n_b is the refractive index and n^ a normal vector to the surface ds, R is the radius vector, and ∂Ω is the volume element. E × B is the Poynting vector, which is taken from the literature (Bohren and Huffman, 1998). The nanostructure model is shown in Figure 1.

The present work focuses on the study of the extinction spectrum of isolated as well as coupled gold nanospheres. The optimized parameters regarding size, shape, and separation gap are explored using MNPBEM toolbox. The simulated extinction spectra of isolated and coupled gold nanospheres surrounded by water cover a broad range of the solar spectrum ranging from the visible to the infrared. The simulation work simply suggests that the scientific energy community should take other metals that can be coated with non-metallic materials and study their extinction or absorption behavior in a solar energy harvesting context. Special emphasis has been given to the gap parameter, which is the distance between the nanospheres. It is concluded that, for the given size, shape, and material, a separation of 3.5 nm is optimal in this simulation. This study also opens up the new idea that researchers should simulate or fabricate a variety of nanostructures and study their extinction spectra for several applications. The model presented is restricted to single and coupled nanospheres, but it can be further extended to study the extinction spectrum of coupled nanospheroids, coupled nanorods, and chains of nanoparticles.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

NP prepared the whole manuscript in terms of the idea and the simulation.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.