Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat The effect of the refractive index profile on the optical response of plasmonic nanostructures inside semiconductors Zachary T. Rex, Marcel Di Vece * Interdisciplinary Centre for Nanostructured Materials and Interfaces (CIMaINa) and Physics Department “Aldo Pontremoli”, University of Milan, Via Celoria 16, 20133, Milan, Italy ARTICLE INFO Keywords: Semiconductors Plasmonic nanostructures Solar cells FDTD simulations ABSTRACT The inclusion of plasmonic nanostructures inside semiconductors has been explored over the last decades with the prominent application of increasing the efficiency in solar cells. The plasmonic properties were often only phenomenologically explained by comparison of experiments with simulations. In this work we investigated the plasmonic properties of a silver particle inside virtual semiconductors with varying band gaps. Because the particle plasmon peaks strictly followed the virtual semiconductor band gap, it was found that only the maxima in the real part of the refractive index were responsible for the presence of particle plasmon peaks. This model, in which the plasmonic response is to a large extent determined by the semiconductor refractive index profile, explains previous investigations of plasmonic nanostructures inside semiconductors in a simple way and will enable a detailed design and explanation of future nanostructured plasmonic systems for solar cells and other devices. 1. Introduction The study of metal nanoparticles inside media has been investigated extensively over the past decades and resulted in a wealth of informa- tion about their optical properties, particularly the plasmon resonance shift as a function of the refractive index [1]. Much of the fundamental work has been performed on metal nanoparticles embedded in large band gap dielectrics because it is technically much more difficult to investigate the optical properties of metal nanoparticles inside semi- conductors, as they absorb much more light. However, because metal nanoparticles have been investigated with respect to light management nanostructures for solar cells [2–7], the optical behaviour of such na- noparticles inside semiconductors, especially the plasmon resonance energy and amplitude, are very important. Many studies explored the inclusion of plasmonic nanoparticles inside solar cell semiconductors, often resulting in efficiency increases, and compared their experiments with simulations. Using the bottom-up metal nanoparticle is only one version of plasmonic light management techniques in solar cells, as for example lithographic methods provided more complicated structures to enhance the light conversion efficiency [8–10]. The composition [11], size and shape [12,13] of plasmonic nanostructures affects their plasmon resonance properties, which for gold and silver lay in the visible range of the optical spectrum, while for aluminium and platinum this is located in the ultra violet [14]. The plasmon resonance of a metal nanostructure can cause increased scattering [15] and local field en- hancement [16–18]. To make optimal use of these properties the plasmon resonance energy needs to be tuned with respect to the band gap of the semiconductor. For example if the semiconductor absorbs sufficiently in the blue but only very little in the red, positioning the plasmon resonance energy in the red could be advantageous for a solar cell. Despite the parasitic conversion of light energy into heat, a careful design can result in improved solar cell performance [19]. A good ex- ample of tuning the plasmon resonance energy is by making plasmonic nanoparticles couple, resulting in a plasmon resonance wavelength red shift [13,20]. The exact mechanism of the plasmon resonance property changes when including them in a semiconductor, remains to be clarified as the explanations with respect to the plasmonic properties were often phe- nomenological. It is therefore important to develop a general theory about the optical properties of the plasmonic nanostructure interacting with the semiconductor. Embedding plasmonic nanostructures inside a dielectric medium has been studied with typically plasmon resonance energy shifts of about 0.2 eV [1]. However, often the refractive index has been assumed constant, which is a good approximation for large band gap dielectrics, but not for commonly used semiconductors. It is therefore very im- portant, not in the least for future nanostructured plasmonic designs, to elucidate the relation between the plasmonic nanostructure and the https://doi.org/10.1016/j.optmat.2019.109314 Received 18 June 2019; Received in revised form 1 August 2019; Accepted 11 August 2019 * Corresponding author. E-mail address: marcel.divece@unimi.it (M. Di Vece). Optical Materials 96 (2019) 109314 0925-3467/ © 2019 Published by Elsevier B.V. T