Plasmonic Cu nanostructures in ZnO as hyperbolic metamaterial thin films

Abstract

Plasmonic metals, such as Cu and Al, have been considered as potential low-loss alternatives for Au and Ag for photonic structures and devices. However, challenges remain in the fabrication and applications of Cu nanostructures, because of its easy oxidation issues. In this work, a new metamaterial structure of plasmonic Cu nanostructures embedded in a dielectric ZnO matrix has been designed and successfully fabricated using a one-step thin film growth method. The Cu–ZnO hybrid thin films present excellent epitaxial quality and exotic optical properties, such as strong localized surface plasmon resonance in the visible regime, and, highly anisotropic and hyperbolic optical response, revealed by angular dependent and polarization resolved reflectivity measurements. This hyperbolic plasmonic metamaterial via the metal-in-oxide matrix form combining low-loss plasmonic Cu nanostructures and extraordinary anisotropic optical properties could be used towards various nanophotonic applications, such as plasmonic solar energy devices and hyperlens.

Introduction

Metallic nanostructures (such as, Au, Ag, and Cu) have been extensively studied owing to their extraordinary optical properties throughout the UV–vis–NIR regime [1], [2], [3], [4], [5], [6]. The light interacts with the free electrons in metallic nanostructures results in collective oscillations of the conduction electrons, which leads to strong enhancement of the local electromagnetic field and hence enhanced light harvesting, so-called localized surface plasmon resonance (LSPR). The intensity, line width, and extinction maximum wavelength of the LSPR have been found to be determined by multiple factors, including size, shape, interspacing, and the surrounding environment of the metallic nanostructures [7], [8]. Furthermore, to achieve the LSPR response, the complex dielectric function with a negative real part (${\epsilon }^{\text{'}}$) and a small imaginary part (${\epsilon }^{\text{'}\text{'}}$) is required. Therefore, various metallic nanostructures with LSPR response have been developed, such as nanoparticle [9], [10], nanowire (nanorod) [11], [12], nanoprism [7], [13], and others [14], [15]. For the metallic material selection, various metals (e.g., Au, Ag, Ga, Na, Al, In, Cu, etc.) theoretically could achieve LSPR in certain wavelength regime [16]. However, difficulties emerge in the process of these metallic nanostructures because of their instability, especially the non-precious metals. For example, Cu is abundant in nature and cost-effective compared with Au and Ag. However, the surface oxidation of Cu hinders its practical application.

Embedding the metallic nanostructures in dielectric matrix has been demonstrated as a viable approach to achieve stable metallic nanostructures and unique optical properties for nanophotonic devices [19], [20], [21]. For example, Ag nanowires have been electrochemically deposited into a porous alumina template to obtain anisotropic light–matter interaction resulting in a hyperbolic dispersion relation, refers to hyperbolic metamaterials (HMMs) [19], [22]. The anisotropic optical response (metallic in one direction and dielectric in other) of HMMs can be realized by material architecture design of multilayers with alternating metallic and dielectric layers [23], [24] or metal nanowire embedded into dielectric matrix [19], [22]. Cu nanostructures have been embedded into anodic alumina oxide (AAO) [17] or solutions [18]. However, most of the metal nanowire synthesis involves the costly AAO template or tedious nanofabrication process [25], [26], which are hard to be integrated with thin film processing procedures for optical devices [20], [21], [27], [28]. Therefore, it is essential to seek such stable Cu nanostructures with desired quality and optical properties via a practical thin film integration process.

In this work, a unique metamaterial of vertically aligned Cu nanostructures embedded into dielectric ZnO matrix (Cu–ZnO) has been designed and grown by a one-step thin film deposition technique on SrTiO₃ (STO) and sapphire substrates, as illustrated in Fig. 1. The advantages of selecting this Cu–ZnO system are as follows: First, ZnO is a well-known transparent semiconductor material with a wide direct bandgap of 3.37 eV and high exciton binding energy of 60 mV, which leads to its various applications in optoelectronic devices [29], [30], [31]. Second, Cu is considered as a low-loss plasmonic metal alternative to Au and Ag. The expected vertically aligned Cu nanostructures could obtain LSPR in interface area and present anisotropic optical performance (e.g., different signs of ε’ at certain wavelength). Third, ZnO could serve as an ideal medium to hinder the oxidation of Cu nanostructures. Detailed microstructure and optical measurements have been conducted to demonstrate the high quality and extraordinary optical properties of this Cu–ZnO metamaterial. This work could present great potential to fabricate and stabilize non-noble metal nanostructures in oxide matrix as new plasmonic and hyperbolic nanostructures for future practical applications.