Nanocomposites as Next-Generation Optical Materials

In this chapter, I give an introduction into the research fields of nanostructured optical materials and provide the key research questions around which this book is structured. These research questions are:

In this chapter, I introduce the fundamental concepts and relationships from the different fields, including electrodynamics, optical design, materials science, and diffractive optics, which are central to the following chapters. This chapter is not intended to replace a textbook but should rather serve as a reference, which provides the readers with the relevant basics of fields with which they are not familiar.

In this chapter, I use large-scale numerical simulations to investigate the transition between the homogeneous and the heterogeneous regimes. To this end, I first introduce a procedure that enables me to obtain reliable refractive index values from numerical simulations of scatterer distributions, which are composed of hundreds of thousands of individual nanoparticles. I then demonstrate that this method indeed enables me to model the regime in which the distribution of scatterers acts as a bulk optical material. This then allows me to quantify how bulk optical nanocomposites must be designed and investigate whether the Maxwell-Garnett-Mie effective medium theory (EMT) is an accurate tool for the design of novel nanocomposite materials. Finally, I also show that the concept of an effective refractive index breaks down on multiple level as a material transitions from the homogeneous into the heterogeneous regime.

In the previous chapter, I have shown that nanocomposites can be used as bulk optical materials. However, this is only possible in the homogeneous regime, which, for applications in the visible spectral range, is reached for particle sizes below 4 nm. The main degrees of freedom that remain available for the design of optical nanocomposites are hence only the constituent materials (host and nanoparticles) and their respective volume fractions. Therefore, the key question is whether significant benefits over conventional materials can be achieved with these degrees of freedom. To answer this question, I, in this chapter, investigate what range of optical properties can be achieved with nanocomposites in the homogeneous regime. Since I have already shown that the Maxwell-Garnett-Mie effective medium theory (EMT) is an accurate tool for the design of nanocomposites in the homogeneous regime, I first use this EMT to investigate the general potential of optical nanocomposites for a wide range of different materials. Subsequently, I present experimental data for specific materials and optical components to confirm these general findings.

The integration of diffractive optical elements (DOEs) into a broadband optical system can often allow for increasing the system’s performance, reducing its size, or its complexity. However, despite considerable efforts to develop different technologies for DOEs, they still remain highly underutilized in broadband imaging system. This is because DOEs that maintain high diffraction efficiencies across the full range of wavelengths, angles of incidence (AOIs), and grating periods required for different optical systems are currently not available. Since the wavelength dependence of the efficiency is fundamentally linked to the dispersion of the phase delay \((\phi (\lambda )\)), this leads to the question of whether the dispersion engineering capabilities of nanocomposites could make such materials an enabling technology for finally unlocking the full potential of DOEs for optical design. In this chapter, I address this question as my first advanced application for nanocomposites. At the same time, my second goal in this chapter is to not restrict myself to one material platform and embodiment of DOEs, but also develop general concepts for how DOEs for broadband systems can be designed.

Nanocomposites allow for controlling the magnitude and dispersion of the effective refractive index within broad regions. Exploiting this tunability offers a high potential for improving the performance or reducing the size of optical systems. This is because the different monochromatic and chromatic aberrations in optical systems depend critically on the materials’ properties. In the previous chapter, I have shown that a key part of this potential is that nanocomposites allow for the design of highly efficient DOEs. However, the full potential of dispersion-engineered nanocomposites goes beyond this application. In fact, I have already demonstrated that the high refractive index of nanocomposites is highly useful for reducing spherical aberration. In this chapter, I now focus on using nanocomposites, or novel dispersion-engineered materials in general, for correcting chromatic aberrations, that is, the design of achromatic optical systems. My goal for this chapter is to advanced concepts on how such materials can be used. I emphasize that evaluating the full potential of dispersion-engineered materials is only possible through systematic optical design studies for different systems. As one promising application for novel dispersion-engineered materials, I investigate the potential of such materials for enabling a new generation of smartphone cameras in the final section of this chapter.

Throughout this book, I have investigated the fundamental properties of optical nanocomposites and their potential as next-generation optical materials. To summarize how my findings bridge different gaps between fundamental research and practice, I, in this chapter, address each of the four main questions that I set out to answer in the introduction. In addition, also provide my perspective on how the challenges that will inevitably arise when the concepts I developed in this book are brought closer to commercialization can be overcome. Finally, I summarize some promising directions for future research.