Robert Warmbier

Optical and other material properties of SiO2 from ab initio studies

Abstract. The optical properties of photonic devices largely depend on the dielectric properties of the underlying materials. We apply modern ab initio methods to study crystalline SiO2 phases, which serve as toy models for amorphous glass. We discuss the dielectric response from the infrared to the VIS/UV, which is crucial for glass based photonic applications. Low density silica, like cristobalite, may provide a good basis for high transmission optical devices.

Ab initio determination of basic dielectric properties

Simulating the performance of fibre optical devices or photonic devices requires an accurate solution of Maxwells equations. However, the resulting predictions often depend quite crucially on the knowledge of the basic dielectric constants within certain frequency ranges, where experimental data could be very inaccurate or simply not available. We illustrate the theoretical and numerical background to work out frequency dependent dielectric constants using modern ab initio methods based on density functional theory. The starting point will be a rough picture of the atomic structure of a given dielectric component. We discuss various technical aspects to determine the electronic and ionic parts of the dielectric constant for a given frequency range, and we give some illustrative examples.

About the Implementation of Frequency Conversion Processes in Solar Cell Device Simulations

Solar cells are electrical devices that can directly convert sunlight into electricity. While solar cells are a mature technology, their efficiencies are still far below the theoretical limit. The major losses in a typical semiconductor solar cell are due to the thermalization of electrons in the UV and visible range of the solar spectrum, the inability of a solar cell to absorb photons with energies below the electronic band gap, and losses due to the recombination of electrons and holes, which mainly occur at the contacts. These prevent the realization of the theoretical efficiency limit of 85% for a generic photovoltaic device. A promising strategy to harness light with minimum thermal losses outside the typical frequency range of a single junction solar cell could be frequency conversion using rare earth ions, as suggested by Trupke. In this work, we discuss the modelling of generic frequency conversion processes in the context of solar cell device simulations, which can be used to supplement experimental studies. In the spirit of a proof-of-concept study, we limit the discussion to up-conversion and restrict ourselves to a simple rare earth model system, together with a basic diode model for a crystalline silicon solar cell. The results of this show that these simulations are very useful for the development of new types of highly efficient solar cells.

Computational plasmonics with applications to bulk and nanosized systems

We present the main features of first principles numerical methods to describe plasmonic excitations in bulk and nanosized materials, and we apply these methods to a number of bulk and lower-dimensional nanosystems. Our main focus lies on graphene, which is an interesting numerical and experimental paradigm to study plasmonic excitations in a nanosystem with anisotropic and lossy dielectric functions. Beyond graphene we also discuss plasmonic excitations in similar two-dimensional nanosystems. In order to analyse more complex collective excitations of the electron gas in nanosystems, we take advantage of a fundamental relation between density fluctuations and the electron energy loss spectra (EELS), and suggest a general method to study noise in nanosystems.

Solar cell device simulations

Device simulations are a crucial part of the development of novel and more efficient types of solar cells. We give a brief introduction to the theoretical and numerical background of solar cell device simulations, and show that most of the key parameters may be taken from ab initio numerical data, rather than experimental data. We also discuss a simple strategy to implement up- and down-conversion layers into solar cell device simulations.

Computational Plasmonics: Numerical Techniques

This is the second Chapter in which we give a detailed introduction into the field of computational plasmonics. While Chap. 11 covered the theoretical background of modern plasmonics, this Chapter provides describtions of the numerical methods involved in computational plasmonics. To this end we use modern ab initio methods, the standard frequency-domain and time-domain methods of computational electromagnetics. Finally we show some applications in the fields of photovoltaics and plasmonic–photonic crystals and close with a discussion of open problems.

Computational Plasmonics: Theory and Applications

In two chapters we will give a detailed introduction into the field of computational plasmonics. The present chapter covers the essential theoretical background of modern plasmonics, based on simple models of light-matter interactions. We will focus on the physical properties of bulk plasmons, surface plasmon polaritons and localized plasmons, and give a number of analytical and numerical examples. As a motivation for the more detailed numerical studies described in Chap. 12, and as an example for new types of technological applications, we also present the field of plasmon enhanced solar cells and other exciting new research directions.

Double-layer capacitance for a charged surface

Supercapacitors are a key technology for the energy storage requirements of future energy systems. A primary problem of supercapacitors is their limited energy density, but new electrode materials and electrode designs might help to overcome this limitation. Numerical modelling can be a valuable tool in this challenge, although realistic ab initio calculations are usually very cumbersome. In this work, we show that electric double-layer capacitors can be modelled to good accuracy by a coarse-grained sampling of the electrolyte’s configuration space rather than using full molecular dynamics simulations. This saves considerable computation time, and it allows for processing more materials and more complicated systems with modest computational effort.

Photovoltaics from first principles

It has been realized in recent years that the efficiency limits for single-junction solar cells can easily be overcome by a number of new design elements, involving plasmonic photonic crystals, plasmonic nanoparticles and up/down-conversion layers. We present a numerical method to analyze the formation of electron-hole pairs in these new types of solar cells, based on density functional theory and the Bethe-Salpeter equation. After discussing the main results of a typical numerical study, we suggest possible extensions and their implementation in typical device modeling programs.

About plasmons and plasmonics in graphene

A number of recent experimental and theoretical studies indicate the existence of THz plasmons in graphene. It is also suggested that the existence of such plasmons should be a generic property of ideal graphene, allowing for the generation of surface plasmons, which are the basis of modern plasmonics technologies. We have approached the problem from a numerical side, using ab initio many-electron simulation methods based on density functional theory. We will discuss the general characterization of plasmonic excitations and surface plasmons using such methods, and show results for ideal and defective graphene, which might shed some light on what is actually detected in such systems.