Quantitative Analysis of Thermomodulated Plasmonic Materials and Multilayers with The Finite-Difference Time-Domain Method

Abstract

Plasmonic and photonic devices are important research areas that have led to breakthroughs in various fields, such as biology, chemistry, material science, and energy applications. However, accurate characterization of plasmonic devices is challenging, as plasmonic effects occur in the near field (NF) and do not propagate into the far field (FF). Currently, there are limited experimental techniques capable of characterizing the optical NF. Near-field scanning optical microscope (NSOM) techniques have probe-sample interaction issues, while photoemission electron microscopy (PEEM) and electron energy loss spectroscopy (EELS) lack established theories for a clear understanding of plasmonic excitations mappings compared to purely optical measurements. Hybrid near field spectroscopic methods like the newly developed Scattering-type Scanning Near field Optical Microscopy (s-SNOM) and Atomic Force Microscopy-based Infrared spectroscopy (AFM-IR) are promising for characterization of plasmonic structures but physically is very similar to NSOM, with a scanning tip, therefore inherently has probe-sample interaction issues.

Computational electromagnetics (CEM) offers a more complete and intuitive method for characterizing plasmonic devices and is frequently used in plasmonic studies. However, although CEM is commonly used for these studies, it has generally failed to achieve its true potential in quantitative characterization.

Spectroscopic ellipsometry (SE) is an optical measurement method that is fast, non-invasive, self-referencing, and has sub-monolayer lateral sensitivity. It is commonly used for determining the optical properties and film thickness of thin films. Due to SE’s unique capabilities for in-situ, non-destructive, sensitive characterization of thin film, SE and related scatterometry techniques have been exploited in the semiconductor industry for precise optical critical dimension (OCD) measurement. However, SE is an indirect measurement technique that requires modeling to retrieve useful physical quantities, such as optical properties, and structural parameters like grating height, pitch, and period. The modeling of OCD measurements in the semiconductor industry is typically done using rigorous coupled-wave analysis (RCWA), as it is efficient for 1D periodic structures.

Using SE, a sensitive FF optical measurement technique, in combination with a CEM such as the finite-difference time-domain method (FDTD), provides a way to quantitatively verify that the NF information provided by FDTD is reliable. Chapter 3 of this work will attempt to characterize the precision and efficiency of FDTD based on this synergistic FDTD-SE method. Meanwhile, Chapter 4 will demonstrate the versatility of the FDTD-SE method on a temperature-dependent plasmonic grating, showing that FDTD-SE is indeed capable of quantitatively characterizing a periodic plasmonic nanostructure.

While SE measurements can be modeled by many CEMs, FDTD’s flexibility and advantages make it a suitable choice for the modeling of plasmonic materials. Firstly, FDTD, being fully vectorial and with simple stability conditions, ensures accurate and reliable simulations. Convergence is almost always guaranteed in FDTD if the stability conditions are met. FDTD is also versatile, capable of handling both non-periodic and periodic structures. One unique feature of FDTD is the ability to obtain broad frequency results from a single simulation, due to its time-domain nature. This also allows for the probing of transient phenomena, which is an important feature that can provide crucial insight in plasmonics. Most importantly, FDTD can handle complex material properties, including anisotropic, dispersive, nonlinear, non-instantaneous or even time-varying metamaterials, further broadening its applicability when required. Moreover, FDTD is well-suited for parallel computing, ensuring that large simulations can be solved if adequate computational resources are assigned, while also having the potential to tap into future technological improvements in parallel computing.

In chapter 5, to explore the quantitative accuracy and practical application of FDTD, particularly in a multi-physics setting, I will demonstrate quantitative accuracy of FDTD using heat transfer analysis of laser-induced heating in optical multilayered thin films, in combination with heat transfer FEM solver. While in chapter 6, I will further apply the quantitative accuracy of FDTD to study the spectral and directional thermal emission of optical multilayered thin films in comparison to emerging theoretical framework on thermal emissions.

Date of Award	8 Nov 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Juan Antonio ZAPIEN (Supervisor)