Vibrational spectroscopy of bio-molecules affords a unique tool to detect chemical fingerprints of a material during the mid-infrared region, prompting it an analytical method of choice for life sciences, medicine, food safety, and biotechnologies. However, the intrinsic cross sections of some molecular vibrations for infrared spectroscopy are rather than small, which limits application of low concentration detection.
To improve the sensitivity, a surface-enhanced infrared absorption (SEIRA) spectroscopy is an engaging method for increasing the prominence of vibrational modes in the infrared spectroscopy. To nowadays, nanostructures with plasmonic resonances have been widely applied to SEIRA spectroscopy such as optical antennas, optical resonators, grating-coupled structures, and so on. These resonant structures can enhance the vibrational features about several orders of magnitude by tuning the plasmonic resonances to the vibrational bands of the molecules. On the other hand, multi-spectral interrogation of vibrational signatures provides an accurate way for determining the molecular composition of a sample in order to confirm the presence of target species, and reduces the number of false positives during bio-sensing experiments. Still, most of plasmonic structures are limited by just a single band response and hence are not suitable for multispectral sensing of different biomolecules at once.
In this research, we will propose a new route to designing a metamaterial-based multi-band infrared absorption device through our home-made genetic algorithm (GA) method in mid-infrared region. This complex structure can provide higher enhancement factor and hot spot density than that of nano-antenna; we started from the optimization process of GA combined with finite-integration-time-domain method, followed by nano-fabrication through electron beam lithography process, and the optical properties of the optimized structure were characterized in infrared region by Fourier transform infrared spectrometer. Finally, the SEIRA activity was demonstrated by spin coating PMMA thin film on the array. Compare the signal with and without an analyte, the enhancement can be achieved about 2.1 at 52 THz.

Contents
Abstract i
Acknowledgements iii
Contents iv
List of Figures vi
List of Tables x
Chapter 1 Introduction 1
1.1 Introduction 1
1.2 Thesis Organization 3
Chapter 2 Literature Review 4
2.1 Introduction to Infrared Absorption and spectroscopy 4
2.1.1 Surface Enhanced Infrared Absorption 7
2.2 Plasmonic and Metamaterial-based SEIRA substrates 9
2.3 Genetic Algorithm for Optimization 27
2.4 Motivation 37
Chapter 3 Optimizing based on Genetic Algorithm 40
3.1 Introduction to Genetic Algorithm 40
3.2 Operators of the Genetic Algorithm 42
3.2.1 The selection operator 43
3.2.2 The crossover operator 45
3.2.3 The mutation operator 48
3.3 Fitness function 49
3.4 Simulation of genetic algorithm in CST 50
3.4.1 Definition of hot spots and simulation setup 50
3.4.2 Selection, Crossover, and Mutation setup 56
Chapter 4 Experimental procedure 58
4.1 Sample fabrication 58
4.1.1 Electron Beam Lithography Process 58
4.1.2 Electron Gun Evaporation of Gold and Lift-off Process 62
4.2 Measurement 64
4.2.1 Micro-Fourier Transform Infrared Spectroscopy (μ-FTIR) 64
Chapter 5 Results and discussion 65
5.1 Simulation results 65
5.1.1 The result of the evolutionary optimization 66
5.1.2 Simplify ‘C-3-4’ 70
5.2 Fabrication and Measurement results 73
5.3 SEIRA experiment 76
Chapter 6 Conclusions and Future Prospects 78
6.1 Conclusions 78
6.2 Future Prospect 79
Appendix 80
References 100

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