Multifunctional metasurfaces for angular-robust operation and electrochemistry

Berger, Luca Maria (2023): Multifunctional metasurfaces for angular-robust operation and electrochemistry. Dissertation, LMU München: Fakultät für Physik

Abstract

Nanophotonics has firmly established itself as a pivotal force in the transformation and shaping of diverse sectors globally, with a pronounced impact on interdisciplinary scientific research. By effectively bridging disciplinary gaps and seamlessly integrating with other scientific fields, nanophotonics has spearheaded significant breakthroughs in information technology, medicine and healthcare, electronics, life sciences, energy, environmental science, and security, among others. As this interdisciplinary trend is expected to persist, the frontier of future research beckons at the interface between nanophotonics and emerging scientific domains or applications, holding the promise of groundbreaking discoveries yet to be unveiled. In this work, we explore light-matter interactions at the interface to electrochemistry and towards advanced sensing applications. The first part of this work investigates nanophotonic metasurfaces based on bound states in the continuum (BICs), which are localized electromagnetic modes within the continuous spectrum of radiating waves. Due to their infinite lifetimes without radiation losses, BICs are driving research directions in lasing, non-linear optical processes, and sensing. However, conventional methods for converting BICs into leaky resonances, or quasi-BICs, with high-quality factors typically rely on breaking the in-plane inversion symmetry of metasurfaces. This results in resonances that are strongly dependent on the angle of incidence, making them unsuitable for many practical applications. Here, an emerging class of BIC-driven metasurfaces is numerically analyzed and experimentally demonstrated, where the coupling to the far field is controlled by the displacement of individual resonators. In particular, both all-dielectric and metallic as well as positive and inverse displacement-mediated metasurfaces sustaining angular-robust quasi-BICs are investigated in the mid-infrared spectral region. Their optical behavior with regard to changes in the angle of incidence is investigated and experimentally shows their superior performance compared to two conventional alternatives: silicon-based tilted ellipses and cylindrical nanoholes in gold. These findings are anticipated to open exciting perspectives for bio-sensing, conformal optical devices, and photonic devices using focused light. In the second part of this work, we explore nanophotonics at the interface of electrochemistry. Electrocatalysis plays a crucial role in realizing the transition toward a zero-carbon future, driving research directions from green hydrogen generation to carbon dioxide reduction. Surface-enhanced infrared absorption spectroscopy (SEIRAS) is a suitable method for investigating electrocatalytic processes because it can monitor with chemical specificity the mechanisms of the reactions. However, it remains difficult to detect many relevant aspects of electrochemical reactions such as short-lived intermediates. Herein, an integrated nanophotonic-electrochemical SEIRAS platform is developed and experimentally realized for the in-situ investigation of molecular signal traces emerging during electrochemical experiments. A platinum nano-slot metasurface xxi featuring strongly enhanced electromagnetic near fields is implemented and spectrally targets the weak vibrational mode of adsorbed carbon monoxide at ≈2033 cm−1. Compared to conventional unstructured platinum films, we show that our nanophotonic-electrochemical platform delivers a 27-fold improvement of the experimentally detected characteristic absorption signals, enabling the detection of new species with weak signals, fast conversions, or low surface concentrations. In the final part of this work, we continue to build on our nanophotonic-electrochemical platform that pioneered a promising narrowband technology to study single vibrational modes of electrochemical interfaces during CO oxidation. However, many reactions involve several species or configurations of adsorption. These have to be monitored, ideally simultaneously, to provide a clear understanding of the underlying electrochemical processes. This requires reproducible and broadband sensing platforms. Here, we experimentally realize multi-band metasurface-driven SEIRAS for the in-situ study of electrochemical CO2 reduction on a Pt surface. By leveraging our reproducible and spectrally tunable platinum nano-slot metasurface, two CO adsorption configurations at 2030 cm-1 and 1840 cm-1 are locally enhanced as a proof of concept that can be extended to more vibrational bands. Our platform provides a 41-fold enhancement in the detection of characteristic absorption signals compared to conventional broadband electrochemically roughened platinum films. Thanks to the signal enhancement provided by our platform, we show that the CO bridge configuration on platinum does not play a significant role during CO2 reduction in an alkaline environment. By providing a deeper understanding of catalytic reactions, the nanophotonic-electrochemical platform is anticipated to open exciting perspectives for electrochemical SEIRAS, surface-enhanced Raman spectroscopy, and other fields of chemistry such as photoelectrocatalysis.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 13. Dezember 2023
• 1. Bericht­erstatter:in: Maier, Stefan A.