PhD defence by Wenwei Lin

Abstract

Light can do much more than illuminate - it can sense the world around us. This PhD project explores how light traveling inside optical fibers can be used to detect tiny changes in the surrounding environment with extraordinary precision. By coating the surface of optical fibers with a thin indium tin oxide (ITO) layer, we developed optical sensors based on two key effects: surface plasmon resonance (SPR) and lossy mode resonance (LMR).

These sensors work by observing how light interacts with the coating and how this interaction changes when the surrounding material varies - for example, when the refractive index or liquid level changes. We first demonstrated a lossy mode resonance sensor capable of simultaneously measuring both refractive index and liquid level with remarkably high resolution. Then, we developed a mid-infrared surface plasmon resonance sensor, which uses longer wavelengths of light to probe deeper into materials. This approach greatly improves the sensitivity of fiber sensors and extends their detection capabilities into the mid-infrared “molecular fingerprint” region.

Finally, by applying this technique to the detection of microscopic molecules, we showed that mid-infrared SPR sensors can achieve up to five times higher sensitivity than previously reported designs. This will open new possibilities for real-time monitoring of gases, liquids, and biological substances with compact, low-cost fiber-optic devices. Overall, the research contributes to the development of next-generation plasmonic optical sensors that can play a crucial role in environmental monitoring, biomedical diagnostics, and industrial process control.

Supervisors

• Principal supervisor: Senior Researcher, Getinet Woyessa, Department of Electrical and Photonics Engineering, DTU
• Co-supervisor: Professor, Ole Bang, Department of Electrical and Photonics Engineering, DTU

Evaluation Board

• Examiner: Professor, Carlos Marques, Universidad de Aveiro, Portugal
• Examiner: Professor, Nunzio Cennamo, University of Campania Luigi Vanvitelli, Italy
• Chairman: Associate Professor, Christos Markos, Department of Electrical and Photonics Engineering, DTU
  
  

Master of the Ceremony

• Senior Researcher Lars Lindvold, Department of Electrical and Photonics Engineering, DTU