PhD defence by Cuiling Zhang

Abstract

Photoacoustic (PA) techniques have emerged as a powerful tool for spectroscopy, biomedical imaging, and environmental sensing, enabling noninvasive retrieval of physicochemical properties through light-induced thermal or pressure waves. Photoacoustic microscopy (PAM), in particular, superiors in label-free and deep-tissue imaging of biological samples by exploiting intrinsic optical absorption contrasts. Concurrently, PA-based gas detection offers exceptional sensitivity for trace analyte monitoring. Both modalities rely on advanced laser systems capable of delivering high-power, precisely modulated optical pulses with extended light-matter interaction. However, conventional solid-core fiber lasers face limitations such as narrow bandwidth and low pulse energy.

Gas-filled anti-resonant hollow-core fibers (ARHCFs) address these challenges by combining fiber-optic portability with a hollow-core design that minimizes nonlinear distortions, enhances high-power handling capability, and extends opticalgas interaction lengths. The tunability of gas composition and pressure further enhances adaptability for spectroscopic and imaging applications. Despite these advantages, the integration of gas-filled ARHCF lasers into PA modalities, particularly for mid-infrared (MIR) and multiwavelength operation, remains underexplored. This thesis bridges this gap by developing gas-filled ARHCF lasers tailored for PA applications. We first established the systems required for gas-filled ARHCFs and PA modalities.

As a proof-of-concept, we demonstrated a MIR Raman laser (3.4 μm) using a cascaded ARHCF structure filled with nitrogen (N2) and hydrogen (H2), achieving high-energy pulses for lipid-rich myelinated brain tissue imaging. To our knowledge, this represents the MIR-PAM system employing a gasfilled ARHCF laser for the first time, addressing the limitations of current MIR lasers used for PAM with its high pulse energy, robust performance, and compact design. Additionally, we introduced a dual-pump multiwavelength laser spanning ultraviolet (UV, ~328 nm) to nearinfrared (NIR, ~2200 nm), enabling broadband chromophore imaging and precision methane (CH4) detection. We validated the practicality of the system through PA imaging of biomedical chromophores and high-sensitivity CH4 detection. These innovations highlight the transformative potential of gas-filled ARHCF lasers in PA technologies, paving the way for portable, high-performance systems in biomedical diagnostics, environmental monitoring, and industrial safety.

Supervisors

• Principal supervisor: Christos Markos, Associate professor, DTU Electro, Denmark
• Co-supervisor: Yazhou Wang, Researcher, DTU Electro, Denmark

Evaluation Board

• Examiner: Miguel Pleitez, Dr. and Group Leader, School of Medicine, TU Munich, Netherland
• Examiner:  David Novoa, Professor, University of Basque Country, Spain
  
  

Chairman

• Jesper Lægsgaard, Associate Professor, DTU Electro, DTU

Master of the Ceremony

• Lars Lindevold, Senior Researcher, DTU Electro, Denmark