| Summary: | Brillouin light scattering (BLS) and time-resolved Brillouin scattering (TRBS) based imaging are new techniques used for imaging and characterizing cells. These methods enable the observation of elasticity-related contrast with optical resolution and label-free operation. Phonon microscopy is a process that can detect Brillouin light scattering (BLS) using laser-generated coherent phonon fields. This approach is particularly appealing for imaging because, at GHz frequencies, the phonon wavelength is sub-optical (approximately hundreds of nm). Previous research has shown that an axial resolution up to 10 times higher than that of the optical system used for the measurements can be achieved. However, the lateral resolution is still limited by the optical systems.
In this thesis, a way to improve the lateral resolution by surpassing the optical diffraction limit is proposed by using multiple designs of optoacoustic lenses to achieve GHz coherent phonon pulse focusing. These optoacoustic lenses are designed to generate a focusing sound field when a pump light illuminates them, and then the probe beam allows the sound field to be continuously monitored in time using TRBS. A numerical model based on the Fourier-Bessel angular spectrum method is used to simulate the distribution of the optoacoustically generated sound field, and the model suggests that a focused acoustic beam down to approximately 200 nm and an increased acoustic intensity are achievable.
A significant portion of this work is devoted to the design and nanoscale fabrication of these lenses, along with detailed simulations that explore their acoustic and optoacoustic properties. These lenses, which include flat Fresnel zone plates and concave structures, are engineered to potentially enhance the lateral resolution in TRBS systems beyond the constraints imposed by optical diffraction limits. The thesis also presents proof-of-concept experimental results that demonstrate the strong focusing effect of these lenses, a crucial step toward their application in high-resolution acoustic microscopy. While full-scale imaging has not yet been realized, these initial findings are promising, indicating the potential of these lenses to achieve superior lateral resolution in TRBS.
Furthermore, this research provides a foundation for future studies to integrate these lenses into TRBS systems for enhanced imaging capabilities, particularly in fields where non-invasive and high-precision observation is essential, such as in biological cell imaging. The thesis encapsulates not only the theoretical and experimental advancements in the field of acoustic microscopy but also lays the groundwork for further exploration and potential applications of these optoacoustic lenses in overcoming resolution barriers in TRBS.
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