Soorani, Mitra
(2018)
Needle-based ultrasonic diagnostic tool.
MPhil thesis, University of Nottingham.
Abstract
Designing extremely small sized photoacoustic transducers can be very important in medical applications to perform in vivo imaging or to measure elastic properties of tissue that is hard to access externally, for example deep-seated tissue or tissue that is in very small and narrow cavities. The medical applications range from in vivo biopsy, in order to prevent of unnecessary biopsies, to the monitoring of cancer therapy, to investigation of the progression of disease in vivo as well as microsurgical guidance. Recently, a three-layered thin-film Fabry-Perot Interferometer (FPI) based structure has been used as a transducer to generate acoustic waves and launch them into cells under study. Brillouin oscillations in the cells were measured in the gigahertz (GHz) range which allowed high resolution imaging of living cells. The approach used in this study demonstrated the potential of providing the new insights in cell biology and it offers advantages over other mechanical imaging techniques in terms of speed, signal amplitude, and resolution. In this thesis, a method for extending this technique is described, where small FPI transducer (sub-mm) is fabricated on the tip of a single mode fibre to produce a wide range of mechanical frequencies with the focus on biomedical applications. The FPI comprises of an optical cavity or spacer sandwiched between two parallel partially reflecting mirrors. In the current study, a novel pump-probe technique is used for the generation and detection of acoustic waves via the same transducer, while two separate transducers are currently used in other studies; one for generation and one for detection of acoustic waves.
In this study, the Brillouin frequencies of the transducers are generated and measured in reflection using a single optical fibre. An opto-thermo-mechanically coupled model was used to predict the behaviour of the transducer when interacting with the pump and probe laser beams. This allows the optimisation of the design parameters for a particular operating frequency range. Different fabrication methods are used based on the utilised materials in the transducers. Sputter coater method is used for deposition of metallic materials such as gold or Indium Tin Oxide (ITO) where spin coating and dip coating methods are used for deposition of polymeric materials such as Polystyrene (PS) or Cellulose Acetate (CA). The aim is to produce acoustic waves in different frequency regions form hundreds of megahertz (MHz) to several GHz, based on the applications. Several approaches are demonstrated to perform this either by using different materials, or/and increasing the spacer thickness.
In this study it was demonstrated that by increasing the thickness of Indium Tin Oxide (ITO) spacer in a FPI-based transducer fabricated on the tip of a single mode fibre from about 150nm to 560nm the frequency decreased from ~9.8GHz to ~4.9 GHz, respectively. Also, by replacing material from ITO to Polystyrene (PS) in spacer as well as increasing its dimension the frequency was reduced from 550 MHz to 430 MHz for the 560nm and 931nm PS devices, respectively.
A tuneable absorbing nanoscale thin-film system was developed experimentally and numerically, comprising of an absorptive dye mixed in dielectric coatings on reflecting surfaces. In this part of study, it was demonstrated for the first time the manipulation of light absorption to tailor the transducer for an available wavelength for the pump laser pulse in order to generate acoustic waves. Specifically, the Cellulose Acetate thin films mixed with Aluminum Phthalocyanine Chloride (APC) absorptive dye with the maximum absorption at 748nm coated on an opaque gold substrate. It was demonstrated that the absorption behaviour of dielectric thin films can be flexibly tuned by adjusting the proper system parameters such as dye solution concentrations and the thickness of dielectric thin film layer coating. From this, to the best of my knowledge, it has been the first time that novel hybrid devices were proposed as the new configurations of FPI-based transducers for the generation and detection of acoustic waves. In the hybrid devices, the absorption can take place within a thin absorptive layer either before or after the metallic layers in the FPI structure for the generation of acoustic waves where the absorption normally takes place within the metallic layers.
The use of sputter assisted spin-coating fabrication method was proposed and demonstrated for the first time the possibility of the fabrication of a multi-layered structure using this method. In this method, different layers from the same dye-CA polymeric solution were deposited layer after layer by coating a protective layer such as ITO between each layer. This method was proposed in order to increase and control the thickness of the transducers more accurately. This method also offered more surface smoothness, where a surface roughness of several nanometres can make significant difference in the optical and mechanical frequency responses of the transducers.
In addition, the used of sputter assisted dip-coating method was proposed for the fabrication of the transducers on the tip of optical fibres using the same procedure. Compared with the repeated dip-coating process used in single coating solutions, this method shows more advantages by virtue of the synergistic effect of the deposited layers. The multi-layered dip-coating differs from the layer-by-layer assembly where monomers from a solution are fabricated layer-by-layer.
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