Abstract
In this thesis the time resolved studies of light propagation through highly scattering media in two extreme cases are reported. The first case concerns media where the dominating process is multiple scattering with negligible absorption, with the second regime being media, which both scatter and amplify light. Light propagation is diffusive in both cases however a very interesting phenomenon takes place in the latter. In this extreme case light emanating from a random, highly disordered medium can have highly ordered (lasing) properties. This phenomenon namely random lasing has raised an enormous research interest. The final goal of the study was the investigation of the possible use of light transport through and the light emission from such media for the improvement of optical biopsy and imaging techniques. In the introductory part of the thesis the most common benign and malignant abnormalities occurring in the female breast as well as a synopsis of the present status on imaging te ...
In this thesis the time resolved studies of light propagation through highly scattering media in two extreme cases are reported. The first case concerns media where the dominating process is multiple scattering with negligible absorption, with the second regime being media, which both scatter and amplify light. Light propagation is diffusive in both cases however a very interesting phenomenon takes place in the latter. In this extreme case light emanating from a random, highly disordered medium can have highly ordered (lasing) properties. This phenomenon namely random lasing has raised an enormous research interest. The final goal of the study was the investigation of the possible use of light transport through and the light emission from such media for the improvement of optical biopsy and imaging techniques. In the introductory part of the thesis the most common benign and malignant abnormalities occurring in the female breast as well as a synopsis of the present status on imaging techniques are given. Then an introduction is also made for the propagation of light through highly scattering amplifying media and the vast number of applications that the phenomenon of random lasing brings into play. After the introductory part the processes governing the interaction of light and matter are described theoretically and focused in the cases that are studied experimentally as well. Diffusive transport of light and lasing theory are the two key aspects in random amplifying media. The coherent properties of the light emitted from random lasers are also investigated. At the same time an overview of the current state of theory describing such media is presented to give a complete picture to the reader. For the time-resolved study of light propagation through turbid matter such as female breast tissue a mode-locked Ti:Sapphire laser emitting 200 fs pulses at 800 nm with an average power of ~1 W and a repetition rate of 82 MHz was used. The detection was performed with a streak camera with a temporal resolution of 2 ps. The tissue samples were excised from female breast during tumor extraction or biopsy operations and concerned normal adipose and fibrous tissue. The theoretical analysis of the temporal spread of the transmitted ultra-fast laser pulses provides the optical properties of the studied medium. Large samples can be studied based on the diffusion theory whereas small biopsy samples are studied based on the Laguerre expansion of the kernels technique. Recently cancer tissue was studied as well. Results are in complete agreement with the histological analysis and thus very promising for the implementation of these techniques in optical biopsy. As far as the scattering gain media is concerned different types of laser systems were used depending on the excitation scheme combined with the same workstation employing a spectrograph - streak camera system for the simultaneous detection of both spectral and temporal information. The temporal resolution is 2 ps and the spectral resolution 1 nm. For one-photon excitation of random lasing materials a Lambda Physik distributed feedback dye laser (DFDL) emitting 500 fs pulses at 496 nm with output energy of 50 μJ pumped by a XeCl excimer laser emitting at 308 nm was used. For two-photon excitation the regeneratively amplified output of the mode-locked Ti:Sapphire laser system emitting 200 fsec pulses at 800 nm with a repetition rate of 1 KHz and an average power of 500 mW was used. The samples were prepared by mixing different lasing dyes with TiO2 microparticles with average diameter of 400 nm in liquid or solid matrices thus providing multiple scattering and gain. As mentioned above light emanating from such media has the temporal and spectral properties of laser light. Pulses of few picoseconds long and few nanometers wide (down to the detection limits of the system) have been detected when the samples were excited above a well-determined energy threshold. This optical explosion provides a beacon like light source, which could be very valuable for detection modalities. Especially the excitation with two near-infrared photons consequently and most importantly provides the necessary penetration inside the strongly scattering media, as well as negligible photobleaching and phototoxication of tissue due to the low energy of the photons. The non-linearity insures that the effect will be confined only on the focal region, thus improving the spatial resolution by minimizing the out of focus fluorescence. The coherent properties of the laser-like emission were studied using the frequency doubled output of the above described laser system employing the regeneratively amplified output of the mode-locked Ti:Sapphire laser emitting at 800 nm. Single photon counting measurements were performed in order to obtain the photon count distributions of the emitted light. Coherent light is described by Poisson distribution whereas chaotic light by Bose-Einstein distribution. The experimental results suggested that depending on the scattering properties of the medium and on the excitation scheme, the emitted light comprised of different amounts of coherent and incoherent components since different modes are overlapping. Similar studies are underway when various fluorophores are embedded in biological tissues. The final goal is to take advantage of this effect towards a more spatially and spectrally confined agent in Photodynamic Therapy of target tissue lesions on skin or other types of superficial lesions. Very promising in the field of skin PDT would be thin patches with various fluophores, which could be applied directly on the lesion and allow the selection of different irradiation wavelengths using the same laser as excitation source. Some of the applications in addition to PDT are photonic marking for identification purposes (photonic codes, search and rescue missions, military applications, marking of hazardous material), substitution of ordinary lasing media in dye lasers and boosting the emission of LEDs and diode lasers in the blue region of the spectrum (if used with electro luminescent polymers). A lot of effort is also put in the study of dye infiltrated opal photonic crystals and the light emitted from such an ordered (crystal structure) and disordered (random photons’ path) structure.
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