Signification of the phase function parameter gamma explored with a fractal distribution of scatterers

The optical properties within limited volumes of diffusive media can be probed by carrying spatially-resolved measurements of diffused light at short source-detector separation (typically one scattering mean free path). At such distance, analytical models only relying on the absorption and reduced scattering coefficients fail at correctly predicting reflectance and it was demonstrated that adding a third optical coefficient γ improves the description of light propagation conditions near the source. In an attempt to relate the γ coefficient to physical properties of turbid media, this paper uses a fractal distribution law for modeling scatterers’ sizes distributions and investigates numerically and experimentally how γ is related to the fractal power α. The results indicate that within the range of γ typically encountered in biological samples, this coefficient is approximately linearly correlated with α.

Optical microscopy for imaging through scattering media

Xin Yang

Optical microscopy has been widely used in the life sciences and biological research for several decades. It possesses many advantages over x-ray radiography, computed tomography, ultrasound imaging and magnetic resonance imaging, in terms of being low cost, high resolution, harmless and capable of multiple contrast mechanism, etc. However, the weakness of optical microscopy is the inability to perform deep tissue imaging. Owing to the short wavelength of optical waves, it experiences much more scattering than longer waves in inhomogeneous materials such as biological tissues. Scattering scrambles the wavefront and limits the penetration depth of optical microscopy to only hundreds of microns. We explore in this thesis, novel optical approaches to image through scattering media. There are two main categories of the state of art techniques: imaging with ballistic photons, which requires separation of ballistic photons from the sneak or multiple scattered ones; and imaging with scattered photons, which relies on manipulation of the scattered photons according to the specific scattering properties. We primarily focus on the second category in this thesis. Taking advantage of the reversibility of light, phase conjugation has a long history in aberration compensation. A deterministic wave is incident on a scattering medium and results in a random scattered field. If we record the full information of the scattered field (amplitude and phase) and generate a phase conjugated replica of it, the phase conjugated wave can follow the same path and reconstruct the original deterministic wave after passing back through the scattering medium. In this work, phase conjugation is achieved using digital method, called digital phase conjugation. The scattered field is recorded through digital holography, and a spatial light modulator is applied to construct the phase conjugated wave. Second harmonic radiation imaging probes are used as the original light source, rendering the generation of a focus behind the scattering medium. The memory effect, which indicates the correlation between speckles with different incident angles, is exploited to move the focus around and obtain three-dimensional scanning microscopy behind scattering medium. Speckle scanning microscopy is the second method demonstrated in this thesis for imaging through turbid media. In contrast to digital phase conjugation, which requires access from both sides of the scatterers to measure the scattered field, in speckle scanning microscopy, excitation and collection can be achieved from the same side. The specific scattering events are not taken into account, instead, the statistic property of the scatterers plays a major role. This technique makes full use of two facts: 1. The statistically averaged autocorrelation of speckle fields approaches a delta function; 2. When a speckle pattern scans across a fluorescence object, the collected fluorescence intensity is proportional to the convolution of the speckle field and the object. Linking these two facts with delicate calculation, we can eventually minimize the influence of the speckle, and reconstruct two-dimensional images of objects behind turbid medium. Encouraged by the success of two-dimensional speckle scanning microscopy, we investigate more advanced possibilities by simulation, including three-dimensional speckle scanning and reference based speckle scanning techniques. [...]

EPFL2014

Method for measuring locally and superficially the scattering and absorption properties of turbid media

Christian Depeursinge

We present a method and apparatus for local and superficial measurement of the optical properties of turbid media. The depth probed is on the order of 1 transport mean free path of the photon. The absorption coefficient, reduced scattering coefficient and the phase function parameter gamma =(1-g2)/(1-g1) are optical parameters computed from a single measurement of the spatially resolved reflectance close to the source. Images of superficial structures of the medium can be obtained by performing multi-site measurements. An important application of this technique is the characterization of biological tissues, for example for medical diagnostic purposes. Measurements on biological tissues can be achieved using a probe of diameter less than 2mm, and the average volume probed is on the order of 1 mm. Separate images of absorption and tissue structure can be achieved with a resolution of approximately one transport mean free path of the considered tissue.

2005

High vacuum chemical vapor deposition (HV-CVD) of alumina thin films

Xavier Multone

We analyzed along this work the feasibility to produce high quality alumina thin films by High Vacuum Chemical Vapor Deposition (HV-CVD). We study the influence of various parameters on the growth process and on the film quality, such as substrate temperature, gas flow or pressure. We aim to highlight the advantages and the limitations of the HV-CVD technique. A high vacuum CVD reactor has been designed, built and optimized. A novel effusing source has been designed to guarantee a molecules distribution uniformity of 95% on the substrate surface. The main technical challenges faced were to reach the maximum substrate temperature in vacuum, to control the precursor flux and to control the temperature of the gas lines to avoid condensation. For the first time, films of Al2O3 are obtained by HV-CVD from aluminum isopropoxide as precursor in a novel HV-CVD reactor. The HV-CVD approach stands out of all the other CVD methods as it allows excellent prediction of growth rates and film thickness homogeneities if the decomposition probability of the precursor is once determined. In this work, the latter was determined to increase with increasing substrate temperature to 0.2, further increase of temperature reduces the decomposition probability due to increased desorption rate. Pure alumina films are obtained in a reliable and reproducible way. The presence of oxidizing agents, as decomposition partners, reduces the activation energy of deposition process from 33.1 ± 8.2 kJ/mol to 11.4 ± 5.3 kJ/mol in presence of sufficient oxidizing agent. The Al2O3 films are growing at deposition rate from 1 to 50 nm/min. It has been found that, the lower the deposition temperature, the higher the density of the deposited films therefore the higher refractive index. The index of refraction varies from 1.65 ± 0.01 to 1.35 ± 0.01 at 632 nm wavelength from lowest to highest deposition temperature. At low deposition temperature, Al2O3 can contain residual OH-groups and Al=O entities. This content is independent of the total flow rates of the precursor in the range of 5·1015 molecules/cm2·s to 3·1016 molecules/cm2·s impinging on the substrate. Substrate effects have not been observed in the limited range of tests on natural oxidized silicon, 3 µm SiO2 on silicon, quartz, stainless steel, fused silica and 100 nm silicon nitride on top of silicon. The chemical composition of the deposited alumina films measured by EDX and XPS is nearly stoichiometric with 35 ± 5 at% Al and 65 ± 5 at% O, without carbon contamination. Remaining hydrogen in the films has not been studied in detail, but the difference of OH- absorption peaks by FTIR indicates that, a low temperature deposition, hydrogen incorporation is possible. Concerning the optical properties of the HV-CVD deposited films, absorption is very low from 250 nm to 1800 nm wavelength, but due to the porosity and granular structure of the films, light scattering can take place. But propagation loss smaller than 2 dB/cm were measured in channel waveguides fabricated from the HV-CVD alumina films. Planar and channel waveguides demonstrate good guiding properties at 670 nm and 1.55 µm, as well. This work also opens new possibilities to deposit in situ local structures of or in transparent alumina on large surface. Light induced high vacuum chemical vapor deposition of alumina microstructures by 248 nm excimer laser is presented and shows an activation energy of 5.2 ± 0.4 kJ/mol, much smaller than for thermal deposition. The influence of the fluence and the repetition rate is discussed. Electron beam assisted HV-CVD of alumina is demonstrated and proves the feasibility of in situ structuration under HV-CVD conditions.