Abstract
We theoretically and experimentally investigate light diffusion in disordered meso-macroporous materials with a cylindrical shape. High Internal Phase Emulsion (HIPE)-based silica foam samples, exhibiting a polydisperse pore-size distribution centered around 19 μm to resemble certain biological tissues, are realized. To quantify the effect of a finite lateral size on measurable quantities, an analytical model for diffusion in finite cylinders is developed and validated by Monte Carlo random walk simulations. Steady-state and time-resolved transmission experiments are performed and the transport parameters (transport mean free path and material absorption length) are successfully retrieved from fits of the experimental curves with the proposed model. This study reveals that scattering losses on the lateral sides of the samples are responsible for a lowering of the transmission signal and a shortening of the photon lifetime, similar in experimental observables to the effect of material absorption. The recognition of this geometrical effect is essential since its wrong attribution to material absorption could be detrimental in various applications, such as biological tissue diagnosis or conversion efficiency in dye-sensitized solar cells.
Highlights
Disordered optical media are ubiquitous in nature, and a proper description of light propagation in such materials is crucial for a number of applications, ranging from the study of radiation transport in the atmosphere [1, 2] to the optical characterization of complex materials [3, 4], to the optical diagnosis of the human body [5, 6]
We have manufactured SiO2(HIPE) monoliths exhibiting a broad pore-size distribution centered around 19 μm
We have developed a model of diffusion in finite cylinders to obtain analytical expressions for the steady-state and time-resolved transmission along the cylinder axis
Summary
Disordered optical media are ubiquitous in nature, and a proper description of light propagation in such materials is crucial for a number of applications, ranging from the study of radiation transport in the atmosphere [1, 2] to the optical characterization of complex materials [3, 4], to the optical diagnosis of the human body [5, 6]. Analytical expressions for the steady-state and time-resolved optical responses in slabs are well-established [7,16,17,18] and such geometries are commonly used in studies on wave phenomena in disordered media [19,20,21,22,23,24]. Our scope is to describe quantitatively the effect of the finite lateral sample size on measurable quantities For this purpose, we merge together knowledge in multiple light scattering and in material design and fabrication. An important finding is that lateral scattering losses can be misinterpreted as material absorption, which would clearly be detrimental to the optical spectroscopy of turbid media in general (biological tissues, colloids, powders, etc). We show that the correct transport parameters (transport mean free path and material absorption length) can be retrieved with good accuracy from fits of the experimental curves with the derived analytical expressions
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