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Light Scattering by Structurally Anisotropic Media: A Benchmark with Transparent Wood
KTH, School of Engineering Sciences (SCI), Applied Physics, Photonics.ORCID iD: 0000-0002-6548-5067
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.ORCID iD: 0000-0002-1591-5815
KTH, School of Engineering Sciences (SCI), Applied Physics.ORCID iD: 0000-0003-2562-0540
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2018 (English)In: Advanced Optical Materials, ISSN 2162-7568, E-ISSN 2195-1071, Vol. 6, no 23, article id 1800999Article in journal (Refereed) Published
Abstract [en]

Transparent wood (TW) is a biocomposite material with hierarchical structure, which exhibits high optical transmittance and anisotropic light scattering. Here, the relation between anisotropic scattering and the internal structure of transparent wood is experimentally studied and the dependence of scattering anisotropy on material thickness, which characterizes the fraction of ballistic photons in the propagating light, is shown. The limitations of the conven-tional haze, as it is implemented to isotropic materials, are discussed, and a modified characteristic parameter of light scattering—the degree of aniso-tropic scattering is defined. This parameter together with the transport mean free path value is more practical and convenient for characterization of the material scattering properties. It is believed that the generic routine described in this paper can be applied for scattering characterization and comparison of other TW materials of either different thickness, optical quality or based on various wood species.

Place, publisher, year, edition, pages
John Wiley & Sons, 2018. Vol. 6, no 23, article id 1800999
Keywords [en]
anisotropic scattering, biocomposites, hierarchical structure, nanocellulose, transparent wood
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-239314DOI: 10.1002/adom.201800999ISI: 000453512700015Scopus ID: 2-s2.0-85055276864OAI: oai:DiVA.org:kth-239314DiVA, id: diva2:1264528
Funder
EU, Horizon 2020, 742733Swedish Research Council, 621-2012-4421Knut and Alice Wallenberg Foundation
Note

QC 20181126

Available from: 2018-11-20 Created: 2018-11-20 Last updated: 2019-01-16Bibliographically approved
In thesis
1. Light propagation in an anisotropically scattering medium
Open this publication in new window or tab >>Light propagation in an anisotropically scattering medium
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Scattering is the main phenomenon of light-matter interaction. In this thesis, we consider one specific case of light propagation in a material whose structure causes anisotropic optical scattering - transparent wood. As a substance with properties interesting for research in optics/photonics, this biocomposite material began to be used quite recently, even though it was initially developed for the study of the internal structure of wood a few decades ago. Besides the anisotropy, the structure has a hierarchal arrangement with features ranging from nanometers up to micrometer sizes, and demonstrates short and long-distance natural ordering which is neither perfect nor totally random. The main interest in transparent wood within the field of optics is due to its remarkable combination of unique structure and optical transparency. There has been much research done on light propagation in diffusive media and/or structurally ordered materials, and in this sense, transparent wood occupies an intermediate niche for investigation. Since transparent wood is a relatively new material, there is a lack of methods of reliable study of its properties. Although there have been some attempts to characterize the optical properties of transparent wood via transmittance and haze (for scattering properties), insufficient understanding of the physical processes behind light propagation within the substance, scattering in particular, leaves a gap in data interpretation for measured parameters and their correlation with the material structure. In this thesis, we present our efforts to fill this gap through means of a more detailed and physically justified description of light propagation in an anisotropically scattering medium. In order to familiarize readers with the subject, we provide a short summary of the structural features, fabrication technology and chemical composition of transparent wood. We discuss issues related to the conventionally applied approach of haze measuring as a characterization method of the scattering in such anisotropic materials as transparent wood. We demonstrate a certain limitation of the haze criterion applicability and instead, suggest a modified characterization routine and parameters, such as transport mean free path, and degree of anisotropic scattering, for estimation of scattering properties of the material. We also discuss the dependence of scattering efficiency on the polarization state of incident light. Due to polarization-dependent scattering, unpolarized light propagating through transparent wood becomes partially polarized, with the angle of polarization oriented perpendicularly to the largest structural components of wood (vessels) which are co-aligned with the wood fibers. At the same time, the polarization degree of completely polarized incident light decreases after propagating through the material. The depolarization of light is attributed to the collective scattering by cellulose fibrils organized in the lamellae of cell walls of the fibers, and is strongly dependent on mutual v orientation of oscillations of the electric field (polarization of light) relative to the long axes of fibers. Transparent wood with its sponge-like structure can be an attractive platform for doping with various additives. In our project, we demonstrate an example of doping of a transparent wood template with organic dye (Rhodamine 6G). The combination of high scattering and optical gain results in the possibility of obtaining laser emission. By analogy with random lasers, there is no external resonator, however, the optical feedback is provided via scattering on inhomogeneity of the internal structure of the medium combining transparent wood and dye. The wood structure has a natural ordering, whereas in random lasers, in general, scatterers are randomly distributed within an active material. Therefore, we refer to such a laser as a “quasi-random” laser, to emphasize this difference. Based on experimental results and the comparative analysis with a reference sample (a polymer-dye), we have shown that wood fibers with highly scattering walls that are the main structural components of the transparent wood can operate as small lasers not correlated with each other. The collective origin of the laser emission of a wood-dye laser is reflected in the emission-line broadening of several nanometers. Measured low spatial coherence and high radiation brightness make this type of laser attractive for the application of speckle-free imaging and illumination. The work demonstrated in this thesis can be useful for further investigation and simulations of light scattering inside the biocomposite and similar anisotropic media. Moreover, the demonstrably successful example of expanding transparent wood functionality definitely creates new opportunities for further research and application of this substance.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2018. p. 53
Series
TRITA-SCI-FOU ; 2018:51
Keywords
anisotropic light scattering, scattering material characterization, polarization dependent scattering, diffusive media, hierarchical structure, biocomposites, wood modification, transparent wood, microfibrils angle, organic materials, polymer active devices, dye lasers, coherence, polarization, degree of polarization.
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-239349 (URN)978-91-7873-033-9 (ISBN)
Public defence
2018-11-14, Sal C, Electrum, våningsplan 2, Kistagången 16, Kista, Stockholm, 10:00 (English)
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Note

QC 20181121

Available from: 2018-11-21 Created: 2018-11-21 Last updated: 2018-11-21Bibliographically approved

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Vasileva, ElenaLi, YuanyuanSychugov, IlyaBerglund, LarsPopov, Sergei

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