This chapter is an integrated overview of organic solid-state tunable laser oscillators that adds an additional and complimentary perspective to the material covered in chapters 5 to 9. Emphases are on miniaturized tunable organic lasers using DFB, VECSOL, and waveguide architectures. Furthermore, the use of dye-doped transparent wood, as an all-organic solid-state gain medium, is described and discussed.

We report on the study of polarization properties of light propagating through transparent wood (TW), which is an anisotropically scattering medium, and consider two cases: completely polarized and totally unpolarized light. It was demonstrated that scattered light distribution is affected by the polarization state of incident light. Scattering is the most efficient for light polarized parallel to cellulose fibers. Furthermore, unpolarized light becomes partially polarized (with a polarization degree of 50%) after propagating through the TW. In the case of totally polarized incident light, however, the degree of polarization of transmitted light is decreased, in an extreme case to a few percent, and reveals an unusual angular dependence on the material orientation. The internal hierarchical complex structure of the material, in particular cellulose fibrils organized in lamellae, is believed to be responsible for the change of the light polarization degree. It was demonstrated that the depolarization properties are determined by the angle between the polarization of light and the wood fibers, emphasizing the impact of their internal structure, unique for different wood species.

Transparent wood (TW) is an emerging optical material combining high optical transmittance and haze for structural applications. Unlike nonscattering absorbing media, the thickness dependence of light transmittance for TW is complicated because optical losses are also related to increased photon path length from multiple scattering. In the present study, starting from photon diffusion equation, it is found that the angle-integrated total light transmittance of TW has an exponentially decaying dependence on sample thickness. The expression reveals an attenuation coefficient which depends not only on the absorption coefficient but also on the diffusion coefficient. The total transmittance and thickness were measured for a range of TW samples, from both acetylated and nonacetylated balsa wood templates, and were fitted according to the derived relationship. The fitting gives a lower attenuation coefficient for the acetylated TW compared to the nonacetylated one. The lower attenuation coefficient for the acetylated TW is attributed to its lower scattering coefficient or correspondingly lower haze. The attenuation constant resulted from our model hence can serve as a singular material parameter that facilitates cross-comparison of different sample types, at even different thicknesses, when total optical transmittance is concerned. The model was verified with two other TWs (ash and birch) and is in general applicable to other scattering media.

We report on the experimental determination of the complete two coordinate spatial coherence function of light emitted by a quasi-random laser, implemented on recently introduced dye-doped transparent wood. The spatial coherence was measured by means of a double grating interferometer, which has some advantages over the standard Young's interferometer. Analysis of the spatial coherence reveals that emission from such a material can be considered as a superposition of several spatial modes produced by individual emitters within semi-ordered scattering medium. The overall degree of coherence, (gamma)over-bar, for this quasi-random laser was found to be 0.16 +/- 0.01, having possible applications in speckle free laser imaging and illumination.

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.

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.

Transparent wood is an emerging load-bearing material reinvented from natural wood scaffolds with added light management functionalities. Such material shows promising properties for buildings and related structural applications, including its renewable and abundant origin, interesting optical properties, outstanding mechanical performance, low density, low thermal conductivity, and great potential for multifunctionalization. In this study, a detailed summary of recent progress on the transparent wood research topic is presented. Remaining questions and challenges related to transparent wood preparation, optical property measurements, and transparent wood modification and applications are discussed.

Polymer materials offer process compatibility, design flexibility, and low cost technology as a multi-functional platform for optical communication and photonics applications. Design and thermal reflowing technology of low loss polymer waveguides, as well as demonstration of transparent wood laser are presented in this paper.

Poly-3-hexylthiophene (P3HT) nanofibers are semiconducting high-aspect ratio nanostructures with anisotropic absorption and birefringence properties found at different regions of the optical spectrum. In addition, P3HT nanofibers possess an ability to be aligned by an external electric field, while being dispersed in a liquid. In this manuscript we show that such collective ordering of nanofibers, similar to liquid crystal material, significantly changes the properties of transmitted light. With a specially fabricated opto-fluidic component, we monitored the phase and transmission modulation of light propagating through the solution of P3HT nanofibers, being placed in the electric field with strength up to 0.1 V/μm. This report describes a technique for light modulation, which can be implemented in optical fiber-based devices or on-chip integrated components.

The report on a study of laser emission from a conceptually new organic material based on transparent wood (TW) with embedded dye Rhodamine 6G molecules is presented in this paper. The lasing performance is compared to a reference organic material containing dye in a poly-methyl-methacrylate matrix. From experimental results, one can conclude that the optical feedback in dye-TW material is realized within cellulose fibers, which play the role of tiny optical resonators. Therefore, the output emission is a collective contribution of individual resonators. Due to this fact, as well as low Q-factor of the resonators/fibers and their length variation, the spectral line of laser emission is broadened up to several nanometers.