Refractive index | EPFL Graph Search

In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or refracted, when entering a material. This is described by Snell's law of refraction, n1 sin θ1 = n2 sin θ2, where θ1 and θ2 are the angle of incidence and angle of refraction, respectively, of a ray crossing the interface between two media with refractive indices n1 and n2. The refractive indices also determine the amount of light that is reflected when reaching the interface, as well as the critical angle for total internal reflection, their intensity (Fresnel's equations) and Brewster's angle. The refractive index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values: the speed of light in a medium is v = c/n, and similarly the wavelength in that medium is λ = λ0/n,

Advanced Techniques in Optical Diffraction Tomography

Ahmed Bassam Sayed Ayoub Mohamed Emam

In this thesis, we study the 3 challenges described above. First, we study different reconstruction techniques and assess the fidelity of each reconstruction results by means of structured illumination and phase conjugation. By reconstructing the 3D refractive index of the sample using different algorithms (i.e. Born, Rytov, and Radon) and then perform a numerical back-propagation of experimentally measured structured illumination pattern we are able to assess the fidelity of each reconstruction algorithms without prior information about the 3D RI distribution of the sample.The second part of the thesis is concerned with the 3D reconstruction of samples using intensity-only measurements which the need to holographically acquire them. We show that using intensity-only measurements, we could still be able to reconstruct the 3D volume of the sample with edge-enhanced effects which was proven useful for drug delivery applications in which nano-particles were identified on the cell membrane of immune T-cells in a drug delivery studies. Such reconstruction technique would result in more robust imaging system where the commercial imaging microscope systems can be incorporated with LEDs for high-quality speckle noise-free imaging systems. In addition, we show that under certain conditions, we can be able to reconstruct the 3D refractive index distribution of different samples.The third part of the thesis is contributing to high-speed complex wave-front shaping using DMDs. In that part, new modulation technique is demonstrated that can boost the speed of the current time-multiplexing techniques by a factor of 32. The modulation technique is based on amplitude modulation where an amplitude modulator is synchronized withvthe DMD to modulate the intensity of each bit-plane of an 8-bit image and then all the modulated bit-planes are linearly added on the detector. Such modulation technique can be used not only for structured illumination microscopy but also for high-speed 3D printing applications as well as projectors.The last part is concerned with using deep learning approaches to solve the missing cone problem usually accompanied with optical imaging due to the limited numerical aperture of the imaging system. Two techniques are discussed; the first is based on using a physical model to enhance the quality of the 3D RI reconstruction and the second is based on using deep neural network to solve the missing cone problem.

EPFL2022

Spatial and Morphological Control of Cyanine Dye Thin Films

Nicolas Alexandre Serge Leclaire

Cyanine dyes are organic semiconductor compounds with light absorption and emission properties useful for emerging technologies such as solar cells and light-emitting devices. The characteristics of these materials in the solid state depend on their organization of the constituting building blocks. This thesis focuses on controlling the morphology of cyanine dye thin films at different length scales and clarifying the resulting properties. When microstructures present features whose size matches visible light wavelengths, new properties may arise from light-matter interactions. Here the properties resulting from the light-matter interactions of cyanine droplet films cast from solution are studied. Based on experimental evidence, it is shown that dye droplet ensembles scatter light with different efficiencies and wavelength ranges depending on their dimensions. FDTD simulations are used to show that this effect results fromscattering enhancement at the absorption edge of the dye where the refractive index varies considerably. Simulations also provide a better understanding of individual dropletsâ interaction with light. While earlier work had hypothesized that the observed scattering phenomenon were due to crystalline clusters within the droplets, this work highlights the contribution of the dye filmmorphology. Cyanines also form single crystals whose fabrication induces molecular-scale order in the material. Previous work demonstrated that thin single crystals could be grown by solvent vapor annealing of dye droplets. Here it is shown that in uncontrolled conditions, cyanine single crystals destabilize to formdendritic crystals. In-situ microscopy observations highlight the solute reservoir role of the droplet distribution surrounding a growing crystal; when the distance between droplets and the crystal front is large, the solute supply is diffusion-limited. Moreover variations in local pressure equilibrium between the droplets and crystal front lead to advection fluxes which perturb the crystal growth. These observations help design configurations to either prevent crystal destabilization or take advantage of the dendritic growth in a controlled manner. In addition, the patterning of crystals on a substrate is relevant for their application in devices. A practical challenge is to induce single crystal growth at specific locations. Here, surfaces patterned with SAMs of hydrophilic and hydrophobic thiols are used to create dye droplet arrays from which crystals can be grown. This method is shown to yield local crystallization of the dye and to prevent crystal destabilization through better control of the droplet distribution. By varying the dye solution concentration, partial control over crystal density is achieved, however it proved difficult to control the number of nuclei per droplet. A more controlled evaporation and solvent vapor annealing system might be necessary to master the nucleation process. Finally the structure and optical properties of cyanine single crystals are addressed. The crystal structure was determined by X-ray diffraction. Structural aspects are shown to lead to excitonic couplings, which are evidenced by orientation-dependent spectroscopic measurements of single crystals. Although further investigation of the absorption band structure is necessary, the results are promising for photovoltaic devices as they might improve exciton transport compared to amorphous layers.

EPFL2018

Free carriers in nanocrystalline titanium dioxide thin films

Simon Springer

This thesis analyzes the properties of heavily doped nanocrystalline titanium dioxide. Thin films were deposited by magnetron sputtering with either water vapor as reactive gas or a periodically interrupted oxygen supply. The samples were at the same time electrically conducting and transparent. They consisted of mixtures of rutile, anatase and amorphous phases. Titanium dioxide is chemically stable, hard, non-toxic, transparent, and inexpensive. Due to a high refractive index it is often found in optical applications as a thin film coating. For many purposes it is interesting to take advantage of a combination of good transparency and electrical conductivity. An even larger field of applications would open if the electric conductivity of TiO2 could be increased in a controlled manner. Conventional bulk doping is limited, however, by low solubility limits. Highly conducting nanocrystalline TiO2 thin films have been obtained recently by sputtering in presence of water vapor. The present project intends to elucidate the doping mechanisms at work in such films. In addition, an alternative process to achieve grain-boundary doping of TiO2 was explored. Spectrophotometry over a wide spectral range (0.05 to 5 eV) was the main tool in probing the mobile electrical charge carriers. The interpretation of these optical measurements required appropriate models. The models developed took many structural properties into account, such as thickness, surface roughness, density, anisotropy and crystalline phase composition. The phase and morphology of the films were analyzed by X-ray diffraction and reflection, scanning probe microscopy, and electron microscopy. The chemical analyses included electron-probe microanalyses, Rutherford backscattering and elastic recoil detection analysis. The DC electrical properties were measured between room temperature and 250 °C. Most water-deposited samples showed semiconducting behavior with an activation energy of the order of 100 meV. The optical measurements revealed a broad absorption band around 1 eV. The absorption coefficient scaled with conductivity. It was successfully modelled as a Lorentzian. Similar absorption bands have been reported for reduced rutile and anatase single crystals. Water-deposited samples containing anatase exhibited a metallic behavior. In these samples the thermal activation energy was reduced to about 30 meV and the charge carriers displayed a high mobility (3 cm2V-1s-1). The infrared absorption band was about 10 times less important than in the samples devoid of anatase. Thin films of TiO2 intercalated with the equivalent of nanometer-thick layers of Ti were prepared. By changing the period in the oxygen supply the grain size, and thereby the film properties, could be modified. The films obtained in this way had many properties in common with water-deposited films. They showed the same absorption band in the infrared, had comparable charge carrier densities and were electrically conductive (102 to 104 Sm-1 at room temperature). To our knowledge, this was the first time that such high electrical conductivities were achieved by sputtering in an oxygen plasma.

EPFL2004