High-refractive-index polymer | EPFL Graph Search

A high-refractive-index polymer (HRIP) is a polymer that has a refractive index greater than 1.50. Such materials are required for anti-reflective coating and photonic devices such as light emitting diodes (LEDs) and s. The refractive index of a polymer is based on several factors which include polarizability, chain flexibility, molecular geometry and the polymer backbone orientation. As of 2004, the highest refractive index for a polymer was 1.76. Substituents with high molar fractions or high-n nanoparticles in a polymer matrix have been introduced to increase the refractive index in polymers. Properties Refractive index A typical polymer has a refractive index of 1.30–1.70, but a higher refractive index is often required for specific applications. The refractive index is related to the molar refractivity, structure and weight of the monomer. In general, high molar refractivity and low molar volumes increase the refractive index of the polymer.

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

Advanced Diamond Microfabrication for Microoptics and Photonics

Marcell Kristof Kiss

Diamond is an exceptional material - hard, stiff, transparent, which makes it ideal for the fabrication of optical and mechanical systems that take advantage of these properties. Diamond is not only "better", but it offers the possibility of integrating bright colour centres. These optically active defects can be exploited for a variety of applications, including the study of fundamental science, magnetometry, biosensing and new types of lasers. Despite the attractive properties of diamond for optics, no standard platform exists to create photonic circuits and devices. This thesis shows the realisation of a diamond-on-insulator photonics platform, that aims to combine the versatility of the silicon-on-insulator photonics with the capability of performing diamond science in single crystal guided-wave devices. The diamond-on-insulator substrate is achieved via ion implantation-based cut of a single crystal membrane and bonding of the membrane to an insulator layer. This approach provides a monolithic fabrication process that scales to commercial exploitation, bringing convenient access to the study of diamond light-matter interactions without requiring custom substrate fabrication. Furthermore, the possibility of releasing the diamond devices adds access to freestanding structures, opening the way to diamond micro-opto-electro-mechanical systems and optomechanics. Diamond also makes an excellent microoptics substrate, due to its high laser damage threshold, transparency and high refractive index, which is an attractive combination for high-power, compact optical systems operating in the UV, visible and near-infrared. Different diamond etching techniques are developed and investigated in this thesis, that enable the realisation of unique features. These techniques are employed to create diamond diffractive microoptical components, which are then characterised, showing high-quality surfaces that closely match the designed features, indicating reliable fabrication and resulting in excellent optical performance. These devices have applications in high power beamsplitters, beam shapers and compact spectrometers.

EPFL2019

On the assessment of aspheric refractive microlenses

Jeremy Béguelin

Refractive microlenses and microlens arrays are key components for many applications such as optical data communication, laser and medical devices, or cameras. In particular, refractive micro-optics enables the miniaturization of high-tech systems but also offers novel optical functionalities. The success of this technology lies in the wafer-level fabrication technique, using the method of photoresist reflow with a subsequent pattern transfer into the substrate by reactive ion etching. Indeed, it allows the manufacturing in parallel of thousands of spherical or aspheric microlenses smaller than 1 mm. Their characterization is usually performed by measuring their surface, as this allows at the same time the evaluation of the microlens performance and feedback for fabrication process optimization. In this thesis, we assess this characterization approach to understand the fabrication process better and to improve the microlenses performance. Concretely, we first study surface form tolerancing, which is crucial to ensure the microlenses quality. However, the link between the surface form of a microlens and its performance is not straightforward, usually resulting in over-restrictive tolerances. Here, we investigate this connection for simple cases and then compare different approaches to tolerance typical micro-optical systems. Practical guidelines are proposed based on the results. Secondly, we present methods to improve surface measurements. For this, we develop an original calibration procedure that takes into account the aberrations of the imaging system. In the presented example, the accuracy is increased by a factor 7, rendering the characterization of diffraction-limited microlenses with high numerical apertures possible. Thirdly, we model the fabrication process to find correlations with the manufactured surface. Thereby, the fabrication optimization is made faster and more accurate. We validate this approach by increasing the uniformity of a large (100mm x 100mm) microlens array by a factor ~3. Finally, we evaluate another microlens characterization that consists of probing the optical functionalities in transmission and compare it to surface measurements. Particularly, we give the reasons for our doubts about its application to wafer-level microlenses. In conclusion, we show that a quantitative analysis of the microlens characterization allows for a significant improvement of the microlens quality and a better understanding of the fabrication process, resulting in lower production cost. For this reason, we believe that the results presented in this thesis will help to render wafer-level refractive micro-optics a more mature technology and build its bright future.

EPFL2020