Ultralow refractive index optical films with enhanced mechanical performance obtained by hybrid glancing angle deposition.

Ultralow refractive index materials (n less than 1.38 at 550 nm) are of particular interest in the context of antireflective coatings, allowing one to enhance their overall optical performance. However, application of such materials is typically limited by their mechanical properties. In this study, we explore the characteristics of a new category of hybrid (organic/inorganic) SiOCH thin films prepared by glancing angle deposition (GLAD) using electron beam evaporation of SiO2 in the presence of an organosilicon precursor. The resulting layers exhibited n as low as 1.2, showed high elastic rebound, and generally better mechanical properties than their inorganic counterparts. In addition, hybrid GLAD films were found to be highly hydrophobic. The performance of the films is discussed in terms of their hybridicity (organic/inorganic) ratio determined by infrared spectroscopic ellipsometry as well as the presence of anisotropy assessed by the nanostructure-based spectroscopic ellipsometry model. Finally, we demonstrate successful implementation of the ultralow-index material in a complete antireflective stack.

TRACK--A new method for the evaluation of low-level extinction coefficient in optical films.

We develop a rigorous methodology named TRACK based on the collection of multi-angle spectrophotometric transmission and reflection data in order to assess the extinction coefficient of quasi-transparent optical films. The accuracy of extinction coefficient values obtained by this method is not affected by sample non-idealities (thickness non-uniformity, refractive index inhomogeneities, anisotropy, interfaces, etc.) and therefore a simple two-layer (substrate/film) optical model can be used. The method requires the acquisition of transmission and reflection data at two angles of incidence: 10° and 65° in p polarization. Data acquired at 10° provide information about the film thickness and the refractive index, while data collected at 65° are used for absorption evaluation and extinction coefficient computation. We test this method on three types of samples: (i) a CR-39 plastic substrate coated with a thick protective coating; (ii) the same substrate coated with a thin TiO(2) film; (iii) and a thick Si(3)N(4) film deposited on Gorilla glass that presents thickness non-uniformity and refractive index gradient non-idealities. We also compare absorption and extinction coefficient values obtained at 410 and 550 nm by both TRACK and Laser Induced Deflection techniques in the case of a 1 micron thick TiO(2) coating. Both methods display consistent extinction coefficient values in the 10(-4) and 10(-5) ranges at 410 and 550 nm, respectively, which proves the validity of the methodology and provides an estimate of its accuracy limit.

Relationships between composition and properties of (Cr/Ti)SiN and (Cr/Ti)CN alloys: an ab initio study.

It has previously been noted that different fcc metal nitrides exhibit different superior properties, including the high hardness of TiN and the excellent corrosion and oxidation resistance of CrN. Si and C have been added into such metal nitrides in order to tailor their functional properties. Contrary to the intensively studied TiSiN and TiCN nanocomposite and solid solution systems, much less is known about the complex relationships between the composition and the properties of CrSiN and CrCN. In fact, there is a qualitative difference between cubic spin-unpolarized materials such as TiN, and spin-polarized materials such as CrN which may exhibit a cubic/orthorhombic structural transformation. In the present work, we report ab initio calculations of the properties of (Cr/Ti)SiN and (Cr/Ti)CN systems of various compositions. We specifically predict the lattice constant, bulk modulus, elastic tensor, shear modulus, Young's modulus, Poisson's ratio, magnetization, electronic structure and preference towards the cubic/orthorhombic structural transformation. Knowledge of the modeled relationships allows one to tailor the material characteristics of various ternary metal nitrides for different technological applications.

Members of the bHLH-PAS family regulate Shh transcription in forebrain regions of the mouse CNS.

The secreted protein sonic hedgehog (Shh) is required to establish patterns of cellular growth and differentiation within ventral regions of the developing CNS. The expression of Shh in the two tissue sources responsible for this activity, the axial mesoderm and the ventral midline of the neural tube, is regulated along the anteroposterior neuraxis. Separate cis-acting regulatory sequences have been identified which direct Shh expression to distinct regions of the neural tube, supporting the view that multiple genes are involved in activating Shh transcription along the length of the CNS. We show here that the activity of one Shh enhancer, which directs reporter expression to portions of the ventral midbrain and diencephalon, overlaps both temporally and spatially with the expression of Sim2. Sim2 encodes a basic helix-loop-helix (bHLH-PAS) PAS domain containing transcriptional regulator whose Drosophila homolog, single-minded, is a master regulator of ventral midline development. Both vertebrate and invertebrate Sim family members were found sufficient for the activation of the Shh reporter as well as endogenous Shh mRNA. Although Shh expression is maintained in Sim2(-)(/)(-) embryos, it was determined to be absent from the rostral midbrain and caudal diencephalon of embryos carrying a dominant-negative transgene that disrupts the function of bHLH-PAS proteins. Together, these results suggest that bHLH-PAS family members are required for the regulation of Shh transcription within aspects of the ventral midbrain and diencephalon.

Interphase in plasma-deposited films on plastics: effect on the spectral properties of optical filters.

The plasma-enhanced chemical vapor deposition of optical coatings on plastic substrates leads to the formation of a physically thick (approximately 50-100-nm) interfacial region (or interphase). We propose, based on our earlier spectroellipsometric (in situ and ex situ) and spectrophotometric (ex situ) studies, an optical model for the description of the interphase refractive-index profile n(z). We study in detail the effect of such an interphase on the spectral performance of various optical filters (antireflective V-coat, W-coat, achromatic W-coat, and minus filter). It is shown that considering the inhomogeneous n(z) profile in the design can improve the optical performance of some filters.

Simple Method for Determining Slowly Varying Refractive-Index Profiles from in situ Spectrophotometric Measurements.

Reliable control of the deposition process of optical films and coatings frequently requires monitoring the refractive-index profile throughout the layer. In the present research a simple in situ approach is proposed that uses a WKBJ matrix representation of the optical transfer function of a single thin film on a substrate. Mathematical expressions are developed that represent the minima and the maxima envelopes of the curves transmittance versus time and reflectance versus time. The refractive index and the extinction coefficient depth profiles of different films are calculated from simulated spectra as well as from experimental data obtained during the PECVD (plasma-enhanced chemical vapor deposition) of silicon-compound films. Variation in the deposition rate with time is also evaluated from the position of the spectra extrema as a function of time. The physical and mathematical limitations of the method are discussed.