Antireflection coating on PMMA substrates by atomic layer deposition

Antireflection coatings (ARC) are essential for various optical components including such made of plastics for high volume applications. However, precision coatings on plastics are rather challenging due to typically low adhesion of the coating to the substrate. In this work, optimization of the atomic layer deposition (ALD) processes towards conformal optical thin films of Al2O3, TiO2 and SiO2 on poly(methyl methacrylate) (PMMA) has been carried out and a five-layer ARC is demonstrated. While the uncoated PMMA substrates have a reflectance of nearly 8% in the visible (VIS) spectral range, this is reduced below 1.2% for the spectral range of 420–670 nm by applying a double-side ARC. The total average reflectance is 0.7%. The optimized ALD coatings show a good adhesion to the PMMA substrates even after the climate test. Microscopic analysis on the cross-hatch areas on PMMA after the climate test indicates very good environmental stability of the ALD coatings. These results enable a possible route by ALD to deposit uniform, crack free, adhesive and environmentally durable thin film layers on sensitive thermoplastics like PMMA

Optical properties of black silicon structures ALD-coated with Al 2 O 3

Atomic layer deposited (ALD) Al 2 O 3 coatings were applied on black silicon (b-Si) structures. The coated nanostructures were investigated regarding their reflective and transmissive behaviour. For a systematic study of the influence of the Al 2 O 3 coating, ALD coatings with a varying layer thickness were deposited on three b-Si structures with different morphologies. With a scanning electron microscope the morphological evolution of the coating process on the structures was examined. The optical characteristics of the different structures were investigated by spectral transmission and reflection measurements. The usability of the structures for highly efficient absorbers and antireflection (AR) functionalities in the different spectral regions is discussed

The Creatine Kinase/Creatine Connection to Alzheimer's Disease: CK Inactivation, APP-CK Complexes, and Focal Creatine Deposits

Cytosolic brain-type creatine kinase (BB-CK), which is coexpressed with ubiquitous mitochondrial uMtCK, is significantly inactivated by oxidation in Alzheimer's disease (AD) patients. Since CK has been shown to play a fundamental role in cellular energetics of the brain, any disturbance of this enzyme may exasperate the AD disease process. Mutations in amyloid precursor protein (APP) are associated with early onset AD and result in abnormal processing of APP, and accumulation of Aβ peptide, the main constituent of amyloid plaques in AD brain. Recent data on a direct interaction between APP and the precursor of uMtCK support an emerging relationship between AD, cellular energy levels, and mitochondrial function. In addition, recently discovered creatine (Cr) deposits in the brain of transgenic AD mice, as well as in the hippocampus from AD patients, indicate a direct link between perturbed energy state, Cr metabolism, and AD. Here, we review the roles of Cr and Cr-related enzymes and consider the potential value of supplementation with Cr, a potent neuroprotective substance. As a hypothesis, we consider whether Cr, if given at an early time point of the disease, may prevent or delay the course of AD-related neurodegeneration

Influence of substrate materials on nucleation and properties of iridium thin films grown by ALD

Ultra-thin metallic films are widely applied in optics and microelectronics. However, their properties differ significantly from the bulk material and depend on the substrate material. The nucleation, film growth, and layer properties of atomic layer deposited (ALD) iridium thin films are evaluated on silicon wafers, BK7, fused silica, SiO2 , TiO2 , Ta2O5 , Al2O3 , HfO2 , Ru, Cr, Mo, and graphite to understand the influence of various substrate materials. This comprehensive study was carried out using scanning electron and atomic force microscopy, X-ray reflectivity and diffraction, four-point probe resistivity and contact angle measurements, tape tests, and Auger electron spectroscopy. Within few ALD cycles, iridium islands occur on all substrates. Nevertheless, their size, shape, and distribution depend on the substrate. Ultra-thin (almost) closed Ir layers grow on a Ta2O5 seed layer after 100 cycles corresponding to about 5 nm film thickness. In contrast, the growth on Al2O3 and HfO2 is strongly inhibited. The iridium growth on silicon wafers is overall linear. On BK7, fused silica, SiO2 , TiO2 , Ta2O5 , Ru, Cr, and graphite, three different growth regimes are distinguishable. The surface free energy of the substrates correlates with their iridium nucleation delay. Our work, therefore, demonstrates that substrates can significantly tailor the properties of ultra-thin films

Enhanced Surface Second Harmonic Generation in Nanolaminates

Second-harmonic generation (SHG) is a second-order nonlinear optical process that is not allowed in media with inversion sym-metry. However, due to the broken symmetry at the surface, surface SHG still occurs, but is generally small. We experimentally investi-gate the surface SHG in periodic stacks of alternating, subwave-length dielectric layers, which have a large number of surfaces, thus enhancing surface SHG considerably. To this end, multilayer stacks of SiO2/TiO2 were grown by Plasma Enhanced Atomic Layer Deposition (PEALD) on fused silica substrates. With this technique individual layers of a thickness of less than 2 nm can be fabricated. We experimentally show that under large angles of incidence (> 20 degrees) there is substantial SHG, well beyond the level, which can be observed from simple interfaces. We perform this experiment for samples with different periods and thickness of SiO2/TiO2 and our results are in agreement with theoretical calculations

Nonlinear polarization holography of nanoscale iridium films

The phasing problem of heterodyne-detected nonlinear spectroscopy states that the relative time delay between the exciting pulses and a local oscillator must be known with subcycle precision to separate absorptive and dispersive contributions. Here, a solution to this problem is presented which is the time-domain analogue of holographic interferometry, in which the comparison of two holograms reveals changes of an objects size and position with interferometric precision (i.e. to fractions of a wavelength of light). The introduced method, called nonlinear polarization holography, provides equivalent information as attosecond nonlinear polarization spectroscopy but has the advantage of being all-optical instead of using an attosecond streak camera. Nonlinear polarization holography is used here to retrieve the time-domain nonlinear response of a nanoscale iridium film to an ultrashort femtosecond pulse. Using density matrix calculations it is shown that the knowledge of the nonlinear response with subcycle precision allows to distinguish excitation and relaxation mechanisms of low-energetic electrons that depend on the nanoscale structure of the iridium film

Materials Pushing the Application Limits of Wire Grid Polarizers further into the Deep Ultraviolet Spectral Range

Wire grid polarizers (WGPs), periodic nano-optical meta-surfaces, are convenient polarizing elements for many optical applications. However, they are still inadequate in the deep ultraviolet spectral range. We show that to achieve high performance ultraviolet WGPs a material with large absolute value of the complex permittivity and extinction coefficient at the wavelength of interest has to be utilized. This requirement is compared to refractive index models considering intraband and interband absorption processes. We elucidate why the extinction ratio of metallic WGPs intrinsically humble in the deep ultraviolet, whereas wide bandgap semiconductors are superior material candidates in this spectral range. To demonstrate this, we present the design, fabrication and optical characterization of a titanium dioxide WGP. At a wavelength of 193 nm an unprecedented extinction ratio of 384 and a transmittance of 10 % is achieved.Comment: 21 pages, Advanced Optical Materials 201

Iridium wire grid polarizer fabricated using atomic layer deposition

In this work, an effective multistep process toward fabrication of an iridium wire grid polarizer for UV applications involving a frequency doubling process based on ultrafast electron beam lithography and atomic layer deposition is presented. The choice of iridium as grating material is based on its good optical properties and a superior oxidation resistance. Furthermore, atomic layer deposition of iridium allows a precise adjustment of the structural parameters of the grating much better than other deposition techniques like sputtering for example. At the target wavelength of 250 nm, a transmission of about 45% and an extinction ratio of 87 are achieved