Ingar Stian Nerbø

Characterization of inclined GaSb nanopillars by Mueller matrix ellipsometry

Inclined GaSb nanopillars prepared by low energy ion sputtering with oblique ion beam incidence have been characterized by two different Mueller matrix ellipsometric tools. The optical properties of the nanopillars were found to be well described by a uniaxial anisotropic graded effective medium model. The pillar height and inclination angle were determined by fitting the parameters of the effective medium model to spectroscopic (1.44–2.88 eV) Mueller matrix measurements at multiple azimuth sample orientations. A set of different samples with various average pillar height and inclination angle was studied; results from the optical characterization correspond well with those from scanning electron microscopy analysis. For samples with nanopillars inclined by 45° or less, the height could be determined from a single Mueller matrix measurement at only one azimuth orientation, allowing real-time in situ observation of the formation. The nanopillars were also studied using a single wavelength angle resolved Mu...

Diffuse-interface model for nanopatterning induced by self-sustained ion etch masking

We construct a simple phenomenological diffuse-interface model for composition-induced nanopatterning during ion sputtering of alloys. In simulations, this model reproduces without difficulties the high-aspect-ratio structures and tilted pillars observed in experiments. We investigate the time evolution of the pillar height, both by simulations and by in situ ellipsometry. The analysis of the simulation results yields a good understanding of the transitions between different growth regimes and supports the role of segregation in the pattern-formation process.

Fast and optimal broad-band Stokes/Mueller polarimeter design by the use of a genetic algorithm

A fast multichannel Stokes/Mueller polarimeter with no mechanically moving parts has been designed to have close to optimal performance from 430-2000 nm by applying a genetic algorithm. Stokes (Mueller) polarimeters are characterized by their ability to analyze the full Stokes (Mueller) vector (matrix) of the incident light (sample). This ability is characterized by the condition number, κ, which directly influences the measurement noise in polarimetric measurements. Due to the spectral dependence of the retardance in birefringent materials, it is not trivial to design a polarimeter using dispersive components. We present here both a method to do this optimization using a genetic algorithm, as well as simulation results. Our results include fast, broad-band polarimeter designs for spectrographic use, based on 2 and 3 Ferroelectric Liquid Crystals, whose material properties are taken from measured values. The results promise to reduce the measurement noise significantly over previous designs, up to a factor of 4.5 for a Mueller polarimeter, in addition to extending the spectral range.

Real-time in situ Mueller matrix ellipsometry of GaSb nanopillars: observation of anisotropic local alignment

The formation of GaSb nanopillars by low energy ion sputtering is studied in real-time by spectroscopic Mueller matrix ellipsometry, from the initial formation in the smooth substrate until nanopillars with a height of 200-300 nm are formed. As the nanopillar height increased above 100 nm, coupling between orthogonal polarization modes was observed. Ex situ angle resolved Mueller polarimetry measurements revealed a 180° azimuth rotation symmetry in the off-diagonal Mueller elements, which can be explained by a biaxial material with different dielectric functions εx and εy in a plane parallel to the substrate. This polarization coupling can be caused by a tendency for local direction dependent alignment of the pillars, and such a tendency is confirmed by scanning electron microscopy. Such observations have not been made for GaSb nanopillars shorter than 100 nm, which have optical properties that can be modeled as a uniaxial effective medium.

Characterization of nanostructured GaSb: comparison between large-area optical and local direct microscopic techniques.

Low energy ion-beam sputtering of GaSb results in self-organized nanostructures with the potential of structuring large surface areas. Characterization of such nanostructures by optical methods is studied and compared to direct (local) microscopic methods. The samples consist of densely packed GaSb cones on bulk GaSb, approximately 30, 50, and 300 nm in height, prepared by sputtering at normal incidence. The optical properties are studied by spectroscopic ellipsometry, in the range 0.6-6.5 eV, and with Mueller matrix ellipsometry in the visible range, 1.46-2.88 eV. The optical measurements are compared to direct topography measurements obtained by scanning electron microscopy, high resolution transmission electron microscopy, and atomic force microscopy. Good agreement is achieved between the two classes of methods when the experimental optical response of the short cones (<55 nm) is inverted with respect to topological surface information, via a graded anisotropic effective medium model. The main topological parameter measured was the average cone height. Optical methods are shown to represent a valuable characterization tool of nanostructured surfaces, in particular when a large coverage area is desirable. Because of the fast and nondestructive properties of optical techniques, they may readily be adapted to in situ configurations.

Well-conditioned multiple laser Mueller matrix ellipsometer

We report on the design and performance test of a multiple laser Mueller matrix ellipsometer (MME). The MME is well conditioned due to the integration of the recently reported achromatic 132-deg compensators based on biprisms, in combination with high-quality Glan-Thompson polarizers. The system currently operates between 300 and 2700 nm, without the need to change any optical components except for the detector. Four lasers are employed as light sources (405, 532, 633, and 1570 nm) to test the performance in both reflection and transmission modes. Thus, the system is used to determine the Mueller matrices and associated optical constants of known optical systems: 1. optical rotatory power of D-glucose in solution, 2. reflection of a native oxide c-Si wafer, and 3. the properties of a liquid crystal spatial light modulator. The results show that the system matrices of the MME have condition numbers between the optimal and 2 during operation, resulting in small experimental errors. To the best of our knowledge, there is no other MME reported with such good conditioning over a comparably wide spectral range.

Characterisation of Liquid Crystals for broadband optimal design of Mueller matrix ellipsometers

The optimal design over a broad spectral range of Liquid Crystal (LC) based Polarisation State Generators (PSG) and Polarisation State Analyzers (PSA), requires detailed knowledge of the spectral behaviour of the LCs. The full Mueller matrix measurement formalism based on the Eigenvalue Calibration Method, does in principle not require an exact modelling of the polarizing components, however, it is required that the condition number for both the PSG and the PSA is close to optimum over a wide spectral range. Two LC technologies are investigated here, Ferroelectric LC (FLC), and Liquid Crystal Variable Retarders (LCVR). In the case of a FLC based PSG, additional components, such as fixed retarders also need to be properly characterised in order for a proper broad spectral optimal design. These issues are here studied by characterizing the FLCs, the LCVRs and the fixed retarders with the help of a commercial visible Mueller matrix ellipsometer, and a polarizer-rotating sample-rotating analyzer near infra red set-up. The results are analyzed in the framework of the optimal condition number over a broad spectral range.

Mueller matrix imaging of plasmonic polarizers on nanopatterned surface

We present the application of a near infra red Mueller matrix imaging ellipsometer to the characterization of plasmonic polarizers. The samples are prepared by evaporation of Au onto SiO2 ripples. The nanostructured ripple surface has been produced by ion beam sputtering at an off normal angle of incidence. Au was thereafter evaporated onto the surface at an grazing angle. As a result, thin lines of nearly connected Au nanoparticles form along the illuminated side of the ripples, resulting in a large in-plane anisotropy of the structure. Mueller matrix imaging is used to determine the lateral uniformity of the optical signal in correlation to the real space topography of the sample, and to determine to what degree the nanoparticles tend to form a connected wire, or whether there are well separated Au particles. The success of this method in order to produce polarizers, lies in controlling the process to allow well connected lines of Au particles along the ripples, with a high degree of homogeneity. Mueller Matrix images of the sample recorded at normal incidence are shown, and the information that can be extracted from such images is discussed.

Genetic invention of fast and optimal broad-band stokes/mueller polarimeter designs

We have applied a genetic algorithm to generate optimal polarimeter designs for a selected wavelength interval, assuming known dispersion relations of the components. Our results are improvements on previous patented designs based on ferroelectric liquid crystals.