Near-normal Incidence Research Articles

Optical anisotropy of wurtzite GaN on sapphire characterized by spectroscopic ellipsometry

Optical anisotropy of wurtzite GaN grown on sapphire was studied by spectroscopic ellipsometry (SE) over the wavelength range of 400–1200 nm at the incident angle of 68°. Both the dispersions of the refractive indices for E ⊥ c (no) and E ∥ c(ne) as well as the film thickness were determined by fitting SE measurement data with a suitable optical model. The average difference between the values of no and ne over the experimental wavelength range is 2.7%. To verify the results of SE measurements, the obtained values of no were used to analyse the interference part of the UV-Vis reflection spectrum of GaN with near-normal incidence geometry and the film thickness was also calculated. The two thickness values obtained by SE and reflection spectra were in good consistence and the relative deviation was found to be only 0.16%.

14.5% near-normal incidence reflectance of Cr/Sc x-ray multilayer mirrors for the water window.

Cr/Sc multilayer mirrors, synthesized by ion-assisted magnetron sputter deposition, are proved to have a high near-normal reflectivity of R = 14.5% at a grazing angle of 87.5 degrees measured at the wavelength lambda = 3.11 nm, which is an improvement of more than 31% compared with previously published results. Elastic recoil detection analyses show that the mirrors contained as much as 15 at. % of N and traces of C and O. Soft x-ray reflectivity simulations reveal interface widths of sigma = 0.34 nm and an exceptionally small layer thickness drift of approximately 1.6 x 10(-5) nm/multilayer period throughout the stack. Simulations show that a reflectivity of R = 25.6% is attainable if impurities and layer thickness drift can be eliminated. The abrupt interfaces are achieved with ion assistance with a low ion energy of 24 eV and high ion-to-metal flux ratios of 7.1 and 23.1 during Cr and Sc sputter deposition, respectively. In addition, a near-normal incidence reflectivity of 5.5% for the C VI emission line (lambda = 3.374 nm) from a laser plasma source was verified.

Anomalous peaks in reflectance spectra of some GaAs substrates

GaAs substrates of various planar orientations were characterized optically by measuring reflectance spectra as near-normal incidence. Anomalous peaks were observed in some spectra in the wavenumber range of 250–550cm−1. Several hypotheses for the origin of these peaks were suggested and investigated.

Wadsworth mount redux: paraboloidal versus spherical gratings.

Spherical and paraboloidal diffraction gratings are considered for use on an experiment involving a collection of satelliteborne spectrographs that will investigate astrophysical plasmas in the spectral region from approximately 100 to 300 A. We shall use eight spectrographs: Wadsworth mounts in near-normal incidence with effective focal lengths of 3 m, each centered on a chosen wavelength in this spectral range. Ray tracings show that paraboloidal gratings used in the Wadsworth mount have a significant increase in resolution over spherical gratings of the same effective focal length, used in the same mounting, as long as the spectral ranges are small (approximately 10% of the central wavelength). This increase in resolution is due to the spatial extent of coma for a paraboloidal grating that is less than the circle of least confusion of spherical gratings.

Exciton–polariton behaviour in bulk and polycrystalline ZnO

We report detailed reflectance studies of the exciton–polariton structure of thin film polycrystalline ZnO and comparison with bulk crystal behaviour. Near-normal incidence reflectance spectra of these samples are fitted using a two-band dielectric response function. Our data show that the reflectance data in polycrystalline ZnO differ substantially from the bulk material, with Fabry–Perot oscillations at energies below the transverse A exciton and above the longitudinal B exciton in the films. In the strong interaction regime between these energies no evidence is seen of the normally rapid oscillations associated with the anomalous waves. We demonstrate that the strong interaction of the damped exciton with the photon leads to polaritons in this region with substantial damping such that the Fabry–Perot modes are eliminated. Good qualitative agreement is achieved between the model and data. The importance of the polariton model in understanding the reflectance data of polycrystalline material is clearly seen.

Derivation of the characteristic transfer matrix of the interfacial layer in superlattices using the characteristic effective medium approximation

This is an analytic derivation of the characteristic matrix of the interfacial layer in a bilayer superlattice using the characteristic matrix technique. The derivation applies for near-normal incidence spectroscopy and presents the interfacial layer as a perturbation matrix. Calculations of the optical constants, absorption coefficient, skin depth, and optical conductivity of superlattices with and without interfacial layers between their primary constituent layers are presented and compared. The calculations employ values of the optical constants of the layered pair and the interfacial layer in between, all obtained from the characteristic effective medium approximation that was established in an earlier communication.

Optimization strategies for parametric analysis of thin-film reflectivity spectra

Near-normal incidence Fourier transform infrared reflection spectra are utilized to determine the optical properties and thickness of thin films. A parametric model of the refractive index for the wavelength range is used in conjunction with three optimization algorithms to determine the global minimum of these complex functions. The simple downhill simplex algorithm can find the global minimum quickest but only if the starting parameters are such that local minima are not encountered on route, and for more complex optical functions, the usefulness decreases. The simulated annealing optimization and genetic algorithms are able to find global minima reproducibly but are seen best as complementary rather than competitive. Genetic algorithms are limited in accuracy according to the number of bits used to encode the solution, and the speed of computation decreases with increasing bits. Simulated annealing is a random process and tends to be slower initially, but any desired accuracy can be achieved by adjusting the random step length as the algorithm proceeds.

Polarized infrared emissivity from dew droplets on a surface.

We model the infrared polarization emissivity from spherical particles on a plane surface. The emissivity and polarization is primarily a function of the density of particles multiplied by their cross-sectional area. The presence of particles tends to reduce the polarization. As the emission angle increases from near-normal incidence, the polarization tends to pass through a maximum, followed by a minimum and a final sharp rise at near grazing angles. The mechanism for this structure is the shadowing of different portions of the spherical particles by other particles.

Photonic bandgap materials and structures by sol–gel processing

The sol–gel method is an inexpensive and flexible liquid phase processing technique that is suitable for the fabrication of 1-D photonic bandgap structures, such as multilayer dielectric mirrors. In the present work, we started by preparing highly reflecting quarter-wave multilayer stacks, composed of alternating films of high refractive index material (TiO2) and low refractive index material (SiO2), possessing well-defined stop bands. For example, for a 11-layer stack, a stop band was obtained with more than 96% reflectance (at near-normal incidence) between ∼600 and 700 nm, with a gap to mid-gap ratio Δλ/λmax=15.4%. Fabry–Perot micro-cavities, containing high or low index defects within the quarter-wave stack, were also prepared and tested. Such structures were optimized with the help of suitable modeling, using transfer matrix techniques. Various experimental techniques were used to characterize these multilayers, including ellipsometry, visible spectroscopy, X-ray diffraction and atomic force microscopy.

Performance bounds for passive sensor arrays operating in a turbulent medium: plane-wave analysis.

The performance bounds of a passive acoustic array operating in a turbulent medium with fluctuations described by a von Kármán spectrum are investigated. This treatment considers a single, monochromatic, plane-wave source at near-normal incidence. A line-of-sight propagation path is assumed. The primary interests are in calculating the Cramer-Rao lower bounds of the azimuthal and elevational angles of arrival and in observing how these bounds change with the introduction of additional unknowns, such as the propagation distance, turbulence parameters, and signal-to-noise ratio. In both two and three dimensions, it is found that for large values of the index-of-refraction variance, the Cramer-Rao lower bounds of the angles of arrival increase significantly at large values of the normalized propagation distance. For small values of the index-of-refraction variance and normalized propagation distance, the signal-to-noise ratio is found to be the limiting factor. In the two-dimensional treatment, it is found that the estimate of the angle of arrival will decouple from the estimates of the other parameters with the appropriate choice of array geometry. In three dimensions, again with an appropriate choice of array geometry, the estimates of the azimuth and elevation will decouple from the estimates of the other parameters, but due to the constraints of the model, will remain coupled to one another.

Effective medium approximation for the effective optical constants of a bilayer and a multilayer structure based on the characteristic matrix technique

This article presents a derivation of the effective optical constants of a bilayer using the characteristic matrix technique. The derivation is simple, and in essence, adds an extra step to an approximate treatment of near-normal incidence spectroscopy, based on the characteristic matrix technique for stratified media. By this method, the effective optical constants of a bilayer consisting of two thin layers, each with a thickness much less than the wavelength of the incident radiation, are determined. This step leads to a modified procedure for calculating the optical properties of a layered stack that consists of thin bilayer units. The method, though still an effective medium approximation, is distinct from previous approximations. It introduces a more consistent parallelism to the fundamental theory of the characteristic matrix technique, and leads to a simplified analog of a multilayer viewed as one system of definite effective optical constants. Such a task has been difficult to achieve for structures with an arbitrary number of layers.

Excitation-induced coherence in a semiconductor microcavity

We measure the dephasing times of high finesse semiconductor microcavities in the strong coupling regime using degenerate four-wave mixing. We find that under certain excitation conditions (near-normal incidence) the dephasing time of the lower polariton branch increases with the excitation intensity. This excitation induced coherence indicates that the lower polariton is acting as a Bose particle.

Simulation of SiO 2 build-up in silicon under oxygen bombardment

We have developed a method to simulate the oxygen saturation in oxygen bombarded silicon at near-normal incidence, which is easily introduced in our implantation/sputtering calculation frame, allowing an efficient simulation of this phenomenon. The method states a limit to the allowed maximum implantation rate, depending on the oxygen concentration and assumes a high mobility of the oxygen in the oxide. Here, we explain this method and show the calculation parameters influence on the 0°, 5 keV, O 2 + case. The obtained oxide width lays in the experimentally measured range, as well as the sputtering yield’s transition dose. We also observe that the latter is related to the formation of the final shape of the concentration profiles at the surface, while the outdiffusion yield’s transition dose is linked to the end of the oxide growing. However, more refined and diverse measurements are needed for a precise determination of the parameters’ values.

Gallium Phase Formation in Cu and Other FCC Metals During Near-Normal Incidence Ga-FIB Milling and Techniques to Avoid this Phenomenon

Gallium Phase Formation in Cu and Other FCC Metals During Near-Normal Incidence Ga-FIB Milling and Techniques to Avoid this Phenomenon

Simultaneous multimode experiments for studies of electrochemical reaction mechanisms: demonstration of concept.

An instrumental setup has been configured for a simultaneous real time recording of electrochemical, spectroelectrochemical, and mass data on an electrode from a single measurement for studies of complex electrochemical reaction mechanisms. This was achieved by combining a few pieces of equipment: a potentiostat/galvanostat for electrochemical measurements, the near-normal incidence reflectance spectrometer for spectroelectrochemical measurements, and a quartz crystal analyzer (QCA) for mass changes on the electrode. The latter two were coupled through a bifurcated optical fiber on the reflective QCA electrode. The experimental setup thus assembled was demonstrated to be very powerful in elucidating reaction mechanisms of the reaction sequence for a complex reaction, for example, electrochemical oxidation of aniline, which accompanies spectral, as well as weight, changes upon oxidation. The system was also applied to the study of the lithium intercalation reaction of tungsten trioxide thin films in a nonaqueous solution.

Atomic-precision multilayer coating of the first set of optics for an extreme-ultraviolet lithography prototype system.

We present our results of coating a first set of optical elements for an extreme-ultraviolet (EUV) lithography system. The optics were coated with Mo-Si multilayer mirrors by dc magnetron sputtering and characterized by synchrotron radiation. Near-normal incidence reflectances above 65% were achieved at 13.35 nm. The run-to-run reproducibility of the reflectance peak wavelength was maintained to within 0.4%, and the thickness uniformity (or gradient) was controlled to within +/-0.05% peak to valley, exceeding the prescribed specification. The deposition technique used for this study is an enabling technology for EUV lithography, making it possible to fabricate multilayer-coated optics to accuracies commensurate with atomic dimensions.

Seismic investigation of thick evaporite deposits on the central and inner unit of the Mediterranean Ridge accretionary complex

Seismic reflection observations from the IMERSE cruise have confirmed the presence of at least two localised thick evaporite basins on the crest and Inner Plateau of the Mediterranean Ridge accretionary complex. The discovery is supported by interval velocity results from both near-normal incidence reflection data and wide-angle seismic data. These evaporite basins are located north of a deep trough known as the Cleft and at a currently active thrust zone south of the Cleft, on the crest of the Mediterranean Ridge. The polarity of the seismic reflections associated with the base of the evaporite shows a phase reversal relative to the seafloor. This high amplitude phase reversed reflection indicates a decrease in velocity between the evaporite and the underlying sediment. The existence of these thick evaporite deposits suggests that the distribution of Messinian salt is much thicker (∼1.8–2 km) on parts of the Ridge crest than initially perceived by previous investigations. We propose that these thick evaporite deposits were deposited in localised basins on the Ridge, formed in response to pre- and syn-Messinian tectonics. The evaporites on the Inner Plateau basin most likely represent the infill of a Messinian forearc basin. The existence of local deep evaporite basins on the crest of the Ridge at depths of 2.7–3.3 km below sea level at the present day supports the hypothesis that the Mediterranean Sea level must have dropped by around 3 km below its present sea level during the Messinian salinity crisis.

Chromium–scandium multilayer mirrors for the nitrogen K _ α line in the water window region

Chromium-scandium (Cr-Sc) is a promising material combination for multilayer mirrors in the water window region. A possible x-ray source for laboratory use in this wavelength range is the nitrogen K(alpha) line at 3.16 nm. High reflectivities at this wavelength can be achieved with Cr-Sc multilayer mirrors if the interfaces between adjacent layers are smooth. The growth parameters of the magnetron sputtering process for these materials have been optimized. It is shown that the reflectivity of such mirrors can be considerably improved by the application of a proper bias voltage during film growth. The high quality of the multilayer films is demonstrated with copper K(alpha) x-ray reflection and transmission electron microscopy. The reflective properties of the multilayers close to the nitrogen K(alpha) line were measured with synchrotron radiation for different angles of incidence. Reflectivities between R = 5.9% for near-normal incidence (theta = 1.5 degrees) and R = 29.6% for theta = 59.9 degrees were measured.

Focusing multilayer mirror detection system for carbon K edge soft x-ray absorption spectroscopy (invited)

Fluorescence yield carbon K edge soft x-ray absorption measurements of mixed element samples are impeded by background scattered x-rays and fluorescence from noncarbon atoms within the sample induced by the incident x-ray beam (first and higher orders). We describe a high efficiency near-normal incidence focusing multilayer mirror detection system for carbon K edge fluorescence yield soft x-ray absorption spectroscopy. A spherical focusing multilayer mirror collects nearly 60% of the available solid angle with a reflectivity of 6% and a resolution of 2% (dE/E) for carbon K radiation in combination with a high efficiency proportional counter detector. The focusing multilayer system is able to effectively discriminate the background fluorescence and scattered light signals originating at the sample during a carbon K edge soft x-ray absorption experiment. A signal-to-background ratio in excess of 100 is possible for very dilute carbon samples, enabling a whole new class of in situ photon-in photon-out soft x-ray absorption measurements of carbon chemistry in catalysts to be made, even in a reactive chemical environment.

Polarization Anisotropy in Electroreflectance Spectrum of Porous Silicon

ABSTRACTLinearly polarized electroreflectance (ER) measurements have been firstly performed on PSi with near-normal incident (NNI) and glancing-angle incident (GAI) light. In the NNI geometry, no significant polarization anisotropy is observed. Meanwhile, in GAI geometry, a clear polarization anisotropy is observed. The s-polarized ER spectrum consists of the ER features corresponding to those found in the NNI geometry, while in the p-polarized ER spectrum, the two features found in s-polarized spectrum in infrared-visible region are not found, while a new feature is observed at ∼2.1 eV. The mechanism of the unusual polarization anisotropy is discussed in association with structural anisotropy of PSi.