Immersion transmission ellipsometry (ITE): a new method for the precise determination of the 3D indicatrix of thin films

The new method of immersion transmission ellipsometry (ITE) [1] has been developed. It allows the highly accurate determination of the absolute three-dimensional (3D) refractive indices of anisotropic thin films. The method is combined with conventional ellipsometry in transmission and reflection, and the thickness determination of anisotropic films solely by optical methods also becomes more accurate. The method is applied to the determination of the 3D refractive indices of thin spin-coated films of an azobenzene-containing liquid-crystalline copolymer. The development of the anisotropy in these films by photo-orientation and subsequent annealing is demonstrated. Depending on the annealing temperature, oblate or prolate orders are generated. PACS 07.60.Fs; 42.70.Gi; 78.66.Qn     

Generalized ellipsometry for biaxial absorbing materials: determination of crystal orientation and optical constants of Sb(2)S(3)

We demonstrate generalized ellipsometry for precise measurement of the principal indices of refraction, the extinction coefficients, and the orientations of the crystal a, b, and c axes of orthorhombic absorbing materials. Stibnite (Sb(2)S(3)) single crystals cut approximately parallel to (100), (010), (001), and (313) are studied at a representative wavelength of 589 nm. The (313) surface is sufficient for retrieval of all optical constants. The expected effects of surface over-layer formation are removed numerically. We propose generalized ellipsometry as a powerful tool for measurement of anisotropic optical function spectra of biaxial materials.     

Spectral ellipsometry as a method for characterization of nanosized films with ferromagnetic layers

Nanosized films with ferromagnetic layers are widely used in nanoelectronics, sensor systems and telecommunications. Their properties may strongly differ from those of bulk materials that is on account of interfaces, intermediate layers and diffusion. In the present work, spectral ellipsometry and magnetooptical methods are adapted for characterization of the optical parameters and magnetization processes in two- and three-layer Cr/NiFe, Al/NiFe and Сr(Al)/Ge/NiFe films onto a sitall substrate for various thicknesses of Cr and Al layers. At a layer thickness below 20 nm, the complex refractive coefficients depend pronouncedly on the thickness. In two-layer films, remagnetization changes weakly over a thickness of the top layer, but the coercive force in three-layer films increases by more than twice upon remagnetization, while increasing the top layer thickness from 4 to 20 nm.     

Reflection and transmission of light by a ferromagnetic structure with a noncollinear orientation of the magnetization of layers

The transmission and reflection of light by a ferromagnetic structure with a noncollinear orientation of the magnetization vectors of layers lying in the plane of the film is considered. The characteristic matrix of the structure that relates the wave amplitudes at the entrance to the system and at the exit from it and that determines the magnetooptical properties of the structure is calculated. The magnetooptical characteristics of this structure are shown to significantly depend on the angle between the magnetization vectors of layers and on the number of layers. Both the magnitude and the character of these dependences are analyzed in relation to the angle of incidence and to the polarization of the incident wave. Such an analysis makes it possible to optimize the magnetooptical properties of this structure.     

Geometrical significance of the orientation of defect-induced electric field gradients

One of the features of defect-induced axial symmetric electric field gradients(efg) is the orientation of their symmetry axis with respect to the crystal lattice. Especially in the case of simple trapped defects(mono-, di-vacancies, single interstitials or single impurity atoms) this orientation should follow the symmetry of the geometrical probe atom defect arrangement. This will be illustrated by PAC measurements at¹¹¹In/¹¹¹Cd for the case of trapped impurity atoms in Cu.This work was supported by the Bundesminister für Forschung und Technologie.     

Analytical solution of the direct problem of ellipsometry for some profiles of optical constants of inhomogeneous layers

Differential equations that describe the reflection of polarized light from an optically inhomogeneous medium are considered. In the approximation of small variations of the refractive index, analytical expressions for the reflection coefficients are obtained for both types of polarization for the exponential and harmonic profiles of the optical constants. The accuracy of the obtained expressions is estimated by numerical simulation. It is found that analytical formulas describe well the behavior of the ellipsometric parameters of periodic structures based on Hg_1–xCd_xTe with a sinusoidal profile of the refractive index.     

Spectroscopic ellipsometry of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)

The optical functions of PTCDA have been determined by spectroscopic ellipsometry. The samples were prepared by thermal evaporation of PTCDA on glass substrates. The influence of the substrate temperature on the properties of PTCDA films has also been investigated. The lowest substrate temperature investigated here was room temperature. At room temperature and higher substrate temperatures, we have not observed ordering of the evaporated films, which would result in high anisotropy, as reported for PTCDA films grown by organic molecular-beam deposition at very low temperatures (so-called quasi-epitaxial growth). Therefore, the samples have been assumed to be isotropic. The obtained imaginary part of the dielectric function exhibits the expected features: a low-magnitude peak around 370 nm and a higher magnitude, two-peak structure in the 400–600 nm spectral region. PACS 78.20.Ci; 78.66.Qn     

The matrix formalism for generalised gradients with time-varying orientation in diffusion NMR

Protein backbone 15N NMR spin relaxation rates are useful in characterizing the protein dynamics and structures. To observe the protein nuclear-spin resonances a pulse sequence has to include a water suppression scheme. There are two commonly employed methods, saturating or dephasing the water spins with pulse field gradients and keeping them unperturbed with flip-back pulses. Here different water suppression methods were incorporated into pulse sequences to measure 15N longitudinal T1 and transversal rotating-frame T1ρ spin relaxation. Unexpectedly the 15N T1 relaxation time constants varied significantly with the choice of water suppression method. For a 25-kDa Escherichiacoli. glutamine binding protein (GlnBP) the T1 values acquired with the pulse sequence containing a water dephasing gradient are on average 20% longer than the ones obtained using a pulse sequence containing the water flip-back pulse. In contrast the two T1ρ data sets are correlated without an apparent offset. The average T1 difference was reduced to 12% when the experimental recycle delay was doubled, while the average T1 values from the flip-back measurements were nearly unchanged. Analysis of spectral signal to noise ratios (s/n) showed the apparent slower 15N relaxation obtained with the water dephasing experiment originated from the differences in 1HN recovery for each relaxation time point. This in turn offset signal reduction from 15N relaxation decay. The artifact becomes noticeable when the measured 15N relaxation time constant is comparable to recycle delay, e.g., the 15N T1 of medium to large proteins. The 15N relaxation rates measured with either water suppression schemes yield reasonable fits to the structure. However, data from the saturated scheme results in significantly lower Model-Free order parameters (=0.81) than the non-saturated ones (=0.88), indicating such order parameters may be previously underestimated.     

c(2 x 2)CO ON Ni(100): Photoemission orientation determination

Angular resolved photoemission studies of c(2 x 2)CO adsorbed on Ni(100) show that the molecule is bound to the surface with the molecular axis normal to the surface. The uncertainty of this determination is approximately 15°, which is consistent with the expected angular broadening due to vibrational modes. This is in distinct contrast to a bend of 34° proposed to explain LEED data on this system.     

Brownian motors in the photoalignment of liquid crystals

Light can change the orientation of liquid-crystal molecules. Usually, the torque that causes the reorientation originates in angular-momentum transfer from the radiation field to the material. If a small amount of dichroic dye is dissolved in the liquid crystal, a light-induced torque can appear essentially without the transfer of angular momentum from light. We show that, in such cases, the dye molecules act as light-driven molecular motors which, via an orientational Brownian ratchet mechanism, transfer angular momentum, which originates at the cell walls, to the liquid crystal. Understanding the details of this mechanism is important for applications ranging from flat-panel displays to optomechanical transducers. Received: 20 October 2001 / Accepted: 14 January 2002 / Published online: 22 April 2002     

Application of spectroscopic ellipsometry to the investigation of the optical properties of cobalt-nanostructured silica thin layers

Spectroscopic ellipsometry is used to investigate optical properties of cobalt-implanted silica thin films. The films under investigation are 250 nm thick thermal SiO₂ layers on Si substrates implanted with Co⁺ ions at energy of 160 keV and at fluences of 10¹⁷ ions/cm² for different temperatures of substrate during implantation (77 and 295 K). Changes due to Co⁺ implantation are clearly observed in the optical response of the films. Optical behaviours are furthermore different for the three implantation temperatures. To understand the optical responses of these layers, the ellipsometric experimental data are compared to different models including interference effects and metal inclusions effects into the dielectric layer. The simulated ellipsometric data are obtained by calculating the interferences of an inhomogeneous layer on a Si substrate. The material within this layer is considered as an effective medium which dielectric function is calculated using the Maxwell-Garnett effective medium approximation. We show that although the structures of these layers are very complicated because of ion-implantation mechanisms, quite simple models can provide relatively good agreement. The possibilities of ellipsometry for the study of the optical properties of such clusters-embedded films are discussed. We especially provide the evidence that ellipsometry can give interesting information about the optical properties of nanostructured layers. This is of special interest in the field of nanostructured layered systems where ellipsometry appears to be a suitable optical characterization technique.     

Real-time in situ spectroscopic ellipsometry investigation of the amorphous to crystalline phase transition in Mo single layers

Molybdenum single layers were grown by ion beam sputter deposition onto [001] Si substrates. Argon or xenon was used as sputter gas. The layer growth was monitored by real-time in situ spectroscopic ellipsometry in the visible spectral region. A volume phase transition from amorphous to polycrystalline molybdenum layer growth was observed during the deposition process. The time regime of the phase transition as well as the layer thickness at which the phase transition occurs, depends on the sputter regime, especially on sputter species and deposition-pressure range. The thermodynamic approach of energy minimisation is discussed as the driving force for the Mo phase transition. A moderate backscattered particle bombardment of the growing molybdenum film provides the activation energy for the recrystallisation process. A self-diffusion-like process is made responsible for atomic rearrangement of the entire as-deposited thin-film volume. The molybdenum phase transition is connected to thin-film densification and therefore volume contraction. PACS 68.65.Ac; 81.15.Cd; 78.67.Pt     

Interference in the ultracold neutron transmission through thin layers

At the time-of-flight spectrometer for ultracold neutrons (UCN) with energies below 10^?4 eV at the FRM, Munich, the perpendicular neutron transmission through carbon and gold films of thickness 450 to 800 Å was measured as a function of neutron velocity in the range 11≧v _z≧3.7 m/s. The transmission curves obtained show the expected interference patterns due to interference between the partial waves transmitted and reflected at the two surfaces. From these curves absolute values for the thickness and density could be obtained. From the UCN transmission through a 4.71 μm thick copper foil a value a_coh=7.5±0.15 F was derived for the nuclear scattering length of Cu.     

Immersion cooling of silicon photomultipliers (SiPM) for nuclear medicine imaging applications

Silicon photomultipliers (SiPM) are compact, high amplification light detection devices that have recently been incorporated into magnetic field-compatible positron emission tomography (PET) scanners. To take full advantage of these devices, it is preferable to cool them below room temperature. Most current methods are limited to the cooling of individual detector modules, increasing complexity and cost of scanners made-up of a large number of modules. In this work we investigated a new method of cooling, immersion of the detector modules in non-electrically conductive, cooled liquid. A small-scale prototype system was constructed to cool a relatively large area SiPM-based, scintillator detector module by immersing it in a circulating bath of mineral oil. Testing demonstrated that the system rapidly decreased and stabilized the temperature of the device. Operation of the detector illustrated the expected benefits of cooling, with no apparent degradation of performance attributable to immersion in fluid.     

Laser imaging system for determination of three-dimensional scalar gradients in turbulent flames

An imaging system for the measurement of three-dimensional (3D) scalar gradients in turbulent hydrocarbon flames is described. Combined line imaging of Raman scattering, Rayleigh scattering, and CO laser-induced fluorescence (LIF) allows for simultaneous single-shot line measurements of major species, temperature, mixture fraction, and a one-dimensional surrogate of scalar dissipation rate in hydrocarbon flames, while simultaneous use of two crossed, planar LIF measurements of OH allows for determination of instantaneous flame orientation. In this manner the full 3D scalar dissipation can be estimated in some regions of a turbulent flame on a single-shot basis.