Slant Incidence Research Articles

High-Order Fiber Mode Beam Parameter Optimization for Transport and Rotation of Single Cells.

Optical tweezers are becoming increasingly important in biomedical applications for the trapping, propelling, binding, and controlled rotation of biological particles. These capabilities enable applications such as cell surgery, microinjections, organelle extraction and modification, and preimplantation genetic diagnosis. In particular, optical fiber-based tweezers are compact, highly flexible, and can be readily integrated into lab-on-a-chip devices. Taking advantage of the beam structure inherent in high-order modes of propagation in optical fiber, LP11, LP21, and LP31 fiber modes can generate structured radial light fields with two or more concentrations in the cross-section of a beam, forming multiple traps for bioparticles with a single optical fiber. In this paper, we report the dynamic modeling and optimization of single cell manipulation with two to six optical traps formed by a single fiber, generated by either spatial light modulation (SLM) or slanted incidence in laser-fiber coupling. In particular, we focus on beam size optimization for arbitrary target cell sizes to enable trapped transport and controlled rotation of a single cell, using a point matching method (PMM) of the T-matrix to compute trapping forces and rotation torque. Finally, we validated these optimized beam sizes experimentally for the LP21 mode. This work provides a new understanding of optimal optical manipulation using high-order fiber modes at the single-cell level.

Modeling Transmission and Reflection Mueller Matrices of Dielectric Half-Wave Plates

We present a simple analytical model describing multiple reflections in dielectric and optically active waveplates, for both normal and slant incidence, including absorption. We compute from first principles the transmission and reflection Mueller matrices of the waveplate. The model is used to simulate the performance of a Stokes polarimeter for mm-waves, in the framework of current attempts to precisely measure the linear polarization of the Cosmic Microwave Background (CMB). We study the spectral response of these optical devices, taking into account band and angle averaging effects and confirm the presence of a much richer spectral dependence than in an ideal phase retarder. We also present the matrix elements for the reflection matrix, which is useful to estimate systematic effects in some polarimeter configurations. The formulas we have derived can be used to quickly simulate the performance of future CMB polarimeters.

Analytical modeling, finite-difference simulation and experimental validation of air-coupled ultrasound beam refraction and damping through timber laminates, with application to non-destructive testing

Reliable non-destructive testing (NDT) ultrasound systems for timber composite structures require quantitative understanding of the propagation of ultrasound beams in wood. A finite-difference time-domain (FDTD) model is described, which incorporates local anisotropy variations of stiffness, damping and density in timber elements. The propagation of pulsed air-coupled ultrasound (ACU) beams in normal and slanted incidence configurations is reproduced by direct definition of material properties (gas, solid) at each model pixel. First, the model was quantitatively validated against analytical derivations. Time-varying wavefronts in unbounded timber with curved growth rings were accurately reproduced, as well as the acoustic properties (velocity, attenuation, beam skewing) of ACU beams transmitted through timber lamellas. An experimental sound field imaging (SFI) setup was implemented at NDT frequencies (120kHz), which for specific beam incidence positions allows spatially resolved ACU field characterization at the receiver side. The good agreement of experimental and modeled beam shifts across timber laminates allowed extrapolation of the inner propagation paths. The modeling base is an orthotropic stiffness dataset for the desired wood species. In cross-grain planes, beam skewing leads to position-dependent wave paths. They are well-described in terms of the growth ring curvature, which is obtained by visual observation of the laminate. Extraordinary refraction phenomena were observed, which lead to well-collimated quasi-shear wave coupling at grazing beam incidence angles. The anisotropic damping in cross-grain planes is satisfactorily explained in terms of the known anisotropic stiffness dataset and a constant loss tangent. The incorporation of high-resolution density maps (X-ray computed tomography) provided insight into ultrasound scattering effects in the layered growth ring structure. Finally, the combined potential of the FDTD model and the SFI setup for material property and defect inversion in anisotropic materials was demonstrated. A portable SFI demonstrator was implemented with a multi-sensor MEMs receiver array that captures and compensates for variable wave propagation paths in glued laminated timber, and improves the imaging of lamination defects.

Acoustic behavior of sandwich plates

Sandwich structures of honeycomb cores and carbon laminate face sheets are ideal to meet low-weight and high-stiffness specifications. However, these structures are susceptible to noise problems. They tend to radiate structure-borne sound very efficiently or be transparent for air-borne sound. The problem’s root is the coincidence frequency, at which the wavelengths of beam and air coincide. In the vicinity of the coincidence frequency, observing wave fields with slanted incidence, the air’s trace wavelength can match the beam’s wavelength. Unlike homogenous structures, where wave trace matching only occurs in a narrow frequency band, sandwich structures can display this effect in a more than two decade wide frequency region. This facilitates noise transmission in a wide frequency band. The first focus of this paper is how to avoid this problem in the first place by designing the sandwich adequately. To check the region of wave trace matching, dispersion curves are used. A simplified but sufficiently accurate dispersion curve is proposed. The second focus lies in the interpretation of the dispersion curve, i.e. what is physically happening in the beam structure. The third focus is placed on the experimental identification of the sandwich beam structure’s shell parameters including its damping. Special attention is placed on which measures are necessary and which not in the experimental setup and experimental procedure to achieve sufficiently accurate results.

Scatterometry of Slant Incidence to Isolated Scatterers for High-Density Memory

The turning on and off of a convex scatterer can be used to store information in digital versatile disc (DVD) or Blu-ray technology. In addition, the distance between scatterers can be used as information, if it is precisely measured. Using a recently developed method, a single Fourier transformation of a diffraction pattern can be used to measure the distance between scatterers with high precision and without a lens. To enhance the precision for high-density memory, we examined a method to improve the resolution of measuring isolated convexes. We show by simulation of RCWA that slant incident light is useful for this purpose. We also show by experiment the validity of the method using gratings and the wing of a butterfly.

Novel slanted incidence air-coupled ultrasound method for delamination assessment in individual bonding planes of structural multi-layered glued timber laminates

Non-destructive assessment of delaminations in glued laminated timber structures is required during their full life cycle. A novel air-coupled ultrasound (ACU) method has been developed, which is able to separately detect delaminations in individual bonding planes of arbitrarily high and long laminated stacks and typically 200mm wide. The 120kHz ACU transmitter–receiver pair is positioned at two opposite lateral faces of the sample, with a small inclination with respect to the inspected bonding planes, so that an ultrasound beam is excited at a user-defined refraction angle within the sample, interacting with defects in a limited height portion of the stack. The attenuation of the ultrasound beam transmitted across the defect (negative detection) provided better sensitivity to defects than the scattered fields (positive detection), which are masked by spurious fields. Dedicated finite-difference time-domain (FDTD) simulations provided understanding on the wave propagation and defect detectability limits, with respect to the heterogeneous anisotropic material structure introduced by the curvature of the annual rings in individual timber lamellas. A simplified analytical expression was derived to calculate refraction angles in timber in function of insonification angle and ring angle. Experimental results show that the method is able to detect >20% wide defects in both isotropic material and in glulam with straight year rings, and >50% wide and 100mm long defects in commercial glulam beams. The discrimination of defects from background variability is optimized by normalizing the images with respect to reference defect-free sample sections (normalization) or previous measurements (difference imaging), and by combining readings obtained with distinct ultrasound beam refraction angles (spatial diversity). Future work aims at the development of a tomographic defect inspection by combining the described theoretical and experimental methods.

Glints from particulate media and wavy surfaces

Glints are bright light spots created by particulate media like cirrus clouds, glaciers, and wavy water surfaces. They are seen around the specular reflection angle. In this paper, the glints from such scattering/reflecting media are described in a unified manner through the probability density for facet tilts. Various kinds of these probability densities for wavy surfaces are defined and classified. The concept of the differential scattering cross section (DSCS) for rough surfaces instead of the conventional bidirectional reflectance distribution function (BRDF) is introduced for characterization of the glints. The simple equations connecting the DSCS and the probability densities for facet tilts are derived. It is shown that the glints from particulate media and wavy surface are very similar at small incidence angles and they are significantly different at slant incidence.

Wavepacket scattering of Dirac and Schrödinger particles on potential and magneticbarriers

We investigate the dynamics of a charged particle moving in a graphene layer and in atwo-dimensional electron gas, where it obeys the Dirac and the Schrödinger equations,respectively. The charge carriers are described as Gaussian wavepackets. The dynamics ofthe wavepackets is studied numerically by solving both quantum-mechanical andrelativistic equations of motion. The scattering of such wavepackets by step-like magneticand potential barriers is analysed for different values of wavepacket energy and width. Wefind: (1) that the average position of the wavepacket does not coincide with the classicaltrajectory, and (2) that, for slanted incidence, the path of the centre of mass of thewavepacket does not have to penetrate the barrier during the scattering process.Trembling motion of the charged particle in graphene is observed in the absence of anexternal magnetic field and can be enhanced by a substrate-induced mass term.

Crystal structure and optical properties of silver nanorings

We report the polyol synthesis and crystal structure characterization of silver nanorings, which have perfect circular shape, smooth surface, and elliptical wire cross-section. The characterization results show that the silver nanorings have well-defined crystal of singly twinned along the whole ring. The spatial distribution of the scattering of a silver nanoring with slanted incidence reveals the unique focus effect of the nanoring, and the focus scattering varies with the incident wavelength. The silver nanorings with perfect geometry and well-defined crystal have potential applications in nanoscaled photonics, plasmonic devices, and optical manipulation.

On Sound Reflection in Superfluid

We consider reflection of the first and the second sound waves by a rigid flat wall in superfluid. Nontrivial dependence of the reflection coefficients on the angle of incidence is obtained. Sound conversion is predicted at slanted incidence.

Laser vibrometry of air-coupled Lamb waves

Abstract A new non-contact methodology combining air-coupled ultrasonic excitation and laser detection of the Lamb waves is developed and applied for advanced material characterization. The flexural modes are generated in a specimen at normal and slanted incidence of a focused air-coupled beam. A laser scanning vibrometer is used for dynamic visualization of the Lamb wave field. As the local flexuaral wave velocity depends on the thickness of a specimen, the meth-odology enables remote imaging and profiling of thickness at high accuracy. Direct imaging of the flexural wave-defect interaction is capable of visualizing the damaged areas in fibre-reinforced composites. The shape of the detected flexural waveforms is shown to reflect the in-plane material anisotropy. The experiments proved a high sensitivity of the technique in remote imaging and measuring of local fibre orientation in composite materials.

Development of Angular Eigenvalue Method for Deep Penetration Problems of Gamma Rays

The present article describes calculations about deep penetration of gamma rays by the angular eigenvalue method, that has been developed as a new semi-analytical method for radiation transport problems in slabs. The angular eigen-value method has a feature that it can be extended to penetration of radiation through thick slabs (deep penetration) more easily than other method, since solutions are given analytically as a function of the slab thickness.. Exposure buildup factors for concrete slabs under slant incidence conditions were calculated by the present method, and were found in good agreement with Monte Carlo calculations by Fournie and Chilton up to the thickness of 10 mfp (mean free paths). Calculations were extended to the thickness of 50 mfp. Exposure buildup factors for point isotropic sources in infinite homogeneous medium of water, iron, and lead were calculated for various source energies by the present method. The buildup factors obtained were found in good agreement with Monte Carlo calculations by Hirayama up to the distance of 40 mfp. Calculations were tried to be extended to 60 mfp.

A Simple Method of Calculating Radiation Streaming through Cylindrical Ducts in a Concrete Shield

A simple method is presented to calculate radiation streaming through cylindrical ducts in a concrete shield. It includes neutron and gamma-ray calculations and can be applied to the problems of practical slant incidence. The method is essentially one of summing the axial dose rates from the different neutron and gamma components which are functions of energy and angle of incident radiation, duct diameter, bend angle, etc. Axial dose rates are calculated by the MCNP Monte Carlo code for straight and bent ducts having diameters of 5,10,20,30 and 40 cm. Radiation sources are plane mono-directional fission neutrons, intermediate neutrons, thermal neutrons and 1.25-MeV gamma rays. Incident angles are 0, 15, 30, 45 and 60 deg for straight ducts. For bent ducts, an isotropic plane source is placed inside the duct mouth to reduce computation time. Bend angles are 30,45,60 and 90 deg. Reliability of the Monte Carlo calculations are assessed by comparison with a 60Co experiment and calculations by the one-dimensional discrete ordinates code.

Physical processes of microwave backscattering from laboratory wind wave surfaces

Physical processes of microwave backscattering from wind wave surfaces are investigated in a wind wave tunnel by using a X‐band (9.6 GHz) microwave scatterometer. Detailed analysis of time series of the backscattered intensity and Doppler spectrum shows that the physical processes of microwave backscattering are closely associated with the processes of the wind wave field. At slant incidence (45°) the backscattered intensity is in phase with the wave profile, and the Doppler velocity also follows the phase of the individual waves with high Doppler velocity observed at the crests of the individual waves. This velocity is equal to the propagating speed of the crests. It is concluded that the fine structures of wind wave surfaces, which are trapped near the crests and propagating with the crests, are the main contributor to microwave backscattering. It is also pointed out that the effect of wave breaking with bubble entrainment on the microwave backscattering is not significant under the condition of the present experiment. At normal incidence the backscattered intensity has a different phase relationship with the wave profile. The intensity has two peaks, one at the crest and the other at the trough of the individual waves. The peak at the trough is stronger than that at the crest. This result is consistent with specular point scattering and also with the asymmetrical features of young laboratory wind waves. At intermediate incident angles, backscattering processes are rather complicated because both specular point scattering and Bragg resonance scattering contribute to the radar backscattering.