Quantifying metarefraction with confocal lenslet arrays

METATOYs can change the direction of light in ways that appear to, but do not actually, contravene the laws of wave optics. This direction change applies only to part of the transmitted light beam; the remainder gets re-directed differently. For a specific example, namely confocal pairs of rectangular lenslet arrays with no dead area between lenslets, we calculate here the fractions of power of a uniform-intensity light beam incident from a specific (but arbitrary) direction that get re-directed in different ways, and we derive an equation describing this redirection. This will facilitate assessment of the suitability of METATOYs for applications such as solar concentration. Finally, we discuss similarities between the multiple refraction of light at the lenslet arrays and multiple refraction and reflection of cold atoms at a barrier in the presence of the light fields.     

Equiangular-spiral bent lightpipes with arbitrary bent angle

A recent study proposed the concept of a leakage-free bent lightpipe with an equiangular-spiral shape [S.-C. Chu, J.-L. Chern, Opt. Lett. 30 (2005) 3006]. This paper extends the design of a leakage-free equiangular-spiral bent lightpipe with arbitrary-bend-angle and addresses in detail the mathematical formalism required to build such an equiangular-spiral bent lightpipe. Furthermore, a comparison of the proposed equiangular-spiral bent lightpipe and a conventional circular bent lightpipe is provided. Numerical verifications are discussed, and experimental explorations with different bent shapes and angles are carried out for comparison. Results show that equiangular-spiral bent lightpipes with different bent angles exhibit a theoretical 100% transmission with more than 76% efficiency when practically propagating, which is much better than conventional circular bent lightpipes. The output irradiance distributions of bent lightpipes with different bent shapes are also investigated.     

Geometric limits to geometric optical imaging with infinite, planar, non-absorbing sheets

Wave optics can limit the ways in which optical components can change light-ray fields. Optical components called METATOYs trade in the continuity of the phase fronts and the precision to which they change light-ray fields in return for additional possibilities when changing light-ray fields. Now only geometry limits the possible mappings between the positions of an object and its geometric image. Here, I study such limitations for the case of an infinite, planar, non-absorbing sheet that images the entire three-dimensional space. The most general case of such a sheet is equivalent to a thin lens with different object- and image-sided focal lengths. Special cases include ordinary thin lenses, confocal lenslet arrays, and negative refration with n₂=-n₁.     

Negative refraction and rough surfaces: A new regime for lensing

The action of a rough, but differentiable, interface upon the passage of rays between air and a left-handed medium is considered. It is shown that negative refraction brings rays to a focus at distances closer to the boundary than can be attained by conventional refraction. This effect enables a new mechanism for reflection to occur, even in media that are impedance matched, caused principally by rays undergoing two interactions with the interface via paths that pass exclusively through air or the left-handed medium.     

Experimental demonstration of ray-optical refraction with confocal lenslet arrays

We observe imaging through windows comprising pairs of confocal lenslet arrays that have different focal lengths but that are otherwise identical. Image space is stretched in the longitudinal direction only. Such windows are examples of METATOYs, optical components that can change light-ray direction in ways that appear wave-optically forbidden.     

Relativistic aberrations in quantum phase space

A phase space treatment of special relativity of quantum systems is developed. In this approach a quantum particle remains localized if subject to inertial transformations, the localization occurring in a finite phase space area. Unlike in the non-relativistic case, relativistic transformations generally distort the phase space distribution function, being equivalent to aberrations in optics. The relativistic aberrations of massive particles are in general different from those of optical images.     

Spectropolarised ray-tracing simulations in densely packed particulate medium

Light scattering from particulate medium is simulated using the Monte Carlo ray-tracing technique. The medium is modelled as a randomly packed medium of ellipsoidal grains with stochastically rough surfaces, with an optional thin coating. Optical properties are modelled using a wavelength-dependent complex refractive index and taking Fresnelian reflections and refractions from the interfaces. The size and shape of the grains are assumed to be large and smooth enough for geometric optics to apply reasonably well.The ray-tracing technique uses parallel, weighted rays for computing simultaneously over a wide wavelength spectrum and a small roughness range, and scaling to obtain a large range of sizes and absorbities simultaneously. Polarisation is fully accounted for. The multiobservation technique is effectively used at each scattering point. The scattering from thinner sample layers is also received as a subresult.Simulations are run for a set of model samples to study the effects and sensitivities regarding the values of certain parameters. It has been found that the size and composition of the grains affect the scattering in a unique and invertible way. The shape of the grain causes similar significant effects that must certainly be taken into account if any accuracy is required, although inverting for the shape is difficult without further constraints. The packing density has a small but observable effect. Polarisation can be used to study the composition of low-albedo objects.     

Ray picture of diffraction gratings

We elaborate on a geometrical picture of diffraction gratings. Exact paraxial propagation including coherence effects is obtained by allowing the geometrical rays to transport Wigner function instead of simply specific intensity. We apply this formalism to perfect and less than perfect gratings, illuminated by plane and Gaussian waves.     

Temporal filtering using time lenses for optical transmission systems

An optical signal processing scheme using time lenses in a 4-f configuration for optical communication systems is proposed. The first time-lens combined with a dispersive element such as an optical fiber produces the Fourier transform of the input signal and the second time lens combined with an optical fiber placed after the temporal filter produces the inverse Fourier transformation. Typically, in an optical signal processing scheme based on space/time-lens, the signal at the output is space/time-reversed because of the direct Fourier transformation after the spatial/temporal filter, which is undesirable for a practical optical communication system. Here, we propose a technique to implement both direct and inverse Fourier transformation using time lenses which has no spatial analogue. As a result, the bit sequence at the output is not time-reversed. Two applications of the proposed scheme, a demultiplexer for wavelength division multiplexing (WDM) systems and a higher-order dispersion compensator, have been discussed and numerically implemented.     

The roles of the exchange and dipole couplings on the magnetoresistance for the nanoparticle arrays

Based on Monte Carlo simulations and resistor network model, the tunneling magnetoresistance (TMR) for the square anisotropic magnetic nanoparticle arrays with the exchange coupling J and dipolar coupling D has been studied. The simulated results reveal that, the coercivity H_c increases with decreasing value of D and increasing magnitude of J; the value of TMR_max decreases with enhancing value of J; for D<0.1, the value of TMR_max decreases with increasing value of D, while in contrast for D>0.1, it increases with increasing value of D. Those behaviours have been interpreted with the competition of the different energies, including the random anisotropy, exchange and dipolar energies.     

Soft magnetic properties of FeCo films with Co underlayer

Bilayered Fe₆₅Co₃₅ (=FeCo)/Co films were prepared by facing targets sputtering with 4πM_s∼24 kg. The soft magnetic properties of FeCo films were induced by a Co underlayer. H_c decreased rapidly when the Co underlayer was 2 nm or more. The films showed well-defined in-plane uniaxial anisotropy with the typical values of H_ce=10 Oe and H_ch=3 Oe, respectively. High frequency characteristics of the films show the films can work at 0.8 GHz with real permeability as high as 250.     

Investigation on the magnetism and transport properties of NiMnSb polycrystalline films with oxidized grain surface

Preferentially oriented half-metallic NiMnSb thin films have been fabricated by pulsed-laser deposition on Si(1 0 0) substrates. A nonmagnetic insulating MnO_x layer was formed on the NiMnSb grain boundaries due to exposure to the air. In addition to the magnetism, transport properties of the grain-surface-oxided NiMnSb films have been investigated. The insulating conduction and significant magnetoresistance of the NiMnSb films were found to be associated with the insulating manganite oxide layer on the surface of NiMnSb grains.     

Optimization of phase objects in 4f coherent imaging system for nonlinear refraction measurements

We numerically analyze the effects of radius and phase shift of phase objects on the diffraction image of the 4f coherent imaging system, a system used for measuring the third-order nonlinear refractive index. The selection of the aperture radius is discussed. We prove that when the phase object radius is 0.1 time of the aperture radius and the phase change of the phase object is 0.57π, one can get the highest sensitivity for nonlinear refraction measurement.     

Debye series expansion of shaped beam scattering by GI-POF

We derive Debye series expansion (DSE) for infinitely long multilayered cylinders normally incident by shaped beam. Typically the interaction between multilayered cylinders and Gaussian beam is derived in detail, and localized approximation is introduced to calculate the beam shaped coefficients. Finally DSE is employed to the study of rainbow scattering by graded-index polymer optical fiber (GI-POF).     

Nonlinear optical properties of Au/ZnO nanoparticle arrays

The triangular-shaped Au/ZnO nanoparticle arrays were fabricated on fused quartz substrate using nanosphere lithography. The structural characterization of the Au/ZnO nanoparticle arrays was investigated by atomic force microscopy. The absorption peak due to the surface plasmon resonance of Au particles at the wavelength of about 570 nm was observed. The nonlinear optical properties of the nanoparticle arrays were measured using the z-scan method at a wavelength of 532 nm with pulse duration of 10 ns. The real and imaginary part of third-order nonlinear optical susceptibility, Re χ⁽³⁾ and Im χ⁽³⁾, were determined to be 1.15 × 10⁻⁶ and −5.36 × 10⁻⁷ esu, respectively. The results show that the Au/ZnO nanoparticle arrays have great potential for future optical devices.     

Correlation between Si-SiO2 heterojunction and Fowler-Nordheim conduction mechanism after soft breakdown in ultrathin oxides

A stress-induced defect band model is proposed to investigate the Fowler-Nordheim tunneling characteristics of ultrathin gate oxides after soft breakdown. Soft breakdown occurs when the average distance between stress-induced defects locally reaches a critical value to overlap the bound electron wavefunction on adjacent defects and to form a defect band. This model shows that an n⁺-poly-Si/N-SiO₂/p-Si heterojunction structure is formed between electrodes at a local area after a soft breakdown in the ultrathin SiO₂ and the soft breakdown current can be described in terms of the Fowler-Nordheim tunneling process with a barrier height of ∼1 eV.     

Giant Faraday rotation in Fe–ZnSe granular films

The giant magneto-optical Faraday effect (giant Faraday rotation) of ferromagnetic metal–semiconductor matrix Fe–ZnSe granular films prepared by radio frequency sputtering is studied. The result shows that the Faraday rotation angle θ_F value of the granular films sample with Fe volume fraction x=35% is of the order of 10⁵ °/cm at room temperature. Temperature dependence of the θ_F of Fe_0.35(ZnSe)_0.65 granular films shows that θ_F value, below 150 K, increases rapidly with the decrease of the temperature, and when T=10 K θ_F value is 6×10⁵ °/cm. Through the study of the structure and dependence of magnetic properties on temperature, it has been found that the remarkable increase of the θ_F value of Fe_0.35(ZnSe)_0.65 granular films below 150 K seems to arise from the sp–d exchange interaction inside the granular films.