Guided-mode resonant subwavelength gratings: effects of finite beams and finite gratings

The effects of finite beams and finite gratings on the performance of guided-mode resonant subwavelength gratings are characterized by using the rigorous boundary element method. The gratings are strongly modulated, have a finite number of periods, and are illuminated by normally incident Gaussian beams. Quantitative results are presented for silicon-on-sapphire resonant gratings and gallium arsenide-aluminum arsenide resonant gratings.     

Symmetric array of off-axis singular beams: spiral beams and their critical points

We consider conditions of structural stability under which the array of singular beams preserves its topological structure and intensity distribution while slightly perturbing its intrinsic parameters. The orbital angular momentum of the array as a function of the array parameters is a characteristic function, and its extreme points correspond to stable and unstable array states.     

Scattering of focused ultrasonic beams by two spherical cavities in close proximity

Ultrasonic fields generated by one and two spherical cavities placed in front of a point-focused acoustic lens are modeled by the semi-analytical distributed point source method (DPSM). Results are generated by properly considering the interaction effect between two cavities placed in the focused ultrasonic field. The interaction effect between the two cavities prohibits the linear superposition of single cavity solutions to obtain the solution for two cavities placed in close proximity. Therefore, although some analytical and semi-analytical solutions are available for the single cavity in a focused ultrasonic field, those solutions cannot be simply superimposed for solving the two-cavity problem even for a linear elastic material. Solution of this problem is necessary to evaluate when two cavities placed in close proximity can be distinguished by an acoustic lens and when it is not possible. The comparison between the reflections of ultrasonic energy from two small cavities versus a single big cavity is also investigated.     

Interaction of the Lissajous singularity and singular Lissajous line carried by non-paraxial vector beams

The interaction of the Lissajous singularity and singular Lissajous line carried by non-paraxial polychromatic beams is studied. It has been found that with propagation the singular Lissajous line breaks into more than two Lissajous singularities, pairs creation and annihilation of Lissajous singularities, the handedness reversal and change in the degree of polarization of Lissajous singularities, as well as the change in the shape of Lissajous figures may take place.     

Polarization-multiplexed diffractive optical elements fabricated by subwavelength structures

Polarization-multiplexed phase-only diffractive optical elements with subwavelength structures are proposed and fabricated. The differences among the phase modulations result from the differences among the effective indices exhibited in the subwavelength structures with various filling factors and surface profiles, and the phase retardations are obtained by the relief depth of the structures. The polarization-selective property is achieved by the polarization dependence of the effective indices exhibited in the one-dimensional subwavelength structures and the polarization independence exhibited in the two-dimensional structures. Additionally, the polarization contrast of our polarization-multiplexed elements, defined as the cross talk between the two polarization incidences, is independent of the relief depth. The principle of the polarization multiplexing by use of the subwavelength structures is described, and the fabrication results for the polarization-multiplexed computer-generated holograms are demonstrated.     

Application of singular elements for fully Lagrangian modeling of dynamics of MEMS with thin beams

The problem of coupled electro-mechanical vibration of a thin micro-electro-mechanical system (MEMS) beam has been recently addressed by Ghosh and Mukherjee (2009) [1]. A singularity in charge density and force usually arises at the corners of a free edge in such beams. This is of little consequence in clamped–clamped beams (addressed in [1], Ghosh and Mukherjee, 2009) because the edges are restrained. For cantilevered beams, however, such singularities on its free edge deserve attention. The present paper addresses the above problem for a cantilevered beam. The boundary element next to the free edge of the beam is treated as singular. Numerical results for static deflections as well as vibrations of the cantilevered beams from the two methods (singular element and nonsingular elements analogous to [1], Ghosh and Mukherjee, 2009) are compared and discussed.     

The initiation, termination, and evolution of nonparaxial optical vortices: High-order singular beams

Exact solutions to the scalar Helmholtz wave equation describing nonparaxial high-order mode beams are considered. To match these solutions to those of the parabolic wave equation, soft approximate boundary conditions are formulated. According to these conditions, in the paraxial limit kz ₀≫1, paraxial and nonparaxial wave beams transform into each other either far off the focal caustic (at z≫z ₀) or near it. The behavior of the mode beams with different azimuthal and radial mode indices is described.     

Experimental characterization of subwavelength diffraction gratings by an inverse-scattering neural method

Characterization of gratings with small period-to-wavelength ratios is difficult to perform but is very helpful in improving the fabrication process. We experimentally tested an inverse-scattering method using a neural network on silicon etched gratings. We also characterized the gratings by using two popular microscopic methods. The validity of each method was determined by comparing measured diffracted intensities with calculated ones obtained from measured profiles. An estimation of accuracy and repeatability was deduced from a scan along a grating sample. This method was thus well validated for nondestructive and noninvasive measurements under experimental conditions that were close conditions of actual usage. This method is easy to implement and requires the measurement of only a few diffracted intensities.     

Improvement of transmittance by fabricating broadband subwavelength anti-reflection structures for polycarbonate

We report on how to increase transmittance of a 0.2 mm thick polycarbonate (PC) film by periodic subwavelength anti-reflection structures in the visible spectral range. Subwavelength anti-reflection structures like moth-eyes are fabricated into the polycarbonate substrate itself by thermal nano-imprinting lithography (TH-NIL), which uses silicon stamps that have periodic structures such as gratings (lines and spaces) and pillared dots, and are fabricated by laser interference lithography (LIL) and transformer coupled plasma etching. To increase transmittance of a polycarbonate film, we control the periods and shapes of patterns, the number of patterned surfaces, and the overlapping direction of patterns that are fabricated into its surfaces. As a result of this, we show that average transmittance improves as the pattern period gets shorter and as both surfaces of the film are patterned. We also show that the spectrum range gets larger as the pattern period gets shorter and is determined by the longer pattern period in the case of designing a film to have different pattern period on its surfaces. The maximum average transmittance of a polycarbonate film increases up to approximately 6% compared to a bare sample in the 470-800 nm spectral range.     

Reduced wavelength-dependent quarter-wave plate fabricated by a multilayered subwavelength structure

We developed a novel fabrication method of a reduced wavelength-dependent quarter-wave plate (QWP) based on form birefringence of a multilayered subwavelength structure. The multilayered structure was constructed by depositing a high-refractive-index thin film on a subwavelength-structured substrate with a low refractive index. The surface structure of the substrate was shallow enough to be formed by a mass replication technology. A high-refractive-index subwavelength grating was formed on ridges of the substrate by sputtering Zn2SnO4 (refractive index of 2.03 at a wavelength of 633 nm). Moreover, since the grooves of the high-refractive-index grating were very deep and narrow, the dispersion of form birefringence suppressed the dependency of phase retardance on the wavelength of light in a limited spectral region. The phase retardance of the fabricated QWP was 89 degrees at a 633 nm wavelength and 79 degrees at a 785 nm wavelength.     

同轴纳米柱对亚波长金属牛眼结构EOT的调控

牛眼结构是一种典型的纳米光学结构。本文设计了一种带有同轴纳米柱的牛眼结构,利用时域有限差分法(FDTD)研究了该结构的增强透射效应。研究发现,柱的半径和高度对透射特性具有显著的影响,恰当选择柱的半径和高度会得到最大的透射强度。另外,牛眼结构对环境折射率有较高的灵敏度。理论分析表明,该种结构的透射增强效应是由局域表面等离激元与表面极化等离激元相互作用产生。这为纳米光学元件的研发与应用提供一个新的思路。     

Scattering of phonons by vacancies

The scattering of phonons by vacancies is estimated by a perturbation technique in terms of the missing mass and the missing linkages. An argument is given why distortion effects can be disregarded. The resonance frequency of the defect is sufficiently high so that resonance effects can be disregarded for phonons in the important frequency range for thermal conduction. The theory is applied to the thermal resistance by vacancies in cases where the vacancy concentration is known: potassium chloride with divalent cations, nonstoichiometric zirconium carbide, and tin telluride.     

Numerical feasibility study of the fabrication of subwavelength structure by mask lithography

Electromagnetic diffraction of a light wave by a single aperture of subwavelength width and subsequent propagation in a lossy medium are numerically investigated. This diffraction problem simulates exposure of a resist with an amplitude mask. It is found that there is the possibility of fabricating a lambda/2 structure on a resist of lambda/4 thickness, where lambda is the wavelength of the exposing light in vacuum, by conventional contact or by proximity lithography. It is also found that an air gap between a mask and a resist of up to lambda/2 does not have a significant effect on resolution. This approach permits easy and cost-effective fabrication of subwavelength structures and leads to wide availability of diffractive optical elements in the nonscalar domain.     

Enhanced confined light transmission by single subwavelength apertures in metallic films

The diffraction of light that emerges from a metallic circular aperture is studied. Near- and far-field results are presented. Spectral angular transmitted intensities are performed versus the incident wavelength for four kinds of aperture. It is shown that, for a definite configuration, a large enhancement of transmission--compared with the basic case of a single hole--occurs combined with a spectacular angular confinement of light. Such effects are, for example, of great interest in optical near-field microscopy for which the probe is a nanosource.     

Polarization conversion in conical diffraction by metallic and dielectric subwavelength gratings

Subwavelength metallic and dielectric diffraction gratings which rotate the linear polarization of incident light by 90 degrees are examined. Using rigorous diffraction theory in total-internal-reflection configuration, it is shown that full conversion from incident transverse electric field to transverse magnetic zero-order field can be achieved with both dielectric and metallic elements, but dielectric gratings provide higher efficiency and are thus preferable. The fabrication aspects and constraints are discussed in detail and the behavior of the gratings over broad wavelength bands is presented.     

Near-field Fourier transform polarimetry by use of a discrete space-variant subwavelength grating

We present a unique method for real-time polarization measurement by use of a discrete space-variant subwavelength grating. The formation of the grating is done by discrete orientation of the local subwavelength grooves. The complete polarization analysis of the incident beam is determined by spatial Fourier transform of the near-field intensity distribution transmitted through the discrete subwavelength dielectric grating followed by a subwavelength metal polarizer. We discuss a theoretical analysis based on Stokes-Mueller formalism, as well as on Jones calculus, and experimentally demonstrate our approach with polarization measurements of infrared radiation at a wavelength of 10.6 microm.     

Effect of ridge morphologies on surface plasmon scattering by subwavelength surface ridges

Surface plasmon scattering by two-dimensional ridges has been studied, mainly in the optical regime. Both the overall width and height of the ridges are fixed. The scattering properties are numerically investigated by the three-dimensional full-vector finite-difference time-domain method, with morphologies regularly changed by subdividing the width and the height of the ridges simultaneously. The results show that the transmission can be improved and the reflection reduced by increasing the degree of subdivision, which makes the surfaces of ridges much smoother. Meanwhile, morphology has little influence on the radiation, which may be attributed to the unchanged total surface area of the ridges. Moreover, we analyze the scattering of SPPs by arrays of ridges.