Anduo Hu

Three-dimensional Dammann vortex array with tunable topological charge

We describe a kind of true 3D array of focused vortices with tunable topological charge, called the 3D Dammann vortex array. This 3D Dammann vortex array is arranged into the structure of a true 3D lattice in the focal region of a focusing objective, and these focused vortices are located at each node of the 3D lattice. A scheme based on a Dammann vortex grating (DVG) and a mirror is proposed to provide a choice for changing the topological charge of the 3D Dammann vortex array. For experimental demonstration, a 5×5×5 Dammann vortex array is implemented by combining a 1×7 DVG, a 1×5 Dammann zone plate, and another 5×5 Dammann grating. The topological charge of this Dammann vortex array can be tuned (from -2 to +2 with an interval of +1) by moving and rotating the mirror to select different diffraction orders of the 1×7 DVG as the incident beam. Because of these attractive properties, this 3D Dammann vortex array should be of high interest for its potential applications in various areas, such as 3D simultaneous optical manipulation, 3D parallel vortex scanning microscope, and also parallel vortex information transmission.

Polarization-independent wideband mixed metal dielectric reflective gratings

A polarization-independent wideband mixed metal dielectric grating with high efficiency of the -1st order is analyzed and designed in Littrow mounting. The mixed metal dielectric grating consists of a rectangular-groove transmission dielectric grating on the top layer and a highly reflective mirror composed of a connecting layer and a metal film. Simplified modal analysis is carried out, and it shows that when the phase difference accumulated by the two propagating modes is odd multiples of π/2, the diffraction efficiency of the -1st order will be high. Selecting grating depth and duty cycle for satisfying the phase difference condition for both TE (electric field parallel to grooves) and TM (magnetic field parallel to grooves) polarizations, a polarization-independent high-efficiency grating can be designed. Using rigorous coupled-wave analysis and a simulated annealing algorithm, geometric parameters of the reflective grating are exactly obtained. The optimized grating for operation around a wavelength of 800 nm exhibits diffraction efficiencies higher than 90% for both TE and TM polarizations over a 120 nm wavelength bandwidth. The simplified modal analysis can be applied in other types of reflective gratings if the top layer is a dielectric transmission grating.

Modal analysis of high-efficiency wideband reflective gratings

Modal analysis of the reflective wideband grating with high efficiency of the negative first order in Littrow mounting is presented. The reflective grating consists of a highly reflective mirror and a transmission grating on the top. The modal analysis is carried out for TE polarization and it reveals that two modes are excited in the transmission grating. Using the two modes and ignoring the absorption of the mirror, a simple expression of diffraction efficiency of the reflective grating is derived and thus the grating depth to achieve high efficiency of the negative first order is obtained. The influence of duty cycle on the difference of effective indices of the two modes for different wavelengths is analyzed, which can explain the wideband behavior of reflective gratings. The modal analysis should be a useful tool for the design of high-efficiency wideband reflective gratings

Generation of dipole vortex array using spiral Dammann zone plates

We propose a new diffractive optical element, called a spiral Dammann zone plate (SDZP), to generate a series of dipole vortices along the optical axis in the focal region of a focusing objective. By combining this SDZP and another Dammann grating, we describe the generation of three-dimensional dipole vortex arrays in the focal volume of an objective. For experimental demonstration, a 1×5 SDZP with base charge of l=1 is fabricated by using lithography and wet-etching techniques, and a 1×5 coaxial dipole vortex array is achieved for an objective of NA=0.127. Furthermore, by combining the 1×5 SDZP and another 5×5 Dammann grating, a 5×5×5 dipole vortex array is also experimentally demonstrated. The results show that topological charges of these 5×5 vortex arrays on five coaxial planes could be tunable by selecting a vortex beam carrying different charge as the incident field.

Beam splitting of a double-groove fused-silica grating under normal incidence

Double-groove fused-silica gratings for 1 × 5 and 1 × 7 TE-polarization beam splitting under normal incidence are studied. The grating profiles are optimized by use of the rigorous coupled-wave analysis (RCWA) and the simulated annealing (SA) algorithm. The average diffraction efficiencies of the two gratings are both more than 95% with uniformity of better than 2%. The diffraction efficiencies of these gratings are approximately 15% more than the conventional Dammann gratings. The physical understanding of the diffraction behaviors taking place inside the beam splitter gratings is presented by the modal method. The mode eigenvalue equation of the double-groove grating is derived theoretically. The eigenmodes of 1 × 5 and 1 × 7 TE-polarization beam splitter gratings are obtained from the eigenvalue equation. Then, we calculate the overlap integral and modal propagation constants. We also present the mode profiles of the propagating modes for TE polarization. The proposed method of analyzing beam splitters under normal incidence should be helpful for developing new double-groove grating-based devices.

Enhancement of absorption in thin-film amorphous silicon solar cell with guided mode resonance

Abstract. The enhancement of absorption in a thin-film amorphous silicon solar cell based on guided mode resonance is theoretically investigated. This is achieved by patterning a single- or double-groove grating with a waveguide layer as the absorbing layer. The optimized grating parameters are obtained by use of rigorous coupled-wave analysis and the simulated annealing algorithm. The averaged integrated absorptions are weakly dependent on the angle of incidence in both grating structures. It is shown the optimized solar cell with double-groove grating has better optical performance than single-groove grating structures. The qualitative understanding of enhanced absorption based on guided-mode resonance and double-groove grating structure is presented. An antireflective grating structure is proposed and discussed for reducing reflection and enhancing absorption. The solar cell with antireflective grating has much better performance than those without an antireflective grating. The designed solar cells have high integrated absorption and are weakly dependent on the incident angle, which should be highly interesting for practical application.

Absorption enhancement in thin-film solar cell using grating structure

The enhancement of absorption in thin-film amorphous silicon solar cell based on guided mode resonance is theoretically investigated. This is achieved by patterning a grating with waveguide layer in the absorbing layer and an antireflective layer on the top. The optimized grating parameters are obtained by use of rigorous coupled-wave analysis and the simulated annealing algorithm in the visible region. The absorption at normal incidence is higher than 50% in the wavelength range 300-660 nm, and the peak absorption is higher than 95% for both TE and TM polarization. We studied the angle dependence of the integrated absorption spectrum in solar cell structures. The integrated absorption for TM polarization is larger than TE polarization in the angular range of 0-88o. In general, the averaged integrated absorption decreases as the incident angle increasing, but it is higher than 60% in the range 0-66o. So it is very weakly dependent on the angle of incidence. A physical understanding of enhanced absorption based on guided mode resonance effect is presented. It is found that the effect can effectively trap light in the absorber layer and enhance absorption in the active layer. Double-groove grating structure is also discussed for the sake of reducing reflection and enhancing absorption. The designed solar cells have high integrated absorption and are weakly dependent on incident angle, which should be of highly practical significance.

Beam splitters of metal-dielectric reflective gratings

Beam splitters are important optical components. In this paper, reflective 1×2 and 1×3 beam splitters based on metaldielectric gratings are designed at wavelength of 1064 nm for TE polarization. Alumina, silver, alumina and fused silica films are coated on substrate in sequence. On the top fused silica film, grating with rectangular grooves are etched. Rigorous coupled-wave analysis (RCWA) and simulated annealing (SA) algorithm are employed to optimize grating parameters such as period, depth, thickness of connecting layer and incident angle. An optimized 1×2 beam splitter can achieve perpendicular beam splitting with diffraction efficiency of 49.2% at the -1st order and 49.1% at the 0th order for incident angle of 30°. When incident angle is 5°, the diffraction efficiencies of the optimized 1×3 beam splitter are 32.66% at the -1st order, 32.71% at the 0th order and 32.72% at the 1st order. To guide the fabrication and operation of beam splitters, the tolerance of grating period, depth, thickness of connecting layer are calculated. The optimized 1×2 and 1×3 beam splitters exhibit high efficiencies and uniformity, which should be useful in applications.

Influence of duty cycle on near-field diffraction of high-density gratings

Near-field intensity distributions of high-density amplitude gratings with period d=2.3λ and different duty cycles illuminated by TE and TM polarized waves are simulated by using the finite-difference time-domain method. Numerical results show that intensity distributions of in the grooves change differently when duty cycle varies for TE and TM polarizations. As duty cycle increases, peak intensity in the middle of the first bright stripes and its locations change obviously. The main reason of location variation of the first bright stripes is the variation of the phase difference between the ±2nd orders and the 0th order, which is verified by using rigorous coupled-wave analysis. The investigation of influence of duty cycle on near-field diffraction of high-density gratings should be helpful for practical applications of high-density gratings.