Junjie Yu

Three-dimensional Dammann array

We demonstrate a scheme that can produce a three-dimensional (3D) focus spot array in a 3D lattice structure, called a 3D Dammann array, in focal region of an objective. This 3D Dammann array is generated by using two separate micro-optical elements, a Dammann zone plate (DZP) that produces a series of coaxial focus spots and a conventional two-dimensional (2D) Dammann grating (DG). A simple, fast, and clear method is presented to design this binary pure-phase (0,π) DZP in vectorial Debye theory regime. Based on this kind of DZP, one can always obtain a 3D Dammann array both for low and high numerical aperture (NA) focusing objectives. For experimental demonstration, an arrangement combining a DZP, a 2D DG, and a pair of opposing lenses is proposed to generate a 5×5×5 Dammann array in focal region of an objective with NA=0.127 and another 6×6×7 Dammann array for an objective of NA=0.66. It is shown that this arrangement makes it possible to achieve 3D Dammann arrays with micrometer-sized focus spots and focus spacings of tens of micrometers for various practical applications, such as 3D parallel micro- and nanomachining, 3D simultaneous optical manipulation, 3D optical data storage, and multifocal fluorescence microscope, etc.

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 Absorber Based on a Cascaded Metal–Dielectric Grating Structure

The spectrum selective absorption effect of a vertically cascaded metal-dielectric grating structure is studied. A polarization-independent spectrum selective absorber based on two pairs of stacked metal-dielectric grating structures is presented for the infrared frequencies. The near-unity absorption with polarization independence is observed under normal incidence. The understanding of such a perfect selective absorption mechanism is illustrated by investigating the electric field distributions and power loss density at the resonant wavelength. The angle independence is also studied for this absorber and it is found that the absorber can maintain high absorbance around large incident angle range (0° -33° ), especially for the TM polarization.

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.

Edge extraction using a time-varying vortex beam in incoherent digital holography

Edge extraction using a time-varying vortex beam (TV-VB) is demonstrated in optical scanning holography (OSH) operating in an incoherent mode. OSH is a two-pupil heterodyne scanning optical system. We propose that one of the pupil functions used is a delta function and the other pupil function is a spiral phase plate (SPP). The interference of these pupils creates a TV-VB to scan over an object to record the edge-only information of an object holographically. We also find that a reconstructed edge with better contrast is achieved by translating the SPP away from the pupil plane. Experimental results are compared with computer simulations and found to be in good agreement.

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

Mode conversion and coupling in a slanted grating

We have proposed a novel transmission slanted grating at the central wavelength of 1550 nm, which can be used in optical communication. We have presented an approximate analytical expression that provides an insightful physical description of the simplified modal method for the slanted grating. The odd grating mode, which only exists in the asymmetric structure under normal incidence, plays the positive role of enhancing the -1st order diffraction efficiency. The analytic expressions of mode conversion and coupling can be obtained to explain the asymmetric field distribution, which cannot occur in the rectangular grating region. Numerical results achieved by the rigorous wave analysis verify the validity of the simplified modal method. We expect that the theoretical modal method set forth in this work will be helpful for the tremendous potential application of the slanted grating.

Distorted Dammann grating

We introduce the Dammann phase-encoding method into original distorted gratings and propose a modified distorted grating, called a distorted Dammann grating (DDG), to realize multiplane imaging of several tens of layers within the object field onto a single image plane. This property implies that the DDG makes it possible to achieve simultaneously high axial resolving power and large axial imaging range without scanning. This DDG should be of high interest for its potential applications in real-time three-dimensional optical imaging and tracking. Multiplane imaging of 7×7 object layers onto a single camera plane is experimentally demonstrated using a 7×7 DDG for an objective of NA=0.127.

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.