Shaoqing Wang

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.

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.

Circular Dammann grating under high numerical aperture focusing

Circular Dammann grating (CDG) under high numerical aperture (NA) focusing is described based on Richards-Wolf vectorial diffraction theory in this paper. Several CDGs are presented under the condition of NA=0.9 with the illumination of circularly polarized plane-wave laser beams. Numerical results show that the sizes of these circular patterns with equal-intensity are in the wavelength scale, and doughnut-shaped central spots and dark rings are in the subwavelength width. To verify this kind of CDG, a binary pure-phase three-order CDG is fabricated to produce a dark center pattern surrounded by three concentric bright rings. The corresponding intensity distribution of the pattern on the focal plane of a high-NA objective (NA=0.9) is measured, and the results agree well with theoretical simulations. This kind of CDG with annular patterns of equal-intensity in the wavelength scale should be highly interesting for its potential applications in optical trapping, stimulated emission depletion (STED) microscopy, and the study of singular optics, as well as annular array illumination.

The study of third order nonlinearities in ZnCdSe-ZnSe/GaAs MQWs using Z-scan

Abstract The third order nonlinearities in ZnCdSeZnSe multiple quantum wells (MQWs) grown by the metal-organic chemical vapour deposition (MOCVD) on GaAs substrates have been studied at room temperature using Z-scan, for the first time. The research result indicates that the nonlinear refractive index n2 in the ZnCdSe-ZnSe/GaAs MQWs is about −5.05×10−5esu. Considering the absorption spectra in ZnCdSe-ZnSe/GaAs MQWs under different pump intensities at room temperature obtained here, the major nonlinear mechanism for the third-order optical nonlinearities in ZnCdSe-ZnSe/GaAs MQWs can be due to band shrinkage effect in the ZnCdSeZnSe MQWs.

Empirical equations for sub-wavelength fused-silica gratings

Two empirical equations are presented to reveal the restrictions on relative grating parameters for 1×2 beam splitter of dielectric rectangular transmission gratings under second Bragg incidence for TE polarized light.

Novel uses for deep-etched, fused-silica diffraction gratings

Diffraction gratings are among the most venerable tools of optical physics, with the first gratings produced in the mid-1700s. They are used in chemical analysis, astronomic spectroscopy, light-based communications, and other applications. Unlike a prism, diffraction gratings operate by interference effects as incident light impinges on rulings or grooves cut or etched onto the grating surface. Over the last 10 years, advances in fabrication techniques—many by our own group—have made possible gratings of unprecedented groove depth and density. These gratings offer new efficiencies, for example, in transmissivity, beam splitting, bandpass filtering, and polarization separation. High density refers to the groove-to-groove distance or period. Groove depth is defined by the gratings’ depth-to-period ratio. We reasoned that optimizing the groove depth to bring the grating period close to the wavelength of the incident light should enable novel functions. Our group faced two fundamental challenges concerning deep-etched, high-density gratings. The first was theoretical: How would such gratings perform? As a practical matter, only when we were fairly sure that the effort would produce useful new devices did we try to fabricate them. Fabrication was the second challenge. Which techniques and materials would be needed to make these gratings? We settled on fused silica, an excellent optical material with good thermal stability, high transmission at our wavelengths of interest, and a high laser-damage threshold. However, fused silica is hard and difficult to etch. Developing ways to work with it took considerable time and experimentation. For the first challenge, it is necessary to understand exactly what is going on inside the grating. Rigorous coupled wave analysis3, 4 is one of the most widely used numerical methods for predicting the performance of gratings, but it is Figure 1. Scanning-electron microscopy image of a deep-etched fusedsilica grating 50mm in diameter.

Application to the design of guide-mode resonance grating filter with using simulated annealing method

Although there are several well -known methods such as RCWA, FMM, for analyzing the diffraction properties of gratings, design of these optical elements with specified spectral properties is commonly a challenging problem. It is relatively not easy for the researchers to design narrow line-with diffraction filters based on guided mode resonance phenomenon with common diffraction algorithm. Simulated Annealing (SA) method is evolutionary, robust technique that has been widely utilized to design optical diffraction components. This method is inspired by the physical process of heating and controlled cooling of metal material to increase the size of its crystals and reduce their defects. The most distinctive features of this method lie in its powerful ability of convergence towards the global minimum in a reasonable computation time and the independence of the initial parameter values. In this paper, first, the physical basis of SA and its mathematical realization are introduced. Then, a Guided-Mode Resonant Grating (GMRG) filters with single layer is designed by using SA algorithm. The central wavelength of GMRG filter is locked at 532nm and its line-width is fixed at 1nm. The plane wave light radiates the grating from air cover with normal incidence. The optimized parameters are refractive indices and thicknesses of high and low material of grating, other parameters are grating period and fill factor of the grating. It is shown from our calculation that an excellent reflection spectrum with narrow line-width, high peak and low sideband can be obtained after optimizing the grating parameters. Next, a double layered GMRG filter with line-width of 4nm, which is relatively easy fabrication in experiment, is designed at central wavelength of 1064nm. The optimized parameters are grating period, groove depth, refractive index of waveguide layer and fill factor respectively. The grating substrate and waveguide layer are Sio2 and Hfo2 respectively, the grating structure is directly etched on the waveguide layer. The above grating values should be included in reasonable ranges in consideration of grating fabrication in our experiment condition. It is demonstrated from the calculations with the parameters obtained from SA optimization algorithm that the peak diffraction efficiency is more than 99% at central wavelength 1064nm and the sideband reflection is depressed at the level bellow 5% in a large wavelength range. Moreover, the parameters of a triple layer GMRG filter structure are also provided with this powerful method. Meanwhile, the results found by SA method are compared with RCWA theory.