Maojin Yun

Wideband ultraflat slow light with large group index in a W1 photonic crystal waveguide

We demonstrate that slow light with large group-index, wideband, and low dispersion can be realized in a silicon-on-insulator w1-type photonic crystal waveguide by simply shifting the first two rows of air-holes adjacent to the waveguide to specific directions. keeping the group index at 46, 60, 86, 111, 151, and 233, respectively, while restricting its variation within a +/- 10% range, we accordingly obtain a slow light bandwidth of 9.0 nm, 6.7 nm, 4.6 nm, 3.3 nm, 2.4 nm, and 1.7 nm, respectively. the normalized delay-bandwidth product keeps around 0.25 for all cases. moreover, we obtain ultraflat slow light with bandwidths over 3.0 nm, 2.4 nm, 1.6 nm, 1.3 nm, 0.93 nm, and 0.6 nm, respectively, where the group index variation is in a range of only +/- 0.8%. numerical simulations are performed, utilizing the 2d plane wave expansion method and the finite-difference time-domain method. (c) 2011 american institute of physics. [doi:10.1063/1.3634074]

Wideband slow light with ultralow dispersion in a W1 photonic crystal waveguide

A dispersion tailoring scheme for obtaining slow light in a silicon-on-insulator W1-type photonic crystal waveguide, novel to our knowledge, is proposed in this paper. It is shown that, by simply shifting the first two rows of air holes adjacent to the waveguide to specific directions, slow light with large group-index, wideband, and low group-velocity dispersion can be realized. Defining a criterion of restricting the group-index variation within a ±0.8% range as a flattened region, we obtain the ultraflat slow light with bandwidths over 5.0, 4.0, 2.5, and 1.0 nm when keeping the group index at 38.0, 48.8, 65.2, and 100.4, respectively. Numerical simulations are performed utilizing the three-dimensional (3D) plane-wave expansion method and the 3D finite-difference time-domain method.

Negative refraction and subwavelength imaging in a hexagonal two-dimensional annular photonic crystal

Negative refraction in a hexagonal two-dimensional (2D) annular photonic crystal (APC) has been studied. The photonic band structure and equal frequency contours of the designed APC are analyzed using the plane wave expansion method, and the subwavelength imaging is simulated using finite-difference time-domain method. Numerical simulation results indicate that negative refraction and subwavelength imaging can be realized in the designed APC. The resolution of the designed APC slab is better than the radiation wavelength.

Broadband and high efficiency metal–multilayer dielectric grating based on non-quarter wave coatings as reflective mirror for 800 nm

A broadband and high efficiency metal–multilayer dielectric grating (MMDG), which was used to compress and stretch an ultra-short laser pulse in a chirped-pulse amplification (CPA) system, was designed. The diffraction characteristics of the MMDG were analyzed using the method of rigorous coupled wave analysis. The reflective mirror used for the broadband and high diffraction grating is made up of non-quarter wave metal–multi-layer dielectric coatings. Taking the diffraction efficiency of the −1 order as merit function, the parameters such as groove depth, residual thickness and reflective mirror were optimized to obtain broadband and high diffraction efficiency. The optimized MMDG shows an ultra-broadband working spectrum with an average efficiency exceeding 97% over 120 nm wavelength centered at 800 nm and TE polarization. The optimized MMDG should have potential application in CPA systems.

Nanoscale light confinement and nonlinearity of hybrid plasmonic waveguide with a metal cap

Abstract. A nonlinear hybrid plasmonic waveguide (HPW) with a metal cap on a nonlinear material-on-insulator rib is proposed. By using a finite-difference time-domain method, its light confinement and effective nonlinearity coefficient of the Kerr effect for all-optical switches are analyzed in detail. Numerical simulations illustrate that the nonlinear HPW structure has nanoscale confinement and high effective nonlinearity coefficient at the wavelength of 1550 nm. Consequently, the HPW can be used in all-optical signal processing of integrated photonics.

Double prisms for two-dimensional optical satellite relative-trajectory simulator

This paper presents the development of an ultra precision optical satellite relative-trajectory simulator. It is commonly known that by combining two prisms of equal apex angle in near contact and by independently rotating them about an axis parallel to the normals of their adjacent faces, a ray can be steered in any direction within a limited cone. Due to the request of high precision, we use the table-looking method rather than the conventional approximately formula to transform the angle between the azimuth and elevation angle of the relative-trajectory and the angle of the servo-motor. To achieve a stable and accurate control of the system, a Proportional-Integral-derivative (PID) controller is used and the controller was optimized use the Genetic Algorithm. Furthermore, we simulated the system with Matlab program under different bandwidth. The results demonstrate that the proposed position control system achieves a good control performance. The simulation indicates that PID controller performed well on the satellite relative-trajectory simulation.

Beam splitter and beam bends based on self-collimation effect in two-dimensional photonic crystals

Basing on the self-collimation effect of photonic crystals, one-to-two beam splitter, beam bend and one-to-three beam splitter are, respectively, designed by introducing a different line defect along the same direction. From the equal-frequency contour plot which is calculated by the plane wave expansion method, we obtain the frequency and the propagate direction of the self-collimated beam. The self-collimated beam propagation in photonic crystals with different line defects is simulated by the two-dimensional finite-difference time-domain method with perfectly matched layer absorbing boundary conditions. The simulation results show that one-to-two beam splitter, beam bend and one-to-three beam splitter can be realized by appropriately arranging the line defect along the proper direction. Such devices can greatly enhance photonic crystals for use in high-density optical integrated circuits.

High diffraction efficiency for multi-layer dielectric gratings with rectangular groove

The purpose of this study is dedicated to the new design of the Multi-layer dielectric grating with the best performance giving a highly diffraction efficiency in the -1 order, by using the rigorous coupled wave Analysis (RCWA). The formulation for the implementation of the RCWA for multi-layer dielectric gratings incorporating the developed enhanced transmittance approach is presented. An optimized design of multi-layer dielectric grating working at 1053-nm with TE polarized light and 51.2° incident can obtain the diffraction efficiency of 99%.

Focal shift and extended focal depth with tunable pupil filter

Extended focal depth and focal shift are very important in microscopy, imaging and optical storage systems, and have attracted much attention in recent years. In order to obtain the extended focal depth and focal shift, a new kind of tunable pupil filter is proposed in this article. It consists of one half-wave plate between two quarter-wave plates, and the half-wave plate is made up of two zones that can rotate with respect to each other. By analyzing the intensity distribution in the focal region of the optical system with such a device, it reveals that focal shift can be realized by rotating any zone of the half-wave plate. When the phase difference of the two zones is π, the extended focal depth and transverse superresolution can be obtained at the same time. Therefore, it may be feasible to use such a tunable pupil filter in optical systems that need focal shift and extended focal depth.

Laser beam shaping system with a radial birefringent filter

Reshaping of a Gaussian laser beam into a uniform or other intensity distribution is required for various applications. The laser beam shaping system with a radial birefringent filter is presented in this paper. With such a system the Gaussian beams can be transformed into uniform or annular beams. The theory and simulation of the proposed systems are described in detail. The primary advantage of such a system is that the out beam profile can be tunable with the rotation of the radial birefringent element.