Xiangyin Li

Genetic optimization of photonic crystal waveguide termination for both on-axis and off-axis highly efficient directional emission

In this work, we use the finite-difference time-domain method in conjunction with a genetic algorithm (GA) to design a photonic crystal waveguide with gratinglike surface added for highly-efficient directional emission. By placing a detector at different locations in the output field, we have obtained both on-axis and off-axis highly-efficient directional emission designs with the help of GA. The interference of light emitted from the outlet of waveguide and surfaces modes in gratinglike surface is believed to account for the directional beaming effect in our designs.

Accurate Modeling of Dark-Field Scattering Spectra of Plasmonic Nanostructures

Dark-field microscopy is a widely used tool for measuring the optical resonance of plasmonic nanostructures. However, current numerical methods for simulating the dark-field scattering spectra were carried out with plane wave illumination either at normal incidence or at an oblique angle from one direction. In actual experiments, light is focused onto the sample through an annular ring within a range of glancing angles. In this paper, we present a theoretical model capable of accurately simulating the dark-field light source with an annular ring. Simulations correctly reproduce a counterintuitive blue shift in the scattering spectra from gold nanodisks with a diameter beyond 140 nm. We believe that our proposed simulation method can be potentially applied as a general tool capable of simulating the dark-field scattering spectra of plasmonic nanostructures as well as other dielectric nanostructures with sizes beyond the quasi-static limit.

Inverse design for directional emitter and power splitter based on photonic crystal waveguide with surface corrugations

In this paper, we report an inverse strategy to design a directional emitter and power splitter based on two-dimensional photonic crystal waveguides (PCWs). Our approach is illustrated by employing a genetic algorithm in conjunction with the finite-difference time domain (FDTD) method to design a corrugated surface added behind the termination of PCWs. By arranging symmetrical or asymmetrical surface corrugations along the axis of a waveguide, we have successfully realized both a highly efficient directional emitter and an open-type Y-shaped power splitter from planar PCWs. Moreover, the angle of directional emission and split beams can be easily controlled by changing the location of the detector in the output area.

Imaging properties of an annular photonic crystal slab for both TM-polarization and TE-polarization

The imaging behaviors for two kinds of polarized waves found by using a two-dimensional annular photonic crystal flat lens are theoretically investigated. For the TE-polarization, subwavelength imaging due to a negative refraction effect is obtained and the focal length can be adjusted by changing the lens thickness. In contrast, for the TM-polarization, because the resulting optical field distribution behind the slab resembles a triple point source due to the effect of channeling, the light can focus again due to the interference effect. Most importantly, using the proper annular photonic crystal lens, the image of an unpolarized wave point source can be realized. These results show that the negative refraction is not the one and only mechanism of imaging by a photonic crystal lens.

Design and fabrication of high efficiency power coupler between different photonic crystal waveguides

Based on inspiration from an inverse optimization strategy and theoretical finite-difference time-domain method simulations, an ultralow loss power coupler between two different photonic crystal waveguides was designed, fabricated and characterized. The experimental results showed that the loss was less than 1 dB for transverse electric polarized light at a wavelength of 1550 nm, which is consistent with expectations from numerical modeling. High efficiency optical couplers are critical for development of integrated optical circuit functionality.

Design and fabrication of high-efficiency photonic crystal power beam splitters

Efficient beam splitters in a planar photonic crystal waveguide with corrugated terminators were designed, fabricated, and characterized. Experimental results showed that these beam splitters have high splitting efficiency and power intensity in the propagation direction, an achievement made possible through a design method employing a genetic algorithm-based optimization method. High-efficiency power beam splitters are useful components in integrated optics, and the design implemented here is particularly suited for integration with other optical components.

Publisher’s Note: “Tunable and absolute electromagnetic vacuum in two-dimensional photonic-band-gap Based on multiferroic materials” [Appl. Phys. Lett. 99, 132903 (2011)]

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Optimization and fabrication of the photonic crystals for large absolute band gaps

In the paper, Optimization algorithm is used to design two-dimensional photonic crystals for large absolute band gaps. By optimizing the location and radius of rods in the unit cell, we present tree designs considering different shape rods. The genetic algorithm finally yielded a photonic crystal with an absolute common band gap of 0.0796(2πc/a) at the mid-frequency of 0.3712(2πc/a). The result have been fabricated using e-beam lithography (EBL) and inductively coupled plasma (ICP). We can find a large absolute band gap can be obtained.

Tunable and absolute electromagnetic vacuum in two-dimensional photonic-band-gap Based on multiferroic materials

When multiferroic terbium manganite (TbMnO3) crystal cylinders are periodically arranged in a square lattice, the resulting two-dimensional (2D) system exhibits photonic band gaps (PBGs). The absolute PBG originating from the Mie resonance is modulated from closed to open by applying an external static magnetic field, which is attributed to the electromagnon depression of the dielectric constant by the rearrangement of antiferromagnetic order. Tunable electromagnetic band structure may be realized by controlling the magnetic transition of manganese spins in TbMnO3.