Liyong Jiang

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.

Polarization-Independent Single-Mode Waveguiding With Honeycomb Photonic Crystals

A line-defect waveguide hosted in a 2D honeycomb photonic-crystal slab is demonstrated theoretically to realize polarization-independent waveguiding. Through an appropriate set of design parameters, a normalized bandwidth of ~2% subject to the condition that the common-frequency region between the transverse-electric and transverse-magnetic polarizations reaches the maximum value for a silicon-on-insulator structure. Moreover, unlike other large-bandwidth polarization-independent photonic-crystal waveguides, our structure has the highest bandwidth for a polarization-independent single-mode photonic-crystal waveguide.

Polarization-independent negative refraction effect in SiO2-GaAs annular photonic crystals

We systematically investigated the negative refraction effect for both TM and TE polarization modes in SiO2-GaAs annular photonic crystals with triangular lattice. It was found that, in comparison with normal triangular-lattice air-holes photonic crystals, the annular photonic crystals have much lower and flatter band structures, which are quite beneficial to the formation of convex equifrequency surfaces for both polarizations. Further analyses on equifrequency surfaces and the electric field distribution of annular photonic crystals with different parameters have not only first demonstrated the possibility of polarization-independent negative refraction effect in annular photonic crystals, but also revealed some important laws to control the working frequency and performance of this remarkable effect.

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.

Polarization-insensitive self-collimation and beam splitter based on triangular-lattice annular photonic crystals

This paper systematically investigates the self-collimation behavior in silicon-based triangular-lattice annular photonic crystals (PCs). It is found that, in comparison with normal air-hole PCs, annular PCs more easily suppress the separation between TE-2 and TM-2 bands along the Γ-M direction by increasing the inner radius of annular air rings. Such a feature is quite beneficial in the formation of a flat equi-frequency contour for both polarizations at the same frequency, which means a polarization-insensitive self-collimation (PISC) effect. Further analysis has shown that, to support PISC, the minimum ratio between the inner and outer radii of annular air rings will gradually increase as the outer radius changes from 0.25a to 0.49a. When the ratio is fixed, the annular air rings with larger outer radius will provide wider common frequency area to realize PISC. We have also investigated the transmission feature for different annular PCs and chosen an optimal structure to illustrate the PISC effect. Finally, a polarization beam splitter has been proposed and demonstrated based on the unique PISC and band-gap feature in triangular-lattice annular PCs.

Design and fabrication of rod-type two-dimensional photonic crystal slabs with large high-order bandgaps in near-infrared wavelengths.

We proposed a novel two-dimensional photonic crystal slab comprised of a number of silicon rods with different radii and locations in the square-lattice unit cell pattern. Such rod-type photonic crystal slabs were automatically optimized by the genetic algorithm and fabricated on the silicon-on-insulator wafer. In particular, the measured transmission spectra of the five-rods sample have shown a large accepted high-order bandgap between 1498 and 1648 nm (gap size is 9.54%). Based on the theories of multiple Bragg and Mie scattering effects, we have given a reasonable explanation to the large high-order bandgaps found in the present study.

Dual-negative-refraction and imaging effects in normal two-dimensional photonic crystals with hexagonal lattices

A novel dual-negative-refraction (DNR) effect is studied in two types of normal two-dimensional photonic crystals (2DPCs) with hexagonal lattices. Systematical analyses of the band structures and equifrequency surfaces indicate that the DNR may be realized when the overlapping second and third bands with relatively flat shapes and only a slight separation are available at some frequencies close to the bands peak of 2DPCs. Further simulations have not only confirmed the DNR and corresponding dual-imaging effects in normal 2DPCs with hexagonal lattices but also revealed some relative rules to the dual images. In particular, the thickness as well the cutoff value at terminations of PCs can strongly influence the performance of dual images and even determine whether the dual images would appear. Moreover, a relatively low working frequency is recommended to minimize the distortion degree of dual images.

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.

Polarization-insensitive and broad-angle self-collimation in a two-dimensional photonic crystal with rectangular air holes.

In this paper, we have systematically investigated the polarization-insensitive and broad-angle self-collimation behavior in a square-lattice two-dimensional photonic crystal (PhC) with rectangular air holes. By analyzing the band structures and the corresponding equi-frequency contours (EFCs), it is found that such PhC can show special dispersion properties when the half-length L and half-width W of rectangular air holes are appropriately changed. First, compared with conventional square-lattice PhCs with circular or square air holes, such PhC is easier to support polarization-insensitive self-collimation (PISC) based on the EFCs for the second band. Meanwhile, the PISC behavior such as working frequency range and effective incident angle can be more flexibly adjusted by changing the structural parameters of rectangular air holes. Second, such PhC can show long flat EFCs for the TM-3 band. This is quite helpful for supporting broad-angle self-collimation. In particular, when L=0.5a and W/L=0.8, this PhC will degenerate to a one-dimensional grating PhC. It can show good all-angle self-collimation behavior with an improved relative bandwidth (about 19.98%) compared with previous works. It also presents advantages in practical applications due to a relatively convenient fabrication process.