Recently it has been shown that it is possible to achieve directional emission out of a subwavelength aperture in a periodically corrugated metallic thin film. We report on theoretical and experimental studies of a related phenomenon concerning light emitted from photonic crystal waveguides that are less than a wavelength wide. We find that the termination of the photonic crystal end facets and an appropriate choice of the wavelength are instrumental in achieving very low numerical apertures. Our results hold promise for the combination of photonic crystal waveguides with conventional optical systems such as fibers, waveguides, and freely propagating light beams.

Highly Directional Emission from Photonic Crystal Waveguides of Subwavelength Width

The add drop filter (ADF) is one of the most significant devices for coarse wavelength division multiplexing (CWDM) systems to add and/or drop a required channel individually from multiplexed output channels without disturbing other channels. The important parameters of the ADF are coupling efficiency, dropping efficiency, passband width and Q factor. Photonic crystal (PC)-based optical devices have attracted great interest due to their compactness, speed of operation, long life period, suitability for photonic integrated circuits, and future optical networks. Here, an extensive overview of a photonic crystal ring resonator (PCRR)-based ADF using a different shape of ring resonator is presented, and its corresponding functional parameters are discussed. Finally, the designed circular PCRR-based ADF for an ITU-T G 694.2 CWDM system is presented. Approximately 100% of coupling efficiency and dropping efficiency, 114.69 of Q factor, and 13 nm of passband width is obtained through simulation, which outperforms the reported one.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-52/issue-6/060901/Photonic-crystal-ring-resonator-based-add-drop-filters--a/10.1117/1.OE.52.6.060901.pdf

Photonic crystal ring resonator-based add drop filters: a review

Novel design of optical channel drop filters based on two-dimensional photonic crystal ring resonators

Abstract We have proposed simple ring resonator 5-channel demultiplexer based on optical channel drop filter analysis that is applicable at third communication window (1550 nm) range. Our proposed base filter is the important part in designing the demultiplexer, inclusive one ring resonator contains one square dielectric rod at core. Demultiplexer structure introduced by arranging five filter with different ring core refractive index. Insomuch every ring core have individual refractive index, thus each ring have diverse resonant wavelength. Numerical results by the finite difference time domain (FDTD) method show quality factor (Q) and transmission efficiency of fundamental channel drop filter are 1038 and 93 %, respectively. It is found that transmission efficiency in designed demultiplexer is more than 90 % for each channel; channel spacing is less than 4.2 nm. The average crosstalk value, total footprint of demutiplexer is −17.85 dB, 689.61 μm2, respectively. Small size with a very simple ring design can be benefit in photonic integrated circuit.

An Optical Five Channel Demultiplexer-Based Simple Photonic Crystal Ring Resonator for WDM Applications

In this paper, we discuss two-dimensional photonic crystals of complex rhombus lattice structures to enable broadband, 'all-angle' light collimation. The proposed structures are made of complex elliptical dielectric rods (e = 12) embedded in an air background. Based on the equi-frequency contours (EFCs) calculated by the plane wave expansion method (PWEM), these structures show a wide range of open, nearly flat EFCs, with the maximum angular deviation from the primary collimating direction less than 2°. The broadest relative range of these open, nearly flat EFCs found is 17% of a chosen central wavelength. For a sample element centered at 1550 nm, this range covers wavelengths from 1429 to 1695 nm; the devices can be scaled to cover alternative wavelength ranges. These structures could benefit applications that require a broad operating wavelength range. Potential applications include collimating beam combiners, multiplexers and solar light collectors.

http://iopscience.iop.org/article/10.1088/2040-8978/12/3/035102/pdf

Complex rhombus lattice photonic crystals for broadband all-angle self-collimation

The position independence of heterostructure coupled waveguides in photonic-crystal switch

We propose a system for achieving highly efficient directional emission using a coupled multi-channel structure to a photonic crystal waveguide with a modified surface region. A highly directional beam with about 50% transmission efficiency at large distances can be obtained. The simulation results show that the surface modes in the modified region are easy to couple to the multi-channel structure, which can generate many light beams in the channels. The interference of the light beams exiting from the channels results in a highly efficient directional emission. Additionally, the size effect of the coupled multi-channel structure on the directional emission is also considered. When the row number of the coupled multi-channel structure is larger than a certain value, a steady state of highly efficient directional emission can be obtained.

https://iopscience.iop.org/article/10.1088/0022-3727/41/10/105110/pdf

Highly efficient directional emission using a coupled multi-channel structure to a photonic crystal waveguide with surface modification

The invention provides a wavelength division multiplexer/demultiplexer based on the multimode interference effect of two-dimensional photonic crystals, comprising a monomode waveguide (1), a multimode interference waveguide (2), a monomode waveguide (3) and a monomode waveguide (4), wherein the monomode waveguide (3) and the monomode waveguide (4) are parallel. The invention confirms the length of the multimode waveguide according to the self-imaging conditions of two preset frequency positions so that after light is input from the monomode waveguide (1) in the two frequency positions, light in one frequency position is imaged on the monomode waveguide (3), and light in the other frequency position is imaged on the monomode waveguide (4) so as to realize the function of a wavelength division demultiplexer. Light with a frequency satisfying an imaging condition is respectively input from the monomode waveguide (3) and the monomode waveguide (4) and output from the monomode waveguide (1) so as to realize the function of a wavelength division multiplexer. The invention has the advantages of compact structure, easy integration, and the like and has wide application prospects in future optical communication.

Wavelength division multiplexer/demodulation multiplexer based on multimode interference effect of two-dimensional photonic crystals

Efficient directional emission is realized by using beam splitting in multi-channel photonic crystal (PC). The periodic channels are introduced on the exit surface of a PC waveguide to create multiple light beams, whose interference leads to the directional emission. In order to improve the quality of the directional emission, coupled PC waveguides with low-loss are designed to make the incident light split into several beams. The simulation results show that the transmission efficiency is greatly enhanced by beam splitting.

Highly directional emission from multi-channel photonic crystal via beam splitting

By means of introducing defect layers into multilayer structures composed by epsilon-negative material and mu-negative
material, the free modulation of single and double localized modes in zero-φ eff gap is realized respectively through
adjusting the thickness of different kinds of defect layers in two designed structures. In addition, the defect modes in the
 zero-φ eff gap, being distinct from those in the Bragg gap, have the property of insensitivity to incident angle and
polarization in some degree, which would be applied widely in tunable omnidirectional filters.

Lingling Zhang

Papers

The position independence of heterostructure coupled waveguides in photonic-crystal switch

Highly efficient directional emission using a coupled multi-channel structure to a photonic crystal waveguide with surface modification

Wavelength division multiplexer/demodulation multiplexer based on multimode interference effect of two-dimensional photonic crystals

Highly directional emission from multi-channel photonic crystal via beam splitting

Free modulation of defect states in multilayer structures consisting of epsilon-negative material and mu-negative material