Rong-Juan Liu

Robust one-way modes in gyromagnetic photonic crystal waveguides with different interfaces

We experimentally realize a one-way waveguide made from a gyromagnetic photonic crystal (GPC) and a normal dielectric photonic crystal in their common photonic band gap. Due to the time reversal symmetry breaking of the GPC under a dc magnetic field, the edge modes exhibit a one-way property. We investigate the unidirectional propagation character originating from the chiral edge states at different widths of the waveguide. Both the forward modes, which are robust to various obstacles, and the backward modes show an obvious frequency shift for different waveguide widths. Numerical simulations by means of the transfer-matrix method and multiple scattering methods agree well with the experimental data.

Surface-plasmon-polariton assisted diffraction in periodic subwavelength holes of metal films with reduced interplane coupling

Metal films grown on an Si wafer perforated with a periodic array of subwavelength holes have been fabricated and anomalous enhanced transmission in the midinfrared regime has been observed. High order transmission peaks up to Si(2,2) are clearly revealed due to the large dielectric constant contrast of the dielectrics at the opposite interfaces. The Si(1,1) peak splits at oblique incidence both in TE and TM polarization, which confirms that anomalous enhanced transmission is a surface-plasmon-polariton (SPP) assisted diffraction phenomenon. Theoretical transmission spectra agree excellently with the experimental results and confirm the role of SPP diffraction by the lattice.

Origin of Shape Resonance in Second-Harmonic Generation from Metallic Nanohole Arrays

Second-harmonic generation (SHG) from periodic arrays of subwavelength rectangular air hole with various aspect ratios perforated in gold thin films can get resonantly enhanced for some specific geometric shapes. Here we clarify the physical origin of this shape resonance effect. A nonlinear coupled-mode theory is set up to solve energy conversion from fundamental wave (FW) mode to second-harmonic wave (SHW) mode within the nanoscale air hole. It reveals that several physical mechanisms, including the FW mode excitation amplitude, FW-SHW modal spatial overlap, FW-SHW mode phase mismatch, and SHW mode attenuation, are all geometric shape sensitive and altogether act to induce the SHG shape resonance effect. The theory agrees well with experimental observations and provides an accurate and complete explanation for the long-emphasized but elusive shape effect. The study may stimulate deeper insights to visualize general nonlinear nanophotonic processes and pave the way to engineering high-efficiency nonlinear nanophotonic devices.

Unidirectional channel-drop filter by one-way gyromagnetic photonic crystal waveguides

We theoretically and experimentally study the side coupling between guided modes and cavity modes in a one-way waveguide that is composed of a regular photonic crystal and a gyromagnetic photonic crystal. At the cavity resonant wavelength, the backward mode can be completely blocked while the forward mode is only slightly influenced in the transmissivity for a specially designed waveguide. This unique light transport property can be exploited to construct a unidirectional band stop filter and a unidirectional channel-drop filter that can selectively process a light signal propagating only along a particular direction.

Multichannel filters via Γ-M and Γ-K waveguide coupling in two-dimensional triangular-lattice photonic crystal slabs

We demonstrate the design, fabrication, and characterization of a multichannel filter in a two-dimensional triangular-lattice photonic crystal slab. The output signal channel, which directs in the Γ-M crystalline direction, is orthogonal to the input signal channel that directs in the Γ-K crystalline direction. In each channel, the filtering function is guaranteed by the indirect resonant coupling between the waveguide and the cavity. Four resonant cavities with different sizes provide the mechanism of finely tuning the output wavelength. Our experimental results are in good agreement with simulations.

Γ–M waveguides in two-dimensionaltriangular-lattice photonic crystal slabs

We propose a line defect waveguide structure along the Γ-? direction in two-dimensional triangular lattice silicon photonic crystal slabs. The modal dispersion relation and the transmission spectra of this waveguide are studied. The results show that by perturbing the width of the line defect and the diameter of the air holes adjacent to the waveguide core, one can control the width of the single mode transmission window and make it far broader than the original one. The proposed Γ-? waveguide will help to build a more flexible network of interconnection channel of light in two-dimensional photonic crystal slabs.

Channel-drop filters in three-dimensional woodpile photonic crystals

Optical waveguides are characterized by high-efficiency transmission of electromagnetic (EM) waves and optical cavities have frequency selective property. The combination of optical waveguides and cavities can result in a large amount of devices in optical integrated circuit, among which a channel-drop filter is an important member. A three-dimensional (3D) woodpile photonic crystal (PC) can possess a complete band gap that allows perfect confinement of EM waves. We theoretically and experimentally study the property of the x-type waveguide and acceptor-type defect cavity that are created in the 3D woodpile PC working in the microwave regime. On the basis of resonant coupling between the x-type wave guide and acceptor-type cavity, a series of three-port in-plane-type channel-drop filters are built: single-cavity, double-cavity, three-cavity, and four-cavity channel-drop filters. All of them exhibit good frequency responding property by changing the size of cavity. The multicavity channel-drop filter demonstrates the possibility to build multichannel wavelength division multiplexers in the 3D PC platform

Experimental demonstration of tunable gyromagnetic photonic crystals controlled by dc magnetic fields

We build a two-dimensional square-lattice gyromagnetic photonic crystal from magneto-optical material of yttrium-iron-garnet (YIG) and demonstrate its tunable electromagnetic properties by placing the photonic crystal in a tunable dc magnetic field and measuring the transmission spectra for microwaves. The magnetic field breaks the intrinsic degeneracy at the M symmetry point between the 2-3 photonic bands and opens a band gap. Near the resonance frequency of YIG, a band gap is also opened due to the metallic properties of YIG. Our work opens a window to exploit magneto-optical photonic crystals as tunable integrated optical devices.

Control and blockage of edge modes in magneto-optical photonic crystals

One-way edge states realized in magneto-optical photonic crystals (MOPCs) is generally believed to be robust to scattering from disorders and even strong perturbations. In this work, we experimentally and theoretically study the influence of obstacles on the transport of forward and backward edge states in two-dimensional MOPCs under a dc magnetic field. We find that the passage of electromagnetic wave around the obstacles mainly takes advantage of the formation of new edge states at the new interfaces formed between the obstacle and bulk MOPC. The forward edge states can be blocked when the new edge state channel is cut off or the coupling with the original edge states to the new edge states is strongly reduced.

Composite metamaterials with dual-band magnetic resonances in the terahertz frequency regime

Composite metamaterials (CMMs) combining a subwavelength metallic hole array (i.e. one-layer fishnet structure) and an array of split-ring resonators (SRRs) on the same board are fabricated with gold films on a silicon wafer. Transmission measurements of the CMMs in the terahertz range have been performed. Dual-band magnetic resonances, namely, an LC resonance at 4.40 THz and an additional magnetic resonance at 8.64 THz originating from the antiparallel current in wire pairs in the CMMs, are observed when the electrical field polarization of the incident light is parallel to the gap of the component SRR. The numerical simulations agree well with the experimental results and further clarify the nature of the dual-band magnetic resonances.