Qinghua Liao

Ultracompact and wideband power splitter based on triple photonic crystal waveguides directional coupler

Three single-mode photonic crystal waveguides (PCWs) are put together closely to construct three PCWs directional coupler (3PCWDC). It is found that the coupling coefficient of two even modes excited by the input field symmetrically entering 3PCWDC may be increased dramatically by reducing the size scales of dielectric rods between the neighbouring PCWs. The coupling length in this structure can be much shorter than that in conventional PCWs within the frequency range of interest. The application of this feature to an ultracompact, wide bandwidth and high-transmission power splitter is addressed. The simulation results by using the finite-difference time-domain method show that the structure exhibits new interesting characteristics.

Design of a Novel Polarized Beam Splitter Based on a Two-Dimensional Photonic Crystal Resonator Cavity

We propose and analyze a novel ultra-compact polarization beam splitter based on a resonator cavity in a two-dimensional photonic crystal. The two polarizations can be separated efficiently by the strong coupling between the microcavities and the waveguides occurring around the resonant frequency of the cavities. The transmittance of two polarized light around 1.55 μm can be more than 98.6%, and the size of the device is less than 15 μm×13 μm, so these features will play an important role in future integrated optical circuits.

Simultaneous large band gaps and localization of electromagnetic and elastic waves in defect-free quasicrystals.

We report numerically large and complete photonic and phononic band gaps that simultaneously exist in eight-fold phoxonic quasicrystals (PhXQCs). PhXQCs can possess simultaneous photonic and phononic band gaps over a wide range of geometric parameters. Abundant localized modes can be achieved in defect-free PhXQCs for all photonic and phononic polarizations. These defect-free localized modes exhibit multiform spatial distributions and can confine simultaneously electromagnetic and elastic waves in a large area, thereby providing rich selectivity and enlarging the interaction space of optical and elastic waves. The simulated results based on finite element method show that quasiperiodic structures formed of both solid rods in air and holes in solid materials can simultaneously confine and tailor electromagnetic and elastic waves; these structures showed advantages over the periodic counterparts.

DESIGN OF HIGH EFFICIENCY AND LARGE SEPARATING ANGLE BEAM SPLITTER BASED ON PHOTONIC CRYSTAL CAVITY RESONATOR

We present the design and simulation of an ultracompact high efficiency beam splitter based on propagation properties of the light waves in waveguide and cavity resonator. The splitting properties of the beam splitter have been numerically simulated and analyzed using the plane wave expansion (PWE) method and finite difference time domain (FDTD) method. Then in order to minimize backward reflections and to obtain equal distribution of power, we placed a cavity resonator waveguides to optimize the devices. It is shown that a beam splitter with high efficiency and large separating angle for TM mode can be achieved. There is no doubt that these excellent features will provide the structure a promising applying prospect for photonic integrated circuit.

Design of a compact polarizing beam splitter based on a photonic crystal ring resonator with a triangular lattice

We propose and simulate a new kind of compact polarizing beam splitter (PBS) based on a photonic crystal ring resonator (PCRR) with complete photonic bandgaps. The two polarized states are separated far enough by resonant and nonresonant coupling between the waveguide modes and the microring modes. Some defect holes are utilized to control the beam propagation. The simulated results obtained by the finite-difference time-domain method show that high transmission (over 95%) is obtained and the polarization separation is realized with a length as short as 3.1 microm. The design of the proposed PBS can be flexible, thanks to the advantages of PCRRs.

Highly Efficient Coupling Between Inner and Surface Fields in Photonic Crystal Waveguides

Connecting inner and surface propagation field in photonic crystal waveguides (PCWs) with an efficient method is critical to achieve flexible designs and applications in photonic integrated circuits. This letter realizes highly efficient coupling between inner and surface PCWs by modifying the structures of waveguides and interface. The aim of the modification is to achieve a good match of modal field profiles between different types of waveguide structures as well as suppress the reflected field at the interface. The numerical results based on finite difference time-domain simulations show that the bandwidth for coupling efficiency larger than 90% can be as broad as over 100 nm.

Bloch oscillations in one-dimensional coupled multiple microcavities containing negative-index materials

We propose a one-dimensional (1D) coupled multiple microcavities (MMCs) structure containing negative-index materials. We obtain the photonic Wannier–Stark ladders (WSLs) both in the ordinary and the well-known omnidirectional band gaps by modulating the widths of the cavities. Due to the omnidirectional gaps having novel characteristics compared with the Bragg gaps, it is meaningful to study the WSL that lies in the omnidirectional gap. We study the temporal–spatial evolutions of Gaussian pulses whose central frequencies are in both gaps as passing through the proposed structure. We show for the first time that the electromagnetic Bloch oscillations can occur in such photonic structures containing negative-index materials.

Modulation of electromagnetic local density of states by coupling of surface phonon-polariton

We studied the electromagnetic local density of state (EM-LDOS) near the surface of a one-dimensional multilayer structure (1DMS) alternately stacked by SiC and Si. EM-LDOS of a semi-infinite bulk appears two intrinsic peaks due to the resonance of surface phonon-polariton (SPhP) in SiC. In contrast with that of SiC bulk, SPhP can exist at the interface of SiC and Si for the 1DMS. The SPhPs from different interfaces can couple together, which can lead to a significant modulation of EM-LDOS. When the component widths of 1DMS are large, the spectrum of EM-LDOS exhibits oscillation behavior in the frequency regime larger than the resonance frequency of SPhP. While the component widths are small, due to the strong coupling of SPhPs, another peak appears in the EM-LDOS spectrum besides the two intrinsic ones. And the position of the new peak move toward high frequency when the width ratio of SiC and Si increases. The influences of distance from the surfaces and period of 1DMS on EM-LDOS have also been studied in detail. The results are helpful in studying the near-field radiative heat transfer and spontaneous emission.

Acoustic multimode interference and self-imaging phenomena realized in multimodal phononic crystal waveguides

We report an acoustic multimode interference effect and self-imaging phenomena in an acoustic multimode waveguide system which consists of M parallel phononic crystal waveguides (M-PnCWs). Results show that the self-imaging principle remains applicable for acoustic waveguides just as it does for optical multimode waveguides. To achieve the dispersions and replicas of the input acoustic waves produced along the propagation direction, we performed the finite element method on M-PnCWs, which support M guided modes within the target frequency range. The simulation results show that single images (including direct and mirrored images) and N-fold images (N is an integer) are identified along the propagation direction with asymmetric and symmetric incidence discussed separately. The simulated positions of the replicas agree well with the calculated values that are theoretically decided by self-imaging conditions based on the guided mode propagation analysis. Moreover, the potential applications based on this self-imaging effect for acoustic wavelength de-multiplexing and beam splitting in the acoustic field are also presented.

A new transfer matrix method to calculate the optical absorption of graphene at any position in stratified media

A new transfer matrix method is developed based on the electromagnetic boundary conditions that Maxwells equations required. Compared with the traditional transfer matrix method, the new method can be used to calculate the optical absorption of any layer at any position in stratified media. By using this new method, we investigate the optical absorption properties of a single graphene sheet in one-dimensional photonic-crystal (1DPC) heterostructures. We demonstrate that the optical absorption of graphene can be greatly enhanced up to almost 100% by the use of the 1DPC heterostructures because of the strong photon localization in the layer of graphene.