Near-field observation of beam steering in a photonic crystal superprism

The optical near-field technique is applied to provide a direct experimental observation of the refracted beam propagation inside a photonic crystal structure displaying a superprism effect. The obtained results show a 35° light beam angle deviation for a wavelength variation from 1500 to 1600 nm. The experimentally determined beam divergence is in good agreement with modeling predictions and previously performed transmittance experiments. A marked self-collimation propagation over a broad 20 nm wide spectral range centered at λ=1550 nm is experimentally demonstrated. The developed technique opens promising perspectives for the invisibility cloaking structures investigation.     

The directional emission sensitivity of photonic crystal waveguides to air hole removal

To obtain highly directional light output from photonic crystal waveguides (PCWs), the emission characteristics of the narrow-width waveguide structures are investigated by tailoring the geometry of the exit sides. The local structural deformations in the form of air hole removal from the triangular-lattice photonic crystal (PC) show the effectiveness of the previously proposed approach that was implemented by us for another type of PC. The spatial broadening of the beam is greatly suppressed. With the modified waveguide exits, highly directional emissions with small side lobes are achieved. The frequency dependency of the directional emissions is evaluated. We show that the divergence angles of the beams depend linearly on the wavelength for a regular type of PCW but the modified PCW exits have local minima with respect to wavelength in terms of the divergence angle. The present work may prove to be helpful in the design of couplers and edge-emitting lasers and in the implementation of free-space optical communications.     

Asymmetric light transport in L-shaped and U-shaped photonic crystal waveguides

We present an asymmetric light transport in a modified U-shaped photonic crystal waveguide (PCW). The compact waveguides consist of coupling two L-waveguides in a PC formed by dielectric rods in air arranged in a square lattice. We show that the transmitted power flow depends on the direction of the incident light, the transmission varies between a high and low value upon switching the position of the source beam from the input to the output of the PCW. Computations are performed using the plane wave expansion and finite-difference time-domain methods.     

Experimental demonstration of self-collimation inside a three-dimensional photonic crystal

We present our experimental demonstration of self-collimation inside a three-dimensional (3D) simple cubic photonic crystal at microwave frequencies. The photonic crystal was designed with unique dispersion property and fabricated by a high precision computer-controlled machine. The self-collimation modes were excited by a grounded waveguide feeding and detected by a scanning monopole. Self-collimation of electromagnetic waves in the 3D photonic crystal was demonstrated by measuring the 3D field distribution, which was shown as a narrow collimated beam inside the 3D photonic crystal but a diverged beam in the absence of the photonic crystal.     

Controlling the self-collimation characteristics of a near-infrared two-dimensional metallic photonic crystal

Self-collimation characteristics of the two-dimensional square-lattice photonic crystal(PC) consisting of metal rods immersed in silicon are studied by the finite-difference time-domain method.The Drude dispersion model is adopted to describe the metal rod,and the self-collimation behaviours of the near-infrared light through the PC are studied.The frequency region and the tolerance of incident angle for the self-collimation behaviour can be controlled by changing the shape of the metal rods.     

Analysis of optical coupling between air-holes photonic crystal and tapered dielectric waveguides

Two types of structures for the optical coupling between photonic crystal waveguide (PCW) with triangular air-holes lattice and tapered dielectric waveguide (TDW) are investigated. The TDWs are coupled with PCW at the input and output connections of the PCW, which can constitute a unidirectional waveguide structure. The transmission efficiencies of the input and output coupling are analyzed theoretically for different coupling structures. It is found that the coupling efficiencies vary with the width and length of coupling region. When the light transmitted from TDW to PCW, the highest transmission efficiency is 92.6%, and it is 99.6% when the light transmitted from PCW to TDW. Then the total transmission efficiency of the entire waveguide is simulated and it can reach 92.3% for one type of structure and is a little lower for the other type of structure under the chosen optimal structure parameters. Considering the current processing accuracy, the structure mentioned above should be an effective coupling manner that can be easily realized.     

Polarization-independent self-collimating bends and beam splitters in photonic crystals

Polarization independent bends and beam splitters for transverse electric (TE) and transverse magnetic (TM) polarizations have been demonstrated in two-dimensional (2D) photonic crystals (PhCs). In virtuel of equi-frequency contour analysis and finite-difference time-domain calculations, self-collimation behaviors for TE- and TM-polarizations are achieved at the same frequency. Simulation results show a 90-degree bend with 90% efficiency and beam splitters with about 96% total efficiency for both TE- and TMpolarizations, where the light is self-guided by the self-collimation effect. Such bends and beam splitters are expected to play important roles in optical devices where polarization insensitivity is needed.     

Broadband large-angle self-collimation in two-dimensional silicon photonic crystal

We report a two-dimensional silicon photonic crystal structure that supports the self-collimation effect with large incident angles over a broad wavelength range. We theoretically and experimentally show the propagation light in the designed and fabricated structure without diffraction. Broadband large-angle self-collimation is observed in infrared wavelength.     

Experimental demonstration of self-collimation beaming and splitting in photonic crystals at microwave frequencies

I studied experimentally beam self-collimation and splitting in two-dimensional microwave photonic crystals. Using a microwave photonic crystal fabricated from alumina rods, I present an experimental proof of principle for an earlier theoretical proposal [A.F. Matthews, S.K. Morrison, Yu.S. Kivshar, Opt. Commun. 279 (2007) 313] of a photonic crystal beam splitter based on the self-collimation effect.     

Amphoteric-like refraction in a two-dimensional photonic crystal

High-index contrast photonic crystals possess an intricate photonic band structure that is responsible for surprising phenomena as the surperprism effect, self-collimation, power splitting or negative refraction. Recently, it was reported that at the interface between an isotropic medium and a uniaxial crystal (or between two uniaxial crystals) a phenomenon known as amphoteric refraction, that is, positive as well as negative refraction, can take place. By means of a equifrequency contours analysis and finite-difference time-domain simulations, we show that a two dimensional photonic crystal can also present amphoteric refraction by properly choosing the lattice orientation and the termination. However, total transmission is difficult to achieve because a Bloch mode is excited inside the photonic crystal and the coupling efficiency from this mode to an external plane wave is lower than one.     

Conditions for self-collimation in three-dimensional photonic crystals

We introduce the theoretical criterion for achieving three-dimensional self-collimation of light in a photonic crystal. Based on this criterion, we numerically demonstrate a body-center-cubic structure that supports wide-angle self-collimation and is directly compatible with the recently developed holographic fabrication technique. We further show that both bends and beam splitters can be introduced into this structure by the use of interfaces.     

Theory and method for enhancing sensitivity of multi-gas sensing based on slow light photonic crystal waveguide

A novel harmonic detection theory and method for multi-component gas sensing based on photonic crystal waveguide (PCW) slow light is proposed. The PCW is used as gas chamber, and harmonic detection method was adopted for signal processing. This system could real-time and remote monitoring multi-component gases simultaneously with sensitivities increased by 10,278, 8650 and 6282 times respectively compared with system PCW not used. The proposed theory and method possesses powerful practicability and favorable application prospects. It could be also applied to other fluid concentration detection system, thus providing a new idea for expanding applications of slow light in sensing fields.     

基于自准直效应的光子晶体异质结偏振分束器

基于光子晶体的自准直效应和禁带特性,提出了一种具有非正交异质结结构的光子晶体偏振分束器.无需引入缺陷或波导,可使光波在该结构中准直无发散地传输并实现分束功能,对制造工艺的要求大大降低.利用Rsoft软件,结合平面波展开法和二维时域有限差分法,对提出的偏振分束器进行了仿真研究.结果表明,该偏振分束器在一个较大的频率范围f=0.275—0.285(a/λ)内可实现横电(TE)和横磁(TM)模的大角度偏振分离,TE和TM模的透过率均在88%以上,偏振消光比分别大于26.57 dB和17.50 dB.该结构可应用到太赫兹波段的传输系统中,a=26μm,尺寸大小为572μm×546μm,在91—95μm波长范围内可实现TE和TM模的分离.利用该结构可设计用于光通信系统(n=3.48)的偏振分束器,a=426.25 nm,结构仅为9.38μm×8.95μm.本方案结构简单,易于集成,有望在集成光路的发展中发挥重要作用.     

Self-collimating photonic crystal polarization beam splitter

We present theoretical and experimental results of a polarization splitter device that consists of a photonic crystal (PhC) slab, which exhibits a large reflection coefficient for TE and a high transmission coefficient for TM polarization. The slab is embedded in a PhC tile operating in the self-collimation mode. Embedding the polarization-discriminating slab in a PhC with identical lattice symmetry suppresses the in-plane diffraction losses at the PhC-non-PhC interface. The optimization of the PhC-non-PhC interface is thereby decoupled from the optimization of the polarizing function. Transmissions as high as 35% for TM- and 30% for TE-polarized light are reported.     

Photonic drop splitters based on self-collimation ring resonators in a silicon photonic crystal

We report here 1 × 3 and 1 × 2 photonic drop splitters (PDSs) with different splitting ratios based on self-collimation ring resonators (SCRRs) in an air-hole silicon photonic crystal. An 1 × 3 PDS consists of four beam splitters and an 1 × 2 PDS consists of three beam splitters and one mirror. Light propagates in the PDSs employing self-collimation effect. The theoretical transmission spectra at different drop ports in PDSs were analyzed with the multiple-beam interference theory. Then they were investigated with the finite-difference time-domain (FDTD) simulation technique. The simulation results agree well with the theoretical prediction. For the drop wavelength 1550 nm, the free spectral range of the PDSs is about 29 nm, which almost covers the whole optical communication C-band window. Because of small dimensions, air-hole structure and whole-silicon material, the proposed PDSs hold great potentials for applications in photonic integrated circuits.     

硅光子晶体TE模式光下路分束器设计

设计了基于自准直效应的硅光子晶体TE模式13和12光下路分束器。13光下路分束器由4个分束镜组成,而12下路分束器由3个分束镜和1个反射镜组成。利用多光束干涉原理推导出光下路分束器各个出口的透射谱理论公式。通过选择合适的分束镜,可以得到不同分束比例的光下路分束器。对于13光下路分束器,设计了1∶1∶1和1∶2∶3两种分束比例;对于12光下路分束器,设计了1∶1和1∶2两种分束比例。再利用时域有限差分软件数值模拟了透射谱,其结果与理论设计一致。当下路波长为1550 nm时,13和12两种光下路分束器的大小均约为10 m 10 m,自由光谱区为36 nm,覆盖了整个光通信C波段。     

Polarization beam splitter based on photonic crystal self-collimation Mach-Zehnder interferometer

A polarization beam splitter based on a self-collimation Mach-Zehnder interferometer (SMZI) in a hole-type silicon photonic crystal was proposed and numerically demonstrated. Utilizing polarization dependence of the transmission spectra of the SMZI and polarization peak matching (PPM) method, the SMZI can work as a polarization beam splitter (PBS) by selecting appropriate path length difference in the structure. Because of its intrinsic operating principle, the PBS possesses high polarization extinction ratios (PERs). As its dimensions are only several operating wavelengths, the PBS may have practical applications in photonic integrated circuits.     

Self-collimation and beam splitting in low-index photonic crystals

We study self-collimation and beam splitting in low-refractive-index photonic crystals created within chalcogenide glass. We propose a beam splitter structure that allows direct experimental verification of photonic-crystal effects at optical wavelengths in a straightforward and definitive manner. The beam splitter provides angular separation of 90° using a highly compact spatial footprint, thus delivering direct application in highly integrated photonic devices.     

Wavelength-division-multiplexing system with photonic crystal self-collimation and co-directional coupling effect

A wavelength-division-multiplexing system with high compactness and extremely simple structures is designed and analyzed theoretically for optical communication wavelengths. The structure consists of a self-collimation region, a coupler, a coupling section, and two arbitrarily bent periodic dielectric waveguides (PDWGs). Operation principle of the devices is based on self-collimation and directional coupling mechanism. The equal-frequency contours (EFCs) are nearly flat from 0.17–0.22 (2πc/a), thus the self-collimation region acts as a multiplexer. Operation principle of the demultiplexer is based on directional coupling in two parallel periodic dielectric waveguides. The device performances have been evaluated by the finite-difference time-domain simulations coupled with perfectly matched layer (PML) boundary conditions.     

A tunable power splitter based on modes coupling and self-collimation effect in two-dimensional photonic crystals

A tunable power splitter is proposed in a two-dimensional photonic crystal with a line defect composed of two rows rods, whose refractive index can be controlled artificially. This device is working at a self-collimation frequency. The finite difference time domain (FDTD) simulations show that the beam splitting ratio is a function of the refractive index of line defect. If the index varies from 3.0 to 3.5, the beam intensity of one output port decreases from 87.2% (normalized by the input beam) to 0 while the intensity of the other output port increases from 8.98% to 100% monotonically. The coupling between the self-collimation mode and the defect mode is used to explain the working mechanism.