Asymmetric Photonic Crystal Waveguide Research Articles

Wideband slow light with low dispersion in lattice-shifted asymmetric photonic crystal waveguides

We propose a different type of asymmetric photonic crystal waveguide to achieve slow light with improved normalized delay-bandwidth product and low group velocity dispersion. The lateral symmetry of the proposed waveguide is broken by periodically shifting the two rows of air holes adjacent to the line defect. Under two simple different procedures of shifting air holes, the group indices of 41, 50, 68, and 114 with bandwidths over 14.9, 11, 7.8, and 4.7 nm, and the group indices of 43, 54, 73, and 124 with bandwidths over 13.9, 10.4, 7.1, and 4.3 nm around 1550 nm are obtained, respectively. Low-dispersion slow light propagation is confirmed by studying the relative temporal pulse-width spreading with the two-dimensional finite-difference time-domain method.

Coupled-mode analysis of contra-directional coupling between two asymmetric photonic crystal waveguides

A self-contained coupled-mode theory for the coupled two asymmetric photonic crystal waveguides (PCWs) is presented. The first-order coupled-mode equations are derived under a weak coupling assumption. The coupling coefficients are obtained systematically by a matrix calculus using the modal solutions of each PCW in isolation. The coupled-mode equations are solved for contra-directional coupling between two asymmetric PCWs formed by a hexagonal lattice of circular air holes in a dielectric medium. The power transmission spectra at different output ports of the coupled PCWs are investigated. It is shown that the self-contained coupled-mode analysis is useful to characterize a peculiar feature of the contra-directionally coupled PCWs as a drop filter.

Flat band slow light in asymmetric photonic crystal waveguide based on microfluidic infiltration

In this paper, an asymmetric photonic crystal waveguide is proposed for flat band slow light transmission. Two structural parameters of one cladding layer of the waveguide are carefully adjusted. They are the refractive index of the liquid infiltrated in the cladding on one side and the radius of the air holes in the infiltrated cladding. In such an asymmetric waveguide, we can achieve a very flat band corresponding to high group index such as 29.4 and low dispersion (10(4)ps(2)/km) for Deltaomega/omega=1.46%. It is found that the value of n(g)Deltaomega/omega of the proposed waveguide can reach 0.429 and this waveguide can also yield a significant increase in the group index-bandwidth product n(g)Deltaomega/omega and improve the bandwidth.

Coupling between even- and oddlike modes in a single asymmetric photonic crystal waveguide

The photonic crystal (PC) is an important structure for realizing optical integrative circuits. Nowadays, most PC based passive devices operate by utilizing the coupling either between several PC waveguides or between PC waveguides and resonance cavities. In this letter, we present a coupling mechanism between even- and oddlike modes in a single asymmetric PC waveguide. Switches based on this coupling phenomenon were validated experimentally. By adjusting the temperature controlled by the thermal resistor, an extinction ratio of 9.1 dB was realized. Our researches provide a potential way of not only guiding but also controlling light in a simple single PC waveguide.

Physical properties of asymmetric photonic crystal waveguides

By considering the next nearest neighboring defects between two photonic crystal waveguides (PCWs) and analytic formulae derived from the tight binding theory, we will explain the physical properties of an asymmetric directional coupler made from two coupled PCWS: (1) The dispersion curves of a photonic crystal coupler will decouple into the dispersion curves of a single line defect, and the electric field will only be localized in one waveguide of the coupler at a particular point, which we name a decoupling point. (2) The parities of the eigenmodes switch at the decoupling point, even though the dispersion curves are not crossing. (3) The eigenfield at a higher (lower) dispersion curve is always mainly localized in the waveguides that have higher (lower) eigenfrequencies of single line defects, even though the eigenmodes are switched. As a given frequency is incident into the coupler, the energy transfer between two waveguides and the coupling length can be expressed analytically. Due to there being no dispersion curve crossing, the coupling length is no longer infinite at the decoupling point in asymmetric PCWs, but it still has the minimal energy transfer between two waveguides when the frequency of the incident wave is close to the decoupling point.

Tight-Binding Theory for Coupling Asymmetric Photonic Crystal Waveguides

Tight-Binding Theory for Coupling Asymmetric Photonic Crystal Waveguides Chih-Hsien Huang and Wen-Feng Hsieh Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan, R.O.C. Szu-Cheng Cheng Department of Physics, Chinese Culture University, Taipei, Taiwan, R.O.C. (Received 21 August 2007) The physical properties of asymmetric photonic-crystal directional couplers are studied under the tight-binding model, which assumes that the eld distribution of photonic-crystal waveguides is localized around periodic defects. The couplings of nearby defects between two asymmetric waveguides cause two dispersion relations to split further whereas the couplings of nearby defects within an individual waveguide cause the sinusoidal modulations of the dispersion curves. The eld-envelope distributions of the coupler are expressed by using the eigenmodes of the tightbinding equations and are consistent with the comparing results from nite-di erence time-domain simulations. PACS numbers: 71.15.Ap, 42.70.Qs, 42.79.Gn, 42.82.Et

Coupling Theory of Asymmetric Photonic-crystal Waveguides

The physical properties of asymmetric photonic-crystal directional couplers are studied by using the tight-binding theory, which considers that the fleld distributions of photonic- crystal waveguides (PCWs) are localized around the periodic defects. From this model, the analytic formulas which describe the dispersion of the coupler and the eigenmode patterns were derived. By considering coupling up to the third nearest-neighboring between two PCWs, analytic expressions of the dispersion relations of asymmetric photonic crystal coupler are consistent with the results from the plane wave expansion method (PWEM). Due to the broken symmetry, two dispersion curves will never cross. Nevertheless, the eigenmode patterns switch their parity as the symmetric coupler does when wavevector k passes over the decoupling point. Photonic crystal waveguide (PCW) is a structure which introduces considerable quantities of point defects in the photonic crystal (PC). The electromagnetic (EM) wave is strongly conflned in the defect channel, allowing low-loss in Y-blanches and a sharp-bending. Furthermore, creating two line defects to form a directional coupler are used as add/drop fllters, switches and multiplexers. To design photonic crystal devices, the simulation tools such as the PWEM and the flnite difierence time domain method (FDTD) have often been used. However, there is no simple analytic for- mula to analyze physical properties of the coupled photonic crystal waveguides (PCWs), especially to analyze coupling between PCWs (1). Tight-binding theory (TBT) is widely used not only in condensed matter physics but also recently in the EM wave propagation in linear and nonlinear single line defects PCW analytically. They usually considered only coupling between the nearest- neighboring defects that may be good enough in the single PCW or conventional waveguides. Nevertheless, it is insu-cient to only consider the nearest-neighboring defects in the directional coupler made of two identical PCWs (2). The dispersion curves will always split or never cross if only the nearest-neighboring coupling is considered, but the dispersion do cross that was resolved by further considering coupling of the second nearest neighboring defects which causes a sinusoidal modulation to the dispersion curves. Similar to the extended TBT (2) which considers three nearby coupling coe-cients, in this paper, we flrstly derive an analytic solution to describe the dispersion of asymmetric coupled PCWs. This formula provides more generalized discussion and physical insight than what is derived for the case of symmetric coupled PCWs. Especially, the coupled identical PCWs should become asymmetric due to the intensity dependent index of refraction in the nonlinear photonic crystal directional coupler. Secondly, by using the analytic formulas, the phenomena of mode switching and mode patterns in the asymmetric PCWs are extensively analyzed. Finally, the simulation results of considering PCWs of triangular lattices are consistent with our theoretic results. 2. COUPLED EQUATIONS FOR ASYMMSTRIC PHOTONIC CRYSTAL WAVEGUIDES