可用于拓宽光波单向传输带宽的光子晶体异质结构界面

摘要

基于光波单向传输的全光二极管在集成光通信、全光网络和光信息处理中有重要应用.基于方向带隙失配设计的光子晶体异质结构可实现光波单向传输,但正向透射率较低,带宽较窄.基于对光子晶体异质界面倾斜角度的研究,根据界面全反射条件,利用可集成材料硅和二氧化硅设计了一种空气孔型二维光子晶体异质结构.异质结构界面两侧的光子晶体对1550 nm波长附近的TE模光波在Γ-X方向均呈导带,避免了方向带隙失配.研究发现当异质界面满足全反射条件时,由于光子晶体的自准直效应,较宽波段的正向光波得以高效传播,而反向光波在界面由于全反射而被禁止传播.光子晶体异质结构界面的全反射效应打破了方向带隙对光波单向传输的限制,使得反向光波在光子晶体中为导带时同样可实现近零透射率,从而拓宽了光波单向传输的波长范围.基于全反射界面的光子晶体异质结构经过优化后,其正向透射率达0.64,透射对比度为0.97,单向传输带宽可达553 nm.%An all-optical diode (AOD) is a spatially nonreciprocal device that in the ideal case and for a specific wavelength allows light to totally transmit along the forward direction but totally inhibits light to propagate along the backward direction, yielding a unitary contrast. AODs are widely considered to be the key components for the next-generation all-optical signal processing, and completely analogous to electronic diodes which are widely used in computers for processing electric signals. Most of AOD designs suffer some serious drawbacks which make them not suitable for commercial and large-scale applications. Relatively large physical sizes are often needed, the balance between figure of merit and optical intensity is usually inadequate, and in some cases cumbersome structural designs are necessary to provide structural asymmetry. Among different approaches, the AOD based on two-dimensional (2D) photonic crystal (PC) heterostructure has shown significant advantages due to the capability of on-chip integration with other photonic devices. However, current PC heterostructure AOD (PCH-AOD) is based on the mismatch of directional bandgaps, which shows poor performance as a result of the relatively low forward transmittance (<0.40) and contrast ratio (<0.75) with a narrow bandwidth (about 10 nm). In order to improve the performance, here we propose a new PCH-AOD design based on the total reflection principle, which is able to achieve high forward transmittance and contrast ratio within a broad wavelength range. Our design is composed of two rectangle lattice 2D PC structures, in which periodically distributed air holes are embedded in silica (PC1);and silicon (PC2) materials, respectively. The two PCs are combined with an inclined interface along the Γ-M direction of both PCs. In this way, the total reflection condition is satisfied when light propagates from silicon to silica material. The forward and backward propagating optical waves are incident along theΓ-X direction of both PCs, in which direction there are transmission bands for TE mode centered at 1550 nm wavelength. A commercial software (R-soft) based on the finite-difference time-domain (FDTD) method is used to study the unidirectional transmission performance of the PCH-AOD. The results show that the forward propagating optical waves (from PC1 to PC2) can transmit efficiently through the device. In addition, we further improve the forward transmittance by exploiting the self-collimation effect of PCs and optimizing the coupling from PC1 to PC2. In the meantime, the light propagating along the backward direction (from PC2 to PC1);is blocked at the total reflection interface with near-zero transmittance. In this way, the unidirectional transmission is achieved without the reliance on the directional bandgap mismatch, and thus broad bandwidth is achieved. The AOD has a forward transmittance of 0.64 and a transmission contrast of 0.97 with a bandwidth of 553 nm at 1550 nm. The equal frequency contours (EFCs) of the PCs is plotted to demonstrate the working principle of the PCH-AOD. Finally, considering the experimental fabrication of the AOD device, we analyze the unidirectional transmission performance of a planar PCH-AOD with a finite thickness of 1500 nm. Despite a small reduction (12.3%) in the forward transmittance, the transmission contrast is maintained at about 0.97, and the unidirectional transmission bandwidth is increased to 600 nm. Therefore, our design can be implemented in practice and our work provides a theoretical framework for designing high performance PCH-AOD. In addition, our design allows an unprecedented high forward transmittance, contrast ratio and broad working bandwidth of the device at extremely low operational optical intensity, due to the total reflection condition, and the optimized forward propagation and coupling condition. The proposed device has a small footprint that is promising for next-generation on-chip applications.