Interface of photonic crystal heterostructure for broadening bandwidth of unidirectional light transmission

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 -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 the -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.