Asymmetric transmission (AT) devices have important applications in integrated optical systems for communication and information processing, the function of which is to allow transmission when a beam of light is incident from a certain direction, but when the beam is incident in the opposite direction, transmission is prohibited. In recent years, AT devices based on the unidirectional excitation of surface plasmon polaritons (SPPs) have also been extensively studied by using metal-insulator-metal asymmetric gratings, multi-layer gradient metasurfaces, and asymmetric metallic gratings coupled with extraordinary optical transmission through subwavelength slits. However, these structures usually exhibit AT in a relatively wide range of wavelengths, not suitable for applications requiring narrow-band or single-wavelength operation.
To solve the above problems, Professor Lirong Huang and her Master student Chunfa Ba, who are from Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, propose a method based on the effect of grating groove depth.
The AT device is of metal-metal-metal structure, which consists of an upper Ag grating, a middle Ag film layer and a lower Ag grating patterned on a SiO2 substrate(as shown in Fig. 1). When forward incident light illuminates it, only a certain narrow bandwidth of light can be converted into SPPs by the upper metal grating, then the SPPs tunnel through the Ag film and are directly and efficiently decoupled into the SiO2 substrate by the lower grating layer. Therefore, the device can achieve high forward transmission and narrow bandwidth transmission. By contrast, under backward illumination, since the designed lower grating cannot excite SPPs in the same frequency vicinity, the backward incident light is hard to penetrate the Ag film. Apart from this, the AT device has the advantages of simple structure, low manufacturing cost, and it may find applications in the field of nanophotonics such as narrow bandwidth unidirectional transmission/detection.
Fig. 1. (a) Schematic diagram of the AT device. (b) Unit cell.
Fig. 2. Electric field distributions for forward/backward incident light. (a) Ez distribution along the x-z plane; (b) The zoomed map of (a); (c) Ez and (d) Ex distributions in the x-z plane for backward incident light.
The paper is published on Optics Express (Vol. 27, No. 18, pp. 25107-25118, 2019) of OSA on 21 Aug 2019. And this word was supported by the National Natural Science Foundation of China (No. 61675074 and 61705127).
Paper website: https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-18-25107
