Porous silicon - calcium fluoride plasma waveguide with asymmetric Ag film and its sensitivity characteristics
Authors: Wang Fang , Chen Ya-Ke , Li Chuan-Qiang , Ma Tao , Lu Ying-Hui , Liu Heng , Jin Chan ,   College of Electronic and Electrical Engineering, Henan Normal University, Xinxiang 453007, China,   Key Laboratory of Interfacial Physics Technology Project, Chinese Academy of Sciences, Shanghai 201800, China,   Key Laboratory Optoelectronic Sensing Integrated Application of Henan Province, Henan Normal University, Xinxiang 453007, China,   Academician Workstation of Electromagnetic Wave Engineering of Henan Province, Henan Normal University, Xinxiang 453007, China
Published: August 2021

In this paper, a porous silicon-calcium fluoride hybrid plasmonic waveguide (PS-CaF<sub>2</sub> HPW) with an asymmetric silver film is studied. The PS-CaF<sub>2</sub> HPW is composed of a PS strip waveguide deposited with asymmetric CaF<sub>2</sub> and Ag thin film layers on an SiO<sub>2</sub> substrate. In the mid-infrared (MIR) region, the mode characteristics and waveguide sensitivity of the mode in the PS-CaF<sub>2</sub> HPW are simulated by using the finite element method (FEM). The results show that there are two fundamental modes (PM 1 and PM 2) with different polarization states in the PS-CaF<sub>2</sub> HPW. The real part of the effective refractive index (Re(<i>n</i><i><sub>eff</sub></i><italic/>)), transmission loss (<i>α</i>), normalized effective mode field area (<i>A</i>), quality factor (<i>FOM</i>) and sensitivity (<i>S</i><sub>wg</sub>) for each of the PM 1 and the PM 2 are studied and optimized. Moreover, the effect of temperature on the performances of the PS-CaF<sub>2</sub> HPW is also analyzed. Firstly, the mode field distributions calculated by the FEM indicate that the mode field energy for each of the PM 1 and PM 2 in the PS-CaF<sub>2</sub> HPW is mostly restricted to the PS layer and CaF<sub>2</sub> layer. Comparing with conventional dielectric waveguides, the mode field energy of the PS-CaF<sub>2</sub> HPW is well confined in the PS layer and CaF<sub>2</sub> layer. The geometric parameters of the PS-CaF<sub>2</sub> HPW are optimized by changing the geometric parameters (<i>W</i><sub>1</sub>, <i>W</i><sub>2</sub>, and <i>W</i><sub>3</sub>). When <i>W</i><sub>1</sub> = 1500 nm, <i>W</i><sub>2</sub> = 300 nm, <i>W</i><sub>3</sub> = 70 nm, and the operating wavelength is ~3.5 μm, <i>α</i> and <i>FOM</i> are 0.019 dB/μm and 1594.99 for the PM 1, and <i>α</i> and FOM are 0.016 dB/μm and 1335.54 for the PM 2, respectively. Secondly, the waveguide sensitivity of the PS-CaF<sub>2</sub> HPW is analyzed. The results show that the size of PS layer has a great influence on the waveguide sensitivity. The waveguide sensitivity decreases with the size of the PS layer increasing. In addition, the PS-CaF<sub>2</sub> HPW has good temperature resistance. Moreover, temperature has almost no effect on Re(<i>n</i><i><sub>eff</sub></i><italic/>), nor <i>α</i> nor <i>A</i> nor FOM nor <i>S</i><sub>wg</sub> in a temperature range from -40 K to 40 K. Finally, the fabrication tolerances of the PS-CaF<sub>2</sub> HPW are demonstrated, and the good properties are maintained in a size tolerance range from -10 nm to 10 nm. With the advantages in propagation property and loss reduction, the PS-CaF<sub>2</sub> HPW provides a feasible label-free biochemical sensing scheme and a method of polarization control devices.