Porous silicon - calcium fluoride plasma waveguide with asymmetric Ag film and its sensitivity characteristics

Abstract

In this paper, a porous silicon-calcium fluoride hybrid plasmonic waveguide (PS-CaF₂ HPW) with an asymmetric silver film is studied. The PS-CaF₂ HPW is composed of a PS strip waveguide deposited with asymmetric CaF₂ and Ag thin film layers on an SiO₂ substrate. In the mid-infrared (MIR) region, the mode characteristics and waveguide sensitivity of the mode in the PS-CaF₂ 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-CaF₂ HPW. The real part of the effective refractive index (Re(<i>n</i><i>_eff</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>_wg) 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-CaF₂ 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-CaF₂ HPW is mostly restricted to the PS layer and CaF₂ layer. Comparing with conventional dielectric waveguides, the mode field energy of the PS-CaF₂ HPW is well confined in the PS layer and CaF₂ layer. The geometric parameters of the PS-CaF₂ HPW are optimized by changing the geometric parameters (<i>W</i>₁, <i>W</i>₂, and <i>W</i>₃). When <i>W</i>₁ = 1500 nm, <i>W</i>₂ = 300 nm, <i>W</i>₃ = 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-CaF₂ 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-CaF₂ HPW has good temperature resistance. Moreover, temperature has almost no effect on Re(<i>n</i><i>_eff</i><italic/>), nor <i>α</i> nor <i>A</i> nor FOM nor <i>S</i>_wg in a temperature range from -40 K to 40 K. Finally, the fabrication tolerances of the PS-CaF₂ 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-CaF₂ HPW provides a feasible label-free biochemical sensing scheme and a method of polarization control devices.