Comparative study of two thermal tunable filters with and without symmetrical linear gradation of thickness

Highlights: • A comparative analysis is made for two thermal tunable metallic photonic crystal filters with and without symmetrical linear graded thickness. • Both of thermal filters are tunable with temperature and angle of incidence. • The wavelength dependence of Silicon refractive index is considered in our calculations. • The gradation of the thickness leads to increasing the height of the transmission peaks. • The wavelength of transmission peaks in the graded structure is larger than that of the non-graded structure. -- Abstract: By using transfer matrix method, a comparative analysis is made for two thermal tunable metallic photonic crystal graded and non-graded filters. The transmission spectra of these two structures are investigated. Three different materials form the structure, Silicon, Silica and Silver as a metallic layer. The dependence of the silicon refractive index on the wavelength is considered in our calculations. In addition, the complex refractive index of Silver is the function of temperature and wavelength simultaneously. The effect of temperature and incident angle on the transmission is considered in this work. The results show that, the transmission peaks (TPs) are tunable with temperature and incident angle. The TPs shift toward the larger wavelength by increasing the temperature. At a constant temperature, TPs shift toward the shorter wavelength by increasing the incident angle. The temperature dependence of TPs is linear in both structures. The effect of the temperature on the height of the TP is also investigated. As the results show, the height of the TPs is also tunable with temperature and decreases linearly by increasing temperature due to the temperature dependence of the Silver loss factor. In order to increase the height of the TPs, we apply the symmetry linear gradation of thickness to one of the constituent layers. The linear gradation of the thickness leads to increase the height of TPs compare with the non-graded structure. Furthermore, the wavelength of TPs in the graded structure is larger and more significant than the non-graded one.