Introduction
Metamaterial-based optical filters are commonly used to address the ‘blue-shift’ issue as they exploit the absorption of the material and interplays of photonic crystals. A shift-free bandpass filter based on a silicon nanosphere (SiNP) with gold as a core--shell array was developed. However, the single-layer design’s blocking performance is relatively weak due to inadequate absorption and scattering. Moreover, it faces pronounced difficulties with transverse electric (TE) polarized light at a high angle of incidence (AOI); therefore, an extra AOI insensitive edge-filter design was employed. The optimized bandpass metamaterial filter demonstrates high insensitivity to AOI and stronger blocking performance, evidenced by its high optical density (OD = 2.35).
Method
A systematic design protocol that combined fullwave simulation software (CST) and transfer-matrix-based thin-film filter design software (Essential Macleod, ESM) was employed to optimize the design.
Results
The fundamental single layer is composed of infinite unit cells in a primitive cubic lattice configuration. Each unit cell comprises silicon nitride (SiN) as the host medium and a SiNP with a gold shell. To address the poor blocking performance problem, a second meta-layer was introduced. Furthermore, to remove the undesired peaks observed at high AOI, an AOI insensitive edge filter was introduced. It incorporates “H/2 L H/2” units, where H and L denote the high (Amorphous silicon) and low (SiN) refractive index materials, respectively, each with a quarter wavelength thickness. The final optimized design significantly enhances the OD up to 2.35 for TE polarization and 1.75 for transverse magnetic polarization.
Conclusion
In conclusion, a multi-layer, wide-angle, shift-free bandpass filter was successfully designed using the combined design protocol and further optimized by introducing an AOI insensitive edge filter. It demonstrates high OD in the stopband at all AOI due to the introduction of the second meta-layer and edge-filter design.
