Tunning the omnidirectional bandgap of nanoporous silicon using a semi-sinusoidal refractive index profile

We report the theoretical comparison of the omnidirectional bandgap in a 1-D photonic crystal using sinusoidal and semi-sinusoidal refractive index profiles. It is found that the omnidirectional bandgap of the semi-sinusoidal profile widens and shifts to a higher wavelength range as a function of the asymmetric ratio of the profile. The asymmetric ratio plays an essential role in tunning the width of the omnidirectional bandgap due to the changed average refractive index and optical thickness. The semi-sinusoidal refractive index is experimentally achieved by changing the current waveform. Novel omnidirectional nanoporous silicon mirrors with an omnidirectional bandgap covering optical communication wavelength (1550 nm) were fabricated using a semi-sinusoidal current waveform. The experimental analogy was carried out by building up the multilayered dielectric structures of omnidirectional mirrors by anodic etching using a semi-sinusoidal current waveform. The experimental results were compared with the theoretical results investigated by the transfer matrix method. It was shown that the distorted current profile impacts the quality of the omnidirectional bandgap although it does not affect the porous size range.