Silicon nitride (SixNy) is a very promising platform for CMOS-compatible integrated photonics, owing to significantly lower nonlinear losses, a broader transparency range, and a much smaller thermo-optic coefficient. To better enhancement nonlinearity, naturally, photonic bandgap (PBG) that could confine light within a small mode volume is frequently introduced into SixNy slabs. However, almost all the PBG in SixNy slabs demonstrated currently only could work for one single-polarization state. In such situation, the asymmetry of the structure or imperfect fabrication would cause polarization mode conversion loss, which will reduce the efficiency of nonlinearity. If complete photonic bandgap (CPBG) could be applied in SixNy slabs, such leaky loss would be restricted, and the nonlinear efficiency could be furtherly enhanced. Moreover, demonstration of CPBG in SixNy slab, also means the ability of polarization multiplexing, polarization management could be benefit from the unique features of CPBG. So CPBG in SixNy slabs is attractive.
Recently, in a photonic crystal (PC) slab with the SixNy refractive index of 2.5, a research group led by Prof. Jin Hou at South-Central MinZu University, achieved the largest normalized CPBG of 5.62%. Previous investigations show that, in two dimensional PCs, TM PBG could be obtained with a lower refractive index contrast than the TE PBG. Meanwhile, in a sandwiched slab structure like SixNy on silica, the slab effective index for TM polarization is also usually lower than that for TE polarization. So the group thinks that, making use of the slab effective index difference between the polarizations, it should be possible to achieve CPBGs in a SixNy slab with lower refractive index. And also, there would be a suitable effective index pair of TE and TM polarizations, with which the coincident frequency range in the TE PBGs and TM PBGs for the same structure parameters could be easily engineered. Through adjusting the effective index pair by changing the thickness of the SixNy core layer or changing the center wavelength, and also optimizing the structure parameters within the PC plane, the largest CPBG of 5.62% was theoretically obtained in their investigation. Moreover, using the same principle, the lowest refractive index of SixNy for a CPBG could be extended to as low as 2. The value, to the best of our knowledge, is also the lowest record refractive index for a PC slab that could support CPBG. Breakthrough of CPBG in SixNy slabs not only enhances nonlinearity, but also indicates that development of various high-performance CPBG devices in SixNy slab would be possible. The work entitled "Complete photonic bandgap in silicon nitride slab assisted by effective index difference between polarizations" was published on Frontiers of Optoelectronics (May 6 2022).
