Polarization rotation using Molybdenum trioxide in 3 µm SOI platform

. We propose a novel polarization rotator concept using two-dimensional (2D) anisotropic materials, such as molybdenum trioxide, on top of 3µm silicon waveguides. The in-plane anisotropic behaviour of 500 nm thick MoO3 flake is analysed and confirmed with Raman spectroscopy.


Introduction
Silicon photonics is one of the most promising integration platforms for optics: it is versatile and is present in many areas of cutting-edge technology.Silicon photonics platforms have many flavours i.e., SiN, InP, thick and thin silicon on insulator (SOI) [1].Among these, 3 µm SOI is well known for its low optical losses, ultra-dense integration, and small polarization dependence [2].Most building blocks in the 3 µm SOI platform support both polarizations (TE and TM), but it becomes a challenge when the two polarizations must be treated independently, for example in telecom and sensing applications [3].Therefore, there is need to develop on-chip polarization management techniques, such as polarization splitters and rotators.
In this paper, our focus is on a compact polarization rotator device.The conventional methods to do polarization rotation in SOI platform are based on mode conversion [4] and mode-evolution [5].However, careful consideration is required to design these building blocks.In this paper, we are exploring the possibility of using 2D materials to achieve polarization rotation.2D materials exhibit diverse optical, electronics and magnetic, and polarization dependent optical properties [6].The polarization dependent optical properties of 2D material arises from the anisotropy in the materials.The in-plane anisotropic 2D materials induces birefringence.A recent study shows that 2D material, Molybdenum trioxide (MoO3) exhibits strong anisotropic behaviour [7] which is 3 times higher ReS2 and ReSe2 [8].The calculated effective index (n+ik) of MoO3 at 1550 nm is 2.3685 + 0.0033811i.

Results and discussion
VTT's 3 µm SOI platform has low-loss optical components, such as up-reflecting mirrors (URMs) [2].The cross section of an URM is shown in Figure 1.These URMs are fabricated with wet etching to achieve smooth 45° angle surfaces.These URMs have been used for coupling light in and out of the chip or wafer.They can be placed on any convenient position, enabling the possibility of using them for wafer level testing (WLT) also.These mirrors have 0.5 dB coupling losses.For the measurements of the URMs, the fibre is edge coupled to the input waveguide and output coupling is done via upreflecting mirror.We select two peaks at 158 and 818 cm -1 which are assigned to Ag c and Ag a mode respectively for observing the in-plane anisotropic behaviour of the material.With the polarization rotation of the excitation, the intensity of both peaks is shown in Figure 3.The strong intensity variation with the polarization angle of the incident beam evidently validates the in-plane anisotropic behaviour of the MoO3 flake.Figure 4 shows the successful transfer of MoO3 flake on top of URM.

Conclusion
In this paper, we proposed the idea of achieving polarization rotation by combining 2D materials with 3 µm SOI platform.The intensity variation was observed when polarization angle changed from 0° to 90°.This analysis would lead to the development of compact polarization rotator in 3 µm silicon waveguides.

Figure 1 :
Figure 1: SEM image and cross section of URM on 3 µm SOI platform.

Figure 3 :
Figure 3: Polar plots of the Raman intensity of two modes Ag c (left) and Ag a (right).