Tunable Modal Birefringence in a Low-Loss Van Der Waals Waveguide
D. Hu, K. Chen, X. Chen, X. Guo, M. Liu and Q. Dai
Adv. Mater. 31, 1807788 (2019)
van der Waals (vdW) crystals are promising candidates for integrated phase
retardation applications due to their large optical birefringence. Among
the two major types of vdW materials, the hyperbolic vdW crystals are
inherently inadequate for optical retardation applications since the supported
polaritonic modes are exclusively transverse-magnetic (TM) polarized
and relatively lossy. Elliptic vdW crystals, on the other hand, represent a
superior choice. For example, molybdenum disulfide (MoS2) is a natural
uniaxial vdW crystal with extreme elliptic anisotropy in the frequency range
of optical communication. Both transverse-electric (TE) polarized ordinary
and TM polarized extraordinary waveguide modes can be supported in MoS2
microcrystals with suitable thicknesses. In this work, low-loss transmission
of these guided modes is demonstrated with nano-optical imaging at the
near-infrared (NIR) wavelength (1530 nm). More importantly, by combining
theoretical calculations and NIR nanoimaging, the modal birefringence
between the orthogonally polarized TE and TM modes is shown to be
tunable in both sign and magnitude via varying the thickness of the MoS2
microcrystal. This tunability represents a unique new opportunity to control
the polarization behavior of photons with vdW materials.