Unraveling Tunable Optical Anisotropy in Colloidal Quantum Wells Using Mueller Matrix Ellipsometry.

Colloidal quantum wells (CQWs) combine atomic-level thickness control with strong excitonic effects, offering a versatile platform for nanophotonics. However, their intrinsic optical anisotropy has not been quantitatively resolved with high precision. Here, we employ Mueller matrix spectroscopic ellipsometry over 350-650 nm to reveal record-high birefringence (Δnmax ≈ 1.95) and dichroism (Δκmax ≈ 1.53) in self-assembled CdSe CQW monolayers─values exceeding those of natural birefringent crystals such as rutile and black phosphorus. The Mueller matrix analysis uncovers pronounced differences between in-plane and out-of-plane dielectric responses, arising from heavy-hole exciton transitions confined to the a-b plane and thickness-tunable quantum confinement along the c-axis. The anisotropy strengthens systematically from 5.5 to 3.5 monolayers due to increased heavy-hole/light-hole splitting and preferential in-plane dipole alignment (∼97% in-plane, confirmed by back focal plane imaging). These results position CdSe CQWs as a solution-processable platform with giant, thickness-tunable optical anisotropy, providing a foundation for advanced polarization-resolved photonic studies.