• • Direct reflectance measurements from edge surfaces yield out-of-plane refractive indices smaller than in-plane for MoS2, NbOCl2, and WTe2, enabling giant birefringence >1.8 (MoS2), >0.6 (NbOCl2), and >0.5 (WTe2) in the 500–1000 nm range, critical for compact waveplates and phase-matching elements.
• • Out-of-plane extinction coefficients are zero for MoS2 and NbOCl2 but nonzero for WTe2, confirmed by transient reflection spectroscopy; this yields linear dichroism up to 100% for MoS2 and NbOCl2 and 40.7% for WTe2, enabling high-efficiency polarizers.
• • WTe2 exhibits enhanced out-of-plane dispersion around 2.14 eV, attributed to increased optical transition probability from larger density of states, which is essential for wavelength-selective photodetectors.
• • The measurement methodology overcomes prior limitations to 2D planes, providing complete 3D optical constants that allow prediction of optical response at arbitrary incidence angles, facilitating design of polarization-sensitive devices.