Optical constants of several multilayer transition metal dichalcogenides measured by spectroscopic ellipsometry in the 300-1700 nm range: high-index, anisotropy, and hyperbolicity
Transition metal dichalcogenides (TMDs) attract significant attention due to their exceptional optical and excitonic properties. It was understood already in the 1960s, and recently rediscovered, that many TMDs possess high refractive index and optical anisotropy, which make them attractive for nanophotonic applications. However, accurate analysis and predictions of nanooptical phenomena require knowledge of dielectric constants along both in- and out-of-plane directions and over a broad spectral range -- information, which is often inaccessible or incomplete. Here, we present an experimental study of optical constants from several exfoliated TMD multilayers obtained using spectroscopic ellipsometry in the broad range of 300--1700 nm. The specific materials studied include semiconducting WS$_2$, WSe$_2$, MoS$_2$, MoSe$_2$, MoTe$_2$, as well as, in-plane anisotropic ReS$_2$, WTe$_2$, and metallic TaS$_2$, TaSe$_2$, and NbSe$_2$. The extracted parameters demonstrate high-index ($n$ up till $\approx 4.84$ for MoTe$_2$), significant anisotropy ($n_{\parallel}-n_{\perp} \approx 1.54$ for MoTe$_2$), and low absorption in the near infrared region. Moreover, metallic TMDs show potential for combined plasmonic-dielectric behavior and hyperbolicity, as their plasma frequency occurs at around $\sim$1000--1300 nm depending on the material. The knowledge of optical constants of these materials opens new experimental and computational possibilities for further development of all-TMD nanophotonics.