Linear and Third-Order Nonlinear Optical Response of Hafnium Disulfide

The linear and third-order nonlinear optical response of thin-films of the transition metal dichalcogenide hafnium disulfide (HfS2) is investigated. Varying angle spectroscopic ellipsometry measurements are performed to obtain the material's in-plane (Formula presented.) and out-of-plane (Formula presented.) refractive indices in the (Formula presented.) - (Formula presented.) wavelength range. HfS2 is found to exhibit a strong, highly anisotropic linear optical response. In particular, it is shown that the material's in-plane refractive index (Formula presented.) exceeds a value of 3 throughout the visible wavelength range, while simultaneously offering a remarkably wide transparency window with (Formula presented.) for (Formula presented.). The absolute value of the in-plane third-order nonlinear susceptibility (Formula presented.) is derived from third-harmonic generation (THG) measurements for fundamental wavelengths of (Formula presented.) to (Formula presented.) and is found to range from (Formula presented.) to (Formula presented.), respectively. The obtained values significantly exceed those of conventional high-index materials, such as silicon or gallium phosphide. Furthermore, the efficiency of the THG process is found to be controllable by varying both the film thickness and the dielectric environment. These findings establish HfS2 as a highly promising candidate for nonlinear optical applications, surpassing the performance of conventional high-index materials.