Compact THz absorption spectroscopy using a LiNbO₃ slot waveguide

THz spectroscopy is a powerful tool for studying a variety of samples, ranging from large biomolecules to solid-state materials. In cases where experimental space is limited or sample volumes are small, THz waveguides have been used to enable compact THz spectroscopy. The THz polaritonics platform is a waveguide-based approach that uses a thin lithium niobate slab to allow direct visualization of THz fields as they interact with structures integrated into the waveguide. Although there have been many successful studies using the platform for integrated photonics, the platform's utility as a spectroscopic tool has been largely unexploited. Here, we use a slot waveguide integrated into the thin lithium niobate slab to measure the absorption spectrum of a sample inserted into the slot. The slot waveguide localizes the THz electric field within a low-index slot where a sample is placed. The THz fields propagate through the slot and are monitored as they interact with the sample. Perturbation theory is then used to extract the absorption spectrum and bulk refractive index of the sample with good sensitivity. These results show much promise for enabling compact linear and nonlinear THz spectroscopy using thin lithium niobate waveguides.