Terahertz spectroscopy of bulk InSb and SrTiO₃ in reflection geometry

Terahertz time-domain spectroscopy (THz-TDS) is a reliable technique for studying the complex optical properties of materials. Its energy range makes it suitable for detecting low energy collective excitations such as phonons, magnons, and plasmons. The technique for THz-TDS in transmission geometry has gained much attention over the years. However, despite the need for exploring reflective samples, the advancement of THz-TDS in reflection geometry has faced several obstacles, mainly due to its strict requirement for high precision in the placement of the sample and reference.

Here we show a technique for measuring samples in reflection geometry using THz-TDS which involves systematically resolving the misplacement issue by first isolating and correcting sources of error in the experimental setup. We then use a novel and robust phase retrieval method to detect and rectify the remaining misplacement with nanometre precision. This provides us with precise values for the phase of the THz pulse which allows us to accurately measure the complex properties of materials. This blend of experimental as well as code-based correction returns highly reliable optical properties of materials.

We use this technique to study non-transparent materials such as InSb and SrTiO₃, which were previously found to be challenging due to their strong spectral features and high reflectivity in the THz frequency range. The experimental results of incident angle and polarization-dependent measurements are shown along with the retrieved complex refractive index and conductivity of these samples. Our method immensely simplifies the procedure for obtaining optical properties of samples in the THz range, thus allowing for more advances in this field by using strong electric fields, magnetic fields, new materials, and more. We anticipate this technique can also contribute substantially to the fundamental understanding of magnetic materials.