Phase retrieval method for quantitative terahertz spectroscopy in reflection geometry

Sammanfattning: Terahertz time-domain spectroscopy (THz-TDS) is a reliable technique used for studying the complex optical properties of materials. Its frequency range makes it suitable for detecting low-energy collective excitations such as phonons, magnons, and plasmons. 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 sub-micron precision in the placement of the sample and reference.Here we demonstrate 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 nanometer precision. This provides us with precise values for the phase of the THz pulse, which in turn allows us to accurately measure the complex optical properties of different types of materials. We use this technique to study bulk semiconducting samples such as Si and InSb. The experimental results of incident angle and polarization-dependent measurements are shown along with the retrieved complex refractive index of these samples. Our method immensely simplifies the procedure for obtaining the optical properties of samples in the THz range. We anticipate that our technique will be applied to spectroscopic studies of many more quantum materials with collective excitations in the THz range.