Study on optical properties of Molybdenum disulfide in terahertz regime

Abstract

In 1984, after the first artificial THz wave from the photoconductive antenna (PCA), active research in the academic field was carried out for about 30 years, and various characteristics and applicability of the THz wave were reported. Through this, THz technology has been applied throughout the industry such as nondestructive testing, cancel detection and treatment, and has gained the position of innovative light source with remarkable growth. However, the research and development of the device that modulates THz itself such as a modulator and polarizer is insufficient compared to such a growth and development of modulation device is essential for advanced THz research. Geim’s research team found graphene as a dream nanomaterial, but discovered a disadvantage that there was no bandgap, and structurally similar two-deminsional materials, the transition metal dichalcogenides (TMDs), were identified as alternatives. TMDs can show various optical and electrical properties from semiconductor to superconductor depending on the choice of transition metal elements, which has proven to be future optoelectronic devices such as solar cell, battery and sensor. In order to realize the possibility of TMDs as devices, it is important to understand the free carriers dynamics that determine the behavior characteristics. The carriers exist in various states, so it is difficult to distinguish them by the general spectroscopic method. However, since carriers have common feature that they can be optically activated and distinguished in the THz frequency region. Therefore, the spectroscopy of materials using THz waves is keyword for device implementation. In this study, the TMDs, MoS2 were analyzed using THz time-domain spectroscopy (THz-TDS) and the possibility of the modulation device operating in the THz frequency region was also proposed. As a basic step, THz was generated from Bulk type inorganic crystals and quasi-phase matched crystal (QPM) to determine the sources suitable for THz-TDS. By using generated THz, we measured the absorption, refractive index and dielectric constant of silicon and confirmed that the constructed THz-TDS system works well by matching with known values. In this study, the optical characteristics of MoS2 were measured through the THz-TDS. Absorption, refractive index and dielectric constant were determined. In addition, the photoconductivity directly related to carrier density and mobility was derived through the thin film approximation. The measured and derived properties are not only valuable to the material study itself, but are also relevant to the THz modulation characteristics of MoS2. Furthermore, by irradiating MoS2 with 532 nm continuous wave laser, the optical characteristics were measured according to the intensity of the irradiation, and the THz transmittance was controlled by exciting the photoelectrons, which resulted in about 30 % modulation depth at 3.8 W/cm2. This suggests the application to optical modulation devices operation through optical reaction at THz frequency.