Characterization of nonlinear/ferrimagnetic materials using THz emission spectroscopy

Abstract

Terahertz wave has been used in many fields due to its unique characteristics. Recently, it is also widely applied to analyze magnetic materials. However, new approaches have been required due to the need for more efficient terahertz wave sources and the limitations of the analysis method using terahertz waves. In this thesis, we propose two approaches to overcome the current issues in the field of nonlinear organic crystal as to develop THz technology. We also propose a noble perspective in magnetic analysis of rare-earth trans-metal ferrimagnets to improve our understanding in material with THz technology. The validity and versatility of the suggested methods are confirmed with electrical and optical measurements, especially THz emission spectroscopy. The first approach is control of refractive index by manipulating the bandgap to improve the phase-matching condition in the near-infrared range. The refractive index and absorption, measured in the optical and the terahertz range, indicated that the phase-matching condition was more improved in the near-infrared range. The effect of the improved phase-matching condition was confirmed through terahertz generation. Furthermore, based on the previous results, we developed new nonlinear organic crystal that can exhibit high efficiency in the near-IR range. The second approach is utilizing off-diagonal nonlinear components for efficient THz generation, including suppressing or eliminating the self-absorption of organic crystals in the THz range. From this, the interaction between phonons and THz waves was minimized and we can confirm the spectrum without absorption by phonons in the below 4 THz. The third approach is an analysis of magnetic properties of rare-earth transition-metal ferrimagnetic material through energy-dependent measurements. We observed clearly different responses to external magnetic fields according to the energy level and measured spin-glass-like behaviors by THz emission spectroscopy.