Emission of Terahertz Electromagnetic Waves: A New Spectroscopic Method to Investigate Physical Properties of Solids

This is the authors' abstract. We don't add key points for this paper.

When a femtosecond laser pulse is applied to a solid with no spatial inversion symmetry, a low-frequency oscillation of polarization sometimes appears via the second-order nonlinear optical effect or equivalently the optical rectification. This results in an emission of terahertz electromagnetic waves via the electric-dipole radiation mechanism, which is usually called a terahertz radiation. In this review, we show that such a terahertz-radiation process itself can be used to reveal physical properties of solids. One example is the detections of ferroelectric domains and domain walls, which are demonstrated in various organic and inorganic ferroelectrics. The amplitude and sign of electric fields of the terahertz radiation are found to reflect the magnitude and direction of the original ferroelectric polarization, respectively. By using these characteristics, ferroelectric domains and domain walls can be visualized by measuring the position dependence of the electric-field waveform of the terahertz radiation over the sample. This method is also applied to determine the polarization vector in three dimensions in ferroelectric films. A similar terahertz radiation can be used as a probe to detect the conducting path in a negative differential resistance state, which is induced by the application of an external electric field on a ferroelectric charge-order material. Besides the electric-dipole radiation, the magnetic-dipole radiation is possible to occur in magnets. Using an emission of the terahertz electromagnetic waves originating from the magnetization modulation, visualization of magnetic domains can be achieved in ferrimagnets. In addition, we review new types of terahertz-radiation mechanisms based upon an impulsive stimulated Raman scattering process, a circular photogalvanic effect, and a photoinduced phase transition. We show that valuable information about the steady-state and dynamical electronic properties of solids can be derived by analyzing the amplitude, phase, and polarization of electromagnetic waves observed in various types of terahertz radiations.