In this paper, the authors discuss the optimization of the Fe(FM-layer)/Pt(NM-layer) for efficient THz emission considering the following points: the FM and NM layer thickness, the optical pump wavelength, the choice of the substrate, and the out-coupling efficiency to the free-space. To improve the out-coupling efficiency, we introduce antenna structures with various shape. It has been demonstrated that a well-designed antenna structure can enhance the THz emission efficiency by several times. In addition, the authors will report a magnetic-field bias modulation, with which the signal detection efficiency can be almost doubled.
Wide-bandgap semiconductors are fundamental components in many optoelectronic and power devices. The free-carrier properties, i.e., carrier density and mobility, are crucial parameters that determine device performance. This paper presents the characterization of a gallium nitride (GaN) wafer with ~1016 cm-3 carrier concentration using terahertz (THz) time- domain ellipsometry. In addition, THz time-domain spectroscopy of monoclinic beta-gallium oxide (β-Ga2O3) semi- insulating bulk and n-type homoepitaxial film are presented. The free-carrier properties are extracted by theoretically fitting the complex refractive index to the Drude and Drude-Lorentz models. THz time-domain techniques are a practical and powerful tool to nondestructively characterize the free-carrier properties of wide-bandgap semiconductors for device development.
Electro-optic (EO) sampling is a powerful non-destructive technique for measuring terahertz (THz) electric field with high temporal resolution. EO sampling is a popular method for monitoring the electron beams in accelerators because the measurement of the electric field pulse width directly corresponds to the width of the electron beam. Previous researches mainly focused on measuring the temporal profile. In this work, we also measure the spatial (radial) profile that is perpendicular to the propagation (longitudinal/temporal direction). The measurement of the electric field profiles in both time and space paves the way to access the spatio-temporal electron beam profile, which makes this method promising for a high-order harmonic generation-seeded free-electron laser (FEL). In this research, we investigated the electric field strength profile and the pulse broadening in the radial direction based on the spatio-temporal electric field around a picosecond relativistic electron beam with an energy of 35 MeV. Special relativity predicts that the electric field contracts in the propagation direction and becomes like a disk with a uniform thickness. With this postulate, the Gauss’ theorem in cylindrical coordinates can be applied to the experimental results to deduce the electron beam size from the electric field profile.
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