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Gallium nitride (GaN) is one of the most technologically important semiconductors and a fundamental component in many optoelectronic and power devices. Low-resistivity GaN wafers are in demand and actively being developed to improve the performance of vertical GaN power devices necessary for high-voltage and high-frequency applications. For the development of GaN devices, nondestructive characterization of electrical properties particularly for carrier densities in the order of 1019 cm−3 or higher is highly favorable. In this study, we investigated GaN single crystals with different carrier densities of up to 1020 cm−3 using THz time-domain ellipsometry in reflection configuration. The p- and s-polarized THz waves reflected off the GaN samples are measured and then corrected based on the analysis of multiple waveforms measured with a rotating analyzer. We show that performing such analysis leads to a ten times higher precision than by merely measuring the polarization components. As a result, the carrier density and mobility parameters can be unambiguously determined even at high conductivities.

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.

We report the synthesis and luminescence properties as well as band structure of a series of layered mixed-anion compounds, including a new layered mixed anion compound, Sr3Sc2Ag2Se2O5. The crystal structure, band structure, and optical properties of this new compound and of related compounds Sr3Sc2Cu2Ch2O5 (Ch:chalcogen = S, Se) are presented. The compounds have a common perovskite-like layer of (Sr3Sc2O5)2+ and different semiconductor layers of (Cu2S2)2−, (Cu2Se2)2− or (Ag2Se2)2−. Similar band structures are found for the valence band maximum (VBM), while the structure of the conduction band minimum (CBM) is different depending on the chalcogen species, resulting in the different band gap energies of the compounds. The band gap energies are estimated to be 3.3, 2.9, and 2.4 eV for Sr3Sc2Cu2S2O5, Sr3Sc2Cu2Se2O5, and Sr3Sc2Ag2Se2O5, respectively. Excitonic luminescence is observed near the band edge for all three compounds, with different wavelengths according to their band gap. These optical observations are consistent with the DFT calculations. The luminescence properties of the system can be controlled by changing the composition of the semiconductor-layers, thereby offering large flexibility in material design advantageous for various applications.

A spintronic heterostructure consisting of Pt and Fe thin films deposited by magnetron sputtering on a convex glass lens is demonstrated as a terahertz emitter. The experimental results show that the convex type spintronic terahertz emitter focuses its radiated terahertz beam without the need for any additional optics, thereby allowing opportunities for the simplification of terahertz setups.

Electric field profiles around a relativistic electron bunch along the propagation and in the radial direction are measured by the electro-optic (EO) sampling. The longitudinal (temporal) profile is obtained by changing the arrival time on the EO crystal between the electron bunch and the pulsed probe laser, while the transverse (radial) profile is investigated by lateral shift of the electron beam path. The validity of the experimental results is confirmed by the three-dimensional particle-in-cell calculations. It is demonstrated that the longitudinal and transverse beam sizes can be obtained by the measured spatio-temporal electric field distribution around the electron bunch.

A 650-nm chaotically oscillating multi-mode semiconductor laser was used to generate and detect a stable continuous terahertz signal from a photoconductive antenna. The intensity of the generated terahertz was ~10 times stronger compared to the excitation without chaotic oscillations. The frequency comb-type spectrum is obtained in the sub-terahertz region, which makes terahertz time-domain spectroscopy systems with this kind of economically superior laser very promising for the evaluation of 5G and 6G components.

An array of tungsten double coils embedded in polyester resin was investigated as an electromagnetic wave absorber. The finite element method was used to numerically find the best structure of the coils. Then, the transmittance response of the sample was evaluated experimentally using terahertz time-domain spectroscopy. Results show that such double-coil array is an effective electromagnetic wave absorber in the sub-terahertz region, and it has two controllable electromagnetic absorption frequency bands.

Spintronic terahertz emitters are terahertz wave emitting elements that utilize spin currents. We have produced Pt/Co spintronic terahertz emitters by epitaxial growth of Pt and Co layers on MgO(110) substrate. The azimuthal angle dependence of Pt/Co spintronic terahertz emission shows characteristic dependences due to magnetic anisotropy. The terahertz emission is useful to evaluate the magnetic anisotropy in spintronic thin films.

Herein we show the terahertz (THz) time-domain ellipsometry investigation of the InSb semiconductor. At room temperature, the carriers of InSb are thermally excited to the conduction band due to its narrow bandgap. We, nevertheless, show that THz ellipsometry can detect subtle differences between carrier densities of undoped and lightly doped InSb bulk semiconductors. The advantage of THz ellipsometry over other THz time-domain spectroscopy techniques is that it does not require reference measurements through an aperture or standard mirrors.

Reflection type measurements were performed on narrow-gap semiconductors such as InSb and InAs bulk samples for harmonic generation using intense terahertz pulses from free electron laser. We successfully measured the characteristic azimuthal angle dependence of the third harmonic generation (THG). The results show cos3θ and cos6θ dependence at different polarization layouts for InAs(111).

We investigated the monoclinic β-Ga 2 O 3 ultrawide-bandgap semiconductor using transmission terahertz time-domain spectroscopy (THz-TDS). To characterize the anisotropic properties, the complex refractive index of a semi-insulating bulk and of an n-type Si-doped homoepitaxial film were obtained along the [100] and [010] directions. The material response can be well-described by the Drude-Lorentz model, from which the static permittivity and the electrical properties were determined in good agreement with electrical contact measurements.

Gallium oxide (Ga 2 O 3 ) bulk substrate and homoepitaxial layer were investigated using terahertz time-domain spectroscopy in the frequency region from 0.2 to 3.0 THz and with polarization along the $a$ crystal axis. From the transmittance measurement, the refractive index spectra of the bulk substrate and the epilayer were obtained. The carrier density, electron mobility, and resistivity of the epilayer were then extracted by employing the Drude-Lorentz model.

Metal oxide-polymer nanocomposites have the combined properties of inorganic and organic materials which can pave the way for a wide array of applications. In short-wavelength optoelectronics, zinc oxide (ZnO) is an attractive component for nanocomposites owing to its stable ultraviolet (UV) luminescence at room temperature. We present herein the observed amplification of the UV emission of oriented ZnO nanorods topped by different polymer layers. The optical emission properties are investigated under different excitation power densities. The as-grown nanorods exhibit UV emission that increases linearly with excitation. On the other hand, the ZnO-polymer nanocomposites exhibit dramatic UV emission enhancement and spectral narrowing. The results show that amplified UV emission can be achieved at a lower excitation threshold by incorporating a polymer layer on top of oriented ZnO nanorods. In this regard, the ZnO-polymer nanocomposites show potential as low-threshold UV light sources attractive for optoelectronic devices.

Homoepitaxial film and semi-insulating bulk β-Ga2O3 with (001) orientation were studied using terahertz time-domain spectroscopy (THz-TDS) in the frequency region from 0.2 to 3.0 THz parallel to the [100] and [010] directions. The static permittivity of the bulk was determined to be 10.0 and 10.4 along the a-axis and b-axis, respectively, and the refractive index values at 0.2 THz are 3.17 and 3.23 for each axis. The electrical resistivity of the epilayer was extracted with good accuracy by employing the Drude–Lorentz model and without the use of electrical contacts. This noninvasive and contact-free material evaluation through THz-TDS proves to be a powerful tool for probing and obtaining various types of information about β-Ga2O3 materials such as bulk and thin films for the development of β-Ga2O3-based device applications.

Density functional theory based calculations with Hubbard correction (DFT + U) were performed to investigate the effects of varying coverage and different adsorption sites on hydrogen (H) adsorption on zinc oxide (ZnO) ( 10 1 ¯ 0 ) surface. Results show that H adsorption on top of oxygen (O) at low coverage (0.25 monolayer, ML) shifts the conduction band below the Fermi level and narrows the band gap. These phenomena are attributed to the charge transfer between H and the surface zinc (Zn) and O atoms. On the other hand, the H adsorption on top of Zn at low coverage (0.25 ML) shows an overlapping of H, Zn, and O states while maintaining the semiconductor nature of the system. At high coverage (1.0 ML), a charge accumulation layer on the surface forms, and the mechanisms that govern the interactions of H atoms when adsorbed exclusively on top of Zn or top of O are found to be similar with the low coverage cases. Lastly, at full coverage (2.0 ML), the effect of H on top of Zn is more evident as the system retained its semiconducting property. The adsorption energy is enhanced due to the reinforced overlapping of the H, Zn, and O states and due to the possible attraction between the adsorbed H atoms. The properties and stability of full-coverage adsorption were explained based on the findings on high- and low- coverages adsorption. The findings of the study will aid in understanding the interaction of H with the ZnO surface toward the further development of ZnO's optoelectronic applications.