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Orientation-Dependent THz Response of <i>m</i> -Plane Sapphire Substrate by Polarization-Sensitive THz Time Domain Spectroscopy
摘要： The dielectric response of m-plane sapphire (m-sapphire) substrates in the terahertz (THz) frequency is investigated using polarized-sensitive femto-second THz time-domain spectroscopy. We examined the transmitted THz time-domain waveforms with respect to the parallel-polarized THz field by rotating phi angle of m-sapphire substrates. The THz amplitude significantly decreased at the azimuth angle of 0°, 90°, and 180° with roughly equal amplitudes and no propagation time delay. The time delay of 480 fs in the THz pulse oscillations was observed between the azimuth angle of 45° and 90°, possibly due to the birefringence of nonpolar m-sapphire substrate. Strong THz field absorptions around ～1 THz frequency at azimuth angles of every 90° could be attributed to low-frequency phonon modes.
关键词： Sapphire,Birefringence,Time-Domain Spectroscopy,THz
更新于2025-09-23 15:23:52
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Complex Permittivity Measurement of Paraffin Phase-Change Material at 26 GHz–1.1 THz Using Time-Domain Spectroscopy
摘要： We report complex permittivity measurement of hexatriacontane films at the frequency range of 26 GHz–1.1 THz. Hexatriacontane (C36H74) has a melting point of 75 ?C that exhibits a 15% volumetric change which is crucial in developing low-loss RF microactuators with large displacement. In this work, we employ time-domain spectroscopy to measure the transmission coefficient of the paraffin samples in the frequency range of 0.3–1.1 THz. In order to extract the dielectric constant and accurately estimate the small values of loss tangent, we developed a propagation model which measured data are fitted to through a new least-squares minimization method. A Debye relaxation model is used to model the frequency dependence of the permittivity. Described method is rapidly convergent with minimum amount of signal processing. This method can be used to determine the complex permittivity of the materials by devising an appropriate function for the frequency dependence of the complex permittivity. Transmission through 20 samples of paraffin with various thicknesses is measured and the average permittivity is found to be 2.25 with standard deviation of 0.028. The loss tangent is monotonically increasing with frequency and the maximum value is 6.32×10?3 at 1.1 THz. Our study demonstrates that paraffin is a low-loss dielectric which makes it an attractive candidate for development of electro-thermo-mechanical actuators for sub-millimeter- and millimeter wave (mmW) variable capacitors, low-loss reconfigurable antennas, and phase shifters.
关键词： Terahertz,Time-domain spectroscopy,Loss tangent,Millimeter wave,Alkane,Paraffin,Complex permittivity
更新于2025-09-23 15:21:21
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Terahertz electromagnetic waves radiated from coherent longitudinal optical (LO) phonons and LO-phonon plasmon coupled modes in (001)-, (110)-, and (111)-oriented semi-insulating GaAs single crystals
摘要： We report on terahertz electromagnetic waves from coherent longitudinal optical (LO) phonons and coherent LO-phonon plasmon coupled (LOPC) modes in (001)-, (110)-, and (111)-oriented semi-insulating GaAs single crystals. The coherent LO phonons and coherent LOPC modes are observed in the three samples. In contrast, coherent transverse optical (TO) phonons are not observed in the (110)- and (111)-oriented samples in spite of that the TO phonon is the Raman active mode. We conclude that only the coherent longitudinal oscillating polarization such as coherent LO phonons and coherent LOPC modes radiate the terahertz wave and that the Raman selection rule does not contribute to the terahertz-wave-radiation process. The photogenerated electron-density dependence of the frequencies of the coherent LOPC mode is almost the same in the three samples. This ?nding indicates that the coupling of the LO phonon with the plasmon is not in?uenced by the surface orientations of the three samples.
关键词： coherent LO-phonon plasmon coupled mode,coherent longitudinal optical (LO) phonon,terahertz time-domain spectroscopy,coherent transverse optical phonon,ultrafast phenomena,Raman scattering effect
更新于2025-09-23 15:21:01
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Terahertz Spectral of Enantiomers and Racemic Amino Acids by Time-Domain-Spectroscopy Technology
摘要： The absorption spectra of four amino acids were studied. These spectra are obtained by terahertz time-domain spectroscopy (TDS) technique. The four amino acids are alanine, methionine, leucine, and valine, respectively. The spectra of their enantiomers (L-, D-) and racemic compounds (DL-) were investigated. Although the two isomers have very similar structures, their absorption spectra are obviously different. The absorption coefficient of each structure is calculated by the density functional theory (DFT), and the simulated spectra of each structure are obtained. It is shown that the number of the calculated peaks is in good agreement with the experimental ones. The experimental spectra were compared with the theoretical spectra. The differences between the absorption spectra of isomers are presented, and the reasons for the differences were analyzed. The vibratory spectra of biomolecules were studied, and the correlation between molecular structure and function was further understood.
关键词： collective vibration,isomers,amino acids,density functional theory,terahertz time-domain spectroscopy
更新于2025-09-23 15:21:01
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Thermal expansion of α-nitrogen ice in terahertz spectroscopy
摘要： The volumetric thermal expansion coef?cient of a-nitrogen (N2) ice is calculated for temperatures between 10 and 25 K using its terahertz transmission data and a phonon model. The observed dip in terahertz transmission around 1.45 THz is modeled as phonon absorption based on an intermolecular potential which is sum of the Lennard-Jones potential and quadrupole–quadrupole interaction term. Using this model and our experimental data, the lattice parameters of a-N2 ice were obtained.
关键词： Thermal expansion,Terahertz,Time-domain spectroscopy,a-Nitrogen ice
更新于2025-09-23 15:19:57
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Nondestructive imaging of hidden defects in aircraft sandwich composites using terahertz time-domain spectroscopy
摘要： Aircraft sandwich composites are superior to ordinary composites and have been largely used in the manufacture of military aircraft. This study attempts to investigate the efficacy of terahertz (THz) time-domain spectroscopy (TDS) imaging technology in detecting hidden defects in aircraft glass fiber (GF) sandwich composites. The defects of various sizes, such as debonding, delamination, and multi-delamination, at different depths in GF A-sandwich structure composites with foam core, C-sandwich structure composites with honeycomb core, and sheet-to-sheet cementing structure composites were evaluated. The THz frequency-domain amplitude and time-domain amplitude imaging algorithms were used to visualize the defects simulated by ultrathin double-layer Teflon inserts. And the suitable image processing methods which include wavelet-based fusion and multiscale edge representation were employed. With a combination of high-resolution THz C-scan and B-scan imaging, both the horizontal size and location, and the vertical depth and thickness of the defects were obtained in three dimensions. This study experimentally demonstrated that THz imaging technology can clearly detect various hidden defects in aircraft GF sandwich composites through reflection or transmission imaging mode.
关键词： image processing,debonding,multi-delamination,Aircraft sandwich composites,terahertz time-domain spectroscopy,nondestructive testing
更新于2025-09-19 17:15:36
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Terahertz and Visible Probing of Particles Suspended in Air
摘要： The attenuation of air suspended particles is measured with a terahertz time-domain spectrometer (THz-TDS). Simultaneously, the attenuation at a wavelength of 650 nm is probed with a laser diode. On the one hand, this dual measurement allows a direct assessment of the visibility evolution in the THz range compared to the visible range. On the other hand, this setup provides an estimation of the scattering strength and the density of particles. Using the Mie theory, the method is successfully applied to experimentally characterize the refractive index of sand grains and glass beads. The refractive indices of sand grains and glass beads, average over the acquisitions, are 1.67 and 2.54, respectively. The estimation of the scattering properties of sand grains are crucial to evaluate the performance of THz systems to image through brownout clouds that are created by helicopter rotors when landing in arid areas.
关键词： THz scattering,propagation,refractive index,THz time-domain spectroscopy
更新于2025-09-19 17:15:36
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Study on Irradiation Effect of Mid-Infrared Free Electron Laser on Hen Egg-White Lysozyme by Using Terahertz-Time Domain Spectroscopy and Synchrotron-Radiation Vacuum-Ultraviolet Circular-Dichroism Spectroscopy
摘要： Mid-infrared free-electron laser (MIR-FEL) is potentially applicable for tissue ablation and dissociation of pathological peptide aggregates in medicine. However, it is still poorly understood how the MIR-FEL irradiation influences on functional proteins such as enzymes. In the current study, the effect of MIR-FEL on both aggregate and non-aggregate (= native) states of hen egg-white lysozyme (HEWL) as a representative enzyme has been investigated. Absorption intensity at terahertz region (0.3–1.2 THz, 10–40 cm?1) of the aggregate of HEWL was lower than that of the native HEWL, but the former was increased nearly to the same level with the latter after the MIR-FEL irradiation tuned to 6 μm that corresponds to carbonyl stretching vibrational mode of amide bonds (amide I). This indicates that the aggregate of HEWL was converted to the native state by the irradiation. On the other hand, synchrotron-radiation vacuum-ultraviolet circular-dichroism spectroscopy showed that protein conformation of the native HEWL, which was rich in α-helix, was little changed after the MIR-FEL irradiation under the same condition with the case of the aggregate of HEWL. Furthermore, the enzymatic hydrolysis activity of the native HEWL against bacterial glycan was not remarkably decreased by the irradiation. Therefore, it can be estimated that the native structure of HEWL is little damaged although the aggregate state can be easily dissociated by the resonant excitation at amide bonds.
关键词： Vacuum-ultraviolet circular-dichroism spectroscopy,Mid-infrared free-electron laser,Protein aggregate,Terahertz-time domain spectroscopy
更新于2025-09-19 17:13:59
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[IEEE 2019 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (IMWS-AMP) - Bochum, Germany (2019.7.16-2019.7.18)] 2019 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (IMWS-AMP) - Characterization and Application of a Commercially Available Laser Diode in a THz System
摘要： In this work, we characterize a commercially available laser diode. The complex optical spectrum shows that most of the laser modes have a fixed phase relation, i.e. the laser is mode-locked. By choosing a single-mode fiber with appropriate length, the intrinsic chirp of the laser can be compensated. As a result, the amplitude of the detected THz signal is increased by a factor of 16.
关键词： mode-locked laser diodes,terahertz technology,time-domain spectroscopy
更新于2025-09-16 10:30:52
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Multi-Energy Valley Scattering Characteristics for a SI-GaAs-Based Terahertz Photoconductive Antenna in Linear Mode
摘要： In this paper, the relationship between the terahertz radiation and the spatial distribution of photogenerated carriers under different bias electric field is studied. Terahertz pulses and the photocurrent of SI-GaAs photoconductive antenna are measured by the terahertz time-domain spectroscopy system. The occupancy rate for photogenerated carriers for different energy valleys is obtained by comparing the photocurrent of terahertz field integrating with respect to time with the photocurrent measured by oscilloscope. Results indicate that 93.04% of all photogenerated carriers are located in the Γ valley when the bias electric field is 3.33 kV/cm, and 68.6% of all photogenerated carriers are transferred to the satellite valley when the bias electric field is 20.00 kV/cm. With the bias electric field increasing, the carrier occupancy rate for the satellite valley tends to saturate at 72.16%. In order to obtain the carrier occupancy rate for the satellite valley and saturate value at the high bias electric field, an ensemble Monte Carlo simulation based on the theory of photo-activated charge domain is developed.
关键词： photoconductive antenna,multi-energy valley scattering,semi-insulating Gallium arsenide (SI-GaAs),terahertz time-domain spectroscopy
更新于2025-09-16 10:30:52