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Classification of Sand Grains by Terahertz Time-Domain Spectroscopy and Chemometrics
摘要： A novel method combining terahertz time-domain spectroscopy (THz-TDS) with chemometrics was proposed for classification of sand samples from different deserts and grain sizes. THz-TDS combined with chemometrics was firstly utilized for identifying the source of sand samples and analyzing the sizes of sand granule. Terahertz time-domain transmittance spectra were collected and then Savitzky–Golay smoothing and orthogonal signal correction (OSC) approaches were used for spectral pretreatment. Principal component analysis, partial least squares discriminant analysis, and support vector machine were used to establish classification models of the sand samples. The prediction correctness ranged between 94 and 97%. Scanning electron microscopy was used to investigate the morphology of the sand samples. The sand samples were analyzed by energy dispersive spectroscopy and the elemental content distributions were obtained. Fourier transform infrared absorption spectra of the sand samples were also collected. THz-TDS combined with multivariate modeling methods provided reliable and useful information to determine the provenance of the sands originated from different deserts in northern China. This work reveals that THz-TDS can be a useful tool for classification of sand from different deserts.
关键词： Terahertz spectroscopy,Chemometrics,Multivariate analysis,Sand source identification
更新于2025-09-10 09:29:36
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Atmospheric Effects on Millimeter and Sub-millimeter (THz) Satellite Communication Paths
摘要： Satellite communications require more bandwidth due to the necessity of increasing the capacity of communication channels and bandwidth to end-users. As a result, looking for new bands is required in the electromagnetic spectrum including millimeter and sub-millimeter wavelengths. Recent technological developments made the extremely high frequencies (EHF) above 30 GHz as a candidate for wireless applications such as the fifth generation (5G) of mobile communications, high resolution radars, and remote sensing. The EHF communication systems are becoming more and more commercial, cheap, and compact. However, the fact that the atmospheric medium is not completely transparent to millimeter waves requires considerations of the frequency selective absorption and dispersion effects emerging in this band. These phenomena affect also remote sensing in the millimeter and sub-millimeter waves (the terahertz frequencies). The atmospheric effects on the propagation of millimeter and sub-millimeter wave transmission from land to satellite are discussed. It is shown that not only atmospheric absorption plays a significant role on the received signal strength but also the refraction index of the atmospheric medium. The inhomogeneous refractivity causes the beam to Bbend^ along the propagation path, and it may even Bmiss^ its destination. This phenomenon should be considered in the design of a link operating in extremely high frequencies involving highly directive antennas.
关键词： Millimeter wave propagation,Terahertz waves,Millimeter wave satellite communications
更新于2025-09-10 09:29:36
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Design of broadband polarization converter for terahertz waves
摘要： A broadband reflective polarization converter is proposed. The unit cell of polarization converter is composed of a single-split resonant ring, a double-split resonant ring, a dielectric substrate and a metallic ground. The simulated results show that the polarization converter can convert x-polarized waves into y-polarized waves and obtain a broadband polarization conversion from 0.501 0 THz to 1.390 0 THz with the polarization conversion ratio (PCR) beyond 80% at normal incidence. Moreover, the surface current distributions are investigated to explain the polarization conversion mechanism. Finally, a good agreement is achieved between simulated and measured results. The polarization converter can be applied in terahertz imaging, communication and stealthy technology.
关键词： metasurface,terahertz waves,polarization conversion ratio,broadband,polarization converter
更新于2025-09-10 09:29:36
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High absorbance performance of symmetrical split ring resonator (SRR) terahertz metamaterial based on paper as spacer
摘要： Paper is a fascinating material for electronic platform devices due to being an inexpensive and environment friendly material. Terahertz wave is able to penetrate paper, thus paper-based terahertz metamaterial will be great terahertz devices. Creating multiple symmetrical Split Ring Resonator (SRR) on upper side paper and depositing metallic plane on back side paper yield terahertz metamaterial absorber. The maximum of absorbance peak for numerical and analytical results are 98% at 0.92 THz and 99% at 0.92 THz, respectively. The superposition of multiple reflections of terahertz wave generated on paper and yielding destructive interference phenomenon. Numerical and analytical works have been carried out in this works and showed good agreement.
关键词： terahertz,paper,multiple reflection,metamaterial absorber,split ring resonator
更新于2025-09-10 09:29:36
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[IEEE 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz2018) - Nagoya, Japan (2018.9.9-2018.9.14)] 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Linear to Circular Polarization Conversion of Terahertz Wave Using Metallic Helix Array
摘要： We developed a 2D array of 3D metallic helix which converts linearly polarized terahertz (THz) wave into circularly polarized THz wave and acts as a broadband THz wave polarizer. It covers the frequency range from 140 GHz to 250 GHz, close to one octave. We made two different arrays which consist of left-handed and right-handed metallic helices and we confirmed that these arrays change the linearly polarized THz wave into right-hand and left-hand circularly polarized THz wave respectively.
关键词： terahertz wave,metallic helix array,broadband polarizer,polarization conversion
更新于2025-09-10 09:29:36
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[IEEE 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz2018) - Nagoya, Japan (2018.9.9-2018.9.14)] 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Terahertz Polarimetric Sensing for Linear Encoder
摘要： We propose a linear encoder which utilizes Terahertz (THz) wave and polarization information. The encoder consists of a polarization angle reader as a read head and a periodic polarizer linear array as a scale. The read head determines its position based on polarization angle on the scale. This technology has possibility to have resistance to fine particles and gap fluctuation between the read head and scale. We experimentally demonstrated that the linear encoder based on the terahertz polarimetric sensing is useful for the industrial applications.
关键词： polarimetric sensing,linear encoder,periodic polarizer linear array,Terahertz,polarization angle reader
更新于2025-09-10 09:29:36
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Isothermal crystallization of poly(glycolic acid) studied by terahertz and infrared spectroscopy and SAXS/WAXD simultaneous measurements
摘要： Isothermal crystallization of poly(glycolic acid) (PGA) has been studied using terahertz (THz) and infrared (IR) spectroscopy and simultaneous small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) measurements. Changes in the intermolecular interactions in PGA during the isothermal crystallization were monitored using THz spectroscopy, which is an efficient technique for analyzing the higher-order structure of polymers. In the THz spectra, the temporal difference in the intensity observed in the isothermal crystallization is due to the difference in the vibrational origins of two bands at 192 and 65 cm?1. The band at 192 cm?1 primarily originates from the intramolecular vibrational mode (twisting of the local structure of the PGA molecular chain). Furthermore, the band at 65 cm?1 exists due to the intermolecular vibration mode (C = O···H-C hydrogen bonds between polymer chains). In addition, these THz bands appeared after the appearance of the SAXS and WAXD peaks. When a lamellar structure is formed and the molecular chains are oriented, the THz band originating from the intermolecular vibration is observed. It is highly possible that the intermolecular vibration appearing in the THz spectra requires the molecular chains to be oriented.
关键词： Terahertz spectroscopy,Intermolecular interactions,Infrared spectroscopy,Isothermal crystallization,SAXS/WAXD,Poly(glycolic acid)
更新于2025-09-10 09:29:36
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A Simple Multi-band Metamaterial Absorber with Combined Polarization Sensitive and Polarization Insensitive Characteristics for Terahertz Applications
摘要： This paper presents a simple multi-band metamaterial absorber for terahertz applications. The unit cell of the proposed structure consists of a single square ring having gaps at the centers on three of its sides. The proposed absorber produces three absorption bands for all polarizations and hence the design can be considered as insensitive to polarization variation. It provides an average absorption of 96.92% for the TE polarization with a peak absorption of 99.44% at 3.87 THz and for the TM polarization, it provides an average absorption of 98.4% with a peak absorption of 99.86% at 3.87 THz. An additional absorption peak is observed for the TE polarization at 1.055 THz that gradually diminishes with the increase in polarization angle and completely vanishes for the TM polarization. Thus, the structure displays a hybrid polarization response with polarization insensitivity in three bands and polarization sensitivity in one band. Parametric analysis has been carried out validating the optimal selection of the design parameters. The simplicity of the design and its combined polarization sensitive and polarization insensitive absorption characteristics can find tremendous applications in the field of terahertz imaging and sensing.
关键词： Metamaterial,Plasma frequency,Terahertz absorber,Multi-band absorber
更新于2025-09-10 09:29:36
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Optical and Electrical Characterization of Pure PMMA for Terahertz Wide-band Metamaterial Absorbers
摘要： The characteristics of relative permittivity and reflectance for poly (methyl methacrylate) (PMMA) films prepared by spin-coating were investigated and an excellent wideband terahertz (THz) metamaterial absorber (MA) had been fabricated by employing PMMA film as the dielectric layer. XRD and AFM indicated that all the PMMA films with thicknesses from 11 to 17 μm were amorphous and extremely smooth with roughness about 0.203 nm. Raman results demonstrated that vibration strength of the same covalent bonds tends to be consistent with an increase in film thickness. The dual beam laser interferometer (DBLI) was employed to study the influence of applied frequency and thickness on the relative permittivity, and the results exhibited dielectric constants increased from 2.88 to 4.04 with the thickness increasing from 9 to 14 μm. And the dielectric constants for PMMA films with a certain thickness (8 μm) gradually declined when the applied frequency increased from 100 to 105 Hz; besides, the dielectric constant in THz band was forecasted approximately 2.5. The terahertz time domain spectrometer (THz-TDS) results revealed that the THz reflectance was of gradual degradation from 80 to 58% at 3.2 THz, as the thickness increased from 11 to 17 μm. In addition, the UV-VIS spectrum showed that thickness had little effect on reflectance and transmittance in visible region, leading to the almost same Eg (energy gap) about 3.7 eV. At last, by employing PMMA film as the dielectric layer, a highly wideband metamaterial absorber with absorption > 80% from 4.1 to 7.4 THz was fabricated.
关键词： Terahertz,Metamaterial absorber,PMMA,Reflectance,Permittivity
更新于2025-09-10 09:29:36
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Temporal stability and absolute composition issues in molecular beam epitaxy of AlGaAs/GaAs THz QCL
摘要： The operation of the terahertz quantum cascade lasers (THz QCL) is strongly dependent on the repeatable fabrication of the well and barrier layers with the certain thicknesses throughout the whole active region stack epitaxial growth. This emphasizes the importance of the strict control of the growth rates and the stability of Si and group III effusion cell fluxes during growth [1]. It was reported [2] that two THz QCLs based on nominally identical multilayer heterostructures Al0.15Ga0.85As/GaAs emit at the different frequencies of 2.59 and 2.75 THz because of unintentional small deviations in the GaAs and AlAs growth rates (4 and 1.6 % respectively). Authors [3] determined the thickness tolerance for working lasing heterostructures to be minimally above 2% while the structures with thickness deviation 4.3 and 6.5 % are not lasing. In [4] the Ga cell temperature has to be increased to maintain a GaAs constant growth rate while the Al cell temperature remains nearly constant because variation in AlAs growth rate can be neglected. By using such growth rate compensation technique the two nominally identical structures approximately 10 mkm overall thick were found to show thickness difference of ~ 1 %. Thus calibration procedures and accurate analysis techniques become crucial to provide enhanced metrology possibilities.
关键词： molecular beam epitaxy,metrology,AlGaAs/GaAs,terahertz quantum cascade lasers,growth rates
更新于2025-09-10 09:29:36