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Using Terahertz Time-Domain Spectroscopy to Discriminate among Water Contamination Levels in Diesel Engine Oil
摘要： Terahertz time-domain spectroscopy (THz-TDS) in the range of 0.5 to 2.0 THz was evaluated for discriminating among water contamination levels (0%, 0.1%, and 0.2%) in diesel engine oil (SAE 15W-40). The absorption coefficient demonstrated potential to discriminate among the three water contamination levels with significant differences among all three levels across the 1.111 to 1.332 THz and 1.669 to 1.934 THz ranges. At each of these frequency ranges, each water contamination level was significantly different from the other two. The 0% water contamination level had the lowest absorption coefficient, while 0.2% water had the highest absorption coefficient. The refractive index demonstrated greater potential to discriminate among water contamination levels with significant differences among all three water levels across the 0.5 to 1.5 THz range. The refractive index of 0% water was the lowest and 0.2% water was the highest across the THz range. Linear regression analysis of the refractive index as a predictor of water contamination level yielded a highly significant equation (p < 0.0001, R2 = 0.99, RMSE = 0.01) when using the refractive indices at 0.5 THz. The refractive indices of these oil samples were promising for discrimination of water contamination. THz spectroscopy should be evaluated for discriminating other engine oil contaminants.
关键词： Terahertz spectroscopy,Diesel,Engine oil,Water contamination
更新于2025-09-09 09:28:46
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Visible Measurement of Terahertz Power Based on Capsulized Cholesteric Liquid Crystal Film
摘要： We demonstrate a new method to detect terahertz (THz) power using a temperature-supersensitive capsulized cholesteric liquid crystal film based on the thermochromic and thermodiffusion effect, which is clearly observed. A quantitative visualization of the THz intensity up to 4.0 × 103 mW/cm2 is presented. The diameter of the color change area is linearly dependent on the THz radiation power above 0.07 mW in the steady state. Moreover, the THz power can be detected for 1 sec of radiation with a parabolic relation to the color change area. The THz power meter is robust, cost-effective, portable, and even flexible, and can be used in applications such as THz imaging, biological sensing, and inspection.
关键词： quantitative visualization,cholesteric liquid crystal,terahertz
更新于2025-09-04 15:30:14
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Plasmon confinement by carrier density modulation in graphene
摘要： We investigate plasmon resonances in graphene with periodic carrier density modulation. The period is 8 μm, and each period consists of 1.7- and 6.3-μm-wide ribbons with di?erent density. Using terahertz spectroscopy, we show two plasmon modes with their electric ?eld mostly localized in the 1.7- or 6.3-μm-wide ribbon arrays. We also show that plasmons are excited only in one of the micro-ribbon arrays when the Fermi energy of the other micro-ribbon array is set close to the charge neutrality point. These results indicate that plasmons can be con?ned by the carrier density modulation. ? 2018 The Japan Society of Applied Physics
关键词： carrier density modulation,plasmon resonances,graphene,terahertz spectroscopy
更新于2025-09-04 15:30:14
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Terahertz Superconducting Radiometric Spectrometer in Tibet for Atmospheric Science
摘要： Terahertz superconducting radiometric spectrometer (TSRS), as one of seven instruments of the atmospheric profiling synthetic observation system (APSOS) project, was completed in the middle of 2017 after 5 years of development. It is a dual-band heterodyne receiver system based on high sensitive superconductor-insulator-superconductor (SIS) mixers which cover the frequency range of 180 to 380 GHz. With fast Fourier transform spectrometer (FFTS) of each band, real-time observation of 2 GHz bandwidth of high spectral resolution atmospheric molecular emission lines has been demonstrated. TSRS has been deployed at Yangbajing site, which stands on the Qinghai-Tibet Plateau at an altitude of 4300 m in southwestern China, since October 2017. It has been worked in a preliminary observation phase along with other active observation equipment of APSOS. Since then, ozone emission lines around 236 GHz and 358 GHz have been monitored simultaneously. Achieved data will be used to retrieve the in situ vertical distribution of ozone and its movement among different layers of the atmosphere.
关键词： Heterodyne receiver,Terahertz,Molecular emission lines,Superconducting
更新于2025-09-04 15:30:14
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Generation of THz radiation by photoconductive antennas on based thin films InGaAs and InGaAs/InAlAs.
摘要： Epitaxial low-temperature grown (LT) semiconductor arsenides (Al, Ga, In)As are widely used as materials for photoconductive antennas (PCA) generators and detectors of pulsed radiation in the terahertz (THz) frequency range [1–3]. It is the combination of subpicosecond carrier lifetime, relatively high mobility and high resistivity that makes LT-materials suitable for PCA applications. Lately, InGaAs has been investigated as a potential candidate for THz-PCA photoconductive material due to room-temperature band gap of 0.74 eV, which allows for 1.56 μm optical excitation with Er3+ fiber laser femtosecond pulses [4–6]. The low substrate temperatures result in a non-stoichiometric growth with the incorporation of excess arsenic in the crystal structure. The most common non-stoichiometry-related point defects in LT-arsenides are arsenic antisites with concentrations in the range 1017–1019 cm-3 depending on the substrate temperature and arsenic overpressure [7–10]. Antisite-related defect band in the semiconductor energy bandgap play a significant role in carrier dynamics. Fast non-radiative recombination of photogenerated electrons and holes through antisite centers results in sub-picosecond carrier lifetimes in LT-materials at optimized growth and annealing conditions [11, 12]. It is generally agreed that main traps of photo-excited electrons are ionized antisite defects [13–15]. A possible approach to increase the resistivity of LT-InGaAs is to employ LT-InGaAs/InAlAs superlattices [6,13,16,17]. LT-InAlAs layers have a higher dark resistivity as compared to LT-InGaAs and exhibit deep trap states that are situated energetically below the antisite defect levels of adjacent InGaAs layers that results in a reduction of residual carrier concentration. The Fig.1 shows the amplitude of THz radiation in time domain. It is seen that the signal from an InGaAs /InAlAs-based structure is 5-6 times higher due to a higher bias voltage, which is possible (without sample breakdown) due to higher sample resistance and lower dark current. Fig.2 shows a comparison of the Fourier amplitude according to the materials of the antennas LT-InGaAs/InAlAs and LT-InGaAs. It is seen that the spectrum of the LT-InGaAs / InAlAs sample is slightly wider in the range from 0.1 THz to 0.6 THz than that of the LT-InGaAs sample. We explain this effect by the difference between the characteristic relaxation times of electrons in the transition from the conduction band to the antisites. We determined the characteristic times of electron relaxation by the 'pump-probe' spectroscopy method. Fig.3 shows the dependence of the normalized transmission in time domain for the samples of LT-InGaAs and LT-InGaAs / InAlAs.We used 2-exponentional model for description experimental curves. On figures τ1 is an electron capture time (capture by charge AsGa defects) [18,19], τ2 is a recombination time of captured electrons and holes [17]. Due to the experimental fact that the characteristic relaxation times for the LT-InGaAs / InAlAs sample are less than for the LT-InGaAs, we observed the difference in the spectra for these samples. Summing up, it was found that THz generation is about 5-6 times more efficient in the case of LT-InGaAs/InAlAs superlattice than LT-InGaAs generation. It is found that due to the shorter electron relaxation time in the superlattice, the spectrum of these samples is wider in the range of 0.1-0.6 THz.
关键词： photoconductive antennas,InGaAs,THz radiation,terahertz frequency range,InGaAs/InAlAs,LT-materials
更新于2025-09-04 15:30:14
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Sign-alternating photoconductivity and magnetoresistance oscillations induced by terahertz radiation in HgTe quantum wells
摘要： We report on the observation of terahertz radiation induced photoconductivity and of terahertz analog of the microwave-induced resistance oscillations (MIRO) in HgTe-based quantum well (QW) structures of different width. The MIRO-like effect has been detected in QWs of 20 nm thickness with inverted band structure and a rather low mobility of about 3 × 105 cm2/V s. In a number of other structures with QW widths ranging from 5 to 20 nm and lower mobility we observed an unconventional nonoscillatory photoconductivity signal which changes its sign upon magnetic ?eld increase. This effect was observed in structures characterized by both normal and inverted band ordering, as well as in QWs with critical thickness and linear dispersion. In samples having Hall bar and Corbino geometries an increase of the magnetic ?eld resulted in a single and double change of the sign of the photoresponse, respectively. We show that within the bolometric mechanism of the photoresponse these unusual features imply a nonmonotonic behavior of the transport scattering rate, which should decrease (increase) with temperature for magnetic ?elds below (above) the certain value. This behavior is found to be consistent with the results of dark transport measurements of magnetoresistivity at different sample temperatures. Our experiments demonstrate that photoconductivity is a very sensitive probe of the temperature variations of the transport characteristics, even those that are hardly detectable using standard transport measurements.
关键词： HgTe quantum wells,terahertz radiation,magnetoresistance oscillations,bolometric mechanism,photoconductivity
更新于2025-09-04 15:30:14
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[IEEE 2018 IEEE International Conference on Computational Electromagnetics (ICCEM) - Chengdu (2018.3.26-2018.3.28)] 2018 IEEE International Conference on Computational Electromagnetics (ICCEM) - Roughness Impact on the RCS of Simple Canonical Objects in the Terahertz Regime
摘要： The higher the frequency is, the greater the influence of the precision and the realism of the CAD models on electromagnetic (EM) scattering characteristics are. In the terahertz (THz) regime, surfaces of most objects can’t be taken as smooth according to Rayleigh criterion. The interaction of EM waves and the surface presents a coherent part in the specular direction and a scattering part in the other directions. Unfortunately, the roughness of surface can’t be represented by the CAD geometry. Based on statistics theory, the rough surface height profile is fully determined by the height probability density function (pdf) and its autocorrelation functions. Without loss of generality, the height pdf of surface is assumed to be Gaussian. Under the assumption, the random Gaussian rough is correspondingly generated. The original CAD surface geometry and the random Gaussian rough surface are superposed as the input of EM computation. To demonstrate the roughness impact on RCS, EM scattering characteristics of simple canonical objects such as plate and dihedral in the THz regime are investigated. Taking into account the statistical surface roughness, the ray-based high-frequency EM method, shooting and bouncing rays (SBR), is utilized to compute the RCS of the objects above in the THz regime. Furthermore, the inverse synthetic aperture radar (ISAR) images are also carried out via filtered back projection (FBP) method. The EM scattering characteristics of the objects above in the THz regime are analyzed. Great differences of the objects EM scattering characteristics between the smooth and rough ones are observed and discussed.
关键词： roughness,shooting and bouncing rays,terahertz (THz),canonical objects
更新于2025-09-04 15:30:14
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CHARACTERISTICS OF MULTILAYERED METAMATERIAL STRUCTURES EMBEDDED IN FRACTIONAL SPACE FOR TERAHERTZ APPLICATION
摘要： This paper discusses the electromagnetic characteristics of a stratified metamaterial structure placed in fractional dimension space. Reflection and transmission coefficients for plane wave incident on multilayered structure in D-dimensional space are computed. Transfer matrix method is used to study the behaviour of different planer multilayered periodic metamaterial structures. The results are compared for integer and fractal dimensional spaces for both the cases of normal and oblique incidences. Classical results are recovered for integer dimensions. This work provides solution for examining the electromagnetic fields and waves in multilayered structures at fractal interfaces.
关键词： multilayered metamaterial structures,transfer matrix method,electromagnetic characteristics,fractional space,terahertz applications
更新于2025-09-04 15:30:14
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Liquid Crystal Tunable Dielectric Metamaterial Absorber in the Terahertz Range
摘要： In this paper, we propose a tunable dielectric metamaterial absorber in the terahertz (THz) range. The absorber is composed of a silicon pillar array embedded in a liquid crystal (LC) layer, which is sandwiched by two graphene electrodes. By way of varying the applied bias, the LC orientation can be continuously tuned. At a saturated bias, all LCs are vertically driven, and an absorption peak of 0.86 is achieved at 0.79 THz. When the bias is turned off, the same LCs are horizontally aligned, and the absorption peak degenerates into two smaller ones. A 47% modulation depth at 0.79 THz is obtained via numerical simulation with experimental feasibility considered. Such an active THz dielectric absorber may be utilized as part of various active THz apparatuses in THz imaging, sensing, switching, and filtering.
关键词： liquid crystal,metamaterial,dielectric absorber,terahertz,tunable
更新于2025-09-04 15:30:14
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Optimum chirp for efficient terahertz generation from two-color femtosecond pulses in air
摘要： We compare detailed spatio-temporal propagation simulations with experiments on generation of terahertz (THz) pulses in single ?laments with two-color optical excitation, which elucidates the mechanism whereby positively chirped optical pulses lead to the maximum THz yield. The long-wavelength components in the leading edge of the positively chirped 2x-pulse preserve the overlap with the x-pulse. The THz yield is extremely sensitive to the temporal overlap: for a 35-fs x-pulse, the yield decreases by an order of magnitude upon a x–2x pulse shift of only 5 fs.
关键词： terahertz,chirp,two-color excitation,femtosecond pulses,THz generation
更新于2025-09-04 15:30:14