Simulations of terahertz signals in the time or frequency domain focus on the photoconductive antenna (PCA). However, they lack a simpler and more accurate simulation technology for PCA, which plays a crucial role in designing and optimizing detectors. Such work is essential in terahertz imaging and time-domain spectroscopy (THz-TDS). This work simulates an incident terahertz wave by introducing a three-dimensional (3-D) finite-difference time-domain (FDTD) simulation in the form of a total field to scattering field. This wave is pretreated as a plane wave that is incident on the receiver. The equation of carrier dynamics with semiconductor charge and transport is solved by using the 3-D full-wave FDTD method. A center surface current method is used to calculate the time-varying conductivity. A sampling electric field was used to evaluate the photocurrent. They were obtained by convolving the main time-varying photoconductivity of the photoexcited carrier distribution on the cross-section in the middle of the gap of a PCA receiver. Then, we compare these simulations with previously reported data from an incident terahertz signal. In addition, we simulated the detection characteristics with other results in the literature. Our simulation tool can accurately reproduce these data sets. These simulations can be used to design and optimize the receiving performance of different PCAs structures before costly fabrication has commenced.
We present the design of an innovative dual-band terahertz sensor based on a metamaterial array and a microfluidic channel. The sensing performance of two ultra-high absorption peaks—with absorption rates of 99.96% and 99.85%—generated in the frequency band of 0.2 to 1.2 THz is analyzed. Refractive indexes of analytes are predicted to be in a specific refractive index range, demonstrating the very low prediction error rate of the proposed sensor. Further analyses show that the designed sensor has application potential in label-free and rapid detection.
In order to obtain the terahertz detection signal as accurately as possible, a numerical simulation tool for terahertz signal reception is developed in this paper. The numerical simulation tool with the full-wave finite-difference time-domain (FDTD) method in three dimensions (3D) that couple multi-physics together is capable of getting the Terahertz detection signals. The carrier distribution effect of the incident femtosecond laser interacting with terahertz on the detector is analyzed briefly, and the simulation tool is validated by comparing the incident terahertz signal with the detected terahertz signal by using the low temperature growth GaAs substrate. The results show that the simulation tool developed in this paper is of great significance to the terahertz detection of micro-structure photoconductive antenna.
Light beams with helical phase-fronts are attractive for many optical applications, such as optical tweezers, particle manipulation, and other optical applications. A superoscillatory metalens has been designed for an azimuthally polarized wave at a wavelength λ = 632.8 nm. Numerical simulation demonstrates that a superoscillation hollow dark spot is generated. The transverse inner FWHM is 0.358λ overcoming the diffraction limit, while the sidelobe ratio can be compressed to 18.3%. More importantly, for such incidence with different orbital angular momentum (OAM) ℓ = 1, 2, and 3, the proposed metalens also presents elegant subwavelength tight focusing performances (the FWHMs are 0.469λ, 0.352λ, and 0.737λ, respectively). Extensive investigations have been made to present the unique characteristics of the generated foci, the phase distribution patterns are given for different OAM values. Small size and ultrathin thickness make such lenses suitable for various optical applications.
The determination of residual water content is vital for the quality control of high explosives, such as octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). However, conventional analytical methods are time-consuming and inaccurate. We have proposed a simple, sensitive, and accurate method for moisture determination based on terahertz (THz) time-domain spectroscopy. A model based on effective medium theory was implemented by a third-order extension of the Landau, Lifshitz, and Looyenga model to predict the moisture content of HMX based on THz spectroscopy. The predicted values of moisture content based on the model were found to be in excellent agreement with the results obtained using gravimetric measurements. Overall, the results proved the effectiveness of the proposed method for rapid and accurate moisture determination of HMX.
We have utilized optical-pump terahertz-probe spectroscopy to analyze the relaxation process of photoinduced nonequilibrium carriers in bulk ZnTe with a crystal orientation of ⟨ 110 ⟩ . The experimental results showed that the concentration of nonequilibrium carriers increases with the increase in the energy of pump laser. When the pump energy calculated by a single exponential fitting function reached 63 μJ, the concentration of nonequilibrium carriers attained saturation. In addition, for low pump energy, the relaxation time did not change with increasing pump energy. On the contrary, when the pump energy was high, the relaxation time increased in proportion to the pump laser energy.
To realize the on-line fluorescence monitoring of mineral oil pollution in water, three-dimensional spectral characteristic of oil-water intermixtures must be studied and the characteristic must be extracted. Using excitation wavelength, fluorescence wavelength and fluorescence intensity as three-dimensional system of coordinate, through sampling and surface fitting, three-dimensional fluorogram is gotten, which can provide gist for oil discrimination when presented in contour chart (finger-print map of oils). But there is little difference between characteristics of three-dimensional fluorogram because of the similarity of constituent and structure of similar oils. Therefore this paper introduces quantitative analysis method-characteristic parameter method which starts with analyzing statistical characteristic of three-dimensional fluorogram. Using RFPC fluorescence spectrometer (Shimadzu, Japan), three-dimensional fluorescence spectra of diesel oil, machine oil, gasoline oil, crude oil are measured and parameterized. The result shows that as a quantitative classified discrimination method of three-dimensional fluorescence spectra, the parameter of characteristic parameter method possesses definiteness for three-dimensional fluorescence spectra, and it is applicable, available when used in oil discrimination.
Using highly pure water disposed by Milli-Qlabo purifying system of United States as background water, employing RF540 fluorescence spectrometer and selecting fourteen wavelengths as excitation wavelengths, this paper measured and analyzed excitation spectra and fluorescence spectra of oil-water intermixtures with different concentrations from eight domestic mineral crude oils, seven imported mineral crude oils and eight mineral product oils. Experiment results show that: all of these oils can emit fluorescence in broad range of excitation spectra, but the fluorescence quantum efficiency is different; optimal excitation wavelength is 254nm, while more effective excitation wavelength is 360nm, and the corresponding optimal fluorescence detection wavelengths are 360nm and 460nm; with the increment of concentration, relative intensities of fluorescence also increase linearly, which shows that they have obvious positive correlation and the correlation coefficient is above 0.9. Thus using fluorescence method to directly measure the content of mineral crude oil and product oil in water is feasible. Based on the experimental work, combining transfer characteristic of optical fiber, the ranges of optimal excitation wavelength and detection wavelength of mineral oil in water are confirmed, which founds for the on-line fluorescence measurement with optical fiber of micro-content of mineral oil in water.
Fourier Transform Infrared gas analyzers have been widely used for speedy quantitative analyses of gases, and it is found that in many cases field maintainability determines the instruments’ online applicability instead of the instrument’s accuracy as is desired. To be maintenance-free is both the target of online instruments and the key to their field applications. Analyses show that if a background can be collected simultaneously with the sample spectrum, the transmittance will be only a function of concentration. Gas spectra collected on Nicolet670 of attenuated inputs and adjusted gains are examined via OMNIC software. Collected data exhibit that the absorbance spectra keep constant when input energy increases 8 times and the instrument gains becomes to 2.0 and 4.0 times. On viewing the absorption peaks vary with wavenumber in high “frequency” and that of the spectrometer itself in low frequency, by subtracting its instrumental response function of a transmittance spectrum extracted by wavelet transform, an absorbance spectrum of air is obtained and comparisons demonstrate that it is a summation of absorbance of the sample and that of gases in the optical path of the spectrometer. Calibration-free and background-free principles are thus exhibited and they construct maintenance-free principles of FTIR gas analyzers.
FTIR spectra have been widely used in quantitative analyses of mixed gases. Although Beer's Law regulates the relationships between absorbency and the product of concentration and path length, its deviations have been found rather complicated. Here we present the complexity of quantitative relationships between methane's infrared spectra and concentrations and resolutions. Measurements of the same methane sample's spectra under different resolutions demonstrate that both area absorbency and height absorbency vary with resolutions; spectra at lower resolution have bigger area absorbency for most of the peaks and are more likely to saturate for peaks of strong absorption. Standard methane sample of certain concentration is diluted with super pure nitrogen via mass control flowmeters and continuously passes through a 2-meter gas cell, such that the spectra of methane of different concentrations are collected. The area absorbencies of different peaks are carefully calculated via OMNIC software and results show that peaks with lower absorption are more likely to fit to linearity but more reluctant to changing concentrations. Area absorbencies are integrated through characteristic absorption regions, height absorbencies and area absorbencies are calculated at the two main absorption peaks and measurements show that approximative linearity fits all the areas and the best linearity appears at 1035.6cm-1.
Laser-induced fluorescence emission contains information about both spectra and time, so the different shapes, intensities and fluorescent lifetimes of fluorescence emission spectra can be used to measure the categories and contents of fluorescent substances with high sensitivity and good selectivity. To measure the oil micro-contamination in water, we utilized femtosecond ultraviolet laser pulse (fs Laser: MaiTai, Spectra Physics, US) as driving source and gated enhanced type ICCD (Time-resolved Fluorescence Spectroscopy, Lavision, German) as detector. We carried through laser-induced fluorescence measurement on DaGang crude oils and machine oils, accomplished data processing, and analyzed the differences of shapes of fluorescence spectra and lifetimes between crude oil and refined oil.
Dedicated experiments were made to clarify interference of the oil vapor, establish the quantitative analysis model and test its effectiveness in analyzing transformer fault gases with Fourier Transform Infrared. Nicolet Nexus 670 FTIR Spectrometer with a 2-meter vacuumable gas cell is used to collect and examine the spectra of the top gap gases and pyrolytic products of the transformer oil and no additional interference other than diagnostic gases were found. Standard gas samples of CH4, CO and CO2 are diluted with super pure nitrogen via mass control flowmeters to get samples with different concentrations dynamically. Spectra of the diluted samples are collected, analyzed and used as calibration standards to measure other samples. It is shown that the absorbance of the spectra can be calculated in height of the peak or in area under the peak and they vary with concentrations in approximate linearity, yet the former is more active and the latter is more passive to be saturated. Repeated measurements say the relative error of the established model is below 4% and typical measured concentrations are 537μL/L for methane and 57μL/L for monoxide.
Fluorescence optical fiber sensor is one of the hot-spot problems in the field of modem optical fiber sensor technology. The concentration of measured materials is decided by measuring its special fluorescence spectrum. This paper presents a novel type of fluorescence optical fiber measurement system for measuring oil concentration in seawater, which combines fluorescence spectrum processing with optical fiber sensing technique. The basic principle and characteristic of fluorescence measurement method are discussed. The fluorescence specific properties of oil in seawater are analyzed. And then the total scheme of the fluorescence optical fiber measurement system is designed. According to the comparative study of energy spectrum distribution of light source, the pulse xenon lamp with UV spectrum, which has low-power consumption, high strength, and low-repeat frequency, is used as the excitation light source. And low-loss silicon optical fiber bundle has been used as transfer channel of light signal. We also redesign the optical fiber probe to effectively eliminate light noise and make seawater exchange real time. In order to effectively separate the fluorescence produced by oil in seawater and the scatter light from the exciting light, high block coefficient, narrow band interference filters are used as spectrum filters. The detecting method of double light route and double channel at the same time is used to eliminate the signal variation due to undulation of the exciting light source. Experiments indicate that the prototype processes many advantages, such as good relativity and repeatability, high-precision, good real time, and anti-jamming. It seems that it will get wide applications foreground in the field of environment measurement.
In this paper, a portable fluorescence optical-fiber measurement system for identification of oil species is presented, which is based on fluorescence spectrum analysis, and combines the optical-fiber sensing technique and computer data processing technique. Fluorescence spectroscopy is enjoying increasing popularity as a technique for qualitative and quantitative analysis. Because of its great sensitivity, it is becoming an important tool for the identification of the type pf oil especially at low concentration levels. Conventional fluorescence methods suffer from low selectivity and are generally ineffective in spectral structural elucidation of carious oils because the different oil has fluorescence spectrum overlapping to great extent. The three-dimensional fluorescence spectroscopy overcomes the above-mentioned shortcomings. In this paper, the principle of 3-D fluorescence spectrum parameterization is introduced as a new spectral analysis method. The main feature parameters are mean, standard deviation, origin moment, marginal distribution, center moment, correlation coefficient and compression ellipse. We obtain the 3-D fluorescence spectrum of four kinds oils (crude oil, diesel fuel, kerosene, machine oil) by using fluorescence spectrophotometer in the lab and extract the feature parameters, which are weighting processed according to their sensitivity to the oil species for identification. The result suggests the degree of accuracy is as high as 95%.
According tp the features of the combustion process of boiler the optimization of combustion is implemented by using fuzzy control principle. The paper states a control strategy implementing different control regulation in different phases (coarse, fine and precision tuning) for enhancing the thermal efficiency of combustion of boiler. The practice shows that the thermal efficiency increased 2.8%.
The main working principles, the instrumental structure, functions and features are described in this paper. Some technical problems of Infrared-Ray Analyzer are discussed in detail. The analyzer can detect carbon dioxide with high precision and efficiency. Meanwhile, it can be applied into many associated fields.
In recent years oil contamination in water is more serious and destroys the mode of life and relation to water body environments. Excitation fluorescence method is one of the main approaches to monitor oil contamination on line. But average intensity of oil fluorescence only indicates its density, not indicates the type of contamination oil. Two-dimensional fluorescence spectrum is more difficult to determine the kind of oil, because the different oil has fluorescence spectrum overlapping to a great extent. In this paper, the 3D fluorescence spectrum parameterization is introduced. It can extract several characteristic parameters to measure the kid of oil to be measured. A prototype of optical fiber 3D fluorescence spectrum meter we developed carries out the identification of different oil types, such as crude oil, diesel oil and kerosene. The experiment arrangement conceived to measure pulse xenon lamp induced of oil component in water. The experiment results state clearly that the 3D fluorescence spectrum parameterization and software are successful to measure oil density and identify the type of oil in situ.
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