Pulsed Terahertz Wave Research Articles

Cylindrical aperture Three-dimensional synthetic aperture imaging with pulsed terahertz waves

Cylindrical aperture Three-dimensional synthetic aperture imaging with pulsed terahertz waves

Changes in the Optical Properties of Rubber Exposed to High-Pressure Hydrogen Using Pulsed Terahertz Waves.

In this study, we investigated how high-temperature, high-pressure hydrogen affects the optical properties of three kinds of sealing rubber (chloroprene rubber, ethylene propylene diene monomer, and acrylonitrile butadiene rubber) using pulsed terahertz waves. The optical properties of the rubber samples were analyzed before and after exposure to hydrogen (80 °C and 200 bar) for 72 h. The results showed that the terahertz waves had a shorter time delay and a lower signal intensity for all rubber types. The exposure response intensity, refractive index, and absorption rate also changed in the frequency domain. Raman and Fourier transform infrared spectroscopy were used for comparison, and a few peak shifts were observed. However, the Raman spectra had low signal quality, and the laser damaged the specimen. The study demonstrates that terahertz waves can be used as a non-contact non-destructive testing technique to evaluate the changes in sealing rubbers after hydrogen exposure.

Tensile loading state sensing and estimation of opaque polymer materials using pulsed-terahertz waves

In this study, we used pulsed terahertz (THz) spectroscopy to analyze the THz optical properties of opaque acrylonitrile–butadiene–styrene (ABS). The phenomenon occurring upon the propagation of pulsed THz waves through ABS was analyzed through measurements of the variations in THz optical parameters (absorption constant, complex dielectric properties, and refractive index). When tensile loading of 0.1–1.5 kN was applied to a specimen, large optical coefficient variations were observed at 0.275, 0.471, and 0.686 THz, regardless of the applied tensile loading. The optical parameter variations were analyzed at 0.275 THz. Moreover, the accuracy of a tensile loading state estimation model established using principal component analysis was analyzed; consequently, random loading was confirmed. The input factor was extracted and calculated, with an error of approximately 3%, except for small loading (100 N).

Improved Numerical Modeling of Terahertz Wave Propagation in Epoxy Coating with the Finite-Difference Time-Domain Method

Electromagnetic theoretical analysis was usually used to investigate the pulsed terahertz wave interaction with the medium. For epoxy coatings, the material dispersion of the coating was often simplified or ignored in the traditional propagation model. It is difficult to elaborate on the propagation mechanism and to differentiate the coating deterioration as serving time increases. An improved propagation model based on the finite-difference time domain was established to characterize the propagation in the epoxy protective coating under broad-band terahertz radiation. Either an intact or defective coating structure was investigated, and the simulated results were compared with the experimental test. The dissipation mechanism was considered in the proposed model. The results indicated that the terahertz reflections varied with the change in optical and dielectric properties caused by coating aging, which influences the intrinsic impedance of the medium. Moreover, it could well elucidate the propagation mechanism of pulsed terahertz waves in rusted, defective coating structures.

Holographic detection of pulsed terahertz waves in terahertz time-domain spectroscopy

In the study of the interaction of terahertz (THz) wave with material, the traditional THz time-domain spectroscopy (THz-TDS) usually only detects the changes in amplitude and phase information of pulsed THz in a certain polarization direction before and after the interaction with the sample to be tested. However, in the detection of material such as anisotropic material and chiral material, only the amplitude and phase information of the sample cannot give the complete internal structure of the sample material. Anisotropic material and chiral material are very sensitive to pulsed THz of different polarization states. In order to reflect the configurations and conformations of these chiral substances through THz spectrum, it is necessary to detect the amplitude, phase and polarization state of the sample before and after pulse THz waves. The pulsed terahertz holographic detector (PTHD) in this work is composed of photoconductive antenna elements that are perpendicular to each other. The quadrature component of the pulsed THz electric field in any direction can be detected by one-time scanning, so that the changes in amplitude, phase and polarization state before and after the pulsed THz wave acts on the sample can be detected at the same time, so it is called pulsed THz wave holographic detection. Both experiments and theoretical analyses verify the reliability of the PTHD for measuring the polarization state of pulsed THz waves. At the same time, the response matrix is used to analyze that the PTHD has good symmetry in a spectral range of 0.1–2.2 THz.

Terahertz irradiation effects on the morphology and dynamics of actin biopolymer

Our recent experimental research on terahertz (THz) irradiation of actin filaments, which serve as representative biopolymer materials, is summarized in this review. We found that pulsed THz waves with energy density of ∼10−4 J cm−2 (∼108 W cm−2 at the peak) generate acoustic waves efficiently in the aqueous media. These acoustic waves propagated deeply into the water and demolished the actin filaments in living HeLa cells that were submerged into the cell culture medium. The results implied that THz pulsed irradiation affects the biomolecules in the tissues, even if these molecules are located a few millimeters away from the body surface. In contrast, irradiation using THz waves with lower peak power of the order of ∼W cm−2 induces the elongation of the actin biopolymer without thermal or acoustic effects. The polymerization of actin molecules plays essential roles in cell motility, growth, differentiation and gene expression. Therefore, our results indicate that THz waves could be applicable to artificial manipulation of cellular functions.

Common plastics THz classification via artificial neural networks: A discussion on a class of time domain features

Pulsed Terahertz waves are widely used as non-destructive technique for dielectric characterization of dielectric materials. In the Terahertz band, a family of materials with a great applicative perspective is represented by some of the most common plastics (including Polymethyl methacrylate, Polyethylene terephthalate, Teflon etc..). In general, lenses for THz frequencies can be fabricated with this type of plastics. Refractive index, absorption and thickness of a material are generally obtained exploiting time and frequency domain measurements in the Terahertz band. These data could be well used for classification purposes combined with chemometric methods. Although this approach is still highly regarded, is gradually being flanked - and sometime even overcome - by introducing intelligent techniques based on artificial neural networks. Since the first step in classification problems is generally represented by features extraction, and this could heavily affect the classification performances, it deserves detailed discussions. In this systematic work, several tuples of features are extracted from Terahertz time domain signals reflected from a set of THz optical materials (common plastics). These tuples are suggested to be used as input data for training and testing an artificial neural network based on Multilayer Perceptron. The performance of the network for materials and thickness classification purposes is hence discussed taking into account the number and type of features, the network architecture and the proportion used to split the dataset into the training and the testing datasets. Extending this approach beyond THz optical materials, an automated methodology for the analysis and classification of potentially each unknown dielectric material could be established.

Spectral Fresnel filter for pulsed broadband terahertz radiation

We proposed a simple and cost-effective method to manipulate the temporal and spectral properties of pulsed terahertz waves. A deep modulation of a pulse spectrum was both numerically and experimentally verified using Fresnel apertures with a radius ranging from several to several tens of the central wavelength of the broadband terahertz radiation. N-fold frequency minima were formed in the spectrum at a specific axial position behind the filter. Non-paraxial properties of this filter were also analyzed. A significant value (35%) of the ratio of the longitudinal to the transverse field component at the filter frequency was obtained. The measured results agree well with the simulation and theoretical predictions. The property of such a diffractive Fresnel notch filter can benefit the generation of longitudinal terahertz fields and relevant applications.

Measuring material parameters using a tunable monochromatic terahertz wave source.

We measured complex refractive indices of materials that transmitted well in the terahertz (THz) range using a tunable, monochromatic pulsed THz wave source and a high-sensitivity semiconductor detector. This approach is based on simple monochromatic transmission power measurements. Contrast enhancement by refractive index cladding was employed to measure the complex refractive indices. The experimental results were in good agreement with previously reported values. These results suggested that our approach could be used to quickly analyze material properties, such as surface roughness, and to determine the complex refractive indices of liquids or gases. In addition, as an application of the findings obtained here, we applied an anti-reflection polymer film to the silicon prism that served as an output coupler of the THz wave source. The film reduced surface reflection by 25%.

A broad range frequency measurement method for continuous and pulsed THz waves.

This paper proposes a method to measure the frequency of terahertz (THz) waves based on the Zeeman effect and the high magnetic field technology with a wideband range from 60 GHz to 3 THz. As the frequency of THz waves absorbed by the sample is linear to the magnetic field in the Zeeman effect, the frequency can be measured by the magnetic field strength. A comparison study of THz frequency measurement was carried out in two magnet systems (a superconducting one and a pulsed one) to investigate the performance in two kinds of high magnetic fields. The experimental results of 60-700 GHz show that this method has high resolution (about 0.001%), excellent linearity, and good repeatability. Moreover, the proposed method can measure polychromatic signals simultaneously as well as the single pulse frequency in the order of tens of microseconds.

Terahertz Displacement and Thickness Sensor with Micrometer Resolution and Centimeter Dynamic Range.

Measuring distance and thickness simultaneously is important in biological, medical, electronic, and various industries. Herein, we propose a method for simultaneously measuring the displacement and thickness of transparent materials using a pulsed terahertz wave. For this technique, a beam splitter was used to design the optical path such that the terahertz wave would incident the specimen vertically to achieve centimeter measurement range and micrometer resolution. The measured terahertz waveform carries peak time information reflected from the upper and lower surfaces of the sample, and the thickness can be calculated using the time difference between the first and second reflected peaks. The displacement can also be calculated using peak time difference when the sample moves from the initial position to the changed position. For validation, an experimental test was performed using aluminum, acrylic, and glass plates. The results confirmed a measurement range of 1 cm with an error of less than 23 μm, and the thickness error was less than 8 μm.

Effects of thermal aging on cellulose pressboard using terahertz time-domain spectroscopy

We demonstrate a method for examining the effects of thermal aging of cellulose pressboard using terahertz time-domain spectroscopy. The change of refractive indexes at the frequencies of pulsed terahertz waves while applying heat reveals that the degree of thermal aging and the condition of thermal breakdown can be clearly analyzed by performing terahertz measurements. The visual examination method only allows evaluation of the degree of blackness on the external surfaces of cellulose pressboard. However, using terahertz techniques for non-destructive inspection enables determination of the dielectric integrity to check the strength of the electrical insulation inside the cellulose pressboard. The results indicate that the terahertz techniques could be very useful, especially in the electric power industry, which requires a variety of methods for internal non-destructive testing to ensure the safety and reliability of insulation materials in electric power facilities and components.

Generation and detection of pulsed terahertz waves in gas: from elongated plasmas to microplasmas

The past two decades have seen an exponential growth of interest in one of the least explored region of the electromagnetic spectrum, the terahertz (THz) frequency band, ranging from to 0.1 to 10 THz. Once only the realm of astrophysicists studying the background radiation of the universe, THz waves have become little by little relevant in the most diverse fields, such as medical imaging, industrial inspection, remote sensing, fundamental science, and so on. Remarkably, THz wave radiation can be generated and detected by using ambient air as the source and the sensor. This is accomplished by creating plasma under the illumination of intense femtosecond laser fields. The integration of such a plasma source and sensor in THz time-domain techniques allows spectral measurements covering the whole THz gap (0.1 to 10 THz), further increasing the impact of this scientific tool in the study of the four states of matter. In this review, the authors introduce a new paradigm for implementing THz plasma techniques. Specifically, we replaced the use of elongated plasmas, ranging from few mm to several cm, with sub-mm plasmas, which will be referred to as microplasmas, obtained by focusing ultrafast laser pulses with high numerical aperture optics (NA from 0.1 to 0.9). The experimental study of the THz emission and detection from laser-induced plasmas of submillimeter size are presented. Regarding the microplasma source, one of the interesting phenomena is that the main direction of THz wave emission is almost orthogonal to the laser propagation direction, unlike that of elongated plasmas. Perhaps the most important achievement is the demonstration that laser pulse energies lower than 1 mJ are sufficient to generate measurable THz pulses from ambient air, thus reducing the required laser energy requirement of two orders of magnitude compared to the state of art. This significant decrease in the required laser energy will make plasma-based THz techniques more accessible to the scientific community, as well as opening new potential industrial applications. Finally, experimental observations of THz radiation detection with microplasmas are also presented. As fully coherent detection was not achieved in this work, the results presented herein are to be considered a first step to understand the peculiarities involved in using the microplasma as a THz sensor.

Detection of Terahertz Pulsed Radiation by Using Heterodyne Electro‐Optic Sampling Scheme

SUMMARYA new electro‐optic (EO) sampling scheme, which we refer to as “heterodyne EO sampling,” for the detection of pulsed terahertz (THz) waves is proposed and demonstrated. In this heterodyne EO sampling scheme, the intensity change in the sampling optical pulse induced by a THz field in a nonlinear crystal is measured without using any polarization optics. Applied in conjunction with the noncollinear Cherenkov phase matching technique, this method allows efficient and easy detection of pulsed THz wave using a simpler optical setup than in the conventional EO sampling method.

Non-ellipsometric detection of terahertz radiation using heterodyne EO sampling in the Cherenkov velocity matching scheme

A new electro-optic (EO) sampling scheme, which we refer to as "heterodyne EO sampling", for detection of pulsed terahertz (THz) waves is proposed and experimentally demonstrated. In this heterodyne EO sampling scheme, the intensity change in the optical probe pulse induced by a THz field in a nonlinear crystal is measured without any polarization optics. Applied in combination with the non-collinear Cherenkov velocity matching technique, this method allows one to detect pulsed THz waves efficiently and easily using a simpler optical setup as compared to the conventional ellipsometric EO sampling method.

Noncontact detection of the location of buried conductive grids with pulsed THz wave

AbstractIn this article, a new method for precisely measuring the location of buried conductive grid based on time‐domain pulsed terahertz (THz) reflectometry is presented. The location of the buried conductive grid is determined by the time difference of reflected THz pulses between the front surface and the buried conductive grid. Since the wavelength of THz wave is usually larger than the roughness of unpolished optical surface and the period of conductive grid, the proposed method is insensitive to the surface condition. Furthermore, this is a non‐contact and real‐time measurement technique. The experimental result indicates that the accuracy of the measurement is at least on the order of 10 μm. Such kind of technique will be very helpful for enhancing the quality and reduce the cost of fabricating electro‐optic windows with embedded conductive grids and/or wires. It can also be beneficial for some other applications. For example, since there are often embedded conductive wires in explosive devices, it can be used to detect the explosive devices because the proposed method can remotely determine the dimensions and locations of conductive wires embedded in dielectric enclosures. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1135–1138, 2012

Comparison of CW THz Wave Spectrometer with Laser Diode Excitation and Pulsed THz Wave Spectrometer with Cr:forsterite Sources Based on Difference Frequency Generation of Near-infrared Lasers in GaP

We constructed two types of terahertz (THz) spectrometers with automatic scanning control based on the difference frequency generation method by the excitation of the phonon-polariton mode in GaP. The pulsed THz wave spectroscopic systems were based on an optical parametric oscillator and Nd: YAG laser sources having a frequency resolution of 1.5 GHz, and on Cr:fosterite sources with a resolution of 20 GHz. Following these, we recently constructed a continuous wave (CW) THz wave spectroscopic system with laser diode excitation. One of the advantages of the CW THz wave spectrometer is its wide frequency tuning range with fine frequency resolution of < 8 MHz. In this study, we compare both types of spectrometers (pulsed versus CW) to show the characteristics of each system in terms of frequency resolution. The absorption spectra of a non-deformed white polyethylene crystal and ultra-high molecular weight polyethylene with/without deformation are measured by using the CW THz wave spectrometer and pulsed THz wave spectrometer. The effect of the high-resolution CW THz wave spectrometer is shown based on the THz spectroscopic results.

Terahertz Wave Gas Photonics: Sensing with Gases

A review of the detection of pulsed terahertz waves via field-induced second harmonic generation in gaseous media is presented. The effects of the DC bias field strength, probe pulse energy, nonlinear susceptibility of the gases, gas pressure, and focusing conditions of the terahertz and optical beams are discussed. A Figure-of-merit, which characterizes the sensitivity of gases media, is introduced.

Broadband terahertz detection with selected gases

We present a combined theoretical and experimental investigation of the detection of pulsed terahertz waves via field-induced second-harmonic generation in gaseous media. The effects of the probe pulse energy, bias field strength, nonlinear susceptibility of the gases, phase matching, and focusing conditions of the terahertz and optical beams are discussed. The analytical calculation, which is based on a Gaussian-beam approximation, allows for the easy identification of the parameters important for the sensitivity of the terahertz gas sensor. A figure of merit is introduced to characterize the sensitivity of gases. The experimental results are found to be in good agreement with the calculation.

Highly sensitive coherent detection of terahertz waves at room temperature using a parametric process

We report the experimental results of coherent detection of narrow-linewidth nanosecond terahertz radiation at room temperature using frequency conversion in a nonlinear MgO:LiNbO3 crystal. Mixing the terahertz radiation with a near-infrared intense pump pulse results in the excitation and amplification of the difference-frequency component, which is detected with an InGaAs-based photodiode. We demonstrate this method in a fast and very sensitive terahertz wave detector. The detector is capable of capturing the temporal profile of terahertz pulses with nanosecond resolution, and is at least one order more sensitive than a typical liquid-He-cooled Si bolometer for detecting nanosecond pulsed terahertz wave beams.