Tuesday, September 17, 2019

An Introduction To Terahertz Technology For Non-Destructive Testing- Terametrix

Applications that benefit from Terahertz technology include aircrafts and fiber-reinforced composites. Shutterstock/Frank_peters

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What is Terahertz technology?
In the electromagnetic spectrum, Terahertz (THz) radiation sits between the infrared and microwave regions with frequencies between 300GHz and 3000GHz (0.3-3THz), and corresponding wavelengths from 1mm to 0.1mm. Historically, difficulties in generating and detecting THz radiation limited research into its interactions with matter [1]. However, recent advances now mean that it is possible to generate and detect 0.1-3THz frequencies.
THz radiation can penetrate a wide variety of non-conducting materials (such as cardboard, plastic, ceramics, clothing, wood, masonry, and paper.)). Penetration depth is relatively large (although usually less than that of microwave radiation), making THz-radiation suitable for probing the inner structure of samples in search of defects and inclusions [2]. Many chemicals have defining fingerprints at THz-frequencies so THz spectroscopy can be used to identify and characterize substances such as drugs and explosives.

Advantages of THz radiation

THz radiation penetrates many materials (more than infrared, for example) and, as an imaging technique, also exhibits good spatial resolution (better than microwaves, for example) [2]. This higher resolution means it is well-suited for analyzing complex systems with a low contrast range where established methods (like an ultrasound) may fail.
As THz pulses carry very low energies (compared to X-ray or UV radiation), they do not ionize and damage the material under study. This makes them particularly suitable for applications such as non-invasive imaging and non-destructive quality control. THz technology can be used where ultrasonic and other inspections cannot, such as when physical contact is not possible or when sensitive materials require examination (such as lightweight aircraft composites) or where materials do not conduct ultrasound [1].
Low energies mean this type of radiation is also safe for people to use; there are no associated health or safety risks.
Terahertz technology provides highly accurate, reliable and repeatable inspection data.


Pulsed terahertz imaging can provide a range of information about a sample. It can measure the thickness of a coating and study how well it is bonded, and give highly precise multi-layer thickness measurements in one go [3].   It can also detect the presence of internal voids or a specific gas, and detail the density distribution in a layer of foam [4]. Its major advantage is that can be used with the most sensitive of materials and testing environments.
Applications include NDT in aircraft and fiber-reinforced composites; ceramic coating thickness measurements; inspection and repair of pipelines; evaluation of seams; detection of voids; inspection of radomes (enclosures that protects radar antennae) and automotive fuel tanks; security applications such as checking for explosives in airports; and imaging of internal structures [1].

TeraMetrix Products

T-Ray 5000 series [1]
TeraMetrix Inc has designed robust compact pulsed terahertz measurement systems for use in an industrial environment. Sensor heads are coupled by an optical fibre to a rack mount package containing the electro-optical acquisition hardware. This gives complete flexibility as sensor heads can be replaced and configured to a user’s specific requirements. Cutting edge research can be performed quickly and easily as the T-Ray 5000 systems can be adapted to handle a large number of experiments thanks to their modular construction.
Sensor Heads
T-Ray 5000 series Control Units can be combined with a handheld gauge or  line scanner for nondestructive testing [4]. The Single Point Gauge (SPG) determines the thickness of several coating layers with unparalleled precision, and the Line Scan Gauge (LSG) produces images of structures under the surface. Measurements with the SPG can be taken either straight-on or at right-angle depending on which tip is selected. While essentially non-contact, a measurement tip helps the user to position the object at the focus of the THz beam when using a hand-held tool. Measurement configuration and mode of operation are selected using the touchscreen display. The system is capable of working on metallic or composite substrates.
Applications areas include aerospace where the system is used to measure coating thickness and panel alignment and identify defects; marine (coating thickness and corrosion under paint); building products (seam inspection and void detection); and petrochemical (steel pipe coatings and pipe repair inspection).
TeraMetrix Online Sensor Head - C1D1 [5]
The sensor head allows the  
T-Ray 5000 systems to work in flammable atmospheres, such as paint spray booths or coating facilities, as the transmitter and receiver are housed within sealed stainless steel and the lens is coated with Teflon to withstand solvents. It can also be mounted on a robot. While its working distance is set to 115 mm, this can be changed if needed.

Applications of TeraMetrix products

A good example is the F-35 Joint Strike Fighter aircraft. Its exterior is coated with specialty chemicals that are particularly challenging to measure. However, THz pulses have proved useful in measuring coating thickness soon after application once the coatings have dried. A handheld THz scanner can then provide the information necessary to assemble the airframe ensuring that parts are properly aligned.
Once in use, the aircraft needs to be inspected and serviced regularly. THz systems can be used to evaluate external coatings, as well as helping with paint removal when coatings need removing. In this case, they can provide information to control laser power and cutting depth.
NASA uses terahertz systems to examine the sprayed-on-foam insulation on the exterior of spacecraft fuel tanks. The Previous model T-Ray 4000 was also used to inspect the tiles of the shuttle heat shield as corrosion can occur underneath. By measuring the layers which attached the tiles to the orbiter, NASA could decide if tiles need replacing.
NASA is also investigating terahertz inspection of heat shields, thermal blankets and ultra-high pressure tanks as it develops the new Ares launch platform; as well as for checking ultra-high pressure gas tanks used for attitude adjustment rockets. Terahertz imaging can monitor the tanks’ Kevlar fiber coatings to check for broken fibers. Any rupture would result in the loss of the orbiter.
THz technology can be used to detect contamination in packaged products such as powdered antibiotics while the product remains inside its packaging. It can also test the weight of tablets in blister packs and powdered antibiotics can be non-destructively examined inside their packages.
Pipelines need regular monitoring for corrosion and leaks and THz technology can be used to measure the thickness of multi-layer coatings and monitor pipe repairs. Usually, fiberglass composite wraps are used as patches and THz imaging can check these patches for degradation. The advantage of THz technology is that it can probe below the patch to the surface of the pipe and check for problems with the pipe surface as well as the patch.

Monday, September 16, 2019

Abstract-Insights into Hydration Dynamics and Cooperative Interactions in Glycerol-Water Mixtures by Terahertz Dielectric Spectroscopy

We report relaxation dynamics of glycerol-water mixtures as probed by megahertz-to-terahertz dielectric spectroscopy in a frequency range from 50 MHz to 0.5 THz at room temperature. The dielectric relaxation spectra reveal several polarization processes at the molecular level with different time constants and dielectric strengths, providing an understanding of the hydrogen-bonding network in glycerol-water mixtures. We have determined the structure of hydration shells around glycerol molecules and the dynamics of bound water as a function of glycerol concentration in solutions using the Debye relaxation model. The experimental results show the existence of a critical glycerol concentration of ~7.5 mol %, which is related to the number of water molecules in the hydration layer around a glycerol molecule. At higher glycerol concentrations, water molecules dispersed in a glycerol network become abundant and eventually dominate and four distinct relaxation processes emerge in the mixtures. The relaxation dynamics and hydration structure in glycerol-water mixtures are further probed with molecular dynamics simulations, which confirm the physical picture revealed by the dielectric spectroscopy.

Abstract-Angle insensitive broadband terahertz wave absorption based on molybdenum disulfide metamaterials

Xiaoyu Wang, Jicheng Wang, Zheng-Da Hu, Guilin Liu, Yan Feng,

Fig. 1. Schematic of a proposed broadband terahertz absorber comprising MCDRs and metal…


Two-dimensional semiconductor materials may be of importance in the field of nanophotonics. We design a broadband terahertz (THz) absorber based on molybdenum disulfide (MoS2) metamaterial, consisting of a monolayer MoS2 concentric double rings and a metal mirror separated by a thin SiO2 layer. The plasma hybridization between two MoS2 rings can effectively increase the operating bandwidth of the device under normal circumstances. The absorption rate of the structure and the absorption bandwidth can be also tuned by temperature or voltage-controlled carrier concentration. The special geometry makes our design exhibit properties that are insensitive to the angle of incidence and the polarization state of the pump source. We believe that our devices may pave the way for the design of tunable sensors in terahertz and other frequency bands.

Sunday, September 15, 2019

Abstract-Ultrafast Terahertz Frequency and Phase Tuning by All‐Optical Molecularization of Metasurfaces

Yuze Hu  Tian Jiang  Junhu Zhou  Hao Hao,  Hao Sun,  Hao Ouyang, Mingyu Tong, Yuxiang Tang, Han Li, Jie You, Xin Zheng,  Zhongjie Xu, Xiangai Cheng,


The integration of photoactive semiconductors exhibiting strong light–matter interactions into functional unit meta‐atoms facilitates effective approaches to dynamically manipulate terahertz (THz) waves. Here, a new metaphotonic modulator is proposed and comprehensively studied, which demonstrates extensive tunability of the resonant frequency and phase with the merit of ultrafast photoswitching. Specifically, parallel silicon (Si) bridges are embedded in metasurfaces to reinforce the connection ability, achieving ultrafast optical molecularization from a magnetic quadrupole into an electric dipole. Under femtosecond pulse excitation, the demonstrated resonant frequency tuning range is as high as 40% (from 1.16 to 0.7 THz) and can be further promoted up to 48% (from 1.56 to 0.81 THz) by varying the Si bridge length. Meanwhile, the phase delay at given frequencies can be controlled up to 53.3° without significantly changing the high transmission. Furthermore, the transient frequency switching and phase shifting dynamics are systematically investigated for the first time, showing a full recovery time within 2 ns. By optically molecularizing metasurfaces, extended tuning ranges with regard to the resonant frequency and phase, as well as an ultrafast switching speed, are simultaneously acquired in the proposed metamodulator, which provides deeper insight into the multifunctional active‐tuning systems

Abstract-Terahertz-Rate Kerr-Microresonator Optical Clockwork

Tara E. Drake, Travis C. Briles, Jordan R. Stone, Daryl T. Spencer, David R. Carlson, Daniel D. Hickstein, Qing Li, Daron Westly, Kartik Srinivasan, Scott A. Diddams, and Scott B. Papp


Kerr microresonators generate interesting and useful fundamental states of electromagnetic radiation through nonlinear interactions of continuous-wave (CW) laser light. With photonic-integration techniques, functional devices with low noise, small size, low-power consumption, scalable fabrication, and heterogeneous combinations of photonics and electronics can be realized. Kerr solitons, which stably circulate in a Kerr microresonator, have emerged as a source of coherent, ultrafast pulse trains and ultra-broadband optical-frequency combs. Using the f2f technique, Kerr combs can support carrier-envelope-offset phase stabilization to enable optical synthesis and metrology. Here, we introduce a Kerr-microresonator optical clockwork, which is a foundational device that distributes optical-clock signals to the mode-difference frequency of a comb. Our clockwork is based on a silicon-nitride (Si3N4) microresonator that generates a Kerr-soliton frequency comb with a repetition frequency of 1 THz. We measure our terahertz clockwork by electro-optic modulation with a microwave signal, enabling optical-based timing experiments in this wideband and high-speed frequency range. Moreover, by EO phase modulation of our entire Kerr-soliton comb, we arbitrarily generate additional CW modes between the 1-THz modes to reduce the repetition frequency and increase the resolution of the comb. Our experiments characterize the absolute frequency noise of this Kerr-microresonator clockwork to one part in 1017, which is the highest accuracy and precision ever reported with this technology and opens the possibility of measuring high-performance optical clocks with Kerr combs.
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Saturday, September 14, 2019

Abstract-Double frequency plasmonic amplification of terahertz radiation in a periodical double-layer graphene

O V Polischuk, I M Moiseenko, M Y Morozov, V V Popov,


The plasmon amplification spectrum of terahertz radiation in a double-layer graphene nanoribbon array is theoretically studied. It is shown that this graphene structure exhibits strong plasmon response and giant amplification at vicinity of the anticrossing regime between optical and acoustic plasmon modes at room temperature.