Abstract-Industrial Applications of Terahertz Sensing: State of Play

Mira Naftaly ,Nico Vieweg,  Anselm Deninger

https://www.mdpi.com/1424-8220/19/19/4203

This paper is a survey of existing and upcoming industrial applications of terahertz technologies, comprising sections on polymers, paint and coatings, pharmaceuticals, electronics, petrochemicals, gas sensing, and paper and wood industries. Finally, an estimate of the market size and growth rates is given, as obtained from a comparison of market reports.

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Abstract-Comparison of Optical Single Sideband Techniques for THz-over-fiber Systems

Luis Gonzalez Guerrero,  Haymen Shams,  Irshaad Fatadin,  Martyn Fice, Mira Naftaly,  Alwyn Seeds, Cyril Renaud

https://ieeexplore.ieee.org/document/8570768

The use of single sideband (SSB) signals and envelope detection is a promising approach to enable the use of economic free-running lasers in photonic THz communications. To combat the signal-signal beat interference (SSBI) associated with envelope detection, broad guard bands (GBs) may be used given the large unregulated spectrum available at THz frequencies (100 GHz - 10 THz). In this scenario, the conventional way of generating SSB signals through a digital SSB filter (here referred to as the CSSB scheme) would require quite high analog digital-to-analog converter (DAC) bandwidths. Digital virtual SSB (DVSSB) and analog virtual SSB (AVSSB) have been proposed in direct-detection optical systems for relaxing the DAC bandwidth requirements. In this paper, we compare the three techniques through simulations and implement them, for the first time, in a THz-over-fiber (ToF) system operating at 250 GHz. For the transmission experiments we employ 5 GBd 16-QAM signals with three different GBs (5.5 GHz, 4.75 GHz and 3.5 GHz). The simulations show that the best performance is obtained with the AVSSB technique, while the worst is obtained with the DVSSB scheme, where the quality of the generated sideband degrades with carrier-to-sideband power ratio. In the experimental transmissions, where receiver noise was the main source of noise, similar behavior was found between the three techniques. At the 3.5 GHz GB, however, the DVSSB exhibited a penalty of 1 dB with respect to the other two. This is likely to be due to nonlinear distortions caused by the increase in the virtual tone power.

Abstract-Terahertz time-domain spectroscopy as a novel metrology tool for liquid-phase exfoliated few-layer graphene

Stefan Skalsky, John Molloy, Mira Naftaly, Toby Sainsbury, Keith R Paton

http://iopscience.iop.org/article/10.1088/1361-6528/aae8ce/pdf

Few-layer graphene (FLG) platelets exfoliated directly from graphite are finding a wide range of potential applications, including composites and printed electronics. However, characterisation of the FLG material following incorporation into polymers, including the quality of the dispersion, remains a challenge. Here, we present the use of terahertz time-domain spectroscopy as a potential solution to this challenge which could form the basis of a rapid characterisation tool. The THz refractive index was found to be highly sensitive to the loading of FLG, opening the route to mapping local FLG concentration within a polymer composite sample. By fitting the measured permittivity of the flakes to the Drude–Smith model of conductivity, we also show that the carrier concentrations of these materials are comparable to un-doped chemical vapour deposition produced materials. The ability to measure electronic properties of FLG following processing is important to ensure that defects have not been introduced or chemical functionalisation removed during processing.

Abstract-Variability of Terahertz Transmission Measured in Live Plant Leaves

Armita Afsharinejad,  Alan Davy,   Mira Naftaly,

http://ieeexplore.ieee.org/document/7892004/

Terahertz (THz) transmission was measured in the live leaves of seven species of common food plants. Multiple leaves of each plant were examined, and large variability of transmission loss was revealed. The results have implications for THz applications for agricultural sensing and for the design of THz communications links.

Abstract-Metrology State-of-the-Art and Challenges in Broadband Phase-Sensitive Terahertz Measurements

 Mira Naftaly,  Roland G. Clarke,   David A. Humphreys, Nick M. Ridler,

http://ieeexplore.ieee.org/document/7829286/

The two main modalities for making broadband phase-sensitive measurements at terahertz (THz) frequencies are vector network analyzers (VNA) and time-domain spectrometers (TDS). These measuring instruments have separate and fundamentally different operating principles and methodologies, and they serve very different application spaces. The different architectures give rise to different measurement challenges and metrological solutions. This article reviews these two measurement techniques and discusses the different issues involved in making measurements using these systems. Calibration, verification, and measurement traceability issues are reviewed, along with other major challenges facing these instrument architectures in the years to come. The differences in, and similarities between, the two measurement methods are discussed and analyzed. Finally, the operating principles of electro–optic sampling (EOS) are briefly discussed. This technique has some similarities to TDS and shares application space with the VNA.

Abstract-Metrology for terahertz time-domain spectrometers

John F. Molloy, Mira Naftaly

National Physical Lab. (United Kingdom)

Proc. SPIE 9810, XII International Conference on Atomic and Molecular Pulsed Lasers, 98101H (December 15, 2015); doi:10.1117/12.2225622

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2478124

In recent years the terahertz time-domain spectrometer (THz TDS) [1] has emerged as a key measurement device for spectroscopic investigations in the frequency range of 0.1-5 THz. To date, almost every type of material has been studied using THz TDS, including semiconductors, ceramics, polymers, metal films, liquid crystals, glasses, pharmaceuticals, DNA molecules, proteins, gases, composites, foams, oils, and many others. Measurements with a TDS are made in the time domain; conversion from the time domain data to a frequency spectrum is achieved by applying the Fourier Transform, calculated numerically using the Fast Fourier Transform (FFT) algorithm. As in many other types of spectrometer, THz TDS requires that the sample data be referenced to similarly acquired data with no sample present. Unlike frequency-domain spectrometers which detect light intensity and measure absorption spectra, a TDS records both amplitude and phase information, and therefore yields both the absorption coefficient and the refractive index of the sample material. The analysis of the data from THz TDS relies on the assumptions that: a) the frequency scale is accurate; b) the measurement of THz field amplitude is linear; and c) that the presence of the sample does not affect the performance characteristics of the instrument. The frequency scale of a THz TDS is derived from the displacement of the delay line; via FFT, positioning errors may give rise to frequency errors that are difficult to quantify. The measurement of the field amplitude in a THz TDS is required to be linear with a dynamic range of the order of 10 000. And attention must be given to the sample positioning and handling in order to avoid sample-related errors.

 © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Challenges Not Insurmountable for Terahertz Spectroscopy

Marie Freebody, Contributing Editor, marie.freebody@photonics.com

Cost, size and power must be addressed before terahertz spectroscopy can broaden its commercial acceptance – but steps are being taken. 
http://www.photonics.com/Article.aspx?AID=56212
Today, terahertz spectroscopy shows promise for some fascinating applications, from detecting illicit drugs and counterfeit pharmaceuticals to measuring the water content of plants and identifying hazardous trace gases. One reason terahertz has gained so much traction for these tasks comes down to the nature of its radiation. Terahertz is ideally placed between IR and microwave radiation, giving it a unique imaging power that crucially lies in the nondestructive region of the spectrum. 

For detecting illicit drugs, for example, time-domain spectroscopy (TDS) systems are ideally suited. Pharmaceutical substances – and in general, most nonpolar organic solids – show fairly broad absorption peaks at terahertz frequencies. These chemical fingerprints are well detected with a time-domain terahertz system. 

Other techniques such as Raman or IR spectroscopy also can identify most of the same substances, of course, but they require direct optical access to the sample. 

“By contrast, one of the key strengths of working at terahertz frequencies is that the terahertz beam penetrates materials such as paper, cardboard or plastics. This opens the possibility to detect chemical agents or illicit drugs even through parcels or sealed mail envelopes,” said Dr. Anselm Deninger, product manager of terahertz technologies at Toptica Photonics AG. “The main advantages of time-domain techniques are their broad bandwidth and the relatively short data-acquisition time: A full spectrum is recorded within subseconds to several 10 s, depending on the number of averages.” 

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Performance of Toptica’s CW terahertz spectrometer: The dynamic range is 80 dB at 500 GHz and still >65 dB at 1.0 THz. The dips are absorption lines of water vapor. Courtesy of © Toptica Photonics AG.

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Drug analysis is not just restricted to the illegal kind; polymorphic forms can also be identified. Various forms of the same chemical compound can exhibit different pharmaceutical properties, which are vital for drug manufacturers to remove from the production line. Different conformations or crystal packing of the same molecules can lead to different optical properties in the terahertz frequency regime, making them ideally suited for spectroscopic detection.

Water: Friend or foe?

At first encounter, water appears to be the enemy for terahertz radiation. Water is a very strong absorber at these frequencies; water vapor has distinct absorption lines more or less throughout the entire terahertz frequency range, and liquid water does not show any sharp peaks but causes broadband attenuation.

Medical research scientists find this a common problem when applying terahertz technology, because anything that takes place more than some hundred microns under the surface of the skin is impossible to reach due to strong water absorption of the terahertz signal.

But turn this around and measure instead the water content of a specimen, and you transform an enemy into a friend. By using the absorbing properties of water as contrast medium for imaging, you can potentially resolve fine differences in water content across a sample.

Terahertz for optimizing irrigation

Professor Martin Koch’s group at University of Marburg in Germany is taking this idea and running with it: The group has been measuring the water status of plants using terahertz TDS.

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Terahertz transmission image of a fresh plant leaf 24 hours after cutting: The dried leaf shows only little remaining water content. (Leaf images are for Dr. Rafal Wilk’s dissertation, available athttp://www.amazon.de/Switchable-THz-Reflectors-Rafal-Wilk/dp/3867273154, ISBN-13: 978-3867273152; Wilk, who completed this Ph.D. thesis under professor Martin Koch, is a leader of the terahertz business unit at Menlo Systems). Courtesy of Menlo Systems.

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The water content in vegetative tissues is of high importance to plant scientists, as it is a good indication of general crop health. Until recently, one of the only ways to evaluate this parameter was to compare the weight of fresh, dry leaves. This does not allow for instantaneous or continuous monitoring of the water content in live tissues.

Terahertz TDS offers a noncontact, nondestructive alternative. “Here, the strong absorption of terahertz radiation by water becomes an advantage, as small changes in water content of a leaf can be detected very sensitively,” said Ralf Gente, a Ph.D. student in Koch’s group. “In future experiments, we will investigate the behavior of agricultural crops like corn, wheat or soy under drought stress and re-irrigation.”

Spectroscopy through smoke

Black smoke is opaque for visible light – there is simply too much scattering. But because terahertz light has a longer wavelength (roughly a factor of 1000), it experiences much less scattering, opening up the possibility of detecting toxic gases released by a fire – in a chemical plant, for example.

“A drawback of traditional techniques – chemical, optical or acoustic sensors – is their limited versatility: A chemical sensor, for instance, requires a dedicated receptor tailored to the gas species to be measured,” Deninger said. “However, the sensor usually saturates after being activated once, and then has to be replaced. If you want to detect several hazardous gases, you may end up with a whole battery of different sensors.”

On the other hand, a single terahertz spectrometer can identify a broad range of substances. It can even do so remotely, if the laser light is guided via optical fibers to the location of interest, transmitting the terahertz beam to exactly where it is needed.

“A CW terahertz source is the instrument of choice here – first, because it is much easier to transmit CW laser light through a long stretch of fiber and, second, because the spectral resolution is just so good,” Deninger said. “Our own CW terahertz systems achieve single-megahertz resolution, which helps you to identify trace amounts of gases unambiguously.”

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Next-generation CW terahertz spectroscopy system: The black cylinders in the foreground are fiber-coupled terahertz antennas, the box on the right-hand side is the laser head, and the “flatpack” box on the left comprises the driver electronics and data-acquisition unit. Courtesy of © Toptica Photonics AG.

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Overcoming the challenges 

Despite the far-reaching potential of terahertz spectroscopy, adoption by the mass market has not yet materialized. But what is stopping industry from turning its back on the established imaging and inspection tools such as Raman spectroscopy, IR and x-ray imaging and replacing them with the versatile, nondestructive terahertz alternative?

Some limits are simply determined by the laws of physics: Atmospheric absorption – especially due to water vapor – limits the transmission of terahertz radiation in air to a few meters at most.

Terahertz transmission image of a fresh plant leaf recorded within 8 hours after cutting. Dark-blue color represents areas of high water content. Courtesy of Menlo Systems.

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“There are different approaches to achieve broadband terahertz operation for spectroscopic applications. Optoelectronic systems have the advantage to be compact and easy to use. Robust and reliable femtosecond lasers allow pulsed terahertz operation where a broad spectral range is available for the measurement at once,” said Dr. Patrizia Krok, who handles international sales of terahertz systems at Menlo Systems GmbH. “Highest reproducibility of the laser parameters such as in our Figure 9 T-Light laser are the key to high-performance THz-TDS systems.”

Other challenges include the need for more compact systems at higher powers. To overcome this, engineering homework and sufficient funding are required.

“We admittedly have some sort of ‘hen and egg’ problem here,” Deninger said. “The price for a fully fledged CW terahertz system is on the US$50,000 - 100,000 level. This is still prohibitively high for broadband industrial use. On the other hand, it is generally agreed that prices will drop significantly and systems will become more mature, if only the sales volume grows large enough.”

Toptica uses optoelectronic terahertz generation techniques in which GaAs- or InGaAs-based antennas convert NIR laser light into terahertz radiation. This works, said Deninger, but the industry still expects smaller, lighter-weight control electronics.

“I think the industry expects more compact systems, more terahertz power, a higher dynamic range, a lower price and ideally a ‘push-button’ level of complexity,” he said. “So our next step is to shrink the size of our CW-terahertz system and make it more efficient at the same time.”

Another challenge is accessing the gap between around 4 THz (time-domain spectrometers) and around 12 THz (Fourier transform IR spectroscopy, or FTIR). Dr. Mira Naftaly and colleagues at the National Physical Laboratory in Teddington, England, are currently developing a terahertz optical parametric oscillator that will operate at 6 to 12 THz. This could enable widespread testing of a greater variety of materials and chemicals in industrial labs as well as in research; it would be comparable to Raman spectroscopy or FTIR.

While the list of hurdles is significant, most in the industry believe that the next five to 10 years will be very exciting for the field.

On-resonance field enhancement factor (a) and terahertz transmission spectra (c) of terahertz resonator pictured in (b). Micron-sized gaps in metamaterial structures can be used to spatially localize terahertz radiation to regions smaller than the wavelength and improve the contrast for biological sensing applications. Courtesy of Lake Shore Cryotronics.

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“The evolution of Raman spectroscopy could be a helpful road map for thinking about the future of terahertz spectroscopy,” said Dr. David Daughton, applications scientist at Lake Shore Cryotronics Inc. in Westerville, Ohio. Lake Shore is a sensors, measurement and control specialist for a variety of research establishments. “The Raman technique is nearly a century old; a generation ago, Raman spectroscopy was isolated to well-equipped laser laboratories, and only in the past decade or so has the emergence of cost-effective diode lasers and filter technology enabled broad adoption of turnkey systems.”

What’s next for terahertz

With the availability of higher-power terahertz sources, materials that partially absorb terahertz radiation or thicker objects can be investigated. For example, Menlo Systems’ fiber-coupled terahertz antennas Tera15-FC already provide high power in their systems, and one of the UK’s terahertz companies, TeraView Ltd., claims to be developing a number of high-power sources that are both cost-effective and usable at ambient temperatures. These systems should be commercially available in the next two to three years.

Among the key developments for bio and medical applications, Daughton believes, are the metal and semiconductor structures that bring spectroscopy down to biological-length scales. To have sufficient sensitivity for bacterial detection, for example, semiconductor antennas can be used to couple free-space terahertz and direct the radiation to a 10-µm gap (which is filled with the specimen).

Also on the horizon is terahertz quasi-TDS, which uses a cheap multimode laser diode to generate terahertz radiation instead of a femtosecond laser. The technique promises to open up a new range of practical applications by massively reducing the cost, while at the same time allowing for much smaller systems that consume less power.

Dense BN is durable terahertz optical material

By John Wallace
Senior Editor                    

Several types of boron nitride (BN) were studied via terahertz time-domain spectroscopy by Mira Naftaly of the National Physical Laboratory (Teddington, England) and Jon Leist of Momentive Performance Materials (Strongsville, OH), with an eye toward terahertz optical materials. The terahertz time-domain spectrometer contained a femtosecond laser and gallium arsenide photoconductive emitter, off-axis parabolic mirrors, and light detection via a zinc telluride crystal and photodiodes. Types of BN tested included HBN, HBR, HBC, and HBT (all versions of BN manufactured using different binders) as well as pBN (pyrolytic BN, fully dense and manufactured by chemical vapor deposition with no binder).

All five types of BN had refractive indices on the order of 2 and were birefringent, with n_e - n_o equaling -0.392 for pBN and ranging between -0.110 and -0.187 for the rest. The one type of BN having a reasonably good optical transmission was pBN, which has measured values of 0.62 cm^-1 for the extraordinary ray and 4.1 cm^-1 for the ordinary ray. Low-cost pBN is hard and durable. It also has low optical dispersion in the terahertz region. Contact Naftaly atmira.naftaly@npl.co.uk.