Supporting the academic and industrial applications of T-waves
NPL is home to the EPSRC THz facility.
Terahertz (THz) waves, also known as T-waves, T-rays or sub-millimetre waves, are the electromagnetic waves in the frequency band of about 0.1 to 10 THz. This frequency band is located between the more well-known bands of microwaves and infrared radiation. It has remained under exploited until very recently due to the lack of practical devices for generation and detection.
The NPL becoming an EPSRC Terahertz facility is an exciting development that will help accelerate R&D in both the technologies and applications in this frequency band. We have had a long and fruitful collaboration with NPL on Terahertz research and, unsurprisingly, our recent grant proposal was the first in include the NPL as an EPSRC Terahertz facility!
Prof Edward Wasige - University of Glasgow
Terahertz waves can penetrate to some degree through most dielectric materials, such as plastics, ceramics, pharmaceuticals, insulators, textiles or wood. This opens new opportunities for non-destructive testing and inspection (NDT) and material characterisation. In addition, many materials have identifiable spectral features at THz frequencies that may allow identification.  Due to their low photon energy, THz waves do not cause ionisation which means that they are regarded as safe and do not modify the material they pass through.
At NPL, we are supporting the academic and industrial applications of terahertz technologies by providing measurement services, characterisation services and consultancy for a range of different applications. We are also actively involved in collaborative research projects and innovative test applications and investigations with both industry and academia.
NPL is home to the EPSRC THz facility, serving the UK academic community. Facilities and services can be accessed through the EPSRC applications process.
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Terahertz wireless communications
The saturation of wireless bandwidth spectrum is driving innovations in wireless technologies, including expansion of the utilised spectrum to higher frequencies. THz frequency links are regarded as the next generation of wireless communications, capable of providing the coveted 1Tbps wireless transmission rate. NPL has developed specialised instrumentation for characterising transmitter and receiver devices used in THz wireless links. Device characterisation is important and necessary for two reasons:
- Accurate and detailed knowledge of device performance is essential in order to be able to design, simulate and build well-functioning wireless links, and to deploy them effectively.
- The ability to specify device performance in accordance with a standardised methodology makes it possible to compare devices reliably. This sets benchmarks for device development and optimisation and makes it possible to select the most suitable components from different manufacturers.
NPL offers device characterisation services and consultancy on characterisation measurements.
Terahertz technologies for non-destructive testing
Non-destructive testing (NDT) is an essential component of every manufacturing process and is a large global industry which incorporates process monitoring, condition monitoring and quality control. Non-contact measurements are strongly preferred and often necessary, therefore optical techniques dominate the field. However, these are limited to either surface inspection or materials with sufficient transparency to visible or near-infrared light. Terahertz waves are particularly well suited to NDT applications because many optically opaque materials have good or moderate THz transparency. Terahertz waves can also be employed in a range of measurement modalities, such as transmission or reflection, time-of-flight or tomography. A summary of THz applications in NDT produced by the British Institute for Non-Destructive Testing (BINDT).
NPL offers a variety of terahertz measurements and services:
- Time-domain spectroscopy in transmission and reflection
- Frequency-domain spectroscopy in transmission
- Consultancy on measurement techniques and implementation
Supporting new terahertz applications
THz technologies are a relatively new area so the instrumentation is developing rapidly and methodologies are being improved and refined. As yet, the techniques, instruments and particular issues of THz measurements are not sufficiently well known and understood in the wider community of non-specialist users. NPL is supporting and encouraging uptake and utilisation of THz waves, and we are engaged in advocacy and dissemination of THz metrology and good measurement practice.
NPL offers a range of THz consultancy services:
- Selection and implementation of appropriate measurement techniques
- Good measurement practice and data analysis
- Selection and setup of appropriate instrumentation.
Terahertz for material characterisation
There are two main drivers for investigating optical properties of materials at THz frequencies:
- The growing use of THz waves in industry and for wireless communications requires the detailed knowledge of transmission and reflection properties of a wide range of materials for optical components and for system design.
- Many aspects of material composition, structure and behavior can be revealed by investigating their response to THz waves.
At NPL we have studied THz optical properties of a great variety of materials, acquiring unparalleled experience and expertise in this area. The materials studied include: polymers, ceramics, glasses, inorganic and organic crystals, liquid crystals, pharmaceuticals, biological materials, and textiles. NPL offers a range of terahertz measurements and services:
- Time-domain spectroscopy in transmission and reflection
- Frequency-domain spectroscopy in transmission
- Consultancy on measurement techniques and implementation
- Consultancy on material selection for THz applications
A recent review paper led by NPL presents 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. It also includes estimates of the market size and growth rates.
THz technologies for materials
Polymers and polymer components - Replacement of established techniques for inline measurements in plastic extrusion processes and inspection of multi-layer paint coatings in the automotive industry using time-of-flight measurements.
Composite materials - Verification of additive content, the distribution and dispersion of fillers, the moisture content, orientation of fibres, particles and molecular chains using THz refractive index and absorption coefficient.
Polymer foams - THz inline sensing during the production process can determine cellular structure and the effective density using the refractive index and evaluating scattering loss.
Adhesives - THz instruments can determine the thickness of adhesive joints and the distribution of adhesives between plastics components.
Paintings and coatings - THz spectroscopy provides a non-destructive non-contact solution for monitoring multi-layer coatings on any substrate, enabling the thickness of each layer to be measured individually.
Paper - THz transmission measurements enable thickness, composition, moisture content and texture to be evaluated.
THz technologies for industries
Pharmaceuticals - Pharmaceutical materials are (semi) transparent at THz frequencies and often possess characteristic spectral signatures, whereas they are opaque in the visible and infrared. High precision monitoring of chemical composition, microstructure and mechanical properties is vital. THz technology can be used for:
- Improving the effectiveness of quality monitoring, reducing waste and increasing product consistency.
- Monitoring porosity and pore size in a continuous in-line non-destructive way
Electronics - THz technology is a useful non-destructive, non-contact inspection tool for semiconductor materials because the electronic properties, such as carrier concentration and mobility, determine their dielectric properties at THz frequencies. THz technology can be used for:
- Characterisation and inspection of solar cells by determining electrical properties, including conductivity, charge carrier density and mobility, in order to predict performance and efficiency.
- Inspection of graphene by mapping conductivity and carrier mobility in graphene sheets.
​Petrochemicals - Petrochemicals have a high transparency at THz frequencies, and the chemical composition and the presence of contaminants affects the THz dielectric properties. THz spectroscopy can be used to identify:
- the composition of crude oil, and determine which oil field it is from
- fuel grades, additives and contaminants, which means it is possible to distinguish and quantify the octane number.
- lubricating and insulating oils based on their viscoelastic or dielectric properties
Gas sensing - Recent developments in terahertz technologies has enabled the detection of various gases in the ambient atmosphere, leading to interest in THz gas detection and monitoring, and a growing awareness of its advantages.
Environmental monitoring - THz technology can be used for:
- detection of pollutants and contaminants to meet the requirements of health and safety
- monitoring of atmospheric components for weather and climate observation.
- Analysing gas exhaled in human breath to aid the early detection of disease biomarkers
- establishing the mixing ratios of natural gases, the refractive index provides a good indicator of the mix, due to the differences in the indices of individual gases.
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NPL is home to the EPSRC THz facility.
Our collaboration with NPL - initiated through an EPSRC industrial CASE studentship - on nondestructive testing of pharmaceuticals tremendously benefits from the world-leading NPL terahertz spectroscopy facility. Specifically, having access to time-domain and continuous wave spectroscopy systems paired with the ability to customise the optical setups provides unique opportunities to perform fundamental and industry-inspired research.
Dr Daniel Markl, - University of Strathclyde