Sensing with terahertz radiation

Research is focused on developing broadband terahertz technologies and novel industrially relevant applications where terahertz sensing can add value.

Key research areas

  • Physics-based modelling of creep behaviour for nuclear cladding materials
  • Dislocation-based modelling for multi-step metal forming process
  • Development of fundamental polycrystal modelling framework

Research themes

The coating on pharmaceutical dosage forms serve various purposes such as taste masking, improving visual appearance and light, moisture protection

Pharmaceutical tablet film coating is typically performed in large batches in perforated pan coaters where the quality of the resulting product shows a very heterogeneous distribution. The lack of fundamental understanding hinders the applicability of the technology for more advanced drug delivery systems. Terahertz in-line sensing has been demonstrated to monitor the film coating of individual tablets in production scale in real-time. We are building on this capability together with optical coherence tomography in a lab-scale process to support process understanding.

In process measurements of coating thickness in turn can be consolidated against mechanistic modelling such as discrete element method to explain process observations. We are undertaking this research in close collaboration with Professor Axel J. Zeitler and Professor James Elliott at University of Cambridge and Professor Yaochun Shen at University of Liverpool. This research is also supported by BASF, Colorcon and Bosch Packaging Technologies.

Figure 1 Schematic of the lab-scale tablet coater for performing combined OCT and THz in-line sensing of the pharmaceutical tablet coating process (top). Time average of the coating thickness measured by the respective sensors independently where thin coatings (10-80 μm) can be reliably resolved with OCT sensor while thick coatings (60–250 μm) are resolved with THz sensor (bottom).

Key publications

  • Measurement of the inter-tablet coating uniformity of a pharmaceutical pan coating process with combined terahertz and optical coherence tomography in-line sensing
    • Lin, H., Dong, Y., Markl, D., Williams, B.M., Zheng, Y., Shen, Y., Zeitler, J.A. 04/2017
    • In: Journal of Pharmaceutical Sciences. 106, 4, p. 1075-1084. 10 p.
  • Pharmaceutical film coating catalogue for spectral-domain optical coherence tomography
    • Lin, H., Dong, Y., Markl, D., Zhang, Z., Shen, Y., Zeitler, J.A. 10/2017
    • In: Journal of Pharmaceutical Sciences. 106, 10, p. 3171-3176. 6 p.
  • Steps towards numerical verification of the terahertz in-line measurement of tablet mixing by means of discrete element modelling
    • Lin, H., Pei, C., Markl, D., Shen, Y., Elliott, J.A., Zeitler, J.A. 23/07/2018
    • In: IET Microwaves, Antennas and Propagation. 12, 11, p. 1775-1779. 5 p.
  • A quantitative comparison of in-line coating thickness distributions obtained from a pharmaceutical tablet mixing process using discrete element method and terahertz pulsed imaging
    • Pei, C., Lin, H., Markl, D., Shen, Y., Zeitler, J.A., Elliott, J.A. 31/12/2018
    • In: Chemical Engineering Science. 192, p. 34-45. 12 p.
  • A Review of the Applications of OCT for Analysing Pharmaceutical Film Coatings
    • Lin H, Zhang Z, Markl D, Zeitler J. A., Shen, Y.
    • Appl. Sci. 2018, 8(12), 2700


  • EPSRC EP/L019922/1 and EP/R019460/1
  • Joy Welch Educational Charitable Trust

Progress in growth of graphene has reached a level where detailed, adequate characterisation over such large areas has become a key challenge.

Among a range of emerging contactless characterisation methods, terahertz time-domain spectroscopy (THz-TDS) operating in transmission has been demonstrated to allow the direct, accurate mapping of graphene conductivity and mobility over large areas. Whilst these demonstrations could potentially enable a rapid in-line graphene monitoring and large-area characterisation, to-date majority of THz-TDS are transmission based that necessitates a THz transparent support. We are developing the technique in reflection geometry applicable for measurement configurations where transmission measurement is not possible.

As a proof of concept, we recently demonstrated quantitative contactless measurement of the electrical conductivity of CVD graphene on a range of application relevant supports using THz-TDS operating in reflection geometry. Even though measurement and analysis are understandably more complex and can be less robust when measuring in reflection compared to transmission, especially considering that there is less terahertz interaction with the sample, THz-TDS in reflection geometry is a highly interesting contactless, quantitative characterisation technique with clear potential to complement existing characterisation techniques for graphene and other related 2D materials. We are undertaking this research in close collaboration with Professor Stephan Hofmann at University of Cambridge.

Raman map of graphene on sapphire substrate, (a) D/G ratio map, (b) D/G frequency distribution, (c) Raman 2D/G map. For the same region a conductivity map of graphene on sapphire substrate was measured with THz-TDS between 0.6-0.9 THz operating in (d) transmission mode with a Tera K15 T-Light setup and (e) in reflection mode with TPI, where (f) shows a spatially filtered map of (e) with a spot size 2.4 times greater. Conductivity histograms for transmission and reflection geometries are compared in (g) before and (h) after filtering. The colour of the histogram is darkened at the overlap between reflection and transmission measurements. Raman and terahertz mapping both resolve a similar shape of the transferred graphene film.

Key publications

  • Contactless graphene conductivity mapping on a wide range of substrates with terahertz time-domain reflection spectroscopy
    • Lin, H., Braeuninger-Weimer, P., Kamboj, V.S., Jessop, D.S., Degl'innocenti, R., Beere, H.E., Ritchie, D.A., Zeitler, J.A., Hofmann, S. 6/09/2017
    • In: Scientific Reports. 7, 1, 9 p.

The development on constitutive modelling frameworks that account for the effects of grain orientation and interaction in metallic materials.

Metallic materials usually consist of large number of grains. It is well-known that the grain orientation and the interaction between grains play an important role in the material properties. Therefore, the development of constitutive modelling frameworks that account for their effects becomes essential.

Self-Consistent (SC) modelling is an advanced solution in this area due to its high accuracy and efficiency in the simulations of mechanical response. This work here focuses on the development and extension of a SC modelling framework, called the Phi-model, which aims to further reduce the computational time and bring it closer to industrial applications. To avoid dealing with the interaction of each grain with each of its neighbours, Self-Consistent (SC) modelling treats each grain as an inhomogeneous inclusion embedded in a homogeneous effective medium (matrix). In this way, only the interaction between the inclusion and the matrix needs to be addressed.

The Phi-model is a unique SC model, with novelty in the way of obtaining the interaction laws of the polycrystal. It makes use of a new intermediate approach that allows spanning from high to low grain interaction using a single parameter Phi. This parameter is naturally introduced in a given potential function that is minimized to obtain the interaction law. With this approach a wide range of results can be obtained by varying Phi. This model is not based on the complex Eshelby-type interaction laws. Therefore, it is more computational efficient which makes it a suitable model for large scale simulations of complex issue via Finite element platforms. The parameter Phi is suggested to be linked with the grain size and the Stacking-Fault-Energy of the material. Our studies show that this model is capable of capturing the behaviours of FCC, BCC and HCP materials in terms of both stress-strain response and texture evolution.

Key publications

  • Coupled effects of the lattice rotation definition, twinning and interaction strength on the FCC rolling texture evolution using the viscoplastic ϕ-model
    • W. Wen, S. M’Guil, S. Ahzi, J.J. Gracio
    • International Journal of Plasticity 46 (2013) 23-36
  • Analysis of shear deformation by slip and twinning in low and high/medium stacking fault energy fcc metals using the ϕ-modelS.
    • M’Guil, W. Wen, S. Ahzi, J.J. Gracio, R.W. Davies
    • International Journal of Plasticity 68 (2015) 132-149
  • Analysis of shear deformation by slip and twinning in low and high/medium stacking fault energy fcc metals using the ϕ-models.
    • M’Guil, W. Wen, S. Ahzi, J.J. Gracio, R.W. Davies
    • International Journal of Plasticity 68 (2011) 132-149
  • Modeling of deformation behavior and texture evolution in magnesium alloy using the intermediate ϕ-modelD.S.
    • Li, S. Ahzi, S. M’Guil, W. Wen, C. Lavender, M.A. Khaleel
    • International Journal of Plasticity 52 (2014) 77-94

Key information


  • Dr Hungyen Lin
  • Dr Jun Song (Nanjing Forestry University, China) – Visiting researcher
  • Mr Decio Alves De Lima – PhD student


  • BASF
  • Colorcon
  • Bosch
  • University of Cambridge
  • University of Liverpool
  • French-German Research Institute of Saint-Louis