Type: 2D/3D Analysis
Terahertz (THz) spectroscopy and imaging is a non-destructive, non-contact, non-invasive technology emerging as a tool for the analysis of cultural heritage. THz Time Domain Spectroscopy (TDS) techniques have the ability to retrieve information from different layers within a stratified sample. The most explored topics—particularly using time-domain terahertz systems—include stratigraphic examinations of wood panel paintings and plaster-covered wall paintings. However, investigations have expanded to include terahertz imaging of ceramic vessels, corroded metal objects and papyrus sheets.
The data can be displayed through a series of parametric images mapping the imaging area with different arguments as parameters (maximum/minimum amplitude, peak to peak, frequency integration etc.) or as a cross section. In this case, if no sample of the wall material is taken, an assumption must be made on the nature of the material constituting the under-layer to determine the thickness and approximate depth location of the different layers.
The technique is be based on a compact free-space, time-domain-terahertz reflectometer consisting of an inter-digitated-metal-finger, semi-insulating-photoconductive-GaAs terahertz emitter and a low temperature-grown-GaAs (LT-GaAs) Hertzian-dipole receiver. The mode-locked, two-stage, amplified, Ytterbium fiber laser operates with a center frequency near 1064 nm, a 100 fs pulse width, a 50 MHz repetition rate and a maximum output power of 400 mW. Under laser irradiation a 100fs femtosecond radiated pulses is emitted with a THz frequency component with a usable bandwidth of 2 THz with a dynamic range of >40 dB for minimal averaging. An acquisition rate of 100 Hz is used for a fixed 320 ps measurement window at a 0.078125 ps time resolution. The THz beam emerging from the emitter, is focused using a high density polyethylene (HDPE) lens. The beam is delivered to the object under study, in these examples in reflection geometry. The fiber-coupled antennas permit rapid modification of the measurement geometry which enables easy in situ examinations.
A single THz waveform is digitally acquired and the beam spot is raster scanned across the object. The stages have a coarser 33.3 µm/step resolution, but their maximum speeds are 16 mm/s and 64 mm/s, respectively.
Acquisition time is strongly dependant on the number of averages, the speed of the translation stages and the pixel acquisition rate; these parameters are adjusted to provide the most optimal results for any particular experimental environment. For example a scan of 200 mm by 150 mm, with 50 averages takes around 5 hours.
- Jackson, J. B., Bowen, J., Walker, G., Labaune, J., Mourou, G., Menu, M., and Fukunaga, K., “A Survey of Terahertz Applications in Cultural Heritage Conservation Science”, IEEE Transactions on Terahertz Science and Technology 1, 220–231, 2011.
- Walker, G.C., Bowen, J.W., Matthews, W., Roychowdury, S., Labaune, J., Labaune, J., Mourou, G., Menu, M.,Hodder, I., and Jackson, J.B., ―Sub-surface terahertz imaging through uneven surfaces: visualizing Neolithic wall paintings in Çatalhöyük, Optics Express, 21(7), 8126-8134 (2013).
- C Walker, B. Jackson, D. Giovannacci, J.W. Bowen, B. Delandes, J. Labaune, G.A Mourou, M. Mene, V. Detalle ” Terahertz analysis of stratified wall plaster at buildings of cultural importancesaccross Europe”, SPIE Optical Metrology, 13-16 May 2013, Munich, Proc. SPIE 8790, Optics for Arts, Architecture, and Archaeology IV, 87900H, 8 pages.
- Giovannacci ; D. Martos-Levif ; G. C. Walker ; M. Menu and V. Detalle ” Terahertz applications in cultural heritage: case studies “, Proc. SPIE 9065, Fundamentals of Laser-Assisted Micro- and Nanotechnologies 2013, 906510 (November 28, 2013); doi:10.1117/12.2049818; http://dx.doi.org/10.1117/12.2049818
MOLAB France: CNRS