IPERION HSIntegrating Platform for the European Research Infrastructure
Type: 2D/3D Analysis

 

Overview

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.

 

Technical details

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.

 

Further readings
  1. 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.
  2. 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).
  3. 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.
  4. 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

 

Providers

MOLAB France: CNRS