Terahertz portable device
Terahertz (THz) spectroscopy and imaging is a non-destructive, non-contract, non-invasing 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-domaub terahertz systems - include stratigraphic examinations of wood panel paintings and plaster-covered wall paintings. However, investigations have expanded to include terahertz imaging of ceramic vesselsn corroded metal objects and papyrus sheets.
The data can be displayed through a serie 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.
TECHNIQUES DETAILS :
The technique is 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 temperture-grown-GaAs (LT-GaAs) Hertzian-dipole receiver. The mode-locked, two stage, amplified, Ytterbium fiber laser operated with a center frequency component with a usable bandwidth of 2 THz with a dunamic 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 on a 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 acroiss the object. The stages have a coarser 33.3 µm/step reosolution, 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 expermental environment. For example, a scan of 200 mm by 150 mm, with 50 averages takes around 5 hours.