We report the use of a terahertz pulsed imaging technique for three-dimensional chemical mapping. Terahertz radiation reflected from a sample was measured pixel-by-pixel in time domain using a terahertz pulsed imaging system developed at TeraView Ltd, UK. The recorded terahertz waveforms were then transformed into frequency domain using time-partitioned Fourier transform. Structural maps of samples were obtained by analyzing the terahertz time-domain data whilst chemical maps were obtained from terahertz spectral data sets. For a sample comprising chemical A at the surface of a polyethylene pellet and chemical B buried inside the pellet, we have separated the component spatial patterns of the two chemicals using their spectral fingerprints. The reconstructed three-dimensional chemical maps not only locate the chemicals in the object, but also identify each chemical. We also demonstrate the capabilities of terahertz pulsed imaging for non-destructive analysis of coating thickness and quality, and for detecting and identifying explosive materials such as RDX.
The terahertz spectrum of the explosive RDX has been measured using a conventional Fourier transform infrared spectroscopy and by terahertz pulse spectroscopy in transmission and reflection modes. Seven absorption features in the spectral range 5-120 cm-1 have been observed and identified as the fingerprints of RDX explosive. Furthermore, a sample consisting of RDX-based explosive, mounted side by side with lactose and sucrose pellets, has been imaged using a terahertz pulse imaging system. The recorded terahertz images and their spectral data have a spectral resolution of 1 cm-1 and cover a spectral range of 5-80 cm-1. This broad spectral coverage enables the spatial distribution of individual chemical substances of the sample to be mapped out. We also discuss the application of Principal Component Analysis and Component Spatial Pattern Analysis to the automatic identification of materials, such as explosives, from terahertz imaging.