The experiments were conducted with a Picometrix T-Ray 4000® THz-TDS imaging system. The system basically consisted in a box, which includes all optical components and an ultrafast laser, an emitter and receiver separately. The mode-locked, two-stage, amplified, Ytterbium fiber pump laser operated with a center wavelength near 1064nm, a 100fs pulse width, a 50MHz repetition rate and a maximum output power of 400mW. The emitter and receiver were low temperature GaAs photoconductive fiber-coupled antennas, which allow rapid modification of the measurement mode (transmission, off-axis and collinear reflection) for in situ analysis. This configuration allowed a two THz spectral bandwidth of the system with a dynamic band ranged to 40dB for minimal averaging with an acquisition rate of 100 Hz for a fixed 320 ps measurement window at a 78.13 fs time resolution. After emission, the THz beam is collimated and focused on the sample with a High Density Poly Ethylene (HDPE) lens and then, the receiver collected the reflected or transmitted THz pulse. All the experiments performed with reflection mode in this article were using off-axis mode. The data is acquired by raster-scanning the emitter and receiver configuration across the region of interest (ROI). A waveform is recorded at each position, including a 4096 point-time series. The parameters to optimize the measurements include the step, the scan size and the speed as they determined the size of each pixel thus the resolution of the image; the scan area as it defined the region of interest; the number of averaging of each pixel as it helped to improve the signal to noise ratio. The acquisition time depends on the number of pixels for the ROI, the resolution of pixel, the displacement speed and the number of averaging.
Electrical strength analysis versus time for a particular position on the specimen (also called A-scan) allows us to highlight several peaks. Each peak reveals difference of the optical distance between the receiver and the sample surface or interfaces. Such identification can provide information along the thickness of a specimen. A visualization of the stratigraphy can be carried out by accumulation of A-scan along a line, (so called a B-scan). Thus a B-scan gives indications of stratified boundaries in the material and features inside the sample, such as cracks, air gaps or layers with the limit of the resolution with approximately 50μm. The 2D cartography, also called C-scan, can be carried out by choosing a particular value of time of flight (it means at constant optical distance from the antennas) or a particular value in electrical field strength for example the maximum (it means at different optical distance from the antenna). Despite the mentioned 2D imaging method, 3D imaging method also applied as it can provide information of the roughness, flatness and layer thickness of the sample structure. This type of 3D image is done by firstly selecting a period of time delay on the TD spectra and defining the maximum or minimum value within the period for the plot.