FFT is an application over a complex space. Don't worry about the imaginary part because physics uses to happen in ℝ^3 and you should take the modulo of the data (in MATLAB is the built in function "abs"). Unpleasantly there are several options to take the modulo or magnitude of the signal (I...
Hi Pene1 (curious name, by the way)
I found this topic searching for other step in this long process of THz-TDS, but I might be able to help you at this step.
The data that you obtain after executing fft(V) is the Spectrum, but with the frequencies disorganized due to the internal behavior...