We present a novel feature extraction technique for identification of explosive and biological materials
using terahertz reflective time domain spectroscopy (THz-RTDS). The absorption signatures of the
materials are extracted directly from the derivative of the amplitude reflectance without measuring the
phase shift caused by the sample's height. This technique provides a straightforward and fast solution
to solve the phase-retrieval problem in RTDS and will benefit the future development of standoff, large
size focal-plane THz sensing and imaging system.