Terahertz (THz) radiation, electromagnetic radiation in a frequency interval from 0.1 THz to 10 THz, occupies the portion of the electromagnetic spectrum between the infrared and microwave bands.

During the past decade, THz waves have been used to characterize the electronic, vibrational and compositional properties of solid, liquid and gas phase materials. Unlike common optical spectroscopes, which measure the intensity of light at specific frequencies, THz time-domain spectroscopy (THz-TDS) measures the electric field of pulsed THz waves as a function of time. Fourier transformation of this time-domain data gives the amplitude and phase of the THz wave pulse, therefore allowing precise measurements of the absorption coefficient of samples that interact with the THz waves. Typical temporal waveform and its spectrum are plotted below. Rotational and vibrational spectra of various explosive molecules lie within the THz frequency band (3-300 /cm, or 100 GHz to 10 THz). THz-TDS allows differentiation between explosives and other molecules in a similar way as Raman Spectroscopy, which allows differentiation of materials based on the lattice vibration spectra.

One of the attractive features of THz time-domain spectroscopy is that it has the capability of high-resolution imaging of dielectric features as well as remote spectroscopy of dielectric materials. It may complement other sensing modalities and provide a non-invasive, non-destructive, non-ionizing, and highly sensitive detection method for the explosives.

Fig. 2 A typical THz waveform and THz spectrum

Motivation

THz waves maintain reasonable penetration depth in certain common materials, such as clothes, plastic, wood, sand and soil. Therefore, THz technology has the potential to detect explosives packaged or buried within these materials because the explosives have unique THz spectral properties when compared to the surrounding materials. We are exploring the spectral fingerprints of explosive materials in the THz band, and apply THz imaging for landmine detection.

Instrumentation



RPI’s THz time-domain spectroscopy system (0.1 – 4 THz) in a vacuum chamber



APL’s experimental setup for a wide-band THz (0.1 – 30 THz) time-domain spectroscopy system

Bruker Optics IFS 66v/S FTIR system (RPI) for mid-IR and far-IR Fourier transform spectroscopy (5-700 cm-1, or 0.15-21 THz, resolution of 0.1 cm-1 or 3 GHz)

Plan and Goals

THz spectroscopy (JHU-APL)
Explore spectral information of explosive materials in the far-infrared region (3 -650 /cm or 100 GHz -20 THz) using FTIR and THz time-domain spectroscopy. The goal is to obtain fingerprints of explosives in the far-infrared region and establish a spectral database of explosive materials. At present, we are exploring the spectral information of a list of explosive materials in the far-infrared region using far-IR FTIR and are developing a wideband THz time domain spectrometer

THz propagation and imaging (RPI)
Develop powerful THz sources and corresponding sensitive THz imaging system to allow fast detection of explosive materials. Investigate THz propagation in air and various soils


Participants

RPI