Terahertz (THz) radiation also known as T-ray lies between the microwave (Electronics) and Optical region of the electromagnetic spectrum which is extended between 100 GHz to 10 THz frequency or 3 mm to 30 µm wavelength range. Terahertz radiation has the capability to interrogate a wide variety of materials, such as semiconductors, gases, polymers, pharmaceutical drugs, chemical mixtures, biological, and environment pollutants and helped in determining the optical properties of many dielectrics and non-metallic materials.
THz ray can penetrate through most of the non-metallic materials such as plastic, paper, wood, rubber and ceramics which are opaque in the visible and infrared region. THz imaging technique acts as a perfect tool for the detection of concealed (hidden) metallic objects like arms and ammunition which may be potential threats, thus making it a suitable tool for defense and homeland security applications. One of the major applications of THz spectroscopy and imaging is in non-invasive detection of materials composition and retrieval of complex surface topography of objects in the form of 2D/3D hyper spectral images. THz-TDS imaging can be carried in transmission and reflection geometries. In transmission geometry, the temporal profile of the electric field transmitted through the investigated sample is measured and compared with a reference spectrum obtained from the free path. However, many samples possess significantly high absorption properties in the THz range or are highly reflective situations that limit or completely impede the use of THz imaging in transmission geometry
The research team led by Prof. A.K. Chaudhary and Mr. P. Naveen Kumar (Sr. Research Scholar) , ACRHEM, School of Physics, University of Hyderabad got success to design and developed an indigenous THz pulsed imaging systems between 0.1 to 2.0 THz range. The designed system is able to detect and identify the concealed metallic, explosives and drugs samples inside the paper envelope. The system provides five numbers of real time images of the given object. We have used an optical fiber coupled femtosecond laser, a voice coil-driven corner mirror with a position sensor and InGaAs / InAlAs photoconductive antennas for THz generation and detection. The project is funded by the D.R.D.O., Ministry of Defense, Govt. of India, ACRHEM-Phase-III. The work is also published in the current issue of In. J. of Physics; March 2023
Fig. 1 Shows the THz imaging system.
Fig. 2 (a) Shows the time domain image of the Key inside envelop and (b) the time domain image of the direct surface of the Key.
Fig.3: Show the time domain spectra domain spectra of RDX, FOX7, TNT, PETN, CL20 and HMX, at different THz frequencies. One week old samples collected from humid atmosphere shows clear cut variation in the colour and texture.
Fig.4: Show the images of metallic Blade under hardboard cover (concealed condition) at different THz frequencies.