ASIAN JOURNAL OF PHYSICS

Terahertz (THz) spectrum broadly covers the electromagnetic region that lies between the microwave and Far-IR region (within the frequency range of 0.1–10 THz). This form of electromagnetic radiation was less known, due to the limited access to technology for generating and detecting radiation and hence was called “THz forbidden gap”. But the spectroscopy in this THz region grew exponentially ever since the table top THz sources and detectors became a reality after the invention of femtosecond lasers. Thus the “THz gap” kept on diminishing and this gap has now completely turned into a most important THz technology. This regime mainly contains various kinds of resonances such as vibrational, translational, rotational, torsional, and conformational states, facilitating it to provide information on molecules that are not accessible with other analytical and imaging techniques in the remaining electromagnetic spectrum. On top of these, this regime allows direct access to numerous other low energy excitations such as spin waves, internal excitations of bound electron-hole pairs and Cooper pairs, through both resonant and non-resonant excitations. These unique characteristics make them apt for identifying, analyzing, or imaging a variety of materials around us. The most notable feature is that the photon energies of THz radiation are small (~0.4-41 meV @ 0.1-10 THz), and hence it is non-ionizing, non-invasive and can penetrate many materials. Thus, THz radiation finds a potential alternate for non-invasive imaging of biological systems, body scanners including surveillance, pharmaceutical quality control, detection of explosives, metrology, the investigation on works of Art and Archaeology, characterization of construction and building materials,… etc to name a few.

The main advantage with this technique is that Kramers-Kronig relation is not needed to estimate the real and imaginary THz conductivity, as one measures the actual THz electric field pulse instead of THz electromagnetic wave intensity. This facilitates one to measure directly the near-DC conductivity from THz spectroscopy. Thus, there have been continuous interests in finding the various sources and detectors in THz regime in the last 20 years, but its spectroscopy is still in its infancy and it is expected that there will be rapid increase in the efficiency and quality of spectroscopic techniques in the coming years. In the last few years, THz spectroscopy has occupied pivotal position in condensed matter physics, chemistry, and biology crossing boundaries in science. Some of the hot areas of research in this topic are (i) to understand how intense THz pulses interact with matter, and this requires thorough theoretical models to be developed and (ii) THz Quantum Cascade Lasers, (iii) THz metamaterials and (iv) THz spectroscopy in condensed matter, biology and chemistry

In this special issue of Asian Journal of Physics, our goal is to summarize the current state of research in THz spectroscopy in the areas mentioned above, and emphasize the present trends and future directions in this field. We plan to invite a series of research articles on THz spectroscopy and these articles may include a short/detailed (tutorial type) review article, or an original paper. The intent of this issue is to brain-storm new ideas and encourage further research into this promising field of THz Spectroscopy in general.

The topics to be considered for this special issue are as follows, but not limited to:

·         THz  spectroscopy

·         THz Metamaterials     

·         THz Plasmonics

·         THz Electronics/ Plasma Wave Detectors and THz mixers

·         THz Sources and detectors

·         THz Imaging in Physics, Chemistry and Biology

·         Quantum Cascade THz lasers

·         Intense THz Radiation

·         THz Spectroscopy of Quantum Materials

·         THz Spectroscopy in High Magnetic fields