AJP	ISSN : 0971 – 3093 Vol 29, Nos 8 & 9, August-September, 2020

Asian
Journal of Physics

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Vol 29, Nos 8 & 9, August-September, 2020

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Special Section
on
Terahertz Spectroscopy

Anita Publications
FF-43, 1st Floor, Mangal Bazar, Laxmi Nagar, Delhi-110 092, India

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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

Vol. 29 Nos 8 & 9, 2020, 00-00

 THz Sources and Detectors

S S Prabhu

The power emitted by THz sources at various frequencies has been quite low (of the order of few micro to milli-Watt) and there is a lot of work going on to increase it to several Watts say at 1 THz frequency. There are sources of THz which can give huge powers but they are to be operated at low temperature or require huge synchrotron like infrastructure or operate at very low frequencies in GHz. Till date, this seems a long dream to have a compact, room temperature operable, stable, economical high-frequency THz source. This paper gives a small review of several different Tera-Hertz (THz) sources and detectors. Different principles and designs are discussed about these sources and detectors. © Anita Publications. All rights reserved.

Keywords: Tera-Hertz, THz Spectroscopy, THz Sources, THz Detectors.

 Vol. 29 Nos 8 & 9, 2020, 00-00

On efficiency of laser beating scheme for tuned THz radiation generation in collisional plasma: Role of periodic magnetic field

Sheetal Punia and Hitendra K. Malik^*

We have analytically investigated the generation of terahertz (THz) radiation in collisional plasma using the lasers beating mechanism. The beating of two doughnut lasers producesa nonlinear current in plasma that oscillates at the beating frequency. Keeping in mind the fact that the magnetic field can compensate the collisional losses, we have applied the magnetic field in the plane perpendicular to the polarization of the incident lasers. The combined configuration of static and periodic fields violently affects the dynamics of the electrons inside the plasma and hence primarily affects the output power and efficiency of the mechanism of THz radiation generation. Besides this, the periodicity of the magnetic field also provides another knob to regulate the generated current and amplitude of the emitted THz radiation. For better understanding, we have exploited various laser and plasma parameters, such as beam width, beam order, and plasma density, to investigate the field of emitted THz radiation and the efficiency of the scheme. Besides, the importance of resonance condition and collisions among the electrons and neutral particles in the laser-plasma interaction region have also been explored.

Keywords: Terahertz radiation, collisions, doughnut lasers, nonlinear current, efficiency, and resonance.

 Vol. 29 Nos 8 & 9, 2020, 00-00

Bright multifocal terahertz (THz) radiation generation

Priyanka Chaudhary^a,b, Durvesh Gautam^a, Manendra^a, and Anil K Malik^a

^aDepartment of Physics, Chaudhary Charan Singh University Campus, Meerut-250 004, India

We propose a theoretical model for bright multifocal terahertz (THz) radiation generation by beating  two colour zeroth order Bessel beams (ZOBBs) in modulated plasma. THz field and efficiency are found maximum near ω₁ – ω₂ ≈ ω_p (i.e., at frequency resonance). The resonance condition is not affected by laser parameter b associated with zeroth order Bessel beams (ZOBBs). THz field amplitude and efficiency depend on the value of laser parameter b. The position and number of peaks can be controlled by wisely choosing the laser parameter b. Multifocal THz are quite useful in various applications like spectroscopy, imaging and medical diagnostics. © Anita Publications. All rights reserved.

Keywords: Terahertz (THz), Laser, Plasmas.

Total Refs : 35