Abstract : Terahertz (THz) refers to the electromagnetic spectrum that lies between microwave and infrared light, spanning frequencies from approximately 0.1 to 10 THz. A terahertz wave has a wavelength ranging between 30 micrometres and 3 millimetres, characterized by its unique ability to penetrate various materials without causing damage or harm. This spectral region has garnered immense attention due to its combination of non-invasive properties and its ability to unveil hidden structural, chemical, and electronic information. Terahertz radiation is a relatively underexplored part of the electromagnetic spectrum. It occupies the frequency range between radio waves and infrared light, often referred to as the “terahertz gap” due to historical challenges in generating and detecting THz waves efficiently. Advances in ultrafast optics and semiconductor technology, enabled the development of terahertz systems that are now critical tools in scientific research and industrial applications.

Abstract : Terahertz (THz) technology has established itself as an effective non-destructive technique for the detection of various materials. Over the last decade, the effectiveness and accuracy of this technology have been demonstrated extensively in various applications starting from structural health monitoring, quality control to non-destructive testing. The THz industry is rapidly moving towards a more customized, rugged, turnkey THz systems that can be used in various industrial applications. This article presents a comprehensive overview of non-destructive testing applications of THz imaging and spectroscopy. It also explains the fundamentals of Terahertz radiation and its applicability in various real-world scenarios such as coating thickness, detection of internal defects, debonds and delamination of composites, quality monitoring in pharmaceutical tablets, defect detection of electronic components etc. This paper also discusses the challenges deploying current THz systems in the real-world applications and way forward to overcome these challenges.

Abstract : β-dicalcium silicate (β-C2S), the second most prevalent constituent of Ordinary Portland Cement (OPC), reacts slowly during the early stage of hydration but is responsible for long-term strength development of concrete. To accelerate the hydration during this stage, nanomaterials are incorporated in the β-C2S matrix which results in pozzolanic reactions. In this work, nanosilica has been added in different percentages, and the acceleration in hydration has been tracked by employing Terahertz spectroscopy for a period of 100 days. This study clearly demonstrates that nanosilica incorporation accelerates the early-stage hydration of β-C2S as seen by the reduction in intensity of the resonances between 500 and 520 cm−1 . Density functional theory (DFT) simulations of β-C2S confirm these resonances to be due to SiO4 and CaO bending modes. The prominence of the resonance around 455 cm−1 for the nanosilica-incorporated β-C2S indicates earlystage formation of the key hydration product, calcium silicate hydrate (C–S–H). Furthermore, sharpening of the resonance around 283 cm−1 during hydration could possibly indicate the formation of more ordered structures of the hydration products. By tracking the shift of these resonances, it can be inferred that the degree of polymerization of silicates during C–S–H formation is different compared to cement and C3S. To track the structural changes during hydration, scanning electron microscopy (SEM) has been carried out which shows the formation of fiber-like morphologies, indicative of early-stage formation of C–S–H for the nanosilicaincorporated samples. This study could potentially help to engineer β-C2S-rich cement matrix with nanomaterials which could aid in reducing CO2 emission during cement manufacturing.

Abstract : N-(pyridin-2-yl) benzamide (Ph2AP)-based organic molecules with prominent terahertz (THz) signatures (less than 5 THz) have been synthesized. The THz resonances are tuned by substituting the most electronegative atom, fluorine, at ortho (2F-Ph2AP), meta (3F-Ph2AP), and para (4F-Ph2AP) positions in a Ph2AP molecule. Substitution of fluorine helps in varying the charge distribution of the atoms forming hydrogen bond and hence strength of the hydrogen bond is varied which helps in tuning the THz resonances. The tuning of lower THz resonances of 2FPh2AP, 3F-Ph2AP, and 4F-Ph2AP has been explained in terms of compliance constant (relaxed force constant). Four-molecule cluster simulations have been carried out using Gaussian09 software to calculate the compliance constant of the hydrogen bonds. Crystal structure simulations of the above molecules using CRYSTAL14 software have been carried out to understand the origin of THz resonances. It has been observed that THz resonances are shifted to higher frequencies with stronger hydrogen bonds. The study shows that 3F-Ph2AP and 4F-Ph2AP have higher hydrogen bond strength and hence the THz resonances originating due to stretching of intermolecular hydrogen bonds have been shifted to higher frequencies compared to 2F-Ph2AP. The methodology presented here will help in designing novel organic molecules by substituting various electronegative atoms in order to achieve prominent THz resonances.

Abstract : Cement hydration is a process involving simultaneous reactions of cement constituents, primarily tricalcium silicate (C3S) and dicalcium silicate (C2S), with the formation of key hydration products, calcium silicate hydrate (C–S–H) and calcium hydroxide (Ca(OH)2 ). Compared to the conventionally explored mid-infrared spectroscopy which is bond specific, terahertz (THz) spectroscopy is highly sensitive to crystalline arrangements and resonances in THz frequency range are primarily due to bulk vibrational modes. Hence, THz spectroscopy can be an effective complimentary tool to study the hydration process as C3S gets converted to different polymorphs of C–S–H. To understand the origin and variation of THz resonances of C3S, C–S–H polymorphs and Ca(OH)2, vibrational modes of C3S, tobemorite 9, tobermorite 14, jennite, and portlandite have been calculated using density functional theory simulations. The origin of the main resonances has been studied using vibrational density of states. Simulations show, for C3S, the resonance around 520 cm−1 appears due to combined effect of symmetric and asymmetric vibrations in SiO4 tetrahedra, the resonance around 450 cm−1 appears due to the combined effect of symmetric and asymmetric SiO4 tetrahedra, and CaO vibrations and the resonance around 318 cm−1 is primarily due to CaO vibrations. THz spectroscopy has been performed to track and understand the contribution of C3S in cement hydration. By combining the simulation and experiments, this work clearly explains the reduction of 520 cm−1 resonance, the constant intensity of 450 cm−1 resonance and frequency shift of the main resonances as C3S is transformed into various polymorphs of C–S–H during hydration.

Abstract : It is in the strategic interest of a nation aspiring to be a regional power to develop an indigenous and internationally competitive defence industry base. Presently, India is one of the largest importers of conventional defence equipment. According to government statistics, roughly 60 percent of India’s defence requirements are met through imports. India has the potential to emerge as a global platform for defence research, manufacturing, supply chain sourcing, software development, and offsets, with the right kind of policy interventions. The Small and Medium Enterprises (SMEs) sector is critical for the success of these flagship initiatives, especially the ‘Make in India’, as a massive number of ancillary units for large manufacturing plants come from this sector. With the increasing technological intensity of defence platforms, expectations for a self-reliant India can be met from the SMEs, provided they can graduate to become reliable suppliers for the defence supply chain and become a source of innovation. This can happen with the right kind of policy intervention and implementation to overcome the woes of the past and to address the new challenges of a “knowledge-based economy”, manifesting in industry through a revolution termed as Industry 4.0.

Abstract : Spectral signatures in the terahertz (THz) frequency region are mainly due to bulk vibrations of the molecules. These resonances are highly sensitive to the relative position of atoms in a molecule as well as the crystal packing arrangement. To understand the variation of THz resonances, THz spectra (2−10 THz) of three structural isomers: 2-, 3-, and 4-cyanobenzaldehyde have been studied. THz spectra obtained from Fourier transform infrared (FTIR) spectrometry of these isomers show that the resonances are distinctly different especially below 5 THz. For understanding the intermolecular interactions due to hydrogen bonds, four molecule cluster simulations of each of the isomers have been carried out using the B3LYP density functional with the 6-31G(d,p) basis set in Gaussian09 software and the compliance constants are obtained. However, to understand the exact reason behind the observed resonances, simulation of each isomer considering the full crystal structure is essential. The crystal structure of each isomer has been determined using X-ray diffraction (XRD) analysis for carrying out crystal structure simulations. Density functional theory (DFT) simulations using CRYSTAL14 software, utilizing the hybrid density functional B3LYP, have been carried out to understand the vibrational modes. The bond lengths and bond angles from the optimized structures are compared with the XRD results in terms of root-meansquare-deviation (RMSD) values. Very low RMSD values confirm the overall accuracy of the results. The simulations are able to predict most of the spectral features exhibited by the isomers. The results show that low frequency modes are mediated through hydrogen bonds and are dominated by intermolecular vibrations.

Abstract : Terahertz (THz) frequency band lies between the microwave and infrared region of the electromagnetic spectrum. Molecules having strong resonances in this frequency range are ideal for realizing "Terahertz tags" which can be easily incorporated into various materials. THz spectroscopy of molecules, especially at frequencies below 10 THz, provides valuable information on the low frequency vibrational modes, viz. intermolecular vibrational modes, hydrogen bond stretching, torsional vibrations in several chemical and biological compounds. So far there have been very few attempts to engineer molecules which can demonstrate customizable resonances in the THz frequency region. In this paper, Diamidopyridine (DAP) based molecules are used as a model system to demonstrate engineering of THz resonances (< 10 THz) by fine-tuning the molecular mass and bond strengths. Density Functional Theory (DFT) simulations have been carried out to explain the origin of THz resonances and factors contributing to the shift in resonances due to the addition of various functional groups. The design approach presented here can be easily extended to engineer various organic molecules suitable for THz tags application.

Abstract : Turmeric is a common spice used as a vital ingredient in Ayurvedic medicines and food. Adulteration of turmeric with chalk powder causes severe health problems in humans. Conventional methods of identifying adulterants via chemical reactions are inaccurate. Here, we use Terahertz spectroscopy to effectively identify adulteration of turmeric with chalk powder. This method shows good potential for nonintrusive detection of adulterated spices and foods in packages.