Employing Terahertz Time-domain Spectroscopy Research Articles

External bias dependent dynamic terahertz propagation through BiFeO3 film

Interactions of terahertz radiations with matter can lead to the realization of functional devices related to sensing, high-speed communications, non-destructive testing, spectroscopy, etc In spite of the versatile applications that THz can offer, progress in this field is still suffering due to the dearth of suitable responsive materials. In this context, we have experimentally investigated emerging multiferroic BiFeO3 film (∼200 nm) employing terahertz time-domain spectroscopy (THz-TDS) under vertically applied (THz propagation in the same direction) electric fields. Our experiments reveal dynamic modulation of THz amplitude (up to about 7% within 0.2–1 THz frequency range) because of the variation in electric field from 0 to 600 kV cm−1. Further, we have captured signatures of the hysteretic nature of polarization switching in BiFeO3 film through non-contact THz-TDS technique, similar trends are observed in switching spectroscopy piezoresponse force microscope measurements. We postulate the modulation of THz transmissions to the alignment/switching of ferroelectric polarization domains (under applied electric fields) leading to the reduced THz scattering losses (hence, reduced refractive index) experienced in the BiFeO3 film. This work indicates ample opportunities in integrating nanoscale multiferroic material systems with THz photonics in order to incorporate dynamic functionalities to realize futuristic THz devices.

Terahertz Spectroscopy Tracks Proteolysis by a Joint Analysis of Absorptance and Debye Model

Terahertz waves have attracted great attention in biomolecule research because of the fact that they cover the range of energy levels of weak interactions, skeleton vibrations, and dipole rotations during inter- and intramolecular interactions in biomacromolecules. In this study, we validated the feasibility of employing terahertz time-domain spectroscopy (THz-TDS) for the nondestructive and label-free monitoring of protein digestion. The acid protease, pepsin, was used at its optimal pH to hydrolyze bovine serum albumin. Correspondingly, the control group experiment was also conducted by adjusting the pH value to inactivate pepsin. The progress of these two experiments was tracked by a compact commercial THz-TDS for 1 h. On one hand, the reaction-time-dependent absorption coefficient was calculated, and a direct absorption coefficient analysis was completed. The results indicate that protein hydrolysis can be easily monitored over time by focusing on the variation tendency of the absorption coefficient from a macroscopic perspective. On the other hand, we explored the use of the Debye model to analyze the dielectric properties of the solution during protein hydrolysis. The results of the Debye analysis prove that it is possible to investigate in detail the microscopic dynamics of biomacromolecule solutions at the molecular level by THz-TDS. Our research examined the process of protein hydrolysis by a combination of absorption spectra and Debye analysis and demonstrated that terahertz spectroscopy is a powerful technology for the investigation of biomolecular reactions, with potential applications in variety of fields.

Magnetic field tuning of spin resonance in TmFeO3 single crystal probed with THz transient

TmFeO3, a canted antiferromagnet, has two intrinsic spin resonance modes in the terahertz (THz) frequency regime: quasi-ferromagnetic (q-FM) mode and quasi-antiferromagnetic (q-AFM) mode. Both the q-FM and q-AFM modes show strong magnetic field and temperature dependence. Hereby, by employing THz time-domain spectroscopy combined with external magnetic field and low temperature system, we systematically investigated the magnetic field induced frequency shift of q-FM and q-AFM modes as well as the temperature driven spin reorientation phase transition in TmFeO3 single crystal. In contrast to the isotropic temperature dependent two-mode, the magnetic field dependence of two-mode is strongly anisotropic: the magnetic field applied along c-axis (a-axis) can harden (soften) the spin resonance frequency of q-FM mode for Γ4 phase of TmFeO3, and the field applied along b-axis shows negligible frequency shift for the q-FM mode, with the q-AFM mode relatively stable. The present study provides solid evidence that the magnetic anisotropy in rare earth orthoferrite plays a dominant role in the q-FM mode and the occurrence of spin reorientation phase transition. With the magnetic anisotropic energy obtained from the temperature dependent q-FM and q-AFM mode frequencies, we can predict both magnetic field and temperature dependence of spin resonance in TmFeO3 single crystal via phenomenological analysis.

Terahertz-infrared spectroscopy of Ti4+-doped M-type barium hexaferrite

We report on the first detailed investigations of the terahertz-infrared dielectric response of potential multiferroic single crystalline Ti-doped M-type barium hexagonal ferrite, BaFe12-xTixO19, with concentrations of the dopant above (x = 1.15) and below (x = 0.62) the critical concentration that corresponds to changes of the charge compensation mechanism. Employing terahertz time-domain spectroscopy and Fourier-transform infrared spectroscopy, we performed measurements of the spectra of complex dielectric permittivity ε*(ν) = ε′(ν)+iε"(ν) at frequencies ν = 2-8000 cm−1 (0.06–240 THz) and in the temperature interval T = 10–300 K. The effect of aliovalent cation substitution on the low-energy response of barium hexaferrite is analyzed and discussed. A rich set of polarization- and temperature-dependent excitations is observed and their origin assigned to the electronic transition within the ground state 5E of the four-fold coordinated divalent iron, to relaxational dynamics of domains, and to phonon resonances.

Hydration shells of carbohydrate polymers studied by calorimetry and terahertz spectroscopy

We present a study of the hydration shells of some carbohydrate polymers of commercial and biological importance, namely, agave fructans, inulin, and maltodextrin, employing terahertz time-domain spectroscopy and differential scanning calorimetry. We observe that the hydration numbers calculated using terahertz spectroscopy are marginally higher than those of the calorimetric values. We attribute this discrepancy to the definition of hydration number, which in a way correlates with the physical process used to quantify it. The aqueous solutions show a non-proportional increase in the absorption coefficient and the hydration number, with a decrease in the carbohydrate concentration. We demonstrate that this behavior is consistent with the “chaotropic” or “structure breaking” model of the hydration shell around the carbohydrates. In addition, the study reveals that agave fructans and inulin have good hydration ability. Given the high glass transition temperature and good hydration ability, these carbohydrates may behave as good bio-protectants and hydrating additives for food and beverages.

Experimental and theoretical study of fingerprint spectra of 2-(4-fluorophenyl)benzimidazole and 2-(4-chlorophenyl)benzimidazole in terahertz range

Due to wide variety of biological and pharmacological activities of benzimidazole derivatives, the terahertz fingerprint spectra of 2-(4-fluorophenyl)benzimidazole and 2-(4-chlorophenyl)benzimidazole are researched by employing terahertz time-domain spectroscopy and density functional theory systematically. Although the substituent at the para position on the benzene moiety are the same family elements (F, Cl) in the periodic table, both experimental and theoretical results show that there are substantial differences in their fingerprint spectra in the range of 0.2–2.5 THz, such as the amount, amplitude and frequency position of absorption peaks. The validity of these results was confirmed by isolated-molecule and solid-state calculations based on density functional theory. The possible reasons of these differences are different intramolecular hyperconjugative interactions, different elongation of the corresponding bond lengths, different HOMO-LUMO energy gaps, as well as different atomic motions within in the unit cell owing to the electron-withdrawing effects of different halogen atoms at the para position on the benzene moiety. These results indicate the importance of this spectral range as a conformational fingerprint region where even minor changes in the molecular configuration lead to major differences in its THz absorption.

Research on the differences between 2-(2-Chlorophenyl)benzimidazole and 2-(4-Chlorophenyl)benzimidazole based on terahertz time domain spectroscopy

Due to wide variety of biological and pharmacological activities of benzimidazole derivatives, the differences between 2-(2-Chlorophenyl)benzimidazole and 2-(4-Chlorophenyl) benzimidazole were researched by employing terahertz time-domain spectroscopy and density functional theory systematically. Although the only difference between their molecular configurations is the arrangement of chlorine atom on chlorophenyl ring, there are distinctive differences in their fingerprint spectra in the range of 0.2–2.5THz, such as amount, amplitude, and frequency position of absorption peaks. The validity of these results was confirmed by the theoretical results simulated by using density functional theory. The possible reasons of these differences originate from the different van der Waals forces and the different dihedral angles of the molecules within crystal cell. These results indicate the importance of this spectral range as a conformational fingerprint region where even minor changes in the molecular configuration lead to major differences in its THz absorption.

Terahertz spectroscopic evidence of low-energy excitations in NdNiO3

Formation of charge-density-waves in ordered electronic phases (such as charge-order) is an emergent phenomenon in the perovskite class of correlated oxides. This scenario is visualized to prevail in exotic RNiO3 (R=rare-earth) nickelates in which the structure controls the incipient charge-order to form in weak localization limit. However, any consequent effect demonstrating these nickelates in rare category of charge-density-waves conductors with controlled charge-lattice interactions has been a fundamental challenge so-far. Here, we present first evidence of the charge-density-waves in a prototypical NdNiO3 system employing terahertz time-domain spectroscopy along selective crystal axes. A finite peak structure at 5meV in the terahertz conductivity displays all the characteristics of a charge-density-wave condensate. Contrasting charge-dynamics of collective charge-density-waves mode and Drude conductivity emerging, respectively, from orthorhombic and cubic symmetries disentangle charge-ordering from the insulating state, establish a novel structure-property cause-effect relationship, and present opportunities to harness these diverse attributes in oxide electronics.

Analysis of calcium carbonate for differentiating between pigments using terahertz spectroscopy

Calcium carbonate that is used as an art pigment exhibits strong absorption at approximately 3 THz. In this study, the authors investigated the relationship between the absorption and the condition of calcium carbonate crystals. By employing terahertz time-domain spectroscopy (0.5–4 THz), they verified that terahertz absorption energy depended on the crystal direction and crystal shape of the powder sample due to large birefringence. Further, the authors observed the difference in the crystal structure (calcite or aragonite) and the presence of impurities in natural calcium carbonate such as shells through terahertz absorbance spectra. The absorbance peak value of calcite at around 3 THz was four times as large as the peak value of aragonite. The absorbance spectral width increased because of the presence of these impurities. From the above observation, this study demonstrated that a certain kind of calcium carbonate crystal could be distinguished by terahertz spectroscopy.

C-axis Josephson plasma resonance observed in Tl_2Ba_2CaCu_2O_8 superconducting thin films by use of terahertz time-domain spectroscopy

We have unambiguously observed the c -axis Josephson plasma resonance (JPR) in high-critical-temperature (T(c)) cuprate (Tl(2)Ba(2)CaCu(2)O(8)) superconducting thin films, employing terahertz time-domain spectroscopy in transmission as a function of temperature in zero magnetic field. These are believed to be the first measurements of the JPR temperature dependence of a high-T(c) material in transmission. With increasing temperature, the JPR shifts from 705 GHz at 10 K to ~170 GHz at 98 K, corresponding to an increase in c-axis penetration depth from 22.4+/-0.6mum to 94+/-9mum . The linewidth of the JPR peak increases with temperature, which indicates an increase in the quasi-particle scattering rate. We have probed the onset of the c -axis phase coherence to ~0.95T(c) . The JPR vanishes above T(c) as expected.