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Anti-counterfeiting technologies of banknotes are essential to avoid financial fraud and maintain the stability of the socio-economic system. In recent years, the technologies used by counterfeiters have become more and more advanced, bringing great challenges to traditional methods of authenticity check. Therefore, the identification technologies of banknotes urgently need to be improved. Terahertz spectroscopy is widely used for analysis and material identification due to its properties (such as biomolecular fingerprinting spectrum, ultra-weak photon emission, non-destructive and time-resolution detection). In this paper, we apply the transmission terahertz imaging to the identification of anti-counterfeit labels of RMB. We designed and built a THz-TDS confocal imaging system, which has higher image resolution than traditional THz-TDS imaging systems. We used this system to perform terahertz imaging on anti-counterfeit labels of banknotes. The results show that our designed and built THz-TDS confocal imaging system not only provides higher image quality, but also can identify more details of the anti-counterfeit labels of banknotes.

We report a terahertz quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) emitting around 1.9 THz with up to 10% continuous fractional frequency tuning of a single laser mode. The device shows lasing operation in pulsed mode up to 102 K in a high-quality beam, with the maximum output power of 37 mW and slope efficiency of 295 mW/A at 77 K. Challenges for up-scaling the operating wavelength in QC metasurface VECSELs are identified.

Firstly, the electron density distribution of inductively coupled plasma (ICP) is measured by laser Thomson scattering (TS) method and the features of the ICP under the same experimental conditions are simulated by finite element method (FEM). The simulated results are in good agreement with the experimental results, which verifies the accuracy of the ICP generation simulation model. Secondly, the propagation characteristics of terahertz wave in ICP are measured by terahertz time domain spectroscopy (THz-TDS) and calculated by FEM according to the electron density distribution of ICP simulated in the first step above. The high consistency between the experimental and simulation results of terahertz wave propagation characteristics in ICP further proves the accuracy of terahertz wave transmission model in plasma and the feasibility of joint simulation with ICP generation simulation model.

In this work, the optical properties of several inorganic contaminants and organic pollutants in soil were characterized by using terahertz time-domain spectroscopy, such as heavy metals and microplastics. The main purpose of the present investigation is to provide a new technical approach for assessing the degree of soil pollutions and remediation. Terahertz wave is a kind of non-ionizing radiation with unique features located between microwave and infrared band. It has been widely utilized in many fields, such as the identification of heavy metals in organic pollutants heptachlor, DDT and inorganic pollutants in environmental science. Terahertz spectrum of soil samples subjected to different drying time was measured at first to explore the duration of moisture effects elimination. Based on the processing and analysis of the time-domain signal, the spectral absorption of wide band pulsed terahertz wave can be modulated by soil, hexavalent chromium and microplastics. The positive experimental results show that the absorption coefficient and refractive index of various samples corresponding to different types and proportion blends. Meanwhile, different samples have various absorption peaks in the terahertz region, which can be used to distinguish different contaminants. At the same time, the content of hexavalent chromium and the number of microplastics in the soil can be judged according to the degree of absorption. The experimental verification confirms that terahertz spectroscopy is a feasible, effective and promising method for the detection of soil pollutants with broad application prospects.

Ultrafast carrier dynamics, including the carrier photoexcitation and relaxation processes, plays an essential role in improving the performance of molybdenum disulfide (MoS2)-based optoelectronic devices. Herein, we investigate the photo-generated carrier dynamics in layered MoS2 crystal using a time-resolved terahertz (THz) spectroscopy. We have analyzed the ultrafast changes of the THz complex photoconductivity deduced from the peak and zero-crossing of THz waveforms. The decay time of the real part of the THz photoconductivity in layered MoS2 crystal is independent with the pump power, while the imaginary part increases with the pump power. We attribute the decay time of the real part to the carrier recombination process via phonon-assistance and the decay time of the imaginary part to the defect-assisted exciton recombination. The peak values of the complex photoconductivity show a trend of saturation with the increase of the pump power because of the many-body effect at high carrier concentration. This work deepens the understanding of the basic ultrafast physical process in MoS2 crystal, which is enlightening for the design of novel optoelectronic devices.

Early diagnosis of cervical cancer is essential for a good prognosis. Terahertz wave detection technology is a nondestructive and label-free physical detection technology, which can detect and monitor the cancer cells in real time, especially for patients with deep or inaccessible tumors. In this study, a single-cell-layer microfluidic device was developed. After replacing the optical clearing agent, the characteristics of H8, HeLa and SiHa cell lines in adherent and suspended states were detected. Additionally, the absorption increased with increasing cell density. For the mixed suspension cell samples, principal component analysis-support vector machine method was used to identify benign and malignant cell component. After living cells formaldehyde, changes in cell membrane permeability were evaluated to identify the cell survival status (i.e., dead or living) based on terahertz spectroscopy amplitude differences. Therefore, extending the terahertz spectrum detection to the molecular level can characterize the life essence of cells and tissues.

In recent years, with the continuous development of terahertz detection technology, the use of terahertz spectroscopy to study chemical samples has become one of the indispensable means in the field of biochemistry. However, the biological activity of most biomolecules can only be expressed in water solution, while water as polar molecules has strong absorption properties of THz wave. Therefore, it is difficult to study the activity of biological samples in water solution by using terahertz technology. In this study, a sandwich terahertz microfluidic chip with high transmission to terahertz wave was designed. The detection area of THz microfluidic chip is COC material, which is colorless and transparent, has no obvious absorption peak in THz frequency range, and has high THz wave transmission. The THz transmission of 2 mm thick COC can reach more than 90%. A valveless micropump was designed on the chip surface to realize automatic injection. The terahertz transmission spectra of distilled water, 0.9 mol/L NH4Cl, (NH4)2SO4, (NH4)2CO3 and CH3COONH4 were measured by combining the chip with terahertz technology in the frequency range of 0.1–0.7 THz. The experimental results show that at the same concentration, the THz spectra of different ammonium salt solutions are obviously different and the spectral intensity is lower than that of distilled water, which indicates that the hydration process of ions has an effect on the hydrogen bond between water molecules, and is reflected in the terahertz transmission spectrum.

The vibrational and rotational energy levels of many biomolecules are in the terahertz band, so terahertz technology can be used to detect biomolecules. In addition, because the photon energy of terahertz wave is low and will not destroy biological samples in the detection process, terahertz technology has a very wide application prospect in the future research fields such as biochemical detection and information communication. As we all know, most biomolecules need to be in liquid environment to give full play to their biological activity. However, the hydrogen bond in aqueous solution will produce strong absorption in terahertz band. In addition, water molecules are polar molecules, and terahertz wave has strong resonance absorption to polar molecules, which makes it very difficult to detect active biomolecules in liquid environment by terahertz technology. Therefore, many research teams combine terahertz spectroscopy with microfluidic technology to reduce the impact of various factors on biomolecular detection. In this study, the THz microfluidic chip was prepared with cycloolefin copolymer (COC). The five potassium salt solutions in a constant electric field at different times were studied, which provided a basis for further strengthening the application of THz technology in biochemistry. What’s more, the electrolyte solution contains a large number of anions and cations, which will move under the action of electric field. And it provides technical support for the study of the dynamic characteristics of electrolyte solution by terahertz technology.

Terahertz spectroscopy, with the advantages of label-free and non-ionizing has been considered as a potential method for biomolecule detection. Hereby we present a promising approach for the label-free protein sensing in liquid environment. In this work, we designed a grapheme-based device to increase the accumulated amount of protein molecules in the solution. The sensor demonstrates a quick and sensitive response to HER2 proteins and gave a linearly response to the concentrations of HER2 from 63 ng/ml to 4.0 μg/mL. The results demonstrated that once HER2 molecules targeting on the surface of graphene, the conductivity of graphene was significantly changed. This provided us a potential method to detect trace biomarker within liquid phase by using terahertz spectroscopy.

We experimentally demonstrated an optical controlled terahertz modulator based on the 3D Dirac semimetal Cd3As2 thin film (~100 nm, λ/3000). Utilizing the time-resolved terahertz spectroscopy, the photoactive characters of Cd3As2 thin film are investigated. The measurements reveal that Cd3As2 modulator exhibits a low optical threshold (< 63.5 μJ/cm2) and ultrafast (< 20 ps) broadband (0.2-1.5 THz) modulation of terahertz waves. Moreover, integrated with the metamaterials structures, Cd3As2 could potentially be used as photo-sensitive and ultrafast reconfigurable terahertz resonant plasmonic/meta-device.

Efficient devices for control properties of electromagnetic waves are essential for the development of terahertz (THz) technologies. But despite the great progress achieved in a study of graphene, the influence of the number of graphene layers on its properties in the THz frequency range has not yet been sufficiently studied. In this work, we experimentally studied properties of multilayer graphene (MLG) films in the frequency range 0.2–0.8 THz, at a room temperature, and a relative humidity of 40%. Using our custom-made THz time-domain spectroscopic polarimetry system, we obtained spectra of the complex relative permittivity and the electrical conductance of the chemical vapor deposition graphene with ∼14, ∼40, and ∼76 layers of graphene on glass substrates. It is shown that the conductance increases nonlinearly with an increase in the graphene layer number and reaches, for ∼76 layers, 0.06 S for the real, and 0.03 S for the imaginary part, respectively.

We demonstrate that titanium-doped lithium niobate strip optical waveguides can act as terahertz emitters. Terahertz pulses were produced by pumping with a femtosecond laser at a wavelength of 1.55 μm via the prism-coupled Cherenkov-phase matching scheme. Near-single cycle time-domain pulses were generated and the terahertz time-domain peak signal from the 40-μm waveguide was found to be ~18% higher than that observed with bulk crystal. The THz power spectra revealed a broad bandwidth and high signal-to-noise ratio. These findings signify the feasibility of titanium-in-diffusion to produce strong terahertz radiation from titanium-doped lithium niobate waveguides.

The linear complex refractive index of a set of borosilicate and tellurite as well as heavy metal oxide silicate, germanate and fluoride glasses has been determined using the Kramers-Kronig analysis on combined data from terahertz time domain (THz-TD) and Fourier transform infrared (FTIR) spectrometers in the ultrabroadband range of 0.15 THz to 200 THz. Debye, Lorentz and shape language modeling (SLM) approaches are applied. Far-infrared absorption power-law model parameters are determined via searching for the largest frequency range that minimizes the root mean squared error (RMSE) of a linear least squares fit for the set of glasses and other glass literature data. Relationships between the absorption parameters, glass properties and compositions are explored.

It is difficult for single-layer metal metasurfaces to excite in-plane component of magnetic dipole moment, so achieving giant intrinsic optical chirality remains challenging. Fortunately, displacement current in dielectric metasurfaces can form the in-plane magnetic moment which is not orthogonal to the electric dipole moment and forms intrinsic chirality. Here, we show a lossless all-silicon metasurface which achieves giant intrinsic chirality in terahertz band. The leaky waveguide mode in the chiral silicon pillars simultaneously excite the in-plane electric and magnetic dipole moments, which triggers the spin-selected backward electromagnetic radiation, and then realizes the chiral response. The theoretical value of circular dichroism in the transmission spectrum reaches 69.4%, and the measured one is 43%. Based on the photoconductivity effect of the silicon metasurface, we demonstrate optical modulation of the intrinsic chirality using near-infrared continuous wave. In addition, by arranging the two kinds of meta-atoms which are enantiomers, we show the spin-dependent and tunable near-field image display. This simple-prepared all-silicon metasurface provides a new idea for the design of terahertz chiral meta-devices, and it is expected to be applied in the fields of terahertz polarization imaging or spectral detection.

We report the magneto-optical Faraday response of bismuth-gadolinium-substituted rare-earth iron garnet at terahertz frequencies ranging from 100 GHz to 1.2 THz. The maximum transmittance of ±45° component is about 60% near the frequency point of 0.63 THz. When the external magnetic field change from –100 mT to +100 mT, the Faraday rotation angle is between –6° and +7.5°. The overall change of ellipticity is relatively small. The maximum value of the Verdet constant is about 260 °/mm/T at 0.1 THz and then gradually decreases to 80 °/mm/T at 1.2 THz. Within the considered frequency range, the thick film exhibits magnetically tunable, non-reciprocal characters and a strong magneto-optical effect within a small external magnetic field at room temperature, which will be widely used for the terahertz isolators, circulators, nonreciprocal phase shifters, and magneto-optical modulators.

THz waves have low photon energies (~ 4.1 meV for 1 THz), which is about 1 million times weaker than the energy of X-ray photons. They do not cause any harmful ionization in biological tissues. The terahertz radiation is strongly attenuated by water and is very sensitive to water content. The interest in terahertz imaging and spectroscopy of biologically related applications is increasing. This paper provides current status and recent advances in terahertz spectroscopy techniques in biological and medical applications. In particular, we report on our designs of THz Quantum-Cascade sources to identify cancerous tissues and other medical issues.

We experimentally demonstrate an electrically tunable superconducting device capable of switching the extraordinary terahertz (THz) transmission. The planar device consists of subwavelength hole arrays with real-time control capability. The maximum transmission coefficient at 0.33 THz is 0.98 and decreases to 0.17 when the applied voltage only increases to 1.3 V. A relative intensity modulation of 82.7% is observed, making this device an efficient THz switch. Additionally, this device exhibits good narrow-bandpass characteristics within 2 THz, which can be used as a frequency-selective component. This study offers an ideal tuning method and delivers a promising approach for designing active and miniaturized devices in THz cryogenic systems.

Water drives the spontaneous self-assembly of lipids and proteins into quasi two-dimensional biological membranes that act as catalytic scaffolds for numerous processes central to life. However, the functional relevance of hydration in membrane biology is only beginning to be addressed, predominantly because of challenges associated with direct measurements of hydration microstructure and dynamics in a biological milieu. Our recent work on the novel interplay of membrane electrostatics and crowding in shaping membrane hydration dynamics utilizing terahertz (THz) spectroscopy represents an important step in this context. In this Perspective, we provide a glimpse into the ever-broadening functional landscape of hydration dynamics in biological membranes in the backdrop of the unique physical chemistry of water molecules. We further highlight the immense (and largely untapped) potential of the THz toolbox in addressing contemporary problems in membrane biology, while emphasizing the adaptability of the analytical framework reported recently by us to such studies.

A versatile setup based on terahertz time-domain spectroscopy was developed to monitor the process of crystallisation and characterise the structure of the crystals as well as the dynamics of the solvent in situ. The setup can be used to investigate crystallisation processes over a wide range of temperature, either under static conditions or during continuous flow. An attached optical imaging probe can be used to monitor the process in parallel to the terahertz spectroscopy investigation. The implementation of terahertz spectroscopy offers a great opportunity to investigate the dielectric relaxation dynamics of the solvent and the vibrational modes characteristic for the crystalline phases simultaneously. The details of the crystallisation setup are demonstrated at the example of the crystallisation process of magnesium sulphate heptahydrate from its aqueous phase.

We experimentally studied the room-temperature polarization-resolved electrical conductance of multilayer graphene-based films using a home-made terahertz time-domain spectroscopic polarimetry system. The conductance increases nonlinearly with an increase in the graphene layer number and reaches, for ~76 layers, 0.06 S for the real, and 0.03 S for the imaginary part, respectively.

Semiconductor microcavities can exhibit various macroscopic quantum phenomena, including Bose-Einstein condensation of polaritons, Bardeen-Cooper-Schrieffer (BCS) states of polaritons, and photon lasing (lasing with negligible Coulombic exciton effects). An important aspect of possible experimental identification of these states is a gap in the excitation spectrum (the BCS gap in the case of a polaritonic BCS state). Similar to the polaritonic BCS gap, a light-induced gap can exist in photon lasers. Although polaritonic BCS states have been observed on the basis of spectroscopy in the vicinity of the laser frequency, the direct observation of polaritonic BCS gaps using light spectrally centered at or around the emission frequency has not been achieved. It has been conjectured that low-frequency (terahertz) spectroscopy should be able to identify such gaps. In this first of two studies, a theory aimed at identifying features of light-induced gaps in the linear terahertz spectroscopy of photon lasers is developed and numerically evaluated. It is shown that spectral features in the intraband conductivity, and therefore in the system's transmissivity and absorptivity, can be related to the light-induced gap. For sufficiently small Drude damping this includes spectral regions of THz gain. A future study will generalize the present formalism to include Coulomb effects.

Using terahertz spectroscopy, we investigate resonant transmission through a dielectric slab with a conducting layer of varied mobility on one side. We observe the well-known Fabry-P\'erot resonances in the transmission. Two fundamentally different regimes of the electrodynamic response of the system are established. In the clean limit, when the radiation frequency exceeds the carrier relaxation rate, $\ensuremath{\omega}\ensuremath{\tau}\ensuremath{\gg}1$, the system exhibits a plasmonic response. As a result, there is a continuous shift in the Fabry-P\'erot resonances with respect to the system parameters. By contrast, in the dirty regime, when $\ensuremath{\omega}\ensuremath{\tau}\ensuremath{\ll}1$, there is an abrupt $\ensuremath{\pi}$ shift in the resonance frequency, as the renormalized film resistance matches the impedance of free space. We find that under this condition, Fabry-P\'erot oscillations disappear entirely over a broad frequency range. Such a matching effect might be of interest in various terahertz applications, for example, involving broadband filters or absorbers.

Terahertz time-domain spectroscopy has been widely applied in performing dielectric analysis for various materials. However, air voids trapped in samples will significantly affect the characterization. In this study, the refractive index of mixture samples consisting of iron trioxide and polytetrafluoroethylene in five different mass ratios were measured with terahertz time-domain spectroscopy. In order to extract the intrinsic refractive index of iron trioxide, the effective medium model of CRI was then applied to remove refractive index of PTFE in the sample. The extracted refractive indices were presented as a variable parameter decrease with the increase of iron trioxide content, which also corresponds to the increase of air voids in the tablets. The correlation between trapped air voids and analyte composition roots from the coarse rust particles used in pellets compression. This study shows that the influence of air porosity should be considered when terahertz waves are utilized to characterize dielectric property of coarse material.

The mineral composition of kaolin is mainly composed of kaolinite, which was widely used in the production of ceramics and refractory materials. In this paper, terahertz time-domain spectroscopy is used to study the optical properties of kaolinite group clay minerals and the absorption coefficient and refractive index of calcined kaolin, washed kaolin and metakaolin were characterized in the terahertz band. The experimental results show that when the type, composition proportion and particle size of kaolin changed, the time-domain signal, absorption coefficient and refractive index of the terahertz wave will show regular changes. Through qualitative and quantitative analysis, the difference which is kaolin of different types, different particle sizes and different concentrations can be effectively distinguished. The research results show that the time-domain terahertz spectroscopy technology can be effectively applied to the mineral component analysis and has important significance in geoscience research.

We report a study of the low-frequency vibrational modes of the crystalline lipid bilayer of sodium 1,2-dimyristoyl-sn-glycero-3-phosphoryl-3′-rac-glycerol (DMPG) through an interplay between the low-temperature THz spectroscopy and solid-state density functional theory. The normal mode analysis shows the optimized structure is stable (but it may be one of the minima), and a consistent assignment of all the calculated THz bands to the observed ones has been achieved. This result represents the successful calculation of the largest molecule crystalline system, having ever been successfully calculated against the THz spectroscopic evidence to the best of our knowledge.

The crystallisation process of MgSO 4 in a crystallisation cell is investigated at different temperatures and concentrations by combining terahertz time-domain spectroscopy with optical analysis. The emergence and change of a spectral feature at 1.6 THz clearly reflect the progress of the crystallisation. Careful spectral investigation allows to extract information about the amorphous phase before and during the crystallisation. The versatile experimental set up is designed to be easily adjusted to the required conditions and can be utilised to investigate other systems. In addition, the set up can be applied to both static and continuous flow.

In this work, we applied terahertz near-field microscope based electro-optic sampling to animal tissue imaging. As a label-free diagnostic technique, fast THz near-field imaging system with a micrometer-scale spatial resolution can be constructed as an intraoperative diagnostic device for cancer clinical detection.

We explore the change in the collective hydration of water during the protein fibrillation (native →intermediate →fibril) of a model protein bovine serum albumin using terahertz spectroscopy. We also establish that the structural makeover of the protein during the conformational transition is followed by the associated hydration.

As researchers have begun to apply THz spectroscopy to increasingly novel materials and THz spectrometers have become more widely available, the need has arisen for data workup techniques that both avoid approximations and are accessible to a wide range of researchers. Here, we present Nelly, a software package that uses a tree-based algorithm to perform complex refractive index extraction for any layered sample geometry without invoking approximations. It is provided as a MATLAB library and graphical user interface (GUI) for ease of use. We have tested it using CST simulations to verify that it gives accurate results for various sample geometries.

Non-enzymatic glucose sensors were developed based on Au nanoparticles embedded in the hydrogel system. The sensors were soaked in glucose aqueous solutions with concentrations ranging from 0 to 1.5 mg/mL for 30 min. Terahertz time domain spectroscopy was then used to measure the absorption of the sensors. The Au nanoparticles can oxidize the glucose molecules to gluconic acid, resulting in a pH change of the samples. The sensitivity of the sensors to glucose levels are therefore enhanced compared to traditional terahertz spectroscopy measurements.

The high-performance broadband terahertz polarizers based on one-dimensional metal grids on carrying thin-film polymeric substrates were developed and manufactured. THz time-domain spectroscopy was used to study their power transmission (PT) and extinction ratio (ER) in the range of 0.2–2 THz. A tandem structure of two polarizers patterned on top of 4-μm-thick polypropylene film showed the best overall characteristics: PT > 95%, while ER exceeded the system dynamic range which spanned from 107:1 to 104:1. The developed polarizers can be used for polarization-sensitive measurements of anisotropic and optically active materials.

Terahertz time domain spectroscopy (THz-TDS) has been demonstrated to be capable of quantifying water uptake and retention properties of Nafion proton exchange membranes (PEMs). With a growing interest in thinner membranes, we analysis thin membranes to reveal its water uptake and retention properties.

Photodiode based emitters are established for continuous-wave terahertz spectroscopy and highly relevant for future wireless communication due to seamless integration in fiber-optical networks. For both applications, the directivity and the beam profile are of major interest. We investigated the radiation pattern experimentally between 100 GHz and 500 GHz. Radiation properties can be linked directly to the antenna structure due to the identical packaging and material of all emitters. We observed a large influence of the feeding point structure that connects photodiode and antenna. We demonstrate a new antenna with increased directivity and lower side lobes.

In this paper, Terahertz time domain spectroscopy system (THz-TDS) is introduced into the nondestructive testing of the defects of power cable insulation materials. Typical defects including air gap, conductive impurities and discharge channel. Samples of cable terminal insulation material with typical defects are tested using THz-TDS. Time-domain and frequency-domain spectrums of different defects are obtain and analysis. Finally, terahertz two-dimensional imaging test is carried out on cable terminal insulation materials with typical defects and defects are clearly visible in the imaging. Experimental results show that terahertz technology can be applied to the field of nondestructive testing of cable terminal insulation.

Three cocrystals of isoniazid with the aromatic carboxylic acids including p-nitrobenzoic acid, p-cyanobenzoic acid and p-aminobenzoic acid under slow evaporation condition have been synthesized by solvent evaporation. The above three cocrystals were analyzed and characterized by THz spectroscopy.

The vibrational and rotational energy levels of many biological macromolecules are in the terahertz band. Therefore, terahertz technology plays an important role in the field of biochemistry. However, due to the strong absorption of THz wave by hydrogen bonds in water, many research groups combine terahertz spectroscopy and microfluidic technology to reduce the influence of hydrogen bonding on biomolecules. We fabricated THz microfluidic chips with COC with high THz transmittance to reduce the interaction distance between THz wave and aqueous solution. Finally, the terahertz transmission characteristics of five different potassium salt solutions were studied by using the chip.

Hyperdoping semiconductor is one of the material candidates for intermediate band photovoltaics. We investigate implantation damage and associated defects. Time resolved terahertz spectroscopy reveals that charge carrier lifetime is hindered by defects not detected by conventional microscopic and spectroscopic techniques.

With the dissemination of new methods and applications in terahertz spectroscopy and terahertz nondestructive testing, the need for compact and low-cost systems is increasing to enable small companies and research groups to work in these fields. Therefore, we present a compact and inexpensive terahertz spectrometer, achieving dimensions of 300 mm x 270 mm x 110 mm, a bandwidth of 1.22 THz, and a peak dynamic range of 70 dB.

New concepts of sensing inspired by quantum optics provide a highly attractive alternative for accessing spectral regions in which photon detection is technically challenging. By using nonlinear interferometers, it is possible to transfer the properties of photons after interaction with a sample to visible photons. Then detection can be realized by widely available and highly developed imaging sensors without the need of cooling or expensive pulsed lasers. We report on the demonstration of spectroscopy in the terahertz frequency range using this new concept measuring absorption features of chemicals in the sub-terahertz frequency range by only detecting visible photons [1].

Frequency-domain terahertz (THz) spectroscopy using a continuously tunable photomixer is a powerful analytical technique, offering unique advantages such as high frequency resolution. Here, we report an improved method to increase the spectral resolution using a pair of fiber stretchers. In this method, the amplitude of transmitted THz waves was determined at each frequency by modulating the optical path difference. As a result, the spectral resolution of about 1 MHz was achieved in this study. We applied this technique to frequency-domain electron paramagnetic resonance (EPR) spectroscopy, and successfully observed EPR spectra with a 20-MHz line width.

The terahertz (THz) band, which corresponds to intermolecular binding energy, is the key to achieving a nonlabeled biosensor. To realize high-sensitivity binding detection, we focused on surface plasmon resonance (SPR) in the THz range. Using THz-SPR enhanced a in topological insulator, we expect to observe the synergistic effects of two resonance phenomena, namely intermolecular vibrational resonance and SPR. In this Letter, we report the nonlabeled detection of biomolecular binding by the topological insulator THz-SPR. B i 2 S e 3 was processed in a microribbon array to enhance the SPR in the THz range. The avidin–biotin specific binding, which is similar to the antigen–antibody reaction mechanism and has powerful interactions, was observed owing to enhancement by B i 2 S e 3 THz-SPR. This work paves the way for a system to directly measure specific molecular bonds using topological insulators.

International audience; The terahertz absorption spectra of sodium magnesium chlorophyllin (Chl-Mg-Na) and sodium copper chlorophyllin (Cu-Chl), two major members of the chlorophyll derivative family, have been measured in the range 0.2−3.0 THz (6.6−100 cm-1), at room temperature. Additionally, surface-enhanced Raman scattering spectroscopy was used to supplement data in the higher frequency range. The capability of terahertz spectroscopy for quantitative characterization of Chl-Mg-Na intermolecular vibrations was investigated and the sensitivity of the 1.82-THz feature with degree of hydration by changes in the molecular environment was examined. For Cu-Chl derivative, a broad feature was observed around 1.8 THz which currently hinders clear Cu-Chl identification and quantification.

We present multi-pulse time-resolved terahertz spectroscopy study of lead halide perovskite materials. By heating carriers with infrared pulse and observed the cooling process, the carrier cooling dynamics are measured in the context of complex photoconductivity as a function of time and frequency. The result indicates non-Dude conductivity and the is discussed in conjunction with theoretical study based on polaron conductivity.

Hydroxyproline (HYP) and pyroglutamic acid (PGA), as amino acid derivatives, are highly similar in structure to proline (Pro). However, their low-frequency vibrations show significant differences in the range of 0.25-2.6 THz. Therefore, this study investigated the reasons for the differences combined with terahertz time domain spectroscopy (THz-TDS) and density functional theory (DFT). The results show that HYP and PGA have stronger absorption of terahertz waves due to the existence of polar substituents. Furthermore, the absorption peaks of HYP and PGA are significant red shifted and blue shifted, respectively. We believe that this is caused by the change in the strength of intermolecular hydrogen bonds. Our findings demonstrate that dipole and hydrogen bond effects play a significant role in low-frequency vibrations.

Metallic nanostructures play an essential role in electromagnetic manipulations due to the localization and enhancement of electromagnetic waves in nanogaps. Scaling down the dimensions of the gap, such as the gap width and the thickness, is an effective way to enhance light-matter interaction with colossal field enhancement. However, reducing the thickness below 10 nanometers still suffers from fabrication difficulty and unintended direct transmission through metals. Here, we fabricate effective-zero-thickness slot antennas by stepping metals in the vicinity of the gaps to confine electromagnetic waves in tiny volumes. We analyze and simulate terahertz transmission, and demonstrate the absorption enhancement of molecules in the slot antennas. Our fabrication technique provides a simple but versatile tool for maximum field enhancement and molecular sensing.

Terahertz spectroscopy, combined with chemometric methods, has proved to be an effective tool in the quantitative analysis of many substances. However, current research has mainly focused on comparing different algorithms under the full spectrum. In fact, the full spectrum is not only composed of characteristic features of the samples, but also many types of noises. Hence, the accuracy of the quantitative analysis may be unsatisfactory if the full spectrum is selected. In this paper, a wavelength selection method based on partial least squares and absorption peak was proposed and an efficient frequency band was determined in the quantitative analysis of three types of pesticides, i.e., 6-benzylaminopurine, 2, 6-dichlorobenzonitrile, and imidacloprid. By introducing two parameters, the sum of peak intervals ( S i ) and peak width, the most efficient peak was selected from multiple peaks and the specific peak width was given with the aid of particle swarm optimization. We concluded that the most efficient absorption peak for quantitative analysis corresponding to the largest S i and full width near one-half maximum could characterize full spectrum information precisely. Comparing before and after wavelength selection, the correlation coefficient ( R ) of the three pesticides have increased from 0.9671, 0.9705, 0.9884 to 0.9921, 0.9934, and 0.9957. In conclusion, the proposed wavelength selection method was demonstrated to be very efficient for the quantitative analysis of the pesticide mixtures, which also could be helpful in the analysis of other multicomponent mixtures with absorption peaks.

Rapid and accurate detection of fluoroquinolones (FQs) antibiotics residues in food substrate are of great significance to food safety. In this study, terahertz spectroscopy was employed to conduct the spectral and imaging analysis to explore its feasibility for detection concentrations of FQs in fish meal feeds (FMF). Four methods (frequency of maximum difference in frequency-domain, characteristic absorption peak, successive projections algorithm and stepwise regression) were used to selected characteristic frequencies of sample. Terahertz imaging was formed at selected characteristic frequency and back propagation neural network was used to establish quantitative evaluation models to select optimal characteristic imaging frequency. Finally, relationship between concentrations of FQs and their gray values was revealed, and each data had a small range of error bar. The results showed that terahertz spectral and imaging technique can visualize norfloxacin and enrofloxacin concentrations in FMF precisely. This study explored a brand-new visualization method for quantitative detection of FQs in FMF based on terahertz spectral and imaging technology.

Terahertz (THz) spectroscopy is an emerging technology that is that is bringing a number of technical breakthroughs in several scientific applications. This review aimed to describe potential applications of THz spectroscopy at the biochemistry and molecules detection for food industry, environment monitoring and diagnostics, and present the importance of such technological platform in disease control and Public Health.

Terahertz technology is receiving increasing attention for its use as an efficient non-destructive, non-contact and label-free optical method for qualitative and quantitative detection. The aim of this study was to develop a chemical analysis methodology based on terahertz time-domain spectra that could be used to detect plant growth regulators, such as glyphosine, naphthaleneacetic acid, daminozide and gibberellic acid. The THz fingerprint spectra of these four PGRs were located in the 0.3–1.8 THz, with the peaks of glyphosine at 0.32, 0.49, 0.74, 0.87, 0.96, and 1.49 THz; daminozide at 0.33, 0.39, 0.55, 0.67, and 1.17 THz; gibberellic acid at 0.46, 0.58, 0.92, and 1.38 THz and naphthaleneacetic acid at 0.43, 0.57, 0.73, and 0.90 THz. The results showed that these four plant growth regulators exhibited numerous distinct spectral features in frequency-dependent absorption spectra, which demonstrated the qualitative capacity of terahertz time-domain. The origin of the observed terahertz absorption peaks of these four plant growth regulators was determined through density functional theory calculations and analysis of absorption spectra. Discriminant analysis method was used to evaluate the classification trends of the four plant growth regulators based on their THz absorbance spectra. Generally, this study provides a reference for the rapid detection of plant growth regulators in fruits and vegetables by using terahertz spectroscopy technology.