Time-domain Spectroscopy Technique Research Articles

Generation of femtosecond spin-polarized current pulses at Fe/MgO interface by quasi-static voltage

The generation of short spin-current pulses is essential for fast spintronic devices. So far, spin current pulses are generated by femtosecond laser pulses which drive spins from a ferromagnetic metal layer. However, the need for miniaturization, simplicity and energy efficiency favour electric-field control of spintronic devices over optic or thermal control. Here, we combine ab initio calculations of electronic density of states at MgO/Fe interface with continuous model for charge transport to investigate the dynamics of the spin-dependent potential. We demonstrate that the voltage-driven instability of the electronic band structure due to the electronic resonant states at the Fe/MgO interface results in the generation of the femtosecond spin-polarized current pulse with the spin polarization up to P=7 00 % that propagates from the interface to the bulk. The dynamics of the current pulses driven by the Stoner instability depends neither on the dielectric relaxation time nor on the details of how the instability is achieved by changing the voltage, i.e. as long as the voltage changes are slow (quasi-static) with respect to the time determined by the spin diffusion constant, being of the order of fs. The presence of the instability can be detected by THz time-domain spectroscopy or pump-probe techniques.

Fano resonance-integrated metal nanoparticles' enhanced sensing for pesticide detection.

The combined application of metasurface and terahertz (THz) time-domain spectroscopy techniques has received considerable attention in the fields of sensing and detection. However, to detect trace samples, the THz wave must still be enhanced locally using certain methods to improve the detection sensitivity. In this study, we proposed and experimentally demonstrated a fano resonance metasurface-based silver nanoparticles (FaMs-AgNPs) sensor. AgNPs can enhance the sensitivity of the sensor by generating charge accumulation and inducing localized electric field enhancement through the tip effect, thereby enhancing the interaction between the THz waves and analytes. We investigated the effects of four different contents of AgNPs, 10 µl, 20 µl, 30 µl and 40 µl, on the detection of acetamiprid. At 30 µl of AgNPs, the amplitude change of the FaMs-AgNPs sensor was more pronounced and the sensitivity was higher, which could detect acetamiprid solutions as low as 100 pg/ml. The FaMs-AgNPs sensor has the advantages of a simple structure, easy processing, and excellent sensing performance, and has a great potential application value in the field of THz trace detection and other fields.

THz dielectric properties of AlPO4-BPO4-SiO2 ternaries

0.05AlPO4-0.05BPO4-0.90SiO2 glass-ceramic and glass-free 0.45AlPO4-0.45BPO4-0.10SiO2 ceramic have been prepared by a solid-state reaction process and their terahertz (THz) dielectric responses were characterized by time-domain spectroscopy (TDS) techniques in the frequency range from 0.2 to 1.6 THz. Both samples demonstrate little dielectric dispersions in the (sub) THz frequency range, having dielectric permittivities (εr) of ∼3.35(7) and ∼4.06(8), respectively. The dielectric responses are dominated by the phonons and electronic polarizations in the (sub) THz frequency range. The contributions to the relative permittivities from electronic polarization εr ∞ are 1.85(4) and 1.99(4), which are ∼55% and ∼49% of the total εr at THz frequencies, respectively. The dielectric losses of both samples continuously increase with increasing frequency in the THz band, while the loss factor of glass-ceramic is one order of magnitude higher than that of glass-free ceramic. The glass-free sample exhibits extremely high Q×f values of 100,000–190,000 GHz in the THz band, being much higher than that of the glass-ceramic (∼7000–25,000 GHz).

Time Domain Reflectometry Technique using for Dielectric Relaxation Study of Biomolecules

We have established the Time Domain Reflectometry (TDR) technique that provides information regarding dielectric permittivity in the frequency range 10 MHz to 30 GHz. A time domain spectroscopy technique has been effectively used to acquire information on the essential and active parameters of molecular dynamics. The dimension of complex permittivity of Biomolecules (i.e. amino acids, enzymes and proteins) in the frequency range from 10 MHz to 30 GHz has been carried out using time domain reflectometry technique.

Inner defect detection of glass fiber reinforced polymer sandwich panel using pulsed terahertz imaging based on smoothing and derivative

We conducted an experimental investigation to localize and detect internal defects in a sandwich panel made of glass-fiber-reinforced polymer (GFRP) using terahertz (THz) time-domain spectroscopy and signal processing techniques, which included smoothing and derivative analysis to enhance the defect detection performance. A simulated defect was created using a peel ply and scanned using a pulsed THz scanner with a reflection geometry specifically designed for this purpose. The reflected THz wave was analyzed to improve defect detection performance through time-domain signal processing using smoothing and derivative analysis. To quantify the performance of the proposed signal processing method, we introduced the Defect to Noise Ratio and evaluated its effectiveness accordingly. As a result, noise was removed and defect detection performance was enhanced, enabling the visualization of foreign insert defects in the GFRP sandwich panel.

Li-ion battery charge transfer stability studies with direct current impedance spectroscopy

The safety of lithium-ion batteries is closely related to the charge transfer process inside the battery. With the rapid increase in battery usage, battery safety issues have become increasingly prominent. Due to technical limitations, the current battery thermal failure management system cannot realize risk early warning. Charge transfer resistance (Rct) is a macroscopic parameter characterizing the battery transport process, which can be detected by electrochemical impedance spectroscopy (EIS). Due to the high cost of EIS test equipment and high signal-to-noise ratio requirements, it is still not applicable to electric vehicles. In addition, Rct is affected by ambient temperature and battery aging, which brings challenges to online safety diagnosis. Here we propose a method to obtain the activation energy of a battery using direct current impedance spectroscopy (DCIS), which enables the stability diagnosis of the charge transport process. DCIS is a time-domain impedance spectroscopy technique. It uses the time constant characteristic of the internal resistor–capacitor network to detect battery parameters. Activation energy is the energy barrier for lithium-ions to cross the electrode/electrolyte interface and can indicate the stability of the charge transport process. Since activation energy is a microscopic parameter, it is not affected by temperature and aging, which is beneficial for battery safety diagnosis. The experimental results show that the charge transfer process of the sample cells with a state of health (SOH) greater than 80% is stable. When the SOH is less than 80%, the activation energy drops rapidly from 0.55 eV to 0.16 eV, indicating an increased risk of thermal runaway. Since DCIS supports online detection, it provides an option for safety diagnosis of electric vehicle batteries.

An Improved Theoretical Model to Extract the Optical Conductivity of Two-Dimensional Material from Terahertz Transmission or Reflection Spectroscopy

The technique of terahertz time-domain spectroscopy (THz-TDS) enables us to simultaneously determine the real and imaginary parts of optical parameters. However, it is still a challenge to extract the optical parameters of a two-dimensional (2D) material (or an ultra-thin film) on a substrate accurately and flexibly for an arbitrary incident angle and different polarization. By treating a 2D material as a conductive boundary without thickness, we propose an improved theoretical model to extract the optical conductivity of the 2D material on a substrate from THz transmission or reflection spectroscopy. Importantly, the effects of wave polarization, incident angle, and multiple reflections in the substrate are considered in our model and the analytical formulae associated with the optical conductivity of the 2D material are provided. Furthermore, we verify the validation of our model based on the THz transmission and reflection experiments for mono- and few-layer MoS2 on sapphire substrates. These results not only are of practical significance for investigating the THz properties of 2D materials but can also be extended to the situations of ultra-thin films and/or incoherent detection such as Fourier transform infrared spectroscopy.

Measurement of Stress Optical Coefficient for Silicone Adhesive Based on Terahertz Time Domain Spectroscopy

The bonding structure is affected by environmental loads during use, causing internal stress in the adhesive layer, which leads to the debonding and expansion of the bonding layer. Therefore, it is important to accurately measure the stress distribution of the bonding layer to assess the life of the bonding structure. In this study, based on the transmission and reflection terahertz time-domain spectroscopy (THz-TDS) technique, the stress optical coefficients of a silicone adhesive were measured, and the calculation models of the transmission and reflection stress optical coefficients were derived. In the reflection calculation model, the caliper THz thickness measurement method was proposed to compensate for the thickness change of the silicone adhesive, under tensile stress. Under the transmission THz-TDS stress optical coefficient calculation model, the stress optical coefficient C of the silicone adhesive is 0.1142 ± 0.0057 MPa−1, and the stress optical coefficient C of the reflective system is 0.1135 ± 0.0051 MPa−1. The test results show that the reflective THz-TDS can also be used to measure the optical stress coefficient of the material, which compensates for the shortcomings of the traditional transmission measurement method, and lays a foundation for the characterization of the internal stress of the adhesive layer of the adhesive structure.

Dielectric Behavior Investigation of Composite Materials Based on Epoxy Resin/Fe3O4/CaTiO3, SrTiO3 Using Mixture Laws

In the same matrix of epoxy resin, Fe3O4 powder was evenly mixed with one of two titanate powders, CaTiO3 or SrTiO3. The dielectric properties of these blends were evaluated using the measurement technique of time domain spectroscopy (TDS). During this work, low-frequency analyzes (DC − 300 MHz) were carried out, mainly focusing on the static permittivity and conductivity properties as a function of titanate concentrations. The results allowed us to confirm the validity and effectiveness of the modified Lichtenecker Laws (MLL). Significant agreement accord can be shown between the MLL-calculated and experimentally determined permittivity values for ternary combinations. The perspective of this research lies in the production of new materials for use in the field of telecommunications and in particular for microelectronic components.

Fabrication of frequency-selective surface by picosecond laser direct writing

In this paper, a terahertz (THz) frequency-selective surface (FSS) is fabricated through picosecond (ps) laser direct writing on the silicon-based silver film. The time-domain spectroscopy (TDS) technique is used to test its electromagnetic modulation performance. Based on the finite-difference time-domain method, CST microwave studio 2016 was used to simulate its resonance characteristic. By optimizing the process parameters, the square ring structures on the FSS with the size error below 2% are fabricated. It is found that the center frequencies of the experimental and simulated resonance peaks are nearly both at 0.293 THz, and the transmission is about 0.01%. From the simulation results, when the size error is controlled below 2 μm, the resonant frequency shift is about 2.2 GHz. Combining the simulation and experimental results, if the size error of inner square is controlled below 2%, it has little effect on the THz metasurfaces’ modulation performance. By optimizing the laser scanning strategy and processing parameters, ps laser direct writing provides a new choice for the convenient fabrication of THz metamaterials.

(Invited) Discovering Bottlenecks and Opportunities in Hybrid Solar Light Harvesting Systems By Ultrafast X-Ray Spectroscopy

A prerequisite for advancing hybrid solar light harvesting systems is a comprehensive understanding of photoinduced electronic dynamics across a wide range of spatial and temporal scales. Time-resolved spectroscopy techniques in the optical domain have proven instrumental in identifying critical timescales of fundamental processes, such as interfacial charge transfer, charge migration, and carrier lifetimes, which are intimately linked to a system's performance. New light source and instrument developments open opportunities to extend such studies into the X-ray regime. Due to their inherent elemental specificity and sensitivity to local electronic environments, time-domain X-ray spectroscopy techniques are uniquely suited to gain a microscopic picture of dynamics from well-defined reporter sites within often complex nanoscale light harvesting architectures. We will present new insights into fundamental photoinduced dynamics in several archetypical heterostructures based on femtosecond and picosecond time-resolved X-ray photoemission spectroscopy (TRXPS).The photoinduced transient charge redistribution in the model hybrid system of nanoporous zinc oxide (ZnO) sensitized by ruthenium bipyridyl chromophores is probed independently from the viewpoints of the molecular electron donor and the semiconductor acceptor.[1] Charge injection from the chromophore into the semiconductor is found to proceed via a transiently populated interfacial charge transfer (ICT) complex on an overall timescale of ~300 ps. Our results show that, even after release from the ICT states into the ZnO conduction band, the injected electrons remain localized within less than 6 nm from the interface, due to enhanced downward band-bending by the photo-injected charge carriers. This spatial confinement suggests that light-induced charge generation and transport in nanoscale ZnO photocatalytic devices proceeds predominantly within the defect-rich surface region, which may lead to enhanced surface recombination and explain their lower performance compared to titanium dioxide (TiO2)-based systems. The quantitative nature and surface sensitivity of TRXPS also provides direct access to the spatial separation of the holes remaining on the bipyridyl chromophores after charge injection from the semiconductor surface with sub-nm precision.Complementary femtosecond and picosecond TRXPS studies of photoinduced dynamics in copper-phthalocyanine(CuPc)-C60 heterojunctions provide a deeper understanding of both exciton migration pathways toward the interface as well as the absolute efficiency by which ICT states dissociate into separate charges.[2] The measurements reveal surprisingly efficient charge generation from triplet excitons and from the lowest lying ICT states, both of which were previously believed to be inactive during light-to-charge conversion.Photoinduced charge transfer dynamics between spherical gold nanoparticles (AuNPs) and a nanoporous film of TiO2 is monitored by picosecond TRXPS, providing direct access to the absolute photon-to-charge conversion efficiency and subsequent electron-hole recombination dynamics.[3] Preliminary results will be presented for the extension of the measurements into the ambient pressure regime, taking a first step toward the study of photoinduced interfacial chemistry by TRXPS in this model system for nanoplasmonic light harvesting applications.[1] S. Neppl et al., “Nanoscale Confinement of Photo-Injected Electrons at Hybrid Interfaces”, J. Phys. Chem. Lett. 12, 11951 (2021).[2] F. Roth et al., “Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS”, Nat. Commun. 12, 1196 (2021).[3] M. Borgwardt et al., “Photoinduced Charge Carrier Dynamics and Electron Injection Efficiencies in Au Nanoparticle-Sensitized TiO2 Determined with Picosecond Time-Resolved X-ray Photoelectron Spectroscopy”, J. Phys. Chem. Lett. 11, 5476 (2020). Figure 1

THz Multi-Mode Q-Plate with a Fixed Rate of Change of the Optical Axis Using Form Birefringence.

We report the design, fabrication and experimental validation of a THz all-dielectric multi-mode q-plate having a fixed rate of change of the optical axis. The device consists of space-variant birefringent slabs manufactured by 3D printing using melt-extruded Acrylonitrile Butadiene Styrene (ABS). The desired form birefringence is analytically evaluated and experimentally measured by the THz time domain spectroscopy technique. The manufactured q-plate design is characterized using a polarization-sensitive imaging setup. The full electric field spatial maps are acquired from the beam propagating through the q-plate. The device enables the realization of both radial and azimuthal vector beams at discrete frequency intervals by controlling the space-dependent orientation of the ordinary and extraordinary axes in the transverse plane with a multi-mode sequence.

Optomechanical Modulation Spectroscopy of Bound States in the Continuum in a Dielectric Metasurface

Elusive features in photonic and electronic devices can be detected by means of advanced, time-domain spectroscopic techniques. In this letter we introduce a novel kind of modulation spectroscopy, based on the optomechanical interaction of photonic and mechanical modes. Applying the technique to a Si metasurface and its drum-like mechanical modes, we detect narrow-band quasi-Bound-State-in-the-Continuum (q-BIC) modes close to normal incidence, where their measurement can be hindered by a high symmetry protection and undesired background modes. Showing a visibility enhancement of more than one order of magnitude, the optomechanical modulation spectroscopy can be an innovative tool for precise spectroscopy of a wide set of photonic devices, including the goal of measuring purely symmetry protected BIC resonances.

Broadband terahertz characterization of graphene oxide films fabricated on flexible substrates

Graphene oxide (GO)fabricated on various flexible substrates (Kapton and lamination sheets) are studied with the help of terahertz time domain spectroscopy (THz-TDS) technique. In this work, GO films are grown using laser scribing technology, popularly names as Laser Induced Graphene (LIG) also, in the ambient conditions making the process cost-effective as well as user friendly. Terahertz transmission study, along with the sample images, suggests the presence of optical anisotropy in the fabricated GO films for different directions of laser scribing. It is further observed that THz transmissions are dominated by the Fabry Perot oscillations along with the wave guide resonance mode within the broadband THz characterization window. Several frequency dependent parameters (refractive index, conductivities, optical densities) essential for designing flexible terahertz devices are experimentally extracted for 0.3 THz to 2.0 THz frequency range. In near future, the outcome of this work can be helpful in integrating GO with flexible terahertz technology.

Studies of leaf water content in smart agriculture using THz technologies: a review

In recent years, the concept of smart agriculture has entered our collective daily routine by radically modifying the methods by which crop monitoring was previously carried out. More precisely, the term smart farming focuses its attention on the use technologies already present on the market like, sensors (multispectral, hyperspectral, thermal and terahertz sensors), WNS (wireless sensor network) and drones capable to reduce the human work in the fields, optimizing at the same time the quantity and quality of the products limiting the use of resources, as water, fertilizers, pesticides and herbicides. Recently, one of the most important problem in the field of precision farming is the availability of water; unfortunately, this factor becomes more critical from year to year. To resolve this problem, the first step is to reduce consumption and rationalize the use of water by adjusting the water supply to the needs of the systems in order to increase its yield while saving money. In this respect, the development of non-destructive techniques operating in the THz spectral region has allowed to monitor in real time the water content present in leaves and plants. In fact, due to the strong water absorption and reflection in this spectral region, this feature can be exploited to detect the water content of leaves and plants helping us to intervene promptly in cases where the plant needs water, avoiding so does water stress, but above all trying to use our primary resource adequately by reducing waste. About this, Imaging and Time-Domain Spectroscopy (THz-TDS) techniques have been applied to monitor soil conditions, drought stress and presence of pathogens on the plants. In this review, we focus our attention on the latest research carried out on monitoring the water content present in leaves through THz technologies. Moreover, we pose particular attention in the description of our system set composed by a 97 GHz transmitter-receiver able to analyze the spatial distribution of the water inside a leaf.

Study of weak intermolecular interactions and vibrational modes in hydroxybenzoic acid isomers using terahertz spectroscopy and density functional theory techniques

In this study, the terahertz (THz) absorption spectra of 2-, 3-, and 4-hydroxybenzoic acid molecules were recorded in the frequency range of 0.6–2.0 THz using the terahertz time-domain spectroscopy (THz-TDS) technique. The characteristic absorption peaks of these three hydroxybenzoic acid isomers were identified. The peak frequencies of the 2-, 3-, and 4-hydroxybenzoic acid molecules were significantly different from each other. Apart from experimental studies, theoretical studies using the density functional theory (DFT) technique were also conducted on the unit cells of the hydroxybenzoic acid molecules for simulating the experimental THz spectral features. The results were in good agreement with the experimentally obtained results. The vibrational characteristics and the weak intermolecular interactions of the three hydroxybenzoic acid isomers were analyzed by studying the potential energy distribution (PED) and using the energy decomposition analysis (EDA) technique based on the molecular force-field (FF; EDA-FF) methods to better understand the THz absorption spectra on a molecular level. The studies helped to systematically and comprehensively interpret the origin of the characteristic absorption peaks. The data obtained by studying the PED were used to explain the absorption peak mechanism in the theoretical spectra of the three isomers. The theoretical spectra were predicted using the DFT technique. The results obtained using the EDA-FF method revealed that the vibrational modes of the 2-hydroxybenzoic acid system are primarily the angle between the vector and the plane, while the vibrational modes of the 3-hydroxybenzoic acid and 4-hydroxybenzoic acid systems were primarily the dihedral angle (torsion). Results obtained using the EDA-FF analysis and the visual molecular dynamics (VMD) visualization methods revealed that the weak interactions exhibited by the 2-, 3-, and 4-hydroxybenzoic acid systems were primarily dispersive in nature. The results presented in this study revealed that the intermolecular hydrogen bonds (HBs) exhibited an obvious resonance response and played key roles in generating the low frequency vibrations in the THz region.

Measurement of Stress Optic Coefficient for Thermal Barrier Coating Based on Terahertz Time-Domain Spectrum

The residual stress introduced inside the thermal barrier coating (TBC) top coating during manufacturing and service processes is one of the main causes of thermal barrier failure. Therefore, a nondestructive and accurate measurement of the residual stress in top coating is essential for the evaluation of TBC life. The terahertz time-domain spectroscopy (THz-TDS) technique, which is based on the calibration or measurement of the stress optical coefficients of the measured materials, is applicable to the measuring of internal stress of nonmetal materials. In this work, to characterize the internal stress in TBC, the stress optic coefficient of the TBC top coating was measured by reflection-type THz-TDS. First, the mechanics model for the internal stress measurement in a TBC top coating was derived based on the photoelastic theory. Then, the THz time-domain spectra of TBC specimens under different loadings were measured in situ by a reflection-type THz-TDS system. Finally, the unimodal fitting, multimodal fitting and barycenter methods were used to carry out the data processing of the THz time-domain spectral-characteristic peaks. By comparing the processed results, the results using the barycenter method were regarded as the calibrated stress optical coefficient of the TBC due to the method’s sufficient accuracy and stability.

Recent Advances in Terahertz Time-Domain Spectroscopy and Imaging Techniques for Automation in Agriculture and Food Sector

Recent Advances in Terahertz Time-Domain Spectroscopy and Imaging Techniques for Automation in Agriculture and Food Sector

Characterization of Proton-Irradiated Polyaniline Nanoparticles Using Terahertz Thermal Spectroscopy

In this study, we investigated the changes in the molecular structure of polyaniline (PANI) nanoparticles illuminated by a proton beam using terahertz (THz) thermal spectroscopy based on the terahertz time-domain spectroscopy technique. PANI nanoparticles in water were exposed to a proton beam of 35 MeV energy with a particle fluence of 1013 particles/cm2. The photothermal properties of this solution of PANI nanoparticles were characterized using THz thermal spectroscopy. We measured the changes in the amplitudes of the reflected THz pulses to identify the variations in temperature induced by the photothermal effects of the PANI nanoparticle solution. The amplitude of a reflected THz pulse of the PANI solution not exposed to the proton beam increased when illuminated by an infrared light source, whereas that of THz signals of the PANI solution exposed to the proton beam hardly exhibited any changes. This implies that the molecular structure of PANI nanoparticles can be varied by a proton beam with a particle fluence above 1013 particles/cm2.

Rapid 3D-Imaging of Semiconductor Chips Using THz Time-of-Flight Technique

In this study, we developed a rapid three-dimensional (3D) time-of-flight imaging tool for inspection of packaged semiconductor chips, using terahertz (THz) time-domain spectroscopy techniques. A high-speed THz system based on the optical sampling by cavity tuning technique is incorporated with a 2-axis galvano scanner to deliver a scanning speed of more than 100 Hz/pixel with a signal-to-noise ratio larger than 20 dB. Through the use of the Hilbert transformation, we reconstruct the 3D structure of the packaged chip in a nondestructive manner. Additionally, the use of frequency-selective imaging allows us to manipulate image resolution; the higher resolution was obtained when monitored using the higher frequency component. Further, using phase information, we were able to detect and identify defects in the packaged chip, such as the delamination area and epoxy-rich regions.