Terahertz Spectroscopy Research Articles

Revealing the frequency-dependent oscillations in the nonlinear terahertz response induced by the Josephson current.

Nonlinear responses of superconductors to intense terahertz radiation has been an active research frontier. Using terahertz pump-terahertz probe spectroscopy, we investigate the c-axis nonlinear optical response of a high-temperature superconducting cuprate. After excitation by a single-cycle terahertz pump pulse, the reflectivity of the probe pulse oscillates as the pump-probe delay is varied. Interestingly, the oscillatory central frequency scales linearly with the probe frequency, a fact widely overlooked in pump-probe experiments. By theoretically solving the nonlinear optical reflection problem on the interface, we show that our observation is well explained by the Josephson-type third-order nonlinear electrodynamics, together with the emission coefficient from inside the material into free space. The latter results in a strong enhancement of the emitted signal whose physical frequency is around the Josephson plasma edge. Our result offers a benchmark for and new insights into strong-field terahertz spectroscopy of related quantum materials.

Ultrafast Long-Distance Electron-Hole Plasma Expansion in GaAs Mediated by Stimulated Emission and Reabsorption of Photons.

Electron-hole plasma expansion with velocities exceeding c/50 and lasting over 10ps at 300K was evidenced by time-resolved terahertz spectroscopy. This regime, in which the carriers are driven over >30 μm is governed by stimulated emission due to low-energy electron-hole pair recombination and reabsorption of the emitted photons outside the plasma volume. At low temperatures a speed of c/10 was observed in the regime where the excitation pulse spectrally overlaps with emitted photons, leading to strong coherent light-matter interaction and optical soliton propagation effects.

Generation of a Chirp-Controlled Terahertz Pulse by a Tapered Corrugated Wakefield Structure

Terahertz pulses with controlled spectral-temporal waveform have great potential in expanding the scope of state-of-the-art terahertz researches, such as terahertz-driven resonant and nonresonant control over matter and light, ultrafast terahertz spectroscopy, and terahertz-driven acceleration. Here, we propose and demonstrate a method to generate a prescribed terahertz chirped waveform with a well-designed tapered corrugated wakefield structure. We develop a deterministic procedure to derive the parameters of the corrugated structure that generates the desired spectral-temporal waveform with certain shape of chirp, central frequency, bandwidth, and pulse duration. The experimental results from ultrashort electron bunches passing through a well-designed tapered corrugated structure are demonstrated. Time-frequency analysis of the terahertz electric field measured by electro-optical sampling shows clear chirp from 0.38 to 0.6 THz. Calibrated measurements of the radiated energy indicate 10 \textmu{}J per pulse for an incident beam charge of 300 pC.

Spectroscopic Investigation of Structural Perturbations in CsPbCl3 Perovskite Nanocrystals: Temperature- and Excitation-Energy-Dependent Study

Inorganic lead halide perovskites are at the frontier of photovoltaic research due to their groundbreaking performance as an absorbing layer in solar cells. Cesium lead chloride (CsPbCl3), known to be a competent material for its blue emission, undergoes subsequent temperature-dependent phase transitions. The photophysical aspects reliant on temperature as well as the phase exhibited by the material are still unknown. Herein, we have studied the temperature- and excitation-energy-dependent underlying photophysics in CsPbCl3 nanocrystals (NCs) employing ultrafast transient absorption (TA) and terahertz (THz) spectroscopy. The direct dependence of carrier relaxation, carrier temperature, and energy loss on the phase transition has been established. Also, to comprehend the outcome of excitation energy, photoconductivity and mobility were determined with the help of optical pump THz probe spectroscopy, and it is established that as the excitation energy is reduced, mobility decay is found to be slow due to diminishing Auger recombination and augmented penetration depth.

Ben Ofori-Okai

Ben Ofori-Okai, a spectroscopist at the SLAC National Accelerator Laboratory, has some big questions about what’s happening at the center of planets. The secret lives of high-pressure and high-temperature environments hold the key to scientists’ long-standing questions about the evolution of our universe and even the elusive chemistry within nuclear reactors. To explore these exotic conditions, Ofori-Okai is taking spectroscopy to the extreme. Ofori-Okai specializes in a field called terahertz spectroscopy, which measures the vibrational energy of molecules as they interact with and emit terahertz radiation. This region of the electromagnetic spectrum, which lies roughly between infrared and microwave wavelengths, captures distinct vibrational modes of molecules that other spectroscopic measurements miss. For example, chemists can use terahertz spectra to reveal the chemical composition of molecular mixtures that might be indistinguishable by nuclear magnetic resonance or infrared spectroscopy, Ofori-Okai says. Yet chemists have been slow to adopt terahertz spectroscopy because “it’s

Simultaneous Terahertz Pulse Generation and Manipulation with Spintronic Coding Surface

AbstractThe utilization of spintronic terahertz (THz) emitters has a significant influence on broadband coherent spectroscopy and imaging at THz frequencies. By using femtosecond light to irradiate the spintronic heterostructures, spin currents from the ultrafast demagnetization effect can be transformed into charge currents through the inverse spin Hall effect, leading to the emission of THz waves. In this work, through the incorporation of sequential and inverse deposition of W‐Co20Fe60B20‐Pt heterostructure films, control is gained over the spin currents and THz waveforms, enabling it to achieve a 180° phase difference across a broad frequency spectrum while efficiently radiating THz waves. This, in turn, allows it to control the spatial phase of THz radiation. To test the device's capabilities, lithography‐based devices capable of focusing, producing vortexes, and dividing beams are manufactured. This investigation aims to advance the development of THz pulse sources while providing essential support for cutting‐edge technologies such as THz spectroscopy, high‐resolution imaging, and high‐speed communications.

Facile application of terahertz spectroscopy in UV-coated and phase change material loaded MPS

Facile application of terahertz spectroscopy in UV-coated and phase change material loaded MPS

Study on the effects of oxidation temperature and ambient humidity on the terahertz spectroscopy characteristics of lignite

In this paper, the terahertz (THz) spectroscopy characteristics of lignite with different oxidation temperatures under different ambient humidity were investigated by terahertz time-domain spectroscopy (THz-TDS), and the mechanism was analyzed by combining the synchronous thermal analyzer. Results indicate that the refractive index of lignite has abnormal dispersion characteristics. THz waves have different transmission characteristics in lignite with different oxidation temperatures, which is reflected in the amplitude difference of time-domain and frequency-domain signals. The complex refractive index of lignite shows obvious oxidation temperature dependence, which is closely related to the physicochemical changes that occur during coal oxidation. Besides, ambient humidity has a significant effect on the THz spectroscopy measurement, which follows the Beer-Lambert law. As humidity increases, the amplitude of time-domain and frequency-domain signals decays linearly, while the complex refractive index increase linearly. Humidity has a similar effect on coal samples with different oxidation temperatures, which means that based on linear effects and good fit results, THz spectroscopy measurements in humid environments can be corrected by a simple linear empirical formula. The results of this study are helpful to study the spectroscopy characteristics of coal in the THz band, and provide guidance for the application of terahertz technology in coal mines.

Transient gap generation in BaFe2As2 driven by coherent lattice vibrations.

Iron-based superconductors provide a rich platform to investigate the interplay between unconventional superconductivity, nematicity, and magnetism. The electronic structure and the magnetic properties of iron-based superconductors are highly sensitive to the pnictogen height. Coherent excitation of the phonon by femtosecond laser directly modulates the pnictogen height, which has been used to control the physical properties of iron-based superconductors. Previous studies show that the driven phonon resulted in a transient increase of the pnictogen height in BaFeAs, favoring an enhanced Fe magnetic moment. However, there are no direct observations on either the enhanced Fe magnetic moments or the enhanced spin-density wave (SDW) gap. Here, we use time-resolved broadband terahertz spectroscopy to investigate the dynamics of BaFeAs in the phonon-driven state. Below the SDW transition temperature, we observe a transient gap generation at early-time delays. A similar transient feature is observed in the normal state up to room temperature.

Quantitative Assessment of Cold Injury in Tea Plants by Terahertz Spectroscopy Method

Cold injury (CI) causes irreversible damage to tea plants, which results in decline in the quality of famous teas and huge economic loss. A new, quick, non-destructive method is provided to assess the CI of tea leaf based on terahertz (THz) spectroscopy. Absorbance of the samples was measured with THz spectroscopy in frequency bands from 0.1 to 1.6 THz under low temperature treatments of 4.0, 0, −2.5, −5.0, −7.5, and −10.0 °C. Fast Fourier transformation was explored to decompose the endpoint signal to improve the ratio of signal to air and convert the time-domain spectra to the corresponding frequency-domain spectra. To improve the separation of overlap signals caused by substantial scattering of injured cells in the leaf, two-dimensional correlation spectroscopy (2DCOS) and average intensity (AI) were introduced into the partial least squares regression (PLSR) to build 2DCOS–PLSR and AI–PLSR models. Quantitative assessments of the 2DCOS–PLSR and AI–PLSR models were conducted to evaluate the three models. The assessment results showed that the correlation coefficients of the 2DCOS–PLSR model (R2D) were 0.7873, 0.8305, and 0.9103, respectively. The root mean square errors of the 2DCOS–PLSR model (RMSE2D) were 0.6032, 0.5763, and 0.5221, respectively. For the AI–PLSR model, RAI values were 0.7477, 0.7691, and 0.8974, respectively. RMSEAI values were 0.6038, 0.5962, and 0.5797. The combination of THz spectroscopy with the 2DCOS–PLSR model provided a better benchmark for the input interval selection and improved the accuracy of cold-injury detection results.

Terahertz Spectroscopy Characterization and Prediction of the Aging Degree of Polyethylene Pipes Based on PLS.

Polyethylene (PE) is widely used in pipeline transportation owing to its excellent corrosion resistance, good stability, and ease of processing. As organic polymer materials, PE pipes inevitably undergo different degrees of aging during long-term use. In this study, terahertz time-domain spectroscopy was used to study the spectral characteristics of PE pipes with different degrees of photothermal aging, and the variation in the absorption coefficient with aging time was obtained. The absorption coefficient spectrum was extracted using uninformative variable elimination (UVE), successive projections algorithm (SPA), competitive adaptive reweighted sampling (CARS), and random frog RF spectral screening algorithms, and the spectral slope characteristics of the aging-sensitive band were selected as the evaluation indices of the degree of PE aging. Based on this, a partial least squares aging characterization model was established to predict white PE80, white PE100 and black PE100 pipes with different aging degrees. The results showed that the prediction accuracy of the absorption coefficient spectral slope feature prediction model for the aging degree of different types of pipes was greater than 93.16% and the verification set error was within 13.5 h.

Single-pulse terahertz spectroscopy monitoring sub-millisecond time dynamics at a rate of 50 kHz

Slow motion movies allow us to see intricate details of the mechanical dynamics of complex phenomena. If the images in each frame are replaced by terahertz (THz) waves, such movies can monitor low-energy resonances and reveal fast structural or chemical transitions. Here, we combine THz spectroscopy as a non-invasive optical probe with a real-time monitoring technique to demonstrate the ability to resolve non-reproducible phenomena at 50k frames per second, extracting each of the generated THz waveforms every 20 μs. The concept, based on a photonic time-stretch technique to achieve unprecedented data acquisition speeds, is demonstrated by monitoring sub-millisecond dynamics of hot carriers injected in silicon by successive resonant pulses as a saturation density is established. Our experimental configuration will play a crucial role in revealing fast irreversible physical and chemical processes at THz frequencies with microsecond resolution to enable new applications in fundamental research as well as in industry.

Free Electron Dynamics and Intrinsic Mobility in Ga2O3 Revealed by Transient Terahertz Conductivity

Here, we investigate the photoconductivity of gallium oxide thin films at different temperatures using time-resolved terahertz spectroscopy. The photogenerated electrons in the conduction band show a monoexponential decay, implying a first-order electron depopulation mechanism. The electron lifetime increases with rising temperature, and this trend coincides with the temperature dependence of the electron mobility rather than diffusion coefficient, suggesting that electron-hole recombination is determined by directional electron drift instead of random diffusion. The electron mobilities extracted from the transient terahertz conductivity are substantially greater than the previously reported Hall mobilities over a wide temperature range, and this is probably because the electron drift in response to the terahertz field is immune from scattering with macroscopic defects. Thus, the mobilities measured here may represent the intrinsic limit of the electron mobility in gallium oxide crystals. Our finding suggests that the current Hall mobility in this wide bandgap semiconductor is still far below the limit, and the long-range electron transport can be further increased by improving the crystalline quality.

Terahertz Dielectric Characterization of Three-Dimensional Organotypic Treated Basal Cell Carcinoma and Corresponding Double Debye Model

The potential of terahertz (THz) spectroscopy and imaging in skin cancer detection and mainly basal cell carcinoma (BCC), the most common type of skin cancer, was previously studied. Thanks to the significant absorption of THz waves by hydrogen bonds of water and the higher hydration of tumors than normal skin, it is considered an ideal non-invasive tool for early cancer detection. Despite this potential, there is a lack of study on the THz characterization of treated tumors with anticancer drugs to constitute innovative approaches for chemotherapeutic research, and this is an inspiration for the current study. Previous THz spectroscopy measurements are based on freshly excised tissue and suffer from environmental impacts on tissue and also limit performing a thorough analysis of the effect of different biological parameters. Thus, we have cultured a three-dimensional (3D) organotypic model of BCC tumor to study the THz electromagnetic parameters of tumor in different stages along with the impact of AG1478 anticancer drug on its THz spectra. Measurements are performed in the frequency range of 0.4 to 1.6 THz, and the corresponding refractive index, absorption coefficient, and complex permittivity of normal skin, BCC, and treated BCC have been compared. Furthermore, an enhanced combinational optimization algorithm is developed to calculate the parameters of the double Debye (DD) dielectric model to explain better the discrimination of treated and untreated tumors with normal skin.

Estimation Method of Molecular Weight of Polyethylene using Terahertz Spectroscopy

This paper uses terahertz (THz) spectroscopy for the first time as a simple method for estimating the molecular weight of a polymer across a very wide range. The viscosity average molecular weight (Mv) was measured and estimated as the molecular weight. Currently, to measure the Mv, a sample is dissolved in a solvent then measured its inherent viscosity. However, this procedure is complicated and time-consuming in practice. Additionally, as the molecular weight increases, the measurement accuracy decreases if Mv of a sample is above 3 million. Therefore, a new simple method is desired. A linear high-density polyethylene sample with an Mv range of 0.1 to 7.4 million was used. THz absorbance spectra of the samples were measured from 50 to 600 cm-1 (1.5 to 18 THz) then the multivariable analysis technique was applied. Consequently, a very high correlation coefficient (R2 ≈ 1) between the estimated and the measured Mv values were obtained across the range of the samples. This method is essential and suitable for industrial applications.

Exciton Formation Dynamics and Band‐Like Free Charge‐Carrier Transport in 2D Metal Halide Perovskite Semiconductors

AbstractMetal halide perovskite (MHP) semiconductors have driven a revolution in optoelectronic technologies over the last decade, in particular for high‐efficiency photovoltaic applications. Low‐dimensional MHPs presenting electronic confinement have promising additional prospects in light emission and quantum technologies. However, the optimisation of such applications requires a comprehensive understanding of the nature of charge carriers and their transport mechanisms. This study employs a combination of ultrafast optical and terahertz spectroscopy to investigate phonon energies, charge‐carrier mobilities, and exciton formation in 2D (PEA)2PbI4 and (BA)2PbI4 (where PEA is phenylethylammonium and BA is butylammonium). Temperature‐dependent measurements of free charge‐carrier mobilities reveal band transport in these strongly confined semiconductors, with surprisingly high in‐plane mobilities. Enhanced charge‐phonon coupling is shown to reduce charge‐carrier mobilities in (BA)2PbI4 with respect to (PEA)2PbI4. Exciton and free charge‐carrier dynamics are disentangled by simultaneous monitoring of transient absorption and THz photoconductivity. A sustained free charge‐carrier population is observed, surpassing the Saha equation predictions even at low temperature. These findings provide new insights into the temperature‐dependent interplay of exciton and free‐carrier populations in 2D MHPs. Furthermore, such sustained free charge‐carrier population and high mobilities demonstrate the potential of these semiconductors for applications such as solar cells, transistors, and electrically driven light sources.

Millimetre wave antenna for biomedical applications-a review

Terahertz represents the portion of electromagnetic wave between the microwave and IR region. Recent research has been focused on the application of Terahertz waves in biomedical applications such as cancer detection. The current diagnostic methods in cancer treatment involve imaging to MRI and biopsy. Due to the low photon energy that terahertz possess, there are no health threats caused in contrast to traditional methods. Cancer cells can be detected by monitoring the blood supply to the affected tissues and water level in the local tissues. This can be diagnosed using Terahertz imaging. THz radiation can be generated and detected using various commercialized systems. On analysis of various literatures, it is observed that Meta material antennas will be the suitable candidate to enhance the existing terahertz spectroscopy techniques since it has better focusing capabilities. This paper reviews the various implementation techniques of an antenna in THz range. The design and properties of the antenna are also investigated.

Observation of negative terahertz photoconductivity in strongly correlated electron-doped CaMnO3 thin film

Electron-doped Ca0.96Ce0.04MnO3 (CCMO) possesses a unique band structure and exhibits a giant topological Hall effect contrary to other correlation-driven manganites known for insulator-to-metal transition, magnetoresistance, complex magnetic order, etc. The interaction mechanisms among the fundamental entities and their dynamical evolutions responsible for this unusual topological phase are yet to be understood. Here, we employ time-averaged and sub-picosecond time-resolved terahertz (THz) spectroscopy to explore the low-energy steady-state and ultrafast carrier dynamics, respectively, to unravel the complexity of charge carriers during their transition from a non-equilibrium state to the ground state in CCMO thin film. The THz optical conductivity confirms the presence of dichotomic charge carriers, i.e. heavy and light carriers throughout the temperature range of 15–300 K. A rare observation of both positive and negative photoconductivities along with a sharp crossover between the two resolved to a few picoseconds of illumination confirms the formation of polaron with a lifetime of a few nanoseconds. These optical evidences of dichotomic charge carriers, along with manipulation of the sign of photoconductivity induced by dynamics of related quasiparticles could facilitate a new mechanism for ultrafast optoelectronic switching devices.

Determining changes in crystallinity of rice starch after heat-moisture treatment using terahertz spectroscopy

To investigate the potential of Fourier-transform terahertz (FT-THz) spectroscopy to follow crystalline structure changes in rice starch after heat-moisture treatment (HMT), we measured the crystallinity by X-ray diffraction (XRD) spectra and found its correlation with THz spectra. According to A-type crystal structure and Vh-type crystalline structure of amylose–lipid complex (ALC) in rice starch, crystallinity is divided into A-type and Vh-type. The intensity of second derivative spectra peak at 9.0 THz was highly correlated with both A-type and Vh-type crystallinity. Additionally, other three peaks at 10.5 THz, 12.2 THz, and 13.1 THz were also sensitive to Vh-type crystalline structure. These results indicate that after HMT, the crystallinity of ALC (Vh-type) and A-type starch can be quantified using THz peaks.

Strong-field THz radiation-induced curing of composite resin materials in dentistry.

Dental caries is listed as one of the three major non-communicable diseases by the World Health Organization, and its main treatment method is to restore it by filling it with resin. At present, the visible light-cure method has the problems of non-uniform curing and low penetration efficiency, which makes the bonding area easy to develop marginal leakages, thus leading to secondary caries and requiring repeated treatment. In this work, through the strong terahertz (THz) irradiation-weak THz detection technique, it is found that the strong THz electromagnetic pulses can accelerate the curing process of the resin, and the weak-field THz spectroscopy can be used to monitor this dynamic change in real time, which will greatly promote the potential application of THz technology in dentistry.