THz Absorption Research Articles

Frequency-resolved measurement of two-color air plasma terahertz emission

We investigated the far-field terahertz beam profile generated from an air plasma induced by two-color femtosecond laser pulses. Under our experimental conditions (filament length shorter than the dephasing length between the two-color pulses), using electro-optic sampling in both ZnTe (0.2-2.2 THz) and GaP (0.4-6.8 THz) crystals, and ultra-broadband ABCD technique (1-17.5 THz), we determined that the THz beam exhibits a unimodal beam pattern below 4 THz and a conical one above 6 THz. This experimental finding is consistent with theoretical studies based on the unidirectional pulse propagation equation, which predict the transition of THz emission from a flat-top profile to a conical one due to the destructive interference of THz waves emitted from the plasma filament. Our results also underscore the importance of accounting for experimental artifacts, such as photo-excited losses in silicon resulting in on-axis THz absorption along with the influence of drilled mirrors, in characterizing the complex spatial and frequency-dependent behavior of two-color plasma-induced terahertz emission.

Stretchable MWCNTs-OH/PDMS composite elastomer with hierarchical porous structure for wideband THz absorption

It is important to develop a wideband THz absorber for the prevention of terahertz electromagnetic pollution and information leakage. Some commonly used methods, such as metamaterials or dynamic modulation technology, have played important roles in the study of wideband THz absorbers. However, most of these absorbers are rigid or non-stretchable, which limits the practical applications in large mechanical deformation and non-plane scenarios. In the paper, we proposed a stretchable MWCNTs-OH/PDMS composite elastomer using the sugar template method. A series of characterization methods were carried out to verify the excellent compatibility of PDMS and MWCNTs-OH. Benefiting from its hierarchical porous structure, the draw-length ratio of sample and tensile strength reaches ∼ 190% and ∼ 0.4 MPa, respectively. More THz waves could be trapped inside the sample for conductive loss because of better impedance matching and conductivity loss (the smallest square value is ∼0.4 kΩ/□), which leads to wideband and high absorptivity (over 98% in 0.2∼2.0 THz range). Furthermore, the tested value of EMI SE was over 10 dB in 0.22∼1.82 THz frequency range at transmission mode. Combined with inherent hydrophobicity characteristics, the material also could be used in humid and rainy environments. The systemic study of stretchable MWCNTs-OH/PDMS composite elastomer will provide theoretical and technical support for subsequent THz absorption in practical scenarios.

Dual-Tuned Terahertz Absorption Device Based on Vanadium Dioxide Phase Transition Properties.

In recent years, absorbers related to metamaterials have been heavily investigated. In particular, VO2 materials have received focused attention, and a large number of researchers have aimed at multilayer structures. This paper presents a new concept of a three-layer simple structure with VO2 as the base, silicon dioxide as the dielectric layer, and graphene as the top layer. When VO2 is in the insulated state, the absorber is in the closed state, Δf = 1.18 THz (absorption greater than 0.9); when VO2 is in the metallic state, the absorber is open, Δf = 4.4 THz (absorption greater than 0.9), with ultra-broadband absorption. As a result of the absorption mode conversion, a phenomenon occurs with this absorber, with total transmission and total reflection occurring at 2.4 THz (A = 99.45% or 0.29%) and 6.5 THz (A = 90% or 0.24%) for different modes. Due to this absorption property, the absorber is able to achieve full-transmission and full-absorption transitions at specific frequencies. The device has great potential for applications in terahertz absorption, terahertz switching, and terahertz modulation.

Fluorescence changes in carbon nanotube sensors correlate with THz absorption of hydration

Single wall carbon nanotubes (SWCNTs) functionalized with (bio-)polymers such as DNA are soluble in water and sense analytes by analyte-specific changes of their intrinsic fluorescence. Such SWCNT-based (bio-)sensors translate the binding of a molecule (molecular recognition) into a measurable optical signal. This signal transduction is crucial for all types of molecular sensors to achieve high sensitivities. Although there is an increasing number of SWCNT-based sensors, there is yet no molecular understanding of the observed changes in the SWCNT’s fluorescence. Here, we report THz experiments that map changes in the local hydration of the solvated SWCNT upon binding of analytes such as the neurotransmitter dopamine or the vitamin riboflavin. The THz amplitude signal serves as a measure of the coupling of charge fluctuations in the SWCNTs to the charge density fluctuations in the hydration layer. We find a linear (inverse) correlation between changes in THz amplitude and the intensity of the change in fluorescence induced by the analytes. Simulations show that the organic corona shapes the local water, which determines the exciton dynamics. Thus, THz signals are a quantitative predictor for signal transduction strength and can be used as a guiding chemical design principle for optimizing fluorescent biosensors.

Label-free detection of inclusion body formation in E. coli with application of terahertz

Extensive production of recombinant protein is essential for use in various fields. Bacterial cells, especially the Escherichia coli (E. coli) expression system, are widely used for the production of recombinant proteins due to their advantages in time and cost. However, the formation of inclusion bodies (IBs) caused by the production of recombinant protein is a significant issue. Therefore, a precise tool for the detection of IBs is in high demand. In this study, we demonstrated a label-free and rapid sensing platform for the detection of IBs formation in E. coli using terahertz-time domain spectroscopy (THz-TDS) system and terahertz metamaterials. The detection was based on the local polarity difference between E. coli samples with induced (positive) and non-induced (negative) protein overexpression. The formation of IBs in positive samples resulted in a decrease in local polarity compared to negative samples. This local polarity difference, strongly related to intermolecular interactions, influenced THz absorption as the THz regime involves weak intermolecular vibrational modes. Experimental results showed that the THz transmittance (ΔT) and resonance frequency shift (Δf) were significantly different between positive and negative samples, confirming the effectiveness of our method. Specifically, positive samples exhibited a ΔT value of 26.57% and a Δf value of 0.12 THz, while negative samples showed a ΔT value of 31.83% and a Δf value of 0.14 THz. These results highlight the capability of our platform to detect IBs rapidly and accurately.

Temperature dependence of THz generation efficiency, THz refractive index, and THz absorption in lithium-niobate around 275 GHz

In this study, we demonstrate the capabilities of the pulse train excitation approach in determining key material properties of nonlinear crystals, such as refractive index, thermo-optic coefficient, and absorption. The method provides reliable results even at relatively low THz frequencies, where other characterization methods, such as THz time-domain spectroscopy, have difficulties. To illustrate the capabilities of our approach, we used pulse trains with 800-fs long pulses and adjustable time delay to investigate the material properties of periodically poled lithium niobate (PPLN) crystal with a poling period of 400 µm. Via scanning the incident pulse-train frequency, we measured the frequency response of the crystal at different temperatures (78-350 K), which enabled us to determine the temperature dependence of the refractive index and thermo-optic coefficient of the PPLN crystal around 275 GHz with very high precision. We further studied the variation of THz generation efficiency with temperature in detail to understand the temperature dependence of THz absorption in PPLN material. The technique employed is quite general and could be applied to both other frequency ranges and nonlinear crystals.

Polarization and Incident Angle Independent Multifunctional and Multiband Tunable THz Metasurface Based on VO2.

Aiming at the limitations of single-functionality, limited-applicability, and complex designs prevalent in current metasurfaces, we propose a terahertz multifunctional and multiband tunable metasurface utilizing a VO2-metal hybrid structure. This metasurface structure comprises a top VO2-metal resonance layer, a middle polyimide dielectric layer, and a gold film reflective layer at the bottom. This metasurface exhibits multifunctionality, operating independently of polarization and incident angle. The varying conductivity states of the VO2 layers, enabling the metasurface to achieve various terahertz functionalities, including single-band absorption, broadband THz absorption, and multiband perfect polarization conversion for linear (LP) and circularly polarized (CP) incident waves. Finally, we believe that the functional adaptability of the proposed metasurface expands the repertoire of options available for future terahertz device designs.

Terahertz time-domain transmission spectroscopy of water and hydrogel thin films: Extraction of optical parameters and application to agarose gel characterization

Terahertz time-domain spectroscopy (THz-TDS) is an emerging optical technique that has potential applications in the characterization of (bio)materials. However, the complicated extraction of optical parameters from multi-layered and optically thin samples is a barrier towards its acceptance by applied scientists. Therefore, the aim of this work is to provide a straightforward approach for the extraction of the THz absorption coefficient and index of refraction profiles of aqueous thin films in a window-sample-window configuration, which is ubiquitous in many laboratories (i.e., sample in a cuvette). A numerical approach-based methodology that accounts for multiple layers, Fabry–Pérot effect, and sample thickness is elaborated which involves an optical interference model based on a tri-layer structure and a simple thickness estimation technique. This method was validated on water samples where a good agreement was found with the THz optical parameters of water reported in the literature, while the use of a commercial software resulted in erroneous optical parameters estimates when used without due regard to its limitations. A case study was then performed to demonstrate the ability of the proposed method to characterize agarose hydrogels with varying degree of sulfation. It was demonstrated that THz-TDS can provide insight into the hydration state of the agarose hydrogels, including the relative number of the hydrogen bonds between the hydroxyl moieties of water and the polysaccharide network which is perturbed by the presence of sulfate. The trend in the index of refraction profiles suggested microstructural differences between the agarose hydrogels, which were confirmed by visualizing the agarose network morphology using cryo-SEM imaging.

Interval-based sparse ensemble multi-class classification algorithm for terahertz data

Terahertz time-domain spectroscopy (THz-TDS) has been widely used for food and drug identification. The classification information of a THz spectrum usually does not exist in the whole spectral band but exists only in one or several small intervals. Therefore, feature selection is indispensable in THz-based substance identification. However, most THz-based identification methods empirically intercept the low-frequency band of the THz absorption coefficients for analysis. In order to adaptively find out important intervals of the THz spectra, an interval-based sparse ensemble multi-class classifier (ISEMCC) for THz spectral data classification is proposed. In ISEMCC, the THz spectra are first divided into several small intervals through window sliding. Then the data of training samples in each interval are extracted to train some base classifiers. Finally, a final robust classifier is obtained through a nonnegative sparse combination of these trained base classifiers. With l1 -norm, two objective functions that based on Mean Square Error (MSE) and Cross Entropy (CE) are established. For these two objective functions, two iterative algorithms based on the Alternating Direction Method of Multipliers (ADMM) and Gradient Descent (GD) are built respectively. ISEMCC transforms the problem of interval feature selection and decision-level fusion into a nonnegative sparse optimization problem. The sparse constraint ensures only a few important spectral segments are selected. In order to verify the performance of the proposed algorithm, comparative experiments on identifying the origin of Bupleurum and the harvesting year of Tangerine peel are carried out. The base classifiers used by ISEMCC are Support Vector Machine (SVM) and Decision Tree (DT). The experimental results demonstrate that the proposed algorithm outperforms six typical classifiers, including Random Forest (RF), AdaBoost, RUSBoost, ExtraTree, and the two base classifiers, in terms of classification accuracy.

Weakly confined silicon nanodiscs as material system for THz absorption: analytical study

A weakly confined silicon based nano structure as a THz absorbing material is introduced. Using an effective mass approximation and 2D hydrogenic solution along with 1st order perturbation correction for truncated potential, we calculated inter-band and intra-band energy spectrum of weakly confined silicon nanodisc system. Variation of inter-band and intra-conduction-band absorption spectrum with the different sizes of the nanodisc are presented for various diameter from 10 nm to 32 nm and constant thickness of 4 nm. Inter-band and intra-band absorption spectrum are simulated using the oscillator strength. These calculation shows that intra-band absorption spectrum of the weakly confined nanodisc can cover a large THz (∼ 0.3–6.5 THz) spectrum range for these set of nanodiscs. The proposed silicon based THz absorbing material can be utilized for CMOS compatible THz technology if design on silicon-on-insulator system.

Graphene Supported Ultrathin VO2 Resonators for Multifunctional THz Absorption

Graphene Supported Ultrathin VO₂ Resonators for Multifunctional THz Absorption

Accurate detection of neotame hydrates and their transformation using terahertz spectroscopy

Neotame as a new non-nutritive sweetener is widely used in food. It has several hydrate forms which can convert into each other at different environmental conditions. In this work, terahertz time domain spectroscopy (THz-TDS) is used to quickly differentiate its two main hydrates and to study their transformation. Neotame monohydrate has obvious terahertz absorption peaks at 0.89, 1.45 and 1.73 THz, whereas neotame anhydrate shows no apparent features in the frequency range of 0.5–3.0 THz due to their different crystalline states. The transformation from monohydrate to anhydrate upon heating is monitored according to the normalized THz absorption peak areas of the monohydrate around 1.45 THz at different heating temperatures and heating times and the activation energy of the transformation is derived to be 67.6 ± 2.0 kJ/mol with a deviation of about 1.7 % from the traditional difference scanning calorimetry (DSC) measurement. Unlike the DSC method, THz spectroscopy method derives the thermodynamic parameters of the transformation without heating the sample until it dissolves. These results indicate that THz-TDS can provide an accurate method to detect molecular hydrates and to study their transformations in a nondestructive manner.

Selection of thin-film-integrable substrates for THz modulator

The integrable substrate for THz modulation directly influences both the quality of films and THz absorption. Currently, the available THz substrate candidate library is still not clear. Here, we have carried out a systematic investigation of commonly used commercial substrates, including Si, quartz SiO2, MgO, Al2O3, GdScO3 and TbScO3 in the range of 0.4–1.6[Formula: see text]THz. It is found that low resistance Si, TSO and GSO are certainly not appropriate for THz light modulation due to their relatively higher absorption and dielectric constant, while the rest show better THz transmittance, low refractive index and loss. However, the dielectric constant and refractive index of high resistance Si are generally two times larger than quartz SiO2, Al2O3 and MgO. Compared with Al2O3 and MgO, quartz SiO2 shows at least 50% lower dielectric constant, refractive index and absorption, making it the best candidate. Our research is believed to build the rich substrate candidate library for THz range light modulation.

The Fabry-Perot effect suppression in gas cells used in THz absorption spectrometers

Gas cells are widely used in THz and IR absorption spectroscopy. However, reflections of an optical wave on cell input and output windows cause a gas sample measured spectrum distortion due to the Fabry-Perot effect. The hardware approach to decrease the Fabry-Perot effect in the THz spectral range is suggested. The approach is based on using a standard gas cell and the additional movable optically transparent window. To illustrate usefulness of the suggested approach, a simulation of registration of the sulfur dioxide absorption band near 660 GHz in the gas mixture consisted of 0.1 % SO2 and 99.9 % N2 was conducted. The absorption band profile fluctuations due to the Fabry-Perot effect were shown to be reduced by almost 4 times with the suggested approach implementation in relation to the default gas cell. This approach can be combined with other hardware and software variants of the Fabry-Perot effect suppression.

Ti3C2Tx MXene Polyester Fiber Mesh Composite as Broadband Terahertz Absorber

AbstractTerahertz (THz) absorbing materials have significant potential applications in THz radar, telecommunications, stealth, detection, and other fields. However, the design and fabrication of THz broadband absorber is still a challenge. In this work, the Ti3C2Tx MXene polyester fiber mesh (MPFM) composite is successfully synthesized using dip‐coating method. By optimizing the fabrication and structures, this study realizes a remarkable absorption rate of over 90% in a wide frequency range of 0.7−2.5 THz. Additionally, Ti3C2Tx loading in the composite is only 0.8 mg, accounting for 6.7 wt.% of the MPFM. The THz absorption capacity of MPFM is minimally affected by the multilayer PFM stacking method and the loading amount of Ti3C2Tx, but rather by the PFM aperture size and stack thickness. The proposed method paves the way for developing THz broadband absorbers using lightweight flexible composite.

Optical tuning of the terahertz response of black phosphorus quantum dots: effects of weak carrier confinement

Abstract In contrast to few-layer black phosphorus (BP) with a relatively larger area, BP quantum dots (BP-QDs) are expected to have distinctive electromagnetic response and carrier behaviors, especially in low-frequency range such as in the THz regime. Herein, we experimentally investigate the THz properties of BP-QDs as well as the optical control of these properties. It is demonstrated that the effects of weak carrier confinement, which is associated with diffusive restoring current in each BP-QD, contribute significantly to the effective THz conductivity of BP-QDs. Instead, spectral features of discretely spaced energy levels as shown for many kinds of semiconductor QDs in UV-visible range are not observed in the THz regime. This indicates an insignificant contribution of strong quantum confinement here. Based on the modified Drude–Smith formula, we show that the optical excitation/pump of a CW laser can induce photogenerated carriers and enhance the effects of weak carrier confinement in BP-QDs. Thus, a nonlinear enhancement of THz absorption can be observed by increasing the power of the excitation laser. These results not only deepen our understanding of the fundamental physics of BP nanomaterials but also provide an alternative approach to realize active control of BP-based THz devices.

Nonlinear photocarrier dynamics in multilayer W S e 2 induced by intense terahertz pulses

Non-equilibrium photocarriers in multilayer WSe2 injected by femtosecond laser pulses exhibit extraordinary nonlinear dynamics in the presence of intense THz fields. The THz absorption in optically excited WSe2 rises rapidly in the low THz field regime and gradually ramps up at high intensities. The strong THz pulses drive the photocarriers into sidebands of higher mobility and release trapped charge carriers, which consequently enhance the transient conductivity of WSe2. The spectrally analyzed conductivity reveals distinctive features, indicating that the photocarriers undergo resonant interactions such as carrier-photon scattering.

Publisher's Note: “Validation of THz absorption spectroscopy by a comparison with ps-TALIF measurements of atomic oxygen densities” [Appl. Phys. Lett. 123, 081107 (2023)

Publisher's Note: “Validation of THz absorption spectroscopy by a comparison with ps-TALIF measurements of atomic oxygen densities” [Appl. Phys. Lett. <b>123</b>, 081107 (2023)

Terahertz Optical Properties of Graphite-Cement Paste

Mixing conducting particles in cement present various applications in electromagnetic shielding and in-situ inspection of structures. In this study, graphite was incorporated in cement paste at varying concentrations which enhanced its EM shielding. The samples were characterized using Terahertz Time-Domain Spectroscopy (THz-TDS) to determine its optical properties and calculate for the conductivity. The Ultraviolet-Visible (UV-Vis) spectroscopy was also used to characterize the sample to confirm the variation of graphite content which showed small peaks at 258 nm caused by the excitation of π electrons in the graphitic structure. The refractive index, absorption coefficient and conductivities were determined from the amplitudes and phase difference obtained in the frequency domain. The spectral cut-off in the THz region decreases with increasing graphite content due to THz absorption of graphite. The THz refractive index appeared to be not frequency-independent while the absorption coefficient showed a power-law behavior. The THz conductivities were calculated and was found to be proportional to the graphite content. This is attributed to an increase in the conducting network of cement paste and increase in the charge carriers in the insulating cement matrix.

Research on Non-destructive Testing Method for Aging of Sheds of Composite Insulators

As an important part of external insulation of transmission and distribution lines and substation equipment, silicone rubber insulators are generally used because of their strong anti-pollution flashover ability, excellent hydrophobicity, and lightweight. It is one of the effective means to obtain the aging degree of sheds quickly and accurately to prevent operation accidents. From the micro and macro perspectives, firstly, this paper uses the traditional scanning electron microscopy and dielectric loss factor test to classify the aging degree of 11 high-voltage side sheds samples of silicone rubber insulators running on-site for 0~11 years. According to the different experimental principles, the aging degree of each experimental method is divided into four grades. Then, the detection method based on terahertz time-domain spectroscopy is proposed to realize a nondestructive evaluation of the aging grades of sheds of silicone rubber insulators. Finally, through comparison, it can be found that the THz absorption coefficient spectra can find the characteristic quantity of classification of aging levels of silicone rubber materials: The aging levels of the silicone rubber samples increase, and the absorption peak at 1.32 THz in the absorption spectra show a downward trend. According to the characteristic quantity, 11 samples can also be divided into four aging grades, and the results are consistent with the previous experiments.