Terahertz Response Research Articles

Influence of SU-8 curing parameters on the terahertz absorption characteristics

A plethora of polymers are excellent candidates that can be utilized to build various terahertz (THz) based systems for sensing and microfluidics. SU-8 is a versatile epoxy-based polymer with excellent properties, such as biocompatibility and good mechanical properties. Nevertheless, the impact of curing parameters on the THz absorption characteristics of SU-8 remains uncertain. This study explores the impact of various curing conditions on the THz absorption properties of SU-8. The aim is to establish a correlation between THz absorption and the polymer's cross-linking characteristics. Therefore, three key curing parameters have been examined: time, temperature, and ultraviolet (UV) exposure dose. The SU-8 samples that are cured under different conditions are examined using THz Time-Domain Spectroscopy (THz-TDS) to estimate the THz absorption coefficient. Next, the swelling experiment is conducted to evaluate the polymer cross-linking of the cured samples. The results show that curing conditions and, thus, cross-linking routines significantly influence THz wave propagation and attenuation within SU-8 samples. The findings recommended using an optimal curing dose for SU-8 spanning 1240 mJ/cm2 to 1860 mJ/cm2 which results in a relatively lower absorption coefficient while still maintaining a higher state of cross-linking. This research establishes a crucial link between SU-8 processing and its THz response. It can lay the foundation for tailored SU-8 polymer with optimized THz transmission for novel and customized biosensing and microfluidic devices.

Exploring the Application of Terahertz Metamaterials Based on Metallic Strip Structures in Detection of Reverse Micelles.

Terahertz spectroscopy has unique advantages in the study of biological molecules in aqueous solutions. However, water has a strong absorption capability in the terahertz region. Reducing the amount of liquid could decrease interference with the terahertz wave, which may, however, affect the measurement accuracy. Therefore, it is particularly important to balance the amount and water content of liquid samples. In this work, a terahertz metamaterial sensor based on metallic strips is designed, fabricated, and used to detect reverse micelles. An aqueous confinement environment in reverse micelles can improve the signal-to-noise ratio of the terahertz response. Due to "water pool" trapped in reverse micelles, the DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) solution and DOPC emulsion can successfully be identified in intensity by terahertz spectroscopy. Combined with the metamaterial sensor, an obvious frequency shift of 30 GHz can be achieved to distinguish the DOPC emulsion (5%) from the DOPC solution. This approach may provide a potential way for improving the sensitivity of detecting trace elements in a buffer solution, thus offering a valuable toolkit toward bioanalytical applications.

Revealing Novel Aspects of Light-Matter Coupling by Terahertz Two-Dimensional Coherent Spectroscopy: The Case of the Amplitude Mode in Superconductors.

Recently developed terahertz (THz) two-dimensional coherent spectroscopy (2DCS) is a powerful technique to obtain materials information in a fashion qualitatively different from other spectroscopies. Here, we utilized THz 2DCS to investigate the THz nonlinear response of conventional superconductor NbN. Using broadband THz pulses as light sources, we observed a third-order nonlinear signal whose spectral components are peaked at twice the superconducting gap energy 2Δ. With narrow-band THz pulses, a THz nonlinear signal was identified at the driving frequency Ω and exhibited a resonant enhancement at temperature when Ω=2Δ. General theoretical considerations show that such a resonance can arise only from a disorder-activated paramagnetic coupling between the light and the electronic current. This proves that the nonlinear THz response can access processes distinct from the diamagnetic Raman-like density fluctuations, which are believed to dominate the nonlinear response at optical frequencies in metals. Our numerical simulations reveal that, even for a small amount of disorder, the Ω=2Δ resonance is dominated by the superconducting amplitude mode over the entire investigated disorder range. This is in contrast to other resonances, whose amplitude-mode contribution depends on disorder. Our findings demonstrate the unique ability of THz 2DCS to explore collective excitations inaccessible in other spectroscopies.

Deep learning-enhanced prediction of terahertz response of metasurfaces

Metasurfaces offer an exciting opportunity to manipulate electromagnetic waves, presenting vast potential across diverse applications. In this study, we introduce a novel deep learning approach that integrates an Autoencoder with a Multi-Layer Perceptron to effectively forecast the Terahertz (THz) spectral response of metasurfaces. By harnessing a large dataset of training examples, our model adeptly captures the intricate correlation between metasurface structures and their optical responses, circumventing the traditionally time-consuming analysis of complex patterns. This proposed methodology furnishes a valuable tool for examining the THz transmission response of metasurfaces and has the potential to expedite metasurface design processes.

Modified Bridgman assisted ZnTe crystal growth for THz device applications

Zinc Telluride (ZnTe) single crystals have enormous potential for generating and detecting broadband terahertz (THz) radiation, making them suitable for a wide range of applications in communication, defence security and biomedical applications. The quality and homogeneities of the crystal plays an important role in obtaining THz response. This paper elaborates on the steps involved in the crystal growth of ZnTe and the characterization technique utilized for the determination of quality of the grown crystal, identifications of defects and qualifications of THz signals. Single crystal was grown by the Bridgman technique in vacuum sealed carbon-coated quartz ampoule with excess tellurium (Zn:Te/3:7). To obtain maximum THz response, crystals were oriented along (110) direction using a Laue diffraction pattern and sliced accordingly. Three different crystals along the length (bottom-middle-top) of size ∼ 10 mm × 10 mm are taken for analysis. The crystallinity of ZnTe was confirmed with X-ray diffraction pattern and lattice parameters were obtained using Rietveld refinement. Raman analysis was carried out at three different regions of the grown crystal which revealed the presence of same high-intensity mode at 413 cm−1 in all the regions confirming the quality of the crystal. The EDS compositional analysis confirms the stoichiometry along the grown crystal length. Optical bandgap of 2.216 eV was measured by UV-Vis-NIR spectra and >55 % transmission was obtained across the grown crystal length. The size and distribution of Te inclusions were measured using IR microscopy. The crystals were subjected to terahertz investigation in the range of 0.1–4 THz using a time domain spectrometer. All the crystals exhibited negligible absorption coefficients in the 0.1–3.75 THz range, confirming the maximum transmission in the THz domain. The crystals also exhibit excellent electro-optic detection which is confirmed by signal-to-noise ratio in the measured range.

Ultrasensitive terahertz response mediated by split ring antenna induced giant resonant field enhancement

Ultrasensitive terahertz response mediated by split ring antenna induced giant resonant field enhancement

Ensemble learning prediction framework for EGFR amplification status of glioma based on terahertz spectral features

Epidermal growth factor receptor (EGFR) plays a pivotal role in the initiation and progression of gliomas. In particular, in glioblastoma, EGFR amplification emerges as a catalyst for invasion, proliferation, and resistance to radiotherapy and chemotherapy. Current approaches are not capable of providing rapid diagnostic results of molecular pathology. In this study, we propose a terahertz spectroscopic approach for predicting the EGFR amplification status of gliomas for the first time. A machine learning model was constructed using the terahertz response of the measured glioma tissues, including the absorption coefficient, refractive index, and dielectric loss tangent. The novelty of our model is the integration of three classical base classifiers, i.e., support vector machine, random forest, and extreme gradient boosting. The ensemble learning method combines the advantages of various base classifiers, this model has more generalization ability. The effectiveness of the proposed method was validated by applying an individual test set. The optimal performance of the integrated algorithm was verified with an area under the curve (AUC) maximum of 85.8 %. This signifies a significant stride toward more effective and rapid diagnostic tools for guiding postoperative therapy in gliomas.

Terahertz nanoscopy: Advances, challenges, and the road ahead

Exploring nanoscale material properties through light-matter interactions is essential to unveil new phenomena and manipulate materials at the atomic level, paving the way for ground-breaking advancements in nanotechnology and materials science. Various elementary excitations and low-energy modes of materials reside in the terahertz (THz) range of the electromagnetic spectrum (0.1–10 THz) and occur over various spatial and temporal scales. However, due to the diffraction limit, a slew of THz studies are restricted to drawing conclusions from the spatially varying THz responses around half of the probing wavelengths, i.e., from tens to a couple of hundred micrometers. To address this fundamental challenge, scanning near-field optical microscopy (SNOM), notably scattering-type SNOM (s-SNOM), combined with THz sources has been employed and is fueling growing interest in this technique across multiple disciplines. This review (1) provides an overview of the system developments of SNOM, (2) evaluates current approaches to understand and quantify light-matter interactions, (3) explores advances in THz SNOM applications, especially studies with THz nano-scale spatial responses employing an s-SNOM, and (4) envisions future challenges and potential development avenues for the practical use of THz s-SNOM.

Light-Induced Melting of Competing Stripe Orders without Introducing Superconductivity in La2−xBaxCuO4

The ultrafast manipulation of quantum material has led to many novel and significant discoveries. Among them, the light-induced transient superconductivity in cuprates achieved by melting competing stripe orders represents a highly appealing accomplishment. However, recent investigations have shown that the notion of photoinduced superconductivity remains a topic of controversy, and its elucidation solely through c-axis time-resolved terahertz spectroscopy remains an arduous task. Here, we measure the in-plane and out-of-plane transient terahertz responses simultaneously in the stripe-ordered nonsuperconducting La2−xBaxCuO4 after near-infrared excitations. We find that although a pump-induced reflectivity edge appears in the c-axis reflectance spectrum, the reflectivity along the CuO2 planes decreases simultaneously, indicating an enhancement in the scattering rate of quasiparticles. This in-plane transient response is clearly distinct from the features associated with superconducting condensation. Therefore, we conclude the out-of-plane transient responses cannot be explained by an equivalent of Josephson tunneling. Notably, those pump-induced terahertz responses remain consistent even when we vary the near-infrared optical pump wavelengths and hole concentrations. Our results provide critical evidence that transient three-dimensional superconductivity cannot be induced by melting the competing stripe orders with pump pulses whose photon energy is much higher than the superconducting gap of cuprates. Published by the American Physical Society 2024

Large area Terahertz digitated photoconductive antennas based on a single high resistivity metal and nanoplasmonic electrode

Optical excited photoconductive antennas are a central technology for the Terahertz (THz) domain, crucial for both emitting and detecting THz radiation. This work proposes and experimentally realises a new approach in digitated photoconductive antennas (d-PCAs) based on a single digitated high resistivity metal contact with integrated resistances as voltage dividers. This permits a uniform applied electric field over a large surface area and a single step device processing procedure, simplifying the device realisation. This concept is further combined with digitated plasmonic nano-antennas that permits to enhance the light-matter interaction. Through femtosecond optical excitation of such structures, THz pulses can be generated efficiently through this device. Further, for the plasmonic d-PCA, the detected THz electric field of the device shows the effect of polarisation of the incident IR beam, highlighting the role of the nanostructured digitated contacts. This work is supported by electromagnetic simulations showing the optical and THz response of this new type of photoconductive antenna with integrated resistances.

Graphene-Based Tunable Dual-Frequency Terahertz Sensor.

A tunable dual-band terahertz sensor based on graphene is proposed. The sensor consists of a metal bottom layer, a middle dielectric layer, and single-layer graphene patterned with four strips on the top. The numerical simulations results show that the proposed sensor exhibits two significant absorption peaks at 2.58 THz and 6.07 THz. The corresponding absorption rates are as high as nearly 100% and 98%, respectively. The corresponding quality factor (Q) value is 11.8 at 2.58 THz and 29.6 at 6.07 THz. By adjusting the external electric field or chemical doping of graphene, the positions of the dual-frequency resonance peak can be dynamically tuned. The excitation of plasma resonance in graphene can illustrate the mechanism of the sensor. To verify the practical application of the device, the terahertz response of different kinds and different thicknesses of the analyte is investigated and analyzed. A phenomenon of obvious frequency shifts of the two resonance peaks can be observed. Therefore, the proposed sensor has great potential applications in terahertz fields, such as material characterization, medical diagnosis, and environmental monitoring.

Spontaneous exciton dissociation in transition metal dichalcogenide monolayers.

Since the seminal work on MoS2, photoexcitation in atomically thin transition metal dichalcogenides (TMDCs) has been assumed to result in excitons, with binding energies order of magnitude larger than thermal energy at room temperature. Here, we reexamine this foundational assumption and show that photoexcitation of TMDC monolayers can result in a substantial population of free charges. Performing ultrafast terahertz spectroscopy on large-area, single-crystal TMDC monolayers, we find that up to ~10% of excitons spontaneously dissociate into charge carriers with lifetimes exceeding 0.2 ns. Scanning tunneling microscopy reveals that photocarrier generation is intimately related to mid-gap defects, likely via trap-mediated Auger scattering. Only in state-of-the-art quality monolayers, with mid-gap trap densities as low as 109 cm-2, does intrinsic exciton physics start to dominate the terahertz response. Our findings reveal the necessity of knowing the defect density in understanding photophysics of TMDCs.

Evidence for d-wave superconductivity of infinite-layer nickelates from low-energy electrodynamics.

The discovery of superconductivity in infinite-layer nickelates established another category of unconventional superconductors that shares structural and electronic similarities with cuprates. However, key issues of the superconducting pairing symmetry, gap amplitude and superconducting fluctuations are yet to be addressed. Here we utilize static and ultrafast terahertz spectroscopy to address these. We demonstrate that the equilibrium terahertz conductivity and non-equilibrium terahertz responses of an optimally Sr-doped nickelate film (superconducting transition temperature of Tc = 17 K) are in line with the electrodynamics of d-wave superconductivity in the dirty limit. The gap-to-Tc ratio (2Δ/kBTc) is found to be 3.4, indicating that the superconductivity falls in the weak coupling regime. In addition, we observed substantial superconducting fluctuations near Tc that do not extend into the deep normal state as the optimally hole-doped cuprates do. Our results support a d-wave system that closely resembles the electron-doped cuprates.

Terahertz response of CdTe/Cd 1-xMg xTe modulation-doped multiple quantum wells

Terahertz response of CdTe/Cd 1-xMg xTe modulation-doped multiple quantum wells

Temperature-dependent charge carrier behavior in phosphorene quantum dots probed by terahertz time-domain spectroscopy.

Although phosphorene quantum dots (PQDs) have gained significant attention in optoelectronics and physics due to their unique optical responses, the low-frequency electromagnetic properties of PQDs and the effects of temperature still remain largely unexplored. Herein, we investigate the temperature-dependent terahertz (THz) response of PQDs by using THz time-domain spectroscopy. Effective THz conductivity of the PQD sample is extracted based on THz measurements to analyze the charge carrier behavior. It is shown that the carriers in the PQDs can be approximated as a weakly confined Drude gas of classical and noninteracting charge particles, which are described by the modified Drude-Smith formula. Then, we also obtain the temperature dependences of the effective characteristic parameters for the charge carriers. As the temperature increases, the plasma frequency linearly enhances whereas both of the carrier diffusion time and the momentum scattering time decrease, which are akin to conventional semiconductors to a large extent. In addition, the confinement factor is closed to 1 and nearly insensitive to temperature. These results are helpful to gain an in-depth understanding of the low-frequency electromagnetic response of charge carriers in PQDs and to explore new applications in photonics and optoelectronics.

Insights into the terahertz response of L-glutamic acid and its receptor.

L-Glutamic acid (L-Glu) is a basic unit of proteins and also serves as an important neurotransmitter in the central nervous system. Its structural properties are critical for biological functions and selective receptor recognition. Although this molecule has been extensively studied, the low frequency vibrational behavior that is closely related to conformational changes and the intermolecular interactions between L-Glu and its receptors are still unclear. In this study, we acquired the fingerprint spectrum of L-Glu by using air plasma terahertz (THz) time-domain spectroscopy in the 0.5-18 THz range. The low frequency vibrational characteristics of L-Glu were investigated through density functional theory (DFT) calculations. The THz responses of the ligand binding domain of the NMDAR-L-Glu complex were studied by the ONIOM method, with a focus on discussing the normal modes and interactions of ligand L-Glu and water molecules. The results illustrate that THz spectroscopy exhibits a sensitive response to the influence of L-Glu on the structure of the NMDAR. The water molecules in proteins have various strong vibration modes in the THz band, showing specificity, diversity and complexity of vibrational behavior. There is potential for influencing and regulating the structural stability of the NMDAR-L-Glu complex through water molecules.

An All-Dielectric Metamaterial Terahertz Biosensor for Cytokine Detection.

In this paper, we report an all-dielectric metamaterial terahertz biosensor, which exhibits a high Q factor of 35 at an 0.82 resonance peak. A structure with an electromagnetically induced transparency effect was designed and fabricated to perform a Mie resonance for the terahertz response. The biosensor exhibits a limit of detection of 100 pg/mL for cytokine interleukin 2 (IL-2) and a linear response for the logarithm of the concentration of IL-2 in the range of 100 pg/mL to 1 μg/mL. This study implicates an important potential for the detection of cytokines in serum and has potential application in the clinical detection of cytokine release syndrome.

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.

Study on the detection method of biological characteristics of hepatoma cells based on terahertz time-domain spectroscopy.

Liver cancer usually has a high degree of malignancy and its early symptoms are hidden, therefore, it is of significant research value to develop early-stage detection methods of liver cancer for pathological screening. In this paper, a biometric detection method for living human hepatocytes based on terahertz time-domain spectroscopy was proposed. The difference in terahertz response between normal and cancer cells was analyzed, including five characteristic parameters in the response, namely refractive index, absorption coefficient, dielectric constant, dielectric loss and dielectric loss tangent. Based on class separability and variable correlation, absorption coefficient and dielectric loss were selected to better characterize cellular properties. Maximum information coefficient and principal component analysis were employed for feature extraction, and a cell classification model of support vector machine was constructed. The results showed that the algorithm based on parameter feature fusion can achieve an accuracy of 91.6% for human hepatoma cell lines and one normal cell line. This work provides a promising solution for the qualitative evaluation of living cells in liquid environment.

Quantitative polarization-sensitive super-resolution solid immersion microscopy reveals biological tissues’ birefringence in the terahertz range

Terahertz (THz) technology offers a variety of applications in label-free medical diagnosis and therapy, majority of which rely on the effective medium theory that assumes biological tissues to be optically isotropic and homogeneous at the scale posed by the THz wavelengths. Meanwhile, most recent research discovered mesoscale (sim lambda ) heterogeneities of tissues; lambda is a wavelength. This posed a problem of studying the related scattering and polarization effects of THz-wave–tissue interactions, while there is still a lack of appropriate tools and instruments for such studies. To address this challenge, in this paper, quantitative polarization-sensitive reflection-mode THz solid immersion (SI) microscope is developed, that comprises a silicon hemisphere-based SI lens, metal-wire-grid polarizer and analyzer, a continuous-wave 0.6 THz (lambda = 500 µm) backward-wave oscillator (BWO), and a Golay detector. It makes possible the study of local polarization-dependent THz response of mesoscale tissue elements with the resolution as high as 0.15 lambda . It is applied to retrieve the refractive index distributions over the freshly-excised rat brain for the two orthogonal linear polarizations of the THz beam, aimed at uncovering the THz birefringence (structural optical anisotropy) of tissues. The most pronounced birefringence is observed for the Corpus callosum, formed by well-oriented and densely-packed axons bridging the cerebral hemispheres. The observed results are verified by the THz pulsed spectroscopy of the porcine brain, which confirms higher refractive index of the Corpus callosum when the THz beam is polarized along axons. Our findings highlight a potential of the quantitative polarization THz microscopy in biophotonics and medical imaging.