Search

A terahertz photonic crystal fiber with two sensing channels was designed. Graphene coated on the micro-grooves in the cladding was used as plasma material to introduce tunability. The dispersion relation, mode coupling, and sensing characteristics of the fiber were studied using the finite element method. Ultrahigh sensitivity of 2.014 THz/RIU and 0.734 GHz/°C were obtained for analytes with refractive index in the range of 1.33 to 1.4 and environment temperature in the range of 10–60 °C, respectively. Refractive index resolution can reach the order of 10−5. The dual parameter simultaneous detection, dynamic tunable characteristics, and working in the low-frequency range of terahertz enable the designed photonic crystal fiber to have application prospects in the field of biosensing. [ABSTRACT FROM AUTHOR]

This study presents a terahertz (THz) resonance gas sensor design incorporating graphene, black phosphorus, and MXene materials in a metasurface structure. The sensor leverages the unique properties of these advanced two-dimensional materials to achieve enhanced sensitivity and versatility in gas detection applications. The proposed design consists of elliptical and square-shaped resonators arranged on a SiO2 substrate with a ground plane back reflector. Comprehensive simulations using COMSOL Multiphysics were conducted to analyze the sensor's performance across various structural parameters and operating conditions. The sensor demonstrates a maximum sensitivity of 400 GHzRIU−1 and a figure of merit up to 0.816 RIU−1 within a refractive index range of 1–1.07 RIU. Electric field distribution analysis validates the sensor's transmittance response at different frequencies. Notably, the design shows potential for 2-bit encoding applications based on transmittance characteristics under varying graphene chemical potential values. Compared to existing studies, the senso's performance is particularly better in terms of sensitivity, offering advantages such as room temperature operation and fast response times. This research contributes to the advancement of THz sensing technology and opens new possibilities for highly sensitive and versatile gas detection in various applications. [ABSTRACT FROM AUTHOR]

With advances in terahertz technology, achieving high and nearly constant birefringence over a wide frequency range plays an extreme role in many advanced applications. In the past decade, significant research efforts have been devoted to creating new systems or elements with high birefringence. To our knowledge, the maximum birefringence attainable using artificial crystals, intrinsic liquid crystals or fiber-based systems has been less than unity. More importantly, the birefringence created in previous studies has exhibited a strong frequency dependence, limiting their practical applications. In this work, we propose a novel approach to achieve extraordinarily high birefringence over a broad terahertz frequency band (>100 GHz). To address the limitation of frequency dependence, we combined the principle of metamaterials with the form birefringence concept. First, we designed a metamaterial with an exceptionally high refractive index, thoroughly characterizing it using simulations and analytical analysis. Next, we systematically investigated the form birefringence concept, exploring its frequency response, geometric limitations, and complex refractive index differences between constituent elements. Finally, we designed a hybrid material system, combining the strengths of both metamaterials and form birefringence. Our results demonstrate the feasibility of achieving a birefringent medium exceeding three orders of magnitude higher than previous reports while maintaining a time-invariant frequency response in the sub-terahertz regime. [ABSTRACT FROM AUTHOR]

Material characterization in the terahertz range is an interesting topic of research due to its great applications in material science, health monitoring, and security applications. Advances in terahertz generation, detection, and data acquisition have contributed to improved bandwidth, signal power, and signal-to-noise ratio. This enables advanced material characterization methods such as ellipsometry, which has been little explored in the terahertz frequency range, yet. Here, we introduce a comparison between material characterization with terahertz time-domain spectroscopy in transmission geometry and ellipsometry reflection geometry. Terahertz ellipsometry images were taken, showing spatially resolved refractive index estimation in the far field and higher image quality compared to single-polarization imaging. [ABSTRACT FROM AUTHOR]

Plastic foams, near-ubiquitous in everyday life and industry, show properties that depend primarily on density. Density measurement, although straightforward in principle, is not always easy. As such, while several methods are available, plastic foam industry is not yet supported with a standard technique that effectively enables to control density maps. To overcome this issue, this paper proposes Terahertz (THz) time-of-flight imaging using normal reflection measurements as a fast, relatively cheap, contactless, non-destructive and non-dangerous way to map plastic foam density, based on the expected relationship between density and refractive index. The approach is demonstrated in the case of polypropylene foams. First, the relationship between the estimated effective refractive index and the polypropylene foam density is derived by characterizing a set of carefully crafted samples having uniform density in the range 70–900 kg/m3. The obtained calibration curve subtends a linear relationship between the density and the refractive index in the range of interest. This relationship is validated against a set of test samples, whose estimated average densities are consistent with the nominal ones, with an absolute error lower than 10 kg/m3 and a percentage error on the estimate of 5%. Exploiting the calibration curve, it is possible to build quantitative images depicting the spatial distribution of the sample density. THz images are able to reveal the non-uniform density distribution of some samples, which cannot be appreciated from visual inspection. Finally, the complex spatial density pattern of a graded foam sample is characterized and quantitatively compared with the density map obtained via X-ray microscopy. The comparison confirms that the proposed THz approach successfully determines the density pattern with an accuracy and a spatial scale variability compliant with those commonly required for plastic foam density estimate. [ABSTRACT FROM AUTHOR]

Metasurfaces suitable for the terahertz gap are alternatives to naturally occurring materials and could accelerate the development of terahertz flat optics integrated with terahertz continuous-wave sources. However, metasurfaces have yet to be commonly adopted in terahertz devices that require a number of specific material properties because of the many choices in meta-atom design. In this paper, we demonstrate that simple dimension control of a single kind of meta-atom enables the design of a wide range of refractive indices from large positive to negative values in the 0.3-THz band. Measurements by terahertz time-domain spectroscopy verify three kinds of metasurfaces with (1) an extremely high refractive index of 12.3 + j0.88 and reflectance of 5.1% at 0.31 THz, (2) a zero refractive index of − 0.44 + j0.12 and reflectance of 2.6% at 0.34 THz, and (3) a negative refractive index of − 5.4 + j0.32 and reflectance of 22.7% at 0.31 THz. The 0.3-THz band is a frequency band for candidates to 6G wireless communications. Our results offer an accessible material platform for terahertz flat optics such as metalenses, antennas, and phase plates. Terahertz flat optics based on our presented metasurfaces would be a welcome contribution to the development of terahertz industries, including 6G wireless communications. [ABSTRACT FROM AUTHOR]

Here, a novel design of a microstructured quad‐band terahertz metamaterial absorber is presented consisting of a copper‐based merged circular and square split ring resonator‐integrated over a lead glass substrate intended to detect changes in the surrounding medium's refractive index for gas sensing applications through THz spectroscopy. Outstanding performance is shown by the suggested absorber, which achieves a maximum absorption rate of 99.4%, 98.7%, 99.2%, and 99.4%, at resonance frequencies of 8.055 (f1$f_1$), 8.423 (f2$f_2$), 9.257 (f3$f_3$), and 9.507 (f4$f_4$) THz, respectively. The quality factors (Q‐factors) attained are 201, 129, 298, and 271 for these four absorption bands. The obtained sensitivity values at these four distinct bands are 3898, 2927, 4719, and 4102 GHz per refractive index unit with high figure of merits (FoM) of 97, 95, 152, and 117, respectively, throughout a range of refractive indices from n = 1.00 to n = 1.05 (with a step size of 0.005). The study's novelty lies in its novel design and remarkable ability to excel in gas sensing with high sensitivity and high FoM across not just one but four distinct THz bands. Furthermore, each band demonstrates significantly superior performance in sensitivity, Q‐factor, and FoM, among other parameters compared to prior research in both gas detection and RI monitoring scenarios, hence exhibiting its potential across a wide range of applications. [ABSTRACT FROM AUTHOR]

Cashmere and wool are both natural animal fibers used in the textile industry, but cashmere is of superior quality, is rarer, and more precious. It is therefore important to distinguish the two fibers accurately and effectively. However, challenges due to their similar appearance, morphology, and physical and chemical properties remain. Herein, a terahertz electromagnetic inductive transparency (EIT) metasurface biosensor is introduced for qualitative and quantitative identification of cashmere and wool. The periodic unit structure of the metasurface consists of four rotationally symmetric resonators and two cross−arranged metal secants to form toroidal dipoles and electric dipoles, respectively, so that its effective sensing area can be greatly improved by 1075% compared to the traditional dipole mode, and the sensitivity will be up to 342 GHz/RIU. The amplitude and frequency shift changes of the terahertz transmission spectra caused by the different refractive indices of cashmere/wool can achieve highly sensitive label−free qualitative and quantitative identification of both. The experimental results show that the terahertz metasurface biosensor can work at a concentration of 0.02 mg/mL. It provides a new way to achieve high sensitivity, precision, and trace detection of cashmere/wool, and would be a valuable application for the cashmere industry. [ABSTRACT FROM AUTHOR]

We investigated the optical constants of thin films of the Nb25Mo25Ta25W25 alloy, which belongs to high-entropy alloys (HEAs), with a transmission terahertz (THz) measurement system and discovered that the refractive index, extinction coefficient, and conductivity of the HEA thin film measured in the range of 0.3–1.0 THz all increased as the thickness of the thin film increased (the thickness was maintained in the range between 10 and 40 nm). According to the experimental results, this alloy with a high refractive index is suitable for producing high-contrast gratings (HCGs). The principle behind achieving a high contrast with a high-reflection HEA grating is to harness the confined plasmonic energy within the grating, thereby enhancing the localized plasmonic energy within the grating. This work elucidates that we can not only fabricate THz components by controlling the thickness of the thin film but also utilize the high refractive index of this material to create HCGs. [ABSTRACT FROM AUTHOR]

In this paper, a high-performance metamaterial sensor with a square split-ring resonator and a double circular split-ring resonator is proposed, which can simultaneously excite broad-band and narrow-band electromagnetically induced transparency-like (EIT-like) effects in the terahertz band. First, the mechanisms of the dual-band EIT-like effects have been revealed by the analyses of the transmission spectra and surface current distributions in the metamaterial. Then, the influences of structural parameters on the dual-band EIT-like are discussed further. Finally, the metamaterial is very sensitive to the change of the refractive index of the surrounding media, and the sensitivity of the dual-band EIT-like resonances can reach 150 GHz/RIU and 237.5 GHz/RIU and the quality factor values are 1.67 and 7.92, respectively. The proposed metamaterial broadens the pathway for the excitation of multi-band EIT-like effects and offers promising applications in multi-frequency sensors, filters and many other aspects. [ABSTRACT FROM AUTHOR]

In this paper, an ultra‐wideband Janus metastructure (MS) utilizing anapole mode for detector in the terahertz (THz) range by graphene is proposed. Specifically, when Fermi level (Ef) is set to 0.9 eV, the MS demonstrates ultra‐broadband absorption exceeding 0.9 from 0.754 to 5 THz in the −z‐direction with a relative bandwidth of 147.6 %, in which perfect absorption of over 98% develops from 3.24 to 5 THz. In the case of the +z‐direction, the absorptivity maintains around 0.6 within the 0.745 ∼ 5 THz range. As Ef equals 0 eV, the difference in absorption between the −z‐direction and +z‐direction exceeds 0.9 from 4.49 to 4.76 THz. The study also explores the MS for refractive index sensing near 3.71 THz by a unique difference detection, measuring two refractive index ranges: 1.2 ∼ 2.6 and 4.5 ∼ 4.7, with corresponding sensitivities of 0.0450 and 0.0304, respectively. Owing to its highly symmetrical structure, the MS is insensitive to the polarization state of the electromagnetic (EM) waves, performing remarkable angular stability as the incident angle varies from 0 to 60 degrees in the −z‐direction. These splendid properties make the design a good candidate for biomedical sensing, EM cloaking, and full‐space EM wave control. [ABSTRACT FROM AUTHOR]

Traditional microwave design and fabrication techniques have been adopted into the terahertz domain. Understanding the properties of microwave dielectric materials at terahertz frequencies is critical for accurate component design. Nevertheless, terahertz properties for common microwave dielectric materials are largely unknown. Hence, this paper presents the relative permittivity, loss tangent, refractive index, and extinction coefficient for such materials, including microwave substrates and low-temperature co-fired ceramics (LTCCs), within the 0.1 to 3.5 THz range. Terahertz time-domain spectroscopy (THz-TDS) and a vector network analyzer produce accurate material parameter results. The material parameters presented in this paper serve as a valuable resource for component design at terahertz frequencies. [ABSTRACT FROM AUTHOR]

In this paper, a refractive index (RI) sensor with superconducting photonic crystal in the terahertz regime is theoretically analyzed by the transfer matrix method. An asymmetric resonance cavity containing gradient thicknesses of the superconducting layer is employed to suppress the resonance absorption linewidth. We present the coupled wave theoretical model to give an optimization scheme for excellent sensing performance. The proposed sensing models can achieve an excellent single resonant peak when the temperature is over 80 K. When the incident angle varies between 50° and 70° in TE mode, the shift of a single resonant peak has a linear relationship with the incident angle. The simulation results report that the sensitivity and figure of merit in the optimal model can reach over 22.2 μm RIU−1 (RIU represents RI unit) and 265 at the ultralow temperature (85 K), respectively. Its performance indicators are dozens of times those of the traditional photonic crystal RI sensor. Our study provides theoretical guidance for the design of a low-temperature RI sensor with a high-performance indicator. [ABSTRACT FROM AUTHOR]

This article presents a novel design for guiding terahertz (THz) waves, which utilises a photonic crystal fibre with a porous core (PC‐PCF). This approach is substantiated by a Topas‐based octagonal‐shaped core with rectangular air slots. By utilising Finite Element Method and Perfectly Matched Layer boundary condition, the guiding properties of a structure consisting of octagonal air holes integrated with circular air slots in the cladding region have been investigated. The proposed structure exhibits a birefringence value of 0.05 and a low effective material loss of 0.013 cm−1. Moreover, it exhibits flattened dispersion measuring 0.6±0.16ps/THz/cm $0.6\pm 0.16\,\mathrm{p}\mathrm{s}/\mathrm{T}\mathrm{H}\mathrm{z}/\mathrm{c}\mathrm{m}$ at a wide‐ranging frequency of 0.4 to 1.6 THz. In addition to that, the proposed structure offers 60% high core power fraction, 10−18 cm−1 confinement loss at 65% porosity of 290 μm core diameter and 1.6 THz frequency. Other significant waveguide properties: effective refractive index, effective area, spot size, and beam divergence of the proposed fibre are numerically analysed and discussed rigorously along with the fabrication viability of the fibre with existing techniques. [ABSTRACT FROM AUTHOR]

We present experimental and theoretical evidence demonstrating the Mie resonance effect in the terahertz (THz) range, utilizing Fe3O4/Kerosene ferrofluid. Our findings indicate a significant and rapid change in the complex refractive indices at 0.5 THz with an increase in the magnetic field strength. Moreover, we observed a prominent absorption peak at 0.5 THz in transmittance and absorption coefficient measurements, corresponding to a magnetic field intensity of 178.0 mT. This phenomenon occurs due to the adjustment of particle spacing, leading to resonance under different magnetic field conditions. These research results hold immense potential in advancing the development of magneto-optical THz modulators for imaging and communication applications. [ABSTRACT FROM AUTHOR]

For the quick identification of diverse types of cancer/malignant cells in the human body, a new hollow-core optical waveguide based on Photonic Crystal Fiber (PCF) is proposed and numerically studied. The refractive index (RI) differs between normal and cancerous cells, and it is through this distinction that the other crucial optical parameters are assessed. The proposed cancer cell biosensor's guiding characteristics are examined in the COMSOL Multiphysics v5.5 environment. The Finite Element Method (FEM) framework is used to quantify the display of the suggested fiber biosensor. Extremely fine mesh elements are additionally added to guarantee the highest simulation accuracy. The simulation results on the suggested sensor model achieve a very high relative sensitivity of 99.9277%, 99.9243%, 99.9302%, 99.9314%, 99.9257% and 99.9169%, a low effective material loss of 8.55 × 10 − 5 cm − 1 , 8.96 × 10 − 5 cm − 1 , 8.24 × 10 − 5 cm − 1 , 8.09 × 10 − 5 cm − 1 , 8.79 × 10 − 5 cm − 1 , and 9.88 × 10 − 5 cm − 1 for adrenal gland cancer, blood cancer, breast cancer type-1, breast cancer type-2, cervical cancer, and skin cancer, respectively, at a 3.0 THz frequency regime. A very low confinement loss of 6.1 × 10 − 10 dB/cm is also indicated by the simulation findings for all of the cancer cases that were mentioned. The straightforward PCF structure of the proposed biosensor offers a high likelihood of implementation when used in conjunction with these conventional performance indexes. So, it appears that this biosensor will create new opportunities for the identification and diagnosis of various cancer cells. [ABSTRACT FROM AUTHOR]

The insight into the composition of iron corrosion products is critical for corrosion studies. A novel and non-destructive method is proposed in this paper to quantitatively analyse the binary iron oxides mixtures based on the terahertz time-domain spectroscopy combined with the effective medium theory. The utility of this method is exemplified by analysing the THz refractive index for a set of samples, including hematite, magnetite, and their mixtures at several volume ratios. The effective medium theory is applied to the raw data of single-component samples to remove the dielectric influence of dilution polymer matrix and trapped air voids. The refractive index of hematite and magnetite is determined to be 4.01 ± 0.04 (1.00%) and 5.6 ± 0.2 (3.6%). The measured refractive index of two iron oxides is further adopted to construct an effective medium model for the binary mixture. The analytical model can predict the volume ratios of two iron oxides within the mixture. Comparison between the theoretical predictions and experimental measurements reveals a relative difference of less than 10.0%. [ABSTRACT FROM AUTHOR]

Reconfigurable metamaterials have immense applications in sensing. A refractive index reconfigurable terahertz metamaterial absorber was investigated in this research for gas sensing applications. The absorption spectrum reconfigures with the changes in the surrounding medium's refractive index. The proposed absorber displays positive permittivity and negative permeability at the resonance frequency of 3.045 THz indicating magnetic resonance. The design consists of concentric U-shaped rings that were optimally designed to perform the parametric analysis using the finite element method (FEM). The absorption bands offered by the structure were found to be insensitive to variation in polarization angles up to 60°. The outcome of this design approach yields a 99.75% absorption rate with a Q-factor of 87. Additionally, the equivalent circuit model of this proposed absorber was analyzed to estimate the resonance frequency, which reveals good agreement with the simulated ones. Moreover, the structure was designed for a refractive index ranging between 1 and 1.03 to detect harmful gases such as methane, chloroform, etc., with a high sensitivity of 3.01 THz/RIU (Refractive Index Unit) and figure of merit (FoM) of 86. This research work is potentially suitable for biological sensing and chemical industry applications. [ABSTRACT FROM AUTHOR]

Metamaterial absorbers are used in the terahertz frequency regime as photo-detectors, as sensing elements, in imaging applications, etc. Narrowband absorbers, on account of their ultra-slender bandwidth within the terahertz frequency spectrum, show a significant shift in the absorption peak when an extrinsic entity relative to the absorber, like refractive index or temperature of the encircling medium, is altered. This property paves the path for the narrowband absorbers to be used as potential sensors to detect any alterations in the encircling medium. In this paper, a novel design of a terahertz metamaterial (MTM) absorber is proposed, which can sense the variations in the refractive index (RI) of the surrounding medium. The effective permeability of the structure is negative, while its permittivity is positive; thus, it is a μ-negative metamaterial. The layout involves a swastika-shaped design made of gold on top of a dielectric gallium arsenide (GaAs) substrate. The proposed absorber achieved a nearly perfect absorption of 99.65% at 2.905 terahertz (THz), resulting in a quality factor (Q-factor) of 145.25. The proposed design has a sensitivity of 2.12 THz/RIU over a range of varied refractive index from n = 1.00 to n = 1.05 with a step size of 0.005, thereby achieving a Figure of Merit (FoM) of 106. Furthermore, the sensor was found to have a polarization-insensitive characteristic. Considering its high sensitivity (S), the proposed sensor was further tested for gas sensing applications of harmful gases. As a case study, the sensor was used to detect chloroform. The proposed work can be the foundation for developing highly sensitive gas sensors. [ABSTRACT FROM AUTHOR]

We propose a type of hollow planar waveguide with corrugated walls, which can realize electrically tunable terahertz (THz) filtering by filling the slit with liquid crystals. When the THz signals propagate in a planar waveguide with periodic corrugations, the transmission spectrum always exhibits many pass and stop bands. Inserting a section of defects in the middle of the periodic corrugations can excite an extremely narrow transmission peak, which would be a very good THz filter for frequency division. To achieve tunability of this narrow linewidth THz filter, we also fill the slit between the two corrugated walls with a nematic liquid crystal. The effective refractive index of liquid crystals will change with the external electric field, thus tuning the frequency of the narrow peak. The simulated results show that the center frequency of the proposed filter can be tuned linearly in the frequency range of 0.984~1.023 THz by the external electric field. Moreover, the bandwidth of the filter can be adjusted from 3.2 GHz to 0.3 GHz by increasing the number of periods in the waveguide, and a maximum Q value of 2556 can be achieved when the number of periods at both sides of the defect is 12. [ABSTRACT FROM AUTHOR]

Metamaterials are artificial structures intentionally designed to possess unique properties that are not naturally occurring in nature. Among their diverse applications, metamaterial absorbers offer advantages over conventional absorbers by effectively attenuating electromagnetic wave energy at different resonance frequencies. Terahertz (THz) metamaterial absorbers operate in the THz spectrum. They find applications in sensing and imaging. A review is presented in this paper on THz metamaterial absorbers mainly focusing on their application in refractive index sensing. The different types of resonances have been identified along with the resonance mechanism behind refractive index sensing. When the refractive index of the encompassing medium changes, the resonance frequency shifts, thereby enabling its sensing. Refractive index sensing finds application in the detection of gases, liquids, biomedical cells, etc. State-of-the-art designs have been explored in terms of absorption characteristics, resonance frequencies, bandwidth, sensitivity, and sensing applications. Graphene-based absorbers have also been studied highlighting their unique properties and reported in this review This paper will inspire future research on THz metamaterial absorbers for sensing and detecting applications. [ABSTRACT FROM AUTHOR]

Identification of biomedical molecules by terahertz (THz) sensing is a quite promising noninvasive technique. Here, we present a THz graphene metamaterial with plasmon-induced transparency (PIT) for high sensitivity biosensing. The strong PIT effect is generated by the destructive interference between the plasmon modes excited by two graphene strips. For testing different biomedical analytes, a distinct frequency shift at PIT resonance can be observed due to the effect of surrounding refractive index variation. The metamaterial scheme for noninvasive biomedical sensing can achieve the sensitivity as high as 2.6 THz/RIU. Particular, it can not only detect the phase of red blood cell infected by malaria but also the concentration of ethanol aqueous solution. The underlying mechanism is discussed for understanding the THz sensing operation. • THz graphene metamaterial for highly noninvasive sensitivity biosensing is present. • The metamaterial can achieve the sensitivity as high as 2.6 THz/RIU. • The metamaterial can defect the different phases of red blood cell. • The metamaterial provides a technique for biomolecular detection and disease diagnosis. [ABSTRACT FROM AUTHOR]

A new design of a triple band perfect metamaterial absorber based on Pythagorean fractal geometry is proposed and analyzed for terahertz sensing applications. The proposed design showed an enhanced sensing performance and achieved three intensive peaks at 33.93, 36.27, and 38.39 THz, corresponding to the absorptivity of 98.5%, 99.3%, and 99.6%, respectively. Due to the symmetrical nature of the recommended design, the structure exhibited the characteristics of independency on the incident wave angles. Furthermore, a parametric study was performed to show the effects of the change in substrate type, resonator material, and substrate thickness on the absorption spectrum. At a fixed analyte thickness (0.5 μm), the resonance frequency of the design was found to be sensitive to the refractive index of the surrounding medium. The proposed design presented three ultra-sensitive responses of 1730, 1590, and 2050 GHz/RIU with the figure of merit (FoM) of 3.20, 1.54, and 4.28, respectively, when the refractive index was changed from 1.0 to 1.4. Additionally, the metamaterial sensor showed a sensitivity of 1230, 2270, and 1580 GHz/μm at the three resonance frequencies, respectively, when it was utilized for the detection of thickness variation at a fixed analyte refractive index (RI) of 1.4. As long as the RI of the biomedical samples is between 1.3 and 1.4, the proposed sensor can be used for biomedical applications. [ABSTRACT FROM AUTHOR]

Graphene monolayer of sub-nanometer thickness possesses strong metallic and plasmonic behavior in a broad terahertz (THz) frequency range. This plasmonic effect can be considerably manipulated when graphene layer is subjected to a variable chemical potential (Ef) via chemical doping or electrical gating. The strong adsorption characteristics of graphene layer is another important advantage. In this work, a photonic spin Hall effect (PSHE) based plasmonic sensor consisting of germanium prism, organic dielectric layer, and graphene monolayer is simulated and analyzed in THz range aiming at highly sensitive and reliable gas sensing. Modified Otto configuration and Kubo formulation for graphene at room temperature are considered. The sensor's performance is examined in terms of figure of merit (FOM). The analysis indicates that under angular interrogation scheme of sensor operation, the FOM improves for smaller chemical potential (moderate doping) and higher THz frequency. Moreover, the influence of temperature on gas sensor's performance (FOM) is negligible, which suggests that the sensor is capable of providing stable sensing performance against temperature variation. The sensor design is highly flexible in terms of selection of THz frequency as an alternative interrogation scheme (i.e., measuring the variation in spin-dependent shift peak value of PSHE spectrum upon change in gas medium refractive index) can also be implemented. It is found that there is no need to change the moderate doping of graphene monolayer (i.e., Ef remains around its normal value ~ 0.1 eV) as the sensitivity achievable with this alternative method has considerably greater magnitude at smaller THz frequency (e.g., 2 THz). The magnitudes of FOM (with angular interrogation method) and sensitivity (with alternative method) are found to be significantly greater for rarer gaseous media, which might possibly assist in early detection of airborne viruses such as SARS-Cov-2 (while using appropriate specificity method) and to measure the concentration of a particular gas in a given gaseous mixture. [ABSTRACT FROM AUTHOR]

This paper introduces a biosensor based on graphene metasurfaces, designed for virus detection in the terahertz (THz) regime. The proposed sensor comprises four resonators arranged in a semicircular configuration, strategically engineered to achieve enhanced sensitivity and overall performance. Computational simulations using COMSOL Multiphysics version 6.2 were employed to optimize geometric parameters and analyze the sensor’s behavior. By integrating Au, SrTiO₃, graphene, and black phosphorus, the biosensor exhibits remarkable sensitivity to refractive index (RI) variations associated with various viruses. The maximum sensitivity demonstrated by the sensor is 4556 GHzRIU−1. Other remarkable performance metrics include a figure of merit of 8.499 RIU−1, a quality factor of 1.131, and a minimum detection limit of 0.149. Electric field distribution analysis reveals optimal absorption at 0.4 THz. Furthermore, the biosensor demonstrates the potential for 2-bit encoding applications. Compared to existing designs, the proposed biosensor offers significantly higher sensitivity for virus detection. The integration of advanced nanomaterials and metasurface design principles presents a promising avenue for rapid, label-free virus sensing, with potential applications in healthcare and biosecurity. [ABSTRACT FROM AUTHOR]

We design and propose a five-band absorber based on graphene metamaterial for the terahertz (THz) sensing field. The localized surface plasmon resonances of patterned graphene are excited, contributing to five tunable ultra-narrow absorption peaks, which are specified by the electric field distributions. Moreover, the absorber is insensitive to different polarization modes and incident angles. When increasing the Fermi level of the patterned graphene, which is composed of a round ring and a square ring connected by four thin wires, the resonant frequencies exhibit distinct blue shifts. For refractive index sensing, due to the addition of a continuous dielectric groove, the theoretical results show that the maximum averaged normalized sensitivity, Q factor, and FOM can reach 0.647 RIUâˆ'1 (refractive index unit, RIU), 355.94, and 215.25 RIUâˆ'1, indicating that the sensing performances are further enhanced compared with previous works. As a result, the proposed structure may provide a new method to realize ultrasensing in the THz region. [ABSTRACT FROM AUTHOR]

In this paper, a novel refractive index sensor in terahertz region is proposed. The proposed structure is prism/(sample/porousTa2O5)15/sample/gyroid metal/substrate. The sensor is based on the Tamm plasmon polariton at the interface between porous one-dimensional photonic crystal and gyroidal metal. The gyroidal metal has been used as an alternative metal and its refraction index can be tuned by the gyroid parameters. The effects of the metal volume fraction and sample refractive index on the performance are studied to improve the ability of the sensor. The proposed sensor achieves high sensitivity of 6.7 THz/RIU, a high figure of merit 6*103 RIU−1, a high-quality factor of 3*103, and a low detection limit of 9*10−6 RIU. The proposed device can be a good candidate for fabricating gyroid metal and porous material-based biosensors, active optoelectronic and polaritonic devices. [ABSTRACT FROM AUTHOR]

Diabetes is one of the biggest health problems in the world; in order to control blood in modern biological detection and analysis, most of them use the method of label determination to realize the detection. This method usually needs to modify and label the samples in advance. This pretreatment process is not only complex, time-consuming, and slow in response, but also has a great risk of damaging and polluting the samples to be tested, which seriously limits the detection accuracy [1-2]. In order to realize the real-time detection and analysis of biological targets without label, high precision, and high sensitivity, the design of liquid refractive index sensor based on terahertz metamaterials is carried out in this paper. Based on terahertz technology and artificial periodic structure design of metamaterials, a liquid refractive index sensor with high precision, high sensitivity, and high stability is successfully designed. Then, based on the equivalent medium theory, the absorption characteristics of the sensor are studied; it is proved that the sensor has excellent polarization-insensitive and wide-angle incident characteristics. It can complete the detection of biological targets in most polarization modes and electromagnetic wave environment with various incident angles, and maintain perfect absorption at the resonance frequency of the sensing structure. The displacement of the resonance absorption peak is 102 GHz, the displacement sensitivity is 51 GHz/RIU, and the detection accuracy is 0.0196 RIU/GHz, which can realize the detection of minimal refractive index change; the average absorption index is high, which is 99.98%, and the displacement of absorption peak has an excellent linear relationship with the change of refractive index, and the linear fitting degree is 98.788%. Through data analysis, it is found that the sensor structure also has a strong bandwidth and absorption stability, which has a certain practical application value in the detection and analysis of biological targets. It provides a new idea for the real-time detection and analysis of biological liquid analytes with no label, high precision, and high sensitivity. Finally, through the data analysis, the change law of the sensor absorption characteristics with different physical dimensions is revealed, which lays a theoretical foundation for the future sensor structure optimization and the improvement of sensing accuracy and sensitivity. [ABSTRACT FROM AUTHOR]

A plasmon-induced transparency (PIT) effect based on an asymmetric graphene loop structure has been proposed and investigated in this paper. The microstructure consists of a pair of graphene square loops and a dielectric substrate. The calculated results show that the transparency peak can be produced at 5.68 THz by the frequency detuning between two different graphene square loops. The geometric parameters of microstructure, such as the coincidence degree between two square loops, the length and the width of two square loops, will affect the position of PIT-window. Moreover, by adjusting the Fermi level of graphene through external gate voltage, the PIT-window can be dynamically tuned. Importantly, the PIT-window in graphene metamaterials can also serve as the amplitude modulator at the fixed frequency and the refractive index sensor. In addition, an improved microstructure is proposed for realizing the multi-PIT-window. The amplitude modulation of multi-PIT-window can be adjusted up to 53% by controlling the coupling distance, which has certain application prospects in the fields of double-channel filters, optical switches, and modulators. [ABSTRACT FROM AUTHOR]

Featured Application: Non-destructive porosity measurements of 3D printed materials by terahertz time-domain spectroscopy. The porosity and inhomogeneity of 3D printed polymer samples were examined using terahertz time-domain spectroscopy, and the effects of 3D printer settings were analysed. A set of PETG samples were 3D printed by systematically varying the printer parameters, including layer thickness, nozzle diameter, filament (line) thickness, extrusion, and printing pattern. Their effective refractive indices and loss coefficients were measured and compared with those of solid PETG. Porosity was calculated from the refractive index. A diffraction feature was observed in the loss spectrum of all 3D printed samples and was used as an indication of inhomogeneity. A "sweet spot" of printer settings was found, where porosity and inhomogeneity were minimised. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel absorber that combines a multi-layer film structure with graphene. The proposed structure is delicately simulated using a commercial finite element method. We combine an equivalent circuit model with parameter inversion to achieve a new method of analyzing the physical mechanism of selective absorption. The results show that four gradually decreasing peaks of ultra-high absorption are formed within 0â€"1.1 THz, and the maximum absorptance is near 100%. Numerical simulation and theoretical calculation are in good agreement. Due to the symmetry of the structure and the locality of surface plasmon resonance, the proposed structure is insensitive to the incident angle and the polarization state of incident light. By changing the Fermi level of the graphene, the coordination of the device is realized. By changing the height of the dielectric material to change the resonance frequency, the working frequency band is increased from 0â€"1.1 THz to 0â€"1.9 THz, and the four absorption peaks become three, which are used as sensor applications. The sensitivity of the sensors is 50 GHz RIUâˆ'1, the coefficient of the determination value (R 2) obtained by linear fitting is 0.9989, and the value of the limit of detection is 5.9 Ă— 10âˆ'5 RIU. The results show that our proposed devices have great potential in the practical application of terahertz technology absorbers and refractive index sensors. [ABSTRACT FROM AUTHOR]

We propose a terahertz hybrid metamaterial composed of subwavelength metallic slits and graphene plasmonic ribbons for sensing application. This special design can cause the interaction between the plasmon resonances of the metallic slits and graphene ribbons, giving rise to a strong coupling effect and Rabi splitting. Intricate balancing in the strong coupling region can be perturbed by the carrier concentration of graphene, which is subject to the analyte on its surface. Thereby, the detection of analyte can be reflected as a frequency shift of resonance in terahertz transmission spectra. The result shows that this sensor can achieve a theoretical detection limit of 325 electrons or holes per square micrometer. Meanwhile, it also works well as a refractive index sensor with the frequency sensitivity of 485 GHz/RIU. Our results may contribute to design of ultra-micro terahertz sensors. [ABSTRACT FROM AUTHOR]

A model of quasi-orthogonal acousto-optic interaction in liquefied SF6 gas has been developed, taking into account the effect of the SF6 gas temperature on the parameters that determine acousto-optic diffraction: density, sound speed, refractive index, radiation, and ultrasound absorption coefficients. It was found that the optimal conditions for the maximum modulation depth of terahertz radiation with a wavelength of 119 μm are a temperature of 0°С and a pressure of 17 bar. [ABSTRACT FROM AUTHOR]

Imidacloprid has been widely used in crop pest control and disease resistance, the highly sensitive detection of imidacloprid concentration is strongly demanded in the field of food safety and the other fields. In this paper, a method to determine the concentration of imidacloprid by using the tablet instead of the liquid on a terahertz metamaterial sensor is proposed, which could avoid the influence of the water absorption in the terahertz band. The physical mechanism of the biosensing performance is verified theoretically and experimentally. It is found that the resonant frequency of the metamaterial has a red shift as the concentration of the imidacloprid tablet increases. The frequency shift is caused by the different concentrations as well as the refractive index of the imidacloprid tablet. The calculated sensitivity is 275.86 GHz/RIU and the figure of merit is 9.61. The proposed method would have important applications in the food safety industry. [ABSTRACT FROM AUTHOR]

Terahertz wave (T-ray) scanning applications are one of the most promising tools for nondestructive evaluation. T-ray scanning applications use a T-ray technique to measure the thickness of both thin Shim stock films and GFRP (glass fiber-reinforced plastics) composites, of which the samples were selected because the T-ray method could penetrate the non-conducting samples. Notably, this method is nondestructive, making it useful for analyzing the characteristics of the materials. Thus, the T-ray thickness measurement can be found for both non-conducting Shim stock films and GFRP composites. In this work, a characterization procedure was conducted to analyze electromagnetic properties, such as the refractive index. The obtained estimates of the properties are in good agreement with the known data for poly methyl methacrylate (PMMA) for acquiring the refractive index. The T-ray technique was developed to measure the thickness of the thin Shim stock films and the GFRP composites. Our tests obtained good results on the thickness of the standard film samples, with the different thicknesses ranging from around 120 μm to 500 μm. In this study, the T-ray method was based on the reflection mode measurement, and the time-of-flight (TOF) and resonance frequencies were utilized to acquire the thickness measurements of the films and GFRP composites. The results showed that the thickness of the samples of frequency matched those obtained directly by time-of-flight (TOF) methods. [ABSTRACT FROM AUTHOR]

In this paper, we analyze the omnidirectional reflection characteristics of a superconductor photonic crystal for TE and TM modes, by using the transfer matrix method and Bloch theorem. SPC is conceived as a periodic media made of superconducting and dielectric materials. By plotting the reflectance spectra and band structures of SPC in THz frequency region, we found two ODR bands in THz frequency range for both TE and TM modes. In our work, the ODR bandgap for TM mode is wider than TE mode, which can be considered as a key feature of such SPCs. Further, we observe the effect of the variations of thicknesses of the dielectric and superconducting materials as well as the variation of the refractive index of dielectric material, on the ODR bandgaps, and some insights are drawn. It is to note that the both ODR bandgaps increase continuously with increasing the thickness of superconducting layer, while on increasing the thickness of dielectric layer, the first bandgap shows reverse effect. Moreover, the first ODR bandgap is found to be more effective at the lower refractive index of dielectric material, while the second ODR bandgap is more effective at the higher refractive index. The novel ideas from the results of ODR bandgaps for such SPCs can be used in designing reflectors or optical mirrors in THz frequency range. [ABSTRACT FROM AUTHOR]

Full phase control of terahertz (THz)‐emitting quantum cascade laser (QCL) combs has recently been demonstrated, opening new perspectives for even the most demanding applications. In this framework, simplifying the set‐ups for control of these devices will help to accelerate their spreading in many fields. This study reports a new way to control the emission frequencies of a THz QCL comb by small optical frequency tuning (SOFT), using a very simple experimental setup, exploiting the incoherent emission of an ordinary white light‐emitting diode. The slightly perturbative regime accessible in these conditions allows tweaking the complex refractive index of the semiconductor without destabilizing the broadband laser gain. The SOFT actuator is characterized and compared to another actuator, the QCL driving current. The suitability of this additional degree of freedom for frequency and phase stabilization of a THz QCL comb is shown and perspectives are discussed. [ABSTRACT FROM AUTHOR]

As a promising terahertz (THz) source, two-color laser filamentation in gases attracts an exponential growth of interest because of the scalability of output THz pulse energy and bandwidth with input parameters. However, nonlinear effects within the laser filament on the THz pulse generation process, such as intensity-dependent refractive index change induced additional phase, have never been studied in depth. In this paper, we experimentally investigate the THz waveform evolution with input infrared pump energy from 1.75 ~ 2.75 mJ were recorded by an air biased coherent detection (ABCD) sampling setup. We observe that the magnitude ratio of the positive and negative peaks increases with the pump pulse energy, and the smooth peak at the tail gradually disappears. Based on the photocurrent model, our calculation shows that the modulation is caused by the plasma-induced additional phase that varies with incident laser intensity. Additionally, the smooth peak at the tail is considered to be caused by the fading of the residual current after the negative peak. [ABSTRACT FROM AUTHOR]

We recently reported that the terahertz (THz)-wave characteristics of pottery bodies varied significantly with firing temperature. This suggested the feasibility of conducting nondestructive inspection of the sinterability and manufacturing-process management of ceramics by THz-wave analysis. However, it is not clear why the THz-wave characteristics of ceramics change with firing temperature. In this study, we evaluated the THz-wave transmission properties of low-temperature-sintered porcelain bodies fired at various temperatures using THz time-domain spectroscopy and investigated the relationship between the crystal phase and microstructure. We found that the THz-wave transmittance increased with the dehydration of kaolinite and decreased with the formation of mullite and glass phases in the body. Also, as the firing temperature increased, the THz-wave transmittance of these bodies was affected by scattering due to the formation of pores. Additionally, the refractive index in the THz range showed a good correlation with the bulk density. We established that the firing-temperature dependence of the refractive index in the THz range was related to the crystal phase and microstructure. [ABSTRACT FROM AUTHOR]

We present a design of terahertz (THz) perfect absorber (TPA) based on thermotropic phase change material vanadium dioxide (VO 2). The TPA device is composed of a metal-insulator-metal (MIM) structure, and the top metal layer is tailored to form an inner VO 2 -based disk structure and an outer Au-based connected-ring structure. The electromagnetic responses of TPA device with different geometric parameters operated in the THz band are investigated, and the perfect absorption is achieved at 2.51 THz. Furthermore, by increasing the temperature applied on VO 2 from 313 K to 363 K with an interval of 10 K, the disk structure changes from an insulating phase to a metallic phase. There is an extra resonance mode arises, which induces the resonance of TPA device can be switched from single to dual. Additionally, being exposed to an ambient environment with different refraction indices, the TPA device shows a high-efficient sensing capability with a sensitivity of up to 398 GHz per refractive index unit (RIU). These results indicate that the proposed design opens an avenue for tunable absorbers, actively controllable switches, THz logic devices, and high-efficiency sensors. • A design of THz perfect absorber (TPA) based on thermotropic phase change VO 2 material is presented. • Perfect absorption is achieved at 2.51 THz. • By increasing the temperature, resonance is switched from single-band to dual-band. • TPA shows a high-efficient sensing capability with a sensitivity of up to 398 GHz/RIU. [ABSTRACT FROM AUTHOR]

In this work, the narrowband and broadband absorption characteristics of single layer graphene-based photonic structures are investigated. The optical features were determined utilizing the 4 × 4 transfer matrix method. The results indicate that the proposed design incorporating monolayer graphene achieves absorption rates exceeding 90% across a bandwidth of 1.8 THz. Additionally, the suggested photonic configuration has the potential to function as a triple-mode narrowband absorber through the alteration of layer positioning or the substitution of materials. The numerical relationships between the refractive indexes of materials A, B, and C have been formulated to facilitate broadband and narrowband absorption. The absorption characteristics of the proposed design have been significantly influenced by the Fermi-level of Graphene and the magnetic field. The results indicate that the absorption value of both broadband and narrowband absorbers can be dynamically adjusted by manipulating the Fermi-level of Graphene and magnetic field. The photonic configuration being proposed presents novel opportunities for the development of electrically and magnetically tunable narrowband and broadband absorbers. • The proposed design can offer both broadband and multi-mode narrowband absorption. • The triple-mode narrowband absorption with a value greater than 95% achieved. • The proposed novel design provided broadband absorption over a bandwidth of 1.8 THz. • Formulated numerical relations for broadband and multi-mode absorption. • The design enables dynamic absorption tuning via electrical and magnetic biassing. [ABSTRACT FROM AUTHOR]

In this study, we numerically investigated mode couplings in terahertz (THz) anti-resonant fibres (ARFs) and found that the mode couplings were determined by the diameter ratio of the core and cladding tubes. Moreover, the optimal diameter ratio was affected by the change in the number of cladding tubes, bent radii and the refractive index of the gas filled in fibre. Mode couplings provide novel theories for fundamental mode suppression and higher-order mode suppression. In THz ARFs, taking the semielliptical tubes as anti-resonant elements can significantly improve the suppression ability of mode couplings. [ABSTRACT FROM AUTHOR]

In this paper, a wheel structured Zeonex-based hexagonal packing photonic crystal fiber (PCF) sensor has been proposed for sensing camel milk with a refractive index of 1.3423 and cow milk with a refractive index of 1.3459. This sensor has been investigated for porosities of 85%, 90%, and 98% within a terahertz (THz) region ranging from 0.2 to 2.0 THz. At an operating frequency of 2 THz, this sensor has shown a maximum sensitivity of 81.16% and 81.32% for camel and cow milk, respectively. EML of 0.033013 cm−1 and 0.03284 cm−1 has been found for camel and cow milk, respectively, at the same operating conditions with negligible confinement losses of 8.675 × 10–18 cm−1 1.435 × 10–18 cm−1. Several other parameters, such as the effective area, flattened dispersion, and numerical aperture, have also been obtained during the investigation. Since considerable attention has not been given yet in detecting various types of dairy products using PCF terahertz sensors, this design will pave a whole new path in further implementing THz sensing in the dairy industry. [ABSTRACT FROM AUTHOR]

Terahertz (THz) quantum-cascade lasers (QCLs) are based on complex semiconductor heterostructures, in which the optical gain is generated by intersubband transitions. Using the spacing of the laser modes in the emission spectra, we have determined the effective group refractive index for more than one hundred THz QCLs of the hybrid design with Fabry–Pérot resonators based on single-plasmon waveguides. The experimentally obtained values of for emission frequencies between 2.5 and 5.6 THz generally follow the trend of derived from electromagnetic simulations. However, for a certain number of QCLs, the experimental values of exhibit a rather large deviation from the general trend and the simulation results. From a thorough analysis, we conclude that differences in the optical gain/loss spectra are responsible for this deviation, which lead to a modification of the dispersion in the active region and consequently to altered values of. The analysis also provides evidence that these differences in the gain/loss spectra originate from both, the details of the design and the gain broadening due to interface roughness. [ABSTRACT FROM AUTHOR]

Strong interaction between terahertz (THz) and matter is a topic of paramount importance, considering continuously enhanced interest in THz photonics as well as condensed matter physics, which can lead to the observation of many linear and nonlinear phenomena in the THz regime. Here, we demonstrate a unique and novel metamaterial-based technique of strong THz matter interaction towards thin film sensing, where the analyte is sensed in between the stacked metasurfaces forming the Fano cavity. Sub-wavelength structures typically overcome the diffraction limit of any optical system, which also possess very high confinement of electromagnetic energy. Fano resonance possesses a sharp asymmetric line shape, low radiation loss and large tuning capability. In addition to them, the material under test is placed in between the metasurfaces to utilize the substantial energy confinement leading to strong light matter interaction, a scheme never explored before. By intelligently exploiting the above characteristics, we have demonstrated a novel way to detect both the refractive index (dielectric constant), thickness and loss factor of the material under test when placed between the array of the meta-resonators forming the Fano metamaterials. Our study revealed that the sensitivity and figure of merit (FOM) are strikingly different for dipole and Fano modes. A maximum sensitivity of >1 THz RIU−1 (1.76 × 105 nm RIU−1) and FOM of around 14.05 are achieved at the Fano mode. Additionally, our sensor shows better performance with decreasing spacer thickness (lesser the material, more the sensitivity). Moreover, the proposed device is passive towards typical ambience temperature variation, and is highly compact because of its stacked configuration. The demonstrated device can be extremely beneficial towards realizing ultra-sensitive meta-sensors and other miniaturized THz meta-photonic devices, and bio-chemical sensing where strong light matter interaction is mandatory. [ABSTRACT FROM AUTHOR]

We report the optical constants of the solid CO2 (dry ice) measured using terahertz time domain spectroscopy. The CO2 ice sample was grown by vapor deposition method. The optical constants were obtained by minimizing the amplitude of the oscillations of refractive index in the spectral domain. We found that the mean refractive index of CO2 ice at 10 K is 1.485 between 0.2 and 1.6 THz with a standard deviation of 0.0137. The experiments were accompanied by first-principles density functional theory calculations using full-potential linear augmented plane wave method. The calculated refractive index is found to be 1.37 which is in reasonable agreement with the experimental value. In addition, the vibrational frequencies of solid CO2 were also studied using density functional perturbation theory, and the corresponding modes were assigned. [ABSTRACT FROM AUTHOR]

It is necessary to know the terahertz dielectric constants of ice and various gas hydrates to understand their fundamental chemical and physical properties and their various possible applications. In this paper, we compare and discuss the frequency dependence and temperature dependence of the optical properties of several hydrogen-bonded solid crystals such as ice, propane hydrate and methane hydrate in the terahertz wave region. The refractive indices of ice and propane hydrate were found to be similar, while that of methane hydrate was slightly lower. The reflectivity of methane hydrate was as low as 15% that of ice, and this difference may be useful in various applications. All samples showed the same tendency with respect to the temperature dependency of their refractive indices. There was no large difference in absorption, and all samples exhibited the same frequency dependence and temperature dependence. [ABSTRACT FROM AUTHOR]

We theoretically investigated a polyethylene-based rectangular and guided mode resonance (GMR) structure with a circular pattern by using the finite-difference time-domain (FDTD) method in the terahertz region. As the refractive index of the grating decreased, the resonance frequency increased, and the Q-factor significantly increased because of the change in the effective refractive index. In addition, GMR was investigated with a sensing layer for sensing applications. The resonance frequency and Q-factor could be perfectly modulated by varying the complex refractive index and thickness of the sensing layer. These results indicate that GMR could be applied to highly sensitive label-free detection, using low-cost GMR sensing platforms based on dielectric materials. [ABSTRACT FROM AUTHOR]

The accurate determination of the photothermal response of nanomaterials represents an essential aspect in many fields, such as nanomedicine. Specifically, photothermal cancer therapies rely on the precise knowledge of the light-to-heat transfer properties of plasmonic nanoparticles to achieve the desired temperature-induced effects in biological tissues. In this work, we present a novel method for the quantification of the photothermal effect exhibited by nanoparticles in aqueous dispersions. By combining the spatial and temporal thermal dynamics acquired at terahertz frequencies, the photothermal conversion efficiency associated with the geometry of the plasmonic nanoparticles can be retrieved in a noncontact and noninvasive manner. The proposed technique can be extended to the characterization of all those nanomaterials which feature a temperature-dependent variation of the refractive index in the terahertz regime. [ABSTRACT FROM AUTHOR]