Terahertz Spectra Research Articles

Near‐Infrared Reflection Modulation Through Electrical Tuning of Hybrid Graphene Metasurfaces

AbstractGraphene, a 2D material with tunable optical properties, has recently attracted intense interest for reconfigurable metasurfaces. So far, the working wavelength of graphene‐based or hybrid graphene metasurfaces has been limited in the mid‐infrared and terahertz spectra. In this paper, by combining graphene with Au nanostructures, the authors demonstrate a near‐infrared tunable metasurface with decent modulation efficiency, weak dependence on graphene's carrier mobility, and small gate voltages, attributing to the unique interband transition of graphene. The experimental results agree well with numerical simulations. It is also shown that by properly designing the structural parameters of Au nanostructures, the hybrid graphene metasurface can be tunable in both near‐infrared and mid‐infrared regions.

Evaluation of formation and proportion of secondary structure in γ-polyglutamic acid by terahertz time-domain spectroscopy

The study of secondary structure is essential for understanding peptides and proteins. Here, we measured the terahertz (THz) spectra of γ-polyglutamic acid (γ-PGA) dominated by α-helix and random coil (RC) respectively. The α-helix has two absorption peaks in the THz region, but no absorption peak is observed in the RC conformation. We believe this is because the hydrogen bonding effect leads to a higher orientation in the helix-dominated γ-PGA. At lower pH, the absorption intensity of γ-PGA increases with the induction time. Similar changes were obtained in the Fourier infrared spectroscopy (FTIR). Through the correlation analysis of THz and IR spectroscopy, it is found that the characteristic peak at 1.2 THz can be used as a sensitive indicator of the intermediate conformation of the α-helical structure. In addition, the transformation of α-helix-RC conformation is related to the peak intensity at 1.99 THz (R2 = 0.991), which preliminarily indicates that terahertz time-domain spectroscopy (THz-TDS) has the potential to become a new effective method for characterizing and evaluating secondary structure.

Nonvolatile Reconfigurable Electromagnetically Induced Transparency with Terahertz Chalcogenide Metasurfaces

AbstractMetasurface analog of electromagnetically induced transparency (EIT) provides a compact platform for generating a narrow‐band transmission window with very sharp spectral features. They hold promise for many appealing applications including ultrasensitive detectors, slow‐light devices, nonlinear optical devices etc. In particular, reconfigurable EIT metasurfaces are crucial for expanding the capability of light field control, which are promising for terahertz (THz) communications and optical networks. Yet, the investigation on reconfigurable EIT metasurfaces with nonvolatile operation remains scarce. Here, reversible switching of the metasurface‐induced transparency in the THz spectrum is experimentally realized. The reconfigurable response (reversible spectral shift) is obtained by integrating a nonvolatile chalcogenide phase change material, Ge2Sb2Te5 (GST225) into the meta‐atoms. A giant reversible switching of EIT takes place under an excitation of nanosecond laser pulses, showing a reconfigurable group delay of the THz waves. The proposed reconfigurable THz metadevices may provide a new route for the ultrafast laser induced switching and reconfigurable slow‐light devices.

Terahertz Fingerprint of Monolayer Wigner Crystals.

The strong Coulomb interaction in monolayer semiconductors represents a unique opportunity for the realization of Wigner crystals without external magnetic fields. In this work, we predict that the formation of monolayer Wigner crystals can be detected by their terahertz response spectrum, which exhibits a characteristic sequence of internal optical transitions. We apply the density matrix formalism to derive the internal quantum structure and the optical conductivity of the Wigner crystal and to microscopically analyze the multipeak shape of the obtained terahertz spectrum. Moreover, we predict a characteristic shift of the peak position as a function of charge density for different atomically thin materials and show how our results can be generalized to an arbitrary two-dimensional system.

Component spectra extraction and quantitative analysis for preservative mixtures by combining terahertz spectroscopy and machine learning

Preservatives are universally used in synergistic combination to enhance antimicrobial effect. Identify compositions and quantify components of preservatives are crucial steps in quality monitoring to guarantee merchandise safety. In the work, three most common preservatives, sorbic acid, potassium sorbate and sodium benzoate, are deliberately mixed in pairs with different mass ratios, which aresupposedto be the “unknown” multicomponent systems and measured by terahertz (THz) time-domain spectroscopy. Subsequently, three major challenges have been accomplished by machine learning methods in this work. The singular value decomposition (SVD) effectively obtains the number of components in mixed preservatives. Then, the component spectra are successfully extracted by non-negative matrix factorization (NMF) and self-modeling mixture analysis (SMMA), which match well with the measured THz spectra of pure reagents. Moreover, the support vector machine for regression (SVR) designed an underlying model to the target components and simultaneously identify contents of each individual component in validation mixtures with decision coefficient R2 = 0.989. By taking advantages of the fingerprint-based THz technique and machine learning methods, our approach has been demonstrated the great potential to be served as a useful strategy for detecting preservative mixtures in practical applications.

Onset of a superconductor-insulator transition in an ultrathin NbN film under in-plane magnetic field studied by terahertz spectroscopy

Optical conductivity of a moderately disordered superconducting NbN film was investigated by terahertz time-domain spectroscopy in external magnetic field applied along the film plane. The film thickness of about 5 nm was comparable with the coherence length, so vortices should not form. This was confirmed by the fact that no marked difference between the spectra with terahertz electric field set perpendicular and parallel to the external magnetic field was observed. Simultaneous use of Maxwell-Garnett effective medium theory and the model of optical conductivity by Herman and Hlubina proved to correctly reproduce the terahertz spectra obtained experimentally in a magnetic field of up to 7 T. This let us conclude that the magnetic field tends to suppress the superconductivity, resulting in an inhomogeneous state where superconducting domains are enclosed within a normal-state matrix. The scattering rate due to pair-breaking effects was found to linearly increase with magnetic field.

Holographic detection of pulsed terahertz waves in terahertz time-domain spectroscopy

In the study of the interaction of terahertz (THz) wave with material, the traditional THz time-domain spectroscopy (THz-TDS) usually only detects the changes in amplitude and phase information of pulsed THz in a certain polarization direction before and after the interaction with the sample to be tested. However, in the detection of material such as anisotropic material and chiral material, only the amplitude and phase information of the sample cannot give the complete internal structure of the sample material. Anisotropic material and chiral material are very sensitive to pulsed THz of different polarization states. In order to reflect the configurations and conformations of these chiral substances through THz spectrum, it is necessary to detect the amplitude, phase and polarization state of the sample before and after pulse THz waves. The pulsed terahertz holographic detector (PTHD) in this work is composed of photoconductive antenna elements that are perpendicular to each other. The quadrature component of the pulsed THz electric field in any direction can be detected by one-time scanning, so that the changes in amplitude, phase and polarization state before and after the pulsed THz wave acts on the sample can be detected at the same time, so it is called pulsed THz wave holographic detection. Both experiments and theoretical analyses verify the reliability of the PTHD for measuring the polarization state of pulsed THz waves. At the same time, the response matrix is used to analyze that the PTHD has good symmetry in a spectral range of 0.1–2.2 THz.

Semi-Empirical model to retrieve finite temperature terahertz absorption spectra using Morse potential

Terahertz (THz) absorption is a fingerprint property of materials, due to the underlying low-frequency vibration/phonon modes being strongly dependent on the chemical constitutions and microscopic structures. The low excitation energies (0.414−41.4 meV) are related to two intrinsic properties of THz vibrations: the potential energy surfaces (PESs) are shallow, and the vibrationally excited states are usually populated via thermal fluctuations. The shallow PESs make the vibrations usually anharmonic, leading to redshifted vibrational excited state absorption; combined with considerable vibrational excited states population, characteristic THz signals are usually redshifted and congested with varying degrees at different temperatures. Combining existing experimental THz spectra at low temperatures, first principles vibration analysis, and the Morse potential, we developed a semi-empirical model to evaluate the anharmonicity of the low-frequency modes. The model was benchmarked with purine molecular crystal to generate THz spectra at different temperatures, the results were consistent with experiments. The good agreement suggests this model would facilitate the application of THz spectroscopy in molecular crystal characterization.

A Heptagonal Pcf-Based Oil Sensor to Detect Fuel Adulteration Using Terahertz Spectrum

A Heptagonal Pcf-Based Oil Sensor to Detect Fuel Adulteration Using Terahertz Spectrum

Qualitative and Quantitative Study on DL-Glutamic Acid and Its Monohydrate using Terahertz Spectra

The rotation, translation, and vibration of many amino acid molecules fall in the terahertz (THz) region, thus qualitative and quantitative researches on amino acids can be carried out through their THz absorption characteristic fingerprint peaks. In this paper, the room temperature THz absorption spectra of DL-glutamic acid and its monohydrate at 0.5-3.0THz were measured by utilizing terahertz time-domain spectroscopy (THz-TDS). It is found that the THz characteristic peaks of these two amino acids are obviously different. Moreover, according to the change of the absorption amplitude of the characteristic absorption peak (1.24THz) of DL-glutamate monohydrate at different sample concentrations, a quantitative analysis was carried out on the mixed samples of DL-glutamate and its monohydrate, and then the quantitative analysis formula was also verified. In addition, the optical mode of DL-glutamic acid and its monohydrate in THz region was predicted by using density functional theory (DFT). Under the condition that the theoretical data matched with the experimental peaks, the origins of THz absorption peaks obtained in the experiment were discussed and summarized. The results show that the THz characteristic peaks (<2.80THz) of DL-glutamic acid and its monohydrate come from the intermolecular interaction modes, and the other absorption peaks are resulted from the combination of intermolecular and intramolecular interaction modes.

Classification of Rubber Vulcanizing Accelerators Based on Particle Swarm Optimization Extreme Learning Machine and Terahertz Spectra

Classification of Rubber Vulcanizing Accelerators Based on Particle Swarm Optimization Extreme Learning Machine and Terahertz Spectra

Исследование спектров отражения и пропускания массивов гетерогенных ферромагнитных нанопроволок в терагерцовом и дальнем инфракрасном диапазонах

In the frequency range from 16 to 50 THz, the transmission, reflection, and absorption spectra of arrays of heterogeneous Ni/Co, FeNi/Co, and Ni/Fe nanowires grown in track polymer membranes by the galvanic method have been studied. The absorption spectra showed that the fraction of the radiation power absorbed by the nanowires and its spectrum depend on the materials of the nanowires. The features of the spectra can be explained by the accumulation of nonequilibrium spin due to electron diffusion and its relaxation stimulated by external terahertz (THz) radiation, which can be used to create a THz radiation detector. In addition, negative absorption was found for FeNi/Co nanowires, which can be explained by the laser effect on spin-flip transitions, which can be used to create a THz radiation source operating at room temperature.

Study of reflection and transmission spectra of arrays of heterogeneous ferromagnetic nanowires in the terahertz and far infrared ranges

In the frequency range from 16 to 50 THz, the transmission, reflection, and absorption spectra of arrays of heterogeneous Ni/Co, FeNi/Co, and Ni/Fe nanowires grown in track polymer membranes by the galvanic method have been studied. The absorption spectra showed that the fraction of the radiation power absorbed by the nanowires and its spectrum depend on the materials of the nanowires. The features of the spectra can be explained by the accumulation of nonequilibrium spin due to electron diffusion and its relaxation stimulated by external terahertz (THz) radiation, which can be used to create a THz radiation detector. In addition, negative absorption was found for FeNi/Co nanowires, which can be explained by the laser effect on spin-flip transitions, which can be used to create a THz radiation source operating at room temperature. Keywords: nanowires, THz radiation, THz spectroscopy, spin-flip transitions, metamaterial.

Terahertz generation from laser-induced plasma

Interest of the research in terahertz (THz) wave has been strongly motivated by its wide applications in the fields of physics, chemistry, biology, and engineering. Developing efficient and reliable THz source is of uttermost priority in these researches. Numerous attempts have been made in fulfilling the THz generation. Greatly benefited from the progress of the ultrafast pulses, the laser-induced-plasma is one of the auspicious tools to provide desirable THz waves, owing to its superiorities in high power threshold, intense THz signal, and ultrawide THz spectrum. This paper reviews the physics and progress of the THz generation from the laser-induced plasmas, which are produced by gas, liquid, and solid. The characteristics of the emitted THz waves are also included. There are many complicated physical processes involved in the interactions of laser-plasma, making various laser-plasma scenarios in the THz generations. In view of this, we will only focus on the THz generation classified by physical mechanisms. Finally, we discuss a perspective on the future of THz generation from the laser-induced plasma, as well as its involved challenges.

Machine-type communications in NOMA-based terahertz wireless networks

Terahertz (THz) band is one of the most promising aspects of wireless communication systems because of its potential to meet the growing demands for the envisioned next-generation of cellular communications. THz band connectivity can alleviate bandwidth shortages and transmit power constraints using multiple-input multiple-output (MIMO) systems. To obtain better throughput and enhanced spectral efficiency, non-orthogonal multiple access (NOMA) configuration can be incorporated into MIMO systems, as NOMA uses non-orthogonal resource allocation by assigning the same carrier frequency to multiple devices in the power domain. In this paper, we focus on 2 × 1 MIMO-NOMA and cooperative 2 × 1 MIMO-NOMA systems for THz spectrum in the downlink transmission. In order to evaluate the effectiveness of the proposed system architecture, we derive accurate data rate, transmission latency, and reliability expressions for 2 × 1 MIMO-NOMA system consists of critical and non-critical devices by leveraging finite block length theory. Moreover, we derive the closed-form expressions for power splitting coefficients, data rate, transmission latency, and reliability for the considered cooperative 2 × 1 MIMO-NOMA system. Extensive numerical results are presented to validate the feasibility of the proposed 2 × 1 MIMO-NOMA as well as the cooperative 2 × 1 MIMO-NOMA systems in the THz band.

Single- Versus Multicarrier Terahertz-Band Communications: A Comparative Study

The prospects of utilizing single-carrier (SC) and multi-carrier (MC) waveforms in future terahertz (THz)-band communication systems remain unresolved. On the one hand, the limited multi-path components at high frequencies result in frequency-flat channels that favor low-complexity wideband SC systems. On the other hand, frequency-dependent molecular absorption and transceiver characteristics and the existence of multi-path components in indoor sub-THz systems can still result in frequency-selective channels, favoring off-the-shelf MC schemes such as orthogonal frequency-division multiplexing (OFDM). Variations of SC/MC designs result in different THz spectrum utilization, but spectral efficiency is not the primary concern with substantial available bandwidths; baseband complexity, power efficiency, and hardware impairment constraints are predominant. This paper presents a comprehensive study of SC/MC waveforms for THz communications, utilizing an accurate wideband THz channel model and highlighting the various performance and complexity trade-offs of the candidate schemes. Simulations demonstrate that discrete-Fourier-transform spread orthogonal time-frequency space (DFT-s-OTFS) achieves a lower peak-to-average power ratio (PAPR) than OFDM and OTFS and enhances immunity to THz impairments and Doppler spreads, but at an increased complexity cost. Moreover, DFT-s-OFDM is a promising candidate that increases robustness to THz impairments and phase noise (PHN) at a low PAPR and overall complexity.

Field-free spintronic terahertz emitters based on IrMn/Fe/Pt exchage bias heterostructures

Spintronic terahertz (THz) emitter has more advantages such as lower cost, broader spectrum and easier operation than the commercial THz emitters, and thus has become a focus of research towards the next-generation THz source. However, in such a spintronic THz emitter, an external magnetic field is technologically required to align the orientation of the magnetization, which is detrimental for practical applications. Here, a spintronic terahertz emitter based on IrMn/Fe/Pt exchange bias structure is presented. By means of ultrafast spin injection on Fe/Pt interface followed by the spin-to-charge conversion in Pt, plus the effective magnetic field originating from the IrMn/Fe interface, the THz pulse with considerable intensity can be generated in such a structure without the assistance of external field. Besides, the remanent magnetization for thin Fe layer is enhanced by inserting an ultrathin Cu layer between the IrMn surface and the Fe surface, which is beneficial to the field-free THz emission. The range of obtained dynamic THz spectrum exceeds 60 dB and the positive saturation field can reach up to ~ –10 mT by optimizing the multilayer thickness, meeting the standard for commercial application. By rotating the sample, it is found that the polarization direction of the generated THz wave circulates simultaneously and keeps perpendicular to the direction of exchange bias field in the film plane. Moreover, we design a spin valve THz emitter based on the structure of IrMn/Fe/Pt/Fe by adding a free ferromagnetic Fe layer into the exchange bias multilayers. The emitted THz pulse amplitude is larger for the antiparallel alignment of the Fe layers at zero field than for the parallel alignment or exchange bias structure. The present work shows that the spin terahertz emitter based on IrMn/Fe/Pt exchange bias structure can produce the considerable terahertz signals without external field. Furthermore, the polarization direction of the emitted THz signal can be easily manipulated by rotating the sample. Because of this series of advantages, such exchange bias heterostructures are expected to play an important role in designing the next-generation THz source.

Detection of myocardial amyloidosis by a small number of terahertz spectra with low signal-to-noise ratio

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed

Terahertz spectra of electrolyte solutions under applied electric and magnetic fields

Most biomolecules require an aqueous environment to fully exert their biological activity. However, the rotation mode, vibration mode, and energy associated with the hydrogen bonding network of water are in the terahertz band, resulting in strong absorption. Therefore, it is difficult to detect liquid biological samples using the terahertz technology. Here, a high-transmittance double-layer microfluidic chip was prepared using a cycloolefin copolymer material with high transmittance of terahertz waves. Combined with terahertz time-domain spectroscopy, the terahertz spectral characteristics of deionized water, NaCl, NaCO3, and CH3COONa solutions were studied. The changes in the terahertz transmission intensity of these electrolyte solutions under constant electric and magnetic fields were measured. The results show that the terahertz spectra of different sodium salt solutions with the same concentration of 0.9 mol/L are different. Furthermore, the terahertz absorption coefficients of the different electrolyte solutions gradually decrease with the increase of their residence time under the electric field, which is contrary to the results obtained under the external magnetic field. This study provides a new idea for the detection of sodium salt solution and lays a foundation for the development of THz technology.

Unfolding the terahertz spectrum of soft porous crystals: rigid unit modes and their impact on phase transitions

The flexibility of soft porous crystals is influenced by RUMs. Via static DFT calculations, the effect of building block substitutions on the RUMs in winerack MOFs is investigated, giving insight in the flexible nature of these frameworks.