High transmittance silicon terahertz polarizer using wafer bonding technology

The -carotene is a short chain polyene molecule containing nine -electron conjugated double-bonds. Because of its special molecular structure, -carotene has been used widely in many fields, including functional materials, optoelectronic devices and biological applications of light collection, light protection, anti-cancer, etc. Recently, new applications of -carotene in generation and detection of terahertz (THz) wave have also attracted great attention. In this work, all-trans -carotene films are prepared by spray coating, and the THz spectra in a wavenumber range of 30-400 cm-1 (a frequency range of 0.9-12 THz) of the as-prepared products are experimentally measured at room temperature by Fourier transform infrared spectroscopy. For comparison, the THz spectra in 0.5-3.0 THz are also characterized at the same temperature by THz time-domain spectroscopy. Based on these measurements, the fingerprint peaks of all-trans -carotene in the THz region are experimentally identified to be located at 54 cm-1 (1.62 THz), 57 cm-1 (1.71 THz), 64 cm-1 (1.91 THz), 77 cm-1 (2.32 THz), 90 cm-1 (2.69 THz), 98 cm-1 (2.95 THz), 115 cm-1 (3.45 THz), 124 cm-1 (3.72 THz), 134 cm-1 (4.02 THz), 170 cm-1 (5.11 THz), 247 cm-1 (7.42 THz), and 279 cm-1 (8.38 THz), respectively. It is worth noting that the recent results about the THz spectra of palm leaves are thus verified. Particularly, the B3 LYP method of density functional theory is further utilized in this work to theoretically simulate the THz spectra of all-trans -carotene molecule. It is revealed that the theoretical simulation results accord well with those experimentally measured data. In addition, we also find that the absorption peaks are caused by the torsion, deformation and rocking vibration of the molecules. Accordingly, the vibrational modes of the measured THz characteristic peaks at 148 cm-1 (4.44 THz), 132 cm-1 (3.96 THz), 115 cm-1 (3.45 THz), 76 cm-1 (2.28 THz) and 52 cm-1 (1.56 THz) are theoretically assigned, which provides a reference to explain the formation mechanism of the THz spectra. The valuable results presented in this work will be helpful for promoting the studies of the THz spectral features and response mechanisms of the organics.

We present frequency-dependent absorption coefficients and refractive indices of sulfur-containing biomolecules in the far-infrared region measured by terahertz (THz) time-domain spectroscopy (TDS). The THz radiation was generated by a surface photocurrent method with a (100)InAs wafer as a THz emitter, and its intensity was enhanced by applying a magnetic field of 1.6 T. The THz radiation was detected by the electro-optic sampling method with a ZnTe crystal. The characterization of the apparatus of THz TDS is discussed in detail. The spectral features in the THz region are different among sulfur-containing biomolecules of L,L-cystine, L-cysteine, L-methionine, and oxidized glutathione in powder. These results demonstrate that THz TDS is a promising method for studies on the low-frequency spectra of complex molecules and identification of materials. We present a calculation method to analyze the THz spectra based on a model function for dynamics of the dipoles, which allows us to obtain spectral information characterizing the low-frequency responses of the molecules. The experimental results are compared with ab initio calculations to discuss assignment of the vibrational modes.

The terahertz (THz) time-domain spectroscopy technique was used to characterize the cooling process of solid n-alkanes. The THz waveforms of n-octadecane, n-nonadecane, n-eicosane, n-heneicosane, n-docosane, and n-pentacosane were obtained with the cooling time using the aforementioned noncontact optical method. The peak values of the THz signal were found to be related to the cooling temperature of n-alkanes. The THz wave was sensitive to the size and structure of particles in the liquid; therefore, the crystallization process of n-alkanes was monitored. An empirical equation based on signal attenuation was proposed to quantitatively distinguish the content change of structural order in the samples. Results present a new noncontact optical approach for characterizing wax crystallization via THz time-domain spectroscopy.

We demonstrate real-time continuous-wave terahertz (THz) line-scanned imaging based on a 1 × 240 InGaAs Schottky barrier diode (SBD) array detector with a scan velocity of 25 cm/s, a scan line length of 12 cm, and a pixel size of 0.5 × 0.5 mm². Foreign substances, such as a paper clip with a spatial resolution of approximately 1 mm that is hidden under a cracker, are clearly detected by this THz line-scanning system. The system consists of the SBD array detector, a 200-GHz gyrotron source, a conveyor system, and several optical components such as a high-density polyethylene cylindrical lens, metal cylindrical mirror, and THz wire-grid polarizer. Using the THz polarizer, the signal-to-noise ratio of the SBD array detector improves because the quality of the source beam is enhanced.

By terahertz (THz) time-domain spectroscopy we have measured low-frequency spectra of amino acid (glycine; Gly), short peptides ((Gly)3and (Gly)4), six globular proteins and bacteriorhodopsin (BR). From the analysis of the THz spectra we defined and obtained the reduced absorption cross sections for these cases, which are proportional to the vibrational density of states. We observed anharmonic behaviors in the low-frequency modes of the short peptides. The globular proteins we investigated show a universal feature in the low-frequency spectra. BR shows the dynamical transition in the temperature dependence of the THz spectrum when the sample is hydrated.

Terahertz (THz) emission from spintronic THz emitters (STEs) has been extensively studied, both for its potential application in THz technologies and as a tool for probing spin dynamics, conductivity, and crystal anisotropy via time-domain THz spectroscopy. However, substrate birefringence can modify the emitted THz polarization, distorting the measured spectra and leading to potential misinterpretation of the results. While some aspects of substrate birefringence on THz emission have been studied in the time domain, its effects on THz spectra have not been discussed in detail. In the THz emission spectrum from STEs, we observe distinct spectral signatures that depend on sample orientation. To understand these signatures, we use STEs grown on c-cut sapphire and systematically vary the orientation of a (100) rutile TiO2 window in both pump-through and THz-through geometries to identify the birefringence-induced effects. These findings demonstrate that substrate birefringence plays a critical role in the emitted THz spectrum and must be carefully considered in the analysis of time-domain THz spectroscopy data. We also show that artifacts from smaller birefringence can have a more misleading effect on the THz spectra.

Given that terahertz (THz) radiation responds to intermolecular forces such as hydrogen bonds, THz time-domain spectroscopy (THz-TDS) has expanded possibilities in cellulose research. In this study, THz-TDS was used to investigate the crystallinity of three types of cellulose-based materials. Microcrystalline cellulose (MCC) and wood were ball milled at different times, and pseudo-wood was a mixture of MCC and lignin of different mass fractions. All the samples showed peaks at 3.04 THz in the THz mass absorption coefficient spectra. Further, the spectra from 2.79 THz to 3.32 THz were cut out and detrended by subtraction from a baseline. The integrated intensity of the detrended spectra showed a correlation with the mass fraction of lignin of the pseudo-wood samples, and ball milling time of the MCC and wood samples. The correlation was similar with the crystallinity index calculated from X-ray powder diffraction. Moreover, the original wood sample without ball milling had an integrated intensity that was about 30% that of the original MCC sample, matching with the cellulose concentration of the wood (about 30% to 40%). We normalized the integrated intensity of 2.79 THz to 3.32 THz into 1 to 0 by a min–max algorithm and proposed a new “index” for evaluating crystallinity.

Terahertz filters based on the frequency selective surfaces (FSS) structure have been modeled and fabricated on the sapphire substrate. The terahertz (THz) transmission properties are investigated using a THz time-domain spectroscopy (THz-TDS) system. The designed two FSS-based THz filters exhibit the THz electromagnetic wave transmission and reflection characteristics, respectively, showing about 70% change of THz transmission at the frequency of 1.3 THz. It is suggested that the THz filtering effect results from the resonance behavior between THz plane-wave and resonant elements of designed metallic FSS structures. The agreement between experiments and simulations indicate that the designed THz filter show a great potential application for high-speed THz modulator.

The knowledge of the surface resistance Rs of superconducting thin film at microwave and terahertz (THz) regions is significant to design, make and assess superconducting microwave and THz electronic devices. In this paper we reported the Rs of MgB2 films at microwave and THz measured with sapphire resonator technique and the time-domain THz spectroscopy, respectively. Some interesting results are revealed in the following: (1) A clear correlation is found between Rs and normal-state resistivity right above Tc, ρ0, i.e., Rs decreases almost linearly with the decrease of ρ0. (2) A low residual Rs ,l ess than 50µ� at 18 GHz is achieved by different deposition techniques. In addition, between 10 and 14 K, MgB2 has the lowest Rs compared with two other superconductors Nb3Sn and the high-temperature superconductor YBa2Cu3O7−δ(YBCO). (3) From THz measurement it is found that the Rs of MgB2 up to around 1 THz is lower than that of copper and YBCO at the temperature below 25 K. (4) The frequency dependence of Rs follows ω n ,w hereω is angular frequency, and n is power index. However, n changes from 1.9 at microwave to 1.5 at THz. The above results clearly give the evidences that MgB2 thin film, compared with other superconductors, is of advantage to make superconducting circuits working in the microwave and THz regions.

Melamine is an organic base and a trimer of cyanamide, with a 1, 3, 5-triazine skeleton. It is usually used for the production of plastics, glue and flame retardants. Melamine combines with acid and related compounds to form melamine cyanurate and related crystal structures, which have been implicated as contaminants or biomarkers in protein adulterations by lawbreakers, especially in milk powder. This paper is focused on developing an available method for quantitative detection of melamine in the fields of security inspection and nondestructive testing based on THz-TDS. Terahertz (THz) technology has promising applications for the detection and identification of materials because it exhibits the properties of spectroscopy, good penetration and safety. Terahertz time-domain spectroscopy (THz-TDS) is a key technique that is applied to spectroscopic measurement of materials based on ultrafast femtosecond laser. In this study, the melamine and its mixture with polyethylene powder in different consistence are measured using the transmission THz-TDS. And we obtained the refractive index spectra and the absorption spectrum of different concentrations of melamine on 0.2-2.8THz. In the refractive index spectra, it is obvious to see that decline trend with the decrease of concentration; and in the absorption spectrum, two peaks of melamine at 1.98THz and 2.28THz can be obtained. Based on the experimental result, the absorption coefficient and the consistence of the melamine in the mixture are determined. Finally, methods for quantitative detection of materials in the fields of nondestructive testing and quality control based on THz-TDS have been studied.

Terahertz (THz) is an electromagnetic spectrum with a frequency range from 0.1 to 10 THz, which is located in between the microwave and infrared regions. The unique features of THz waves make them eligible for use in various medical applications. The THz imaging is one of them, which is mainly based on the analysis and processing of the transmission and reflection spectrum information of the sample. This chapter mainly presents the research status and prospects of several THz medical imaging systems and their applications for medical imaging in biological tissues. As the demand for technology grows, high-performance THz imaging systems are becoming indispensable. The ability of the THz time-domain spectroscopy (THz-TDS) system to extract spectra from amplitude and phase information opens virtually unlimited possibilities for imaging applications. The performance achievements of THz-TDS-based imaging with potential research on its fast-imaging components for solving the existing limitations of imaging speed are also presented. Furthermore, the latest developments of several THz-TDS-based imaging methods, including tomography imaging and near-field imaging, are highlighted with their performance improvement. Additionally, the rapid development of THz-TDS as a highly versatile analytical tool for the characterisation of pharmaceutical materials has drawn considerable attention. One of the greatest biomedical potentials of THz imaging is the use of molecular spectroscopy for diagnostics, which is exponentially advanced and moving closer to progress. It is now evident that different types of biomolecules leave distinctive spectral fingerprints in the THz region, which considerably widens the coverage of its technology application including in-vitro and in-vivo measurements of small molecules of clinical importance in point of care and diagnostic systems. In-vivo molecular imaging is considered the next frontier in medical diagnostics, which would be ideally performed non-invasively. Recent achievements in the field of medical imaging have dramatically enhanced the early detection and treatment of many pathological conditions. THz imaging systems can help in detecting early cancer before it is visible or sensitive to any other identification resources. The THz images can distinguish between healthy tissue and basal cell carcinoma and therefore help in mapping the exact margins of early-stage tumours. By obtaining both frequency and time-domain information, the THz imaging can ensure enhanced detection of cancer and provide sharper imaging and molecular fingerprinting. THz biomedical imaging has become a modality of interest due to its ability to simultaneously acquire both image and spectral information. Advanced digital image processing algorithms are greatly needed to assist in screening, diagnosis and treatment. Finally, we summarise the obstacles in the way of the application of THz biomedical imaging application technology in clinical detection, which need to be investigated and overcome in the future.

Incoherent terahertz (THz) sources can be compact, stable, reliable and cheaper alternative to coherent emitters for compact THz systems. Low power of incoherent THz emitters can be compensated by benefit from extremely high sensitivity of THz micro-detectors. Incoherent THz torch device based on Ga(As,Bi)/AlGaAs parabolic quantum well (PQW) is modeled, fabricated and experimentally investigated paying special attention to the THz spectral range and optical properties. The structures for optical characterization were grown using MBE technique on GaAs substrates. The experimental study was carried out to measure the absorption by intersubband transition of electrons in PQW using vacuum Fourier transform spectroscopy, conventional farinfrared Fourier transform spectroscopy and THz time-domain spectroscopy. Transmittance spectra dependence on temperature and photo-excitation of undoped structures for THz torch device is studied exploring impact of the PQW on THz spectra.

Electron beam (e-beam) irradiations are often involved for characterizing graphene-based terahertz (THz) devices or realizing graphene surface plasmons. Here, based on THz time-domain spectroscopy (TDS) and two-dimensional scanning system, the time dependence and the in situ mapping images of changed THz conductivity of graphene induced by e-beam irradiations are studied. The change in THz signals with irradiation indicates a decrease in the THz conductivity as a result of electron doping in graphene through irradiation. And the spatial imaging maps of the decreased THz conductivity reveal diverse electron doping speeds in graphene during different current e-beam irradiations. Additionally, different theoretical doping models are given for explanations of the imaging maps and the calculated results by doping models are in good accordance with the experimental results. Our findings are of significance for understanding the change in THz conductivity and carrier transport of graphene under e-beam irradiations.

Despite significant advancements in terahertz (THz) generation and detection technologies, the practical deployment of portable THz systems remains constrained by existing modulator limitations, particularly stringent temperature requirements, inefficient thermal management, and high power consumption. Here, we demonstrate a breakthrough in magnetically controlled THz modulation using a CoFe2O4/MgO/F-Mica composite structure. The tunable optical response and dielectric properties of the composite under varying magnetic fields were systematically investigated via THz time-domain spectroscopy. Experimental results revealed a magnetic field-dependent THz transmission attenuation, with systematic suppression of transmission spectra proportional to the applied magnetic field intensity. A theoretical model accounting for external magnetic field variations was proposed, which agrees well with the experimental results regarding the imaginary component. Remarkably, the composite material realizes phase modulation simultaneously under magnetic field. This exceptional magnetic responsiveness significantly broadens the application potential of ferrites in the THz regime. These findings provide critical insights for designing tunable multifunctional THz magnetic devices in 6G communications, medical diagnostics, and nondestructive testing applications.

Terahertz time-domain spectroscopy has emerged as a competent technique for real-world applications such as quality control in pharmaceutical and food industry, biomedical and precision-agriculture. However, investigating the optical response of the practical composite samples such as pharmaceutical tablets, biological composites and nano-composites using Terahertz (THz) time-domain spectroscopy (TDS) is quite challenging. In the present work, we have prepared two series of binary composites, with different volume fraction of constituent materials. These composites were probed using pulsed THz TDS and their dielectric response was modeled using the effective medium theory (EMT). Based on the volume fraction of the constituent material, three EMT models, including Maxwell-Garnet (MG), Polder-van-Santen (PvS) and Landau-Lifshitz-Looyenga (LLL), were compared empirically with the conventional analysis at different frequencies; and the accuracy and ambiguity of the extracted parameters using these quasi-static models were determined. Hence, the feasibility study of these theoretical models was discussed to assess their practicality.