Terahertz Time-domain Research Articles

Nature of Dielectric Relaxation in SrTiO<sub>3</sub>:Mn Single Crystals

Dielectric spectra of SrTiO3and SrTiO3:Mn single crystals have been studied in the frequency range of 10‒3000 cm–1and in the temperature range of 5–297 K using time-domain terahertz spectroscopy and Fourier-transform infrared spectroscopy. A comparative analysis of the experimental results made it possible to detect a significant broadening of the absorption lines corresponding to the Slater and Last phonon modes, while the parameters of the Axe mode when replacing Ti with Mn (2 at %) stay invariant. This effect is associated with an enhance in structural disorder in the cation subsystem (B-sublattice) of the SrTiO3crystal. It has been established that doping with Mn ions reduces the antiferrodistortive phase transition temperature by about 20 K, but hardly affects the character of the temperature dependence of the parameters of a ferroelectric soft mode at temperatures of about 60–297K. It has been found that an additional excitation with the frequency below the frequency of the ferroelectric soft mode should be taken into account for an appropriate model description of the dispersion of the permittivity of SrTiO3:Mn in the terahertz frequency range. The results obtained in this work indicate that dielectric relaxation in the SrTiO3:Mn crystal is due to thermally activated hops of Mn atoms between displaced (noncentral) crystallographic sites; i.e., the mechanism of radiofrequency relaxation in SrTiO3:Mn is hopping rather than polaronic, which is also actively discussed in the literature.

Demonstration of high-throughput magnetic hysteresis measurements based on spintronic THz emission

Spintronic terahertz (THz) emitters have been shown to be a cost-efficient source for use in time-domain THz spectroscopy. The use of external magnetic fields to control the polarity of the THz emission provides an opportunity to measure the magnetization of spintronic materials as well as shaping THz emission. Here, we demonstrate an efficient method of measuring magnetic hysteresis with material sensitivity and speed several orders of magnitude greater than typical magnetometry methods. In addition, we utilize the rapid control of material magnetization for lock-in detection in time-domain THz spectroscopy of spintronic emitters. The ability to rapidly control and measure the material magnetization on very small volumes provides an opportunity to study magnetic hetero-structures with sub-micron spatial resolution.

Influence of neutron irradiation on the electronic properties of hexagonal boron nitride measured by terahertz time-domain spectroscopy

Due to the low atomic number of B, hexagonal boron nitride (hBN) has a large neutron scattering cross section and, therefore, is an ideal material for the realization of solid-state neutron detector. Here we apply the THz time-domain spectroscopy to study the effect of neutron irradiation on electronic properties of pyrolytic (PBN) and hot-pressed boron nitride (HBN). The key electronic parameters of these samples, such as the static dielectric constant ε b , the effective carrier density N*, the carrier relaxation time τ, and the electronic localization factor α, are determined optically, and their dependences upon the neutron irradiation fluence (NIF) are examined. We find that for hBN,N* and ε b decrease while τ and |α| increase with increasing NIF. These results can be used to further understand the neutron irradiation effects on the basic physical properties of hBN material. We believe that the results obtained from this work can benefit to the design and application of hBN material for neutron detectors.

Ionic doping engineered Ce2(Zr1−xBx)3(MoO4)9 (B[dbnd]Cd1/3Nb2/3, Cd1/3Ta2/3) ceramics: Structural, chemical bond, and microwave/terahertz dielectric performance

A series of Ce2(Zr1−xBx)3(MoO4)9 ceramics, where BCd1/3Nb2/3, Cd1/3Ta2/3 (abbreviated as CZ1−xNx, CZ1−xTx), were synthesized using the classical solid-state reaction method. This study focused on the intrinsic mechanisms affected by doping in terms of chemical bond parameters, polarizability, lattice parameters, and bond valence. X-ray diffraction (XRD) results showed that Cd/Nb and Cd/Ta were solidly dissolved into the matrix lattice, which crystallized in a pure trigonal structure (Space group of R-3c, No. 167). According to the Phillips-van Vechten-Levine (P–V-L) theory, the chemical bond features have been parameterized for exploring the relationships between crystal structure and property. Interestingly, the findings indicated that the dielectric parameters varied closely with the chemical bond features of Zr1–O4. Far-infrared reflectivity (FIR) spectrum and terahertz time-domain (THz-TD) were used to obtain information on lattice vibration. The results showed that the complex permittivity extrapolated from THz-TD was closer to the measured value (at microwave) than the former. The relaxation polarization mechanism may be a key intrinsic factor affecting dielectric loss. Additionally, the bond valence results showed a linear relationship between Zr1–O4 and τf. CZ1−xNx and CZ1−xTx exhibited Qf values that were 2.86 and 5.46 times greater than the matrix, respectively, with low permittivity (εr, roughly 10) suitable for high-frequency communication materials. Notably, CZ0.94T0.06 ceramic achieved excellent performance with Qf = 103,974 GHz, εr = 10.63, and τf = −17.68 ppm/°C at 725 °C.

A reconstructing approach to reduce distortion in detection of THz pulses via electro-optic sampling

Terahertz (THz) band is an important range in photonics, and provides numerous advantages in various applications. One of the most popular detection methods for THz pulses is electro-optic sampling (EOS). EOS provides many benefits; however, in this method distortion damages the output signal, and limits the bandwidth of this technique. In this article, a calculation-based approach is proposed to remove the effect of distortion in EOS detection process. This manner is based on definition of a base-level spectrum which modifies the output of EOS to eliminate unwanted disorders. The introduced method is computational, inexpensive, feasible, fast, adaptable, and effective. Moreover, a detailed comprehensive step-by-step model of THz time-domain spectroscopy (THz-TDS) with a simple and obvious perspective for ZnTe and GaP crystals is provided.

Design and fabrication of a microcoil metamaterial absorber for the sub-terahertz region.

The development of electromagnetic wave absorbers operating in the sub-terahertz (sub-THz) region is necessary in 6G communications. We designed and fabricated a sub-THz metamaterial absorber based on metal microcoils embedded and periodically arranged in a dielectric substrate. The microcoil parameters were optimized by calculating the electromagnetic response of the metamaterial using finite element analysis. An actual metamaterial was then fabricated based on the optimized parameters and characterized using THz time-domain spectroscopy. Our microcoil absorber exhibits an absorptance of >80% and a high shielding performance at about 250 GHz. The resonance frequency can be precisely adjusted by modifying the microcoil array dimensions.

Optical analysis of aligned Ni nanowire arrays with different degree of oxidation for terahertz polarizer application

The optical properties of aligned nickel nanowire arrays (NiNWAs) with different degrees of oxidation for terahertz (THz) polarizer applications have been investigated by using THz time-domain spectroscopy. In frequency-domain spectra, the full width at half maxima of transmitted peaks was broadened and the peak positions have a blue shift with increasing oxidation levels, besides the enhancement in peak intensity. It is indicated that the oxidation of Ni nanowires (NWs) has a significant influence on the interaction between Ni NWs and THz wave. The transmittance of the aligned NiNWAs increases with annealing temperature increasing. Conversely, the degree of polarization and extinction ratio (ER) decreases. A corresponding relationship between the change of ER and degree of oxidation is summarized by means of thermogravimetric analysis. The change of ER for the annealing sample with the degree of oxidation of 0.507% is 27.32%, which induced the polarization properties of aligned NiNWAs to be sensitive to the oxidation of Ni NWs. These findings can provide new positive features in the development of future polarization-based device applications for THz electronics and photonics.

Propagation of THz radiation in air over a broad range of atmospheric temperature and humidity conditions

As the need for higher data rates for communication increases, the terahertz (THz) band has drawn considerable attention. This spectral region promises a much wider bandwidth and the transmission of large amounts of data at high speeds. However, there are still challenges that need to be addressed before the THz telecommunications technology hits the consumer market. One of the recurring concerns is that THz radiation is greatly absorbed by atmospheric water-vapor. Although many studies have presented the attenuation of THz signals under different atmospheric conditions, these results analyze specific temperature or humidity values, leaving the need for a more comprehensive analysis over a wider range of climate conditions. In this work, we present the first study of the attenuation of THz radiation over a broad range of temperatures and humidity values. It is worth noticing that all of our measurements have been undertaken at atmospheric pressure unlike many previous studies where the pressure was not kept constant for various temperatures. Furthermore, we extend our analysis beyond the impact of absolute humidity on the bit error rate in THz communications. We also discuss the refractivity of the atmosphere, examining its variations across different temperatures and humidity levels. THz propagation is studied using two different measurement systems, a long-path THz time-domain spectrometer as well as a quasi-optic setup with vector network analyze. We also compare the results with the ITU-R P.676-13 propagation model. We conclude that the attenuation at the absorption peaks increases linearly with water content and has no dependence on the temperature, while the refractive index, away from absorption lines, namely at 300 GHz shows a sub-linear increase with humidity.

Continuous-wave terahertz in-line holographic diffraction tomography with the scattering fields reconstructed by a physics-enhanced deep neural network

Diffraction tomography is a promising, quantitative, and nondestructive three-dimensional (3D) imaging method that enables us to obtain the complex refractive index distribution of a sample. The acquisition of the scattered fields under the different illumination angles is a key issue, where the complex scattered fields need to be retrieved. Presently, in order to develop terahertz (THz) diffraction tomography, the advanced acquisition of the scattered fields is desired. In this paper, a THz in-line digital holographic diffraction tomography (THz-IDHDT) is proposed with an extremely compact optical configuration and implemented for the first time, to the best of our knowledge. A learning-based phase retrieval algorithm by combining the physical model and the convolution neural networks, named the physics-enhanced deep neural network (PhysenNet), is applied to reconstruct the THz in-line digital hologram, and obtain the complex amplitude distribution of the sample with high fidelity. The advantages of the PhysenNet are that there is no need for pretraining by using a large set of labeled data, and it can also work for thick samples. Experimentally with a continuous-wave THz laser, the PhysenNet is first demonstrated by using the thin samples and exhibits superiority in terms of imaging quality. More importantly, with regard to the thick samples, PhysenNet still works well, and can offer 2D complex scattered fields for diffraction tomography. Furthermore, the 3D refractive index maps of two types of foam sphere samples are successfully reconstructed by the proposed method. For a single foam sphere, the relative error of the average refractive index value is only 0.17%, compared to the commercial THz time-domain spectroscopy system. This demonstrates the feasibility and high accuracy of the THz-IDHDT, and the idea can be applied to other wavebands as well.

Ultrafast spin-to-charge conversion in antiferromagnetic (111)-oriented L12-Mn3Ir

Antiferromagnetic L12-Mn3Ir combines outstanding spin-transport properties with magnons in the terahertz (THz) frequency range. However, the THz radiation emitted by ultrafast spin-to-charge conversion via the inverse spin Hall effect remains unexplored. In this study, we measured the THz emission and transmission of a Permalloy/(111)-oriented L12-Mn3Ir multilayer by THz time-domain spectroscopy. The spin Hall angle was determined to be approximately constant at 0.035 within a frequency range of 0.3–2.2 THz, in comparison with the THz spectroscopy of a Permalloy/Pt multilayer. Our results not only demonstrate the potential of L12-Mn3Ir as a spintronic THz emitter but also provide insights into the THz spin transport properties of L12-Mn3Ir.

Terahertz Metasurfaces Exploiting the Phase Transition of Vanadium Dioxide.

Artificially designed modulators that enable a wealth of freedom in manipulating the terahertz (THz) waves at will are an essential component in THz sources and their widespread applications. Dynamically controlled metasurfaces, being multifunctional, ultrafast, integrable, broadband, high contrasting, and scalable on the operating wavelength, are critical in developing state-of-the-art THz modulators. Recently, external stimuli-triggered THz metasurfaces integrated with functional media have been extensively explored. The vanadium dioxide (VO2)-based hybrid metasurfaces, as a unique path toward active meta-devices, feature an insulator-metal phase transition under the excitation of heat, electricity, and light, etc. During the phase transition, the optical and electrical properties of the VO2 film undergo a massive modification with either a boosted or dropped conductivity by more than four orders of magnitude. Being benefited from the phase transition effect, the electromagnetic response of the VO2-based metasufaces can be actively controlled by applying external excitation. In this review, we present recent advances in dynamically controlled THz metasurfaces exploiting the VO2 phase transition categorized according to the external stimuli. THz time-domain spectroscopy is introduced as an indispensable platform in the studies of functional VO2 films. In each type of external excitation, four design strategies are employed to realize external stimuli-triggered VO2-based THz metasurfaces, including switching the transreflective operation mode, controlling the dielectric environment of metallic microstructures, tailoring the equivalent resonant microstructures, and modifying the electromagnetic properties of the VO2 unit cells. The microstructures' design and electromagnetic responses of the resulting active metasurfaces have been systematically demonstrated, with a particular focus on the critical role of the VO2 films in the dynamic modulation processes.

Refractive index measurement of IP-S and IP-Dip photoresists at THz frequencies and validation via 3D photonic metamaterials made by direct laser writing

Direct laser writing (DLW) is widely used to fabricate complex metamaterials (MMs) and photonic devices for nanoscale applications across the electromagnetic frequency spectrum. While the optical properties of conventional photoresists used in DLW are well studied in the visible and infrared range, it is still unclear how they behave at lower frequencies. Here, we measure the refractive index and absorption of IP-S and IP-Dip photoresists within the THz range of the electromagnetic spectrum. Further, we utilize THz time-domain spectroscopy (THz-TDS) to experimentally measure the laser-processed three-dimensional (3D) MM structures. We conduct full-wave electromagnetic simulations using the measured refractive index values to validate our experiments. The THz-TDS measurements are in excellent agreement with the theoretical predictions verifying the validity of our refractive index measurements. This study aims to support and lead future investigations utilizing standard DLW photoresists for photonic applications in the THz range.

Nondestructive evaluation of aircraft stealth coating by Terahertz-time domain spectroscopy: experimental and numerical investigation

ABSTRACT Particle-reinforced polymer matrix composites (PMCs) are often used as radar-absorbing materials (RAM) in aircraft stealth applications. The thickness evaluation of such multilayered coatings with micrometre-level thickness is highly challenging from single-side access. In this study, we used Terahertz Time-domain Spectroscopy (THz-TDS) in the transmission mode to extract the refractive index of the dielectric coatings for measuring the thickness using the time-of-flight method. A numerical study based on the Finite Element method (FEM) has also been developed to validate the transmission experiments. A frequency-dependent complex dielectric parameters must be considered for coatings with high absorption in the THz regime. This was addressed by performing the finite element simulations in a frequency domain. The thickness of each layer of a multi-layered coating is estimated by carrying out the experiments in reflection mode. Since the interface echoes were overlapping, a deconvolution algorithm and frequency thresholding were employed to reconstruct the signals reflected from the different interfaces. Using this technique, the thickness of each layer of coating is estimated accurately in a single measurement, which was challenging to measure using other conventional non-destructive testing (NDT) methods.

Investigation of resonant properties of metamaterial THz filters fabricated from vanadium dioxide thin films

Vanadium dioxide (VO2) is a promising candidate for electronic and optical switching applications in the terahertz frequency range due to the metal to insulator transition (MIT). In this study, the use of VO2 patterned as a metamaterial surface or coupled as a homogeneous layer with a metallic metamaterial surface on top is investigated in terms of performance. High-quality VO2 thin films were deposited on c-cut sapphire substrates by using the dc magnetron sputtering technique. A change in resistivity by a factor of 104 MIT in VO2 was observed allowing to investigate its use as a controllable metamaterial. The layer was patterned using a unique geometry (four-cross shaped) that operates in the THz frequency range. To understand its performance as a tunable THz filter, the four-cross structure fabricated from VO2 is compared to one fabricated from Au on VO2 bare film using UV lithography and ion beam etching techniques. The spectral performances of metamaterials were assessed using THz-Time Domain Spectroscopy (THz-TDS) and results were compared with simulations based on CST Microwave Studio. Absence of the resonant effects in the purely developed VO2 device, while clear observation of the MIT behavior shows the strong dependency of the inductive and/or capacitive effects of the four-cross structure on conductivity of the surface metamaterial, which is clearly observable for the Au-based device. In the latter case, the resonant transmittance of the filter can be effectively modulated by change in temperature.

Magnetic-field-induced spin reorientation in a c-cut TmFeO3 single crystal observed by terahertz spectroscopy

In this study, we utilized time-domain terahertz magneto-optical spectroscopy to investigate the spin resonance in a c-cut TmFeO3 single crystal at T = 1.6 K under different magnetic fields and characterized its intricate internal interactions. By selectively exciting the magnetic resonance modes in the crystal, we found that as the magnetic field increases, the quasi-ferromagnetic (q-FM) mode of TmFeO3 single crystal shifts towards higher frequencies, while the quasi-antiferromagnetic (q-AFM) mode transforms into the q-FM mode at low critical magnetic fields (2.2–3.6 T). Through magnetic structure analysis and theoretical calculations, it is confirmed that the magnetic moment of TmFeO3 undergoes a magnetic-field-induced spin reorientation from Γ2 to Γ4 phase. Materials with magnetic moment flipping at low magnetic fields may find applications for antiferromagnetic spintronic devices in the future.

Enhanced terahertz magneto-optical performance in substrate-free ultra-thick TbErBi:RIG crystal films

A wafer-scale single crystal thick film of rare-earth iron garnet (RIG) has been successfully produced on a 3-in. gadolinium gallium garnet (GGG) substrate using the liquid phase epitaxy method. The RIG crystal's thickness measures ∼550 μm. By removing the GGG substrate through polishing, we improved the terahertz (THz) transmittance of the RIG crystal. In the frequency range of 0.1–1.0 THz, the RIG material exhibits a large refractive index of around 4.50, with a transmittance of ∼60% and an absorption rate of only 10–50 cm−1. Furthermore, we investigated the THz magneto-optical effect in the RIG material through THz time-domain spectroscopy. The observed results demonstrate the presence and significance of the magneto-optical effect in the RIG crystal. To provide further insights, we measured the THz Faraday rotation angle of the 550 μm-thick RIG crystal using the THz-TDS system under an external magnetic field of 0.17 T. The measured Faraday rotation angle reached 22°, and the calculated Verdet constant for the RIG sample was ∼120°/mm/T. Considering these findings, our study highlights the unique properties of this wafer-scale single crystal thick film of RIG, including its low loss, high transmission, and strong magneto-optical effect in the THz range. These characteristics make it a promising candidate for various applications, such as THz magnetic polarization conversion, non-reciprocal phase shifters, and isolators.

Solvation Plays a Key Role in Antioxidant-Mediated Attenuation of Elevated Creatinine Level: An In Vitro Spectroscopic Investigation.

An elevated level of creatinine (CRN) is a mark of kidney ailment, and prolonged retention of such condition could lead to renal failure, associated with severe ischemia. Antioxidants are clinically known to excrete CRN from the body through urine, thereby reducing its level in blood. The molecular mechanism of such an exclusion process is still illusive. As the excretion channel is urine, solvation of the solute is expected to play a pivotal role. Here, we report a detailed time-domain and frequency-domain terahertz (THz) spectroscopic investigation to understand the solvation of CRN in the presence of two model antioxidants, mostly used to treat elevated CRN level: N-Acetyl-l-cysteine (NAC) and ascorbic acid (ASC). FTIR spectroscopy in the mid-infrared region and UV absorption spectroscopy measurements coupled with quantum chemical calculations [at the B3LYP/6-311G++(d,p) level] reveal that both NAC and ASC form HBonded complexes with CRN and rapidly undergo a barrier-less proton transfer process to form creatinium ions. THz measurements provide explicit evidence of the formation of highly solvated complexes compared with bare CRN, which eventually enables its excretion through urine. These observations could provide a foundation for designing more beneficial drugs to resolve kidney diseases..

Frequency selective fingerprint sensor: the Terahertz unity platform for broadband chiral enantiomers multiplexed signals and narrowband molecular AIT enhancement

Chiral enantiomers have different pharmacological and pharmacokinetic characteristics. It is important to strictly detect chiral component for avoiding being harmful to the human body due to side effects. Terahertz (THz) trace fingerprint detection is essential because the molecular vibrations of various biological substances such as chiral enantiomers are located in THz range. Recent reported enhanced trace fingerprint technologies have some drawbacks. For instance, multiplexing technology suffered from narrow operation range and limitation by frequency resolution of commercial THz time domain spectroscopy; Absorption induced transparency (AIT) identification for narrowband molecular oscillations suffered from random resonance frequency drift due to fabrication error. In this paper, we proposed frequency-selective fingerprint sensor (FSFS), which can experimentally achieve enhanced trace fingerprint detection by both broadband multiplexing technology and robust AIT identification. Such FSFS is based on polarization independent reconfiguration metasurfaces array. Broadband absorption lines of trace-amount chiral carnitine were boosted with absorption enhancement factors of about 7.3 times based on frequency-selective multiplexing at 0.95–2.0 THz. Enhanced trace narrowband α-lactose fingerprint sensing can be observed at several array structures with absorption enhancement factors of about 7 times based on AIT, exhibiting good robustness. The flexibility and versatility of proposed FSFS has potential applications for boosting trace chiral enantiomer detection as well as diversity of molecular fingerprints identification by both multiplexing and AIT.

Terahertz emission with remarkable efficiency through spherical nanoparticles using dark-hollow-Gaussian beams

Simultaneous tuning of Terahertz (THz) radiation with respect to its frequency, power and focus is still a challenge for the scientific community, because of which its use is lacking in several areas. To achieve such properties, the present work proposes monodisperse graphite nanoparticles (NPs) of spherical shape dispersed in two orthogonal directions with normal vector of their basal planes parallel and perpendicular to the electric field of the dark-hollow-Gaussian beams (DHGBs) used for the generation of THz radiation. The NPs are impinged upon by two DHGBs of slightly different frequencies, which exert a ponderomotive force on the conduction electrons of the NPs. The motion of these electrons under the action of high intensity laser beams constitutes a macroscopic nonlinear current that produces the THz radiation. The well-structured THz emission occurs with the efficiency reaching 10−2 when the NPs resonate with the beating frequency, and they are arranged in their parallel orientations with respect to the electric field of the lasers. The proposal and the obtained results will contribute to the THz science and technology along with their use in medical science and THz time-domain spectroscopy.

THz time-domain spectroscopy modulated with semiconductor plasmonic perfect absorbers.

Terahertz time-domain spectroscopy (THz-TDS) at room temperature and standard atmosphere pressure remains so far the backbone of THz photonics in numerous applications for civil and defense levels. Plasmonic microstructures and metasurfaces are particularly promising for improving THz spectroscopy techniques and developing biomedical and environmental sensors. Highly doped semiconductors are suitable for replacing the traditional plasmonic noble metals in the THz range. We present a perfect absorber structure based on semiconductor III-Sb epitaxial layers. The insulator layer is GaSb while the metal-like layers are Si doped InAsSb (∼5·1019 cm-3). The doping is optically measured in the IR with polaritonic effects at the Brewster angle mode. Theoretically, the surface can be engineered in frequency selective absorption array areas of an extensive THz region from 1.0 to 6.0 THz. The technological process is based on a single resist layer used as hard mask in dry etching defined by electron beam lithography. A wide 1350 GHz cumulative bandwidth experimental absorption is measured in THz-TDS between 1.0 and 2.5 THz, only limited by the air-exposed reflectance configuration. These results pave the way to implement finely tuned selective surfaces based on semiconductors to enhance light-matter interaction in the THz region.