Terahertz Time-domain Research Articles

A magnetic continuum in the cobalt-based honeycomb magnet BaCo2(AsO4)2.

Quantum spin liquids (QSLs) are topologically ordered states of matter that host fractionalized excitations. A particular route towards a QSL is via strongly bond-dependent interactions on the hexagonal lattice. A number of Ru- and Ir-based candidate Kitaev QSL materials have been pursued, but all have appreciable non-Kitaev interactions. Using time-domain terahertz spectroscopy, we observed a broad magnetic continuum over a wide range of temperatures and fields in the honeycomb cobalt-based magnet BaCo2(AsO4)2, which has been proposed to be a more ideal version of a Kitaev QSL. Applying an in-plane magnetic field of ~0.5 T suppresses the magnetic order, and at higher fields, applying the field gives rise to a spin-polarized state. Under a 4 T magnetic field that was oriented principally out of plane, a broad magnetic continuum was observed that may be consistent with a field-induced QSL. Our results indicate BaCo2(AsO4)2 is a promising QSL candidate.

Experimental Detection and Simulation of Terahertz Spectra of Aqueous L-Arginine.

Terahertz (THz) wave is a good candidate for biological sample detection, because vibration and rotation energy levels of biomolecule are in THz band. However, the strong absorption of THz wave by water in biological samples hinders its development. In this paper, a method for direct detection of THz absorption spectra of L-arginine suspension was proposed by using a strong field THz radiation source combined with a polyethylene cell with micrometer thickness in a THz time-domain spectroscopy system. And the THz absorption spectrum of L-arginine solution was simulated by the density functional theory and the simulation result is in good agreement with the experimental results. Finally, the types of chemical bond interaction that cause the absorption peak are identified based on the experimental and simulation results. This work paves a way to investigate the THz absorption spectra and intramolecular interactions of aqueous biological samples.

Disentangling complex current pathways in a metallic Ru/Co bilayer nanostructure using THz spectroscopy

Many modern spintronic technologies, such as spin valves, spin Hall applications, and spintronic THz emitters, are based on electrons crossing buried internal interfaces within metallic nanostructures. However, the complex current pathways within such nanostructures are difficult to disentangle using conventional experimental methods. Here, we measure the conductivity of a technologically relevant Ru/Co bilayer nanostructure in a contact-free fashion using THz time-domain spectroscopy. By applying an effective resistor network to the data, we resolve the complex current pathways within the nanostructure and determine the degree of electronic transparency of the internal interface between the Ru and Co nanolayers.

Evolution of the cyclotron mass with doping in La2−xSrxCuO4

The recent observation of cyclotron resonance in optimally-doped La$_{2-x}$Sr$_x$CuO$_4$ using time-domain THz spectroscopy in high magnetic field has given new possibilities for the study of cuprate superconductors. One can measure the cyclotron mass in more disordered cuprates possesing short scattering times therefore expanding the study to materials and dopings in which quantum oscillations have not been observed. Here we present the measurement of the carrier mass of the hole doped cuprate La$_{2-x}$Sr$_x$CuO$_4$ across a a range of dopings spanning the slightly underdoped ($x=0.13$) to highly overdoped ($x=0.26$), near to the termination of the superconducting dome. These results reveal a systematic increase of $m_c$ with doping, up to values greater than thirteen times the bare electron mass. This is in contrast with those masses extracted from the heat capacity, which show a peak near the pseudogap critical point $p^*$ and/or Lifshitz transition. The cyclotron frequency is linear in field up to 31 T for all dopings giving no evidence for field induced Fermi surface reconstructions. The cyclotron mass is found to be positive for all dopings, but with a magnitude systematically below the heat capacity mass for under and optimally doped samples, while exceeding it for overdoped samples. Among other aspects, these results are surprising as photoemission reveals a Lifshitz transition in the middle of our doping range and the sign of the cyclotron mass determined from a finite frequency resonance is -- in conventional theories -- a {\it topological quantity} only sensitive to whether or not the Fermi surface is closed around holes or electrons. We see no sign of a divergence of the mass near $p^*$ nor near the Lifshitz transition, showing that any singularity -- if it exists -- is not strong enough to affect the cyclotron mass.

Broadband spectroscopy of astrophysical ice analogues

Context. Broadband optical constants of astrophysical ice analogues in the infrared (IR) and terahertz (THz) ranges are required for modeling the dust continuum emission and radiative transfer in dense and cold regions, where thick icy mantles are formed on the surface of dust grains. Such data are still missing from the literature, which can be attributed to the lack of appropriate spectroscopic systems and methods for laboratory studies. Aims. In this paper, the THz time-domain spectroscopy (TDS) and the Fourier-transform IR spectroscopy (FTIR) are combined to study optical constants of CO and CO2 ices in the broad THz-IR spectral range. Methods. The measured ices were grown at cryogenic temperatures by gas deposition on a cold silicon window. We developed a method to quantify the broadband THz-IR optical constants of ices, based on the direct reconstruction of the complex refractive index of ices in the THz range from the TDS data and the use of the Kramers-Kronig relation in the IR range for the reconstruction from the FTIR data. Uncertainties introduced by the Kramers-Kronig relations were eliminated by merging the THz and IR spectra. Finally, the reconstructed THz-IR response was analyzed using classical models of complex dielectric permittivity. Results. The complex refractive index of CO and CO2 ices deposited at the temperature of 28 K was obtained in the range of 0.312.0 THz and fitted using the analytical Lorentz model. Based on the measured dielectric constants, opacities of the astrophysical dust with CO and CO2 icy mantles were computed. Conclusions. The method developed in this work can be used for a model-independent reconstructions of optical constants of various astrophysical ice analogs in a broad THz-IR range. Such data can provide important benchmarks for interpreting broadband observations from existing and future ground-based facilities and space telescopes. The reported results will be useful in modeling sources that exhibit a drastic molecular freeze-out, such as the central regions of prestellar cores and mid-planes of protoplanetary disks, as well as CO and CO2 snow lines in disks.

Field-resolved THz-pump laser-probe measurements with CEP-unstable THz light sources

Radiation sources with a stable carrier-envelope phase (CEP) are highly demanded tools for field-resolved studies of light-matter interaction, providing access both to the amplitude and phase information of dynamical processes. At the same time, many coherent light sources, including those with outstanding power and spectral characteristics lack CEP stability, and so far could not be used for this type of research. In this work, we present a method enabling linear and non-linear phase-resolved terahertz (THz) -pump laser-probe experiments with CEP-unstable THz sources. THz CEP information for each pulse is extracted using a specially designed electro-optical detection scheme. The method correlates the extracted CEP value for each pulse with the THz-induced response in the parallel pump-probe experiment to obtain an absolute phase-resolved response after proper sorting and averaging. As a proof-of-concept, we demonstrate experimentally field-resolved THz time-domain spectroscopy with sub-cycle temporal resolution using the pulsed radiation of a CEP-unstable infrared free-electron laser (IR-FEL) operating at 13 MHz repetition rate. In spite of the long history of IR-FELs and their unique operational characteristics, no successful realization of CEP-stable operation has been demonstrated yet. Being CEP-unstable, IR-FEL radiation has so far only been used in non-coherent measurements without phase resolution. The technique demonstrated here is robust, operates easily at high-repetition rates and for short THz pulses, and enables common sequential field-resolved time-domain experiments. The implementation of such a technique at IR-FEL user end-stations will facilitate a new class of linear and non-linear experiments for studying coherent light-driven phenomena with increased signal-to-noise ratio.

A measurement framework using THz Time-Domain sensing for wood quality assessment across tree ring samples

To investigate the effectiveness of selective tree improvement work, an accurate and calibration-free framework based on THz time-domain sensing for intrinsic wood quality traits assessment was specifically designed for tree ring core sample analysis. This study contains a global phase unwrapping strategy, effective medium theory (EMT)-based measurement, combined measurement, and resolution enhancement, all of which aim to solve and discuss several challenges in THz time-domain sensing. Namely, the global phase unwrapping strategy eliminates the global jumps in phase difference by analysing the principle of jump generation; EMT-based THz measurement can assess properties of hundreds of consecutive sampling points on wood cores without model training; the combined measurement adjusts relative permittivity and dielectric loss relative to one other in a coordinate system; the resolution enhancement converts 3-D PSF into 1-D PSF, and extracts attenuation to achieve resolution enhancement of target properties. The results show that the global phase unwrapping strategy has the potential to deal with phase differences of both homogeneous and heterogeneous samples and can be considered a general unwrapping method. All R2 obtained by combined measurement are higher than that obtained by separate relative permittivity measurement. The resolution enhancement can achieve more accurate and faster measurement. The framework presented here offers significant potential in promoting the application of THz in wood quality traits assessment by providing a low-cost and high precision analysis system.

Temperature tunable electromagnetically induced transparency in terahertz metasurface fabricated on ferroelectric platform

The integration of active materials in terahertz (THz) metasurfaces is pivotal for the realization of functional device applications in diverse fields like sensing, imaging, communication, etc. In this context, ferroelectric materials endowed with tunable electro-optic properties have recently emerged as a novel candidate for achieving actively tuned THz metasurfaces. Here, we experimentally investigate temperature tuning of electromagnetically induced transparency (EIT) effects in a THz metasurface based on ferroelectric barium titanate (BaTiO3 (BTO)) thin film. We characterize tunable dielectric properties of the BTO thin film under variable temperatures (25 °C–100 °C) at THz frequencies by utilizing THz-time domain spectroscopy technique. Based on this aspect, we design a THz metasurface capable of displaying the EIT effect. THz transmissions through the metasurface sample are then probed for different applied temperatures. The EIT features undergo frequency shifts along with amplitude modulations owing to the temperature induced variations of the dielectric properties of the BTO thin film. A total red shift ∼27 GHz in EIT resonance dip is observed experimentally as the temperature increases from 25 °C to 100 °C. Therefore, we demonstrate utilities of ferroelectric platform toward the development of temperature tunable EIT metasurfaces.

Microstructure, Raman spectroscopy, THz time domain spectrum and microwave dielectric properties of Li2Ti1-x(Zn1/3Ta2/3)xO3 ceramics

Li2Ti1-x(Zn1/3Ta2/3)xO3 (0.00 ≤ x ≤ 0.30) (LTZTx) ceramics were successfully prepared by the traditional solid phase method. The order-disorder phase transition for LTZTx ceramics was detected by the X-ray diffractometer (XRD). The scanning electron microscope (SEM) was used to measure the microstructure for LTZTx ceramics. The evolution in crystal structure for LTZTx ceramics was studied by Raman spectroscopy. The reduction of dielectric loss for LTZTx ceramics was caused by the weakening of asymmetric vibration in Li–O bond. The excellent dielectric properties (εr = 18.69, Q×f = 97,763 GHz, τf = +11.08 ppm/°C) were obtained in the LTZT0.15 ceramic fired at 1250 °C. Besides, by the addition of 5 wt % LiF, the sintering temperature of LTZT0.15 ceramic was reduced. The optimal dielectric properties (εr = 19.4773, Q×f = 102,531 GHz, τf = - 8.08 ppm/°C) of LTZT0.15 + 5 wt % LiF were obtained at 1100 °C. The THz time domain spectrum (THz-TDS) was implemented to investigate the polarization mechanism and dielectric properties during 0.5–1.5 THz. The LTZT0.15 and LTZT0.15 + 5 wt % LiF ceramics still own the high Q×f values at the band of THz, which is expected to be used in the future of 6G communication.

Magneto-optical Spectroscopy with RAMBO: A Table-Top 30 T Magnet

Optically probing materials in high magnetic fields can provide enlightening insight into field-modified electronic states and phases, while optically driving materials in high magnetic fields can induce novel nonequilibrium many-body dynamics of spin and charge carriers. While there are high-field magnets compatible with standard optical spectroscopy methods, they are generally bulky and have limited optical access, which prohibit performing state-of-the-art ultrafast and/or nonlinear optical experiments. The Rice Advanced Magnet with Broadband Optics (RAMBO), a unique 30-T pulsed mini-coil magnet system with direct optical access, has enabled previously challenging experiments using femtosecond optical pulses, including time-domain terahertz spectroscopy, in cutting-edge materials placed in strong magnetic fields. Here, we review recent experimental advances made possible by the first-generation RAMBO setup. After summarizing technological aspects of combining optical spectroscopic techniques with the mini-coil magnet, we describe results of magneto-optical studies of a wide variety of materials, providing new insight into the states and dynamics of four types of quasiparticles in solids - excitons, plasmons, magnons, and phonons - in high magnetic fields.

Vanadium-doped CuO: Insight into structural, optical, electrical, terahertz, and full-spectrum photocatalytic properties

Aiming to widen the band gap, minimize the charge-carrier recombination, and enhance the transport properties, we reported the first synthesis of high purity vanadium-doped CuO as a promising photocatalyst nanopowder for water purification. Pure CuO and Cu1-xVxO (where x = 0.00, 0.01, 0.03, 0.05, and 0.07 mol) were synthesized by a facile sol-gel technique. The structural investigation performed by both X-ray and selected area electron diffraction patterns evidenced the formation of highly pure monoclinic CuO. The vanadium inclusion caused a noticeable red shift in Bragg's positions. The grain size was estimated by both Debye-Scherrer and Halder-Wagner methods and ranged from 21 to 28 nm. All the vibrational absorption bands disclosed by FTIR measurement are affiliated with monoclinic CuO exhibiting a small shift for V-doped nanopowders. The stoichiometry of the constituent elements (Cu, O, and V) was assured by energy dispersive X-ray analysis (EDX). The presence of agglomeration is the main feature in field emission scanning electron microscope images revealing the nanostructure property of CuO. The band gap energy of Cu0.93V0.07O determined from diffuse reflectance is 1.44 eV which meets exactly the peak of the solar spectrum rendering it a promisingly photocatalytic material. The rise of absorption in the terahertz range as measured by THz time-domain spectroscopy (THz-TDS) established the increase of density of states owing to the inclusion of vanadium. The incorporation of 0.07 mol of V considerably increases the free sunlight photocatalytic activity of CuO for high concentration Congo red pollutant (20 ppm) from 30 to 76% at the same time of 100 min (over 1.53 times as pure CuO)

Broadband and efficient asymmetric wavefront manipulation via terahertz polarization-selective metasurface

Asymmetric manipulation of electromagnetic waves exhibits tremendous capabilities in the applications of encryption, ultrafast information processing, and communication. However, compact and broadband terahertz (THz) components for asymmetric applications are still scarce. Here, we demonstrate broadband, ultrathin, and efficient metasurface-based asymmetric transmission and wavefront manipulation in the THz region. The metasurface consists of tri-layer metallic structures separated by two flexible polyimide dielectrics. This ultrathin structure exhibits superior asymmetric response which can act as an efficient device with a high linear polarization conversion ratio over 0.9 ranging from 0.5 to 1.1 THz. We also develop THz asymmetric applications based on the flexible metasurface for beam deflector and orbital angular momentum generation. The THz time-domain spectroscopy system and spectral imaging system are used to characterize the broadband asymmetric properties. The experimental results confirm that the beam deflection and vortex waves can be realized over broadband under one linearly polarized incidence while blocked under its orthogonal incidence. This work may provide a promising route to achieve broadband asymmetric applications in information encryption, THz wireless communication, and spectroscopy applications.

Extraction of Fickian water diffusion in polymers using terahertz time-domain spectroscopy

The dynamic behavior of polycarbonate and poly(methyl methacrylate) upon water immersion at temperatures of 40 °C and 70 °C was studied by gravimetric measurements and by terahertz time-domain spectroscopy. Time-resolved measurements of the terahertz refractive index and absorption coefficient show a good agreement to the gravimetric measurements. Assuming a Fickian diffusion mechanic the diffusion coefficients were extracted from both methods. It was found that the terahertz measurements provide a reliable non-contact way to study the water diffusion in polymers. Especially, the THz absorption coefficient seems to be a robust method to determine the diffusion coefficients. • Extraction of Fickian diffusion constants in polymers by THz time domain spectroscopy. • Terahertz optical constants are in good agreement against gravimetric analysis. • A contactless technique to investigate water mobility in polymers.

Nonvolatile chirality switching in terahertz chalcogenide metasurfaces

Actively controlling the polarization states of terahertz (THz) waves is essential for polarization-sensitive spectroscopy, which has various applications in anisotropy imaging, noncontact Hall measurement, and vibrational circular dichroism. In the THz regime, the lack of a polarization modulator hinders the development of this spectroscopy. We theoretically and experimentally demonstrate that conjugated bilayer chiral metamaterials (CMMs) integrated with Ge2Sb2Te5 (GST225) active components can achieve nonvolatile and continuously tunable optical activity in the THz region. A THz time-domain spectroscopic system was used to characterize the device, showing a tunable ellipticity (from ‒36° to 0°) and rotation of the plane polarization (from 32° to 0°) at approximately 0.73 THz by varying the GST225 state from amorphous (AM) to crystalline (CR). Moreover, a continuously tunable chiroptical response was experimentally observed by partially crystallizing the GST225, which can create intermediate states, having regions of both AM and CR states. Note that the GST225 has an advantage of nonvolatility over the other active elements and does not require any energy to retain its structural state. Our work allows the development of THz metadevices capable of actively manipulating the polarization of THz waves and may find applications for dynamically tunable THz circular polarizers and polarization modulators for THz emissions.

Dissociation and Recombination Processes in Lactose Monohydrate Detected by THz Time-Domain Spectroscopy

The terahertz (THz) region has much appeal for differentiating between hydrate systems and for physically characterizing pharmaceutical drug materials. The present study employs THz absorption spectroscopy to investigate the effect of heating on dehydration and hydration in α-lactose monohydrate. Distinctive THz absorption spectra were observed following various heating durations. The THz absorption spectra for α-lactose monohydrate and anhydrous α-lactose exhibit clear differences. Pure α-lactose monohydrate displays clear absorption peaks at 0.53, 1.05, 1.11, 1.33, and 1.56 THz. The complete dehydration of α-lactose monohydrate takes only 15 mins at 145°C (418 K). Moreover, the THz refractive index of α-lactose monohydrate decreases during dehydration. The dehydration of α-lactose monohydrate was also studied using Beer–Lambert law to compare THz absorption spectra as functions of the heating time. The absorption coefficient spectra recorded at 0.53 and 1.35 THz for α-lactose monohydrate after different dehydration times vary linearly with the remaining water content.

Terahertz Spectroscopy and Ab-Initio Vibrational Analysis of Two Crystalline Forms of 5,5-Diethylbarbituric Acid.

Terahertz (THz) absorption spectra of the two crystalline forms of 5,5-diethylbarbituric acid (barbital) were measured with THz time-domain spectroscopy (THz-TDS). The spectra exhibited the dissimilarity between the two forms. Identification of the polymorphs and quantitative analysis of the two forms are possible by THz-TDS. Further, we performed ab-initio calculations of the vibrational modes considering the crystal structures of the two forms, and the results were in good agreement with the experimental data. We discuss the difference between the THz absorption spectra of the two forms.

Integrated system of traditional THz time-domain spectroscopy and asynchronous optical sampling

Terahertz time-domain spectroscopy (THz-TDS) system, as a new means of spectral analysis and detection, plays an increasingly pivotal role in basic scientific research. However, owing to the long scanning time of the traditional THz-TDS system and the complex control of the asynchronous optical scanning (ASOPS) system, which requires frequent calibration, we combine traditional THz-TDS and ASOPS systems to form a composite system and propose an all-fiber trigger signal generation method based on the time overlapping interference signal generated by the collinear motion of two laser pulses. Finally, the time-domain and frequency-domain spectra are obtained by using two independent systems in the integrated systems. It is found that the full width at half maximum (FWHM) of the time-domain spectra and the spectral width of the frequency-domain spectra are almost the same, but the sampling speed of the ASOPS system is significantly faster than that of the traditional THz-TDS system, which conduces to the study of the transient characteristics of substances.

Disorder-enhanced effective masses and deviations from Matthiessen's rule in PdCoO2 thin films

The observation of hydrodynamic transport in the metallic delafossite PdCoO$_2$ has increased interest in this family of highly conductive oxides, but experimental studies so far have mostly been confined to bulk crystals. In this work, the development of high-quality thin films of PdCoO$_2$ has enabled a thorough study of the conductivity as a function of film thickness using both dc transport and time-domain THz spectroscopy. With increasing film thickness from 12 nm to 102 nm, the residual resistivity decreases and we observe a large deviation from Matthiessen's rule (DMR) in the temperature dependence of the resistivity. We find that the complex THz conductivity is well fit by a single Drude term. We fit the data to extract the spectral weight and scattering rate simultaneously. The temperature dependence of the Drude scattering rate is found to be nearly independent of the residual resistivity and cannot be the primary mechanism for the observed DMR. Rather, we observe large changes in the spectral weight as a function of disorder, changing by a factor of 1.5 from the most disordered to least disordered films. We believe this corresponds to a mass enhancement of $\geq 2$ times the value of the bulk effective mass which increases with residual disorder. This suggests that the mechanism behind the DMR observed in dc resistivity is primarily driven by changes in the electron mass. We discuss the possible origins of this behavior including the possibility of disorder-enhanced electron-phonon scattering, which can be systematically tuned by film thickness.

Terahertz probe assisted failure mechanism analysis of silver nanowire film heaters under high working power

The silver nanowire film heaters have the breakdown problem under high working power. It is related with the Rayleigh instability of the silver nanowires and with the electromigration of silver atoms under high electric field localized within nanowire diameter due to the percolation conduction of the silver nanowire films. During the instability of the two structures, the carrier is confined to the nanostructure and suffers different backscattering, which is studied by non-contact Terahertz (THz) spectroscopy. In this paper, structural collapse of heaters presented by THz spectroscopy is to be studied in depth, and the Rayleigh instability and electromigration on device failure are paid attention to by extracting the characteristic parameters of THz time domain spectroscopy and by evaluating the changing trend of the characteristic parameters during device failure. Failure mechanism of heaters under high working power is to be revealed and countermeasures are to be developed. The implementation of this project could drive forward further research on intelligent heaters, which is valuable for scientific research and industrial applications.

Multifrequency Contrast Source Inversion-Based Permittivity Estimation for Terahertz Multilayer Analysis

In this study, the method of estimating permittivity using inverse scattering is presented for multilayered analysis in the terahertz (THz) frequency band. While many studies have been conducted to determine the relative permittivity estimation of multilayer media, most of the studies are based on the time-of-flight (TOF) analysis, which requires the careful consideration of multiple reflection factors and a sufficiently short pulse, i.e., a wider frequency bandwidth, to discriminate the number of TOF from each layer. Therefore, the contrast source inversion-based permittivity estimator exploiting multiple frequency cost function analysis has been introduced. The efficiency of our proposed approach is shown using THz time-domain spectroscopy measurements on two thin layer objects.