Terahertz Response Research Articles

Mapping active strain using terahertz metamaterial laminates

The ability to image strain fields in composite materials is an indispensable necessity for structural health monitoring. Embedded electromagnetic metamaterials sensitive to applied stresses and operating in the terahertz regime have been proposed as a solution, but they have traditionally relied on slight amplitude or frequency shifts of their terahertz spectral resonance relative to a presumably unchanging, unstrained reference spectral map. Here, we demonstrate a facile, reference-free imaging technique to map the currently active local strain throughout a composite structure using a reversible passive terahertz metamaterial laminate with a tailored polarimetric signature. Only two orthogonal polarizations and a few frequencies are required to measure this strong terahertz response, from which the local strain environment may be rapidly and quantitatively mapped over large areas.

A Terahertz Detector Based on Double-Channel GaN/AlGaN High Electronic Mobility Transistor.

A double-channel (DC) GaN/AlGaN high-electron-mobility transistor (HEMT) as a terahertz (THz) detector at 315 GHz frequency is proposed and fabricated in this paper. The structure of the epitaxial layer material in the detector is optimized, and the performance of the GaN HEMT THz detector is improved. The maximum responsivity of 10 kV/W and minimum noise equivalent power (NEP) of 15.5 pW/Hz0.5 are obtained at the radiation frequency of 315 GHz. The results are comparable to and even more promising than the reported single-channel (SC) GaN HEMT detectors. The enhancement of THz response and the reduction of NEP of the DC GaN HEMT detector mainly results from the interaction of 2DEG in the upper and lower channels, which improves the self-mixing effect of the detector. The promising experimental results mean that the proposed DC GaN/AlGaN HEMT THz detector is capable of the practical applications of THz detection.

Impact of nitrogen doping on the linear and nonlinear terahertz response of graphene

It is well known that impurities play a central role in the linear and nonlinear response of graphene at optical and terahertz frequencies. In this work, we calculate the bands and intraband dipole connection elements for nitrogen-doped monolayer graphene using a density functional tight-binding approach. Employing these results, we calculate the linear and nonlinear response of the doped graphene to terahertz pulses using a density-matrix approach in the length gauge. We present the results for the linear and nonlinear mobility as well as third-harmonic generation in graphene for adsorbed and substitutional nitrogen doping for a variety of doping densities. We show that the conduction bands are more parabolic in graphene structures with substitutional nitrogen doping than for those with adsorbed nitrogen. As a result, substitutional doping has a greater impact on the terahertz mobility and nonlinear response of graphene than adsorbed nitrogen does.

Anomalous nonlinear terahertz transmission of photoexcited carbon nanotubes

We demonstrate that free-standing multi-walled carbon nanotubes exhibit extraordinary nonlinear terahertz responses upon optical excitation. Terahertz transmission of the photoexcited nanotubes rises in a narrow range of intermediate intensity with increasing intensity, while falling in the regime of low and high intensities. A theoretical analysis shows that the nanotube conductivity drops sharply in the region of intermediate intensity and soars elsewhere. Field-effect mobility and field-induced carrier multiplications are considered to be competing processes governing the rise and fall of the conductivity.

Terahertz response of monolayer and few-layer WTe2 at the nanoscale

Tungsten ditelluride (WTe2) is an atomically layered transition metal dichalcogenide whose physical properties change systematically from monolayer to bilayer and few-layer versions. In this report, we use apertureless scattering-type near-field optical microscopy operating at Terahertz (THz) frequencies and cryogenic temperatures to study the distinct THz range electromagnetic responses of mono-, bi- and trilayer WTe2 in the same multi-terraced micro-crystal. THz nano-images of monolayer terraces uncovered weakly insulating behavior that is consistent with transport measurements. The near-field signal on bilayer regions shows moderate metallicity with negligible temperature dependence. Subdiffractional THz imaging data together with theoretical calculations involving thermally activated carriers favor the semimetal scenario with Delta approx -10,{{{rm{meV}}}} over the semiconductor scenario for bilayer WTe2. Also, we observed clear metallic behavior of the near-field signal on trilayer regions. Our data are consistent with the existence of surface plasmon polaritons in the THz range confined to trilayer terraces in our specimens. Finally, data for microcrystals up to 12 layers thick reveal how the response of a few-layer WTe2 asymptotically approaches the bulk limit.

A Universal Approach for Maximizing Terahertz Wave Absorption in Graphene Cut-Wires

Graphene micro-/nanostructures and their arrays have attracted considerable attention in infrared (IR) and terahertz (THz) applications due to their strong plasmon responses. However, as too many parameters, including geometry, carrier concentration, frequency, and adjacent substrate, can affect the plasmonic behaviors of the micro-/nanostructures, the optimization of the THz-IR responses, such as absorption and reflection, of these structures and their arrays require tremendous computations on parameter scanning. Here, we propose a theoretical approach to design graphene cut-wires with maximized THz wave absorption. Analytical expression describing the THz absorption/reflection of graphene cut-wires is derived. Accordingly, a maximum THz wave absorption of the array, regardless of its operating frequencies and geometrical parameters, can be achieved by simply tuning the cut-wires duty ratio. The analytical results are further validated by numerical simulations. This intuitive design manner is of significance for the design of graphene arrays with high-efficiency THz responses as well as promoting their practical applications in THz functional devices.

Comprehensive analysis of Terahertz frequency response of Bi2Se3 and Bi2Te3 single crystals using Terahertz time-domain spectroscopy

We report the Terahertz time-domain spectroscopy (THz-TDS) dielectric response of the large single crystal of Bi2Se3 and Bi2Te3, which are grown in-house through a solid-state reaction route via the self-flux method technique. We fit the obtained experimental complex dielectric constant to the Lorentz oscillator model and obtain various resonating mode strengths in both materials. The strength of dielectric constants is an order of magnitude different while comparing the Terahertz response of both the crystals. However, the dielectric relaxation times are similar for stronger modes, depicting the similarity between the two crystals.

Microwave and terahertz properties of porous Ba4(Sm,Nd,Bi)28/3Ti18O54 ceramics obtained by sacrificial template method

AbstractMicrowave and terahertz communications are increasingly significant, however, the lack of material information in terahertz band limits their development. Moreover, few lightweight materials with a high relative dielectric constant () are found suited for satellite communication and wearable devices. In this study, we developed lightweight porous Ba4[(Sm0.1Nd0.9)0.9Bi0.1]28/3Ti18O54 (BSNBT) ceramics exhibiting a total porosity ranging from 6.3% to 26.5% (bulk density ranging from 5.47 to 4.29 g/cm3) and relatively high ranging from 85.6 to 56.8, which were obtained by sacrificial template method using polymethyl methacrylate spheres (PMMAs) of varying average particle sizes, from 9 to 34 μm, as sacrificial materials. A high refractive index ranging between 7.5 and 8.9 and a low absorption coefficient of approximately 17 cm−1 at 0.3 THz were obtained for the porous ceramics with different total porosities derived from PMMAs with average particle sizes of 9 and 19 μm. Furthermore, effective medium and Mie scattering theories were applied to understand the effects of porous structure on the dielectric properties in microwave and terahertz frequency ranges, respectively, owing to the different wavelengths in the BSNBT matrix. The results of this study suggest that introducing a porous structure can effectively exploit lightweight microwave dielectric ceramic materials and provide valuable information on their terahertz response mechanism.

Rapid prototyping of a bispectral terahertz-to-infrared converter.

Conversion of terahertz radiation into thermal radiation is a promising approach for the development of low cost terahertz instruments. Here, we experimentally demonstrate bispectral terahertz-to-infrared conversion using metamaterials fabricated using a rapid prototyping technique. The converter unit cell is composed of two metal-insulator-metal (MIM) antennas absorbing independently the terahertz radiation at 96 and 130 GHz and a thin carbon nanotubes (CNT) layer used as a thermal emitter. The converter unit cell has a typical λ/100 thickness and sub-wavelength lateral dimensions. The terahertz absorption of the converter was observed by monitoring its thermal emission using an infrared camera. Within the first hundred milliseconds of the terahertz pulse, thermal radiation from the CNTs only increases at the location of the MIM antennas, thus allowing to record the terahertz response of each MIM antenna independently. Beyond 100 ms, thermal diffusion causes significant cross-talk between the pixels, so the spectral information is more difficult to extract. In a steady state regime, the minimum terahertz power that can be detected is 5.8 µW at 130 GHz. We conclude that the converter provides a suitable low-cost solution for fast multi-spectral terahertz imaging with resolution near the diffraction limit, using an infrared camera in combination with a tunable source.

Strong confinement-induced nonlinear terahertz response in semiconductor nanostructures revealed by Monte Carlo calculations

Nonlinear terahertz conductivity spectra of charges confined in semiconductor nanostructures were calculated using a semiclassical Monte Carlo method. The confinement-induced nonlinear response per charge carrier is much stronger than the intrinsic nonlinearity of common bulk semiconductors and more than 20 times stronger than in graphene, which has been considered as a material with one of the highest terahertz nonlinearities. Moderate intensities of the terahertz radiation are thus sufficient to achieve efficient frequency mixing or high-harmonics generation. Enclosing the nanostructures into metallic nanoslits concentrates the electric field into the semiconductor and thus easily provides nonlinear terahertz signal strength comparable to the linear one.

Characterization and discrimination of human colorectal cancer cells using terahertz spectroscopy

Terahertz technology has been widely used in biomedical research. Herein, terahertz time-domain attenuated total reflection (THz TD-ATR) spectroscopy was employed to characterize and discriminate human cancer cell lines (DLD-1 and HT-29). Terahertz responses of the cell lines were measured and Savitzky-Golay algorithm was applied to smooth the spectra of refractive index, absorption coefficient and dielectric loss tangent in terahertz regime. Principal component analysis (PCA) was then adopted for feature extraction and cell characterization. Based on the processed data, cancer cell lines were discriminated by applying random forests (RF) method to analyze three characteristic parameters separately and the results from them were compared. Results indicate that absorption coefficient was the most sensitive parameter for cancer cell discrimination. Our study suggests great potential for human cancer cell recognition and provides experimental basis for liquid biopsy.

Manipulating terahertz responses of GdFeO3 ceramics by thickening and thermal methods

Electric field-induced antiferromagnets with ultrahigh resonant frequencies without current flow in insulating materials is highly desired. This paper reports the terahertz (THz) responses of Γ4 in rare-earth orthoferrite GdFeO3 ceramics, which were manipulated using thickening and thermal methods. As the thickness was increased, the noise was greatly reduced, and increased peak heights of both the ferromagnetic resonances and antiferromagnetic resonances were observed. For the thermal control of THz responses in GdFeO3, the resonant frequencies of both the ferromagnetic resonances and antiferromagnetic resonances experienced a blue shift, and their peak heights increased. The detected shifts of the responses were determined by the changes in thermodynamic potential. Moreover, macroscopic properties were collected to verify the coexistence of ferromagnetism and antiferromagnetism.

Heterointerface-Enhanced Ultrafast Optical Switching via Manipulating Metamaterial-Induced Transparency in a Hybrid Terahertz Graphene Metamaterial.

We have demonstrated the active manipulation of metamaterial-induced transparency (MIT) in a terahertz hybrid metamaterial with graphene overlayer under photoexcitation. It is found that the introduction of graphene can greatly modify the resonant dips and transparency window through the formed depolarization field around unequal-length double bars to weaken dipole resonances and their destructive interference. Transient control of MIT behaviors is determined by the photogenerated carrier dynamics, which influences the distributions of currents and electric fields in the resonant region to hinder the near-field coupling of two bright modes. Optical modulation depth is sensitive to bar spacing due to an anomalous increased double-bar coupling involving intracell and intercell interaction. Heterointerface formed by the added graphene with substrate could further enhance terahertz response via effective separation of the photoexcited carriers. Theoretical calculation based on the coupled Lorentz oscillator model reveals that the photoinduced terahertz response mainly originates from the coupling and damping in hybrid structures. Our findings could facilitate the development of graphene-based dynamical terahertz modulators and optoelectronic devices.

Probing lattice vibration of alkali halide crystals by broadband terahertz spectroscopy

Terahertz spectral features of alkali halide crystals were studied with the combination of broadband terahertz time-domain spectroscopy and the solid-state-based density functional theory calculations. To understand the particular modes of the observed terahertz features of the alkali halide crystals, the resonant modes of KCl and CsCl were analyzed using face-centered cubic and body-centered cubic lattice models, respectively. The results show that the characteristic terahertz absorption peaks could be assigned to the lattice vibration of the ionic crystals. Furthermore, the terahertz responses of a series of alkali halides were recorded, and obvious absorption peaks were observed in each salt in the frequency region below 8.5 THz. What is more interestingly is that the frequencies of these observed peaks are red-shifted with the increases of the mass and radius of the ions. This correlation between the resonant frequency of the lattice vibration, the reduced atomic mass, and the equilibrium distance between the ions agrees well with the harmonic oscillator model.

Terahertz Polaron Oscillations of Electrons Solvated in Liquid Water.

The terahertz (THz) response of solvated electrons in liquid water is studied in nonlinear ultrafast pump-probe experiments. Free electrons with concentrations from c_{e}=4 to 140×10^{-6} moles/liter are generated by high-field THz or near-infrared multiphoton excitation. The time-resolved change of the dielectric function as mapped by broadband THz pulses exhibits pronounced oscillations persisting up to 30ps. Their frequency increases with electron concentration from 0.2 to 1.5THz. The oscillatory response is assigned to impulsively excited coherent polarons involving coupled electron and water shell motions with a frequency set by the local electric field.

Smoothing method to directly denoise terahertz signals in rare‐earth orthoferrite antiferromagnets

AbstractRare‐earth orthoferrites with weak ferromagnetism have become one of the most fascinating topics in antiferromagnet research because of their potential application in future information technologies. However, an appropriate and direct way to reduce the noise in the intrinsic but weak resonances of rare‐earth orthoferrites is currently lacking, creating difficulties in analysis. Here, we report a numerical smoothing method to directly denoise the detected terahertz (THz) responses of orthoferrites using a B‐spline algorithm. For comparison, the Savitzky–Golay smoothing method, which is a typical and widely used numerical smoothing method, was also used to process the data. The thickness‐dependent signals of DyFeO3 were processed by the above methods. LaFeO3, HoFeO3, and DyFeO3 were chosen and prepared, and their thermally tunable signals were processed. The B‐spline signal smoothing algorithm was shown to have stability and robustness to noise and was more effective than the Savitzky–Golay smoothing method at reducing noise. This work on smoothing thickness‐ and temperature‐dependent THz signals may help provide a promising approach to reduce noise in the intrinsic but weak resonances of rare‐earth orthoferrites.

A Novel Method of On-Line Coal-Rock Interface Characterization Using THz-TDs

The core problem in unmanned/intelligent working face of coal mining is the automatic adjustment of shearer arm where the coal-rock interface detection is the key. The cutting location of shearer drum affect the proportion of coal and rock powder around the cutting teeth of shearer drum. Therefore, the method of on-line coal rock interface characterization using Terahertz Time Domain spectroscopy (THz-TDs) we proposed aims to estimate the ratio of rock by Terahertz response. Firstly, anthracite and quartz sandstone were uniformly mixed according to 39 different ratios in this study, the samples' responses were obtained by terahertz system, and then the obtained time domain data was converted into frequency domain data by fast Fourier transform. The absorption coefficient spectrum and the refractive index profile of the 39 samples were calculated by optical parametric model. Secondly, corresponding quantitative model between mixed coal/rock powder and THz signal was built by using back propagation neural network (BPNN) and least squares support vector machine (LSSVM). We expected to use the ratio of rock powder detected by the model to estimate the depth of shearer drum teeth embedded in the rock layer. Finally, we found that both two mathematical arithmetic is feasible to quantitatively detect different proportion of coal and rock mixtures. The results show that the depth of shearer drum teeth embedded in the rock layer could be estimated by the novel method, which means the coal-rock interface could be on-line characterized by using THz-TDs and the height of the drum could be adjusted in time.

TCAD Modeling for Si–Ge HBT THz Detectors

Terahertz (THz) response of transistor and integrated circuit yields important information about device parameters and has been used for distinguishing between working and defective transistors and circuits. Using a TCAD model for SiGe HBTs we simulate their current-voltage characteristics and their response to sub-THz (300\,GHz) radiation. Applying different mixed mode schemes in TCAD, we simulated the dynamic range of the THz response for SiGe HBTs and showed that it is comparable with that of the TeraFET detectors. The HBT response to the variations of the detector design parameters are investigated at different frequencies with the harmonic balance simulation in TCAD. These results are useful for the physical design and optimization for the HBT THz detectors and for the identification of faulty SiGe HBT and Si BiCMOS circuits using sub-THz or THz scanning.

A Study of the Characteristics of a Terahertz Radiation Detector for the Solntse–Terahertz Scientific Apparatus

This paper provides a brief description individual elements that can be included in the Solntse–Terahertz scientific apparatus that was designed for the first time to carry out an extra-atmospheric experiment onboard the International Space Station. Its purpose is to measure terahertz electromagnetic radiation both from the quiet Sun and during active processes on the Sun (solar flares, coronal mass ejections, etc.), which is necessary to establish the physical nature of solar activity and solar flares. The possibility is discussed of using optoacoustic converters (Golay cells) as receivers of terahertz radiation, the sensitivity, stability, and response time of which were determined in the course of preliminary laboratory studies under terrestrial conditions.

Terahertz response of DL-alanine: experiment and theory.

The terahertz (THz) spectrum of dl-alanine has been measured for the first time at cryogenic temperatures and with a pure sample. Several sharp absorptions are observed, over a wide frequency range (0.8-4.8 THz), at 8 K. The sample structure and purity were confirmed with both Raman spectroscopy and X-ray diffraction. Temperature dependent spectra revealed redshifting, with increasing temperature, for all modes except one at 2.70 THz. This mode exhibits blueshifting until ≈120 K, where it starts to redshift. A Bose-Einstein distribution has been used to model the frequency shift with temperature for the four lowest energy modes. Strong correlations between the fits and data indicate that these modes are caused by phonon excitation in an anharmonic potential. Density functional theory has also been used to identify the origin of these low frequency modes. They are attributed to large scale molecular vibrations.