Terahertz Wave Absorption Research Articles

A Terahertz-Microfluidic Chip with a Few Arrays of Asymmetric Meta-Atoms for the Ultra-Trace Sensing of Solutions

Biosensing with terahertz (THz) waves has received large amounts of attention due to its potential to detect the functional expression of biomolecules in a label-free fashion. However, many practical challenges against the diffraction limit of THz waves and the strong absorption of THz waves into polar solvents still remain in the development of compact biosensors. Here, we present a non-linear, optical, crystal-based THz-microfluidic chip with a few arrays of asymmetric meta-atoms, an elementary unit of metamaterials, for the measurement of trace amounts of solution samples. A near-field THz emission source, that is locally generated in the process of optical rectification at a fs (femtosecond) laser irradiation spot, induces a sharp Fano resonance and modifies the resonance frequency of the meta-atoms when the channel is filled with solution samples of different concentrations. Using this chip, we successfully detected minute changes in the concentration of trace amounts of mineral water and aqueous sugar solutions by monitoring the shift in the resonance frequency. A higher detectable sensitivity of 1.4 fmol of solute in a 128 pL volume of solution was achieved. This was an improvement of one order of magnitude in the sensitivity compared to our previous experiment.

Bond theory, terahertz spectra, and dielectric studies in donor‐acceptor (Nb‐Al) substituted ZnTiNb2O8 system

AbstractAs a donor and acceptor separately, Nb5+ and Al3+ are used to substitute Ti4+ in the ixiolite ZnTiNb2O8 system for the first time. The dielectric responses in the terahertz range of this system are initially studied based on the data from terahertz time‐domain transmitted spectroscopy. Combined with ligand field theory, the formation of a secondary phase of ZnAl2O4 is reasonably explicated. Then, the origins of the lower dielectric loss and the terahertz wave absorption coefficient are determined to be the high chemical bond covalency and effective phase control. For the composition of ZnTi0.85(Al0.5Nb0.5)0.15Nb2O8 sintered at 1180°C, a low dielectric loss (<5 × 10−3 @ 0.5 THz) and absorption coefficient (<10/cm @ 0.5 THz) are obtained, which make this kind of material propitious for developing terahertz dielectric devices.

Terahertz wave absorption for alkylcyclohexyl-isothiocyanatobenzene liquid crystal materials

According to density functional theory, in this paper we report a simulation result obtained by using the Gaussian09 package. Adopted in the calculation are an optimized Opt Freq and a base group of B3LYP/6-311g to simulate the absorption of 16 kinds of liquid crystal (LC) molecules of 4-(trans-4-n-alkylcyclohexyl) isothiocyanatobenzenes (CHBT) in a 0.1−5.0 terahertz band (THz). The results show that in the low terahertz band, the absorption is caused mainly by the vibration and rotation of the molecules. So for convenience, we present an novel analytical method of studying the influence of molecular moment of inertia and mass center of gravity shift on absorption. An important result is found that the length of the molecular alkyl chain can lead to different molecular mass, mass center of gravity and moment of inertia, which causes the rotation and vibration of the molecule to be different. These factors lead to the difference in terahertz wave absorption. In the 0.1−5.0 terahertz band, the molecules with 3−7 alkyl chain carbon atoms show a strong absorption. As a reference, reducing and increasing the carbon atoms in the alkyl chain will cause the molecules to reduce the absorption of terahertz waves . In the end, the calculated results are compared with the experimental results obtained from 10 molecules according to the reference data in a frequency range of 0.3−3.0 terahertz. It is found that in the low frequency band there exist some differences between the calculation results and the experimental measurements, in which the difference in the position of the absorption peak may originate from a hydrogen bond. Comparing the relative magnitudes of the absorption intensities, it is found that the experimental measurements are consistent with the calculated results, indicating that the absorption intensity comes from the absorption of dipole vibration and rotation, which demonstrates the positive significance of computational simulation. We look forward to the experimental measurements in the future, and correct the calculation methods and keywords as well as the parameters such as temperature calculation that is to be done in future work. As a theoretical basis, the calculation results can better reflect the absorption of molecular materials, and it is expected to provide useful suggestions for designing and synthesizing the liquid crystal molecules.

室温微测辐射热计太赫兹探测阵列技术研究进展(特邀)

Terahertz (THz) detector with thermal sensitive microbolometer structure indicates many prominent features, such as wide band detecting region, large array pixels, high integrated device, real-time imaging, comparing with some other room-temperature THz technologies. Microbolometer structure using VOx thin film as sensitive material has been successfully fabricated uncooled infrared focal plane array detector, but unsuitable for THz detection for very low THz wave absorption ratio. So some special design should be presented in the microbolometer structure to achieve high performance THz detector. Room-temperature THz technologies and development of microbolometer type THz detector array were introduced in this article briefly, and also the research about THz absorption material or structure in University of Electronic Science and Technology of China was presented in the paper.

Giant impact of self-photothermal on light-induced ultrafast insulator-to-metal transition in VO2 nanofilms at terahertz frequency.

Ultrafast detection and switching of light are key processes in high-speed optoelectronic devices. However, the performances of VO2-based optoelectronics are strongly degraded by photothermal. The mechanism of the latter is still unclear. Here, by using femtosecond-laser (fs-laser) driven kinetic terahertz wave absorption, we quantitatively separate slow photothermal response and ultrafast photodoping response (e.g. light-induced insulator-to-metal transition) from second- to picosecond-timescales, and discover the competing interplay between them. With self-photothermal (mainly determined by fs-laser pulse repetition rate and pump fluence), the ultrafast transition time was degraded by 190% from 50 ps to 95 ps, the ultrafast transition threshold was decreased to 82% from 11mJ/cm2 to 9mJ/cm2, while the amplitudes of the two photoresponse are competing. Percolation theory, along with the macroscopic conductivity response, is used to explain the competing interplay. Our findings are relevant for designing and optimizing VO2-based ultrafast optoelectronic devices.

Nanofluidic terahertz metasensor for sensing in aqueous environment

The terahertz spectral region has received tremendous attention for label free chemical and biological sensing, due to the presence of molecular fingerprints, low energy characteristics, and remote sensing capabilities. However, a major hindrance for the realization of a high performance terahertz bio-chemical sensor comes from the large absorption of terahertz waves by aqueous solution. Here, we overcome this limitation by confining the analyte-aqueous solution in a nanovolumetric fluidic chamber, integrated on metamaterial resonant cavities. The metamaterial resonators confine electromagnetic fields in extremely subwavelength space and hence allow for the enhanced interaction between the nanovolumetric analyte-aqueous solution and terahertz waves, while minimizing the absorption loss. We compare the sensing performance of split ring resonator and Fano resonator systems as metamaterial resonators. As a demonstration of chemical sensing, three alcoholic solutions with different concentrations were measured. Selective adenosine triphosphate (ATP) sensing capability was examined through ATP aptamer functionalization on gold metamaterials, where a decrease in the transmittance value was observed as the ATP concentration increased. The proposed sensing approach has the potential to be an effective tool for molecular analysis through exploiting the advantages offered by low energy terahertz, subwavelength metamaterial resonators and nanofluidic technologies.

Hybridization-induced broadband terahertz wave absorption with graphene metasurfaces

Electromagnetic (EM) wave absorption plays a vital role in photonics. While metasurfaces are proposed to absorb EM waves efficiently, most of them exhibit limited bandwidth and fixed functionalities. Here, we propose a broadband and tunable terahertz (THz) absorber based on a graphene-based metasurface, which is constructed by a single layer of closely patterned graphene concentric double rings and a metallic mirror separated by an ultrathin SiO2 layer. Plasmonic hybridization between two graphene rings significantly enlarges the absorption bandwidth, which can be further tuned by gating the graphene. Moreover, the specific design also makes our device insensitive to the incident angle and polarization state of impinging EM waves. Our results may inspire certain wave-modulation-related applications, such as THz imaging, smart absorber, tunable sensor, etc.

Modified model of equivalent height for predicting atmospheric attenuation at frequencies below 350 GHz

Due to atmospheric absorption, the signal attenuation must be considered when designing communication links in the terahertz (THz) band. The equivalent height described in International Telecommunication Union Recommendation (Rec. ITU)-R P.676 can be used to evaluate the signal attenuation over satellite links by meteorological parameters of the Earth's surface, which are relatively convenient to obtain. Since both temperature and water vapour content can greatly influence the absorption of THz waves and have temporal and spatial variability, the equivalent height should be a combined value representing the influences of temperature, water vapour, and pressure. However, only pressure is considered in the ITU-R P.676 model. As a result, it will cause inaccuracies for different regions and seasons. In this study, a modified model of the equivalent height with temperature and water vapour near the Earth's surface is proposed. Compared with the forward equivalent heights, the equivalent heights based on both models are evaluated. Analyses of the window frequencies are performed, and improved results are achieved with the modified model, especially for mid- and high-latitude winter profiles. The accuracy of the modified model for other profiles is also analysed and the results indicate its suitability.

Measurement of Absorption Coefficient of Paraformaldehyde and Metaldehyde with Terahertz Spectroscopy

The characteristic absorption spectra of paraformaldehyde and metaldehyde in the terahertz frequency region are obtained by terahertz time-domain spectroscopy (THz-TDS). In order to reduce the absorption of terahertz (THz) wave by water vapor in the air and the background noise, the measurement system was filled with dry air and the measurements were conducted at the temperature of 24°C. Meanwhile, the humidity was controlled within 10% RH. The THz frequency domain spectra of samples and their references from 0 to 2.5 THz were analyzed via Fourier transform. The refractive index and absorption coefficients of the two aldehydes were calculated by the model formulas. From 0.1 to 2.5 THz, there appear two weak absorption peaks at 1.20 and 1.66 THz in the absorption spectra of paraformaldehyde. Only one distinct absorption peak emerges at 1.83 THz for metaldehyde. There are significant differences between the terahertz absorption coefficients of paraformaldehyde and metaldehyde, which can be used as "fingerprints" to identify these substances. Furthermore, the relationship between the average absorption coefficients and mass concentrations was investigated and the average absorption coefficient–mass concentration diagrams of paraformaldehyde and metaldehyde were shown. For paraformaldehyde, there is a linear relationship between the average absorption coefficient and the natural logarithm of mass concentration. For metaldehyde, there exists a simpler linear relationship between the average absorption coefficient and the mass concentration. Because of the characteristics of THz absorption of paraformaldehyde and metaldehyde, the THz-TDS can be applied to the qualitative and quantitative detection of the two aldehydes to reduce the unpredictable hazards due to these substances.

不同光功率激励下石墨烯的太赫兹波吸收特性

不同光功率激励下石墨烯的太赫兹波吸收特性

Filament characterization via resonance absorption of terahertz wave

In this work, we characterize the transmission properties of the focusing-laser-induced filament by using terahertz (THz) time domain spectroscopy. Significant resonance absorption of the THz wave is identified, which is attributed to the dynamic screening of the ionized electrons. The polarization and intensity of the filament-generated pulse have been varied to study their influences on the resonance absorption. Our results suggest that the electron density of the filament can be determined by fitting the phase shifts and absorption properties around the absorption. This method provides not only an alternative to the plasma diagnostics with plasma frequency at the THz range, but also a supplement to the existing generation theory of THz waves from plasma.

Truncated pyramidal-shaped InAs/GaAs quantum dots in the presence of a vertical magnetic field: An investigation of THz wave emission and absorption

The authors report on the calculation of energy eigenvalues, transition energies, and transition dipole moments (TDMs) of truncated pyramidal-shaped InAs/GaAs quantum dots (QDs) coupled to their wetting layers (WL) under a varying and vertical magnetic field. The aim of this work is to explore a tunable source for terahertz (THz) emission and absorption through the change in dimensions and magnetic field. Through the solving the Schrodinger equation in the single-band effective mass approximation for QDs, seven initial states were extracted and the corresponding envelope functions and their changes under a magnetic field were inspected. Two groups of QDs were considered: group A of QDs, in which the height length was taken to be constant as 5nm and the base length was varied from 10 to 30nm, and group B of QDs, in which the base length was taken to be constant as 25nm and the height length was varied from 1 to 6nm. The results showed that the transition frequencies of group B of QDs lay dominantly in the terahertz region and are shifted towards lower frequencies with decreasing the height and/or increasing the magnetic field. With applying a varied magnetic field up to 25 T, a ~6THz shift in s-to-pX transition frequency was seen. Also, the polarizations of pronounced transitions were also investigated. The s-to-px, s-to-pY, and pY-to-dXY transitions were shown to be all in-plan-polarized transitions. The s-to-dXY transition is not an allowed transition.

Giant Terahertz-Wave Absorption by Monolayer Graphene in a Total Internal Reflection Geometry

We experimentally demonstrated significant enhancement of terahertz-wave absorption in monolayer graphene by simply sandwiching monolayer graphene between two dielectric media in a total internal reflection geometry. In going through this structure, the evanescent wave of the incident terahertz beam interacts with the sandwiched graphene layer multiple (up to four) times at varying incidence angles. We observed extremely large attenuation (up to ∼70% per reflection), especially for s-polarized radiation. The experimental results are quantitatively consistent with our calculations, where we modeled the experiment as an electromagnetic wave reflection process in monolayer graphene. We also derived analytical expressions for the absorptance, showing that the absorptance is proportional to the amount of Joule heating on the graphene surface induced by the terahertz radiation.

Coherent Excitation of Optical Phonons in GaAs by Broadband Terahertz Pulses.

Coherent excitation and control of lattice motion by electromagnetic radiation in optical frequency range has been reported through variety of indirect interaction mechanisms with phonon modes. However, coherent phonon excitation by direct interaction of electromagnetic radiation and nuclei has not been demonstrated experimentally in terahertz (THz) frequency range mainly due to the lack of THz emitters with broad bandwidth suitable for the purpose. We report the experimental observation of coherent phonon excitation and detection in GaAs using ultrafast THz-pump/optical-probe scheme. From the results of THz pump field dependence, pump/probe polarization dependence, and crystal orientation dependence, we attributed THz wave absorption and linear electro-optic effect to the excitation and detection mechanisms of coherent polar TO phonons. Furthermore, the carrier density dependence of the interaction of coherent phonons and free carriers is reported.

Wavelength Scaling of Terahertz Wave Absorption via Preformed Air Plasma

Two-color laser mixing in gases can result in the emission of broadband, single-cycle terahertz (THz) pulses. In this study, we find that THz wave absorption is observed when the THz wave and preplasma beam are focused on the same point, and that the wavelength of the preplasma beam can influence the absorption. The experimental results reveal that the THz wave absorption becomes less pronounced with increasing preplasma wavelength. We found that the absorption decreased sharply at a wavelength of 1400 nm, which may be related to the preplasma density, and the preplasma fluorescence profiles were measured using a charge-coupled device arrangement to prove our conjecture. In addition, the full-width at half-maximum of the frequency of the THz wave becomes broadened during absorption. The experimental results agree well with our presented model.

Broadband terahertz absorption enabled by coating an ultrathin antireflection film on doped semiconductor.

We show that perfect absorption of terahertz wave can be achieved in a compact system where an ultrathin film of lossless dielectric is coated on a doped semiconductor substrate. Due to the nontrivial reflection phase shift at the interface between the two media, strong resonant behavior and the concomitant antireflection occur at wavelengths that are much larger than the thickness of the dielectric film, resulting in strong absorption of the incident wave in a wide frequency range. Using this mechanism, we design a broadband terahertz absorber by coating a Ge film on a highly doped GaAs substrate. We show that such a system not only has a perfect absorption peak, but also exhibits high absorptance (over 0.9) within a fractional bandwidth of over 20%. By varying the free carrier density in the GaAs substrate, the central frequency of the absorption band can be tuned from 1.79 to 2.69 THz. In addition, the absorption performance of the proposed system is shown to be insensitive to both incident angle and polarization. Our results offer a low-cost way for the design of absorption-based THz devices.

Complex extraordinary dielectric function of Mg-doped lithium niobate crystals at terahertz frequencies

We study the dispersion of the extraordinary dielectric function real and imaginary parts in the wide terahertz-frequency range of the lowest polariton branch for bulk LiNbO3 and Mg:LiNbO3 crystals. At frequencies 0.1–2.5 THz, both dispersion parts are measured by means of standard time-domain terahertz spectroscopy, and at higher frequencies up to 5.5 THz, the dielectric function real part is determined using a common scheme of spontaneous parametric down-conversion under near-forward Raman scattering by phonon polaritons. A special approach is applied for measuring of the dielectric function imaginary part at frequencies 1–3 THz, based on the analysis of visibility of three-wave second-order interference under spontaneous parametric down-conversion. The generalized approximate expressions are obtained for complex dielectric function dispersion within the lower polariton branches of LiNbO3 and Mg:LiNbO3. It is shown that the well-known decrease in terahertz-wave absorption of lithium niobate crystals under Mg-doping is caused by changes in the defect structure and reduction of coupling of the terahertz-frequency polaritons with Debye relaxational mode.

Terahertz wave absorption via preformed air plasma

Terahertz wave generation from laser-induced air plasma has continued to be an exciting field of research over the course of the past decade. In this paper, we report on an investigation concerning terahertz wave absorption with preformed plasma created by another laser pulse. We examine terahertz absorption behavior by varying the pump power and then analyze the polarization effect of the preplasma beam on terahertz wave absorption. The results of experiments conducted in which a type-I beta barium borate (BBO) crystal is placed before the preformed air plasma indicate that the fundamental (ω) and second harmonic (2ω) pulses can also influence terahertz absorption.

The application of the fractional calculus model for dispersion and absorption in dielectrics II. Infrared waves

In the first paper of this series, an empirical formula based on viscoelastic analysis techniques that employs concepts from the fractional calculus originally used to model the dielectric behavior of materials exposed to oscillating electromagnetic fields in the radiofrequency band was applied to do the same for electromagnetic fields oscillating in the terahertz frequency range. The empirical formula was integrated into Maxwell’s equations producing a fractional order Ampere’s law whereof a fractional order wave equation was derived. This wave equation was used to describe the absorption and dispersion of terahertz waves in a dielectric medium. In this work, the empirical formula is extended again for application in the infrared frequency spectrum. The fractional calculus dielectric model is adapted to curve fit the complex refractive index data of a variety of semiconductors and insulators. Following the same procedure used in the first paper of this series, the fractional calculus dielectric model is again integrated in Maxwell’s equations with the same dispersion and absorption analysis performed using the newly derived fractional order wave equation. The mathematical consequences of extending this model into infrared frequencies are also discussed.

太赫兹波对肾癌组织的光谱检测

In this paper, terahertz spectroscopy technique was used for the investigation of human renal cancer by comparing the response of tumor tissue and normal tissue adjacent to tumor. Based on the measurement and analysis of fresh tissue and lyophilized tissue respectively, the absorption coefficient and refractive index from 0.2 THz to 1.2 THz were obtained. The result shows that water content of biological tissue has great influence on terahertz wave absorption signal amplitude. Meanwhile, difference of cell structure and composition also affect the transmission of terahertz signal. For example, as the different structure and composition of the cancer and normal cell, the absorption coefficient and refractive index of these tissues are completely different. The result indicates that terahertz spectroscopy technique is an important technology and has a broad prospect of application in biological tissue investigation and clinical medicine such as tumor disease diagnosis.