Absorption Of Terahertz Radiation Research Articles

Ultrafast dynamics of Dirac surface and bulk photocarriers in topological-insulator bismuth telluride nanocrystals using terahertz spectroscopy

Unlike conventional semiconductors, Dirac fermions in surface states of topological insulators (TIs) exhibit many fascinating electronic properties relevant for several optoelectronic and spintronic applications. We investigate the relaxation dynamics of photoexcited carriers in bismuth telluride (${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$) nanocrystals---a prominent candidate among the three-dimensional (3D) topological insulators, using ultrafast time resolved terahertz (THz) spectroscopy. Upon photoexcitation using an 800 nm pump pulse, we observe a decrease in the THz transmission due to the absorption of THz radiation by the photoexcited carriers. The time evolution of the real part of pump induced conductivity [$\mathrm{\ensuremath{\Delta}}{\ensuremath{\sigma}}_{R}({\ensuremath{\tau}}_{\text{pp}})=\phantom{\rule{4pt}{0ex}}{\ensuremath{\sigma}}_{\text{pump}}{\phantom{\rule{4pt}{0ex}}}_{\text{on}}({\ensuremath{\tau}}_{\text{pp}})\ensuremath{-}{\ensuremath{\sigma}}_{\text{pump}}{\phantom{\rule{4pt}{0ex}}}_{\text{off}}$] with respect to the pump-probe delay time (${\ensuremath{\tau}}_{\text{pp}}$), exhibits a sign change from positive to negative as carriers relax back to their equilibrium distribution, which is also evident from the spectral signatures of $\mathrm{\ensuremath{\Delta}}\ensuremath{\sigma}(\ensuremath{\omega})$, measured at various ${\ensuremath{\tau}}_{\text{pp}}$. While the positive sign of $\mathrm{\ensuremath{\Delta}}{\ensuremath{\sigma}}_{R}({\ensuremath{\tau}}_{\text{pp}})$ is attributed to the contribution from the semiconducting bulk states, the negative sign of $\mathrm{\ensuremath{\Delta}}{\ensuremath{\sigma}}_{R}({\ensuremath{\tau}}_{\text{pp}})$ at longer delay times is shown to arise from the Dirac surface states of TIs due to increased scattering rate of photoexcited charge carriers. We model the differential conductivity spectrum [$\mathrm{\ensuremath{\Delta}}\ensuremath{\sigma}(\ensuremath{\omega})$] and the transient dynamics [$\mathrm{\ensuremath{\Delta}}{\ensuremath{\sigma}}_{R}({\ensuremath{\tau}}_{\text{pp}})$] quantitatively using the Boltzmann transport equation applied to the surface and bulk states. This includes the energy dependence of scattering rates along with the dynamics of hot electron temperature (${T}_{e}$) evaluated in a self-consistent manner. Such an approach enables us to understand various scattering mechanisms associated with the photoexcited carriers both in Dirac surface as well as in bulk electronic states.

Spectral and intensity control of high energy terahertz radiation from bulk liquids

High power, broadband terahertz (THz) radiation from liquids, excited by intense, femtosecond 800 nm laser pulses has been demonstrated recently, overturning the long held belief that liquids would not give such emission due to their absorption in the THz region. Given the widespread interest in the use of THz radiation for several applications and the energy and bandwidth limitations of existing sources, liquids are expected to attract great attention in the future. While the emission at tens of microjoules from liquids is very promising, it is important to explore whether control of the THz flux and spectrum is achievable by manipulating the laser or liquid parameters. In this paper we present results on manipulating the spectrum of THz radiation from liquids by chirping the input laser pulse and optimizing the THz output energy by laser chirp as well as optimizing the focal position. We demonstrate tunability by varying the chirp of the laser pulse and show that the THz emission predominantly comes from the region near the end of the liquid path, consistent with the absorption of THz radiation by liquids. This control gives us an opportunity to tune the THz radiation to suit experimental needs. We present simulations that support the results.

Numerical modelling and experimental verification of thermal effects in living cells exposed to high-power pulses of THz radiation

Exposure of cells or biological tissues to high-power pulses of terahertz (THz) radiation leads to changes in a variety of intracellular processes. However, the role of heating effects due to strong absorption of THz radiation by water molecules still stays unclear. In this study, we performed numerical modelling in order to estimate the thermal impact on water of a single THz pulse as well as a series of THz pulses. A finite-element (FE) model that provides numerical solutions for the heat conduction equation is employed to compute the temperature increase. A simple expression for temperature estimation in the center of the spot of THz radiation is presented for given frequency and fluence of the THz pulse. It has been demonstrated that thermal effect is determined by either the average power of radiation or by the fluence of a single THz pulse depending on pulse repetition rate. Human dermal fibroblasts have been exposed to THz pulses (with an energy of 15,upmu hbox {J} and repetition rate of 100 Hz) to estimate the thermal effect. Analysis of heat shock proteins expression has demonstrated no statistically significant difference (p < 0.05) between control and experimental groups after 3 h of irradiation.

FEM Simulation of Frequency-Selective Surface Based on Thermoelectric Bi-Sb Thin Films for THz Detection

Terahertz (THz) filters and detectors can find a wide application in such fields as: sensing, imaging, security systems, medicine, wireless connection, and detection of substances. Thermoelectric materials are promising basis for THz detectors’ development due to their sensitivity to the THz radiation, possibility to be heated under the THz radiation and produce voltage due to Seebeck effect. Thermoelectric thin films of Bi-Sb solid solutions are semimetals/semiconductors with the band gap comparable with THz energy and with high thermoelectric conversion efficiency at room temperature. Detecting film surface can be transformed into a periodic frequency selective surface (FSS) that can operate as a frequency filter and increases the absorption of THz radiation. We report for the first time about the simulation of THz detector based on thermoelectric Bi-Sb thin-filmed frequency-selective surface. We show that such structure can be both detector and frequency filter. Moreover, it was shown that FSS design increases not only a heating due to absorption but a temperature gradient in Bi-Sb film by two orders of magnitude in comparison with continuous films. Local temperature gradients can reach the values of the order of 100 K·mm−1. That opens new perspectives for thin-filmed thermoelectric detectors’ efficiency increase. Temperature difference formed due to THz radiation absorption can reach values on the order of 1 degree. Frequency-transient calculations show the power dependence of film temperature on time with characteristic saturation at times around several ms. That points to the perspective of reaching fast response times on such structures.

Design of the vanadium dioxide temperature self-regulator in the terahertz regime

A temperature self-regulator can keep the particle temperature almost constant over a wide range of light intensities. In this study, we propose a temperature self-regulator design under terahertz radiation. A key ingredient in this proposal is vanadium dioxide (VO$_2$), whose properties in the terahertz regime vary drastically around its phase transition temperature. One would expect that the particle temperature may be self-adjusted in the phase change process. To verify this notion, we employ the Mie theory for the theoretical analysis. The results show that the absorption exhibits nonlinear dependence with different parameters. At optimized sizes, the absorption of terahertz radiation can be significantly suppressed once the phase change occurs. Consequently, the temperature does not show a significant increment with increasing intensity. These results can facilitate the future development of temperature self-regulator using phase change materials in the terahertz regime.

The Influence of Defects on the Absorption of Terahertz Radiation in a CdSiP2 Single Crystal

The transmission and reflection spectra of a CdSiP2 single crystal are measured in the temperature interval from 80 to 300 K using the terahertz (THz) pulsed and infrared Fourier spectroscopy methods. A significant influence of postgrowth defects on the absorption in the THz frequency range is revealed. This absorption is found to depend weakly on temperature compared to that observed previously for other chalcopyrite crystal with substantially lower concentration of defects. Upon cooling, intrinsic absorption mechanisms are minimized, and the contribution of defects to absorption is separated.

Utilization of Oil Shale as an Electromagnetic Wave Absorbing Material in the Terahertz Range

This study demonstrates that oil shale subjected to pyrolysis at 1000 °C strongly absorbs electromagnetic radiation in the terahertz (THz) range. Analyses reveal obvious differences between oil shale samples with and without pyrolysis, as well as between low-middle mature shale and oil shale samples. The electromagnetic absorption effect of oil shale subjected to pyrolysis conditions is demonstrated to be strongly related to its relatively small concentration of pyrite, which progressively yields pyrrhotite and elemental Fe during pyrolysis that facilitates the absorption of THz radiation. Accordingly, oil shale can be used not only as a resource for generating oil, natural gas, and petrochemical products, but the material obtained after processing can also be employed for the absorption of THz radiation as a means of converting incident electromagnetic radiation into heat or other forms of energy, which offers substantial prospects in many engineering and technology applications.

Synergy effects in dielectric and thermal properties of layered ethylene vinyl acetate composites with carbon and Fe3O4 nanoparticles

ABSTRACTProperties of layered composites with carbon nanotube (CNT), GnP, Fe3O4 nanoinclusions, and hybrid composites with these nanoinclusions were investigated in the wide frequency range (from 20 Hz to 2 THz). All investigated composites (except with single Fe3O4 inclusions) are above percolation threshold. The strong enhancement of microwave and terahertz radiation absorption is observed in hybrid composites in comparison with composites with single inclusions. The synergy effect is discussed in terms of the complex impedance and the distribution of relaxation times. At low temperatures (below 200 K), the electrical conductivity of all hybrid composites follows the tunneling law model and the lowest potential barrier was estimated for composites with CNT inclusions. The best thermal properties are observed for composites with CNT inclusions. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48814.

Terahertz field depolarization and absorption within composite media

In this work, we pursue a deeper understanding of the expression of the inclusion morphology and index contrast in the refraction and absorption characteristics of composites within the terahertz (THz) spectrum. The composites are composed of SiO2 and Si nanoparticles as well as SiO2 and Si microparticles functioning as deeply subwavelength inclusions in a polydimethylsiloxane (PDMS) host. Terahertz time-domain spectroscopy is used for experimental characterization of the composites over a wide range of volumetric fractions, and the trends that emerge are contrasted to theoretical predictions from the Bruggeman model. It is found that the refraction characteristics have a heightened dependence on the inclusions' shape when their index contrast with respect to the host becomes sufficiently large. We attribute such a correlation to terahertz field depolarization that occurs within inclusions at high index contrasts and the dependence of the fields to the inclusions' shape—as defined by a depolarization factor in the generalized form of the Bruggeman model. Moreover, it is found that the absorption characteristics have a heightened dependence on the inclusions' size when that size becomes sufficiently small. We attribute this to the manifold of surface states that form in small inclusions, due to their high surface-to-volume ratio, which raises the absorption beyond that of the bulk. It is concluded that the Bruggeman model can accurately characterize the refraction and absorption of THz radiation within composites having deeply subwavelength inclusions if their (shape-dependent) polarization and (size-dependent) absorption are suitably defined.

Absorption of terahertz radiation by a thin metal absorber in conventional and inverted bolometers

The absorption of terahertz radiation by a thin metal absorber in the bolometers of five different designs is analyzed. Main attention is paid to bolometers of three basic configurations: (i) conventional bolometers with a double-layer main optical cavity, (ii) conventional bolometers with an additional optical cavity, and (iii) inverted bolometers with an additional optical cavity. Analytical expressions that allow calculating the absorption spectra in all the considered bolometers are obtained. It is shown that conventional bolometers only with the main optical cavity made up of two dielectric layers, as well as conventional and inverted bolometers with both main and additional optical cavities, can have the absorptance close to unity in the terahertz range. At the same time, conventional terahertz bolometers are mainly selective, whereas inverted ones are broadband.

Terahertz Imager Based on a THz-to-IR Converter

A new terahertz (THz) imager based on THz-to-IR conversion has been proposed and studied. The THz-to-IR converter consists of an ultra-thin resonant THz absorber (meta-absorber) whose backside is coated with an emission layer with an emission factor close to unity. The absorption of THz radiation leads to converter heating, which is recorded by an IR camera from the emission layer side. The small thickness of the converter (more than 50 times smaller than the working wavelength of THz radiation) determines its low heat capacity, resulting in an increase in the sensitivity and operating speed of the imager. Optimization of the optics of the THz imager, making cuts in the converter structure to reduce the blooming and increase the response, and the IR image processing method increasing the signal-to-noise ratio, provided the sensitivity of the THz imager similar to the sensitivity of thermal detectors in the 8–12 μm IR range.

Efficient Broadband Terahertz Radiation Detectors Based on Bolometers with a Thin Metal Absorber

The matrix method has been used to calculate the coefficients of absorption of terahertz radiation in conventional (with radiation incident from vacuum adjacent to the bolometer) and inverted (with radiation incident from the substrate on which the bolometer was fabricated) bolometric structures. Near-unity absorption coefficients were obtained when an additional cavity in the form of a gap between the bolometer and the input or output window was introduced. Conventional bolometers then became narrowband, while inverted-type devices remained broadband.

Measurements of glucose concentration in aqueous solutions using reflected THz radiation for applications to a novel sub-THz radiation non-invasive blood sugar measurement method.

The terahertz (THz) frequency range corresponds to molecular vibrations or relaxation modes such as those for the hydrogen bond. Most biomolecules are activated only in aqueous solutions, thus, to understand the function and structure of biomolecules, it is necessary to investigate the characteristics of electromagnetic waves in hydrated samples. THz radiation causes little damage to the human body, thus it is expected that it can be applied for noninvasive examinations. However, spectrometry of the transmitted light is difficult, since the absorption of THz radiation in water is extremely high. In this study, we used sub-THz radiation (frequencies near to 0.1 THz), where the absorption is lower than for THz radiation, to measure the reflectance of a glucose water solution. We found that the reflectance decreases in proportion with the glucose concentration. These results suggest that sub-THz radiation can be used in the noninvasive measurement of blood glucose levels.

Wide-aperture total absorption of a terahertz wave in a nanoperiodic graphene-based plasmon structure

The terahertz absorption spectrum in a periodic array of graphene nanoribbons located on the surface of a dielectric substrate with a high refractive index (terahertz prism) is studied theoretically. The total absorption of terahertz radiation is shown to occur in the regime of total internal reflection of the terahertz wave from the periodic array of graphene nanoribbons, at the frequencies of plasma oscillations in graphene, in a wide range of incidence angles of the external terahertz wave even at room temperature.

Resonance absorption of terahertz radiation in nanoperforated graphene

Recent measurements of the conductivity of nanoperforated graphene are interpreted in terms of edges states existing near the edge of each nanohole. The perimetric quantization of edge states should result in the formation of a quasi-equidistant ladder of quasistationary energy levels. Dirac fermions filling this ladder rotate about each nanohole in the direction determined by the valley index. It is shown that the irradiation of this system by circularly polarized terahertz radiation leads to a resonance in absorption in one of the valleys. The magnitude of absorption at the resonance frequency can be controlled by means of gate voltage.

Effects of plasmonic absorption of terahertz radiation in silica nanocomposite doped with erbium silicate

This paper studies terahertz transmission spectra of silicon and erbium silicides which can be used to create uncooled bolometers for terahertz frequency domain.

On the natural magnetization of ideal quantum dots and the possibility of detection of terahertz radiation in the magnetic field

A wave function is constructed and the binding energy of electron localized at an ideal quantum dot in the external magnetic field is calculated. It is shown that the energy of the Zeeman splitting in moderate magnetic fields corresponds to photons from the terahertz band, i.e., processes with the electron spin flip lead to radiation or absorption of terahertz radiation. It is established that external terahertz radiation changes the spin equilibrium distribution and, as a result, the magnetization of the system, thus creating the possibility of detection of this radiation by measuring the magnetization of the system of quantum dots.

Terahertz electron transport in a two-dimensional topological insulator in a HgTe quantum well

The terahertz response of a two-dimensional topological insulator in a HgTe quantum well to radiation with wavelengths of 118 and 184 μm is investigated. It is found that the photoconductivity is rather high (up to a few percent of dark conductivity) and is manifested in both the local and nonlocal responses of the system. This fact proves that the observed photoconductivity is caused by changes in the transport via edge current-carrying states. The sign and nonresonant character of the photoconductivity indicate that it is caused by the heating of electrons in the system. The analysis of experimental results makes it possible to suggest that this heating originates from the Drude absorption of terahertz radiation by metallic “droplets” appearing owing to fluctuations in the impurity potential and the gap and located in direct proximity to edge states.

Global Distribution of Water Vapor and Cloud Cover—Sites for High-Performance THz Applications

Absorption of terahertz radiation by atmospheric water vapor is a serious impediment for radio astronomy and for long-distance communications. Transmission in the THz regime is dependent almost exclusively on atmospheric precipitable water vapor (PWV). Though much of the Earth has PWV that is too high for good transmission above 200 GHz, there are a number of dry sites with very low attenuation. We performed a global analysis of PWV with high-resolution measurements from the Moderate Resolution Imaging Spectrometer (MODIS) on two NASA Earth Observing System (EOS) satellites over the year of 2011. We determined PWV and cloud cover distributions and then developed a model to find transmission and atmospheric radiance as well as necessary integration times in the various windows. We produced global maps over the common THz windows for astronomical and satellite communications scenarios. Notably, we show that up through 1 THz, systems could be built in excellent sites of Chile, Greenland and the Tibetan Plateau, while Antarctic performance is good to 1.6 THz. For a ground-to-space communication link up through 847 GHz, we found several sites in the Continental United States where mean atmospheric attenuation is less than 40 dB; not an insurmountable challenge for a link.

Terahertz metamaterial absorbers with an embedded resistive layer

A conductive layer of Ti, with a sheet resistance of about 220 Ω/sq, was placed in the dielectric spacer of an Al/SiOx/Al metamaterial terahertz absorber at various depths to probe the effect on the absorption of terahertz radiation. For a square size of 15 µm and a periodicity of 21 µm, and dielectric thickness approximately 1.6 µm, maximum absorption was 60%, 88%, and 94% for Ti layers 297, 765, and 1270 nm deep into the SiOx. Finite element simulations of the absorption correlated well with that of the measurements. This indicates that metamaterials with an embedded high temperature coefficient of resistance (TCR) conducting layer can be used for fabrication of microbolometers with tuned spectral sensitivity.