We have solved the long-standing problem of the mechanism of terahertz (THz) generation by a two-color filament in air and found that both neutrals and plasma contribute to the radiation. We reveal ...that the contribution from neutrals by four-wave mixing is much weaker and higher in frequency than the distinctive plasma lower-frequency contribution. The former is in the forward direction while the latter is in a cone and reveals an abrupt down-shift to the plasma frequency. Ring-shaped spatial distributions of the THz radiation are shown to be of universal nature and they occur in both collimated and focusing propagation geometries. Experimental measurements of the frequency-angular spectrum generated by 130-fs laser pulses agree with numerical simulations based on a unidirectional pulse propagation model.
The main reason for the alternation of terahertz (THz) response in biological samples is the transition of a part of water from its free to bound state and back. To analyze such results, a precise ...spectrum of bound water within THz range should be known. The suggested wet powder spectroscopy method is optimal for this task. We measured the THz transmission of dry glucose pressed into pellets. Then, we added a small amount of water to these pellets and again measured the THz spectra. The contribution from the free water state starts to appear in resulting THz spectra after water concentration in glucose samples reaches 14–16%. Just below is the optimal point to extract the spectral contribution from bound water. We extracted the absorption and refraction spectra of bound water and confirmed that they are an order of magnitude weaker than those of free water within 0.07–1-THz range.
We report a study of aqueous solutions of glucose and bovine serum albumin using THz time-domain spectroscopy. To describe the permittivity of the solutions of these substances, we use a simplified ...model being applicable in the frequency range of . On the assumption that most of the water molecules become bound at high concentrations of glucose and protein in the solution, the changes in water characteristics are investigated. To improve the reliability of the results, the measurements are performed by two independent methods: the method of attenuated total internal reflection and the transmission method. Combination of the results obtained by these two methods allows expanding the spectral range towards lower frequencies.
Sensitivity of the THz frequency range to the solutions of biomolecules originates from the decrease of absorption and dispersion of water in its bound state. Correct measurement and interpretation ...of the THz spectra of water-containing samples is still a challenging task because the reliable relaxation model for such spectra is not well established. The transmission and the attenuated total internal reflection geometries data were combined for precise analysis of the spectra of the aqueous solutions of bovine serum albumin within the range 0.05–3.2 THz. We compare the concentration dependencies of the dielectric function at “low,” “middle,” and “high” frequency and do not confirm an anomalous increase in absorption for concentrations below 17 mg/mL published by other teams.
Terahertz time-domain spectroscopy has been used for measuring of bovine serum albumin and glucose solutions response. The transmission and the attenuated total internal reflection geometries have ...been combined for analyzing the dielectric properties of aqueous solutions spectra at 0.07-3.2 THz.
We report the results of experimental studies of the photoelectric properties of a
p–i–n
GaAs photodiode with InAs/GaAs(001) double asymmetric quantum dots (DAQDs) grown by self-assembling in the ...metal–organic vapor-phase epitaxy process. Three peaks were observed in the dependence of the photocurrent on the reverse bias measured at monochromatic photoexcitation of the DAQDs at the wavelength corresponding to the energy of interband optical transitions between the ground hole and electron states in the larger quantum dots (QDs). These peaks were related to the tunneling of the photoexcited electrons between the QDs, including a dissipative one (with emission and absorption of optical phonons). The experimental results agree qualitatively with the theoretical field dependence of the probability of 1D dissipative tunneling between QDs.
In this paper, we measure broadband terahertz spectra from a single 744-nm filament produced in tight, medium, and loose geometrical focusing conditions. Terahertz spectra are measured using ...interferometer coupled to a helium-cooled bolometer avoiding any cutoff frequencies. Based on THz spectrum maximum position and spectral width, we estimate electron-heavy particle (neutrals and ions) collision rate. Numerical simulations are performed using the state-of-the-art model of unidirectional femtosecond pulse propagation and convergence/divergence of optical radiation at large angles.
Abstract
Adhesion of the spike protein of the SARS-CoV-2 virus is studied by vibrational spectroscopy using terahertz metamaterials. The features of metastructure absorption upon the deposition of ...histidine, albumin, and the receptor-binding domain of the spike protein films are investigated. An original technique for quantitative assessment of the efficiency of virus adhesion on the metamaterial surfaces are proposed and experimentally tested.
Using the effective medium model, the complex permittivity of the ocular surface system in the THz frequency range is considered. The penetration depth of THz radiation and the sensitivity of the ...reflected component to the dynamics of evaporation of the tear film are discussed. The results of in vivo experiments aimed to determine the reflection coefficient of the ocular surface using continuous THz reflectometers are well described using the developed model.
Terahertz (THz) emission from a single-color ultraviolet (UV) (248 nm) filament in air is studied experimentally and compared to that of an infrared (IR) (744 nm) filament for similar pulse ...durations, focusing conditions, and excess of pulse power over the critical power for self-focusing. An angular distribution of the THz emission for both UV and IR filaments is conical with the close cone angle. In contrast, the THz radiation energy and spectrum differ significantly. The energy of THz emission from the UV filament is 1-2 orders of magnitude lower than the THz yield from the IR filament. The THz spectrum of the UV filament is shifted to the low-frequency range and is narrower compared to the spectrum of THz emission from the IR filament. We explain qualitatively the difference in THz yields and spectra by lower intensity and plasma density in the UV filament.
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