Terahertz Wave Frequency Research Articles

Study on the generation of terahertz waves in collision plasma

Terahertz communication technology holds promise for solving the challenging “blackout” problem. Research has revealed that the spectrum exhibits new peaks when terahertz waves propagate through plasma. In the realm of communications, elucidating the underlying physical mechanisms responsible for these novel peaks is of paramount importance. In this paper, a combination of theoretical analysis and simulation is utilized to investigate the causes of these spectral changes. Theoretically, the expression for the relative dielectric constant of plasma has been refined by incorporating collision terms. Drawing on radiation theory, a particle whose motion is interrupted by collisions can be considered equivalent to a damped harmonic oscillator. Thus, the frequency expression of the new peak was derived. Furthermore, the simulation involved the interaction of a terahertz wave, which had a frequency of 1 THz and lasted for 2 ps, and plasma characterized by a central electron density of 1021 m−3 and a collision frequency of 0.114 THz. The simulation results are consistent with the frequency expression of the new peak. In sum, this study found that collisions between electrons and neutral particles within plasma cause a shift in the frequency of terahertz waves, and has derived the frequency expression. This plasma model can be used to estimate the frequency range of terahertz waves that are generated as they propagate through collisional plasmas.

Microstructure and dielectric properties of low-temperature sintered MgO-based ceramics at millimeter wave and terahertz frequencies

Low-temperature co-fired ceramics (LTCC) applied in millimeter/microwave and terahertz frequencies (5G/6G) have attracted a lot of attention recently. In this study, MgO-based dielectric ceramics were successfully sintered at 950∘C with the sintering aids: x wt.% of LiF fluoride ([Formula: see text], 4, 6, 8, 10) and 0.5[Formula: see text]wt.% of BBSZ (Bi2O3–B2O3–SiO2–ZnO) glass. BBSZ glass was introduced as another sintering aid to facilitate the sintering and densification. Crystalline structure and micro-morphology were investigated and analyzed. Dielectric properties ([Formula: see text], [Formula: see text], [Formula: see text]) at millimeter/microwave and terahertz wave frequencies were also studied. The ionic characteristics of Mg–O bond ([Formula: see text]), the lattice energy (U) and the bond energy (E) were calculated and analyzed. It is suggested that the optimal [Formula: see text], where [Formula: see text], [Formula: see text][Formula: see text]GHz (@12[Formula: see text]GHz) and [Formula: see text][Formula: see text]ppm/∘C at millimeter/microwave range. When the frequency was up to terahertz (1.0[Formula: see text]THz), the [Formula: see text] values were 8.8–9.35 and the tan[Formula: see text] were [Formula: see text]–[Formula: see text]. The experimental results indicated that the low-temperature sintered MgO-based ceramics have potential for millimeter/microwave and terahertz communication applications.

Design Optimization of Silicon-Based Optically Excited Terahertz Wave Modulation

The modulation of a terahertz (THz) wave on amplitude, phase and polarization is important for the application of THz technology, especially in the field of imaging, and is one of the current research hotspots. Silicon-based, optically excited THz modulator is a wavefront modulation technique with a simple, compact and reconfigurable optical path. It can realize the dynamic modulation of THz wavefronts by only changing the projected two-dimensional pattern, but it still suffers from the problems of lower modulation efficiency and slower modulation rates. In this article, the Drude model in combination with the multiple thin layers structure model and Fresnel matrix method is used to compare the modulation efficiencies of three modulation modes and more factors. The method is more accurate than the popular proposed method, especially when the thickness of the excited photoconductive layers reaches a few hundred microns. In comparing the three modes, namely transmission, ordinary reflection and total internal reflection, it is found the total internal reflection modulation mode has the best modulation efficiency. Further, under this mode, the effects of three factors, including the lifetime of photo-excited carriers, the wavelength of pump light and the frequency of THz wave, on the performance of THz modulator are analyzed. The simulation results show that the realization of total internal reflection using silicon prisms is a simple and effective method to improve the modulation efficiency of a silicon-based optically excited THz modulator, which provides references for the design of a photo-induced THz modulator.

Theoretical study of influence of laser pulse chirp on terahertz emission characteristics of gas induced by two-color laser field

With the development of terahertz (THz) wave research, the demand for controllable THz sources is increasing. How to obtain the regulated THz waves has been one of the research hotspots and key problem in the field of THz science. There have been researches in which the resulting THz wave is modulated by changing the wavelength, relative phase, energy, or chirp of the laser produced by a two-color laser. In this work, we establish a three-dimensional theoretical model of THz wave generation and subsequent propagation induced by two-color laser. And we investigate the influence of chirp modulation of different laser on THz wave by chirp modulation of the fundamental wave (FW) and the second harmonic wave (SHW) of two-color laser, including THz wave amplitude, THz wave center frequency and spectrum width, and analyze the physical mechanism of related phenomena. At the same time, the effects of different orders of magnitudes of laser chirp parameters (femtosecond and picosecond) and initial phase of laser pulse on THz wave parameters are also studied. The results are shown below. 1) In the two-color laser, the chirp of FW mainly affects the shape of THz wave when the initial phase is unchanged. The chirp modulation of SHW can cause the amplitude of THz wave to change significantly, and affect the center frequency and spectrum width of THz waves. 2) In the case of laser pulse width of femtosecond order, 40 fs is taken as an example. When the chirp parameter is of femtosecond magnitude, the chirp parameter has a great influence on the THz wave generation efficiency of two-color laser filament. At the picosecond magnitude, the chirp parameter has a weak effect on the THz wave energy and mainly affects the phase of the THz wave. 3) The initial phase of the two-color laser can aid in chirp modulation of THz wave to optimize the energy generated. 4) The initial phase of two-color laser can assist in the process of chirped laser modulation of terahertz waves to optimize the energy generated. Our research shows that the chirp modulation, as a controllable parameter, has multiple regulation effect on the properties of radiated THz waves. The related research results provide a new idea and basis for studying the generation and regulation of THz waves.

Effect of Terahertz Waves on the Structure of the Aβ42 Monomer, Dimer, and Protofibril: Insights from Molecular Dynamics Simulations.

Amyloid-β (Aβ) and its assemblies play important roles in the pathogenesis of Alzheimer's disease (AD). Recent studies conducted by experimental and computational researchers have extensively explored the structure, assembly, and influence of biomolecules and cell membranes on Aβ. However, the impact of terahertz waves on the structures of Aβ monomers and aggregates remains largely unexplored. In this study, we systematically investigate the molecular mechanisms by which terahertz waves affect the structure of the Aβ42 monomer, dimer, and tetramer through all-atom molecular dynamics (MD) simulations. Our findings indicate that terahertz waves at a specific frequency (42.55 THz) can enhance intramolecular and intermolecular interactions in the Aβ42 monomer and dimer, respectively, by resonating with the symmetric stretching mode of the -COO- groups and the symmetric bending/stretching mode of -CH3 groups. Consequently, the β-structure content of the Aβ42 monomer is greatly increased, and the binding energy between the monomers in the Aβ42 dimer is significantly enhanced. Additionally, our observations suggest that terahertz waves can mildly stabilize the structure of tetrameric protofibrils by enhancing the interactions among peripheral peptides. Furthermore, we also investigated the effect of the frequency of terahertz waves on the structure of Aβ42. The present study contributes to a better understanding of the impact of external fields on the biobehavior of Aβ42 peptides and may shed some light on the potential risks associated with electromagnetic field radiation.

Terahertz radiation by quantum interference of excitons in a one-dimensional Mott insulator

Nearly monocyclic terahertz waves are used for investigating elementary excitations and for controlling electronic states in solids. They are usually generated via second-order optical nonlinearity by injecting a femtosecond laser pulse into a nonlinear optical crystal. In this framework, however, it is difficult to control phase and frequency of terahertz waves. Here, we show that in a one-dimensional Mott insulator of a nickel-bromine chain compound a terahertz wave is generated with high efficiency via strong electron modulations due to quantum interference between odd-parity and even-parity excitons produced by two-color femtosecond pulses. Using this method, one can control all of the phase, frequency, and amplitude of terahertz waves by adjusting the creation-time difference of two excitons with attosecond accuracy. This approach enables to evaluate the phase-relaxation time of excitons under strong electron correlations in Mott insulators. Moreover, phase- and frequency-controlled terahertz pulses are beneficial for coherent electronic-state controls with nearly monocyclic terahertz waves.

The laws and effects of terahertz wave interactions with neurons.

Introduction: Terahertz waves lie within the energy range of hydrogen bonding and van der Waals forces. They can couple directly with proteins to excite non-linear resonance effects in proteins, and thus affect the structure of neurons. However, it remains unclear which terahertz radiation protocols modulate the structure of neurons. Furthermore, guidelines and methods for selecting terahertz radiation parameters are lacking. Methods: In this study, the propagation and thermal effects of 0.3-3THz wave interactions with neurons were modelled, and the field strength and temperature variations were used as evaluation criteria. On this basis, we experimentally investigated the effects of cumulative radiation from terahertz waves on neuron structure. Results: The results show that the frequency and power of terahertz waves are the main factors influencing field strength and temperature in neurons, and that there is a positive correlation between them. Appropriate reductions in radiation power can mitigate the rise in temperature in the neurons, and can also be used in the form of pulsed waves, limiting the duration of a single radiation to the millisecond level. Short bursts of cumulative radiation can also be used. Broadband trace terahertz (0.1-2THz, maximum radiated power 100μW) with short duration cumulative radiation (3min/day, 3days) does not cause neuronal death. This radiation protocol can also promote the growth of neuronal cytosomes and protrusions. Discussion: This paper provides guidelines and methods for terahertz radiation parameter selection in the study of terahertz neurobiological effects. Additionally, it verifies that the short-duration cumulative radiation can modulate the structure of neurons.

Terahertz wave excitation by nonlinear coupling of intense laser field with magnetized plasma

In this article, the plasma density, the laser intensity and the external magnetic field are playing vast roles firstly to generate a coherent THz wave then to enhance the stability of generated THz. Due to the relativistic increase of electron mass, the relativistic self-focusing of a right circular polarized (RCP) laser beam inside magnetized plasma will occur which leads to raising the laser power to enough limits for exciting the terahertz wave. By fulfilling the energy–momentum conservation conditions, a terahertz wave frequency at the difference between the laser pump wave frequency and plasma wave frequency is obtained. More stabilization and higher power (reaching to tens of gigawatts) of terahertz field amplitude have been observed whenever the plasma density, the laser intensity, and the external magnetic field are increased. Better results are recorded at high THz frequency (5 THz) compared with low THz frequency (1 THz).

Photo-Excited Silicon-Based Spatial Terahertz Modulators

The increasing development of terahertz (THz) technology has led to various potential applications in THz imaging, spectroscopy and communications. These devices capable of actively manipulating the amplitude, phase and frequency of THz waves are thus gaining numerous interests. All-optical silicon-based spatial terahertz modulators (STMs), as a simple, cost-effective, and reconfigurable technique, are standing the focus of research. Beginning with a fundamental concept of THz radiation, this paper systematically summarized the modulation mechanism and theoretical model for this kind of STM, reviewed the recent advancements in THz functional devices implemented by this optical method and yet, discussed the performance-improved measures with an emphasis on the reflection reduction. Despite that, there has been considerable progress in realizing high-performance STMs, and novel design is urgent to realize higher modulation rate and more functionality.

Impact of Half-Angles on the Transmission of Terahertz Wave in Inhomogeneous Plasma Sheath

Terahertz communication is considered as a potential solution to alleviate the communication blackout. However, it is not clear how the shape parameters of the vehicle affect the propagation of terahertz communications in the plasma sheath. In this article, the influence of half-angle on the plasma flow and terahertz propagation characteristics around blunt cone vehicle was investigated using the numerical hypersonic hydrodynamic model and multilayer transmission model. The maximum electron number density in the plasma sheath near the antenna decreases with the decreasing half-angle. The thickness of the high-density section in the plasma sheath is almost unaffected by the half-angle. As a result, the terahertz wave attenuation decreases with the decreasing half-angle. The half-angle is reduced from 20° to 5°, and the attenuation of electromagnetic waves is reduced by about 25 dB. Therefore, reducing the half-angle of blunt cone vehicle can effectively reduce the attenuation of electromagnetic waves. Finally, the feasibility of lowering the half-angle to solve the blackout problem during the whole reentry process was discussed; the results show the operating frequency of terahertz wave that maintains normal communication when the half-angle is 1° is reduced by 0.1 THz compared with the half-angle of 9°. Based on the study, a new scheme combining the reduction of half-angle of blunt cone vehicle and terahertz communication was proposed in this article to mitigate the RF blackout.

Spoof surface plasmon analysis based on Marcatili’s method

Motivated by surface plasmon polariton waveguides in the optical regime, spoof surface plasmon (SSP) waveguides have received a lot of attention in terahertz and millimeter wave frequencies. Most research on these kinds of waveguides is numerical. However, some limited analytical work can be seen in the literature. In this paper, one type of SSP waveguide that is composed of a rectangular corrugation with finite lateral width on the ground is considered, and an analytical method, which is inspired by Marcatili’s method, is proposed in order to calculate the dispersion curve of the first mode. The results of this analytical method and a numerical commercial eigenmode solver are compared. The accuracy of this method by varying different geometrical parameters is shown. Finally, it is shown that this method works well in predicting the electromagnetic fields of the first mode.

A 350-GHz Coupled Stack Oscillator with −0.8 dBm Output Power in 65-nm Bulk CMOS Process

This paper presents a push-push coupled stack oscillator that achieves a high output power level at terahertz (THz) wave frequency. The proposed stack oscillator core adopts a frequency selective negative resistance topology to improve negative transconductance at the fundamental frequency and a transformer connected between gate and drain terminals of cross pair transistors to minimize the power loss at the second harmonic frequency. Next, the phases and the oscillation frequencies between the oscillator cores are locked by employing an inductor of frequency selective negative resistance topology. The proposed topology was implemented in a 65-nm bulk CMOS technology. The highest measured output power is −0.8 dBm at 353.2 GHz while dissipating 205 mW from a 2.8 V supply voltage.

A broad range frequency measurement method for continuous and pulsed THz waves.

This paper proposes a method to measure the frequency of terahertz (THz) waves based on the Zeeman effect and the high magnetic field technology with a wideband range from 60 GHz to 3 THz. As the frequency of THz waves absorbed by the sample is linear to the magnetic field in the Zeeman effect, the frequency can be measured by the magnetic field strength. A comparison study of THz frequency measurement was carried out in two magnet systems (a superconducting one and a pulsed one) to investigate the performance in two kinds of high magnetic fields. The experimental results of 60-700 GHz show that this method has high resolution (about 0.001%), excellent linearity, and good repeatability. Moreover, the proposed method can measure polychromatic signals simultaneously as well as the single pulse frequency in the order of tens of microseconds.

Propagation characteristics of terahertz wave in hypersonic plasma sheath considering high temperature air chemical reactions

In this work, the propagation characteristics of terahertz wave in the hypersonic plasma sheath is demonstrated and analyzed based on the realistic vehicle model and the high temperature air thermal chemical reactions. Based on the simulated flow field distribution around the hypersonic vehicle, the electron densities of the plasma sheath at different Mach numbers are calculated by adopting the equilibrium constant method of the 7-species reacting air model. And then the propagation model of terahertz wave in plasma sheath is established using the scattering matrix method. The calculated and simulated results show that the plasma electron density is strongly influenced by Mach number, and the higher frequency and smaller incident angle of terahertz wave are advantageous for a better transmittance and lower loss. Furthermore, the effects of an external magnetic field on the propagation characteristics of left-hand and right-hand polarized terahertz wave in plasma sheath are studied. The external magnetic field indeed reduces the transmission loss of the left-handed wave. This work promises that terahertz waves have good potential applications in the detection and the communication for the hypersonic vehicles in plasma sheath.

Terahertz wave generation using a soliton microcomb.

The Terahertz or millimeter wave frequency band (300 GHz - 3 THz) is spectrally located between microwaves and infrared light and has attracted significant interest for applications in broadband wireless communications, space-borne radiometers for Earth remote sensing, astrophysics, and imaging. In particular optically generated THz waves are of high interest for low-noise signal generation. Here, we propose and demonstrate stabilized terahertz wave generation using a microresonator-based frequency comb (microcomb). A unitravelling-carrier photodiode (UTC-PD) converts low-noise optical soliton pulses from the microcomb to a terahertz wave at the soliton's repetition rate (331 GHz). With a free-running microcomb, the Allan deviation of the Terahertz signal is 4.5×10-9 at 1 s measurement time with a phase noise of -72 dBc/Hz (-118 dBc/Hz) at 10 kHz (10 MHz) offset frequency. By locking the repetition rate to an in-house hydrogen maser, in-loop fractional frequency stabilities of 9.6×10-15 and 1.9×10-17 are obtained at averaging times of 1 s and 2000 s respectively, indicating that the stability of the generated THz wave is limited by the maser reference signal. Moreover, the terahertz signal is successfully used to perform a proof-of-principle demonstration of terahertz imaging of peanuts. Combining the monolithically integrated UTC-PD with an on-chip microcomb, the demonstrated technique could provide a route towards highly stable continuous terahertz wave generation in chip-scale packages for out-of-the-lab applications. In particular, such systems would be useful as compact tools for high-capacity wireless communication, spectroscopy, imaging, remote sensing, and astrophysical applications.

Study of the Absorption Properties of Terahertz Wave in the Plasma Medium

Abstract In this article, we study about the absorption properties of terahertz (THz) wave in the magnetic plasma medium. Terahertz wave has strong transmission in plasma. Generally speaking, with the increase in THz wave frequency, the transmission in plasma is stronger. Thus, we can consider raising carrier frequency to THz band to solve the communication of “blackout.” We found that the absorption in magnetic plasma is greatly affected by magnetic field. The changes on the power absorption coefficient of the two kinds of eigen wave in magnetic plasma vary with how the outside magnetic field, incident angle, and the thickness of the plasma are obtained. The study found that following the increase in magnetic field, the absorption of left circularly polarised waves (L-wave) gradually reduces, and the right circularly polarised waves (R-wave) will produce two absorption peaks, and these two absorption peaks move to high frequency as a whole. With the increase in incident angle and the high spectrum absorption of L-wave enhancement, the reflection between the two absorption peaks of R-wave broadens. The reflection area on the left side of the low-frequency area to absorb has a little change. The absorption on the right side of the high-frequency area is enhancement. With the increase in the plasma thickness, the L-wave absorption peak on the right side of the high-frequency area absorbs enhancement; the R-wave reflection between the two absorption peak areas is impregnability, the second absorption peak absorption enhancement on the right side of the high-frequency area. The study has shown the different absorption mechanisms of the L-wave and the R-wave and shown different absorption features in magnetic plasma.

Effects of thermal aging on cellulose pressboard using terahertz time-domain spectroscopy

We demonstrate a method for examining the effects of thermal aging of cellulose pressboard using terahertz time-domain spectroscopy. The change of refractive indexes at the frequencies of pulsed terahertz waves while applying heat reveals that the degree of thermal aging and the condition of thermal breakdown can be clearly analyzed by performing terahertz measurements. The visual examination method only allows evaluation of the degree of blackness on the external surfaces of cellulose pressboard. However, using terahertz techniques for non-destructive inspection enables determination of the dielectric integrity to check the strength of the electrical insulation inside the cellulose pressboard. The results indicate that the terahertz techniques could be very useful, especially in the electric power industry, which requires a variety of methods for internal non-destructive testing to ensure the safety and reliability of insulation materials in electric power facilities and components.

Ponderomotive force induced nonlinear interaction between powerful terahertz waves and plasmas

We have investigated the ponderomotive force induced nonlinear interaction between terahertz (THz) wave and plasmas. The effects of ponderomotive force on the THz beam-width and spatial electric intensity distribution are investigated in different incident THz wave frequencies. It is observed that ponderomotive force can make THz beam focusing, and the higher the THz wave frequency, the more stable the focusing effect will be. The characteristics of THz beam will also be affected by plasma density. This unique nonlinear interaction may be exploited in novel THz driven optical modulators and switches.

Tunable terahertz wave difference frequency generation in a graphene/AlGaAs surface plasmon waveguide

Graphene-based surface plasmon waveguides (SPWs) show high confinement well beyond the diffraction limit at terahertz frequencies. By combining a graphene SPW and nonlinear material, we propose a novel graphene/AlGaAs SPW structure for terahertz wave difference frequency generation (DFG) under near-infrared pumps. The composite waveguide, which supports single-mode operation at terahertz frequencies and guides two pumps by a high-index-contrast AlGaAs/AlOx structure, can confine terahertz waves tightly and realize good mode field overlap of three waves. The phase-matching condition is satisfied via artificial birefringence in an AlGaAs/AlOx waveguide together with the tunability of graphene, and the phase-matching terahertz wave frequency varies from 4 to 7 THz when the Fermi energy level of graphene changes from 0.848 to 2.456 eV. Based on the coupled-mode theory, we investigate the power-normalized conversion efficiency for the tunable terahertz wave DFG process by using the finite difference method under continuous wave pumps, where the tunable bandwidth can reach 2 THz with considerable conversion efficiency. To exploit the high peak powers of pulses, we also discuss optical pulse evolutions for pulse-pumped terahertz wave DFG processes.

Transmission characteristics of terahertz wave in high temperature plasma

In the hypersonic flight, the air surrounding an aircraft under the effect of high temperature will be ionized. The ionized gas is called plasma. Because of the influence of interaction between electromagnetic wave, in some cases the communication will be interrupted. High temperature effect is an important characteristic of the plasma. Therefore, the study of terahertz wave propagation in high temperature plasma is of great significance. In this paper, the transmission of terahertz wave in a high temperature plasma slab is studied. Generally, high temperature plasma is an anisotropic medium. The electromagnetic wave propagates in anisotropic high-temperature plasma and forms left-hand circular polarization mode or right-hand circular polarization (RCP) mode. It is found that the RCP wave can exhibit some novel characteristics, such as the forbidden band transmission characteristics, which is discovered in this paper. The transmission characteristics of terahertz wave in high temperature plasma are studied analytically. The results show that when the frequency of terahertz wave is lower than plasma frequency, the wave cannot be propagated in high temperature plasma, and it shows a stopband characteristic. When the frequency is higher, it can be transmitted through the plasma, and it presents a passband characteristic. These are consistent with the propagation characteristics of electromagnetic waves in cold plasma. However, some characteristics in high temperature plasma are different from those in the cold plasma. In high temperature plasma, the transmission characteristics are influenced by the electron temperature and external magnetic field. When the two parameters are chosen appropriately, a sharp transmission peak will be produced in the stopband. This phenomenon has never been found in cold plasma models before. And the paper will discuss this problem by the two influencing factors. It is also found that the frequency of the transmission peak is affected by magnetic field, and the peak amplitude is influenced by electron temperature. The electron temperatures at high transmittance (transmittance is about 1) under different applied magnetic fields are calculated. In order to study the law embodied in the data, the method of data fitting is adopted. And the formula of transmission peak frequency is obtained by curve fitting. The fitting results show that the transmission peak frequency is proportional to the external magnetic field. The relationship between peak electron temperature and external magnetic field is exponential. Finally, the fitting formula is verified by the finite-difference time-domain method. The numerical results are in good agreement with the analytical solution results, which proves the correctness of the work.