High-power Terahertz Radiation Research Articles

Observation of THz surface waves escaping from metal gratings through a dielectric substrate

Extensive research has been conducted on generating THz waves using Smith-Purcell radiation, yet a portion of the electron bunch’s interaction energy with the gratings is confined to the metal gratings’ surface, leading to a low THz radiation power. This paper experimentally demonstrates that metal gratings with a dielectric substrate can emit the resonant modes in surface waves when excited by relativistic femtosecond electron bunches. The observed spectra of the resonant THz waves align well with the theoretical estimations derived from the configuration’s dispersion relation and 3D simulations. In comparison to traditional Smith-Purcell radiation generated by the grating, these resonant THz waves exhibit significantly higher intensity and improved orientation. Additionally, we investigated the radiation characteristics of the resonant THz waves, including radiation angle, beam-grating distance, beam energy, and bunch length. This innovative approach presents a novel method for generating high-power coherent terahertz radiation.

0.263 terahertz irradiation induced genes expression changes in Caenorhabditis elegans

The biosafety of terahertz (THz) waves has emerged as a new area of concern with the gradual application of terahertz radiation. Even though many studies have been conducted to investigate the influence of THz radiation on living organisms, the biological effects of terahertz waves have not yet been fully revealed. In this study, Caenorhabditis elegans (C.elegans) was used to evaluate the biological consequences of whole-body exposure to 0.263 THz irradiation. The integration of transcriptome sequencing and behavioral tests of C.elegans revealed that high-power THz irradiation damaged the epidermal ultrastructures, inhibited the expression of the cuticle collagen genes, and impaired the movement of C.elegans. Moreover, the genes involved in the immune system and the neural system were dramatically down-regulated by high-power THz irradiation. Our findings offer fresh perspectives on the biological impacts of high-power THz radiation that could cause epidermal damage and provoke a systemic response.

Investigation of nonlinear optical properties in α-A2BB'O6 (A = Li, Na, K; B = Ti, Zr, Hf; B' = Se, Te) by first-principles calculations.

Nonlinear optical (NLO) crystals based on oxides typically have wide bandgaps and large laser damage thresholds (LDTs), which are important for generating high-power and continuous terahertz radiation. Recently, a new family of NLO materials α-A2BB'O6 including Li2TiTeO6 (LTTO) with a strong second harmonic generation (SHG) efficiency of 26 × KH2PO4 (KDP) and a large LDT of 550 MW cm-2 were reported. Herein, we systematically study the electronic structures and NLO properties of α-A2BB'O6 (A = Li, Na, K; B = Ti, Zr, Hf; B' = Se, Te) to explore the relationship between the structure and SHG coefficient. First, 15 members of the A2BB'O6 family are demonstrated to be highly stable and NLO materials, excluding K2TiTeO6, K2TiSeO6 and K2ZrSeO6. Then, the electronic band structure, dipole moment and distortion of BO6/B'O6 octahedrons, SHG coefficient and terahertz absorption spectrum are calculated comprehensively with the element variation of A-site, B-site and B'-site. Finally, the magnitude of the SHG coefficient is found to be directly proportional to the value of total dipole moment and distortion, and inversely proportional to the bandgap value. Most importantly, among the A2BB'O6 materials, K2HfSeO6 shows the smallest direct bandgap of 2.99 eV, the largest SHG coefficient d33 of about 5 × LTTO and low terahertz absorbance from 0.1 to 9 THz. Our results provide new NLO crystals that may have potential application in terahertz radiation sources and other nonlinear electronics.

Toward high-power terahertz radiation sources based on ultrafast lasers

A breakthrough to achieve high-power THz sources is needed. This review covers recent advancements in laser-based THz generation, specifically using ultrafast lasers.

Perspectives on ultraintense laser-driven terahertz radiation from plasmas

High-power terahertz (THz) radiation is fundamental to numerous applications in many fields. Ultraintense laser-produced plasmas have attracted ever-increasing interest as a damage-free medium for generating high-peak-power THz pulses. This article gives the authors' perspectives on how the field of ultraintense laser-driven THz radiation from plasmas developed and where the field is headed. In particular, recent advances and some new ideas are outlined in terms of THz genesis, metrology, and applications. In addition to pushing the limits of achievable THz pulse energies and peak powers, much attention will be paid on the tunability of THz properties. Single-shot THz metrology will develop toward multi-dimensional resolution. The resulting extreme THz radiation offers immense opportunities in the THz control over matter and THz-driven strong-field physics. A selection of illustrative application cases in the field of materials, chemistry, and biology are briefly discussed. In the authors' opinion, the concerted advances in these aspects will propel this field into the bright future.

Two coupled resonant-tunneling-diode oscillators with an air-bridged transmission line for high-power coherent terahertz radiation

We developed a two-coupled resonant-tunneling-diode (RTD) terahertz (THz) oscillator with a high output power to fill the THz gap. We arranged two RTD mesas in a low-loss air-bridged transmission line for strong mutual coupling and generation of a unique operation mode and integrated a planar ring-slot antenna for efficient THz radiation while satisfying the impedance matching condition. The device structure was fabricated using a multilayer resist process and a carefully controlled wet etching process. The fabricated device exhibited coherent operation and a high output power of approximately 0.24 mW at a high frequency of 925 GHz.

Electron Beam Third Harmonic Amplification Generates High-Power Tunable Terahertz Radiation

A novel approach to extract and amplify the third harmonic of electron beam to generate high-power terahertz radiation is presented. Three planar grating structures operating in the backwave region are used for the extraction and amplification of the electron beam from the fundamental modulation, the fundamental amplification, and the third harmonic. This results in an amplified output of the fundamental wave and a high power output of the fundamental triplet frequency. The planar grating structure is easier to machine and assemble than conventional harmonic amplifiers and increases the flow rate of the electron beam. Calculations show that with an input signal of 119.5–120.6 GHz at 80 mW and a 35-A/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text{2}}$</tex-math> </inline-formula> electron beam in adjustment from 27.5 to 29.3 keV, the structure has an output signal of hundreds of milliwatts in the range of 358.5–361.8 GHz, with a power output of 583.2 mW at the central frequency point of 360 GHz. The harmonic signal is output while a watt-level fundamental signal can be output.

Transmission grating couples and enhances the second harmonic of the electron beam to generate tunable high-power terahertz radiation.

Vacuum electronic devices utilizing free-electron-based mechanisms are a crucial class of terahertz radiation sources that operate by modulating electron beams. In this study, we introduce what we believe is a novel approach to enhance the second harmonic of electron beams and substantially increase the output power at higher frequencies. Our method employs a planar grating for fundamental modulation and a transmission grating operating in the backward region to augment the harmonic coupling. The outcome is a high power output of the second harmonic signal. Contrasting with traditional linear electron beam harmonic devices, the proposed structure can achieve an output power increase of an order of magnitude. We have investigated this configuration computationally within the G-band. Our findings indicate that an electron beam density of 50 A/cm2 at 31.5 kV can produce a 0.202 THz center frequency signal with an output power of 4.59 W. As the electron beam voltage is adjusted from 23 kV to 38.5 kV, the output signal frequency shifts from 0.195 THz to 0.205 THz, generating several watts of power output. The starting oscillation current density at the center frequency point is 28 A/cm2, which is significantly lower in the G-band compared to conventional electron devices. This reduced current density has substantial implications for the advancement of terahertz vacuum devices.

Improving the efficiency of an optical-to-terahertz converter using sapphire fibers

Objectives. The study aims to improve the efficiency of a large-area photoconductive terahertz (THz) emitter based on an optical-to-terahertz converter (OTC) having a radiating area of 0.3 × 0.3 mm2 for generating high-power THz radiation by using an array of close-packed profiled sapphire fibers having a diameter in the range of 100–300 μm as focusing optics.Methods. As a photoconductive substrate, we used a semi-infinite LT-GaAs layer (low-temperature grown GaAs; GaAs layer grown by molecular beam epitaxy at a low growth temperature). Additional Si3N4 and Al2O3 layers are intended for reducing leakage currents in the OTC and reducing the reflection of the laser pump pulse from the air/semiconductor interface (Fresnel losses), respectively, at a gap width of 10 μm. For forming the antenna electrodes and feed strips, the Ti/Au metal system was used. The simulation was carried out by the finite element method in the COMSOL Multiphysics environment.Results. The use of a profiled sapphire fiber whose diameter has been optimized with respect to the gap parameters to significantly increase the concentration of charge carriers in the immediate vicinity of the electrodes of an OTC is demonstrated. The integrated efficiency of a large-area photoconductive THz emitter was determined taking into account the microstrip topology of the array with a characteristic size of feed strips proportional to the gap width in the OTC and with the upper (masking) metal layer. The maximum localization of the electromagnetic field in close proximity to the edges of electrodes at the “fiber–semiconductor” interface is achieved with a profiled sapphire fiber diameter of 220 μm.Conclusions. By optimizing the diameter of the sapphire fiber, the possibility of improving the localization of incident electromagnetic waves in close proximity to the edges of the OTC electrodes by ~40 times compared to the case without fiber, as well as increasing the overall efficiency of a large-area emitter by up to ~7–10 times, was demonstrated.

Detection of the minimum concentrations of α-lactose solution using high-power THz-ATR spectroscopy.

Terahertz (THz) technology has emerged as a promising tool for the qualitative and quantitative identification of markers containing major diseases, enabling early diagnosis and staged treatment of diseases. Nevertheless, the detection of water-containing biological samples is facing significant challenges due to limitations in high-power THz radiation sources and high-sensitivity detection devices. In this paper, we present a designed and constructed set of Terahertz-Attenuated Total Reflection (THz-ATR) spectrometer for high-sensitivity detection of liquid biological samples, which can dynamically maintain the signal-to-noise ratio (SNR) of THz detection signal of liquid biological samples at 40-60dB. Our high-power THz-ATR spectroscopy can identify and quantitatively detect α-lactose aqueous solution with a minimum concentration of 0.292mol/L. Moreover, we observed that the rate of change in the absorption peak position varied greatly between high and low concentration samples. Our high-power, high-sensitivity THz-ATR spectroscopy detection provides a rapid, accurate, and low-cost method for detecting disease markers such as blood and urine indicators. Additionally, this approach offers new perspectives for the refinement and in-depth detection of biomedical samples.

Multi-millijoule terahertz emission from laser-wakefield-accelerated electrons

High-power terahertz radiation was observed to be emitted from a gas jet irradiated by 100-terawatt-class laser pulses in the laser-wakefield acceleration of electrons. The emitted terahertz radiation was characterized in terms of its spectrum, polarization, and energy dependence on the accompanying electron bunch energy and charge under various gas target conditions. With a nitrogen target, more than 4 mJ of energy was produced at <10 THz with a laser-to-terahertz conversion efficiency of ~0.15%. Such strong terahertz radiation is hypothesized to be produced from plasma electrons accelerated by the ponderomotive force of the laser and the plasma wakefields on the time scale of the laser pulse duration and plasma period. This model is examined with analytic calculations and particle-in-cell simulations to better understand the generation mechanism of high-energy terahertz radiation in laser-wakefield acceleration.

Microsphere-based photoexcited efficient terahertz radiation at room temperature enhanced by Ag/PI/PMMA/ZnO circle hollow waveguide resonance

The discovery and development of efficient terahertz sources at room temperature could lead to comprehensive application foreground. Here, ZnO mesocrystal microspheres were prepared by hydrothermal method through self-assembly of nanosheets. High-frequency mechanical vibrations of charged nanostructures excited by a continuous 514.5 nm laser at room temperature can lead to radiative emission at 0.36 THz, conversion efficiency of which was only 0.015%. A sandwich-structured Ag/PI/PMMA/ZnO circle hollow terahertz waveguide was fabricated to obtain high-power terahertz radiation source. Nano Ag thin film effectively localized terahertz light in the waveguide and reduced diffusion losses because of high reflection. Then, Optical lenses were placed before and after the waveguide to form a resonant cavity, so that terahertz light was resonantly enhanced at certain length. Meanwhile, the thickness of the PI thin film was designed to enhance the interference of terahertz light. Besides, the incident angle of green laser had a great influence on its reflection path within the waveguide, which in turn affected the number of excitations of ZnO. The above comprehensive reasons have successfully improved the conversion efficiency by 5.5 and 8.3 times when the angle of incident laser was 0° and 30°, respectively. This work revealed a new strategy to design and fabricate terahertz sources with excellent properties simultaneously such as room temperature operation, stable performance, and high power output.

Widely tunable electron bunch trains for the generation of high-power narrowband 1–10 THz radiation

Terahertz radiation enables the coherent excitation of many fundamental modes, such as molecular rotation, lattice vibration and spin precession. To excite the transient state of matter far out of equilibrium, high-power and tunable narrowband terahertz radiation sources have been in great demand for years. However, the terahertz source available at present cannot meet these tunability and power demands, leaving a large scientific gap that has yet to be fully explored. Here we convert the energy modulation induced by the nonlinear longitudinal space charge force to a density modulation and experimentally demonstrate the generation of electron bunch trains with modulation frequencies that are adjustable between 1 and 10 THz. The electron bunch trains can be directly used to produce tunable high-power narrowband terahertz radiation that fully covers the long-standing ‘terahertz gap’, which will open up many more possibilities in terahertz science. A tunable terahertz radiation pulse is demonstrated based on a linear accelerator. The emission frequency of this terahertz radiation is tunable from 1 to 10 THz by changing the bunching frequency of a 34 MeV electron beam. The pulse energy is at the submillijoule level.

High-selective spatially-extended Bragg resonators implementing three-dimensional distributed feedback for powerful free electron lasers

Novel scheme of Bragg resonators implementing three-dimensional distributed feedback is proposed. The study of their electrodynamic characteristics in the framework of the coupled wave approach has been carried out and high selective properties for the three mode indices have been demonstrated under conditions of substantial oversize. The results of the theoretical analysis are corroborated by the 3D simulations. The prospects of using these resonators in the project of a super-power terahertz-band free electron laser, which is currently being developed at the BINP RAS (Novosibirsk) in collaboration with the IAP RAS (Nizhny Novgorod), are discussed. Keywords: free-electron lasers, high-power terahertz radiation, Bragg resonators, three-dimensional distributed feedback, mode selection.

Considered design and radiation calculation for a compact THz coherent radiation source

Since the light obtained from the Siam Photon Source (SPS) in Thailand is the high-intensity radiation in only the region of the infrared (IR) to high-energy X-rays, it has great attention in many areas of scientific research and industrial development. In order to increase in the number of users and applications, the terahertz (THz) region covering the frequency range from 0.1 to 10 THz needs to be fulfilled. Therefore, a compact accelerator based free-electron laser or coherent radiation is particularly interesting for providing an intense THz radiation. A 1.6-cell photocathode RF gun is used as an electron source to generate a high-brightness electron beam. The energy of electron beam can be raised up to meet the target of 30 MeV in a linear accelerator (Linac) before moving to a chicane bunch compressor. The THz radiation is produced when a short bunch electron beam propagates through an undulator. In order to achieve the intense narrow-band THz coherent radiation at tunable frequencies between 0.5 THz and 5.0 THz with MW peak power, a THz radiation source needs to be studied and developed. Design of high-power THz radiation source is described in this paper.

All-dielectric metalens for quasi-optical mode and polarization conversion.

Quasi-optical mode conversion technology plays a very important role in the development of high-power terahertz radiation sources. The ability of metamaterials to manipulate wave-front paves a new way in the field of quasi-optical mode conversion. In this paper, the approach for quasi-optical mode conversion by all-dielectric metalens and polarization conversion is proposed and investigated. Three metalens are designed to converter cylindrical waveguide TE01 mode to linear polarized (LP), left-hand circularly polarized (LHCP), and right-hand circularly polarized (RHCP) Gaussian beams at 350 GHz. Electromagnetic simulations show that the Gaussian mode contents of output waves from three metalens are all over 98% with high polarization contents. Furthermore, a metalens is designed for dual circularly polarized (DCP) which could convert cylindrical waveguide TE01 mode to LHCP and RHCP simultaneously. This work unveils the potential application for metalens in terahertz region.

High power terahertz radiation generated by beam-plasma system in multi-filament regime

In the particle-in-cell simulation study of this paper, it is found that the high-power terahertz radiation at twice the plasma frequency can be generated by the interaction between a weakly relativistic, large-radius, focused electron beam and an overdense plasma. The terahertz radiation is emitted from a multi-filament structure in the electron beam, which is caused by the coupling of the longitudinal two-stream instability and the transverse filamentation instability. The analysis of the k space of the plasma waves indicates that the radiation can be explained by the mode coupling among a forward oblique mode, a backward oblique mode, and a second harmonic radiation mode. The influences of the beam Lorentz factor and plasma density on the radiation power, efficiency, and coherence are also investigated. The present results not only contribute to the development of novel megawatt-level narrowband terahertz radiation sources, but also propose a potential terahertz radiation mechanism for the fundamental research of the beam-plasma system.

A piecewise sine waveguide for terahertz traveling wave tube

In this paper, a piecewise sine waveguide (PWSWG) is proposed as the slow-wave structure (SWS) to develop high-power terahertz (THz) traveling wave tubes (TWTs). The PWSWG is an improvement over the rectangular waveguide wherein its two E-planes simultaneously oscillate up and down along the longitudinal direction. The oscillation curve in the H-plane is a piecewise sine curve formed by inserting line segments into the peaks and troughs of the sine curve. The simulation analysis and experimental verification show that the PWSWG offers the advantages of large interaction impedance and excellent electromagnetic transmission performance. Furthermore, the calculation results of beam–wave interaction show that the TWT based on PWSWG SWS can generate a radiated power of 253.1 W at the typical frequency of 220 GHz, corresponding to a gain of 37.04 dB and an interaction efficiency of 6.92%. Compared with the conventional SWG TWTs, the PWSWG TWT has higher interaction efficiency and shorter saturation tube length. In conclusion, the PWSWG proposed in this paper can be considered a suitable SWS for high-power THz radiation sources.

Rectangular Grating Slow-Wave Structure With a Slot Embed Electron Beam for High-Power Terahertz Radiation

To enhance the output power of 0.5-Terahertz (THz) backward wave oscillator (BWO), a grating loaded rectangular waveguide (GLRW) slow-wave structure (SWS) with a slot embed electron beam (SEEB) is proposed in present work. For the designs and optimizations of BWO GLRW SWS with SEEB, the dispersion equations are derived with a field matching method (FMM) and solved by a numerical method; the results show that the coupling impedances are remarkably enhanced. Moreover, the output performances are studied with a help of particle-in-cell (PIC) simulation, and the simulated results show that the BWO operation voltage is 25 kV and the current is 40 mA; the output power 18.9 W of the perfect electronic conductor SWS with SEEB is obtained, which is improved about 1.5 times. On the other hand, the time of start oscillation is decreased to 50&#x0025;, which is compared with that the conventional SWS without slot. As for the THz vacuum electronic devices (VED), the amplitudes of output power are remarkably affected by the conductivity and roughness of SWS; therefore, the determinate factors are studied, and the PIC results show that the maximum power is about 5&#x2013;7 W when the conductivity is 3&#x2013;<inline-formula> <tex-math notation="LaTeX">$5\times10$ </tex-math></inline-formula>⁷ S/m. The proposed scheme affords a promising option for the developing of high-power THz source.

Holography with high-power CW coherent terahertz source: optical components, imaging, and applications

This paper presents the results of 15 years of studies in the field of terahertz holography at the Novosibirsk free electron laser. They cover two areas: research on obtaining holographic images in the terahertz range and the use of diffractive optical elements to form high-power terahertz radiation fields with specified characteristics (intensity, phase, and polarization), using well-studied and widely applied in the optical range methods of optical (analog), digital, and computer-generated holography. All experiments were performed with the application of high-power coherent monochromatic frequency-tunable radiation from the Novosibirsk free electron laser. The features of hologram registration in the terahertz range are described. Methods, technologies, and optical materials for terahertz holographic elements are discussed. A wide range of promising applications of high-power terahertz fields with a given spatial structure is considered. The results of the study of terahertz holograms recorded as digital holograms, as well as radiation-resistive optical elements realized as computer-synthesized holograms, are presented.