Terahertz Free Electron Laser Research Articles

Complex anisotropy ratio of third harmonic generation in a semiconductor using a terahertz free electron laser.

The nonlinear susceptibility in the terahertz region is expected to have a non-negligible imaginary part originating from the momentum-dependent scattering time of free carriers, but it has been scarcely reported. By utilizing an intense 4 THz beam from a terahertz free electron laser, we investigated the azimuth angle dependence of the third harmonic generation (THG) from semiconductors. The observed angular anisotropy of THG revealed the contribution of the imaginary part of the nonlinear susceptibility originating from the momentum-scattering time relation in addition to its real part originating from the band nonparabolicity. The results provide a deeper understanding of nonlinear optics in the terahertz region.

Optical design of the time-resolved ARPES beamline of the new material spectroscopy experimental station for the update of CAEP THz-FEL facility

Abstract The Chinese Academy of Engineering Physics Terahertz Free Electron Laser Facility (CAEP THz FEL, CTFEL) is the only high-average power free electron laser terahertz source based on superconducting accelerators in China. The update of the CTFEL is now undergoing and will expand the frequency range from 0.1-4.2 THz to 0.1-125 THz. Two experimental stations for material spectroscopy and biomedicine will be built. A high harmonic generation (HHG) lightsource based beamline at the material spectroscopy experimental station for time-resolved angle-resolved photoemission spectroscopy (ARPES) research will be constructed and the optical design is presented. The HHG lightsource covers the extreme ultraviolet (XUV) photon energy range of 20-50 eV. A Czerny-Turner monochromator with two plane gratings worked in conical diffraction configuration is employed for maintaining the transmission efficiency and preserving the pulse time duration. The calculated beamline transmission efficiency is better than 5% in the whole photon energy range. To our knowledge, this is the first time in China to combine THz-infrared FEL with HHG light source, and this experimental station will be a powerful and effective instrument and give new research opportunities in the future for users doing research on dynamic evolution of excited electron band structure of material surface.

Out-coupling characteristics of partial resonator mirror for compact terahertz free-electron laser

Out-coupling characteristics of partial resonator mirror for compact terahertz free-electron laser

Inductive detection of temperature-induced magnetization dynamics of molecular spin systems

The development and technological applications of molecular spin systems require versatile experimental techniques to characterize and control their static and dynamic magnetic properties. In the latter case, bulk spectroscopic and magnetometric techniques, such as AC magnetometry and pulsed electron paramagnetic resonance, are usually employed, showing high sensitivity, wide dynamic range, and flexibility. They are based on creating a nonequilibrium state either by changing the magnetic field or by applying resonant microwave radiation. Another possible source of perturbation is a laser pulse that rapidly heats the sample. This approach has proven to be one of the most useful techniques for studying the kinetics and mechanism of chemical and biochemical reactions. Inspired by these works, we propose an inductive detection of temperature-induced magnetization dynamics as applied to the study of molecular spin systems and describe the general design and construction of a particular induction probehead, taking into account the constraints imposed by the cryostat and electromagnet. To evaluate the performance, several coordination compounds of VO2+, Co2+, and Dy3+ were investigated using low-energy pulses of a terahertz free electron laser of the Novosibirsk free electron laser facility as a heat source. All measured magnetization dynamics were qualitatively or quantitatively described using a proposed basic theoretical model and compared with the data obtained by alternating current magnetometry. Based on the results of the research, the possible scope of applications of inductive detection and its advantages and disadvantages in comparison with standard methods are discussed.

Characterization of RF System for MIR/THz Free Electron Lasers at Chiang Mai University

The establishment of the mid-infrared and terahertz free-electron laser (MIR/THz FEL) facility is ongoing at the PBP-CMU Electron Linac Laboratory (PCELL) in Chiang Mai University. The facility utilizes an S-band radio-frequency (RF) gun and a linear accelerator (linac) to generate and accelerate electron bunches. These electron bunches are accelerated in the RF gun and the linac using RF pulses with a frequency of 2856 MHz. Measuring the RF properties becomes essential, as the RF pulse information can be utilized to estimate the electron beam properties. To achieve the measurement results, we employed an RF measurement system comprising directional couplers, coaxial cables, attenuators, a crystal detector, and an oscilloscope. Prior to conducting measurements, the crystal detector and RF equipment were calibrated and characterized to ensure precise and reliable results. The electron beam energy estimation using the measured RF power was compared with the measured beam energies. The gun and the linac were operated with an absorbed RF power of 1.52 MW and an input power of 1.92 MW, respectively. The estimated electron beam energies were found to be 2.18 MeV and 15.0 MeV, respectively, closely aligning with the measured beam energies of 2.1 MeV and 14.0 MeV after the gun and linac acceleration. These consistent energy values support the reliability of our RF power measurement system and procedure.

Third harmonic generation due to free carrier in InSb using a terahertz free electron laser.

We report on the third harmonic generation (THG) in InSb semiconductor irradiated by a terahertz (THz) free electron laser (FEL). The conversion of 4 THz (wavelength 70 µm) FEL outputs into its third harmonic 12 THz was observed. We found that by tuning the sample temperature to 360 K, high conversion efficiency up to 1% can be obtained and is the highest in the THz and FIR regions below 10 THz. We also discuss the observed intensity dependence of the THG with the nonlinear order lower than 3 when the pumping intensity was high.

Numerical Simulation of an Electron Beam for Magnetic Bunch Compressor Commissioning at PITZ

A THz free electron laser (FEL) prototype has been developed at the Photo Injector Test Facility at DESY in Zeuthen (PITZ) for obtaining high intensity radiation for THz-pump and X-ray-probe experiments at the European XFEL. In this development, a magnetic chicane was recently installed to optimize the THz FEL performance. The aim of this study is to investigate the beam dynamics in the chicane for a trajectory commissioning by tracking the electron beam via ASTRA using a 3-dimensional (3-D) magnetic field of the chicane simulated with CST-EM Studio. The simulated results indicate the possibility to obtain a minimum-momentum dispersion of an electron beam after chicane with the high bunches charge of electron beam transportation.

Photon Diagnostics for the High-Gain THz FEL at PITZ

Research and development of an accelerator-based THz source prototype for pump-probe experiments at the European XFEL are ongoing at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). Proof-of-principle experiments have been performed to generate a high-gain THz Free-electron Laser (FEL) based on the Self-Amplified Spontaneous Emission scheme. The FEL radiation pulses with a central wavelength of about 100 μm (3 THz) and single pulse energy of several tens μm can be generated. In this paper, we present and discuss the photon diagnostic setup for the THz FEL together with examples of diagnostic results, including pulse energy and an FEL gain curve. The upgraded photon diagnostic setup, capable of measuring pulse energy, transverse distribution, and spectral distribution, is expected to be operational in the spring of 2023.

Effective DC acceleration of charged particles in a circular ring and its potential application

A novel scheme is proposed for realizing effective DC acceleration of charged particles in a circular ring. The key is to use an induction acceleration cell with multi-insulating gaps that allow forced short circuit using solid-state switching devices such as SiC-MOSFET or unforced short circuit using diodes. A baseline design of the induction acceleration cell is shown. A coasting beam is accelerated in the circular ring without bunching. As its possible application, a continuous-wave terahertz free-electron laser (FEL) is proposed with numerical parameters, where this effective DC acceleration compensates for energy loss of the electron beam occupying the entire isochronous storage ring due to synchrotron radiation and FEL interaction. As a result, FEL microbunching of a coasting beam can be created, resulting in a high average output power that has not been possible with existing linac-driven FELs or RF storage-ring FELs.

High-intensity single-pulse extraction using a laser-activated GaAs reflective switch for a terahertz free-electron laser

Single-pulse extraction from a terahertz (THz) pulse train is demonstrated for a free-electron laser oscillator using the laser-activated semiconductor switching technique with gallium arsenide (GaAs). Using a GaAs wafer as the switching substrate and a titanium sapphire laser (Ti:sapphire laser), a single THz pulse can be extracted with a high contrast from the pulse train at intervals of 37 ns in a simple experimental setup due to a short decay time of the electron–hole plasma. For a single THz pulse with a duration of a few to a few tens of picoseconds and a nominal wavelength of 70μm, we achieve an extracted single pulse energy of 76 μJ.

Analysis of the beam loading effect in a compact linac with long bunch tails

Beam loading effect is an important issue in all kinds of accelerating structures, especially in a high current case, which might result in a significant difference between the actual output energy and the designed energy. In this paper, we perform simulations and experimental studies on the beam loading effect in an S-band traveling wave (TW) linac with a maximum acceleration gradient of 19.4 MV/m for an electron bunch larger than 200 pC. The TW linac is designed to drive a terahertz free electron laser (THz-FEL) and can produce beam energies varying from 8 ∼ 14 MeV. The main challenge of the beam loading analysis come from the long bunch tail produced by the thermionic cathode gun. The long bunch tail causes considerable beam loss in the linac and brings great difficulties in the beam commissioning, especially in the beam energy tuning. So, the macro-particle tracking (MPT) algorithm is proposed to accurately analyze the beam loading effect of the long tail beams. The MPT algorithm uses energy conservation in each cell of the TW linac to take into account the particle loss and phase shift during the acceleration process. Based on the THz-FEL test facility, beam energies were measured under different loading cases. The experimental results confirmed that the new algorithm could predict the beam energy with a relative error less than 1.4%, while that of the traditional method is 4% ∼ 6%.

Design, construction, and measurement of electromagnetic steering magnet for 25-MeV electron accelerator system

Mid-infrared and terahertz free-electron laser (MIR/THz FEL) facility is under the construction at the PBP-CMU Electron Linac Laboratory (PCELL) in Chiang Mai University. For producing MIR-FEL, electron beams with an energy of 25 MeV accelerated by a radio-frequency linear accelerator (RF linac) are required. Transporting the 25-MeV electron beam from the linac to the MIR-FEL station requires small dipole magnets for steering the electron beams along the beamline. This work focuses on the design, construction, and measurement of an electromagnetic steering magnet for controlling the electron beam trajectory. A computer software, CST Studio Suite 2022 is used for the 3D magnetic field simulation of the magnet. The physical length of the magnet and the diameter of copper coil are decided based on the simulation and calculation results. After the construction, a hall probe with a measuring system controlled by a computer program is used to measure the magnetic field for comparison with the simulation results. From simulated and calculated results, the steering magnet with a length of 40 mm and the coil with a diameter of 1.06 mm was chosen. At an applied current of 1.8 A, this steering magnet can bend the 25-MeV electron beam with a bending angle of 13.7 mrad. The measured magnetic field is consistent with the simulated results. The advantage of this steering magnet is that it can be assembled on a square frame for steering the beam in both horizontal and vertical direction. It can also be used for the THz-FEL beamline, which requires the electron beam with an energy of 10-16 MeV.

Construction and tests of phosphor view screen station for monitoring transverse profile of electron beam at PCELL

Development of an accelerator-based light source for generating coherent terahertz (THz) and mid-infrared (MIR) free-electron laser (FEL) is ongoing at the PBP-CMU Electron Linac Laboratory (PCELL). For producing high quality radiation, suitable electron beam property is necessary. In particle accelerator technology, transverse beam distribution and profile can be measured by using several techniques. The beam monitoring using view screen station is one of popular and effective methods. At PCELL, we use stations consisting of phosphor screen equipped with CCD camera to monitor, measure and record the beam image along the accelerator system and beamlines. The phosphor screen was chosen due to its high spatial resolution. The phosphor film is uniformly coated on a thin aluminium plate. Its green emission when electrons hitting the screen is matched well with the respond of the CCD camera. The screen can be moved in and out from the vacuum chamber using the air pressurized linear actuator. This paper presents about design, construction and tests of the screen stations at PCELL. The resolution of measurements for beam transverse profile and emittance is also be discussed.

Specific Features of the Proteomic Response of Thermophilic Bacterium Geobacillus icigianus to Terahertz Irradiation

Studying the effects of terahertz (THz) radiation on the proteome of temperature-sensitive organisms is limited by a number of significant technical difficulties, one of which is maintaining an optimal temperature range to avoid thermal shock as much as possible. In the case of extremophilic species with an increased temperature tolerance, it is easier to isolate the effects of THz radiation directly. We studied the proteomic response to terahertz radiation of the thermophilic Geobacillus icigianus, persisting under wide temperature fluctuations with a 60 °C optimum. The experiments were performed with a terahertz free-electron laser (FEL) from the Siberian Center for Synchrotron and Terahertz Radiation, designed and employed by the Institute of Nuclear Physics of the SB of the RAS. A G. icigianus culture in LB medium was THz-irradiated for 15 min with 0.23 W/cm2 and 130 μm, using a specially designed cuvette. The life cycle of this bacterium proceeds under conditions of wide temperature and osmotic fluctuations, which makes its enzyme systems stress-resistant. The expression of several proteins was shown to change immediately after fifteen minutes of irradiation and after ten minutes of incubation at the end of exposure. The metabolic systems of electron transport, regulation of transcription and translation, cell growth and chemotaxis, synthesis of peptidoglycan, riboflavin, NADH, FAD and pyridoxal phosphate cofactors, Krebs cycle, ATP synthesis, chaperone and protease activity, and DNA repair, including methylated DNA, take part in the fast response to THz radiation. When the response developed after incubation, the systems of the cell's anti-stress defense, chemotaxis, and, partially, cell growth were restored, but the respiration and energy metabolism, biosynthesis of riboflavin, cofactors, peptidoglycan, and translation system components remained affected and the amino acid metabolism system was involved.

Enhancement of Smith-Purcell radiation from cylindrical gratings by quasi-bound states in the continuum.

Smith-Purcell radiation (SPR) is an important means of generating terahertz waves, and the enhancement of SPR is an attractive topic nowadays. Inspired by the phenomenon of special SPR, where the enhancement is achieved by using a high-duty-cycle grating, we describe a new, to the best of our knowledge, but more effective approach to this challenging problem. By deriving a simple analytical solution for the SPR from an annular electron beam passing through a cylindrical metallic grating, we show that the inverse structure, a low-duty-cycle grating can exhibit rather high SPR efficiencies in the presence of quasi-bound states in the continuum (quasi-BICs). The analytical prediction is supported by particle-in-cell simulations, which show that the quasi-BICs can enhance the superradiant SPR generated by a train of electron bunches by orders of magnitude. These results present an interesting mechanism for enhancing the SPR from metallic gratings, and may find applications in terahertz free-electron lasers.

Nonlinear terahertz transmission spectroscopy on Ga-doped germanium in high magnetic fields

The authors report the observation of cyclotron resonance transitions of holes in the magnetotransmission spectra of gallium-doped germanium at low temperatures, using intense, pulsed terahertz free-electron laser radiation with a photon energy lower than the ionization energy of the Ga dopants. The spectra are very sensitive to THz intensity and provide compelling results about ionization mechanisms and saturation of Landau level transitions. The spectroscopy technique introduced here offers an exciting perspective for researching nonlinear magneto-optical processes in semiconductors.

Single-pass high-efficiency terahertz free-electron laser

Single-pass high-efficiency terahertz free-electron laser

Radioprotective effect of X-ray abdominal FLASH irradiation: Adaptation to oxidative damage and inflammatory response may be benefiting factors.

Ultrahigh dose-rate irradiation (FLASH-IR) was reported to be efficient in tumor control while reducing normal tissue radiotoxicity. However, the mechanism of such phenomenon is still unclear. Besides, the FLASH experiments using high energy X-ray, the most common modality in clinical radiotherapy, arerarely reported. This study aims to investigate the radiobiological response using 6 MV X-ray FLASH-IR or conventional dose-rate IR (CONV-IR). The superconducting linac of Chengdu THz Free Electron Laser (CTFEL) facility was used for FLASH-IR, a diamond radiation detector and a CeBr3 scintillation detector were used to monitor the time structure and dose rate of FLASH pulses. BALB/c nude mice received whole abdominal 6 MV X-ray FLASH-IR or CONV-IR, the prescribed dose was 15Gy or 10Gy and the delivered absolute dose was monitored with EBT3 films. The mice were either euthanized 24 h post-IR to evaluate acute tissue responses or followed up for 6weeks to observe late-stage responses and survival probability. Complete blood count, histological analyses, and measurement of cytokine expression and redox status were performed. The mean dose rate of>150Gy/s and instantaneous dose rate of>5.5 × 105 Gy/s was reached in FLASH-IR at the center of mice body. After 6 weeks' follow-up of mice that received 15Gy IR, the FLASH group showed faster body weight recovery and higher survival probability than the CONV group. Histological analysis showed that FLASH-IR induced less acute intestinal damage than CONV-IR. Complete blood count and cytokine concentration measurement found that the inflammatory blood cell counts and pro-inflammatory cytokine concentrations were elevated at the acute stage after both FLASH-IR and CONV-IR. However, FLASH irradiated mice had significantly fewer inflammatory blood cells and diminished pro-inflammatory cytokine at the late stage. Moreover, higher reactive oxygen species (ROS) signal intensities but significantly reduced lipid peroxidation were found in the FLASH group than in the CONV group in the acute stage. The radioprotective effect of 6 MV X-ray FLASH-IR was observed. The differences in inflammatory responses and redox status between the two groups may be the factors responsible for reduced radiotoxicities following FLASH-IR. Further studies are required to thoroughly evaluate the impact of ROS on FLASH effect.

Prospects of a terahertz free-electron laser for field application

Prospects of a terahertz free-electron laser for field application

Low-loss and small-cross-section waveguide for compact terahertz free-electron laser

The demand for terahertz (THz) sources that can be used in practical implementations has not yet been fully met. Through the development of small-scale free-electron lasers (FEL), we investigated a solution to overcome the low lasing gain of the FEL, which is caused by a small-current accelerator and a short-length undulator. To enhance the FEL gain, the FEL interaction between the electron beam and radiation was increased by reducing the cross-sectional area of the FEL oscillator mode. We developed a waveguide for the FEL oscillator, which has a remarkably small eye-shaped cross section with low wave loss. The mode cross-sectional area was calculated to be just 4 mm2 (full width at half maximum), which is considerably smaller than those of the free-space Gaussian mode and other waveguide modes. Using COMSOL Multiphysics simulation code, we calculated and analyzed the attenuation loss and mode cross-section area for waveguides of different shapes and sizes, and we observed that the dielectric coated eye-shaped waveguide has the lowest attenuation loss, that is, less than 2.5% for 1-m propagation at an operating wavelength of 300–600 µm. These results are in good agreement with our requirements for the tabletop THz FEL. Finally, we showed that the calculated operating wavelength range of the waveguide-mode THz FEL is 300–600 µm using the machine parameters of an accelerator and undulator that were developed by considering the dispersion relations of the eye-shaped waveguide and undulator radiation.