Far-infrared Free-electron Laser Research Articles

Generation of ∼100 kW narrow-line far-infrared radiation from a KTP off-axis THz parametric oscillator

In the far-infrared spectrum between 20 and 60 μm, the free-electron laser (FEL) is the only wavelength-tunable coherent radiation source capable of generating kilowatt to megawatt peak powers with a linewidth of the order of 1%. Here, we report the detection of >70 kW radiation power at about 52 μm in a 45 kW radiation power for the far-infrared radiation. With 63% coupling efficiency of the silicon prism atop the KTP crystal, the measured >70 and >45 kW far-infrared radiation correspond to >111 and >71 kW powers extracted from the KTP crystal of the seeded off-axis THz parametric oscillator. The radiation source accomplished in this work has great potential to become a tabletop and economical alternative for the bulky and expensive far-infrared FELs in national facilities.

Enhancement of the Perfomence in a Far-infrared Free-electron Laser Oscillator Using a Harmonic Wiggler

Enhancement of the Perfomence in a Far-infrared Free-electron Laser Oscillator Using a Harmonic Wiggler

Evidence for competition modes in a partially guided far-infrared free-electron laser

The infrared free-electron laser (FEL) offers a large tunability since the FEL gain remains high throughout the infrared spectral range, and the reflectivity of metal mirrors remains also close to unity. The main limitation comes from the diffraction of the optical beam due to the finite size of the vacuum chamber of the undulator. A solution is to use this chamber as a waveguide by adapting the radius of curvature of the cavity mirrors to this regime. Then, as has been shown before, a minimum appears in the spectrum that can be produced by the FEL. We discuss the physical mechanism of this particular regime and compare it to experiments using vacuum chambers of different transverse sizes. A good agreement is found with results of simulations and with a simple analytical formula.

Ultrabright continuously tunable terahertz-wave generation at room temperature.

The hottest frequency region in terms of research currently lies in the ‘frequency gap' region between microwaves and infrared: terahertz waves. Although new methods for generating terahertz radiation have been developed, most sources cannot generate high-brightness terahertz beams. Here we demonstrate the generation of ultrabright terahertz waves (brightness ~0.2 GW/sr·cm2, brightness temperature of ~1018 K, peak power of >50 kW) using parametric wavelength conversion in a nonlinear crystal; this is brighter than many specialized sources such as far-infrared free-electron lasers (~1016 K, ~2 kW). We revealed novel parametric wavelength conversion using stimulated Raman scattering in LiNbO3 without stimulated Brillouin scattering using recently-developed microchip laser. Furthermore, nonlinear up-conversion techniques allow the intense terahertz waves to be visualized and their frequency determined. These results are very promising for extending applied research into the terahertz region, and we expect that this source will open up new research fields such as nonlinear optics in the terahertz region.

Free-electron laser operation with a superconducting radio-frequency photoinjector at ELBE

At the radiation source ELBE a superconducting radio-frequency photoinjector (SRF gun) was developed and put into operation. Since 2010 the gun has delivered beam into the ELBE linac. A new driver laser with 13MHz pulse repetition rate allows now to operate the free-electron lasers (FELs) with the SRF gun. This paper reports on the first lasing experiment with the far-infrared FEL at ELBE, describes the hardware, the electron beam parameters and the measurement of the FEL infrared radiation output.

Spectral properties of a far-infrared free-electron laser oscillator based on a two-frequency wiggler

The spectral properties and the nonlinear interactions between an electron beam and the emitted radiation field were studied in a far-infrared free-electron laser oscillator. A two-frequency wiggler was used to obtain a high efficiency and a narrow spectrum, and a multi-particle simulation code was developed using a modified free-electron laser model based on the two-frequency wiggler. Numerical simulations were performed to obtain the optimal condition for the two-frequency wiggler, and the emitted field amplitude and the spectral properties were found to be improved when the two-frequency ratio parameter of the wiggler was optimized.

Nonlinear interaction between radiation and an electron beam in a free-electron laser oscillator based on a two-frequency wiggler

A two-frequency wiggler was used to enhance the efficiency and the radiation power in a far-infrared free-electron laser oscillator. The interaction between the electron beam and the radiation was numerically studied using multi-particle and multi-pass simulation coding to understand the effect of the two-frequency wiggler, and the wiggler’s frequency ratio parameter was optimized. The influence of the electron beam energy spread was also investigated in the two-frequency wiggler for a monochromatic laser field.

A tapered undulator experiment at the ELBE far infrared hybrid-resonator oscillator free electron laser

A tapered undulator experiment was carried out at the ELBE far-infrared free electron laser (FEL). The oscillator FEL makes use of a hybrid optical resonator. The main motivation was to see whether the presence of a dispersive medium in the form of a waveguide in the resonator has any effect on the outcome. The FEL saturated power and the wavelength shifts have been measured as a function of both positive as well as negative undulator field amplitude tapering. In contrast to the typical high-gain FELs where positive tapering proves beneficial for the output power we observed an improvement of performance at negative taper. During the same experiments we studied the characteristics of the detuning curves. The width of the curves indicates a maximum small signal gain for zero taper while the output peak power increases with negative taper. The saturated power output, the detuning curve characteristics, and the wavelength shifts agrees with the theoretical predictions. Details of the experiment are presented.

Effects of Optical Parameters in a Free-Electron Laser Oscillator

We have developed a three-dimensional simulation code to study free-electron laser optical resonator parameters for a proposed far-infrared free-electron laser facility. We have investigated the effects of Rayleigh length, electron beam spot size, power extraction, and waist position by using the modified coupled equations, including a wave equation that is expanded in axisymmetric Gauss–Laguerre modes. The modified coupled equations are also used in the simulation to obtain the effects for fundamental and higher order modes. The optimum value for the oscillator mirror radius of curvature is found, and the effects of Rayleigh length, electron beam spot size, power extraction, and waist position in a free-electron laser oscillator with a short undulator are investigated.

Far-infrared free-electron lasers and their applications

Free-electron lasers based on radio-frequency linear accelerators provide an important source of far-infrared radiation which allow exciting experiments that cannot be performed in any other way. Facilities such as FELIX (Nieuwegein, The Netherlands), JFEL (Newport News VA, USA), FELBE (Dresden, Germany), CLIO (Paris, France) and others provide mid- and far-infrared output in picosecond pulses with micro-joules of energy. They give continuous, wide tuning in far-IR for resonant pumping of discrete transitions (with simultaneous coverage of mid-IR) from around 3 to 250 μm wavelength. This enables time-resolved spectroscopy, non-linear optics and spectroscopy of weak absorptions. They have been applied to a wide variety of problems in condensed matter physics, physical chemistry and biophysics. We review the physics applications of these sources.

Analysis of periodic spectral gaps observed in the tuning range of free-electron lasers with a partial waveguide

A phenomenon of ``spectral gaps'' is observed in far-infrared free-electron lasers using partial waveguiding: the laser power falls down to zero at some particular wavelengths, whatever the beam adjustments are. We show that this effect is related to a periodic increase of the cavity losses, with a period depending on the dimensions of the waveguide. A code including diffraction effects exhibits good agreement with the measurements.

The Effect of the Tapered Wiggler in a Free-Electron Laser Oscillator with the Multi-Transverse Modes

In a far-infrared free-electron laser oscillator, the nonlinear interaction between the electrons and the radiation field has been studied. We have developed a simulation code which is based on the coupled Maxwell–Lorentz equations of motion, and this code enables us to describe the transverse mode competition and the effect of the tapered wiggler. In order to improve the power and efficiency, we have performed numerical simulation with higher transverse modes of the radiation field, and the tapering parameter is optimized.

Development of far-infrared FEL with needle photo-RF-gun

Far-infrared free electron lasers (FELs) with central wavelengths of 11.6 and ∼70 μm were obtained by using an S-band RF linac (LEENA) at electron energies of 14.4 and ∼5.5 MeV, respectively. Small-signal gain and cavity loss were measured to be 8.2% and 3.3%, respectively, at a peak current of 4.4 A, an electron energy of 15 MeV, and a central wavelength of 10 μm. The pulsewidth of the FEL signal could be lengthened by tailoring the waveform of klystron drive voltage, since uniformity of electron energy in a macropulse was improved. A tungsten needle photo-RF gun was investigated using the field-dependence of QE for a third-harmonics Nd:YAG laser (wavelength of 355 nm). Numerical calculations of electron beam performances in a needle photo-RF-gun were made. A tungsten needle photocathode, with a tip radius of 10 μm, was introduced into the RF-gun cavity of LEENA.

Period-doubling and Hopf bifurcations in far-infrared driven quantum well intersubband transitions

The behavior of n-δ-doped wide quantum well (QW) heterostructures in the presence of intense far-infrared (FIR) radiation is studied using the semiconductor Bloch equations, in the time-dependent Hartree version and without the rotating-wave approximation. A QW is designed where one can either obtain a strong subharmonic (period doubling) or a strong incommensurate (Hopf) frequency response by varying the sheet density and field strength. These strong responses should be attainable with current technology, and the field amplitudes and frequencies of the drive are well within the range of FIR free-electron lasers.

Upgrade of a compact FIR FEL driven by a magnetron-based microtron for the wavelength range of 100–300 μm

A compact far infrared (FIR) free-electron laser (FEL) driven by a magnetron-based microtron has been upgraded to extend the wavelength range from 100–160μm to reach 300μm. The energy of the electron beam from the microtron can be varied from 6.5 to 4.3MeV by changing the position of an RF cavity inside the microtron. We have extracted a macropulse current of the electron beam of more than 50mA for 5.4MeV, and the signals in the wavelength range of 160–280μm have been observed with the electron beam. The radiation is transported to an experimental stage through a 10-m-length vacuum pipe with a collimating lens and gold-coated mirrors. The measured spatial distribution of the transported radiation shows a small diverging angle less than 1mrad and spot size (FWHM) less than 20mm. The pulse width and macropulse power of the lasing signals measured at the experimental stage are approximately 3μs and 10W, respectively. The main features of the FEL have been investigated in this report. The possibility of the FEL covering the wavelength range up to 1mm is discussed and investigated by the measured signal of the coherent radiation. The application experiments of the FEL to the FIR imaging, molecular spectroscopy, and solid-state physics are under preparation.

Structure determination of gas phase aluminum oxide clusters

Neutral aluminum oxide clusters are produced in a molecular beam by laser vaporization in a pulsed-nozzle cluster source. These clusters are ionized via (multi-) photon absorption from either an ultraviolet excimer laser or from a far-infrared free electron laser. Ultraviolet (multi-) photon ionization produces sparse mass spectra with only relatively light aluminum oxide clusters, while infrared ionization produces a smooth distribution of higher molecular weight ions from the same nascent source distribution. Tuning the IR wavelength, multiphoton infrared spectra are recorded pointing to the γ-Al2O3 structure for a whole series of AlO·(Al2O3)n clusters, n≤34.

Infrared resonance-enhanced multiphoton ionization spectroscopy of magnesium oxide clusters

Neutral (MgO)n clusters are produced in a molecular beam by laser vaporization in a pulsed-nozzle cluster source. These clusters are ionized via multiphoton absorption from either an ultraviolet excimer laser or a far-infrared free electron laser. While ultraviolet ionization produces mass spectra consistent with previous measurements, infrared ionization produces higher molecular weight ions from the same nascent source distribution. Ultraviolet ionization occurs by direct electronic excitation/ionization, while infrared ionization occurs by vibrational excitation followed by thermionic electron emission. In both cases, prominent masses are observed corresponding to cubic nanocrystals with near equal x:y:z dimensions. By tuning the IR wavelength while recording the mass-resolved ion yield, vibrational spectra are obtained revealing two resonances near 16 and 22 microns. Clusters up to 300 atoms in size are studied, and spectra exhibit a gradual variation with size, converging to positions near to, but not matching the bulk phonon frequencies. Structural implications of these vibrational spectra are investigated.

A 100 MW far-infrared free-electron laser driven by a 180 MeV RF-linac

The development of a 100 MW far-infrared free-electron laser oscillator is reported. By using a 180 MeV electron beam, this system will provide the high power and the rapid scanning of the broad wavelength range, 10–100 μm, to enhance the user programs. The intracavity power of several GWs will be used for Compton backscattering X- or γ-ray generation. To store such an intense radiation, the dynamics of the FEL oscillator with a coupling parameter μ c⩽1, which is defined as the ratio of the slippage distance and the electron bunch length, was investigated experimentally on the operating mid-infrared system.

Designing a grating based free electron laser

A compact far-infrared free electron laser (FEL) driven by a moderate energy beam would have many applications in academic and industrial research. Our goal is to find the optimal operating parameters producing the highest gain for a grating FEL with either a SASE or oscillator configuration. The basic structure consists of a grating bounded above by a roof extending over a portion of its length. The dispersion relation for such a system can be found by expanding the electromagnetic fields in a series of resonant modes. When the fields and the beam have near-synchronous velocities, the fields grow at a rate determined by the imaginary component of ω ( k for spatial growth) and group velocity. Relations between gain and beam energy, roof height, tooth depth and angle of curve intersection have been investigated.

Simulation and design of the CAEP far-infrared FEL

A far-infrared free-electron laser oscillator with a wavelength of 100 μm, which is driven by a L-band RF accelerator and has a linear planar wiggler, has been built in China Academy of Engineering Physics (CAEP) and is now ready to undergo the first lasing experiments. The simulation and design of the CAEP FIR FEL has been made with the help of our 3D FEL code. The results show that the energy spread and emittance of the electron beam must be <1% and 1.5 π mm mrad, respectively. It is also found that the total loss of the optical cavity is about 30% due to the loss on the wall of the beam duct, which must be reduced or overcome by larger gain.