Researchers report sub-angstrom fundamental-wavelength lasing at the SPring-8 Angstrom Compact Free-Electron Laser in Japan. The output has a maximum power of more than 10 GW, a pulse duration of 10−14 s and a lasing wavelength of 0.634 A.

A compact X-ray free-electron laser emitting in the sub-ångström region

Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

https://iopscience.iop.org/article/10.1088/0034-4885/70/8/R02/pdf

Imaging with terahertz radiation

Recent research and development has been incredibly successful at advancing the capabilities for vacuum electronic device (VED) sources of powerful terahertz (THz) and near-THz coherent radiation, both CW or average and pulsed. Currently, the VED source portfolio covers over 12 orders of magnitude in power (mW-to-GW) and two orders of magnitude in frequency (from ; 10 THz). Further advances are still possible and anticipated. They will be enabled by improved understanding of fundamental beam-wave interactions, electromagnetic mode competition and mode control, along with research and development of new materials, fabrication methods, cathodes, electron beam alignment and focusing, magnet technologies, THz metrology and advanced, broadband output radiation coupling techniques.

Vacuum Electronic High Power Terahertz Sources

This article provides an overview and illustrative examples of how the electric and magnetic fields of intense terahertz transients can be used to resonantly, and even nonresonantly, control matter and light. It discusses the fundamental interaction mechanisms of intense terahertz radiation with matter.

Resonant and nonresonant control over matter and light by intense terahertz transients

Researchers demonstrate the FERMI free-electron laser operating in the high-gain harmonic generation regime, allowing high stability, transverse and longitudinal coherence and polarization control.

Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet

A free-electron laser oscillator has been operated above threshold at a wavelength of 3.4 \ensuremath{\mu}m.

https://link.aps.org/pdf/10.1103/PhysRevLett.38.892

First Operation of a Free-Electron Laser

Gain has been observed for optical radiation at 10.6 \ensuremath{\mu}m due to stimulated radiation by a relativistic electron beam in a constant spatially periodic transverse magnetic field. A gain of 7% per pass was obtained at an electron current of 70 mA. The experiments indicate the possibility of a new class of tunable high-power free-electron lasers.

https://link.aps.org/pdf/10.1103/PhysRevLett.36.717

Observation of Stimulated Emission of Radiation by Relativistic Electrons in a Spatially Periodic Transverse Magnetic Field

Using beams of low-energy electrons ($El5$ MeV), a free-electron laser can be operated as a continuously tunable ($1000 \AA{}l\ensuremath{\lambda}l50 \ensuremath{\mu}$m), high-power ($Pg10$ kW, cw) source of laser radiation. With electrostatic accelerators the electron beam can be recycled to increase the overall efficiency of the laser. Wall power to laser power efficiencies greater than 10% are possible.

High-Power, cw, Efficient, Tunable (uv through ir) Free-Electron Laser Using Low-Energy Electron Beams

Observation of single-mode operation in a free-electron laser.

A technique is described which allows a free‐electron laser to utilize an electron beam with a wide energy spread. A laser using this technique can be powered by a greater variety of electron sources than would otherwise be possible. In addition, the techniques ease some of the problems encountered when designing a laser to operate in conjunction with a storage ring.

Luis R. Elias

Papers

First Operation of a Free-Electron Laser

Observation of Stimulated Emission of Radiation by Relativistic Electrons in a Spatially Periodic Transverse Magnetic Field

High-Power, cw, Efficient, Tunable (uv through ir) Free-Electron Laser Using Low-Energy Electron Beams

Observation of single-mode operation in a free-electron laser.

Reducing the sensitivity of a free‐electron laser to electron energy