What Are Free Electron Lasers?

Free electron lasers are coherent light sources that generate radiation by passing a relativistic electron beam through a periodic magnetic structure called an undulator, rather than through the bound atomic transitions used in conventional lasers. First demonstrated in 1976 by John Madey and colleagues at Stanford University, free electron lasers occupy a distinct class among photon sources because their output wavelength is continuously tunable across a broad spectrum, from microwaves through hard X-rays, by adjusting the electron beam energy or the undulator magnetic field strength.

The physics underlying free electron lasers differs fundamentally from that of gas or solid-state lasers. Rather than stimulating transitions between discrete energy levels in atoms or molecules, the free electron laser exploits the interaction between a high-energy electron beam and the transverse electric field of the radiation it produces inside the undulator. This interaction, described by the Free Electron Laser Collaboration and LEAPS initiative, causes the electrons to self-organize into thin current bunches called microbunches, spaced at the wavelength of the emitted light.

Undulators and the SASE Mechanism

The undulator is the central component of a free electron laser. It consists of an array of alternating magnetic poles arranged so that an electron traversing the device follows a sinusoidal path in the transverse plane. As the electron oscillates, it emits synchrotron radiation at a wavelength that depends on the undulator period and the electron's relativistic Lorentz factor. In modern X-ray free electron lasers, undulators can span hundreds of meters. The SwissFEL facility at the Paul Scherrer Institute uses a 400-meter linear accelerator to boost electrons to 6 GeV before injecting them into the undulator beamlines.

Coherence in a free electron laser typically arises through a process called self-amplified spontaneous emission (SASE). Initial spontaneous radiation from the electron beam acts back on those same electrons, reinforcing the microbunching through repeated passes down the undulator. Because all microbunched electrons radiate in phase, the emitted intensity scales with the square of the number of participating electrons rather than linearly, yielding peak brightness many orders of magnitude above conventional synchrotron sources. X-ray free electron lasers generate light millions of times more powerful than standard X-ray tubes.

Electron Beams and Relativistic Effects

The performance of a free electron laser depends critically on the quality of the electron beam supplied by the accelerator. Beam quality is characterized by the transverse emittance, a measure of the spread in position and angle of the electron distribution. Lower emittance allows tighter microbunching and higher gain. Achieving the low emittance needed for hard X-ray production requires photocathode injectors that release electrons via ultrashort laser pulses and radio-frequency accelerating cavities that capture and compress the bunch before any significant emittance growth occurs.

Relativistic effects enter directly into the wavelength equation: as the electron energy increases, the relativistic Doppler shift compresses the emitted wavelength, shifting radiation from infrared toward extreme ultraviolet and X-ray regimes. This tunability is what makes free electron lasers uniquely versatile as research instruments. Detailed treatments of the relativistic kinematics and gain theory are available in lectures published by CERN and in the comprehensive review by Stohr at SLAC National Accelerator Laboratory.

Applications

Free electron lasers have applications in a wide range of scientific and technical disciplines, including:

• Structural biology: determining protein and virus structures at atomic resolution with femtosecond X-ray pulses
• Materials science: probing atomic-scale composition and dynamics in new materials
• Chemical dynamics: filming the formation and breakup of chemical bonds in real time
• Extreme-conditions physics: studying matter under planetary-interior pressures and temperatures
• Medical imaging research: coherent X-ray imaging techniques for soft-tissue contrast
• Defense and security: infrared free electron laser systems for directed-energy and countermeasure research