First operation of a two-color mode in an infrared free-electron laser

A technological milestone has been achieved at the Max Planck Society’s Fritz Haber Institute (FHI) in Berlin. For the first time, an infrared free-electron laser (FEL) has been operated in two-color mode. This globally unique technology enables experiments with synchronized two-color laser pulses, opening up new possibilities in research.

Free-electron lasers, of which there are more than a dozen worldwide, vary significantly in size (from a few meters to several kilometers), Range of wavelengths (from microwaves to hard x-rays), and cost (millions to over a billion). However, they all produce intense, short pulses of radiation. Free-electron lasers have become important radiation sources in the past decades, finding wide applications in basic research and applied sciences.

FHI researchers have now developed a method with American partners that allows the simultaneous generation of infrared pulses in two different colors. This innovation is particularly important for the study of transient processes in solids and molecules.

In a FEL, Bunches of electrons First, an electron is accelerated to very high kinetic energies by an accelerator, almost reaching the speed of light. Next, the accelerated electrons pass through an oscillator, where they are forced down a slalom-like path. Strong magnetic field of varying polarity from time to time.

The oscillations of the electrons lead to the emission of electromagnetic radiation, the wavelength of which can be varied by adjusting the energy of the electrons and/or the strength of the magnetic field. For this reason, FELs can be used to produce laser-like radiation in almost all parts of the electromagnetic spectrum, from long terahertz to short X-ray wavelengths.

Since 2012, FHI has operated a FEL, which produces intense, pulsed radiation in the mid-infrared (MIR) range, continuously tunable in wavelength from 2.8 to 50 micrometers. In recent years, FHI scientists and engineers have worked on a two-color extension that installed a second FEL branch to produce radiation in the far infrared (FIR) at wavelengths between 5 and 170 micrometers.

The FIR-FEL branch includes a new hybrid magnet oscillator, specially developed at FHI. Additionally, a 500-MHz cooker cavity was installed behind the electron linear accelerator (LINAC) for transverse deflection of electrons. The cooker cavity can deflect high-energy electron beams at a rate of 1 billion times per second.

In June 2023, the FHI team performed the first “leasing” of the new FIR-FEL, directing all incoming electron beams from the LINAC to the FIR-FEL. In December 2023, they were able to demonstrate two-color operation for the first time. In this mode, the strong oscillating electric field created in the cooker cavity deflects every other electron bunch to the left and every other bunch to the right.

Thus, a high repetition rate (1 GHz; 1 bunch per ns) electron bunch train from a LINAC is split into two bunch trains of half the repetition rate each. One is led to the old MIR-FEL and the other to the new FIR-FEL. In each FEL, the wavelength is continuously tuned by a factor of four by varying the strength of the undulator magnetic field.

For nearly a decade, the FHI-FEL has enabled research groups at FHI to conduct experiments ranging from gas-phase spectroscopy of clusters, nanoparticles, and biomolecules to nonlinear solid-state spectroscopy and surface science, resulting in I have been peer-reviewed nearly 100 times. Publications so far.

The new two-color mode, not available at any other IR FEL facility worldwide, will enable new experiments such as MIR/MIR and MIR/FIR pump-probes. This is expected to open up new opportunities for experimental studies in fields ranging from physical chemistry, materials science, catalysis research to biomolecular studies, thereby contributing to the development of new materials and drugs.