X-ray Free Electron Lasers: What Are They and How Do They Work?

An X-ray laser is a laser that provides X-ray light instead of visible or infrared light. These lasers are often used to study the structure of matter at an atomic level with unprecedented detail.

The basic idea is that these lasers use high-energy electrons to generate X-ray photons. The process that enables this is called SASE (self-amplified spontaneous emission). These lasers are also called XFELs (X-ray free-electron lasers).

Most commonly, these lasers are used in fields like structural biology, materials science, and molecular imaging. Here’s a bit more about these lasers and how they’re used.

How does an X-ray laser work? 

The process starts with the acceleration of electrons to high kinetic energies. This is achieved through various particle accelerator technologies, usually via a linear accelerator. The goal is for the electrons to gain sufficient energy, often measured in gigaelectronvolts (GeV). Is specific X-ray laser facilities, it’s not uncommon for electrons to reach speeds more than 99% of the speed of light. 

After this, an undulator is used to wiggle the accelerated electrons back and forth which causes them to emit photons in the form of a synchrotron radiation. This is a device that consists of alternating magnetic poles aligned in a straight line over 100s of meters. These poles are arranged so that they create a beamline. Over the course of their transports in this periodic arrangement of magnets, the electrons will wiggle more and more coherently, ultimately reaching spatial coherence at the end of the undulator, leading to a free-electron laser.

Ongoing high-power laser research makes the technology more reliable and potent each day.

What is X-ray photon wavelength? 

The x-ray laser wavelength is the wavelength of the photons generated by an x-ray laser. The wavelength of a photon is directly related to the energy it carries. X-ray laser photons have wavelengths in the range of 0.1 to 10 nanometers (sometimes expressed in angstroms, 1Å = 1e-10 m) which corresponds to energies from 100s eV (electronvolts) to 10s keV.

Generally speaking, there are two major types of X-ray lasers are based on wavelength.

• Soft X-rays: Typically in the range of 1 to 10 nanometers.
• Hard X-rays: Commonly in the range of 0.1 to 1 nanometer.

An example of this can be seen at the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory, which produces X-ray photon wavelengths as low as 0.05 nanometers (0.5 Å).

What’s the difference between an X-ray and an optical laser? 

Since we’re discussing the topic of X-ray lasers, it’s better to ask what the difference is between an X-ray and an optical (visible or infrared) laser. There are a few differences, and here are a few worth pointing out:

• Generation mechanism: Optical lasers operate via the stimulated emission of photons from a specific atomic transition in an amplification medium, while X-ray lasers are usually created by the saturation of the self-amplified spontaneous emission originating at the start of the undulator.
• Amplification medium: In traditional optical lasers, the amplification medium is a gas or a solid bulk material which emits the laser light. In X-ray lasers, the amplification medium is the magnetic field produced within the undulator.
• Interaction with matter: Optical lasers require indirect mechanisms to heat, ionize, and excite matter. X-rays, on the other hand, do it more directly by interacting with electrons inside materials since they penetrate deeper.

What about gamma ray lasers? 

Another confusion comes from comparing X-ray lasers with gamma-ray lasers. While the concepts are related, they greatly differ in two crucial categories:

• Electromagnetic spectrum
• Generating mechanism

The wavelengths of gamma rays are even shorter than X-ray photons.

The most important thing to remember is that gamma-ray lasers still don’t exist. This is a hypothetical device that would, in theory, produce coherent gamma rays. Since gamma rays are the form of electromagnetic radiation with the highest energy, this would have so many potential applications, from scanners in airports to product quality checks.

What is an X-ray laser used for? 

X-ray lasers have a wide industrial application and a pivotal role in scientific research. Some of the fields where they are used are:

• Structural biology: With the help of X-ray lasers, biologists can study molecules like proteins, viruses, and nucleic acids at the atomic level. This allowed breakthroughs in understanding processes like protein folding and enzymatic reactions, which are important for drug development and disease mechanisms.
• Materials science: We’re developing more sophisticated materials like nanoparticles and nanocrystals. These materials are used for everything from medicine and healthcare all the way to electronics and solar energy. With the use of a free-electron laser, we can develop a better understanding of how these materials behave at the nanoscale.
• Chemistry: With the help of free-electron lasers, chemists can study molecular structure and dynamics, as well as probe ultrafast processes, which they were never able to do before. A big advantage is that these lasers can send incredibly short pulses (often in femtoseconds or attoseconds), which helps scientists track immediate states and reaction pathways in chemical reactions.
• Astrophysics: There are many applications of X-ray lasers in astrophysics but one of the biggest is the ability to study the composition of stars through their evolution. With the help of an X-ray laser, physicists have approached similar conditions in a laboratory setting. This has provided them with a much greater insight into the dense and hot plasma conditions existing in these stellar objects, a pivotal capability for understanding this topic.

In other words, X-ray lasers are currently reshaping some of the most important industries for our growth and development as a species. There are constant upgrades and developments made in the field of X-ray free-electron lasers, which will make this even more prominent.

Wrap up

X-ray lasers are a relatively new technology and already occupy an important place in scientific fields like medicine, physics, and chemistry. Due to their usefulness in studying topics like nuclear processes, materials behavior, and more, they could potentially bring some civilization-changing discoveries.

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