free-electron lasers (original) (raw)

Acronym: FEL

Definition: laser devices where light amplification occurs by interaction with fast electrons in an undulator

Category: article belongs to category laser devices and laser physics laser devices and laser physics

Related: ultraviolet lightultraviolet lasersX-ray lasers

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What is a Free-electron Laser?

A free-electron laser is a relatively exotic type of laser where optical amplification is achieved in an undulator, fed with high energy (relativistic) electrons from an electron accelerator. Such devices have been demonstrated with emission wavelengths reaching from the terahertz region via the mid- and near-infrared, the visible and ultraviolet range to the X-ray region, even though no single device can span this whole wavelength range.

free-electron lasers

Figure 1: Setup of an undulator, as used in a free-electron laser. The periodically varying magnetic field forces the electron beam (blue) on a slightly oscillatory path, which leads to emission of radiation.

In the undulator, a periodic arrangement of magnets (permanent magnets or electromagnets) generates a periodically varying Lorentz force, which forces the electrons to radiate with a frequency which depends on the electron energy, the undulator period, and (weakly) on the magnetic field strength. Both spontaneous and stimulated emission occur, allowing for optical amplification in a certain wavelength range.

The greatest attractions of free-electron lasers are:

Compared with other synchrotron radiation sources (pure undulators and wigglers), FELs can generate an output with a much higher spectral brightness and coherence. This is very useful for a number of applications, including fields such as atomic and molecular physics, ultrafast X-ray science, advanced material studies, ultrafast chemical dynamics, biology and medicine.

Various FELs also exhibit extremely high precision, e.g. concerning beam position stability and focusing of generated X-rays down to spots with only a couple of nanometers diameter [16], or concerning temporal positioning of pulses.

The big drawback of FELs is that their setups are very large and expensive, so that they can be used only at relatively few large facilities in the world. A highly ambitious free-electron laser project is pursued in Hamburg (European XFEL, originally within the TESLA project, now within a European project) [20]. That 3.4 km long XFEL generates hard X-ray output with unprecedented performance features: wavelengths down to 0.05 nm, pulse durations below 100 fs, and extremely high brilliance. The LCLS at SLAC has already achieved lasing wavelengths below 0.15 nm, corresponding to a photon energy of 10 keV. The substantially upgraded version LCLS-II (with an immensely improved radiance) has become operational in 2023.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a free-electron laser?

A free-electron laser (FEL) is a type of laser where the gain medium is not a material but rather high-energy electrons from an accelerator, which are forced to radiate in a magnetic structure called an undulator.

How do free-electron lasers work?

In an undulator, a periodic magnetic field forces high-energy electrons onto an oscillating path, causing them to radiate. This process includes both spontaneous and stimulated emission, which leads to optical amplification in a certain wavelength range.

What are the main advantages of free-electron lasers?

Free-electron lasers offer exceptional wavelength tuning over very wide regions, including extreme spectral ranges like X-rays. Compared to other synchrotron sources, they provide much higher spectral brightness and coherence.

What are typical applications of FELs?

Due to their high coherence and spectral brightness, FELs are used in fields such as atomic and molecular physics, ultrafast X-ray science, advanced material studies, ultrafast chemical dynamics, biology, and medicine.

What are the disadvantages of free-electron lasers?

The main drawback of FELs is their immense size and cost, which means they can only be built and operated at a few large-scale research facilities worldwide.

Bibliography

[1] L. R. Elias et al., “Observation of stimulated emission of radiation by relativistic electrons in a spatially periodical transverse magnetic field”, Phys. Rev. Lett. 36, 717 (1976); doi:10.1103/PhysRevLett.36.717
[2] D. A. G. Deacon et al., “First operation of a free-electron laser”, Phys. Rev. Lett. 38 (16), 892 (1977); doi:10.1103/PhysRevLett.38.892
[3] A. M. Kondratenk and E. L. Saldin, “Generating of coherent radiation by a relativistic electron beam in an ondulator”, Part. Accel. 10, 207 (1980)
[4] C. A. Brau, “Free-electron lasers”, Science 239 (4844), 1115 (1988); doi:10.1126/science.239.4844.1115
[5] K.-J. Kim and A. Sessler, “Free-electron lasers: present status and future prospects”, Science 250, 88 (1990); doi:10.1126/science.250.4977.88
[6] G. R. Neil and L. Merminga, “Technical approaches for high-average-power free-electron lasers”, Rev. Mod. Phys. 74, 685 (2002); doi:10.1103/RevModPhys.74.685
[7] W. Ackermann et al., “Operation of a free-electron laser from the extreme ultraviolet to the water window”, Nature Photon. 1, 336 (2007); doi:10.1038/nphoton.2007.76
[8] P. Emma et al., “First lasing and operation of an Ă„ngstrom-wavelength free-electron laser”, Nature Photon. 4, 641 (2010); doi:10.1038/nphoton.2010.176
[9] W. A. Barletta et al., “free-electron lasers: Present status and future challenges”, Nuclear Instruments and Methods in Physics Research A 618, 69 (2010); doi:10.1016/j.nima.2010.02.274
[10] J. N. Galayda et al., “X-ray free-electron lasers — present and future capabilities”, J. Opt. Soc. Am. B 27 (11), B106 (2010); doi:10.1364/JOSAB.27.00B106
[11] E. C. Snively et al., “Broadband THz amplification and superradiant spontaneous emission in a guided FEL”, Opt. Express 27 (15), 20221 (2019); doi:10.1364/OE.27.020221
[12] N. S. Mirian et al., “Generation and measurement of intense few-femtosecond superradiant extreme-ultraviolet free-electron laser pulses”, Nature Photonics 15, 523 (2021); doi:10.1038/s41566-021-00815-w
[13] E. Prat et al, “A compact and cost-effective hard X-ray free-electron laser driven by a high-brightness and low-energy electron beam”, Nature Photonics 14, 748 (2020); doi:10.1038/s41566-020-00712-8
[14] W. Decking et al., “A MHz-repetition-rate hard X-ray free-electron laser driven by a superconducting linear accelerator”, Nature Photonics 14, 391 (2020); doi:10.1038/s41566-020-0607-z
[15] A. Fisher et al., “Single-pass high-efficiency terahertz free-electron laser”, Nature Photonics 16, 441 (2022); doi:10.1038/s41566-022-00995-z
[16] J. Yamada et al., “Extreme focusing of hard X-ray free-electron laser pulses enables 7 nm focus width and 1022 W cm−2 intensity”, Nature Photonics 18, 685 (2024); doi:10.1038/s41566-024-01411-4
[17] P. Franz et al., “Terawatt-scale attosecond X-ray pulses from a cascaded superradiant free-electron laser”, Nature Photonics 18, 698 (2024); doi:10.1038/s41566-024-01427-w
[18] The World Wide Web Library on free-electron lasers, http://sbfel3.ucsb.edu/www/vl_fel.html
[19] The LCLS (Linac Coherent Light Source) at SLAC (Stanford), https://lcls.slac.stanford.edu/
[20] The European X-ray laser project XFEL, https://www.xfel.eu/

(Suggest additional literature!)

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