Free electron light sources are featured by extraordinary luminosity, directionality, and coherence, which has enabled significant scientific progress in fields including physics, chemistry, and biology, et al. The next generation of light sources has aimed at compact radiation sources driven by free electrons, with the advantages of reduction both in space and cost.
Nowadays, with the rapid development of ultra-intense and ultra-short lasers, great efforts have been devoted to the quest for compact free-electron lasers (FELs). This review focuses on the current efforts and advancements in the development of compact FELs, with a particular emphasis on two notable paths: the development of compact accelerators and the construction of micro undulators based on innovative materials/structures or optical modulation of electrons. In addition, the physical essence of inverse Compton scattering (ICS) is discussed, which offers remarkable capability to develop an optical undulator with a spatial period that matches the optical wavelength.
Recent scientific developments and future directions for miniaturized and integrated free-electron coherent light sources are also reviewed. In the future, the prospect of generating ultrashort electron pulses provides fascinating means of producing super radiant radiation, promising high brilliance and coherence even on a micro-scale using optical micro undulators.
Recently, a team of scientists led by Prof. Ruxin Li and Ye Tian at Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, was invited by Co-Editors-in-Chief to contribute a review paper entitled "Coherent Free-electron Light Sources", which was published in the second issue of Photonics Insights. (Dongdong Zhang, Yushan Zeng, Ye Tian, Ruxin Li. Coherent free-electron light sources[J]. Photonics Insights, 2023, 2(3): R07).
In this paper, they summarize the development of free-electron radiation, focus on the latest research progress of compact free-electron lasers based on free-electron drives, and conclude with an outlook on the future development of this compact free-electron source, as well as the free-electron laser high-gain radiation source.
Free electron radiation can be traced back to the discovery of cathode luminescence radiation in the 19th century, while free electron laser was first experimentally verified by Madey. In recent years, with the development of ultra-intense and ultra-short laser technology, emerging trends in this area have led to new concepts of optical micro-undulator, laser-plasma acceleration, and the quest for compact free-electron lasers. Essentially, the core of these researches involve the control of coherent energy transfer between free electrons and photons. In the future, the prospect of generating ultrashort electron pulses offers fascinating means of producing superradiation, with the promise of high brightness and coherence even at the microscale using an optical microwave undulator.
1. Multiple Radiation Mechanisms of Free Electrons
Cherenkov radiation (CR) was first discovered by the Soviet physicist P. A. Cherenkov in 1934, followed by the classical description of the phenomenon based on Maxwell's equations by I. M. Frank and I. Y. Tamm in 1937. Cherenkov radiation, also known as "superluminal radiation," occurs when a free electron moves through a medium at a speed greater than the phase velocity of light in that medium, i.e., when ve > vph = c/n. Cherenkov radiation has significant applications in particle detection, imaging, astrophysics and nuclide detection.
In 1945, Ginzburg and Frank predicted the phenomenon of transition radiation (TR). Transition radiation can be intuitively understood as the difference in wave vectors of the electromagnetic fields generated by an electron when it transitions between two different media. This difference is released into free space as far-field radiation. Currently, radiation sources developed using the TR radiation mechanism, especially in scarce spectral regions such as X-rays and THz radiation, are a simple and efficient means of generating radiation.
In 1988, Coherent synchrotron radiation (SR) was first introduced at a conference by Professor Paul Hartman of Cornell University. SR is the collective effect of free-electron radiation, which occurs mainly when a beam of free electrons passes through a dipole magnet. Because particle paths in most accelerators are deflected by magnetic fields. Currently, there are storage ring-based sources and linear accelerator based sources. More than 50 synchrotron radiation sources are currently in operation in 23 countries worldwide.
In 1953, the American physicists S. J. Smith and E. M. Purcell discovered for the first time that a free electron passing over the surface of a metal grating could excite radiation whose wavelength depends on the velocity of the free electron and the geometry of the periodic structure, known as Smith-Purcell Radiation (SPR). It can be anticipated that SPR would generate radiation with even shorter wavelengths when produced in photonic crystals or metasurfaces that share periodic characteristics with the material. This is due to the spatial periodicity of photonic crystals at the sub-nanometer scale, allowing low-energy electrons to emit ultraviolet or X-ray radiation.
Figure 1. Free electron radiation mechanism (a) Cherenkov radiation; (b) Transit radiation; (c) Synchrotron radiation, (d) Smith Purcell effect, (e) undulator radiation, (f) inverse Compton scattering
Stepping into the 21st century, as the demand for scientific tools increases with advancements in productivity, lasers with shorter wavelengths and higher radiation power have become essential scientific goal. Free Electron Lasers (FELs) emerged in this context. In 1971, Professor Madey of the United States put forward the theoretical concept of free electron laser, with the expectation of laser in the X-ray band. In 1997, the first free electron laser device was constructed.
Since then, five Nobel Prizes have been awarded in the field of free-electron lasers. Due to the employment of relativistic energy electrons, the radiation wavelengths generated through frequency conversion can reach the X-ray range. X-rays has important applications in areas such as biochemistry, materials science, information technology, and energy research. The core principle of FEL is the resonance between free electrons and the radiation field, which can be expressed as λ ≈ λu(1 + K2/2)/(2mγ2 ) , which is also known as the free-electron radiation relation. In this mathematical formula, the energy of the free electrons (γ) and the wavelength of the undulator radiation (λu) are two key physical quantities. We need to start with these two physical quantities and develop the corresponding theories and technologies to achieve miniaturization of electron coherent radiation source.
Fig. 2 Schematic diagram of the harmonic resonance relationship of free electron laser wave oscillation and radiation
In the last two decades, with the development of ultra-intense and ultra-short lasers (femtosecond laser systems on the PW scale), free-electron lasers or free-electron coherent light sources have ushered in a new era of compact FELs. Pioneering works include novel acceleration systems and miniature wave undulators based on new nanophotonic platforms. The emergence of new principles and new technologies promise to develop a new generation of miniaturised high-brightness free-electron laser devices. This could serve as an effective complement to existing large-scale devices.
2. Compact FEL driven by laser plasma accelerator
Reducing the size of free-electron lasers has been dreamed by scientists. In recent years, with the construction and application of PW laser facilities, the LWFA program based on the development of laser tail-field electron acceleration is considered to be the optimal path for the next generation of compact particle acceleration technology, and has rapidly attracted widespread attention and research in the international arena. Especially in 2004, the proposal of "Dream Beam" showed people the hope of LWFA-driven FEL.
In 2020, Shanghai Institute of Optical Machinery (SIOM) experimentally achieved, for the first time, the laser wake field acceleration (LWFA) driven SASE-FEL exponential amplification at the 27 nm, as shown in Fig. 2(a). It marks a milestone for the LWFA in the adventure towards a practical FEL light source, which provided an important reference for the development of compact free-electron laser devices. Subsequently, in 2021 a group from the National Laboratory of Frascati, Italy, demonstrated a photocathode electron source combined with a laser plasma accelerator (LPA) driven FEL exponential gain in the 830 nm and validate this viewpoint once again.
Fig. 3 Compact laser plasma acceleration (LPA) electron source-driven FEL.(a) LWFA-driven SASE-FEL realizes exponential gain amplification in the EUV band. (b) Compact laser plasma acceleration (LPA) device realizes SASE-FEL operation in the 830 nm IR band, (c) LWFA electron source-driven seed-FEL realizes high-gain coherent radiation amplification at 270 nm, and (d) proposed experimental scheme for PWFA electron source-driven X-ray FELs
In 2022, a group from SOLEIL Laboratory, France, validated the achievement of LWFA driven Seeded-FEL. They achieved control over the wavelength of free electron laser radiation at 270nm using external seeding. In 2023, a team at the University of Strathclyde, England, proposed a PWFA-driven FEL scheme, which is aimed at increasing the brightness of sub-fs electron beams. In addition, considering the cascade acceleration scheme of LWFA and PWFA will greatly improve the plasma free-electron acceleration quality and facilitate the subsequent driving of radiation sources and other related applications.
Thus, LWFA provides a viable to access compact FEL through shrinking the accelerator. Despite the current LWFA performance is not yet a perfect match to that of existing free-electron lasers, it represent a technological breakthrough in terms of stability, repetition rate, and the efficiency of the electron beam transfer to the radiation that may be improved in the future.
3. Micro undulator based on surface electromagnetic fields
In addition to optimize undulator structure can also developing new types of coherent free electron radiation sources with table-top footprint and high brightness. Specifically, if a novel electromagnetic modes could serve to effectively modulate the free electrons, the conventional centimeter-scale modulation period could be reduced to micrometer-nanometer scale with the benefits of both smaller undulator and slower electrons.
Such idea has become viable since the rapid development of ultra-intense and ultrashort lasers has made it possible to construct optical strong fields on the micrometer-nanometer scale in free space or on the material surfaces. A series of new optical materials, such as two-dimensional surfaces with strongly bound and surface plasmon polariton (SPP), multilayer vdW, and exciton polaritons, surface phonon excitons, is found to have the ability to compress and enhance the optical field at the micro-nano scale.
This novel mode of electromagnetic fields could support sub-wavelength modulation of low energy electrons, enabling higher harmonic generation to obtain short-wave radiation, etc. Moreover, such novel optical micro-undulator can be customized by tuning the excitation light frequency, material dielectric response function, and metasurface engineering for optimizing radiation properties.
Fig. 4 Miniature undulators modulating low-energy free-electron radiation output (a) nanowire array-level optical undulator, (b) wire waveguide helical undulator, (c) graphene SPP optical undulator (d) multilayer graphene SPP undulator concept (e) vacuum fluctuating polariton undulator (f) metasurface SPP undulator.
The experimental demonstration of ultrafast laser-driven nanoarray micro-undulators (Fig. 4a) was reported in 2004 by I.A. Andriyash et al. A strong electrostatic field is established on the surface of a metal wire, and the equivalent undulator can be as small as a few micrometers in this configuration. The radiated photon energy reaches 12 keV to 106 keV.
In 2017, our research team demonstrated the concept of a miniature helical undulators (Fig. 4b). By illuminating a thin metal wire with a femtosecond laser, the generated free electron emit coherent terahertz radiation by the helical trajectories around the wire. In 2022, we further revealed the underlying mechanism of this coherent amplification, which relies on the deceleration field of the TM mode of the THz SPP and acquires energy from the net energy transfer from the electrons to the SPP. To extend the idea, we also proposed a novel concept of SPP-FEL.
In 2016, Liang Jie Wong and colleagues from the US and Singapore proposed a scheme of graphene as a micro-undulator to realize EUV-X-ray radiation (Fig. 4c). Graphene SPP exhibit strong confinement of light. In 2019, to further increase the radiative photon yield, Andrea Pizzi et al. successively proposed a multilayer "graphene SPP undulator" scheme to increase the electron-plasma interaction distance. Theoretical calculations have shown that photon radiation with energies of 2.7-12 keV can be obtained under 5 MeV electron-driven conditions.
In addition, study in 2019 demonstrated that the vacuum force constituted by the vacuum fluctuations can be used to modulate electrons to achieve emission of photon-polariton. It can thus be assumed that many types of photonic quasiparticles can exert a periodic modulation force on free electrons, and that such electromagnetic field modulation schemes can all be subsumed into the concept of micro-undulator.
4. All optical undulator free electron radiation
The review also describes the Inverse Compton Scattering (ICS) radiation as a mechanism for optical undulator. The electric and magnetic field components of strong laser pulses in free space can also be considered as a general "optical undulator". In the rest frame of free electron, the laser field modulates the electron's transverse momentum when the electron interacts with the oncoming laser field. That is, the corresponding ICS equivalent oscillator period is half of the collision laser wavelength, i.e., , and the equivalent magnetic field strength is twice the laser magnetic field strength 2B.
When the laser normalized vector potential is comparable to the static magnetic field oscillator parameter , the free-space laser field can produce energetically effective undulator modulation of free electrons. The oscillation period is determined by the laser wavelength, which is 4-5 orders of magnitude smaller than that of the static magnetic field oscillator. Therefore, the high-energy electrons can radiate more easily in the X-ray and γ-band.
Fig. 5 All-optical inverse Compton configuration of an optical undulator. (a) Concept of linear accelerator-based ICS X-ray source proposed by MIT (b) "ICS-configured undulator". (c) Surface plasma polariton constructed "Compton scattering undulator" (d) Surface plasma polariton based "optical undulator"
5. Future Prospects and Conclusion
In this review, the research team led by Ruxin Li and Ye Tian has overviewed the latest progress of coherent radiation generated by free electrons. Starting from the phase-matching condition of the energy exchange between free electrons and light fields, the review article elaborates in detail on the development of the next generation of coherent free-electron light sources with laser modulation of the free electrons. In particular, the potential of micro-undulator holds the promise for future development of compact free-electron coherent light sources which can be super small, and even be integrated on chip scale. The feasibility of miniaturization of free electron lasers constructed by laser wake field electron acceleration and optical near-field has been discussed.
In the future, free-electron super radiation is indispensable for obtaining higher energy power output. Ultrafast laser modulation of the electron pulses is expected to pave the way for the future free electron light sources with advanced footprint, expenses, radiation wavelength and brightness.
