Particle accelerators are scientific facilities that utilize electromagnetic waves to accelerate charged particles to speeds close to that of light. Over the last century,accelerator development has contributed significantly to the advancement of science,particularly for the investigation of microscopic constituents that comprise macroscopic matter,and broader application communities such as the construction of advanced light sources,material science,and medical therapy. Conventionally,radio frequency (RF) is employed to power mature accelerators. However,this acceleration scheme is constrained by an acceleration gradient of 100 MV/m and encounters challenges such as high price,significant footprint,and a lengthy construction time,all of which impede its promotion for wider scientific and technological applications. As a result,the search for new electron acceleration technologies has intensified in the pursuit of advanced accelerators.
To reduce accelerator size,increasing operation frequency provides an effective path for enhancing accelerator performance and availability. In such cases,energy transfer from electromagnetic waves to electrons can occur over shorter distances,resulting in larger accelerating gradients and a smaller device footprint. In particular,contrary to the long wavelength associated with RF,electron acceleration driven by terahertz (THz) waves and lasers on a dielectric grating can generate acceleration gradients up to GV/m in these higher frequency bands. Recent progress utilizing terahertz and optical waves has already demonstrated non-relativistic and relativistic electron acceleration and phase space manipulation. Moreover,the combination of a light-wave-driven electron source with a novel accelerator enables realization of an all-optical electron source,which unlocks new directions for small-scale and even integrated accelerator development.
Research on optical-field-driven electron acceleration originates from the laser invention in the last century. However,owing to electron beam distortion by scattering in the air,dielectric laser acceleration takes advantage of phase-matching between the electron and laser on a grating surface,which has only emerged over the past two decades,and was experimentally demonstrated by Byer et al. and Hommelhoff et al. in 2013. Subsequently,advancement toward the concept of an integrated accelerator has been proposed and extensively studied worldwide. Currently,two primary directions have emerged that utilize the distinct electromagnetic spectral bands of THz and laser waves. In dielectric laser acceleration,the average acceleration gradient witnessed rapid growth from 25 MV/cm (for sub-relativistic electrons) and 300 MV/m (for relativistic electrons) to near GV/m by careful design of the accelerator materials and structures (Figs.3 and 5). In general,the established designs rely on single- and double-sided grating,with the leading arrangement exploring flexible dual pillar rows and inverse-design structures. This could facilitate access to the required integrated electron accelerator.
Although increasing the operation frequency can significantly reduce accelerator size,the laser’s short wavelength also poses significant challenges for synchronization,stability,and acceleration of a substantial amount of charge. Therefore,THz radiation emerges as a suitable operating band for compact accelerators that alleviate these stringent requirements and the accelerator structure processing precision. Nanni et al. reported the first experimental demonstration of THz-driven acceleration in 2015. Meanwhile,developments in electron energy gain (Fig.1) and phase space manipulations (Fig.5) have emerged by increasing THz wave energy and optimizing accelerator structures,which also increases interaction distance. To further improve THz-driven electron acceleration,however,additional energy must be made available,necessitating the development of new THz wave generation and transmission techniques. The recently unraveled amplification of THz surface plasmons via free electron pumping (Fig.2) provides a novel method for accelerating electrons,enabling energy gain in the order of MeV.
This review examines research progress on electron acceleration driven by THz to optical bands in the electromagnetic spectrum,including related advances of coherent electron sources and beam control,and investigates the emerging novel concept of integrated electron accelerators. Despite significant progress in both the acceleration/manipulation of non-relativistic and relativistic electrons,current advances remain insufficient to enable the development of a compact accelerator for mature applications. It is anticipated that higher accelerator pump power availability will lead to breakthroughs in terms of energy gain reaching MeV or even GeV scales. Recent progress in the generation of millijoule or even subjoule scale THz radiation has already illuminated such prospects in THz-driven acceleration research. The intense surface wave,in particular,allows access to an integrated electron source device,which eliminates the necessity for bulky optics for free-space THz wave generation,transport,and mode conversion. Future processing precision refinement,on the other hand,would enable more precise control of tip-generated electrons and their phase-matching with optical waves. This is particularly critical for dielectric laser acceleration,which is dependent on electron generation and steering. In the future,integrated accelerators are anticipated as a viable alternative to large-scale RF particle accelerators in university laboratories. This will potentially trigger significant research development in physics,chemistry,medicine,and other disciplines.
