Amplification of the Evanescent Field of Free Electrons

Abstract

A nanoscale plasmonic metal film can amplify free-electron evanescent fields, leading to strongly enhanced light emission via Smith-Purcell effect through a mechanism analogous to the 'poor-man's superlens' for optical evanescent field enhancement. We demonstrate that a layer of silver a few tens of nanometers thick can amplify the evanescent field of free electrons in the same way that the plasmonic metal acts on the evanescent field of light. The enhancement of the evanescent field of free electrons is studied by analyzing the far-field emission from nano-gratings on the tips of optical fibers in a scanning electron microscope ( Fig. 1) and through numerical simulations showing an order of magnitude increase in Smith-Purcell radiation. The enhancement exhibited here for Smith-Purcell radiation may readily be applied to other forms of electron-beam-induced light emission.The electromagnetic energy of the evanescent field of free electrons exists in the form of evanescent waves and can be decoupled into light only when they are in close proximity to a slow-wave medium or optical inhomogeneity. The former gives rise to what is famously known as Cerenkov radiation, the latter to diffraction or Smith-Purcell radiation [1]. Since the exponential decay rate of such evanescent waves is proportional to the ratio of the frequency of the electromagnetic field to the velocity of the particle, high velocity, relativistic particles are more efficient in generating light in the optical part of the spectrum. Here, rather than increasing the energy of particles to achieve higher levels of conversion, we utilize a thin silver layer to amplify the rapidly decaying evanescent waves of nonrelativistic free electrons. In analogy to the 'poor-man's superlens' where the evanescent component of light from an object is restored by a thin silver layer to beat the diffraction limit [2], we consider a thin silver layer for the amplification of the free electron evanescent field before it is decoupled into light by a nano-grating. The period of the grating is chosen so that the Smith-Purcell emission wavelength in the surface-normal direction coincides with the plasmonic resonance

Spectrally and Spatially Resolved Smith-Purcell Radiation in Plasmonic Crystals with Short-Range Disorder

“…The Smith-Purcell effect critically depends on precise fabrication of such a periodic structure, which is why the substantial progress in nanofabrication techniques in recent years has revealed a large variety of new applications (e.g., Refs. [14][15][16][17][18][19][20]) based on the SmithPurcell effect. Interestingly, the inverse Smith-Purcell effect has also shown exciting possibilities [21,22], which consequently led to more advanced photonic crystal structures being used as promising platforms for particle acceleration by light excitation of a periodic structure [23][24][25].…”

“…Such a measurement is particularly interesting for light-electron interactions involving structural defects, or when resolving the dynamics of surface-emitted light. In contrast, other CL setups collect the emitted light with a fiber [18,47] or a parabolic mirror, with the latter allowing angular-resolved CL collection [19,[48][49][50][51]. Yet, despite these techniques, no direct spatially resolved imaging of the light emission has been performed before, to the best of our knowledge.…”

“…These include the excitation of dipoles in localized resonances and other kinds of CL-related effects [18,[47][48][49][50][51], some coming from atomic transitions of The sample is held so that its surface is almost parallel to the path of the electron beam. The emitted light is collected by an objective, directing the emitted light into an enclosure box holding the rest of the optical setup.…”

Generation of convergent light beams by using surface plasmon locked Smith-Purcell radiation

“…Especially, the SPR-based terahertz (THz) light source has drawn a lot of attention in recent research 14 – 17 . Very recently, it has been found that SPR can be locked and enhanced at some emission wavelength and angle by excitation of SPs on the substrate 18 , 19 . This phenomenon is attributed to that the energy from electron concentrated in the excited SPs and then transformed into radiation via SPR mechanism.…”

Metamaterials at the University of Southampton and beyond

“…Concurrently with Albert Polman's group in AMOLF, but a few days earlier, we published on the generation of travelling surface plasmon waves by free-electron impact [92]. We also demonstrated amplification of the evanescent field of free electrons [93]. We developed a nanoscale analogue of a free electron light source by demonstrating that the passage of a free-electron beam through a nanohole in a periodically layered metal-dielectric structure (nano undulator) creates a new type of tunable, nanoscale radiation source that we termed a 'light well' [94].…”