Electron Acceleration by Relativistic Surface Plasmons in Laser-Grating Interaction

International audienceThe generation of energetic electron bunches by the interaction of a short, ultraintense (I > 10 19 W=cm 2) laser pulse with "grating" targets has been investigated in a regime of ultrahigh pulse-to-prepulse contrast (10 12). For incidence angles close to the resonant condition for surface plasmon excitation, a strong electron emission was observed within a narrow cone along the target surface, with energy spectra peaking at 5-8 MeV and total charge of ∼100 pC. Both the energy and the number of emitted electrons were strongly enhanced with respect to simple flat targets. The experimental data are closely reproduced by three-dimensional particle-in-cell simulations, which provide evidence for the generation of relativistic surface plasmons and for their role in driving the acceleration process. Besides the possible applications of the scheme as a compact, ultrashort source of MeV electrons, these results are a step forward in the development of high-field plasmonics. Surface plasmons [1,2], also named surface waves, are electromagnetic (EM) modes localized at the interface of different media which allow local field confinement and enhancement. Surface plasmons are the core of the vibrant research field of plasmonics [3], with applications ranging from light concentration beyond the diffraction limit [4], to biosensors [5] and plasmonic chips [6]. The extension of plasmonics into the regime of high fields, where nonlinear and relativistic effects arise, is largely unexplored. An example is provided by the multiterawatt laser-driven excitation of unipolar surface plasmons by transient charge separation [7,8], with potential application to the generation of intense THz pulses [8,9]. In the optical or near-infrared frequency range, surface plasmons can be excited by laser light incident on a sharp material interface having a periodic modulation, e.g., a grating, to allow phase matching. However, most experiments so far have been restricted to intensities below 10 16 W=cm 2 [10] because of the prepulses inherent in high-power laser systems which can lead to an early disruption of the target structuring. The development of devices for ultrahigh contrast pulses [11,12] now allows us to explore the interaction with targets structured on a submicrometric scale at laser intensities high enough for the electron dynamics to become relativistic [13,14]. In particular, a strong increase of the cutoff energy of protons accelerated from the rear surface of grating targets was observed and related to surface plasmon-enhanced absorption [15]. While a detailed theory is still lacking for nonlinear and relativistic surface plasmons, numerical simulations also showed surface plasmon-related effects in this regime [16,17], including electron acceleration at weakly relativistic intensities [18] and, more recently, surface plasmon-enhanced high harmonics [19] and synchrotron radiation [20] in gratings. In this Letter, we demonstrate that relativistic surface plasmons accelerate high-energy electrons along a grating surface. The acceleration process is related to two basic surface plasmon properties, i.e., the subluminal phase velocity and the longitudinal field component. The energy and number of electrons in gratings irradiated at an incidence angle close to the resonant value for surface plasmon excitation are strongly enhanced with respect to flat targets. At intensities I ¼ 5 × 10 19 W=cm 2 , corresponding to a relativistic parameter a 0 ≃ 5 [where a 0 ¼ ðIλ 2 =10 18 W cm −2 μm 2 Þ 1=2 and λ isthelaserwavelength]theelectronemissionwasconcentrated in a narrow cone with energy spectra peaking at 5-8 MeVand reaching up to ∼20 MeV. The basics of surface plasmon generation and electron acceleration may be described as follows. At high laser intensities (I > 10 18 W=cm 2) a solid target is ionized within one laser cycle, thus the interaction occurs with a dense plasma. Assuming a dielectric function εðωÞ ¼ 1 − ω 2 p =ω 2 ≡ 1 − α (where ω p is the plasma frequency) the phase velocity of a surface plasmon is v p ¼ ω=k ¼ cðα − 2Þ 1=2 =ðα − 1Þ 1=2 where k is the surface plasmon wave vector PRL 116