Ponderomotive electron acceleration by polarization-gated surface-enhanced optical fields

Peifen Lu, Jian Wu, Hongxing Qi, Heping Zeng
2008 Applied Physics Letters  
We study ponderomotive electron acceleration with polarization-gated surface-plasmon-resonance enhanced optical fields excited by two counter-incident femtosecond laser pulses. Such a polarization-gated excitation scheme creates an intense linearly polarized high-gradient evanescent field for electron acceleration. The maximum kinetic energy of the accelerated ultrafast electrons is doubled with a symmetric angular distribution about the normal of the metal surface as compared with the
more » ... nal one-pulse excitation scheme. Ultrafast electron pulses with ultrahigh spatial and temporal resolutions are significant to reveal the underlying dynamics in the microworld through experiments such as timeresolved electron diffraction 1-4 and electron microscopy. [5] [6] [7] [8] Femtosecond electron pulses with a kinetic energy up to 200 keV can be generated by exciting the photocathode with frequency-tripled femtosecond laser pulses and accelerated with a dc accelerator. 6 Alternatively, ultrafast electron pulses with keV kinetic energy can be produced using surfaceplasmon-resonance ͑SPR͒ enhanced optical fields excited by femtosecond laser pulses with spatially high-gradient intensity distribution. 9-11 This provides a simple all-optical process for synchronous generation and acceleration of ultrafast electron pulses, which represents the potential for the development of compact optical-pump electron-probe systems. As the incident optical field resonates with the surface plasmon ͑SP͒, 12 it is significantly enhanced in intensity close to the metal film and exponentially decreases away from the surface as E SP ͑y , t͒ = E l ͑t͒exp͑−␣y͒, which leads to a highly spatial gradient evanescent field and can be used for ponderomotive electron acceleration. Here, is the electric field enhancement factor, ␣ −1 ͑Ͻ l ͒ is the evanescent decay length into the vacuum side, E l and E SP are the incident and SPRenhanced optical fields, respectively. However, the electron acceleration is dramatically influenced by the elliptically polarized feature of the SPR-enhanced optical field, which limits the effective field intensity and thus the kinetic energy of the emitted electron pulses. Recently, it was demonstrated that polarization of the SPR-enhanced optical field can be controlled to be linear by using polarization-gated excitation scheme. 13 In this paper, we show that the polarization-gated SPR excitation scheme can be used to efficiently accelerate electrons on the basis of ponderomotive force. In contrast with the conventional one-pulse excitation scheme, linear polarization mode can be created as the counter-propagating SPRenhanced optical pulses with counter-rotating elliptical polarizations are matched in both time and space, which increases the effective field intensity as well. As excited by two synchronized femtosecond laser pulses, the emitted electrons can be accelerated to a doubled maximum kinetic energy with a symmetric angular distribution about the normal of the metal surface. As shown in Fig. 1 , the Kretschmann configuration is considered for the coupling of optical field and SP waves. In order to have an intuitive picture of the polarization-gated excitation scheme under the driving of two synchronized p-polarized incident femtosecond laser pulses ͑femtosecond pulses A and B as shown in Fig. 1͒ , we first consider the simplified formulae 14 for the SPR-enhanced optical fields on the vacuum side of the metal film, E SP,x_A ͑x,y,t͒ = ␤E͑x,t͒cos͓k SP ͑x + L/2͒ − 0 t − /2 + 0 ͔e −␣y , E SP,y_A ͑x,y,t͒ = E͑x,t͒cos͓k SP ͑x + L/2͒ − 0 t + 0 ͔e −␣y , E SP,x_B ͑x,y,t͒ = ␤E͑− x,t͒cos͓− k SP ͑x − L/2͒ − 0 t + /2 + 0 ͔e −␣y , E SP,y_B ͑x,y,t͒ = E͑− x,t͒cos͓− k SP ͑x − L/2͒ − 0 t + 0 ͔e −␣y , ͑1͒ where E SP,x͑y͒_A͑B͒ is the x͑y͒ component of the SPRenhanced optical field excited by incident laser pulse A ͑or a͒ Electronic
doi:10.1063/1.3020711 fatcat:ua2yifiokrbifggnjhhkdgzesy