Electron acceleration driven by ultrashort and nonparaxial radially polarized laser pulses

Abstract

Exact closed-form solutions to Maxwell's equations are used to investigate the acceleration of electrons in vacuum driven by ultrashort and nonparaxial radially polarized laser pulses. We show that the threshold power above which significant acceleration takes place is greatly reduced by using a tighter focus. Moreover, electrons accelerated by tightly focused single-cycle laser pulses may reach around 80% of the theoretical energy gain limit, about twice the value previously reported with few-cycle paraxial pulses. Our results demonstrate that the direct acceleration of electrons in vacuum is well within reach of current laser technology.

Direct Electron Acceleration with Radially Polarized Laser Beams

“…This is mainly a consequence of the fact that shorter pulses allow the electron to move close to the pulse peak; in longer pulses, the electron is trapped and accelerated by the front edge of the pulse. As explained in [75], where additional details about the results discussed in this paragraph may be found, the data shown in Figure 6 is completely independent of the dominant wavelength λ 0 of the TM 01 laser pulse. Table 1.…”

“…Rigorous numerical simulations show that, instead, the maximum energy gain is much less unless the beam parameters are carefully optimized [67,68,[73][74][75]. We will see in Section 4 that exceptional conditions are provided by ultrashort and tightly focused pulses.…”

“…In this limit, the sub-cycle longitudinal acceleration regime is reached only if the average beam power is in the PW range (see also [18,67]). Alternatively, the beam can be tightly focused to increase the peak intensity and lower the threshold down to the TWs and GWs [68,75]. The sub-cycle regime characterized by a To illustrate the different regimes of longitudinal electron acceleration in RPLP, we performed a series of simulations, where the time-dependent Lorentz force equation was integrated.…”

“…A rigorous solution for the lowest-order radially polarized isodiffracting pulsed beam may therefore be obtained by taking a coherent superposition of TM 01 beams [see Equations (1)] with different frequencies but identical confocal parameter z 0 [77]. If we weight each frequency component by the spectral amplitude function F (ω) in Fourier space and then calculate the inverse Fourier transform, we obtain the following field components in complex notation for a TM 01 pulsed beam [75,77]:…”

“…Note that as s increases, the width of the spectrum F (ω) decreases while the pulse duration increases. The spectrum F (ω) and the corresponding temporal profile of the E z field component are shown in Figure 5 for different values of s. The direct acceleration of an on-axis electron by an ultrashort and nonparaxial TM 01 pulsed beam was investigated by some of us in a recent contribution [75]. More specifically, the laser power dependence of the maximum final kinetic energy that an electron initially at rest at r = 0 can acquire was studied for different pulse durations and degrees of focusing.…”