Anticorrelated Emission of High Harmonics and Fast Electron Beams From Plasma Mirrors

Abstract

We report for the first time on the anticorrelated emission of high-order harmonics and energetic electron beams from a solid-density plasma with a sharp vacuum interface−plasma mirror−driven by an intense ultrashort laser pulse. We highlight the key role played by the nanoscale structure of the plasma surface during the interaction by measuring the spatial and spectral properties of harmonics and electron beams emitted by a plasma mirror. We show that the nanoscale behavior of the plasma mirror can be controlled by tuning the scale length of the electronic density gradient, which is measured in-situ using spatial-domain interferometry.PACS numbers: 52.38. Kd,52.38.Ph Over the past 30 years, solid-density plasmas driven by intense femtosecond (fs) pulses, so-called plasma mirrors, have been successfully tested as a source of high-order harmonics and attosecond XUV pulses in a number of experiments [1][2][3][4][5][6][7][8][9][10], where the laser intensity typically exceeds a few 10 14 W/cm 2 . Other experiments have shown it is also possible to accelerate energetic electrons from plasma mirrors for intensities above 10 16 W/cm 2 [11-13]. Attempting to understand each of these experimental observations invariably points to the key role played by the plasma-vacuum interface during the interaction both on the nanoscale spatially and on the sub-laser-cycle scale temporally [14,15].It is commonly assumed that the electronic density at the plasma mirror surface decreases exponentially from solid to vacuum over a distance L g , also called density gradient. When the laser pulse reflects on this plasma mirror, for every oscillation of the laser field, some electrons are driven towards vacuum and sent back to the plasma [16,17]. These bunches of so-called Brunel electrons [18] impulsively excite collective high-frequency plasma oscillations in the density gradient that lead to the emission of XUV radiation through linear mode conversion [19]. As illustrated in Fig 1(a), each position x of the plasma behaves as a nanoscale oscillator of frequency ω p (x) = ω 0 n e (x)/n c where ω 0 is the driving laser angular frequency, n e the local electronic density at position x and n c the critical density. This periodic mechanism, called Coherent Wake Emission (CWE), leads to efficient high harmonics generation for very short plasma scale lengths, typically L g ∼ λ/100 [19], even for subrelativistic intensities, a 0 < 1, where a 0 = eA 0 /mc is the normalized vector potential, e and m the electron charge and mass and c the speed of light. However, the efficiency significantly drops for L g >> λ/20

On the physics of electron ejection from laser-irradiated overdense plasmas

“…In a very recent work, 17 it was observed that electron ejection and harmonic generation were anti-correlated, as opposed to our present findings. This is because in Ref.…”

“…This is because in Ref. 17, the laser intensity was not relativistic (I < 10 18 W=cm 2 ) and harmonics were generated via a completely different mechanism, the so-called coherent wake emission. 42 In coherent wake emission, the electrons return to the dense plasma and excite a plasma wake which in turn radiates.…”

“…These experiments usually produce large divergence electron beams with energies ranging from hundreds of keV (Refs. [13][14][15][16][17] to few MeV. 18 Some experiments reported on beams emitted in the specular direction, 18 while others reported on beams emitted along the surface 19 or in the normal direction.…”

Amplification of Relativistic Electron Bunches by Acceleration in Laser Fields

“…The efficiency of the relativistic backscattering process is dependent on a high density and narrow spectral distribution of the electron layer. These requirements reason recent interests in investigating laser accelerated electron bunches from solid bulk targets [2,3,18] and from ultra-thin foil targets [9,19,20].…”

Influence of laser contrast on high-order harmonic generation from solid-density plasma surfaces

“…Recently it is further extended to the generation of intense isolated attosecond pulses [5,6] with near diffraction-limited spatial beam quality [7] , which can be regarded as a powerful tool for diagnosing the properties of plasmas [8,9] and imaging science [10,11] . Since HOHG from solid targets was observed with CO 2 laser for the first time by Carman et al in 1981 [12] , it has been extensively investigated both experimentally [13][14][15][16] and theoretically [17][18][19][20][21][22] . Until now, there are two well recognized mechanisms of HOHG from plasma surfaces distinguished by the laser intensities, coherent wake emission (CWE) [17][18][19] and relativistically oscillating mirror (ROM) [20][21][22] .…”