Coherent synchrotron emission from electron nanobunches formed in relativistic laser–plasma interactions

Abstract

Extreme ultraviolet (XUV) and X-ray harmonic spectra produced by intense laser-solid interactions have, so far, been consistent with Doppler upshifted reflection from collective relativistic plasma oscillations-the relativistically oscillating mirror mechanism 1-6. Recent theoretical work, however, has identified a new interaction regime in which dense electron nanobunches are formed at the plasma-vacuum boundary resulting in coherent XUV radiation by coherent synchrotron emission 7,8 (CSE). Our experiments enable the isolation of CSE from competing processes, demonstrating that electron nanobunch formation does indeed occur. We observe spectra with the characteristic spectral signature of CSE-a slow decay of intensity, I, with high-harmonic order, n, as I(n) ∝ n −1.62 before a rapid efficiency rollover. Particle-in-cell code simulations reveal how dense nanobunches of electrons are periodically formed and accelerated during normal-incidence interactions with ultrathin foils and result in CSE in the transmitted direction. This observation of CSE presents a route to high-energy XUV pulses 7,8 and offers a new window on understanding ultrafast energy coupling during intense laser-solid density interactions. Relativistic laser plasmas have been shown to support the generation of bright beams of XUV radiation with attosecond duration 9 and high spatial quality 10 in reflection. Experimental observations 5,10,11 in the spectral region above the maximum plasma frequency of the interaction 12 (ω pmax) are consistent with a relativistic Doppler effect provided by a highly reflective critical density plasma surface collectively oscillating in the intense field of the incident laser pulse 1-4. This periodic upshifting of the driving radiation into the specularly reflected direction results in harmonic spectra consistent with the relativistically oscillating mirror (ROM) mechanism characterized by I (n) ROM ∝ n −8/3 scaling 4,5 , where n is the harmonic order. It should be noted that there is another mechanism that can dominate for frequencies below ω pmax of the target-coherent wake emission. Here we exclusively discuss harmonic frequencies above ω pmax for our interaction densities. Studies into how laser energy couples to relativistic plasmas, however, have revealed a fundamentally different mechanism for coherent XUV/X-ray generation. Simulations show that dense nanobunches of electrons can be periodically formed

Optically controlled dense current structures driven by relativistic plasma aperture-induced diffraction

“…The physics underpinning relativistic induced transparency (RIT) in thin foils is discussed by Vshivkov et al [15]. It has been shown that the effect can modify the rising edge profile, duration and polarization of the laser pulse [16][17][18][19] and that it is important in driving new ion acceleration [20,21] and radiation production [22][23][24] mechanisms.…”

“…The physics underpinning relativistic induced transparency (RIT) in thin foils is discussed by Vshivkov et al [15]. It has been shown that the effect can modify the rising edge profile, duration and polarization of the laser pulse [16][17][18][19] and that it is important in driving new ion acceleration [20,21] and radiation production [22][23][24] mechanisms.For the first time, we demonstrate that an ultraintense laser pulse induces a 'relativistic plasma aperture' in a thin foil and as a result undergoes the fundamental optic 2 process of diffraction. It is demonstrated, both numerically and experimentally, that the collective electron motion (including angular frequency of rotation) is determined by the resulting near-field diffraction pattern and can be controlled by simply varying the polarization of the laser.…”

Generation of Attosecond Light Pulses from Gas and Solid State Media

“…Due to its superior properties and predominance at high intensities, the basis for the generation of single asec light pulses [87,119] will most probably be the ROM mechanism. More recently, theoretical work has revealed that, under certain conditions, another mechanism can dominate and produce harmonic radiation with superior efficiency [120][121][122][123][124][125]. In this mechanism dense electron nanobunches are formed at the plasma vacuum boundary giving rise to XUV radiation by coherent synchrotron emission (CSE).…”

Intense circularly polarized attosecond pulse generation from relativistic laser plasmas using few-cycle laser pulses

“…In fact, the field has become a focus of research in the nonlinear optics of HHG. Compared to HHG from noble gases, HHG from plasma surfaces [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] does not subject to the limitation of maximum applied laser intensity and can thus use the state-of-the-art terawatt and petawatt laser technology, which will improve attosecond pulse energy, making it potentially useful to pump-probe experiments [29]. There is a strong motivation to seek circularly polarized attosecond XUV light source by HHG from plasma surfaces.…”