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Ultrastrong terahertz pulses produced by relativistic laser-matter interaction
Luc Bergé1, Emilien Denoual2, Xavier Davoine2 and Laurent Gremillet2
1Centre des Lasers Intenses et Applications, Université de Bordeaux-CNRS-CEA, F-33405 Talence Cedex, France
2CEA, DAM, DIF, F-91297 Arpajon, France & Université Paris-Saclay, CEA, LMCE, F-91680 Bruyères-le-Châtel, France
Petawatt laser sources deliver optical pulses lasting a few tens of femtoseconds with an intensity larger than 1020 W/cm2. When such a light beam interacts with a gas or a solid target, the electrons accelerated by the laser ponderomotive force become relativistic and acquire high energies, in excess of the GeV. Such laser systems also produce various radiations such as hard X photons or electron-positron pairs by quantum conversion of gamma photons. If this extreme light makes it possible to generate radiation in the highest frequency regions of the electromagnetic spectrum, it also fosters, through the production of plasma waves and particle acceleration, conversion processes towards much lower frequencies belonging to the gigahertz and terahertz (THz) ranges.
Intense sources of terahertz radiation are drawing growing interest for, e.g., atom probe tomography [1], particle acceleration or modification of condensed matter properties [2,3]. An ever-increasing number of strong-field applications require driver pulses that are both high power and spectrally tunable. While intense lasers offer promising prospects for developing compact, ultrashort THz sources, a current challenge nowadays is to produce broadband THz pulses with mJ-level energies. This is a nontrivial task as the most widely explored THz generation mechanisms for this purpose, namely optical rectification in asymmetric or organic crystals [4] or photoionization of gases by two-color, moderate-intensity (∼ 1014 Wcm−2) femtosecond laser pulses [5] are to date limited to tens of μJ THz pulse energies and ∼1-GVm-1 field strengths.
A more auspicious approach is to irradiate gaseous targets at relativistic laser intensities (>1018 Wcm−2). In this regime, coherent transition radiation (CTR) from wakefield-accelerated relativistic electron bunches at the rear plasma boundary can lead to intense THz emissions, characterized by a few-100-μJ energy yield and > 10-GVm−1 field strength [6]. Such a radiation is coherent because the typical dimensions of the electron bunches (∼ a few μm) are smaller than the THz radiation wavelengths (>10–100 μm). The THz pulse energy essentially scales as the square number of fast electrons escaping the plasma, which makes it a potentially very efficient mechanism.
CTR also operates in relativistic laser-solid interactions, whereby, compared to gas targets, it benefits from a stronger absorption of the laser energy into MeV-range electrons and hence from an increased number of radiating particles. However, because different acceleration mechanisms are at play, these energetic electrons are generally characterized by a much larger angular divergence than those generated by laser wakefield in gas plasmas. Yet, owing to its high density (∼1019<−1021 cm−3), the hot-electron population does not only radiate via CTR when exiting a solid foil. Less energetic electrons actually get reflected in the strong charge-separation field that they set up in vacuum. This results in an additional coherent, synchrotron-type radiation of polarity opposite to that of CTR. An additional complication follows from the fraction of fast electrons that are able to escape the target and thus just emit a single burst of CTR. The sheath electric field induced by the hot electrons on both sides of the target subsequently sets into motion the surface ions, a process known as target normal sheath acceleration (TNSA) [7]. Because of their highest charge-to-mass ratio, the protons, generally present as contaminants, react the fastest to that field over ∼1 ps timescales and lead to a dipole-type, low-frequency radiation able to contribute to the THz spectrum (see Figure 1).
A few years ago, experimental measurements [8] reported an efficient production of terawatt (TW)-level, mJ-level THz pulses, whose spectrum could be manipulated by tuning the laser pulse duration or target size, from high-intensity picosecond laser irradiating metal foils. More recently, TW, joule class THz radiation sources generated from microchannel targets driven by 100s of joule, picosecond lasers were associated to an increased conversion efficiency compared to that reached from planar foil targets, with laser-to-THz energy conversion up to 0.9% [9].
In the present talk, using high-resolution, two-dimensional particle-in-cell simulations, we numerically investigate the mechanisms of terahertz emissions in sub-micrometer-thick solid foils driven by ultraintense (∼1020 Wcm-2), ultrashort (30 fs) laser pulses at normal incidence. The considered range of target thicknesses extends from 0.5 μm down to the relativistic transparency regime (∼ 15 nm) that is known to optimize fs laser-driven ion acceleration. By disentangling the fields emitted by longitudinal and transverse currents, our analysis reveals that, within the first picosecond after the interaction, THz emission occurs in bursts as a result of coherent transition radiation by the recirculating hot electrons and antenna-type emission by the shielding electron currents traveling along the fast-expanding target surfaces [10].
Next, we theoretically focus on the radiation from the energetic electrons exiting the backside of a solid target. Our model takes account of the coherent transition radiation due to electrons crossing the plasma-vacuum interface as well as of the synchrotron radiation due to their deflection and deceleration in the sheath field they set up in vacuum. After showing that both mechanisms tend to largely compensate each other when all the electrons are pulled back into the target, we then demonstrate the sensitivity of this radiation to a percent-level fraction of escaping electrons. The same sheath field that confines most of the fast electrons around the target rapidly sets into motion the surface ions. We describe the THz emission from these accelerated ions and their accompanying hot electrons by means of a plasma expansion model that allows for finite foil size and multidimensional effects. Under conditions typical of current ultrashort laser-solid experiments, we find that the THz radiation from the expanding plasma is much less energetic—by one to three orders of magnitude—than that due to the early-time motion of the fast electrons [11].
Figure 1: (left) coherent transition and synchrotron radiations created by electrons escaping from and returning to the plasma, respectively, and (right) TNSA-induced “sheath” – or plasma expansion – radiation.
References
[1] A. Vella et al., “High resolution terahertz-driven atom probe tomography”, Sci. Adv.7, eabd7259 (2021).
[2] X. Li et al. “Terahertz field-induced ferroelectricity in quantum paraelectric SrTiO3”, Science 364, 1079 (2019).
[3] P. Salén et al., “Matter manipulation with extreme terahertz light: Progress in the enabling THz technology”, Phys. Rep. 838-837, 1 (2019).
[4] C. Vicario, B. Monoszlai, and C. P. Hauri, “GV/m single-cycle terahertz fields from a laser-driven large-size partitioned organic crystal”, Phys. Rev. Lett. 112, 213901 (2014).
[5] T.I. Oh et al., “Intense terahertz generation in two-color laser filamentation: energy scaling with terawatt laser systems”, New J. Phys 15, 075002 (2013).
[6] J. Déchard, A. Debayle, X. Davoine, L. Gremillet, and L. Bergé, “Terahertz pulse generation in underdense relativistic plasmas: From photoionization-induced radiation to coherent transition radiation”, Phys. Rev. Lett. 120, 144801 (2018).
[7] P. Mora, “Plasma expansion into a vacuum”, Phys. Rev. Lett. 90, 185002 (2003).
[8] G.-Q. Liao et al., “Towards terawatt-scale spectrally tunable terahertz pulses via relativistic laser-foil interactions”, Phys. Rev. X 10, 031062 (2020).
[9] G. Bruhaug et al., “Terawatt, Joule-class pulsed THz sources from microchannel targets”, arxiv.org/abs/2311.07718v1 (2023).
[10] J. Déchard, X. Davoine, L. Gremillet, and L. Bergé, “ Terahertz emission from submicron solid targets irradiated by ultraintense femtosecond laserpulses”, Phys. Plasmas 27, 093105 (2020).
[11] E. Denoual, L. Bergé, X. Davoine, and L. Gremillet, “Modeling terahertz emissions from energetic electrons and ions in foil targets irradiated by ultraintense femtosecond laser pulses”, Phys. Rev. E 108, 065211 (2023).