Attosecond phase locking of harmonics emitted from laser-produced plasmas

Abstract

Laser-driven coherent extreme-ultraviolet (XUV) sources provide pulses lasting a few hundred attoseconds 1,2 , enabling real-time access to dynamic changes of the electronic structure of matter 3,4 , the fastest processes outside the atomic nucleus. These pulses, however, are typically rather weak. Exploiting the ultrahigh brilliance of accelerator-based XUV sources 5 and the unique time structure of their laser-based counterparts would open intriguing opportunities in ultrafast X-ray and high-field science, extending powerful nonlinear optical and pump-probe techniques towards X-ray frequencies, and paving the way towards unequalled radiation intensities. Relativistic laser-plasma interactions have been identified as a promising approach to achieve this goal 6-13 . Recent experiments confirmed that relativistically driven overdense plasmas are able to convert infrared laser light into harmonic XUV radiation with unparalleled efficiency, and demonstrated the scalability of the generation technique towards hard X-rays 14-19 . Here we show that the phases of the XUV harmonics emanating from the interaction processes are synchronized, and therefore enable attosecond temporal bunching. Along with the previous findings concerning energy conversion and recent advances in high-power laser technology, our experiment demonstrates the feasibility of confining unprecedented amounts of light energy to within less than one femtosecond.The nonlinear response of matter exposed to intense femtosecond laser pulses gives rise to the emission of highfrequency radiation at harmonics of the laser oscillation frequency. If the harmonics are phase-locked, their superposition results in a train of attosecond bursts 20 . The concept has been so far successfully implemented in atomic gases 21 , and culminated in isolated attosecond pulses by using few-cycle laser drivers 1,2 . The low generation efficiency of harmonic radiation from atoms has motivated research into alternative concepts. Dense, femtosecond-laser-produced plasmas hold promise of converting laser light into coherent harmonics with much higher efficiency and of exploiting much higher laser intensities, because the plasma medium-in contrast to the atomic emitters-imposes no restriction on the strength of the laser field driving the harmonics [6][7][8][9][10][11][12][13] . Recent experimental studies of harmonics produced from overdense plasmas impressively corroborate several theoretical predictions: the high conversion efficiency 19 , the favourable scalability of the generation technique towards high photon energies 14,16,19 and excellent divergence due to the spatial coherence of the generated harmonics 19,22 . Whether the high-order harmonics that are produced in overdense plasmas • gold-coated off-axis parabolic mirror with the same focal length as the laser focusing parabola. The recollimating mirror is mounted on a flipper stage for easy withdrawal, thus enabling the spectral characterization of the emitted XUV light. Thin metal filters (typically 150 nm Al, In or Sn)...

Attosecond control of collective electron motion in plasmas

“…This can be seen as the time-reverse of resonant absorption [22], where laser light is partially converted into collective electron oscillations in a gradient of plasma density (both effects correspond to linear mode conversion mechanisms [22,23]). This emission, known as coherent wake emission or CWE [24], consists of a sub-femtosecond burst of coherent radiation, superimposed with the laser light reflected at the plasma surface [25], with a spectrum extending into the extreme ultraviolet (XUV) up to ωmax = N solid /Nc ωL, emitted from the region of solid plasma density N solid (Fig. 1b).…”

Multi-μJ harmonic emission energy from laser-driven plasma

“…For relatively low laser intensities, the so-called coherent wake emission mechanism (CWE) dominates [11,12]. Under these circumstances, it has been recently demonstrated that the harmonic emission leads to temporal bunching with attosecond pulse durations [13,14]. At higher intensities, the relativistic oscillating mirror (ROM) mechanism becomes dominant [15][16][17].…”

“…It is therefore essential that the efficiency measurement is accompanied by a simultaneous determination of the beam divergence. In the past, a number of efficiency measurements mainly for individual harmonics and for different spectral ranges have been reported [13,18,21,[24][25][26]. The solid angle of the emission in these reports was mostly estimated without concurrent determination of the actual emission cone.…”

Streaking at high energies with electrons and positrons

“…Interestingly, there are already suggestions to produce zeptosecond pulses of keV-energy photons by employing relativistic laser-plasma interactions [12,13,14], and short pulses of multi-MeV energy photons can be produced via nonlinear Thomson/Compton backscattering [15,16,17]. At even shorter timescales, there is a proposal for an imploding ultrarelativistic flying mirror which can be created by a megajoule energy laser pulse at the ultrarelativistic intensity of 10 24 W/cm 2 [18,8].…”