Apparatus for soft x-ray table-top high harmonic generation

Abstract

There has been considerable recent interest in tabletop soft X-ray attosecond sources enabled by the new generation of intense, few-cycle laser sources at operating wavelengths longer than 800 nm. In our recent work [Johnson , Sci. Adv.(5), eaar3761 (2018)], we have demonstrated a new regime for the generation of X-ray attosecond pulses in the water window (284-540 eV) by high-harmonic generation, which resulted in soft X-ray fluxes of ≈10 photons/s and a maximum photon energy of 600 eV, an order of magnitude and 50 eV higher, respectively, than previously attained with few-cycle drivers. Here we present the key elements of our apparatus for the generation and detection of soft X-ray high harmonic radiation in the water window. Of critical importance is a differentially pumped gas target capable of supporting the multi-atmospheric pressures required to phase-match the high energy emission while strongly constraining the gas density, suppressing the effects of ionization and absorption outside the interaction region.

Onset of ionic coherence and ultrafast charge dynamics in attosecond molecular ionisation

“…During the past few years, considerable effort has been made to produce sufficiently intense, fully coherent, ultrashort vacuum (VUV), extreme (XUV) ultraviolet and X-ray pulses, a Department of Physics, Imperial College London, Prince Consort Road, London SW7 2AZ, United Kingdom. Tel: 0044 20 7589 47728; E-mail: m.ruberti11@imperial.ac.uk using both table-top high harmonic generation (HHG) [30][31][32][33][34][35][36] and free-electron laser (FEL) sources [37][38][39][40][41][42][43] , that would open the way to attosecond pump-attosecond probe experiments. Following this rapid progress 44,45 , several time-resolved spectroscopic techniques have been developed 18,19,[46][47][48][49][50][51][52][53][54][55] and have demonstrated unprecedented temporal resolution 14,57,58 in real-time tracing of both valence and core coherent electron dynamics as well as nuclear motion 56 .…”

“…As a proof-of-principle, the new methodology is applied to the study of the attosecond ionisation of the C 2 H 2 and C 2 H 4 molecules by an ultrashort XUV pulse, of the kind which can be produced in the attosecond laboratories from table-top HHG sources [30][31][32][33][34][35][36] . In this work, the effects of electron correlation are shown by comparing the ultrafast charge dynamics calculated by using two different methods of the RCS-ADC(n) hierarchy for the neutral molecule, which improve and go beyond the ADC(1) method, where the ionic system is described at the Koopmans approximation level and no shakeup effects are included: RCS-ADC(2), which improves the description of the Koopmans one hole (1h) ionic states by taking into account their couplings to the two hole -one particle (2h1p) configurations manifold, but does not provide a qualitatively good description of the relevant shakeup satellite states; and RCS-ADC (2)x, which fixes the limitations of both aforementioned methods by giving a qualitatively accurate description of the shakeup processes.…”

Attosecond soft X-ray high harmonic generation

“…The time of each half-cycle relative to the field envelope prior to propagation is indicated above; negative times indicate half-cycles before the peak of the field. Inset arrows show the direction in which phasematching is locally satisfied; this angle has been exaggerated such that 45 • corresponds to 10 mrad divergence, approximately the maximum angle detectable by our soft X-ray flat-field spectrometer [32]. The phasematching maps are furthermore clipped to the region where the intensity is high enough that harmonics at 350 eV are emitted.…”

Transient extreme ultraviolet measurement of element-specific charge transfer dynamics in multiple-material junctions

“…The high harmonics are generated by focusing the intense visible or NIR beam into a semi-infinite or infinite gas cell, which is flowed with a noble gas. Due to the energy scaling of high harmonic generation, the use of a NIR driving pulse allows access to higher energy x-ray edges in the water window 8,29 . A metal filter then removes residual visible or NIR light to select the desired harmonics.…”

“…Recent advances in high harmonic generation have allowed the production of attosecond and longer XUV pulses in table-top instrumentation [7][8][9][10][11] . In the extreme ultraviolet (XUV) region, a photoexcited core to valence level transition is measured.…”