Simultaneous VUV and XUV pulse generation and characterization for attosecond pump probe experiments
D. Fabris, W. A. Okell, D. Walke, T. Witting, J. P. Marangos, J. W. G. Tisch
aa r X i v : . [ phy s i c s . op ti c s ] N ov Simultaneous VUV and XUV pulse generation andcharacterization for attosecond pump probe experiments
D. Fabris, , ∗ W. A. Okell, D. Walke, T. Witting, J. P. Marangos, and J. W. G. Tisch Blackett Laboratory, Imperial College, London SW7 2AZ, UK compiled: September 15, 2018We report the generation and characterization of isolated attosecond XUV and VUV pulses generated simul-taneously via HHG driven by few-cycle pulses using an in-line dual gas target system. One gas jet target wasoperated with Kr gas that optimized HHG in the 15-25 eV photon energy range (VUV), whilst the second gasjet target was operated in Ne gas to optimize the high harmonic generation around 90 eV (XUV). Appropriatefilters were used to isolate the required spectral components to synthesize isolated pulses. Sn and In filters wereused for the VUV region while a Zr filter was used for the XUV. We characterized both the XUV and VUVpulses independently using the attosecond streaking technique and the LSGPA retrieval algorithm obtaininga 1.7 ± . ±
50 as pulse using Sn, while preserving a 266 ±
10 as isolatedXUV pulse.
OCIS codes: (320.7090) Ultrafast lasers; (320.7100) Ultrafast measurements; (190.7110) Ultra-fast nonlinear optics; (190.7220) Upconversion; (190.4180) Multiharmonic generation; (260.7190) Ultraviolet;(260.7200) Ultraviolet, extreme; (320.7160) Ultrafast technologyhttp://dx.doi.org/10.1364/XX.99.099999
1. Introduction
A significant part of future development in attosecondscience [1] depends on attosecond light sources in newspectral regions to be applied in novel pump probe ex-periments [2]. The production of isolated attosecondpulses (IAP) in the XUV spectral region (30-150 eV) isnowadays a robust process. It requires gating the highharmonics (HH) emission produced with an intense laserusing different techniques. Most common are polariza-tion gating [3], ionization gating [4] and amplitude gat-ing [5]. However the yield of photons of such pulses istypically too low for them to be used in an attosecond-pump attosecond probe experiment. Therefore the driv-ing IR field is usually employed with the XUV pulse ina femtosecond-attosecond pump-probe scheme. Usingthe IR field, however, lowers the time resolution of theexperiment, moreover it is not always the best choiceto study attosecond dynamics as many target moleculesand atoms of interest show strong perturbation in a IRfield of sufficient strength to induce photoionisation.The results presented in this paper focus on the VUVspectral region (10-25 eV). The use of isolated pulses inthis spectral region is advantageous for use in the pumpstep of a pump-probe measurement. Compared to HHgenerated in higher energy spectral regions ( >
50 eV)the HH generation in the 10-25 eV region is expected toprovide a far higher photon yield and have a much higher ∗ Corresponding author: [email protected] interaction cross-section (e.g. for photoionisaton) witha typical atomic or molecular species[6].For these reasons there is a great deal of interest indeveloping reliable higher power attosecond sources bothat higher [7] or lower [8–11] energies with respect to thestandard XUV region.The metallic filters used to select a portion of the HHspectrum generated with a few-cycle IR pulse are in-dium (central energy 15 eV) and tin (central energy 20eV) [12]. Using a collinear target geometry has beenproven to be a promising approach [13]. We have usedcollinear pulse gas targets to simultaneously generate anisolated XUV pulse ( τ XUV = 266 ±
10 as) together withVUV pulses ( τ = 1 . ± . τ = 616 ±
50 as).The pulses were characterised using IR-field dressed pho-toionisation and the FROG-CRAB method [14] for boththe XUV-IR and VUV-IR configurations.
2. Experiment
The experiment reported in this paper was performedusing the Imperial College attosecond beamline [15].A commercial chirped pulse amplification laser system(Femtolaser GmbH, Femtopower HE CEP) providedpulses at 1 kHz, 28 fs duration with up to 2.5 mJ perpulse. The CPA pulses were compressed to 3.5 fs du-ration and pulse energy of up to 800 µ J in a differen-tially pumped hollow core fibre (HCF)[16] in order toimplement amplitude gating in the HHG process. Thecarrier-envelope phase (CEP) of the pulses was lockedafter the fibre output with a feedback system on the os-cillator pump [17]. Laser pulses of 400 µ J were focusedwith a 70 cm focusing mirror (FM1) onto two in linepulsed gas targets as shown in Figure 1.
GT1GT2FM1 IR F 2PM e - T O F GT3FKa Zr In KaSn
Fig. 1. Schematics of experimental setup. FM1,FM2: Fo-cusing mirrors. GT1, GT2: pulsed gas targets. IR: iris. F:filters, GT3: effusive gas target, 2PM: MoSi 2 part mirror,e-TOF: electron time of flight spectrometer.
The pulsed gas target GT1 was Ne (at a gas density of ≈ cm − ) for the production of the XUV radiation.The second pulsed gas target GT2 was Kr (at a gas den-sity of ≈ cm − ), whose lower ionization potentialresults in a more efficient production of VUV radiation.The position of the two targets with respect to the laserfocus and with respect to each other was optimized forphase-matching by performing position scans of the jetsalong the propagation axis. To filter the HH radiation aset of metallic filters was mounted on a stage for conve-nient interchange. Kapton (7.5 µ m thickness) was usedto reject the HH radiation providing the IR pulse for thepump probe experiment. A Zr filter (200 nm thickness)rejected the IR and lower harmonics filtering the XUVpulse. An In filter (200 nm thickness) selected a VUVpulse centered at 15 eV while Sn (200 nm thickness)filtered a VUV pulse centered at 20 eV. The mountinggeometry allowed us to select all pairs of pump-probepulses in combination with the filters. The filter mountwas positioned so that the beam was intercepted by twoadjacent filters (e.g. Kapton and In to streak the VUVat 15 eV), leading to side-by-side co-propagating por-tions of the beam that have been differently filtered.The streaking setup [15] included a MoSi multilayertwo part mirror mounted on a piezo stage that selected a8.6 eV bandwidth around 92 eV for the XUV pulse andprovided the time delay between the pulses. The twotime delayed pulses were focused onto the effusive gastarget in front of the electron time of flight spectrome-ter (e-TOF). Photoelectron spectra were recorded as afunction of time delay between the XUV/VUV pulsesand the IR pulse.Though we have not yet made an absolute measure-ment of photon fluxes of the pulses, based on known pho-toionisation cross-sections [18], filter trasmission and thecollection efficiency of our electron spectrometer, and as-suming the MoSi multilayer mirror to have a reflectivityin the VUV of few % (2% at 30 eV [12]), we estimatethat we generate ≈
10 nJ of VUV per pulse.
3. Results3.A. XUV-VUV compatibility
The first result achieved is the proof that the inlinegeometry is compatible with simulataneous attosecondXUV pulse generation and VUV pulse generation. Forthis reason XUV-IR streaking traces in Ne were recordedboth with the gas target for the VUV pulse (which wesubsequently refer to as GT2) on and off. The resultsare shown in Figure 2. Panels (a) and (b) show the mea-sured and retrieved XUV-IR traces with GT2 off, whilein panel (c) and (d) the same situation is illustrated forGT2 on. Panel (e) shows the retrieved IR pulses, show-ing that the effect of the VUV gas target is a CEP shift,while its duration is not altered. Panel (d) shows the re-trieved XUV pulses. The two traces have been analyzedwith exactly the same smoothing parameters. The algo-rithm implemented for the retrieval is LSGPA [19] andit has been applied in the same way to both traces pro-viding a time duration of 266 ±
10 as and 253 ±
10. Thestated errors include both the retrieval and experimentaluncertainties. A difference in time duration of ≈
13 at-toseconds is therefore within the errors showing that theXUV pulse was not strongly perturbed by the presenceof the VUV gas target maintaining a pulse duration suf-ficiently short for attosecond pump probe experiments. E ne r g y ( e V ) (a)708090100110 E ne r g y ( e V ) (b) 708090100110 E ne r g y ( e V ) delay (fs) (c)−5 0 5708090100110 E ne r g y ( e V ) delay (fs) (d)−5 0 5 708090100110−5 0 500.51 I n t en s i t y ( a r b . un i t s ) time(fs) →→ ←← (e) −1000 0 1000 00.51time (as) →→ ←←
253 as266 as I n t en s i t y ( a r b . un i t s ) (d) Fig. 2. Effect of second gas target. XUV-IR measured andretrieved traces with gas target off (a-b) and on (c-d). (e) IRpulses, with gas target off (blue line) and on (green dashedline). (d) XUV retrieved pulses from (a) (blue line) and (c)(green dashed line).
A 200 nm thick foil of indium provides a transmissionof about 25% at 15 eV, with a bandwidth of ≈ delay (fs) E ne r g y ( e V ) (a)−10 0 101012141618 delay (fs) E ne r g y ( e V ) (b)−10 0 10101214161810 1500.20.40.60.81 Energy (eV) I n t en s i t y ( a r b . u . )
10 15 0.10.20.30.40.50.6 P ha s e (r ad ) (c) −5 0 500.20.40.60.81 → ← I n t en s i t y ( no r m ) (d) Fig. 3. Results using an indium filter. (a) Measured trace.(b) Retrieved trace. (c) Spectral intensity, retrieved (blue),measured (black dashed), phase (dashed blue), transmissionof In filter (red). (d) Temporal intenisty, retrieved (blue)FTL (black dashed).
The traces were obtained using xenon as gas target toallow single photon ionization by the VUV pulse. Pan-els (a) and (b) are the measured and retrieved tracerespectively. The agreement between measured and re-trieved data is good with a FROG error of 1.7% on agrid 160 × . ± . / √ q where q is the harmonic order, the ex-pected VUV pulse duration is ≈ ≈ The 200 nm tin foil provides 22% transmission at 21 eV,with a bandwidth of ≈ delay (fs) E ne r g y ( e V ) (a)−5 0 514161820222426 delay (fs) E ne r g y ( e V ) (b)−5 0 5 1416182022242615 20 2500.20.40.60.81 15 20 25 00.10.20.30.4 P ha s e (r ad ) −5 0 5 00.20.40.60.81 → ←
616 ast (fs) I n t en s i t y ( no r m ) (d) Fig. 4. Results using a tin filter. Plots as in figure 3.
The target was Xe in this case as well. The plotsand color code of Figure 4 are the same as in Figure 3.The agreement between measured and retrieved tracesis good with a FROG error of 3% on a grid 160 × ±
50 as.The presence of two harmonics in the spectral windowprovided by tin leads to the presence of satellite pulsesin the time domain. However they are a factor of 5lower in intensity with respect the main pulse. Againthe absence of some features in the experimental datawith respect to the retrieval can be attributed to thespectrometer resolution, even though in this case, giventhe photoelectron energies measured are higher ( ≈
4. Conclusions
The compatibility of VUV and XUV IAP in a collineargas target geometry has been proven by measuring anIAP in the XUV both with the gas for the VUV gener-ation present and absent with no appreciable change inthe measured XUV pulse duration.Indium has been used to filter the HH spectrum, pro-viding a 1 . ± . / √ q (1.2 fs).Tin has been used to filter a pulse at 20 eV. The mea-sured pulse duration is 616 ±
50 as.This is the first time, to best of our knowledge, thatIAP pulses in the 15-25 eV spectral region have beengenerated and characterized together with an IAP inthe XUV. The results demonstrate the feasibility ofa HH based XUV-VUV attosecond-attosecond pump-probe experiments at these photon energies.
Acknowledgments
This work was financially supported by EPSRC throughgrants EP/I032517/1 and EP/F034601/1. We acknowl-edge technical support from Andrew Gregory and PeterRuthven.
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