Long-term operation of surface high-harmonic generation from relativistic oscillating mirrors using a spooling tape Jana Bierbach,1,2,* Mark Yeung,2 Erich Eckner,1 Christian Roedel,1,2,3 Stephan Kuschel,1,2 Matt Zepf,1,2,4 and Gerhard G. Paulus1,2 1
Institut fuer Optik und Quantenelektronik, Friedrich-Schiller-Universitaet Jena, Max-Wien-Platz 1, 07743 Jena, Germany GSI Helmholtzzentrum fuer Schwerionenforschung GmbH, Helmholtz-Institut Jena, Froebelstieg 3, 07743 Jena, Germany 3 SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA 4 Centre for Plasma Physics, School of Mathematics and Physics, Queen’s University Belfast, BT7 1NN, UK * [email protected]
2
Abstract: Surface high-harmonic generation in the relativistic regime is demonstrated as a source of extreme ultra-violet (XUV) pulses with extended operation time. Relativistic high-harmonic generation is driven by a frequency-doubled high-power Ti:Sapphire laser focused to a peak intensity of 3·1019 W/cm2 onto spooling tapes. We demonstrate continuous operation over up to one hour runtime at a repetition rate of 1 Hz. Harmonic spectra ranging from 20 eV to 70 eV (62 nm to 18 nm) were consecutively recorded by an XUV spectrometer. An average XUV pulse energy in the µJ range is measured. With the presented setup, relativistic surface highharmonic generation becomes a powerful source of coherent XUV pulses that might enable applications in, e.g. attosecond laser physics and the seeding of free-electron lasers, when the laser issues causing 80-% pulse energy fluctuations are overcome. ©2015 Optical Society of America OCIS codes: (190.4160) Multiharmonic generation; (340.7480) X-rays, soft x-rays, extreme ultraviolet (EUV).
References and links 1. 2. 3. 4. 5. 6. 7. 8.
9.
P. M. Paul, E. S. Toma, P. Breger, G. Mullot, F. Augé, Ph. Balcou, H. G. Muller, and P. Agostini, “Observation of a train of attosecond pulses from high harmonic generation,” Science 292(5522), 1689–1692 (2001). M. Drescher, M. Hentschel, R. Kienberger, M. Uiberacker, V. Yakovlev, A. Scrinzi, Th. Westerwalbesloh, U. Kleineberg, U. Heinzmann, and F. Krausz, “Time-resolved atomic inner-shell spectroscopy,” Nature 419(6909), 803–807 (2002). A. L. Cavalieri, N. Müller, Th. Uphues, V. S. Yakovlev, A. Baltuska, B. Horvath, B. Schmidt, L. Blümel, R. Holzwarth, S. Hendel, M. Drescher, U. Kleineberg, P. M. Echenique, R. Kienberger, F. Krausz, and U. Heinzmann, “Attosecond spectroscopy in condensed matter,” Nature 449(7165), 1029–1032 (2007). G. Sansone, L. Poletto, and M. Nisoli, “High-energy attosecond light sources,” Nat. Photonics 5(11), 655–663 (2011). F. Krausz and M. Ivanov, “Attosecond physics,” Rev. Mod. Phys. 81(1), 163–234 (2009). P. B. Corkum and F. Krausz, “Attosecond science,” Nat. Phys. 3(6), 381–387 (2007). R. L. Sandberg, A. Paul, D. A. Raymondson, S. Hädrich, D. M. Gaudiosi, J. Holtsnider, R. I. Tobey, O. Cohen, M. M. Murnane, H. C. Kapteyn, C. Song, J. Miao, Y. Liu, and F. Salmassi, “Lensless diffractive imaging using tabletop coherent high-harmonic soft-x-ray beams,” Phys. Rev. Lett. 99(9), 098103 (2007). G. Lambert, T. Hara, D. Garzella, T. Tanikawa, M. Labat, B. Carre, H. Kitamura, T. Shintake, M. Bougeard, S. Inoue, Y. Tanaka, P. Salieres, H. Merdji, O. Chubar, O. Gobert, K. Tahara, and M.-E. Couprie, “Injection of harmonic generated in gas in a free-electron laser providing intense coherent extreme-ultraviolet light,” Nat. Phys. 4(4), 296–300 (2008). Z. Huang and K.-J. Kim, “Review of x-ray free-electron laser theory,” Phys. Rev. ST Accel. Beams 10(3), 034801 (2007).
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Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12321
10. P. Heissler, P. Tzallas, J. M. Mikhailova, K. Khrennikov, L. Waldecker, F. Krausz, S. Karsch, D. Charalambidis, and G. D. Tsakiris, “Two-photon above-threshold ionization using extreme-ultraviolet harmonic emission from relativistic laser-plasma interaction,” New J. Phys. 14(4), 043025 (2012). 11. A. Borot, A. Malvache, X. Chen, A. Jullien, J.-P. Geindre, P. Audebert, G. Mourou, F. Quéré, and R. LopezMartens, “Attosecond control of collective electron motion in plasmas,” Nat. Phys. 8(5), 416–421 (2012). 12. J. A. Wheeler, A. Borot, S. Monchocé, H. Vincenti, A. Ricci, A. Malvache, R. Lopez-Martens, and F. Quéré, “Attosecond lighthouses from plasma mirrors,” Nat. Photonics 6(12), 829–833 (2012). 13. M. Krebs, S. Hädrich, S. Demmler, J. Rothhardt, A. Zaïr, L. Chipperfield, J. Limpert, and A. Tünnermann, “Towards isolated attosecond pulses at megahertz repetition rates,” Nat. Photonics 7(7), 555–559 (2013). 14. R. Lichters, J. Meyer-ter-Vehn, and A. Pukhov, “Short-pulse laser harmonics from oscillating plasma surfaces driven at relativistic intensity,” Phys. Plasmas 3(9), 3425 (1996). 15. G. D. Tsakiris, K. Eidmann, J. Meyer-ter-Vehn, and F. Krausz, “Route to intense single attosecond pulses,” New J. Phys. 8, 19 (2006). 16. C. Rödel, D. an der Brügge, J. Bierbach, M. Yeung, T. Hahn, B. Dromey, S. Herzer, S. Fuchs, A. G. Pour, E. Eckner, M. Behmke, M. Cerchez, O. Jäckel, D. Hemmers, T. Toncian, M. C. Kaluza, A. Belyanin, G. Pretzler, O. Willi, A. Pukhov, M. Zepf, and G. G. Paulus, “Harmonic generation from relativistic plasma surfaces in ultrasteep plasma density gradients,” Phys. Rev. Lett. 109(12), 125002 (2012). 17. J. Bierbach, C. Rödel, M. Yeung, B. Dromey, T. Hahn, A. Galestian Pour, S. Fuchs, A. E. Paz, S. Herzer, S. Kuschel, O. Jäckel, M. C. Kaluza, G. Pretzler, M. Zepf, and G. G. Paulus, “Generation of 10 µW relativistic surface high-harmonic radiation at a repetition rate of 10 Hz,” New J. Phys. 14(6), 065005 (2012). 18. B. Dromey, S. Kar, C. Bellei, D. C. Carroll, R. J. Clarke, J. S. Green, S. Kneip, K. Markey, S. R. Nagel, P. T. Simpson, L. Willingale, P. McKenna, D. Neely, Z. Najmudin, K. Krushelnick, P. A. Norreys, and M. Zepf, “Bright multi-keV harmonic generation from relativistically oscillating plasma surfaces,” Phys. Rev. Lett. 99(8), 085001 (2007). 19. www.eli-beams.eu 20. A. Borot, A. Malvache, X. Chen, D. Douillet, G. Iaquianiello, T. Lefrou, P. Audebert, J.-P. Geindre, G. Mourou, F. Quéré, and R. Lopez-Martens, “High-harmonic generation from plasma mirrors at kilohertz repetition rate,” Opt. Lett. 36(8), 1461–1463 (2011). 21. A. Borot, D. Douillet, G. Iaquaniello, T. Lefrou, P. Audebert, J.-P. Geindre, and R. Lopez-Martens, “High repetition rate plasma mirror device for attosecond science,” Rev. Sci. Instrum. 85(1), 013104 (2014). 22. E. Fill, J. Bayerl, and R. Tommasini, “A novel tape target for use with repetitively pulsed lasers,” Rev. Sci. Instrum. 73(5), 2190 (2002). 23. B. H. Shaw, J. van Tilborg, T. Sokollik, C. B. Schroeder, W. R. McKinney, N. A. Artemiev, V. V. Yashchuk, E. M. Gullikson, and W. P. Leemans, “High-peak-power surface high-harmonic generation at extreme ultra-violet wavelengths from a tape,” J. Appl. Phys. 114(4), 043106 (2013). 24. J. van Tilborg, B. H. Shaw, T. Sokollik, S. Rykovanov, S. Monchocé, F. Quéré, Ph. Martin, A. Malvache, and W. P. Leemans, “Spectral characterization of laser-driven solid-based high harmonics in the coherent wake emission regime,” Opt. Lett. 38(20), 4026–4029 (2013). 25. C. Thaury and F. Quéré, “High-order harmonic and attosecond pulse generation on plasma mirrors: basic mechanisms,” J. Phys. At. Mol. Opt. Phys. 43(21), 213001 (2010). 26. F. Quéré, C. Thaury, P. Monot, S. Dobosz, Ph. Martin, J.-P. Geindre, and P. Audebert, “Coherent wake emission of high-order harmonics from overdense plasmas,” Phys. Rev. Lett. 96(12), 125004 (2006). 27. K. Eidmann, R. Rix, T. Schlegel, and K. Witte, “Absorption of intense high-contrast sub-picosecond laser pulses in solid targets,” Europhys. Lett. 55(3), 334–340 (2001). 28. M. Zepf, G. D. Tsakiris, G. Pretzler, I. Watts, D. M. Chambers, P. A. Norreys, U. Andiel, A. E. Dangor, K. Eidmann, C. Gahn, A. Machacek, J. S. Wark, and K. Witte, “Role of the plasma scale length in the harmonic generation from solid targets,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 58(5), R5253– R5256 (1998). 29. C. Rödel, M. Heyer, M. Behmke, M. Kübel, O. Jäckel, W. Ziegler, D. Ehrt, M. C. Kaluza, and G. G. Paulus, “High repetition rate plasma mirror for temporal contrast enhancement of terawatt femtosecond laser pulses by three orders of magnitude,” Appl. Phys. B 103(2), 295–302 (2011). 30. E. Sidick, A. Knoesen, and A. Dienes, “Ultrashort-pulse second-harmonic generation. I. Transform-limited fundamental pulses,” JOSA B 12(9), 1704–1712 (1995). 31. S. Fuchs, C. Rödel, M. Krebs, S. Hädrich, J. Bierbach, A. E. Paz, S. Kuschel, M. Wünsche, V. Hilbert, U. Zastrau, E. Förster, J. Limpert, and G. G. Paulus, “Sensitivity calibration of an imaging extreme ultraviolet spectrometer-detector system for determining the efficiency of broadband extreme ultraviolet sources,” Rev. Sci. Instrum. 84(2), 023101 (2013). 32. S. Fourmaux, C. Serbanescu, L. Lecherbourg, S. Payeur, F. Martin, and J. C. Kieffer, “Investigation of the thermally induced laser beam distortion associated with vacuum compressor gratings in high energy and high average power femtosecond laser systems,” Opt. Express 17(1), 178–184 (2009). 33. S. Kahaly, S. Monchocé, H. Vincenti, T. Dzelzainis, B. Dromey, M. Zepf, P. Martin, and F. Quéré, “Direct observation of density-gradient effects in harmonic generation from plasma mirrors,” Phys. Rev. Lett. 110(17), 175001 (2013).
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Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12322
1. Introduction The generation of broadband coherent extreme ultraviolet (XUV) radiation using ultra-short laser pulses has opened new areas of research and applications. Most prominently, the process of high-harmonic generation (HHG) has facilitated attosecond pulses [1] which provide an instrument for time-resolved XUV spectroscopy of electronic processes [2–6]. Moreover, HHG has been used for coherent diffraction imaging [7] and the seeding of free-electron lasers (FELs) [8, 9]. In almost all experiments reported so far, high-harmonic generation from gases has been used. Only few experiments have been reported that utilize high-harmonic generation from plasma surfaces [10–12]. This is mainly due to the technical advantages of gas harmonics resulting from the fact that the interaction medium is getting continuously renewed. Accordingly, experiments can be performed using small-sized femtosecond laser systems with repetition rates in the order of several kilohertz or even up to megahertz [13]. The drawbacks of HHG in gases, however, are intrinsic limitations in terms of intensity and efficiency as the applicable intensity is limited by the ionization potential of the gas. Therefore, the highest available peak powers of several tens of terawatts up to petawatts cannot be exploited with gas harmonics unless impractically long focal lengths are used. Such an intensity restriction does not exist for relativistic surface high-harmonic generation (SHHG). Relativistic SHHG requires a normalized vector potential of a02 = (I·λ2)/(1.37·1018 W cm−2 µm2) ≥ 1 where I is the intensity in W/cm2 and λ is the wavelength of the laser in µm. Accordingly, the intensity should be higher than 1018 W/cm2 for 800-nm pulses. The generation of relativistic surface harmonics can be described by the model of a relativistic oscillating mirror (ROM) [14]. Here, a laser pulse with a0 > 1 interacts with a plasma surface. For oblique incidence and p-polarization, the electric field component normal to the plasma surface drives an oscillation that can reach relativistic velocities. As the laser pulse is reflected, its electric field undergoes a strong modulation resulting in a train of attosecond pulses emitted in the specular direction [15]. In the spectral domain, this corresponds to highorder harmonics. In experiments using moderate relativistic intensities, conversion efficiencies in the range of 10−5 to 10−6 and energies at the µJ level have been measured for XUV wavelengths [16, 17]. In terms of efficiency and XUV pulse energy ROM harmonic generation emerges as a competitive source to HHG from gases [4]. For ultra-relativistic peak intensities, a shallow spectral slope has been recorded up to keV harmonic frequencies [18]. Accordingly, the generation of ROM harmonics will strongly benefit from advancing highpower laser technology such that ultra-relativistic peak powers can be provided at high repetition rates [19]. However, for SHHG the optical surface is destroyed after each laser pulse and a new surface with optical quality must be delivered for each subsequent laser pulse. Rotating glass disks are typically used which provide a target for a couple of hundreds to thousands of shots [17, 20, 21]. The complicated target positioning and motion as well as the limited amount of shots are the main reason why relativistic surface harmonics have not been used for advanced experiments so far. A strategy to provide surface harmonics over hours of operation is the use of a spooling tape as a target [22]. Recently, surface harmonics have been generated from spooling tapes [23, 24] utilizing the non-relativistic generation mechanism of coherent wake emission (CWE) [25, 26]. However, CWE harmonics already occur at intensities of 1016 W/cm2 and have less favorable properties as compared to ROM harmonics. For example, only
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Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12323
Fig. 1. Experimental setup for surface high-harmonic generation from a spooling tape target. A λ/2 wave plate rotates the polarization of the 800 nm pulses such that the KDP crystal generates horizontally polarized SHG pulses via type-I phase matching. The residual 800 nm radiation is attenuated by more than 5 orders of magnitude by two dichroic turning mirrors, which are highly reflective at 400 nm and antireflection coated for 800 nm. An f/3 aluminumcoated off-axis parabola focuses the SHG pulses to an intensity of about 2.9·1019 W/cm2 onto a spooling tape. In the specular direction a calibrated spectrometer system [31] measures the XUV radiation.
harmonic frequencies up to the maximum plasma frequency of the surface plasma can be produced [26] which limits their applications. In contrast, the spectral extend and energy of ROM harmonics can be scales up with the laser intensity. Furthermore, as compared to CWEs, the harmonics from relativistic oscillating mirrors possess a smaller divergence thus providing a highly directional beam [25]. These favorable characteristics make ROM harmonics one of the most promising sources of intense attosecond pulses. Here, we present ROM harmonic generation from a spooling tape containing frequencies measured up to the aluminum edge at 17 nm (72 eV). We show consecutively measured shots over run-times of up to one hour, featuring µJ average XUV pulse energies in the spectral range of 62 nm down to 17 nm. 2. Experimental setup The experiment was carried out at the Ti:sapphire laser system “JETI” at Friedrich Schiller University Jena, which delivers 0.7-J, 35-fs pulses at a maximum repetition rate of 10 Hz. As schematically shown in Fig. 1, the 800-nm laser pulses are frequency doubled using a 0.7 mm thick potassium dihydrogen phosphate (KDP) crystal. The contrast is significantly enhanced by second-harmonic generation (SHG) [17, 27]. Two dichroic mirrors suppress the intensity of the fundamental 800-nm pulses by more than five orders of magnitude. Thus, significant target ionization and plasma expansion before the arrival of the main pulse is avoided, which is a prerequisite for surface high-harmonic generation [28, 29]. An f/3 off-axis parabolic mirror coated with aluminum focuses the SHG pulses at an angle of incidence of 45° and ppolarization onto a spooling tape target. The focus is imaged using a microscope objective. It is found that 35% of the entire pulse energy of 120 mJ is contained in a 4.2 µm2 focal spot (defined by the full-width at half maximum). For our laser parameters, the pulse duration of the second harmonic is similar to the one of the fundamental [30]. Thus, a peak intensity on the target surface of 2.9·1019 W/cm2 (a0 ≈1.8) is estimated. The experiment was performed using two different tape materials: 60 µm thick acrylic glass tape (Plexiglas, ThyssenKrupp) and commercially available video tape (VHS, consisting
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Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12324
Fig. 2. (a) XUV spectra from a VHS tape are consecutively measured during a continuous operation mode at a repetition rate of 1 Hz. Each vertical line corresponds to a single spectrum. The color map indicates the number of counts on the CCD camera. (b) Shows the temporally averaged and calibrated energy spectrum. (c) Measured XUV spectra and (d) averaged spectral energy for a comparable long-term operation over one hour using an acrylic glass tape
of an iron-oxide layer on a Mylar substrate). Both targets are very cost efficient and provide good optical surface quality [23]. More than 200 m of tape can be wound up on the target holder (Fig. 1). The spooling velocity is continuously adjustable such that an adequate distance between consecutive laser shots can be chosen for laser repetition rates up to a few kHz. As soon as the tape is uncoiled, the holder automatically translates by a given distance to a new row and starts to rewind. For the used acrylic glass tape of 5 cm width, a continuous generation of hundred thousands of XUV pulses can be achieved without replacement. The harmonic radiation is measured in the specular direction using a calibrated XUV spectrometer [31]. It consists of a nickel-coated toroidal mirror which images the XUV emission onto a CCD camera (back-thinned Andor DO940N) and a 1000-lines/mm transmission grating with freestanding gold bars for dispersion. Two 200-nm thick aluminum foils at the entrance aperture of the spectrometer block the intense visible radiation. 3. Experimental results For both tape materials we recorded SHHG spectra in the spectral range of ~20 to 70 eV (62 – 18 nm). Figure 2(a) displays spectra taken consecutively with a video tape (VHS) over a duration of 33 minutes at a laser repetition rate of 1 Hz. Each vertical line corresponds to a single spectrum. In general, the detected signal consists of a broad and continuous XUV background and harmonic lines of the orders 8 to 23 of the driving 400 nm laser pulse. The cutoff for CWE is at the 7th – 8th harmonic for plastic materials such as the applied foils [16, 23]. Accordingly, the harmonic signal for higher orders can be attributed to the ROM process. For the determination of the detected XUV energy, the filter transmission and spectrometer calibration [31] are taken into account. An average yield of 1 µJ per pulse in the spectral range of 20 eV to 70 eV has been recorded for the ~30 minutes of runtime (Fig. 2(b)). As the continuous background signal is presumably incoherent, it is subtracted for the determination of the SHHG energy, leaving an average of 0.17 µJ per XUV pulse. We note that the calculated energy values represent only the fraction of the harmonic beam that enters the spectrometers aperture (25 mrad x 10 mrad). For a typical ROM harmonic divergence in the range of 20 – 40 mrad [16, 17], the pulse energy per harmonic will thus be higher than the above stated values by up to a factor of two.
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Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12325
Fig. 3. (a) At the full lasers repetition rate of 10 Hz the ROM harmonic signal is found to recede within minutes. (b) A running average of the energy of the 10th harmonic shows a reduction by more than one order of magnitude within two minutes.
For comparison to the VHS tapes, which are coated with iron-oxide on the backside, we applied a transparent acrylic glass tape as a target material using identical laser conditions. Consecutive spectra of an experiment running one full hour are shown in Fig. 2(c). It is noticeable at first glance that the background level from the acrylic glass tape is much lower than from the VHS tapes. This is due to reduced plasma radiation from a target without heavy elements. Such a low background would be beneficial, e.g. for ionization experiments requiring high-quality XUV pulses and low background signal. Furthermore, the run with the acrylic glass tape shows a higher average XUV pulse energy as compared to the VHS tape. An integrated and rms-averaged energy of 0.90 µJ is measured (Fig. 2(d)). After subtracting the continuous background, the ROM harmonic energy yields 0.34 µJ per XUV pulse. Nevertheless, the stability of the harmonic signal remains an issue. A standard deviation of 85% (75%) of the energy of the harmonic signal after background subtraction has been recorded for the acrylic glass (VHS) target. On the one hand, intensity fluctuations of the laser (2% rms) have a strong influence on the fluctuations of the XUV source since cascaded nonlinear processes have been used (SHG + ROM harmonic generation). Notably, ROM harmonic generation is extremely nonlinear at the low relativistic intensities used in our experiment. Even for mechanically stable glass targets, we measured a stability of 30% rms under similar laser conditions [17]. Using higher peak energies and an energy-stabilized laser system would increase the stability of the ROM harmonic source significantly. On the other hand, using a pilot laser, we observed noticeable motions of the tape due to the impact of the laser pulse and the ablating plasma during this experiment. For this reason and in order to shield the adjacent surface area from debris, the target mount is equipped with mechanical guides. We expect that the performance can be improved when the tension of the tape target is increased and thicker foils are used. Furthermore, interferometric online monitoring and active stabilization of the surface position in a feedback loop could be used in order to increase the mechanical stability [21]. When the laser system is operated at the full repetition rate of 10 Hz, it was observed that the ROM harmonic signal started decreasing within minutes (Fig. 3). As an example, Fig. 3(b) shows the energy of the 10th order harmonic over time, which is already reduced by one order of magnitude after two minutes. This is due to absorption and heat accumulation at the gold gratings in the compressor of the laser stage. As a consequence of the thermally induced laser beam distortions, the focal spot quality is deteriorated [32]. The use of gratings with dielectric coating inside the compressor would significantly reduce the heat load and thus allows higher repetition rates. This should be considered in the design of high intensity lasers that operate at high repetition rate. 4. Conclusion In summary, we demonstrated relativistic surface high-harmonic generation in a continuous operation mode over runtimes of up to one hour from spooling tape targets. XUV spectra containing µJ energies and harmonic frequencies up to 70 eV are measured using a calibrated
#225842 - $15.00 USD © 2015 OSA
Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12326
spectrometer setup. The prospect of generating ROM harmonics at laser systems with higher peak intensity and under optimization of the plasma scale length, e.g. by utilizing a controlled prepulse [33], promises an increase in XUV pulse energy and spectral extend. Although technically challenging, advancing ROM harmonic sources from single-shot mode to a stable long-term operating XUV source will pave the way for applications such as highly timeresolved XUV spectroscopy, nonlinear attosecond experiments or the seeding of free-electron lasers. For laser systems exceeding the moderately relativistic intensities used in this experiment, relativistic surface high-harmonic generation using a spooling tape configuration might become an essential part of an attosecond beam line. Acknowledgments This work was supported by Deutsche Forschungsgemeinschaft (SFB TR18). C.R. acknowledges support from the VolkswagenStiftung. Burgard Beleites and Falk Ronneberger have contributed to this work by operating the JETI facility.
#225842 - $15.00 USD © 2015 OSA
Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12327
Institut fuer Optik und Quantenelektronik, Friedrich-Schiller-Universitaet Jena, Max-Wien-Platz 1, 07743 Jena, Germany GSI Helmholtzzentrum fuer Schwerionenforschung GmbH, Helmholtz-Institut Jena, Froebelstieg 3, 07743 Jena, Germany 3 SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA 4 Centre for Plasma Physics, School of Mathematics and Physics, Queen’s University Belfast, BT7 1NN, UK * [email protected]
2
Abstract: Surface high-harmonic generation in the relativistic regime is demonstrated as a source of extreme ultra-violet (XUV) pulses with extended operation time. Relativistic high-harmonic generation is driven by a frequency-doubled high-power Ti:Sapphire laser focused to a peak intensity of 3·1019 W/cm2 onto spooling tapes. We demonstrate continuous operation over up to one hour runtime at a repetition rate of 1 Hz. Harmonic spectra ranging from 20 eV to 70 eV (62 nm to 18 nm) were consecutively recorded by an XUV spectrometer. An average XUV pulse energy in the µJ range is measured. With the presented setup, relativistic surface highharmonic generation becomes a powerful source of coherent XUV pulses that might enable applications in, e.g. attosecond laser physics and the seeding of free-electron lasers, when the laser issues causing 80-% pulse energy fluctuations are overcome. ©2015 Optical Society of America OCIS codes: (190.4160) Multiharmonic generation; (340.7480) X-rays, soft x-rays, extreme ultraviolet (EUV).
References and links 1. 2. 3. 4. 5. 6. 7. 8.
9.
P. M. Paul, E. S. Toma, P. Breger, G. Mullot, F. Augé, Ph. Balcou, H. G. Muller, and P. Agostini, “Observation of a train of attosecond pulses from high harmonic generation,” Science 292(5522), 1689–1692 (2001). M. Drescher, M. Hentschel, R. Kienberger, M. Uiberacker, V. Yakovlev, A. Scrinzi, Th. Westerwalbesloh, U. Kleineberg, U. Heinzmann, and F. Krausz, “Time-resolved atomic inner-shell spectroscopy,” Nature 419(6909), 803–807 (2002). A. L. Cavalieri, N. Müller, Th. Uphues, V. S. Yakovlev, A. Baltuska, B. Horvath, B. Schmidt, L. Blümel, R. Holzwarth, S. Hendel, M. Drescher, U. Kleineberg, P. M. Echenique, R. Kienberger, F. Krausz, and U. Heinzmann, “Attosecond spectroscopy in condensed matter,” Nature 449(7165), 1029–1032 (2007). G. Sansone, L. Poletto, and M. Nisoli, “High-energy attosecond light sources,” Nat. Photonics 5(11), 655–663 (2011). F. Krausz and M. Ivanov, “Attosecond physics,” Rev. Mod. Phys. 81(1), 163–234 (2009). P. B. Corkum and F. Krausz, “Attosecond science,” Nat. Phys. 3(6), 381–387 (2007). R. L. Sandberg, A. Paul, D. A. Raymondson, S. Hädrich, D. M. Gaudiosi, J. Holtsnider, R. I. Tobey, O. Cohen, M. M. Murnane, H. C. Kapteyn, C. Song, J. Miao, Y. Liu, and F. Salmassi, “Lensless diffractive imaging using tabletop coherent high-harmonic soft-x-ray beams,” Phys. Rev. Lett. 99(9), 098103 (2007). G. Lambert, T. Hara, D. Garzella, T. Tanikawa, M. Labat, B. Carre, H. Kitamura, T. Shintake, M. Bougeard, S. Inoue, Y. Tanaka, P. Salieres, H. Merdji, O. Chubar, O. Gobert, K. Tahara, and M.-E. Couprie, “Injection of harmonic generated in gas in a free-electron laser providing intense coherent extreme-ultraviolet light,” Nat. Phys. 4(4), 296–300 (2008). Z. Huang and K.-J. Kim, “Review of x-ray free-electron laser theory,” Phys. Rev. ST Accel. Beams 10(3), 034801 (2007).
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Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12321
10. P. Heissler, P. Tzallas, J. M. Mikhailova, K. Khrennikov, L. Waldecker, F. Krausz, S. Karsch, D. Charalambidis, and G. D. Tsakiris, “Two-photon above-threshold ionization using extreme-ultraviolet harmonic emission from relativistic laser-plasma interaction,” New J. Phys. 14(4), 043025 (2012). 11. A. Borot, A. Malvache, X. Chen, A. Jullien, J.-P. Geindre, P. Audebert, G. Mourou, F. Quéré, and R. LopezMartens, “Attosecond control of collective electron motion in plasmas,” Nat. Phys. 8(5), 416–421 (2012). 12. J. A. Wheeler, A. Borot, S. Monchocé, H. Vincenti, A. Ricci, A. Malvache, R. Lopez-Martens, and F. Quéré, “Attosecond lighthouses from plasma mirrors,” Nat. Photonics 6(12), 829–833 (2012). 13. M. Krebs, S. Hädrich, S. Demmler, J. Rothhardt, A. Zaïr, L. Chipperfield, J. Limpert, and A. Tünnermann, “Towards isolated attosecond pulses at megahertz repetition rates,” Nat. Photonics 7(7), 555–559 (2013). 14. R. Lichters, J. Meyer-ter-Vehn, and A. Pukhov, “Short-pulse laser harmonics from oscillating plasma surfaces driven at relativistic intensity,” Phys. Plasmas 3(9), 3425 (1996). 15. G. D. Tsakiris, K. Eidmann, J. Meyer-ter-Vehn, and F. Krausz, “Route to intense single attosecond pulses,” New J. Phys. 8, 19 (2006). 16. C. Rödel, D. an der Brügge, J. Bierbach, M. Yeung, T. Hahn, B. Dromey, S. Herzer, S. Fuchs, A. G. Pour, E. Eckner, M. Behmke, M. Cerchez, O. Jäckel, D. Hemmers, T. Toncian, M. C. Kaluza, A. Belyanin, G. Pretzler, O. Willi, A. Pukhov, M. Zepf, and G. G. Paulus, “Harmonic generation from relativistic plasma surfaces in ultrasteep plasma density gradients,” Phys. Rev. Lett. 109(12), 125002 (2012). 17. J. Bierbach, C. Rödel, M. Yeung, B. Dromey, T. Hahn, A. Galestian Pour, S. Fuchs, A. E. Paz, S. Herzer, S. Kuschel, O. Jäckel, M. C. Kaluza, G. Pretzler, M. Zepf, and G. G. Paulus, “Generation of 10 µW relativistic surface high-harmonic radiation at a repetition rate of 10 Hz,” New J. Phys. 14(6), 065005 (2012). 18. B. Dromey, S. Kar, C. Bellei, D. C. Carroll, R. J. Clarke, J. S. Green, S. Kneip, K. Markey, S. R. Nagel, P. T. Simpson, L. Willingale, P. McKenna, D. Neely, Z. Najmudin, K. Krushelnick, P. A. Norreys, and M. Zepf, “Bright multi-keV harmonic generation from relativistically oscillating plasma surfaces,” Phys. Rev. Lett. 99(8), 085001 (2007). 19. www.eli-beams.eu 20. A. Borot, A. Malvache, X. Chen, D. Douillet, G. Iaquianiello, T. Lefrou, P. Audebert, J.-P. Geindre, G. Mourou, F. Quéré, and R. Lopez-Martens, “High-harmonic generation from plasma mirrors at kilohertz repetition rate,” Opt. Lett. 36(8), 1461–1463 (2011). 21. A. Borot, D. Douillet, G. Iaquaniello, T. Lefrou, P. Audebert, J.-P. Geindre, and R. Lopez-Martens, “High repetition rate plasma mirror device for attosecond science,” Rev. Sci. Instrum. 85(1), 013104 (2014). 22. E. Fill, J. Bayerl, and R. Tommasini, “A novel tape target for use with repetitively pulsed lasers,” Rev. Sci. Instrum. 73(5), 2190 (2002). 23. B. H. Shaw, J. van Tilborg, T. Sokollik, C. B. Schroeder, W. R. McKinney, N. A. Artemiev, V. V. Yashchuk, E. M. Gullikson, and W. P. Leemans, “High-peak-power surface high-harmonic generation at extreme ultra-violet wavelengths from a tape,” J. Appl. Phys. 114(4), 043106 (2013). 24. J. van Tilborg, B. H. Shaw, T. Sokollik, S. Rykovanov, S. Monchocé, F. Quéré, Ph. Martin, A. Malvache, and W. P. Leemans, “Spectral characterization of laser-driven solid-based high harmonics in the coherent wake emission regime,” Opt. Lett. 38(20), 4026–4029 (2013). 25. C. Thaury and F. Quéré, “High-order harmonic and attosecond pulse generation on plasma mirrors: basic mechanisms,” J. Phys. At. Mol. Opt. Phys. 43(21), 213001 (2010). 26. F. Quéré, C. Thaury, P. Monot, S. Dobosz, Ph. Martin, J.-P. Geindre, and P. Audebert, “Coherent wake emission of high-order harmonics from overdense plasmas,” Phys. Rev. Lett. 96(12), 125004 (2006). 27. K. Eidmann, R. Rix, T. Schlegel, and K. Witte, “Absorption of intense high-contrast sub-picosecond laser pulses in solid targets,” Europhys. Lett. 55(3), 334–340 (2001). 28. M. Zepf, G. D. Tsakiris, G. Pretzler, I. Watts, D. M. Chambers, P. A. Norreys, U. Andiel, A. E. Dangor, K. Eidmann, C. Gahn, A. Machacek, J. S. Wark, and K. Witte, “Role of the plasma scale length in the harmonic generation from solid targets,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 58(5), R5253– R5256 (1998). 29. C. Rödel, M. Heyer, M. Behmke, M. Kübel, O. Jäckel, W. Ziegler, D. Ehrt, M. C. Kaluza, and G. G. Paulus, “High repetition rate plasma mirror for temporal contrast enhancement of terawatt femtosecond laser pulses by three orders of magnitude,” Appl. Phys. B 103(2), 295–302 (2011). 30. E. Sidick, A. Knoesen, and A. Dienes, “Ultrashort-pulse second-harmonic generation. I. Transform-limited fundamental pulses,” JOSA B 12(9), 1704–1712 (1995). 31. S. Fuchs, C. Rödel, M. Krebs, S. Hädrich, J. Bierbach, A. E. Paz, S. Kuschel, M. Wünsche, V. Hilbert, U. Zastrau, E. Förster, J. Limpert, and G. G. Paulus, “Sensitivity calibration of an imaging extreme ultraviolet spectrometer-detector system for determining the efficiency of broadband extreme ultraviolet sources,” Rev. Sci. Instrum. 84(2), 023101 (2013). 32. S. Fourmaux, C. Serbanescu, L. Lecherbourg, S. Payeur, F. Martin, and J. C. Kieffer, “Investigation of the thermally induced laser beam distortion associated with vacuum compressor gratings in high energy and high average power femtosecond laser systems,” Opt. Express 17(1), 178–184 (2009). 33. S. Kahaly, S. Monchocé, H. Vincenti, T. Dzelzainis, B. Dromey, M. Zepf, P. Martin, and F. Quéré, “Direct observation of density-gradient effects in harmonic generation from plasma mirrors,” Phys. Rev. Lett. 110(17), 175001 (2013).
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Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12322
1. Introduction The generation of broadband coherent extreme ultraviolet (XUV) radiation using ultra-short laser pulses has opened new areas of research and applications. Most prominently, the process of high-harmonic generation (HHG) has facilitated attosecond pulses [1] which provide an instrument for time-resolved XUV spectroscopy of electronic processes [2–6]. Moreover, HHG has been used for coherent diffraction imaging [7] and the seeding of free-electron lasers (FELs) [8, 9]. In almost all experiments reported so far, high-harmonic generation from gases has been used. Only few experiments have been reported that utilize high-harmonic generation from plasma surfaces [10–12]. This is mainly due to the technical advantages of gas harmonics resulting from the fact that the interaction medium is getting continuously renewed. Accordingly, experiments can be performed using small-sized femtosecond laser systems with repetition rates in the order of several kilohertz or even up to megahertz [13]. The drawbacks of HHG in gases, however, are intrinsic limitations in terms of intensity and efficiency as the applicable intensity is limited by the ionization potential of the gas. Therefore, the highest available peak powers of several tens of terawatts up to petawatts cannot be exploited with gas harmonics unless impractically long focal lengths are used. Such an intensity restriction does not exist for relativistic surface high-harmonic generation (SHHG). Relativistic SHHG requires a normalized vector potential of a02 = (I·λ2)/(1.37·1018 W cm−2 µm2) ≥ 1 where I is the intensity in W/cm2 and λ is the wavelength of the laser in µm. Accordingly, the intensity should be higher than 1018 W/cm2 for 800-nm pulses. The generation of relativistic surface harmonics can be described by the model of a relativistic oscillating mirror (ROM) [14]. Here, a laser pulse with a0 > 1 interacts with a plasma surface. For oblique incidence and p-polarization, the electric field component normal to the plasma surface drives an oscillation that can reach relativistic velocities. As the laser pulse is reflected, its electric field undergoes a strong modulation resulting in a train of attosecond pulses emitted in the specular direction [15]. In the spectral domain, this corresponds to highorder harmonics. In experiments using moderate relativistic intensities, conversion efficiencies in the range of 10−5 to 10−6 and energies at the µJ level have been measured for XUV wavelengths [16, 17]. In terms of efficiency and XUV pulse energy ROM harmonic generation emerges as a competitive source to HHG from gases [4]. For ultra-relativistic peak intensities, a shallow spectral slope has been recorded up to keV harmonic frequencies [18]. Accordingly, the generation of ROM harmonics will strongly benefit from advancing highpower laser technology such that ultra-relativistic peak powers can be provided at high repetition rates [19]. However, for SHHG the optical surface is destroyed after each laser pulse and a new surface with optical quality must be delivered for each subsequent laser pulse. Rotating glass disks are typically used which provide a target for a couple of hundreds to thousands of shots [17, 20, 21]. The complicated target positioning and motion as well as the limited amount of shots are the main reason why relativistic surface harmonics have not been used for advanced experiments so far. A strategy to provide surface harmonics over hours of operation is the use of a spooling tape as a target [22]. Recently, surface harmonics have been generated from spooling tapes [23, 24] utilizing the non-relativistic generation mechanism of coherent wake emission (CWE) [25, 26]. However, CWE harmonics already occur at intensities of 1016 W/cm2 and have less favorable properties as compared to ROM harmonics. For example, only
#225842 - $15.00 USD © 2015 OSA
Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12323
Fig. 1. Experimental setup for surface high-harmonic generation from a spooling tape target. A λ/2 wave plate rotates the polarization of the 800 nm pulses such that the KDP crystal generates horizontally polarized SHG pulses via type-I phase matching. The residual 800 nm radiation is attenuated by more than 5 orders of magnitude by two dichroic turning mirrors, which are highly reflective at 400 nm and antireflection coated for 800 nm. An f/3 aluminumcoated off-axis parabola focuses the SHG pulses to an intensity of about 2.9·1019 W/cm2 onto a spooling tape. In the specular direction a calibrated spectrometer system [31] measures the XUV radiation.
harmonic frequencies up to the maximum plasma frequency of the surface plasma can be produced [26] which limits their applications. In contrast, the spectral extend and energy of ROM harmonics can be scales up with the laser intensity. Furthermore, as compared to CWEs, the harmonics from relativistic oscillating mirrors possess a smaller divergence thus providing a highly directional beam [25]. These favorable characteristics make ROM harmonics one of the most promising sources of intense attosecond pulses. Here, we present ROM harmonic generation from a spooling tape containing frequencies measured up to the aluminum edge at 17 nm (72 eV). We show consecutively measured shots over run-times of up to one hour, featuring µJ average XUV pulse energies in the spectral range of 62 nm down to 17 nm. 2. Experimental setup The experiment was carried out at the Ti:sapphire laser system “JETI” at Friedrich Schiller University Jena, which delivers 0.7-J, 35-fs pulses at a maximum repetition rate of 10 Hz. As schematically shown in Fig. 1, the 800-nm laser pulses are frequency doubled using a 0.7 mm thick potassium dihydrogen phosphate (KDP) crystal. The contrast is significantly enhanced by second-harmonic generation (SHG) [17, 27]. Two dichroic mirrors suppress the intensity of the fundamental 800-nm pulses by more than five orders of magnitude. Thus, significant target ionization and plasma expansion before the arrival of the main pulse is avoided, which is a prerequisite for surface high-harmonic generation [28, 29]. An f/3 off-axis parabolic mirror coated with aluminum focuses the SHG pulses at an angle of incidence of 45° and ppolarization onto a spooling tape target. The focus is imaged using a microscope objective. It is found that 35% of the entire pulse energy of 120 mJ is contained in a 4.2 µm2 focal spot (defined by the full-width at half maximum). For our laser parameters, the pulse duration of the second harmonic is similar to the one of the fundamental [30]. Thus, a peak intensity on the target surface of 2.9·1019 W/cm2 (a0 ≈1.8) is estimated. The experiment was performed using two different tape materials: 60 µm thick acrylic glass tape (Plexiglas, ThyssenKrupp) and commercially available video tape (VHS, consisting
#225842 - $15.00 USD © 2015 OSA
Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12324
Fig. 2. (a) XUV spectra from a VHS tape are consecutively measured during a continuous operation mode at a repetition rate of 1 Hz. Each vertical line corresponds to a single spectrum. The color map indicates the number of counts on the CCD camera. (b) Shows the temporally averaged and calibrated energy spectrum. (c) Measured XUV spectra and (d) averaged spectral energy for a comparable long-term operation over one hour using an acrylic glass tape
of an iron-oxide layer on a Mylar substrate). Both targets are very cost efficient and provide good optical surface quality [23]. More than 200 m of tape can be wound up on the target holder (Fig. 1). The spooling velocity is continuously adjustable such that an adequate distance between consecutive laser shots can be chosen for laser repetition rates up to a few kHz. As soon as the tape is uncoiled, the holder automatically translates by a given distance to a new row and starts to rewind. For the used acrylic glass tape of 5 cm width, a continuous generation of hundred thousands of XUV pulses can be achieved without replacement. The harmonic radiation is measured in the specular direction using a calibrated XUV spectrometer [31]. It consists of a nickel-coated toroidal mirror which images the XUV emission onto a CCD camera (back-thinned Andor DO940N) and a 1000-lines/mm transmission grating with freestanding gold bars for dispersion. Two 200-nm thick aluminum foils at the entrance aperture of the spectrometer block the intense visible radiation. 3. Experimental results For both tape materials we recorded SHHG spectra in the spectral range of ~20 to 70 eV (62 – 18 nm). Figure 2(a) displays spectra taken consecutively with a video tape (VHS) over a duration of 33 minutes at a laser repetition rate of 1 Hz. Each vertical line corresponds to a single spectrum. In general, the detected signal consists of a broad and continuous XUV background and harmonic lines of the orders 8 to 23 of the driving 400 nm laser pulse. The cutoff for CWE is at the 7th – 8th harmonic for plastic materials such as the applied foils [16, 23]. Accordingly, the harmonic signal for higher orders can be attributed to the ROM process. For the determination of the detected XUV energy, the filter transmission and spectrometer calibration [31] are taken into account. An average yield of 1 µJ per pulse in the spectral range of 20 eV to 70 eV has been recorded for the ~30 minutes of runtime (Fig. 2(b)). As the continuous background signal is presumably incoherent, it is subtracted for the determination of the SHHG energy, leaving an average of 0.17 µJ per XUV pulse. We note that the calculated energy values represent only the fraction of the harmonic beam that enters the spectrometers aperture (25 mrad x 10 mrad). For a typical ROM harmonic divergence in the range of 20 – 40 mrad [16, 17], the pulse energy per harmonic will thus be higher than the above stated values by up to a factor of two.
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Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12325
Fig. 3. (a) At the full lasers repetition rate of 10 Hz the ROM harmonic signal is found to recede within minutes. (b) A running average of the energy of the 10th harmonic shows a reduction by more than one order of magnitude within two minutes.
For comparison to the VHS tapes, which are coated with iron-oxide on the backside, we applied a transparent acrylic glass tape as a target material using identical laser conditions. Consecutive spectra of an experiment running one full hour are shown in Fig. 2(c). It is noticeable at first glance that the background level from the acrylic glass tape is much lower than from the VHS tapes. This is due to reduced plasma radiation from a target without heavy elements. Such a low background would be beneficial, e.g. for ionization experiments requiring high-quality XUV pulses and low background signal. Furthermore, the run with the acrylic glass tape shows a higher average XUV pulse energy as compared to the VHS tape. An integrated and rms-averaged energy of 0.90 µJ is measured (Fig. 2(d)). After subtracting the continuous background, the ROM harmonic energy yields 0.34 µJ per XUV pulse. Nevertheless, the stability of the harmonic signal remains an issue. A standard deviation of 85% (75%) of the energy of the harmonic signal after background subtraction has been recorded for the acrylic glass (VHS) target. On the one hand, intensity fluctuations of the laser (2% rms) have a strong influence on the fluctuations of the XUV source since cascaded nonlinear processes have been used (SHG + ROM harmonic generation). Notably, ROM harmonic generation is extremely nonlinear at the low relativistic intensities used in our experiment. Even for mechanically stable glass targets, we measured a stability of 30% rms under similar laser conditions [17]. Using higher peak energies and an energy-stabilized laser system would increase the stability of the ROM harmonic source significantly. On the other hand, using a pilot laser, we observed noticeable motions of the tape due to the impact of the laser pulse and the ablating plasma during this experiment. For this reason and in order to shield the adjacent surface area from debris, the target mount is equipped with mechanical guides. We expect that the performance can be improved when the tension of the tape target is increased and thicker foils are used. Furthermore, interferometric online monitoring and active stabilization of the surface position in a feedback loop could be used in order to increase the mechanical stability [21]. When the laser system is operated at the full repetition rate of 10 Hz, it was observed that the ROM harmonic signal started decreasing within minutes (Fig. 3). As an example, Fig. 3(b) shows the energy of the 10th order harmonic over time, which is already reduced by one order of magnitude after two minutes. This is due to absorption and heat accumulation at the gold gratings in the compressor of the laser stage. As a consequence of the thermally induced laser beam distortions, the focal spot quality is deteriorated [32]. The use of gratings with dielectric coating inside the compressor would significantly reduce the heat load and thus allows higher repetition rates. This should be considered in the design of high intensity lasers that operate at high repetition rate. 4. Conclusion In summary, we demonstrated relativistic surface high-harmonic generation in a continuous operation mode over runtimes of up to one hour from spooling tape targets. XUV spectra containing µJ energies and harmonic frequencies up to 70 eV are measured using a calibrated
#225842 - $15.00 USD © 2015 OSA
Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12326
spectrometer setup. The prospect of generating ROM harmonics at laser systems with higher peak intensity and under optimization of the plasma scale length, e.g. by utilizing a controlled prepulse [33], promises an increase in XUV pulse energy and spectral extend. Although technically challenging, advancing ROM harmonic sources from single-shot mode to a stable long-term operating XUV source will pave the way for applications such as highly timeresolved XUV spectroscopy, nonlinear attosecond experiments or the seeding of free-electron lasers. For laser systems exceeding the moderately relativistic intensities used in this experiment, relativistic surface high-harmonic generation using a spooling tape configuration might become an essential part of an attosecond beam line. Acknowledgments This work was supported by Deutsche Forschungsgemeinschaft (SFB TR18). C.R. acknowledges support from the VolkswagenStiftung. Burgard Beleites and Falk Ronneberger have contributed to this work by operating the JETI facility.
#225842 - $15.00 USD © 2015 OSA
Received 28 Oct 2014; revised 30 Jan 2015; accepted 30 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012321 | OPTICS EXPRESS 12327
