Enhanced High Harmonic Generation from Multiply Ionized Argon

PRL 103, 143901 (2009)

PHYSICAL REVIEW LETTERS

week ending 2 OCTOBER 2009

Enhanced High Harmonic Generation from Multiply Ionized Argon above 500 eV through Laser Pulse Self-Compression P. Arpin,1,* T. Popmintchev,1 N. L. Wagner,1,2 A. L. Lytle,1,3 O. Cohen,1,4 H. C. Kapteyn,1 and M. M. Murnane1 1

JILA and Department of Physics, University of Colorado, Boulder, Colorado 80309-0440, USA 2 NOAA/ESRL/CSD, 325 Broadway, Boulder, Colorado 80305, USA 3 Physics Department, Hamilton College, Clinton, New York 13323, USA 4 Physics Department, Technion, Haifa, Israel (Received 20 December 2008; published 29 September 2009)

By combining laser pulse self-compression and high harmonic generation within a single waveguide, we demonstrate high harmonic emission from multiply charged ions for the first time. This approach enhances the laser intensity and counteracts ionization-induced defocusing, extending the cutoff photon energy in argon above 500 eV for the first time, with higher spectral intensity and cutoff energy than He for the same input laser parameters. This Letter demonstrates a pathway for extending high harmonic emission to very high photon energies using large, multiply charged, ions with high ionization potentials. DOI: 10.1103/PhysRevLett.103.143901

PACS numbers: 42.65.Ky, 42.65.Sf, 52.38.r

High-order harmonic generation (HHG) is a useful source of ultrafast [1,2], fully spatially coherent [3], soft x-ray beams with applications in ultrafast molecular and materials spectroscopy [4–7] as well as in high resolution imaging [8]. Although harmonics have been generated at photon energies of a few keV [9], to date most applications have been limited to the extreme ultraviolet region of the spectrum because of the challenge in extending bright harmonic emission beyond 100 eV. Hence, by enhancing high harmonics at shorter wavelengths, the potential exists to extend applications to wavelengths spanning the entire soft and hard x-ray regions of the spectrum. Two main challenges to extending bright harmonic emission to shorter wavelengths prevail: first, demonstrating full or quasiphase-matching techniques over extended distances at high photon energies [10,11] and, second, the extension of the maximum photon energy cutoff for HHG from large atoms and ions [12,13]. HHG from large ions with high ionization potentials would have several advantages: high effective nonlinear susceptibilities, compatibility with a guiding geometry for the laser, and very high photon cutoffs [14] given by hc ¼ Ip þ 3:2Up . Here Ip is the ionization potential of an atom or ion and Up is the ponderomotive energy that characterizes the energy gained by an electron in an oscillating electric field [Up / I2 , where I is the peak intensity, and is the laser wavelength]. This cutoff rule scales linearly with laser intensity, and has been observed to be valid in all experiments in both neutral atoms and ions to date [12,13]. Using few-cycle laser pulses, harmonics generated in He and Ne, have been extended to >2 keV [9]. For large atoms such as Ar, Kr, and Xe, however, harmonics at high photon energies (e.g., >150 eV in Ar) must be generated from ions. This presents a significant challenge because the presence of a free-electron plasma in an ionized medium dramatically reduces the coherence length for HHG emission, while also leading to ionization-induced laser beam defocusing. As a 0031-9007=09=103(14)=143901(4)

result, significant HHG from Ar ions has only recently been observed in either gas-filled hollow waveguides (250 eV) [15] or plasma waveguides (275 eV) [13], since these geometries counteract plasma-induced defocusing to maintain high laser intensity. Recent work reported an extension of the cutoff from Arþ ions to 360 eV [16]. In that work, multimode quasiphase matching was proposed as a mechanism that allowed for significant coherent buildup of harmonics at high driving intensities. However, the spatiotemporal evolution of the laser pulse in the rapidly ionizing medium was not considered [17,18], which as we show here, can significantly extend the HHG cutoff. Moreover, the observation of discrete harmonic peaks in the water window or in fully ionized media is unexpected, because phase modulation of the laser as it rapidly ionizes the medium usually leads to a quasicontinuum HHG emission. In past work, discrete harmonic peaks were observed from ions only when the medium was preionized by a capillary discharge prior to harmonic generation [12,13]. In this Letter, we demonstrate experimentally that by combining laser pulse self-compression with high harmonic generation within a single gas-filled waveguide, we can generate harmonics from multiply ionized species for the first time. The increase in laser intensity as a result of pulse compression allows us to extend the cutoff photon energy in Ar (specifically Ar2þ ) to >540 eV, which represents a significant extension of the maximum photon emission by 180 eV over previously reported observations. We note that these cutoff photon energies in Ar are higher than has been achieved using any other approach to date— including the use of very short driving pulses or midinfrared laser drivers. We show that the pulse selfcompression mechanism that operates in an ionizing medium reduces the driving laser pulse duration from 38 to 19 fs, while increasing the peak intensity by 20% and counteracting ionization-induced defocusing of the laser.

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Ó 2009 The American Physical Society

PRL 103, 143901 (2009)


These observations are significant for several reasons. First, they demonstrate HHG emission from multiply charged ions for the first time. Large ions can be used to extend the HHG brightness and cutoff photon energy significantly using quasiphase matching schemes. Second, our observations represent a significant extension of the highest HHG photon energies generated from Ar—from 360 to 540 eV. Finally and most importantly, through our direct measurements of pulse compression of the laser, we identify a dominant mechanism for how very high-order harmonics are generated from ions. Laser pulse self-compression has been observed in both short gas-filled hollow waveguides [17] and in filaments [19,20]. In both cases, nonlinear processes such as selfphase modulation broaden the spectrum of the pulse, while spatiotemporal effects compress the pulse in space and time without the need for external dispersion compensation. Past work that first investigated self-compression in a hollow waveguide observed a reduction in pulse duration from 30 to 13 fs [17]. This scheme was then used to enhance the flux of selected harmonics generated in neutral Ar at photon energies near 95 eV, using a self-quasiphase matching process [18]. The use of near-single-cycle pulses, compressed in a filamentation process, to generate harmonics has also been theoretically investigated [21]. No work to date has explored how the pulse self-compression mechanism can be used to extend the HHG cutoff photon energy or to generate harmonics from multiply charged ions. In our experiment, 3 mJ pulses from a Ti:sapphire laser amplifier system with transform-limited pulse duration of 24 fs, at a repetition rate of 1 kHz, were focused into a 150 m diameter, 2 cm long, hollow-core waveguide. To isolate the influence of the self-compression from the influence of the mode beating on the increase of the laser peak intensity, we implemented two schemes. First, the beam waist at the entrance of the fiber was selected to be 65% of the radius of the waveguide. This configuration ensures optimum coupling into the EH11 mode of the hollow waveguide [22], thus reducing energy loss and suppressing significant excitations of high-order modes. Second, the HHG emissions from Ar, He, and Ne gases were compared. Since Ar can ionize rapidly to Ar2þ at the laser intensities used in this experiment (2:2 1015 W cm2 ), the self-compression mechanism will primarily operate in Ar. In contrast, both Ne and He are singly ionized to 100% and 95% ionization levels, respectively. In these experiments, the gas was inserted into the waveguide through a small hole placed 5 mm from the entrance of the waveguide. A pressure gradient was maintained in the waveguide by evacuating through another hole placed 5 mm from the exit of the waveguide, in addition to the differential pumping to vacuum at both ends of the waveguide. This resulted in an interaction region of 1 cm with a pressure ramp, where the laser entered through the high pressure end.


The laser pulse was characterized both before and after the waveguide using second-harmonic frequency-resolved optical gating [23]. In order to generate the brightest harmonics, the laser pulse width was adjusted to optimize the flux of the highest harmonics generated in Ar. Under these conditions, the pulse width measured after the waveguide was 38 fs, chirped from the transform limit of 24 fs, when no gas was present in the waveguide. (The additional linear chirp required to optimize the harmonics may improve the self-pulse compression process by increasing the spectral broadening [24].) The pulse spectrum and duration did not change significantly when low pressure He was introduced into the waveguide (10 torr). In He, the ionization peaks around 95% at the maximum laser intensity of 2:2 1015 W cm2 present in the waveguide without pulse compression. In contrast, when the waveguide is filled with 4 torr of Ar, the rapid ionization (peaking around 300%, corresponding to fully depleted Ar2þ ) leads to strong spatiotemporal self-compression of the pulse [17]—from 38 fs with no gas present in the wave-

FIG. 1 (color online). (a) Comparison of the pulse temporal profile and phase measured with the waveguide filled with no gas and with 4 torr of Ar. (Inset) Corresponding spectra, illustrating the blue shift and broadening that reduce the transform-limited pulse duration. (b) Measured pulse width at various pressures showing that as the gas pressure is increased, the duration continues to decrease to 14 fs at 9–10 torr.

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guide to 2:6 1015 W cm2 . Under optimal conditions of self-compression, the highest photon energies generated in Ar extend up to 543 eV (see Fig. 2), representing an increase of >180 eV from that previously observed [16]. At these high ionization levels, the HHG spectrum forms a quasicontinuum as expected, and discrete peaks are not visible [12,13]. The spectrum was calibrated using the Sc absorption edge at 400 eV and the Ti edge at 454 eV, and the oxygen edge at 543 eV can also be seen due to contamination on the optics and filters. Numerical simulations show that harmonic emission from Ar above 370 eV must emerge from Ar2þ ions [see Fig. 2(c)]. For comparison, HHG spectra were taken in He and Ne for the same input laser energy and with the pressure optimized for highest harmonic generation in He and Ne (10 torr). Figure 3(b) shows that the photon energy cutoff in Ar extends 100 eV beyond that in He and 200 eV beyond the cutoff in Ne. For the He and Ar data shown in Fig. 3, integration times of 30 minutes were used, because of the weak, nonphase matched, HHG signals. The HHG spectra of Fig. 3 can be understood by considering Ammosov-Delone-Krainov (ADK) ionization given the successive ionization potentials of the three atoms—He: 24.6 eV, 54.4 eV; Ne: 21.6 eV, 41.0 eV; and Ar: 15.8 eV, 27.6 eV, and 40.7 eV. In the case of He, the ionization is not saturated (95%), and the 450 eV maximum represents the cutoff photon energy for the laser intensity in the absence of pulse compression. The ionization rate depends on both the ionization potential and the charge of the remaining ion. Thus, although the ionization potential of doubly ionized Ar (Ar2þ I.P. 40.7 eV) is very close to that of singly ionized Ne (Neþ I.P. 41.0 eV), the Ar2þ is significantly further ionized, while the Neþ is not. According to ADK calculations, a peak intensity high enough to fully ionize Ar2þ would result in 25% ionization of Neþ . Therefore, the lower HHG cutoff observed in Ne at 280 eV indicates that both the neutral Ne is fully depleted (100% ionized) and the laser intensity is too low to ionize Neþ . In contrast, the pulse self-compression


FIG. 2 (color online). (a) Transmission curves for the filters: 0:5 m Sc (green), 0:4 m Ti (blue), and two 0:2 m Cr assuming a 20 nm layer of Cr2 O3 at each surface (red). (b) Harmonic emission from a 150 m inner diameter, 2 cm long fiber filled with low pressure Ar (4 torr), driven by a self-compressed 19 fs pulse. The dashed black line indicates background noise level. The edge around 543 eV is due to oxide contamination layers formed on both the filters and optics. (c) Calculated harmonic emission from neutral (cyan), singly ionized Ar (black), and doubly ionized Ar (magenta) for a 19 fs laser pulse centered at 800 nm, at a laser intensity of 2:5 1015 W cm2 , showing that all harmonics above 370 eV in Ar are generated in Ar2þ . Each spectrum has an arbitrary offset for comparison.

mechanism in the multiply ionizing Ar (300%) enhances the laser peak intensity in Ar compared with Ne, allowing higher HHG cutoffs of 543 eV to be reached. As discussed below, HHG between 370 eVand 520 eV must emerge only from doubly ionized Ar2þ being further ionized to Ar3þ . Moreover, for photon energies greater than 380 eV, HHG emission from Ar ions is brighter ( 5) than that observed from He, although He is significantly more transparent in this photon energy range, and the high levels of ionization in Ar significantly reduce the coherence length. Assuming that the coherence length (Lc ) is dominated by the freeelectron plasma [25] and that the observed HHG emission in this nonphase matched regime will be proportional to ½Lc ðdensity of emittersÞ2 , we conclude that the effective HHG yield from Ar2þ is 45 times larger than that of He.

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bright harmonics can be emitted from multiply charged ions, in this case fully depleted Ar2þ . The cutoff photon energy in argon was significantly extended to 540 eV photon energies for the first time, with a higher brightness than that observed in neutral He for the same laser input energy. In the future, high harmonic emission can be extended to very high photon energies using multiply charged ions in combination with quasiphase matching and midinfrared techniques [11,28,29]. The authors gratefully acknowledge support for this work from the NSF Engineering Research Center on EUV Science and Technology and the Department of Energy. P. A. acknowledges support from the NSF IGERT program. FIG. 3 (color online). (a) Filter transmission for the Ar data shown in (b). (b) Comparison of spectra taken in 10 torr He (black), 4 torr Ar (blue), and 10 torr Ne (red). The spectra for He and Ar compare the same number of shots with the same filters, which provides a direct comparison of brightness. The data taken in Ne are a comparison of cutoff photon energies and not of absolute brightness.

Figure 4 plots the ADK [26] argon ion populations as a function of time within the pulse, with the HHG cutoff energy plotted on one of the vertical axes. A significant Ar3þ population starts to appear at laser intensities corresponding to a single-atom cutoff of 240 eV. Even taking into account that the ADK rates might overestimate the ionization levels by 10% [27], we can nevertheless conclude that harmonic emission between 370 and 520 eV must emerge only from doubly ionized Ar2þ being further ionized to Ar3þ . In conclusion, by combining laser pulse selfcompression with high harmonic generation within a single gas-filled waveguide, we demonstrate for the first time that

FIG. 4 (color online). Calculated ionization levels in argon for a 19 fs laser pulse at a peak laser intensity of 2:5 1015 W cm2 , using ADK rates. Laser pulse (black); Ar (blue); Arþ (green); Ar2þ (red); Ar3þ (pink); Ar4þ (brown). The right axis shows the predicted HHG cutoff energy for a particular laser intensity, calculated from the cutoff rule.

*[email protected] [1] I. P. Christov, M. M. Murnane, and H. C. Kapteyn, Phys. Rev. Lett. 78, 1251 (1997). [2] A. Baltuska et al., Nature (London) 421, 611 (2003). [3] R. A. Bartels et al., Science 297, 376 (2002). [4] L. Miaja-Avila et al., Phys. Rev. Lett. 101, 046101 (2008). [5] E. Gagnon et al., Science 317, 1374 (2007). [6] A. S. Sandhu et al., Science 322, 1081 (2008). [7] W. Li et al., Science 322, 1207 (2008). [8] R. L. Sandberg et al., Proc. Natl. Acad. Sci. U.S.A. 105, 24 (2008). [9] E. Seres, J. Seres, and C. Spielmann, Appl. Phys. Lett. 89, 181 919 (2006). [10] T. Popmintchev et al., Opt. Lett. 33, 2128 (2008). [11] O. Cohen et al., Phys. Rev. Lett. 99, 053902 (2007). [12] D. M. Gaudiosi et al., Phys. Rev. Lett. 96, 203001 (2006). [13] B. A. Reagan et al., Phys. Rev. A 76, 013816 (2007). [14] K. C. Kulander, K. J. Schafer, and J. L. Krause, in SuperIntense Laser-Atom Physics, edited by B. Piraux, A. L’Huillier, and K. Rzazewski (Plenum Press, New York, 1993), p. 95. [15] E. A. Gibson et al., Phys. Rev. Lett. 92, 033001 (2004). [16] M. Zepf et al., Phys. Rev. Lett. 99, 143901 (2007). [17] N. L. Wagner et al., Phys. Rev. Lett. 93, 173902 (2004). [18] X. S. Zhang et al., Opt. Lett. 30, 1971 (2005). [19] C. P. Hauri et al., Appl. Phys. B 79, 673 (2004). [20] A. Mysyrowicz, A. Couairon, and U. Keller, New J. Phys. 10, 025 023 (2008). [21] H. S. Chakraborty, M. B. Gaarde, and A. Couairon, Opt. Lett. 31, 3662 (2006). [22] R. L. Abrams, IEEE J. Quantum Electron. 8, 838 (1972). [23] K. W. DeLong et al., J. Opt. Soc. Am. B 11, 2206 (1994). [24] J. Y. Park, J. H. Lee, and C. H. Nam, Opt. Express 16, 4465 (2008). [25] E. A. Gibson et al., Science 302, 95 (2003). [26] M. V. Ammosov, N. B. Delone, and V. P. Krainov, Zh. Eksp. Teor. Fiz. 91, 2008 (1986) [Sov. Phys. JETP 64, 1191 (1986)]. [27] V. P. Krainov, J. Opt. Soc. Am. B 14, 425 (1997). [28] H. Kapteyn et al., Science 317, 775 (2007). [29] T. Popmintchev et al., Proc. Natl. Acad. Sci. U.S.A. 106, 10 516 (2009).

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