OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 1 — Jan. 2, 2012
  • pp: 90–100
« Show journal navigation

Enhancement of high harmonics from plasmas using two-color pump and chirp variation of 1 kHz Ti:sapphire laser pulses

R. A. Ganeev, C. Hutchison, A. Zaïr, T. Witting, F. Frank, W. A. Okell, J. W. G. Tisch, and J. P. Marangos  »View Author Affiliations


Optics Express, Vol. 20, Issue 1, pp. 90-100 (2012)
http://dx.doi.org/10.1364/OE.20.000090


View Full Text Article

Acrobat PDF (1331 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We have investigated resonance effects in high-order harmonic generation (HHG) within laser-produced plasmas. We demonstrate a significantly improved harmonic yield by using two-color pump-induced enhancement and a 1 kHz pulse repetition rate. Together with an increased HHG output, the even harmonics in the cutoff region were enhanced with respect to odd harmonics. We report the observation of a resonance-induced growth in intensity of 20th harmonic in silver plasma (2×), 26th harmonic in vanadium plasma (4×), and 28th harmonic in chromium plasma (5×).

© 2011 OSA

1. Introduction

Enhancement of HHG in gaseous media as a result of atomic and ionic resonances has been the subject of extensive theoretical work over the last ten years [1

1. E. S. Toma, P. Antoine, A. Bohan, and H. G. Muller, “Resonance-enhanced high-harmonic generation,” J. Phys. B 32(24), 5843–5852 (1999). [CrossRef]

5

5. R. Taïeb, V. Veniard, J. Wassaf, and A. Maquet, “Roles of resonances and recollisions in strong-field atomic phenomena. II. High-order harmonic generation,” Phys. Rev. A 68(3), 033403 (2003). [CrossRef]

]. The most significant experimental results in this field were, however, reported for HHG in plasma plumes. Weakly-ionized plasmas from some solid targets displayed resonances in the excitation of neutrals at certain harmonic wavelengths [6

6. M. Suzuki, M. Baba, R. Ganeev, H. Kuroda, and T. Ozaki, “Anomalous enhancement of a single high-order harmonic by using a laser-ablation tin plume at 47 nm,” Opt. Lett. 31(22), 3306–3308 (2006). [CrossRef] [PubMed]

8

8. M. Suzuki, M. Baba, H. Kuroda, R. A. Ganeev, and T. Ozaki, “Intense exact resonance enhancement of single-high-harmonic from an antimony ion by using Ti:Sapphire laser at 37 nm,” Opt. Express 15(3), 1161–1166 (2007). [CrossRef] [PubMed]

]. These experiments also showed that singly charged ions can enhance the yield of specific harmonics. As a much wider range of solids are available compared with gaseous target materials, HHG studies in plasma plumes dramatically increase the chance of finding an ionic transition, which is resonant with a harmonic wavelength.

Several mechanisms explaining the origin of resonant harmonics in laser-produced plasmas have recently been proposed [9

9. I. A. Kulagin and T. Usmanov, “Efficient selection of single high-order harmonic caused by atomic autoionizing state influence,” Opt. Lett. 34(17), 2616–2618 (2009). [CrossRef] [PubMed]

13

13. M. Tudorovskaya and M. Lein, “High-order harmonic generation in the presence of a resonance,” Phys. Rev. A 84(1), 013430 (2011). [CrossRef]

]. In [9

9. I. A. Kulagin and T. Usmanov, “Efficient selection of single high-order harmonic caused by atomic autoionizing state influence,” Opt. Lett. 34(17), 2616–2618 (2009). [CrossRef] [PubMed]

], it was shown that the influence of atomic autoionizing states on the phase matching of HHG in calcium plasma may result in efficient selection of a single harmonic. This was the first report of efficient high-order harmonic selection by autoionizing states. The four-step model, developed in [10

10. V. Strelkov, “Role of autoionizing state in resonant high-order harmonic generation and attosecond pulse production,” Phys. Rev. Lett. 104(12), 123901 (2010). [CrossRef] [PubMed]

], predicts the enhanced generation efficiency for the harmonic resonant with the transition between the ground and the autoionizing state of the ion. In this model, the third (recombination) step of the three-step model of HHG [14

14. J. L. Krause, K. J. Schafer, and K. C. Kulander, “High-order harmonic generation from atoms and ions in the high intensity regime,” Phys. Rev. Lett. 68(24), 3535–3538 (1992). [CrossRef] [PubMed]

,15

15. P. B. Corkum, “Plasma perspective on strong field multiphoton ionization,” Phys. Rev. Lett. 71(13), 1994–1997 (1993). [CrossRef] [PubMed]

] is divided into two steps: the capture of a laser-accelerated electron into an autoionizing state of the parent ion, and the relaxation of this state to the ground state accompanied by the emission of a harmonic photon.

It was found by Milošević [12

12. D. B. Milošević, “Resonant high-order harmonic generation from plasma ablation: Laser intensity dependence of the harmonic intensity and phase,” Phys. Rev. A 81(2), 023802 (2010). [CrossRef]

] that the laser intensity dependence of the intensity and phase of the single harmonic generated in the resonant HHG from plasma ablation is different than that of the standard plateau and cutoff high harmonics. The resonant harmonic intensity increases continuously (i.e., without rapid oscillations) with the increase of the laser intensity, while the resonant harmonic phase is almost constant. Such unusual for HHG behaviour of the harmonic phase requires a detailed experimental investigation. Namely, the harmonic phase dependence is important for synchronization of high-order harmonics. The importance of the results [12

12. D. B. Milošević, “Resonant high-order harmonic generation from plasma ablation: Laser intensity dependence of the harmonic intensity and phase,” Phys. Rev. A 81(2), 023802 (2010). [CrossRef]

] was highlighted by the recent reconstruction of a train of attosecond pulses produced through HHG in an ablation plasma [16

16. L. B. Elouga Bom, S. Haessler, O. Gobert, M. Perdrix, F. Lepetit, J. F. Hergott, B. Carré, T. Ozaki, and P. Salières, “Attosecond emission from chromium plasma,” Opt. Express 19(4), 3677–3685 (2011). [CrossRef] [PubMed]

]; the group of odd harmonics responsible for the pulse train encompassed a resonant harmonic. In order to understand HHG in the presence of such a resonance, a time-frequency analysis was performed in [13

13. M. Tudorovskaya and M. Lein, “High-order harmonic generation in the presence of a resonance,” Phys. Rev. A 84(1), 013430 (2011). [CrossRef]

]. Consistent with the predictions of the four-step model [10

10. V. Strelkov, “Role of autoionizing state in resonant high-order harmonic generation and attosecond pulse production,” Phys. Rev. Lett. 104(12), 123901 (2010). [CrossRef] [PubMed]

], it was found that the resonance gives rise to a single harmonic in the HHG spectrum.

Resonant enhancements in HHG from metal and semiconductor targets ablated by picosecond pulses [17

17. R. A. Ganeev, “High-order harmonic generation in a laser plasma: a review of recent achievements,” J. Phys. B 40(22), R213–R253 (2007). [CrossRef]

,18

18. R. A. Ganeev, “Generation of high-order harmonics of high-power lasers in plasmas produced under irradiation of solid target surfaces by a prepulse,” Phys. Usp. 52(1), 55–77 (2009). [CrossRef]

] have been previously studied using a single pump pulse at λ ≈800 nm. However, when HHG is driven by a two-color pump scheme, emission spectra can display both odd and even harmonics, increasing the chance of a spectral overlap with an ionic resonance. Another promising route towards locating additional resonances is to tune the harmonic wavelength. This could be achieved by various methods: tuning the fundamental wavelength of laser pulse [7

7. R. A. Ganeev, M. Suzuki, M. Baba, H. Kuroda, and T. Ozaki, “Strong resonance enhancement of a single harmonic generated in the extreme ultraviolet range,” Opt. Lett. 31(11), 1699–1701 (2006). [CrossRef] [PubMed]

,19

19. B. Shan, A. Cavalieri, and Z. Chang, “Tunable high harmonic generation with an optical parametric amplifier,” Appl. Phys. B 74(9), s23–s26 (2002). [CrossRef]

]; chirping the laser radiation [20

20. Z. Chang, A. Rundquist, H. Wang, I. Christov, H. C. Kapteyn, and M. M. Murnane, “Temporal phase control of soft-x-ray harmonic emission,” Phys. Rev. A 58(1), R30–R33 (1998). [CrossRef]

23

23. R. A. Ganeev, P. A. Naik, H. Singhal, J. A. Chakera, and P. D. Gupta, “Strong enhancement and extinction of single harmonic intensity in the mid- and end-plateau regions of the high harmonics generated in weakly excited laser plasmas,” Opt. Lett. 32(1), 65–67 (2007). [CrossRef] [PubMed]

]; altering the laser intensity to control the ionization rate of the nonlinear medium [24

24. H. Kim, D. Lee, K.-H. Hong, J.-H. Kim, I. Choi, and C. Nam, “Continuously tunable high-order harmonics from atoms in an intense femtosecond laser field,” Phys. Rev. A 67(5), 051801 (2003). [CrossRef]

27

27. R. A. Ganeev, M. Suzuki, P. V. Redkin, M. Baba, and H. Kuroda, “Variable pattern of high-order harmonic spectra from a laser-produced plasma by using the chirped pulses of narrow-bandwidth radiation,” Phys. Rev. A 76(2), 023832 (2007). [CrossRef]

]; and adaptive pulse-shaping [28

28. D. H. Reitze, S. Kazamias, F. Weihe, G. Mullot, D. Douillet, F. Aug, O. Albert, V. Ramanathan, J. P. Chambaret, D. Hulin, and P. Balcou, “Enhancement of high-order harmonic generation at tuned wavelengths through adaptive control,” Opt. Lett. 29(1), 86–88 (2004). [CrossRef] [PubMed]

,29

29. T. Popmintchev, M.-C. Chen, P. Arpin, M. M. Murnane, and H. C. Kapteyn, “The attosecond nonlinear optics of bright coherent X-ray generation,” Nat. Photonics 4(12), 822–832 (2010). [CrossRef]

].

An attractive feature of resonant enhancement is that it can increase the conversion efficiency of a specific harmonic by more than an order of magnitude [17

17. R. A. Ganeev, “High-order harmonic generation in a laser plasma: a review of recent achievements,” J. Phys. B 40(22), R213–R253 (2007). [CrossRef]

]. In this paper, we combine resonant enhancement with two-color pumping at 1 kHz pulse repetition rate to generate strong harmonics in different spectral ranges. This unique source will be ideal for various applications in physics, chemistry and biology, and for advancing nonlinear x-ray optics and attosecond physics.

2. Experimental setup

These experiments were performed using a 1 kHz Ti:sapphire chirped pulse amplification (CPA) laser (Red Dragon, KML Inc.) delivering 2.5 mJ pulses of 40 fs duration at 780 nm. In these experiments a portion (1.5 mJ, 20 ps) of the uncompressed pulse was split from the beam line prior to the laser compressor stage. This pulse was focused to an intensity of Ips = 5 × 109 W cm−2 to 3 × 1010 W cm−2 on the target using a f = 400 mm lens, as shown in Fig. 1
Fig. 1 Experimental setup: HPP, heating pump pulse; FPP, femtosecond probe pulse; M, mirrors; VC, vacuum chamber; T, target; FM, focusing mirror; FFG, flat field grating; MCP, microchannel plate; CCD, charge coupled device.
. The beam spot of the ablation laser was measured to be 0.4 mm at the target surface. The target was moved constantly up and down manually during these experiments.

The compressed pulse (30 fs, 1 mJ) was focused into the plasma in a direction orthogonal to that of the picosecond pulse using a f = 200 mm mirror. The position of the focus with respect to the plasma plume was chosen to maximize the harmonic signal. The intensity at the focus of the femtosecond pulse was estimated to be Ifs = 5×1014 W cm−2. The delay between plasma initiation and femtosecond pulse was varied in the range of 6 – 57 ns using an optical delay line. A flat-field grating (1200 lines/mm, Hitachi) and imaging microchannel plate (Photonis USA, Inc.) with a CCD camera were used to record the high-harmonic spectrum.

To drive HHG using two colors, the second harmonic (2ω) of the fundamental (ω) femtosecond pulse was generated using a 0.5-mm-thick BBO crystal in a type-I phase matching scheme. Group velocity dispersion between the ω and 2ω pulses in nonlinear crystal was compensated for using a calcite plate. The second harmonic conversion efficiency was 4%. HHG was enhanced by the presence of the 2ω field despite the 25:1 energy ratio between the ω and 2ω pulses.

The chirp of the femtosecond laser pulse was tuned by adjusting the separation of the two gratings in the CPA compressor. Only the wavelength at the leading edge of the pulse contributes significantly to HHG because at the intensity used the strong field induced plasma grows increasingly with time, eventually preventing HHG; thus the wavelengths of the high-harmonic comb from a chirped pulse can be controlled through this ionization gating effect [20

20. Z. Chang, A. Rundquist, H. Wang, I. Christov, H. C. Kapteyn, and M. M. Murnane, “Temporal phase control of soft-x-ray harmonic emission,” Phys. Rev. A 58(1), R30–R33 (1998). [CrossRef]

,21

21. H. T. Kim, J. H. Kim, D. G. Lee, K. H. Hong, Y. S. Lee, V. Tosa, and C. H. Nam, “Optimization of high-order harmonic brightness in the space and time domains,” Phys. Rev. A 69(3), 031805 (2004). [CrossRef]

,24

24. H. Kim, D. Lee, K.-H. Hong, J.-H. Kim, I. Choi, and C. Nam, “Continuously tunable high-order harmonics from atoms in an intense femtosecond laser field,” Phys. Rev. A 67(5), 051801 (2003). [CrossRef]

,25

25. V. Tosa, H. T. Kim, I. J. Kim, and C. H. Nam, “High-order harmonic generation by chirped and self-guided femtosecond laser pulses. II. Time-frequency analysis,” Phys. Rev. A 71(6), 063808 (2005). [CrossRef]

]. We calibrated the chirp by measuring the spectrum, spectral phase, and pulse duration of the laser pulses as a function of grating separation using SPIDER [32

32. C. Iaconis and I. A. Walmsley, “Spectral phase interferometry for direct electric-field reconstruction of ultrashort optical pulses,” Opt. Lett. 23(10), 792–794 (1998). [CrossRef] [PubMed]

].

3. Results

In this section we present the results of single- and two-color HHG studies using three different target materials: silver, chromium and vanadium. The used targets showed best stability at high pulse repetition rate among other materials, where resonance induced enhancement has previously been observed using the low (10 Hz) pulse repetition sources. The optimum delay between heating and driving pulses was found to be 40 ns for these three targets. Through the chirping technique, we are able to locate resonant enhancements of single odd, and in some cases even, harmonics.

Figures 2a
Fig. 2 Harmonic spectra from Ag plasma in the cases of (a) apertureless and (b) apertured single color pump (780 nm). (c) Tuning of 17th and 19th harmonics by changing the distance between the gratings in the compressor stage. Positive and negative values of pulse duration correspond to positively and negatively chirped pulses. Dotted lines show the tuning of 17th and 19th harmonics with different chirps. Black lines show the wavelengths of these harmonics at chirp-free conditions. Thick black lines on the left side of bottom graph show the tuning range of 17th harmonic (2.8 nm).
and 2b show harmonic spectra generated in silver plasma in the cases of apertureless and apertured single color pump (780 nm). Harmonics above the 50th order were routinely generated. Apertureless pump means the single color pump without introduction of aperture in front of input window of vacuum chamber. Apertured pump means that the 7 mm diameter pump beam propagates through the 4 mm aperture before entering the vacuum chamber. Introduction of 4 mm aperture on the path of driving radiation in front of vacuum chamber changed the relative distribution of harmonics from being stronger for lower orders in the case of apertureless beam (Fig. 2a) to stronger higher order harmonics in the case of apertured beam (Fig. 2b) due to the propagation effects changing the phase matching conditions for different groups of harmonics.

Variation of the laser chirp allowed a considerable tuning of harmonic wavelengths (2.8 nm in the case of 17th harmonic, Fig. 2c), while the relative intensities of harmonics in the plateau region remained approximately the same over a broad range of the driving laser chirps, which led to variation of pulse duration in the range of −97 fs to +110 fs (positive and negative values of pulse duration correspond to positively and negatively chirped pulses). Note that effective tuning of harmonic wavelengths through chirp can be achieved only for broadband pulses, where a pronounced difference in the wavelength components at the leading and trailing parts of the pulse can be produced. The pulses used in our experiments have a bandwidth of ~40 nm FWHM. In the case of variable chirp conditions, the conversion efficiency decreased by a factor 6 in the case of 97 fs negatively chirped pulses compared with 30 fs chirp-free pulses.

An interesting feature of the spectrum in Fig. 3a is the enhanced even (20th) harmonic compared with neighboring even harmonics, although this enhancement (2×) was not as pronounced as those previously reported in studies of single odd harmonics from some plasmas [17

17. R. A. Ganeev, “High-order harmonic generation in a laser plasma: a review of recent achievements,” J. Phys. B 40(22), R213–R253 (2007). [CrossRef]

]. No spectral line was identified from the NIST tables in this part of the spectrum, but the behavior was similar to resonance enhancements in other metal plasmas. One can note that this ionic line appeared along with harmonic spectra at stronger excitation of silver target.

We identified two regimes for HHG in chromium plasma displaying different spectral features. For a weak excitation of the target, a cutoff was observed at 31.2 nm (E = 39.74 eV, 25H). With increased target excitation and increased femtosecond pulse intensity inside the plume by moving the plasma towards the focus of 780 nm radiation, a second plateau appeared with strong 27th and 29th harmonics, and the cutoff was extended toward the range of E = 58.81 eV (37th harmonic). For zero-chirp the 27th and 29th harmonics were approximately equal in intensity. Varying the laser chirp changed the relative intensities of these two harmonics, while the intensities of the other harmonics remained approximately the same. As the chirp was varied and the pulse duration vas changed from +114 fs to −92 fs, the intensity of 27th harmonic considerably increased (15×), while the 29th harmonic became weaker, as seen in Fig. 4a
Fig. 4 (a) Harmonic spectra from chromium plasma at different chirps of laser radiation. Positive and negative values of pulse duration correspond to positively and negatively chirped pulses. (b) Harmonic spectrum at over-excited conditions of Cr plasma formation, with ionic lines appearing close to the enhanced 27th and 29th harmonics. The arrow shows one of these lines close to the 27th harmonic.
. This is because the 27th harmonic (λ = 28.88 nm, E = 42.91 eV for zero-chirp) is shifted towards the ionic resonance. The presence of such a resonance is confirmed by over-exciting the chromium plasma; the ionic line can be seen at the blue side of 27th harmonic on Fig. 4b.

The meaning of “weak” excitation stays for intensity of heating pulse on the target surface below 1×1010 W cm−2. In that case, chromium plasma emission shows only spectral lines from excited neutrals and singly charged ions. With increase of intensity (≈2×1010 W cm−2 - 3×1010 W cm−2), the doubly charged ions appear in the plasma plume, which led to their involvement in harmonic generation. Over-excitation of plasma at these conditions means even stronger heating of target when high concentration of free electrons appearing during laser ablation causes the phase mismatching of HHG.

These results are consistent with other HHG studies in Cr plasma, which also displayed a variation in the 27th harmonic intensity for different chirps [40

40. R. A. Ganeev, M. Suzuki, M. Baba, and H. Kuroda, “Harmonic generation from chromium plasma,” Appl. Phys. Lett. 86(13), 131116 (2005). [CrossRef]

]. In those studies, the 27th harmonic almost disappeared from the harmonic spectra, and a strong 29th harmonic was observed for zero-chirp. The experiments in [40

40. R. A. Ganeev, M. Suzuki, M. Baba, and H. Kuroda, “Harmonic generation from chromium plasma,” Appl. Phys. Lett. 86(13), 131116 (2005). [CrossRef]

] were performed using 800 nm laser pulses, so the 29th harmonic was closest to the resonance.

Calculated gf values at photon energies range of 40–60 eV (λ = 20.66 – 31 nm) clearly show a group of transitions from 44.5 to 44.8 eV with very strong oscillator strengths (gf between 1 and 2.2; the gf value is the product of the oscillator strength f of a transition and the statistical weight g of the lower level.) [42

42. C. McGuinness, M. Martins, P. Wernet, B. F. Sonntag, P. Kampen, J.-P. Mosnier, E. T. Kennedy, and J. T. Costello, “Metastable state contributions to the measured 3p photoabsorption spectrum of Cr+ ions in a laser-produced plasma,” J. Phys. B 32(20), L583–L591 (1999). [CrossRef]

]. These transitions are much stronger than those in the range of 40 – 60 nm, and are likely to be responsible for the enhancement of the 27th and 29th harmonics we observed in our experiments. Furthermore, strong photoabsorption lines within the 41–42 eV region reported in [42

42. C. McGuinness, M. Martins, P. Wernet, B. F. Sonntag, P. Kampen, J.-P. Mosnier, E. T. Kennedy, and J. T. Costello, “Metastable state contributions to the measured 3p photoabsorption spectrum of Cr+ ions in a laser-produced plasma,” J. Phys. B 32(20), L583–L591 (1999). [CrossRef]

] could decrease the yield of the 25th harmonic.

In our case, the dynamics of vanadium harmonics variations was the same as in the case of chromium plasma once the chirp and corresponding redistribution of laser spectrum along the pulse caused the tuning of harmonic wavelength. Only low-order harmonics (up to the 23rd order) were obtained when the V target was weakly excited (Ips = 6×109 W cm−2, Fig. 6a
Fig. 6 Variations of harmonic spectra at (a) weak excitation of V target (Ips = 6×109 W cm−2), (b) stronger excitation of target (Ips = 1×1010 W cm−2), and (c) application of two-color pump.
). With a stronger excitation (Ips = 1×1010 W cm−2), the spectrum extended to higher energies, with the appearance of a “second plateau” beginning with a strong 27th harmonic, as shown in Fig. 6b. Two-color pumping led to an increase in the harmonic yield (Fig. 6c), accompanied by the appearance of an enhanced (4×) 26th harmonic (λ = 30 nm, E = 41.33 eV), which is attributed to the influence of a strong ionic transition at this energy. The background that appears in spectra of Fig. 6 is induced by a scattering from some stray light, which appeared from case to case during different alignments of our registration system.

We have not yet identified the ionic transition responsible for this enhancement. The NIST database contains one relatively strong transition (3p63d - 3p5(2P°)3d2(1G); λ = 31.2 nm) in this spectral region for quadruply ionized vanadium, but it is unlikely that such a high level of ionization was achieved using our laser ablation parameters.

5. Discussion and conclusions

Our two-color HHG studies confirm previously reported spectral features in the vicinity of the 3p - 3d transitions of the Cr II ions [40

40. R. A. Ganeev, M. Suzuki, M. Baba, and H. Kuroda, “Harmonic generation from chromium plasma,” Appl. Phys. Lett. 86(13), 131116 (2005). [CrossRef]

] and reveal a strong even harmonic generated close to those transitions. The observed enhancement of the 28th harmonic of the 780 nm pump (corresponding to the 14th harmonic of 390 nm radiation) is attributed to the influence of these transitions, although it is not as pronounced compared with the 29th harmonic of 800 nm radiation. The enhancement of the 27th and 28th harmonics from vanadium plasma can also be attributed to an overlap with ionic transitions. Although we were unable to identify these ionic transitions, the ionic lines were seen by over-exciting the target.

Calculations of harmonic enhancement for some plasmas, in particular for the chromium ion, can be found in [10

10. V. Strelkov, “Role of autoionizing state in resonant high-order harmonic generation and attosecond pulse production,” Phys. Rev. Lett. 104(12), 123901 (2010). [CrossRef] [PubMed]

,11

11. M. V. Frolov, N. L. Manakov, and A. F. Starace, “Potential barrier effects in high-order harmonic generation by transition-metal ions,” Phys. Rev. A 82(2), 023424 (2010). [CrossRef]

]. The coincidence of the enhanced harmonics observed in our experiments with the giant 3p - 3d resonance implies the involvement of this transition in the enhancement. The resonant enhancement can also be considered from a macroscopic perspective. At resonance conditions, when the harmonic frequency is close to the atomic transition frequency, the variation in the wave number of a single harmonic could be large, and the influence of dispersion from free-electrons can be compensated by the atomic dispersion for a specific harmonic order [9

9. I. A. Kulagin and T. Usmanov, “Efficient selection of single high-order harmonic caused by atomic autoionizing state influence,” Opt. Lett. 34(17), 2616–2618 (2009). [CrossRef] [PubMed]

]. In this case, improvement of the phase matching conditions for single harmonic can be achieved.

We would like to emphasize the importance of using high pulse repetition rate lasers for improving the average power of harmonics, which could be additionally enhanced in the presence of resonance effects and two-color field induced enhancement. This is a main motivation of present work and has been demonstrated for the first time.

From the point of view of optical technology where the average power of the XUV radiation may be critical the demonstration at a 1kHz is essential. So not only does the result ratify earlier findings but it paves the way to concrete applications. Some estimations show that, once usual non-resonant conversion efficiency is of order of ~10−6, with resonance this may be increased by an order of magnitude or higher (~10−5). So for a mJ class laser we expect at least 10nJ per pulse i.e. average power 10 microwatts of XUV at 1kHz, but only 0.1 microwatt at 10Hz. Present work is a further advancement in this field, which shows considerable improvement of the characteristics of plasma harmonics compared with results presented in [31

31. R. A. Ganeev, C. Hutchison, T. Siegel, M. E. López-Arias, A. Zaïr, and J. P. Marangos, “High-order harmonic generation from metal plasmas using 1 kHz laser pulses,” J. Mod. Opt. 58(10), 819–824 (2011). [CrossRef]

].

In conclusion, we have studied HHG from laser-produced plasmas using three recently introduced techniques to improve the plasma harmonic yield: resonance-induced and two-color pump-induced enhancement, together with the application of a high repetition rate laser source. Together with an increased HHG output, we observed an enhanced yield of even harmonics compared with odd ones, and resonance-induced enhancement in the intensity of some single even harmonics when the ratio between the 780 and 390 nm pulse energies was 25:1.

During our experiments, several new resonance-enhanced processes were revealed, mostly due to the modification of electron trajectories in HHG by the presence of a weak second harmonic field. This led to the generation of enhanced odd and even harmonics in different regions of the plateau. In particular, we observed an enhanced 20th harmonic in silver plasma (2×), 26th harmonic in vanadium plasma (4×), and 28th harmonic in chromium plasma (5×). These results support theoretical predictions that the involvement of autoionizing states of atoms and ions will enhance nonlinear optical processes in the vicinity of a resonance [9

9. I. A. Kulagin and T. Usmanov, “Efficient selection of single high-order harmonic caused by atomic autoionizing state influence,” Opt. Lett. 34(17), 2616–2618 (2009). [CrossRef] [PubMed]

,10

10. V. Strelkov, “Role of autoionizing state in resonant high-order harmonic generation and attosecond pulse production,” Phys. Rev. Lett. 104(12), 123901 (2010). [CrossRef] [PubMed]

]. Crucially, using broadband (40 nm) pulses for plasma HHG permitted tuning of the spectral position of the harmonic comb over a broad range (2.8 nm in the case of 17th harmonic), thus allowing us to ensure that a particular harmonic coincided with the resonance wavelength of an autoionizing state.

Acknowledgments

This research was supported by a Marie Curie International Incoming Fellowship within the 7th European Community Framework Programme and EPSRC programme (grants No. EP/F034601/1, EP/I032517/1, and EP/E028063/1).

References and links

1.

E. S. Toma, P. Antoine, A. Bohan, and H. G. Muller, “Resonance-enhanced high-harmonic generation,” J. Phys. B 32(24), 5843–5852 (1999). [CrossRef]

2.

M. B. Gaarde and K. J. Schafer, “Enhancement of many high-order harmonics via a single multiphoton resonance,” Phys. Rev. A 64(1), 013820 (2001). [CrossRef]

3.

Z. Zeng, R. Li, Y. Cheng, W. Yu, and Z. Xu, “Resonance-enhanced high-order harmonic generation and frequency mixing in two-color laser field,” Phys. Scr. 66(4), 321–325 (2002). [CrossRef]

4.

R. Bartels, S. Backus, E. Zeek, L. Misoguti, G. Vdovin, I. P. Christov, M. M. Murnane, and H. C. Kapteyn, “Shaped-pulse optimization of coherent emission of high-harmonic soft X-rays,” Nature 406(6792), 164–166 (2000). [CrossRef] [PubMed]

5.

R. Taïeb, V. Veniard, J. Wassaf, and A. Maquet, “Roles of resonances and recollisions in strong-field atomic phenomena. II. High-order harmonic generation,” Phys. Rev. A 68(3), 033403 (2003). [CrossRef]

6.

M. Suzuki, M. Baba, R. Ganeev, H. Kuroda, and T. Ozaki, “Anomalous enhancement of a single high-order harmonic by using a laser-ablation tin plume at 47 nm,” Opt. Lett. 31(22), 3306–3308 (2006). [CrossRef] [PubMed]

7.

R. A. Ganeev, M. Suzuki, M. Baba, H. Kuroda, and T. Ozaki, “Strong resonance enhancement of a single harmonic generated in the extreme ultraviolet range,” Opt. Lett. 31(11), 1699–1701 (2006). [CrossRef] [PubMed]

8.

M. Suzuki, M. Baba, H. Kuroda, R. A. Ganeev, and T. Ozaki, “Intense exact resonance enhancement of single-high-harmonic from an antimony ion by using Ti:Sapphire laser at 37 nm,” Opt. Express 15(3), 1161–1166 (2007). [CrossRef] [PubMed]

9.

I. A. Kulagin and T. Usmanov, “Efficient selection of single high-order harmonic caused by atomic autoionizing state influence,” Opt. Lett. 34(17), 2616–2618 (2009). [CrossRef] [PubMed]

10.

V. Strelkov, “Role of autoionizing state in resonant high-order harmonic generation and attosecond pulse production,” Phys. Rev. Lett. 104(12), 123901 (2010). [CrossRef] [PubMed]

11.

M. V. Frolov, N. L. Manakov, and A. F. Starace, “Potential barrier effects in high-order harmonic generation by transition-metal ions,” Phys. Rev. A 82(2), 023424 (2010). [CrossRef]

12.

D. B. Milošević, “Resonant high-order harmonic generation from plasma ablation: Laser intensity dependence of the harmonic intensity and phase,” Phys. Rev. A 81(2), 023802 (2010). [CrossRef]

13.

M. Tudorovskaya and M. Lein, “High-order harmonic generation in the presence of a resonance,” Phys. Rev. A 84(1), 013430 (2011). [CrossRef]

14.

J. L. Krause, K. J. Schafer, and K. C. Kulander, “High-order harmonic generation from atoms and ions in the high intensity regime,” Phys. Rev. Lett. 68(24), 3535–3538 (1992). [CrossRef] [PubMed]

15.

P. B. Corkum, “Plasma perspective on strong field multiphoton ionization,” Phys. Rev. Lett. 71(13), 1994–1997 (1993). [CrossRef] [PubMed]

16.

L. B. Elouga Bom, S. Haessler, O. Gobert, M. Perdrix, F. Lepetit, J. F. Hergott, B. Carré, T. Ozaki, and P. Salières, “Attosecond emission from chromium plasma,” Opt. Express 19(4), 3677–3685 (2011). [CrossRef] [PubMed]

17.

R. A. Ganeev, “High-order harmonic generation in a laser plasma: a review of recent achievements,” J. Phys. B 40(22), R213–R253 (2007). [CrossRef]

18.

R. A. Ganeev, “Generation of high-order harmonics of high-power lasers in plasmas produced under irradiation of solid target surfaces by a prepulse,” Phys. Usp. 52(1), 55–77 (2009). [CrossRef]

19.

B. Shan, A. Cavalieri, and Z. Chang, “Tunable high harmonic generation with an optical parametric amplifier,” Appl. Phys. B 74(9), s23–s26 (2002). [CrossRef]

20.

Z. Chang, A. Rundquist, H. Wang, I. Christov, H. C. Kapteyn, and M. M. Murnane, “Temporal phase control of soft-x-ray harmonic emission,” Phys. Rev. A 58(1), R30–R33 (1998). [CrossRef]

21.

H. T. Kim, J. H. Kim, D. G. Lee, K. H. Hong, Y. S. Lee, V. Tosa, and C. H. Nam, “Optimization of high-order harmonic brightness in the space and time domains,” Phys. Rev. A 69(3), 031805 (2004). [CrossRef]

22.

R. A. Ganeev, L. Bom, J.-C. Kieffer, and T. Ozaki, “Systematic investigation of resonance-induced single-harmonic enhancement in the extreme-ultraviolet range,” Phys. Rev. A 75(6), 063806 (2007). [CrossRef]

23.

R. A. Ganeev, P. A. Naik, H. Singhal, J. A. Chakera, and P. D. Gupta, “Strong enhancement and extinction of single harmonic intensity in the mid- and end-plateau regions of the high harmonics generated in weakly excited laser plasmas,” Opt. Lett. 32(1), 65–67 (2007). [CrossRef] [PubMed]

24.

H. Kim, D. Lee, K.-H. Hong, J.-H. Kim, I. Choi, and C. Nam, “Continuously tunable high-order harmonics from atoms in an intense femtosecond laser field,” Phys. Rev. A 67(5), 051801 (2003). [CrossRef]

25.

V. Tosa, H. T. Kim, I. J. Kim, and C. H. Nam, “High-order harmonic generation by chirped and self-guided femtosecond laser pulses. II. Time-frequency analysis,” Phys. Rev. A 71(6), 063808 (2005). [CrossRef]

26.

C. A. Froud, E. T. F. Rogers, D. C. Hanna, W. S. Brocklesby, M. Praeger, A. M. de Paula, J. J. Baumberg, and J. G. Frey, “Soft-x-ray wavelength shift induced by ionization effects in a capillary,” Opt. Lett. 31(3), 374–376 (2006). [CrossRef] [PubMed]

27.

R. A. Ganeev, M. Suzuki, P. V. Redkin, M. Baba, and H. Kuroda, “Variable pattern of high-order harmonic spectra from a laser-produced plasma by using the chirped pulses of narrow-bandwidth radiation,” Phys. Rev. A 76(2), 023832 (2007). [CrossRef]

28.

D. H. Reitze, S. Kazamias, F. Weihe, G. Mullot, D. Douillet, F. Aug, O. Albert, V. Ramanathan, J. P. Chambaret, D. Hulin, and P. Balcou, “Enhancement of high-order harmonic generation at tuned wavelengths through adaptive control,” Opt. Lett. 29(1), 86–88 (2004). [CrossRef] [PubMed]

29.

T. Popmintchev, M.-C. Chen, P. Arpin, M. M. Murnane, and H. C. Kapteyn, “The attosecond nonlinear optics of bright coherent X-ray generation,” Nat. Photonics 4(12), 822–832 (2010). [CrossRef]

30.

R. A. Ganeev, H. Singhal, P. A. Naik, J. A. Chakera, H. S. Vora, R. A. Khan, and P. D. Gupta, “Systematic studies of two-color pump-induced high-order harmonic generation in plasma plumes,” Phys. Rev. A 82(5), 053831 (2010). [CrossRef]

31.

R. A. Ganeev, C. Hutchison, T. Siegel, M. E. López-Arias, A. Zaïr, and J. P. Marangos, “High-order harmonic generation from metal plasmas using 1 kHz laser pulses,” J. Mod. Opt. 58(10), 819–824 (2011). [CrossRef]

32.

C. Iaconis and I. A. Walmsley, “Spectral phase interferometry for direct electric-field reconstruction of ultrashort optical pulses,” Opt. Lett. 23(10), 792–794 (1998). [CrossRef] [PubMed]

33.

E. Cormier and M. Lewenstein, “Optimizing the efficiency in high order harmonic generation optimization by two-color fields,” Eur. Phys. J. D 12(2), 227–233 (2000). [CrossRef]

34.

I. J. Kim, C. M. Kim, H. T. Kim, G. H. Lee, Y. S. Lee, J. Y. Park, D. J. Cho, and C. H. Nam, “Highly efficient high-harmonic generation in an orthogonally polarized two-color laser field,” Phys. Rev. Lett. 94(24), 243901 (2005). [CrossRef]

35.

J. Mauritsson, P. Johnsson, E. Gustafsson, A. L’Huillier, K. J. Schafer, and M. B. Gaarde, “Attosecond pulse trains generated using two color laser fields,” Phys. Rev. Lett. 97(1), 013001 (2006). [CrossRef] [PubMed]

36.

T. Pfeifer, L. Gallmann, M. J. Abel, D. M. Neumark, and S. R. Leone, “Single attosecond pulse generation in the multicycle-driver regime by adding a weak second-harmonic field,” Opt. Lett. 31(7), 975–977 (2006). [CrossRef] [PubMed]

37.

D. Charalambidis, P. Tzallas, E. P. Benis, E. Skantzakis, G. Maravelias, L A A. Nikolopoulos, A. Peralta Conde, and G. D. Tsakiris, “Exploring intense attosecond pulses,” New J. Phys. 10(2), 025018 (2008). [CrossRef]

38.

I. J. Kim, G. H. Lee, S. B. Park, Y. S. Lee, T. K. Kim, C. H. Nam, T. Mocek, and K. Jakubczak, “Generation of submicrojoule high harmonics using a long gas jet in a two-color laser field,” Appl. Phys. Lett. 92(2), 021125 (2008). [CrossRef]

39.

R. A. Ganeev, H. Singhal, P. A. Naik, I. A. Kulagin, P. V. Redkin, J. A. Chakera, M. Tayyab, R. A. Khan, and P. D. Gupta, “Enhancement of high-order harmonic generation using a two-color pump in plasma plumes,” Phys. Rev. A 80(3), 033845 (2009). [CrossRef]

40.

R. A. Ganeev, M. Suzuki, M. Baba, and H. Kuroda, “Harmonic generation from chromium plasma,” Appl. Phys. Lett. 86(13), 131116 (2005). [CrossRef]

41.

M. Suzuki, L. B. Elouga Bom, T. Ozaki, R. A. Ganeev, M. Baba, and H. Kuroda, “Seventy-first harmonic generation from doubly charged ions in preformed laser-ablation vanadium plume at 110 eV,” Opt. Express 15(7), 4112–4117 (2007). [CrossRef] [PubMed]

42.

C. McGuinness, M. Martins, P. Wernet, B. F. Sonntag, P. Kampen, J.-P. Mosnier, E. T. Kennedy, and J. T. Costello, “Metastable state contributions to the measured 3p photoabsorption spectrum of Cr+ ions in a laser-produced plasma,” J. Phys. B 32(20), L583–L591 (1999). [CrossRef]

OCIS Codes
(190.2620) Nonlinear optics : Harmonic generation and mixing
(190.4160) Nonlinear optics : Multiharmonic generation

ToC Category:
Nonlinear Optics

History
Original Manuscript: October 21, 2011
Revised Manuscript: November 21, 2011
Manuscript Accepted: November 21, 2011
Published: December 19, 2011

Citation
R. A. Ganeev, C. Hutchison, A. Zaïr, T. Witting, F. Frank, W. A. Okell, J. W. G. Tisch, and J. P. Marangos, "Enhancement of high harmonics from plasmas using two-color pump and chirp variation of 1 kHz Ti:sapphire laser pulses," Opt. Express 20, 90-100 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-1-90


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. E. S. Toma, P. Antoine, A. Bohan, and H. G. Muller, “Resonance-enhanced high-harmonic generation,” J. Phys. B32(24), 5843–5852 (1999). [CrossRef]
  2. M. B. Gaarde and K. J. Schafer, “Enhancement of many high-order harmonics via a single multiphoton resonance,” Phys. Rev. A64(1), 013820 (2001). [CrossRef]
  3. Z. Zeng, R. Li, Y. Cheng, W. Yu, and Z. Xu, “Resonance-enhanced high-order harmonic generation and frequency mixing in two-color laser field,” Phys. Scr.66(4), 321–325 (2002). [CrossRef]
  4. R. Bartels, S. Backus, E. Zeek, L. Misoguti, G. Vdovin, I. P. Christov, M. M. Murnane, and H. C. Kapteyn, “Shaped-pulse optimization of coherent emission of high-harmonic soft X-rays,” Nature406(6792), 164–166 (2000). [CrossRef] [PubMed]
  5. R. Taïeb, V. Veniard, J. Wassaf, and A. Maquet, “Roles of resonances and recollisions in strong-field atomic phenomena. II. High-order harmonic generation,” Phys. Rev. A68(3), 033403 (2003). [CrossRef]
  6. M. Suzuki, M. Baba, R. Ganeev, H. Kuroda, and T. Ozaki, “Anomalous enhancement of a single high-order harmonic by using a laser-ablation tin plume at 47 nm,” Opt. Lett.31(22), 3306–3308 (2006). [CrossRef] [PubMed]
  7. R. A. Ganeev, M. Suzuki, M. Baba, H. Kuroda, and T. Ozaki, “Strong resonance enhancement of a single harmonic generated in the extreme ultraviolet range,” Opt. Lett.31(11), 1699–1701 (2006). [CrossRef] [PubMed]
  8. M. Suzuki, M. Baba, H. Kuroda, R. A. Ganeev, and T. Ozaki, “Intense exact resonance enhancement of single-high-harmonic from an antimony ion by using Ti:Sapphire laser at 37 nm,” Opt. Express15(3), 1161–1166 (2007). [CrossRef] [PubMed]
  9. I. A. Kulagin and T. Usmanov, “Efficient selection of single high-order harmonic caused by atomic autoionizing state influence,” Opt. Lett.34(17), 2616–2618 (2009). [CrossRef] [PubMed]
  10. V. Strelkov, “Role of autoionizing state in resonant high-order harmonic generation and attosecond pulse production,” Phys. Rev. Lett.104(12), 123901 (2010). [CrossRef] [PubMed]
  11. M. V. Frolov, N. L. Manakov, and A. F. Starace, “Potential barrier effects in high-order harmonic generation by transition-metal ions,” Phys. Rev. A82(2), 023424 (2010). [CrossRef]
  12. D. B. Milošević, “Resonant high-order harmonic generation from plasma ablation: Laser intensity dependence of the harmonic intensity and phase,” Phys. Rev. A81(2), 023802 (2010). [CrossRef]
  13. M. Tudorovskaya and M. Lein, “High-order harmonic generation in the presence of a resonance,” Phys. Rev. A84(1), 013430 (2011). [CrossRef]
  14. J. L. Krause, K. J. Schafer, and K. C. Kulander, “High-order harmonic generation from atoms and ions in the high intensity regime,” Phys. Rev. Lett.68(24), 3535–3538 (1992). [CrossRef] [PubMed]
  15. P. B. Corkum, “Plasma perspective on strong field multiphoton ionization,” Phys. Rev. Lett.71(13), 1994–1997 (1993). [CrossRef] [PubMed]
  16. L. B. Elouga Bom, S. Haessler, O. Gobert, M. Perdrix, F. Lepetit, J. F. Hergott, B. Carré, T. Ozaki, and P. Salières, “Attosecond emission from chromium plasma,” Opt. Express19(4), 3677–3685 (2011). [CrossRef] [PubMed]
  17. R. A. Ganeev, “High-order harmonic generation in a laser plasma: a review of recent achievements,” J. Phys. B40(22), R213–R253 (2007). [CrossRef]
  18. R. A. Ganeev, “Generation of high-order harmonics of high-power lasers in plasmas produced under irradiation of solid target surfaces by a prepulse,” Phys. Usp.52(1), 55–77 (2009). [CrossRef]
  19. B. Shan, A. Cavalieri, and Z. Chang, “Tunable high harmonic generation with an optical parametric amplifier,” Appl. Phys. B74(9), s23–s26 (2002). [CrossRef]
  20. Z. Chang, A. Rundquist, H. Wang, I. Christov, H. C. Kapteyn, and M. M. Murnane, “Temporal phase control of soft-x-ray harmonic emission,” Phys. Rev. A58(1), R30–R33 (1998). [CrossRef]
  21. H. T. Kim, J. H. Kim, D. G. Lee, K. H. Hong, Y. S. Lee, V. Tosa, and C. H. Nam, “Optimization of high-order harmonic brightness in the space and time domains,” Phys. Rev. A69(3), 031805 (2004). [CrossRef]
  22. R. A. Ganeev, L. Bom, J.-C. Kieffer, and T. Ozaki, “Systematic investigation of resonance-induced single-harmonic enhancement in the extreme-ultraviolet range,” Phys. Rev. A75(6), 063806 (2007). [CrossRef]
  23. R. A. Ganeev, P. A. Naik, H. Singhal, J. A. Chakera, and P. D. Gupta, “Strong enhancement and extinction of single harmonic intensity in the mid- and end-plateau regions of the high harmonics generated in weakly excited laser plasmas,” Opt. Lett.32(1), 65–67 (2007). [CrossRef] [PubMed]
  24. H. Kim, D. Lee, K.-H. Hong, J.-H. Kim, I. Choi, and C. Nam, “Continuously tunable high-order harmonics from atoms in an intense femtosecond laser field,” Phys. Rev. A67(5), 051801 (2003). [CrossRef]
  25. V. Tosa, H. T. Kim, I. J. Kim, and C. H. Nam, “High-order harmonic generation by chirped and self-guided femtosecond laser pulses. II. Time-frequency analysis,” Phys. Rev. A71(6), 063808 (2005). [CrossRef]
  26. C. A. Froud, E. T. F. Rogers, D. C. Hanna, W. S. Brocklesby, M. Praeger, A. M. de Paula, J. J. Baumberg, and J. G. Frey, “Soft-x-ray wavelength shift induced by ionization effects in a capillary,” Opt. Lett.31(3), 374–376 (2006). [CrossRef] [PubMed]
  27. R. A. Ganeev, M. Suzuki, P. V. Redkin, M. Baba, and H. Kuroda, “Variable pattern of high-order harmonic spectra from a laser-produced plasma by using the chirped pulses of narrow-bandwidth radiation,” Phys. Rev. A76(2), 023832 (2007). [CrossRef]
  28. D. H. Reitze, S. Kazamias, F. Weihe, G. Mullot, D. Douillet, F. Aug, O. Albert, V. Ramanathan, J. P. Chambaret, D. Hulin, and P. Balcou, “Enhancement of high-order harmonic generation at tuned wavelengths through adaptive control,” Opt. Lett.29(1), 86–88 (2004). [CrossRef] [PubMed]
  29. T. Popmintchev, M.-C. Chen, P. Arpin, M. M. Murnane, and H. C. Kapteyn, “The attosecond nonlinear optics of bright coherent X-ray generation,” Nat. Photonics4(12), 822–832 (2010). [CrossRef]
  30. R. A. Ganeev, H. Singhal, P. A. Naik, J. A. Chakera, H. S. Vora, R. A. Khan, and P. D. Gupta, “Systematic studies of two-color pump-induced high-order harmonic generation in plasma plumes,” Phys. Rev. A82(5), 053831 (2010). [CrossRef]
  31. R. A. Ganeev, C. Hutchison, T. Siegel, M. E. López-Arias, A. Zaïr, and J. P. Marangos, “High-order harmonic generation from metal plasmas using 1 kHz laser pulses,” J. Mod. Opt.58(10), 819–824 (2011). [CrossRef]
  32. C. Iaconis and I. A. Walmsley, “Spectral phase interferometry for direct electric-field reconstruction of ultrashort optical pulses,” Opt. Lett.23(10), 792–794 (1998). [CrossRef] [PubMed]
  33. E. Cormier and M. Lewenstein, “Optimizing the efficiency in high order harmonic generation optimization by two-color fields,” Eur. Phys. J. D12(2), 227–233 (2000). [CrossRef]
  34. I. J. Kim, C. M. Kim, H. T. Kim, G. H. Lee, Y. S. Lee, J. Y. Park, D. J. Cho, and C. H. Nam, “Highly efficient high-harmonic generation in an orthogonally polarized two-color laser field,” Phys. Rev. Lett.94(24), 243901 (2005). [CrossRef]
  35. J. Mauritsson, P. Johnsson, E. Gustafsson, A. L’Huillier, K. J. Schafer, and M. B. Gaarde, “Attosecond pulse trains generated using two color laser fields,” Phys. Rev. Lett.97(1), 013001 (2006). [CrossRef] [PubMed]
  36. T. Pfeifer, L. Gallmann, M. J. Abel, D. M. Neumark, and S. R. Leone, “Single attosecond pulse generation in the multicycle-driver regime by adding a weak second-harmonic field,” Opt. Lett.31(7), 975–977 (2006). [CrossRef] [PubMed]
  37. D. Charalambidis, P. Tzallas, E. P. Benis, E. Skantzakis, G. Maravelias, L A A. Nikolopoulos, A. Peralta Conde, and G. D. Tsakiris, “Exploring intense attosecond pulses,” New J. Phys.10(2), 025018 (2008). [CrossRef]
  38. I. J. Kim, G. H. Lee, S. B. Park, Y. S. Lee, T. K. Kim, C. H. Nam, T. Mocek, and K. Jakubczak, “Generation of submicrojoule high harmonics using a long gas jet in a two-color laser field,” Appl. Phys. Lett.92(2), 021125 (2008). [CrossRef]
  39. R. A. Ganeev, H. Singhal, P. A. Naik, I. A. Kulagin, P. V. Redkin, J. A. Chakera, M. Tayyab, R. A. Khan, and P. D. Gupta, “Enhancement of high-order harmonic generation using a two-color pump in plasma plumes,” Phys. Rev. A80(3), 033845 (2009). [CrossRef]
  40. R. A. Ganeev, M. Suzuki, M. Baba, and H. Kuroda, “Harmonic generation from chromium plasma,” Appl. Phys. Lett.86(13), 131116 (2005). [CrossRef]
  41. M. Suzuki, L. B. Elouga Bom, T. Ozaki, R. A. Ganeev, M. Baba, and H. Kuroda, “Seventy-first harmonic generation from doubly charged ions in preformed laser-ablation vanadium plume at 110 eV,” Opt. Express15(7), 4112–4117 (2007). [CrossRef] [PubMed]
  42. C. McGuinness, M. Martins, P. Wernet, B. F. Sonntag, P. Kampen, J.-P. Mosnier, E. T. Kennedy, and J. T. Costello, “Metastable state contributions to the measured 3p photoabsorption spectrum of Cr+ ions in a laser-produced plasma,” J. Phys. B32(20), L583–L591 (1999). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited