High-order Harmonic Generation of Laser Radiation in Plasmas:

1. Imperial College London, 9 November 2010 High-order Harmonic Generation of Laser Radiation in Plasmas: Recent Achievements and Perspectives R. A. Ganeev Institute of Electronics, Tashkent 100125, Uzbekistan BRING YOUR MUGS!!

2. Outline • Introduction: why, who, what? • Recent achievements: attosecond pulses, two-color pump-induced odd and even harmonics, broadband harmonics, nanoparticle/fullerene/nanotube harmonics, extension of harmonic cutoff, etc • Perspectives: high-power harmonics from nanoclusters at 1 kHz pulse repetition rate, few-cycle pump-induced harmonics, attosecond experiments, IR pump studies, etc

3. Introduction Plasma harmonics: • Why? Resonances, nanoparticles, fullerenes, nanotubes, variable plasmas, various solid-state elements of periodic table • Who? Experiment: ISSP (Japan), RRCAT (India), INRC (Canada),Instituto de Quìmica Física (Spain), LOA (France), Kansas University (USA) Imperial College London (UK), Politecnico di Milano (Italy), DCU (Ireland), WWU (Germany) Theory:GPI (Russia), Voronezh SU (Russia), Moscow SU (Russia), Max-Born-Institut (Germany), Universitá di Palermo (Italy), Samarkand SU (Uzbekistan) • What? 101st harmonic, 7.9 nm, 10-4 conversion efficiency for the single harmonic, two-color pump-induced enhancement of odd and even harmonics, SPR-induced enhancement of fullerene harmonics, harmonics in nanoparticle plasma, resonance enhancement, etc

4. What it looks like? Harmonic spectrum from silver plasma

5. Recent achievements: Attosecond pulses Two-color pump-induced odd and even harmonics Broadband harmonics Nanoparticle/fullerene/nanotube harmonics Extension of harmonic cutoff Resonant harmonics

6. Attosecond pulses The first attosecond pulse train reconstruction of high-order harmonics generated from plasma has been performed. It was demonstrated that, as with gas medium, plasma medium can also be used to generate attosecond pulses in the extreme ultraviolet range. The harmonics from chromium plasma were generated, with five harmonics between the 11th to the 19th order showing attosecond pulse duration. By using the method of Reconstruction of Attosecond Beating by Interference of Two-photon Transitions (RABITT), attosecond pulse trains were characterized with 300 as duration. AIP Conf. Proc. LASER-DRIVEN RELATIVISTIC PLASMAS APPLIED TO SCIENCE, INDUSTRY AND MEDICINE, 254 (2009).

7. Attosecond pulses

8. Phys. Rev. A 80, 033845 (2009), Phys. Rev. A (2010) (to be published) Two-color pump-induced odd and even harmonics

9. Broadband harmonics We demonstrate a considerable broadening of the harmonics generating in laser plasma using the laser pulses propagating through the filaments in air. A fourfold increase of harmonic bandwidth was achieved in the case of phase-modulated laser pulses, alongside with an increase in harmonic conversion efficiency (from 510−6 to 1.210−5). APL 95, 201117 (2009)

10. Nanoparticle harmonics Typical HHG spectra from silver nanoparticles (solid line) and bulk silver target (dashed line). It may be noted that the intensity of HHG spectrum from bulk Ag target is 10 multiplied for better visibility. The intensity of the 9th H from Ag nanoparticles is ~ 40 times higher compared to the 9th H from bulk silver. Harmonic generation from two-color laser pulse (solid line) and only fundamental laser pulse (dashed line). Both even and odd orders of harmonics are generated from two-color laser pulse. J. Phys. B 43, 025603 (2010) Phys. Rev. A 82, 043812 (2010)

11. Nanoparticle harmonics J. Phys. B 42, 055402 (2009) APL 94, 111108 (2009) Laser Part. Beams 28, 69 (2010) J. Appl. Phys. 106, 023104 (2009)

12. Fullerene harmonics A comparison between the harmonic generation in single-atom/ion-containing plasmas (using bulk carbon, silver, and indium targets) and fullerene-rich plasma plumes showed better conversion efficiency for the latter medium. J. Appl. Phys. 106, 103103 (2009) Appl. Phys. B 100, 581 (2010)

13. Fullerene harmonics Phys. Rev. Lett. 102, 013903 (2009) Phys. Rev. A 80, 043808 (2009)

14. Carbon nanotube harmonics ( a ) ( b ) ( c ) TEM image of the deposited CNTs. The length of marked line is 20 nm. Harmonic and plasma spectra obtained from the CNT plasma during focusing of femtosecond pulse: a) before the plasma plume, b) inside the plasma plume, and c) after the plasma plume. Phys. Rev. A, submitted

15. 95H 31H 17H Extension of harmonic cutoff Cd Cr Mn V Harmonic spectral patterns obtained during “over-excitation” of Cd, Cr, Mn, and V plumes APL 94, 051101 (2009)

16. Extension of harmonic cutoff Harmonic spectra from (a) V and (b) Mn plasmas obtained after optimization of harmonic excitation using the time-resolved technique. (1) Optimization for the first plateau distribution. (2) Optimization for the second plateau distribution.

17. Resonant harmonics It was suggested the 4-step model of the resonant HHG due to the autoionizing state, and developed the numerical and analytical approaches to describing this process. This model predicts the phase-locking of the resonant harmonics which leads to production of intense but relatively long attosecond pulses. PRL 104, 123901 (2010)

18. Resonant harmonics Experimentally observed strong enhancement of a single high-order harmonic in harmonic generation from low-ionized laser plasma ablation is explained as resonant harmonic generation. The resonant harmonic intensity increases regularly with the increase of the laser intensity, while the phase of the resonant harmonic is almost independent of the laser intensity. This is in sharp contrast with the usual plateau and cutoff harmonics whose intensity exhibits wild oscillations while their phase changes rapidly with the laser intensity. The results were illustrated using examples of tin (Sn) and antimony (Sb) plasmas. Phys. Rev. A 81, 023802 (2010)

19. Resonant harmonics The study was devoted to finding an explanation of the observed phenomena of resonance enhancement of high-order harmonics in indium plasma. The computations were performed on the base of time-dependent density functional theory (TDDFT). The results of TDDFT calculation of HHG for indium ion were found in good correspondence with experimental data. This allowed us for the first time to introduce a theory of resonant recombination for HHG. It can also be used to predict the most promising targets for resonant HHG and to increase its efficiency by control of the pump radiation’s parameters. Phys. Rev. A, 81, 063825 (2010)

20. Perspectives • High-power harmonics from nanoclusters at 1 kHz pulse repetition rates • Few-cycle pump-induced harmonics • Molecular orientation in plasma plumes • Attosecond experiments • Plasma manipulations • Carbon-containing plasma: perspectives of application for plasma HHG • Long-wavelength femtosecond pump

21. Westfäliche Wilhelms-Universität , Münster High-power harmonics from nanoclusters at 1 kHz pulse repetition rates For soft x-ray microscopy and (imaging) photoelectron spectroscopy high repetition rate sources are desired. The Münster group has also ample experience in laser-produced plasmas. Such plasmas generated from intense (1016 to 1017 W/cm2) Ti:sapphire radiation are used to produce incoherent hard x-ray radiation. Although such high intensities are not required in the present application the experience how to handle debris and other plasma related problemsis well-known to the group. In all cases a movable tape target is employed. Laser plasma generated on a moving tape target at 1 kHz repetition rate.

22. Politecnico di Milano Attosecond experiments This radiation will be used for attosecond plasma HHG studies. We may expect an improvement of plasma HHG due to the provision of a regime of waveguide pump propagation through the plasma medium, feasibility analysis of attosecond pulse generation in laser-produced plasma, etc. All these concepts are new and did not applied previously for the improvement of plasma harmonic generation efficiency. Together with old approaches, such as search of the attosecond pulse generation by achieving the continuum in the harmonic emission near the cutoff, this allows for the first time the systematic analysis of generation of ultrashort laser pulses in the plasma plumes. It was demonstrated a novel configuration that allows one to significantly enhance the spectral broadening characteristics of the hollow-fiber technique. Using two hollow fibers separated by a compression stage, a supercontinuum has been generated, which extends to a bandwidth exceeding 510 THz, with excellent spatial beam quality. High-peak-power, sub-7-fs light pulses tunable from the visible to the near infrared have been generated by compression of portions of the supercontinuum, employing different sets of chirped mirrors.

23. Raja Ramanna Centre for Advanced Technology Further improvements of plasma HHG We intend to carry out the following studies of plasma HHG to further investigate the nonlinear optical properties of laser plumes: 1. Two-color pump HHG: search of new schemes 2. Double pump by heating pulses: search for modification of dispersion of laser plasma 3. Applications of plasma harmonics: irradiation of organic and nanoparticle-containing films 4. Manipulation of plasma characteristics: nanoparticle formation at high fluences

24. Imperial College London Few-cycle pump-induced harmonics The milestones of the proposed studies include • search of the continuum in the plasma harmonic emission near the cutoff (a characteristic signature for attosecond pulse generation), • comparison of the HHG from gas jets and from laser plasma, • search for the conditions of attosecond pulse trains generation, • measurements of the HHG from fullerenes and nanoparticles employing a sub-10fs drive field, • influence of molecular orientation in plasma plumes on the HHG, • application of longer wavelength sources for plasma HHG, • etc.

25. Acknowledgements Author thanks H. Kuroda, P. D. Gupta, P. A. Naik, and T. Ozaki for fruitful discussions and support at various stages of these studies. Author is indebted to M. Suzuki, H. Singhal, L. B. Elouga Bom, J. A. Chakera, M. Baba, U. Chakravarty, I. A. Kulagin, P. V. Redkin, R. A. Khan, M. Raghuramaiah, V. Arora, S. R. Kumbhare, R. P. Kushwaha, M. Tayyab, V. P. Bhardwaj, and D. B. Milosevic for their contribution during these studies. Author acknowledges the support from Japanese Society for the Promotion of Sciences, Raja Ramanna Centre for Advanced Technology, National Sciences and Engineering Research Council of Canada, TWAS-UNESCO Research Grant, and Marie Curie Fellowship. Author enjoyed the discussions with J. Marangos (Imperial College London), M. Danailov (ELETTRA, Trieste), H. Zacharias (Westfäliche Wilhelms-Universität , Münster), P. De Silvestri (Politechnico di Milano), J. Costello (DCU, Dublin), E. Fiordilino (Universita di Palermo), and V. Strelkov (General Phisics Institute, Moscow) regarding future joint experimental and theoretical studies of plasma harmonics.

26. Thank you for your attention!