PIs: Louis DiMauro & Pierre Agostini

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1 Interaction of Clusters with Intense, Long Wavelength Fields PIs: Louis DiMauro & Pierre Agostini project objective: explore intense laser-cluster interactions in the strong-field limit project approach: use intense, ultra-fast long wavelength (1-4 µm) drivers and charged particle detection vary cluster size (atomic to nano-scales) & constituents theoretical collaboration with Prof. J. M. Rost (MPI-Dresden)

2 ultra-fast nano-plasma picture ATOMIC MANY-BODY & PLASMA NEUTRAL INNER IONIZATION HEATING + EXPANSION RELAXATION laser pulse initiates atomic-ionization seeding electrons free electrons are heated by quiver motion quiver induces many-body interactions laser off, cluster relaxes via Coulomb or hydrodynamics several studies in the near-visible and EUV, even in the hard-x-rays

3 ultra-fast nano-plasma picture ATOMIC MANY-BODY & PLASMA control NEUTRAL INNER IONIZATION HEATING + EXPANSION RELAXATION laser pulse initiates atomic-ionization seeding electrons free electrons are heated by quiver motion quiver induces many-body interactions laser off, cluster relaxes via Coulomb or hydrodynamics intense, ultrafast (2-8 cycles) mid-infrared (1 4 µm) pulses

4 motivation: accessing the classical limit a quantum view: argon, 200 TW/cm µm the classical behavior mimics the quantum picture 2 longer wavelength the classical picture coalesces around the quantum result Tate et al. PRL 98, (2007)

5 λ-scaling for strong-field parameters strong-field regime Keldysh (ionization mode) optical frequency γ = (I tunneling frequency p /2U p ) 1/2 λ -1 Ι -1/2 ponderomotive potential (quiver energy) U p λ 2 Ι attochirp (harmonic atomic dispersion) 2 φ/ 2 ω τ f /U p λ -1

6 λ-scaling for strong-field parameters strong-field regime Keldysh (ionization mode) optical frequency γ = (I tunneling frequency p /2U p ) 1/2 λ -1 Ι -1/2 ponderomotive potential (quiver energy) U p λ 2 Ι bound-state & continuum intensity parameters (Krainov, Reiss, Smirnov) z 1 γ -2 λ 2 and z U p /hν λ 3 strong-field condition: γ < 1 and z 1 & z >> 1

7 intensity (au) strong-field scaling in the optical regime au bound continua perturbative limit frequency (au) W/cm 2 tunneling U p > hν e - quiver photon energy (ev) AFOSR Meeting 19 December 2012

8 intensity (au) strong-field scaling in the optical regime au bound continua perturbative limit frequency (au) W/cm 2 tunneling U p > hν e - quiver photon energy (ev) MIR accesses the SF regime at low intensity before neutral depletion AFOSR Meeting 19 December 2012

9 elastic rescattering: wavelength scaling ponderomotive potential: U p λ 2 Ι measured xenon photoelectron energy constant Ι electron counts (au) 0.8 µm 1.3 µm 2.0 µm 4.0 µm γ = 0.3 γ = 1.3 longer wavelengths produce higher electron energies! P. Colosimo et al. Nat. Phys. 4, 386 (2008)

10 elastic rescattering: wavelength scaling ponderomotive potential: U p λ 2 Ι measured xenon photoelectron energy constant Ι electron counts (au) 0.8 µm 1.3 µm 2.0 µm 4.0 µm γ = 0.3 γ = 1.3 classical limit (no interaction) xenon evolves towards the tunneling limit P. Colosimo et al. Nat. Phys. 4, 386 (2008) classical limit (elastic scatter)

11 HHG frequency comb for different λ, constant Ι argon HHG, neon PES detector 0.8 µm 110 TW/cm µm 2 µm at constant intensity, the photon energy is increased by λ 2

12 ion signal AFOSR Meeting 19 December 2012 rescattering: wave packet and inelastic cross-sections 1E+6 helium, 0.8 µm neon, 0.8 µm 1E+4 He + He 2+ 1E+2 E m = 150 ev 1E+0 1E-2 1E-4 NS ADK (He µm inelastic (e,2e) collisions results in double ionization of helium maximum return energy: 3U p 150 ev > I p (He + ) 1E-6 1E+14 1E+15 1E+16 intensity (W/cm 2 ) B. Walker group, PRL 94, (2005)

13 ion signal AFOSR Meeting 19 December 2012 rescattering: wave packet and inelastic cross-sections 1E+6 helium, 0.8 µm neon, 0.8 µm 1E+4 He + He 2+ 1E+2 1E+0 1E-2 NS ADK (He + ) 1E-4 1E-6 1E+14 1E+15 1E+16 intensity (W/cm 2 ) B. Walker group, PRL 94, (2005)

14 rescattering: wave packet and inelastic cross-sections γ xenon, 4 µm ADK (Xe) E m = µm highly charged ion (n = 2-6) production before neutral depletion ADK (Xe + 4 µm inelastic (e,ne) collisions results in multiple ionization of xenon maximum return energy: 3U p 300 ev > ΣI p (O shell) 244 ev

15 so we think we know everything? photoelectron 2 µm, 0.15 PW/cm 2 normalized electron counts U p Ar H 2 N 2 ADK U p 60 ev γ electron energies (ev)

16 existence of a low-energy structure (LES) photoelectron 2 µm, 0.15 PW/cm 2 normalized electron counts (linear scale) Ar H 2 N 2 SFA γ electron energies (ev) the low-energy structure is not predicted by SFA or KFR LES is universal feature of tunnel ionization large effect: ½ the electrons Blaga et al., Nature Physics 5, (2009)

17 OSU cluster apparatus design initial studies on inert gas clusters mono/bichromatic fields & pump-probe configurations Even-Lavie supersonic valve, M 100 differentially pumped chamber for e - and m/q analysis