Generation of short pulses 2.7 fs Ultrashort pulse generation 15 fs pulse Single cycle pulse Time [fs] Wavelength [m] Time [fs] Wavelength [m] Raman scattering and attosecond pulses Input two frequencies nearly resonant with a Raman resonance. At high intensity, the process cascades many times. Input pulses Output pulse as input to a second process Output pulse of second process as input to a third process Output pulse of third process as input to a fourth process Etc. 1 0 Raman processes can cascade many times, yielding a series of equally spaced modes =1+/- n0 S. E. Harris and A. V. Sokolov PRL 81, 2894 frequency Cascaded Raman generation A. V. Sokolov et al. PRL 85, 85 562 Dba = 2994 cm-1 This can be done with nanosecond laser pulses! Experimental demonstration of cascaded Raman scattering 2994 cm-1 Detuning from 2-photon resonance - 400MHz + 100MHz + 700MHz 75,000 cm-1 (2.3 x 1015 Hz) of bandwidth has been created! A. V. Sokolov et al. PRL 85, 562 Experimental demonstration of cascaded Raman scattering The different frequencies are locked Pulses with 1 fs duration are measured The spectrum is discrete: the pulses are emitted in a pulse train, separated by the vibrational period. The main advantage of this process: high efficiency The main drawback: the carrier frequency is in the visible regime We cannot produce an isolated pulse. A. V. Sokolov et al. PRL 85, 562 Breaking the femtosecond limit 2001: First observation of an attosecond pulse (650 as) M. Hentschel et al., Nature 414, 509-513 (2001) G. Sansone et al., Science 314, 443 (2006) 2006: (130 as) Our main tool: intense laser pulses Field Intensity: 1014 –1015 W/cm2 2.7 fs The force is comparable to the force binding the electrons in the atom or molecule. Attosecond pulse generation process Re-collision E>100eV Acceleration by the electric field Tunnel ionization I p Ek With I~1014 W/cm2 ~ 1000 cutoff Fundamental frequency E2 Ip 3 2 40 Attosecond pulse generation process Acceleration by the electric field Tunnel ionization Optical radiation with attoseconds duration Attosecond pulse generation process Classical model F ma eE E t E cos0t vt0 , t eE sin 0t sin 0t0 me0 eE cos0t0 cos0t 0 sin 0t0 t t0 xt0 , t me0 Ek t , t0 2U p sin 0t sin 0t0 2 e2 E 2 Up 4me02 Attosecond pulse generation process Classical model vt0 , t eE sin 0t sin 0t0 me0 eE cos0t0 cos0t 0 sin 0t0 t t0 xt0 , t me0 Attosecond pulse generation process Classical model The return times are determined such that x0(t,t0)=0 Ek t , t0 2U p sin 0t sin 0t0 2 Long trajectories Short trajectories Ek is the instantaneous frequency of the attosecond pulse Attosecond pulse generation process Quantum model 1 i x, t 2 V x E cos0t x x, t 2 The electron’s wavefunction t , x g x c x, t The induced dipole moment xt g x c The dynamics of the free electron is mapped into the optical field I FFTxt Electron wave packet dynamic Attosecond pulse Electron wave packet dynamic XUV field: Husimi reprsentation H , t F xx e t 2 / b2 Attosecond pulse generation process Classical model Elliptically polarized light: eE sin 0t sin 0t0 v x t0 , t me0 v y t0 , t eE cos0t cos0t0 me0 The electron is shifted in the lateral direction: the recollision probability reduces significantly Isolating a single attosecond pulse The multi-cycle regime E t E t E t 0.5 0 t n 0 E t E t 0.5 0 t n 0 n n E E E exp i 0.5 0 n0 E 1 exp in n0 n E n0 n 0 n odd harmonics even harmonics Isolating a single attosecond pulse The multi-cycle regime Femtosecond pulse High harmonics 20 fs, 800nm I~1014 W/cm2 H15 23.3eV H21 32.6eV H27 41.9eV H39 60.5eV Attosecond pulse generation process M. Hentschel et al., Nature 414, 509-513 (2001) Attosecond pulse generation process G. Sansone et al., Science 314, 443 (2006) Time resolved measurements in the attosecond regime Attosecond pulses generation Measurement How to measure an attosecond pulse? XUV Autocorrelation NLO effects: 2-photon absorption 2-photon ionization focusing NL Problems: low XUV flux small sabs Kobayashi et al., Opt. Lett. 23, 64 (1998) Attosecond streak camera F ma momentum Dpti e El t dt eAl ti ti E t A t Laser field Photo-electrons Attosecond pulse M. Hentschel et al., Nature 414, 509-513 (2001) Electron release time Momentum transfer depends on instant of electron release within the wave cycle Dp (t ) e EL (t )dt Mapping time to momentum Momentum change along the EL vector 800-nm laser electric field Δp(t7) Δp(t6) Δp(t5) t1 t2 t3 t4 t5 t6 t7 instant of electron release Δp(t4) Δp(t3) Δp(t2) Δp(t1) Incident X-ray intensity -500 as 0 Δpi 500 as Optical-field-driven streak camera J. Itatani et al., Phys. Rev. Lett. 88, 173903 (2002) M. Kitzler et al., Phys. Rev. Lett. 88, 173904 (2002) Electron counts per bin Electron countsper per bin counts Electron counts counts per per bin Electron bin Electron counts bin Electron per bin Full characterization of a sub-fs, ~100-eV XUV pulse 2 1 1 xuv = 250as 0 -1 -2 0 -3 -0.4 -0.2 0.0 Time [fs] 0.2 -0.4 Instantaneous energy shift [eV] Intensity [arb. u.] Reconstructed temporal intensity profile and chirp of the xuv excitation pulse: 00 100 100 100 100 Measurement Measurement Measurement Simulation Simulation Simulation 50 50 50 50 000 100 100 100 100 100 100 100 Measurement Measurement Simulation Simulation Field-free spectrum 50 50 50 50 50 50 000 00 100 100 100 100 t60 = -T0/4 60 d60 70 70 70 80 80 80 90 90 90 100 100 100 t60 60= +T0/4 d 70 70 80 80 90 90 100 100 Measurement Measurement Electron [eV] Electron Electronkinetic kinetic kineticenergy energy energySimulation [eV] [eV] Simulation 50 50 50 0 0 0 100 100 100 Electron Electron kinetic kinetic energy energy [eV] [eV] 50 50 0 0 60 60 70 70 80 80 90 90 Electron kinetic energy [eV] = 250 attoseconds!! 100 100 Energy shift of sub-fs electron wave-packet As we vary the relative delay between the XUV pulse and the 800-nm field, the direction of the emitted electron packet will vary. ΔW +10 eV 0 -10 eV tD dN/dW Photoelectron kinetic energy [eV] Attosecond streak camera trace 90 80 70 60 50 0 2 4 6 8 10 12 14 16 18 20 22 Delay Dt [fs] E. Goulielmakis et al., Science 305, 1267 (2004) RABITT (Reconstruction of Attosecond Beating by Interference of Two-photon Transition) q2 xuv Narrow one photon transition xuv 0 xuv xuv 0 q q 2 Two photon transition The different paths interfere with a relative phase of: q 1 20 q q 2 RABITT RABITT (Reconstruction of Attosecond Beating by Interference of Two-photon Transition) RABBITT takes advantage of the interference of the even-harmonic sidebands created when the XUV pulse interacts with the intense IR laser pulse. RABITT results for a 250-as pulse Time resolved measurements Can we performed an attosecond pump probe measurement? The main problem is the low photon flax focusing NL One solution is to use the strong IR field as either the pump or the probe Attosecond streaking spectroscopy Valence level ionization M. Drescher et al, Nature 419, 803 (2002) Core level ionization Auger Decay Time resolved atomic inner shell spectroscopy Time resolved atomic inner shell spectroscopy Time resolved atomic inner shell spectroscopy Oscillating dipole The attosecond pulses contains the spatial information of the ground and the free electron wavefunctions. Imaging the ground state c ~1A d(t)= a(k) <g|er|eikx-()t> The free electron act as a probe - the re-collision step maps the ground state wave function to the spectrum Harmonic intensities 5 9 0 22 45 67 90 8 7 Harmonic Intensity Harmonic intensities from N2 at different molecular angles x 10 6 5 4 3 2 1 0 15 EL 20 25 30 Harmonic Number 35 40 45 Tomographic image reconstruction Reconstructed Molecular Orbital - N2 Reconstructed orbital Calculated orbital J. Itatani, et al., Nature 432, 867 (2004).