3 Raman scattering and attosecond pulses Input two frequencies nearly resonant with a Raman resonance.At high intensity, the process cascades many times.Output pulse of second process as input to a third processOutput pulse of third process as input to a fourth processOutput pulse as input to a second processEtc.Input pulses10Raman processes can cascade many times, yielding a series of equally spaced modesfrequency=1+/- n0S. E. Harris and A. V. Sokolov PRL 81, 2894
4 Cascaded Raman generation Dwba = cm-1A. V. Sokolov et al. PRL 85,This can be done with nanosecond laser pulses!
5 Experimental demonstration of cascaded Raman scattering Detuning from 2-photon resonance2994 cm-1- 400MHz+ 100MHz+ 700MHz75,000 cm-1 (2.3 x 1015 Hz) of bandwidth has been created!A. V. Sokolov et al. PRL 85, 562
6 Experimental demonstration of cascaded Raman scattering The different frequencies are lockedPulses with 1 fs duration are measuredThe spectrum is discrete: the pulses are emitted in a pulse train, separated by the vibrational period.The main advantage of this process: high efficiencyThe main drawback: the carrier frequency is in the visible regimeWe cannot produce an isolated pulse.A. V. Sokolov et al. PRL 85, 562
7 Breaking the femtosecond limit 2001: First observation of an attosecond pulse (650 as)M. Hentschel et al., Nature 414, (2001)G. Sansone et al., Science 314, 443 (2006)2006: (130 as)
8 Our main tool: intense laser pulses Field Intensity: 1014 –1015 W/cm22.7 fsThe force is comparable to the force binding the electrons in the atom or molecule.
9 Attosecond pulse generation process Re-collisionAcceleration by the electric fieldE>100eVTunnel ionizationWith I~1014 W/cm2Fundamental frequency
10 Attosecond pulse generation process Acceleration by the electric fieldTunnel ionizationOptical radiation with attoseconds duration
11 Attosecond pulse generation process Classical model
12 Attosecond pulse generation process Classical model
13 Attosecond pulse generation process Classical modelThe return times are determined such that x0(t,t0)=0Long trajectoriesShort trajectoriesEk is the instantaneous frequency of the attosecond pulse
14 Attosecond pulse generation process Quantum modelThe electron’s wavefunctionThe induced dipole momentThe dynamics of the free electron is mapped into the optical field
16 Electron wave packet dynamic XUV field:Husimi reprsentation
17 Attosecond pulse generation process where only the gas was changed in between.0,0 0,1 0,2 0,3 0,4 0,50,010,11H21ArN2normalized signal at H21elAttosecond pulse generation processClassical modelElliptically polarized light:The electron is shifted in the lateral direction: the recollision probability reduces significantly
18 Isolating a single attosecond pulse The multi-cycle regime
19 Isolating a single attosecond pulse The multi-cycle regimeFemtosecond pulse20 fs, 800nmHigh harmonicsI~1014 W/cm2H1523.3eVH2132.6eVH2741.9eVH3960.5eV
20 Attosecond pulse generation process M. Hentschel et al., Nature 414, (2001)
21 Attosecond pulse generation process G. Sansone et al., Science 314, 443 (2006)
22 Time resolved measurements in the attosecond regime Attosecond pulses generationMeasurement
23 How to measure an attosecond pulse? XUV AutocorrelationNLO effects: 2-photon absorption2-photon ionizationtProblems: low XUV flux small sabsfocusingNLKobayashi et al., Opt. Lett. 23, 64 (1998)
24 Attosecond streak camera momentumLaser fieldPhoto-electronsElectron release timeAttosecond pulseM. Hentschel et al., Nature 414, (2001)
25 Momentum transfer depends on instant of electron release within the wave cycle
26 Mapping time to momentum change along the EL vector800-nm laser electric fieldΔp(t7)Δp(t6)Δp(t5)t1t2t3t4t5t6t7Δp(t4)instant ofelectronreleaseΔp(t3)Δp(t2)ΔpiΔp(t1)Incident X-rayintensity-500 as500 asOptical-field-driven streak cameraJ. Itatani et al., Phys. Rev. Lett. 88, (2002)M. Kitzler et al., Phys. Rev. Lett. 88, (2002)
27 Full characterization of a sub-fs, ~100-eV XUV pulse Field-freespectrumtd = -T0/4Reconstructed temporal intensity profile and chirp of the xuv excitation pulse:Time [fs]Intensity [arb. u.]1Instantaneous energy shift [eV]-3-2-12-0.4-0.20.00.2txuv= 250astd = +T0/4 = 250 attoseconds!!
28 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.+10 eV-10 eVΔWtDdN/dW
29 Attosecond streak camera trace 908070Photoelectron kinetic energy [eV]60Goul bild beim 3. mausklick, Titel: Direct measurement of light waves50246810121416182022DelayDt[fs]E. Goulielmakis et al., Science 305, 1267 (2004)
30 RABITT (Reconstruction of Attosecond Beating by Interference of Two-photon Transition) The different paths interfere with a relative phase of:Two photon transitionNarrow one photon transition
32 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.
34 Time resolved measurements Can we performed an attosecond pump probe measurement?The main problem is the low photon flaxtfocusingNLOne solution is to use the strong IR field as either the pump or the probe