Polarization of the fastest flashes of light

1、Oren CohenExtreme nonlinear optics groupPolarization of the fastest flashes of lightDr. Avner Fleischer, Ofer Kfir, Tzvi Diskin, Pavel SidorenkoWhat are the (controlled) fastest flashes? Why do we care?Bullet Piercing an Apple, 1964 (H.E. Edgerton). Stroboscopic photographyControlled microsecond fla

2、sh Millisecond flashThe “Galloping Horse” ControversyPalo Alto, CA 1872The Birth of Ultrafast TechnologyLeland StanfordEadweard MuybridgeTime Resolution:1/60th of a secondBet: Do all four hooves of a galloping horse ever simultaneously leave the ground?/artists/muybridge_eadweard.htmlDecember 25, 20

3、12 Ort Braude Avner Fleischer188719601990: FemtoChemistryUltra-short laser pulsesAttosecond pulses are produced through high-harmonic generation 800 nm30 nm2.7 fsWhat is high harmonic generation (HHG)?How it produces attosecond pulses?Ruby laserLensQuartz crystalPrismPhotographic plate694nm347nmP.A.

4、 Franken et al, PRL 7, 118 (1961) Second-Harmonic GenerationEpumpEatomicLog intensityFrequencyPhoton energyLog(Intensity)PlateauSoft X-ray beamHigh-harmonic generationMcPherson et al., JOSA B 4, 595 (1987)Ferray et al., J. Phys. B 21, L31 (1988) I1014 - 1016 W/cm2Typically =800 nmCut-off energy Epum

5、pEatomicHigh harmonic generation & attosecond pulses10Mller, M., et al. PRA 86, 011401 (2012)Electron jiggles in the driving oscillating electric field. Recollision emission of attosecond burst of radiationMultiple re-collisions train of attosecond pulses & comb of high harmonics.Linear polarization

6、Weihe, F.A., et al., PRA 51, R3433 (1995)AtomOscillating Electric fieldA train of linearly polarized attosecond pulsesSpectrumMcPherson et al., JOSA B 4, 595 (1987)Ferray et al., J. of Phys. 21, L31 (1988).High harmonic generation (HHG)a unique light source ControlCoherence (laser-like)Spectrum broa

7、d / single harmonicSpatial profile Temporal Attosecond sciencePolarization: our groupA. Fleischer et al., Nature Photonics 8, 543 (2014).O. Kfir et al., Nature Photonics 9, 99 (2015).HHGVis/IRExtreme UVHelicity: h= 1h= +1PolarizationQuarter-wave plates: visible vs extreme UVVisibleControl polarizati

8、on Lossless Extreme UVVodungbo et al., Opt. Exp. 19, 4346 (2011)LossyLimited spectral regionsSensitiveNo applicationsExtreme-UV “wave-plate”Why control polarization?Why was it difficult to control the polarization of high-order harmonics and attosecond pulses? AtomEllipticity effect in HHG16Elliptic

9、ity diminishes HHG efficiencyEllipticity of HHG field 0.4Mller, M., et al. PRA 86, 011401 (2012)Our method to produce circularly polarized high harmonicsCounter-rotating bi-chromatic drivers18Long et al., PRA 52 2262 (1995). Bi-chromatic field800 nm + 400 nmCircularly polarized harmonicsThe left-rot

10、ating harmonics have similar intensity as the right-rotating average chirality 0.High harmonics with zero average chirality corresponds to a train of linearly polarized attosecond pulses.AtomtHHG orderSpectrumHigh harmonic generation with counter-rotating circularly-polarized bi-chromatic fields Com

11、parable efficiency to HHG by lineally polarized driver Harmonic OrderH20H21H22H23H24H19H18H17Linear-linearCircular-circularMeasured circularitymeasuredfitBright circularly-polarized HHG at Co M-edge-Magneto-Optical dichroic absorption coefficient.Opposite helicity for consecutive harmonics. First do

12、wnstream experiment of magnetism using our source.X-ray Magnetic Circular Dichroism (XMCD) XMCD of ferromagnet transition metals at the M-edge (60eV) :Absorption spectra of magnetized Cobalt:O. Kfir et al. Nature Photonics 9 99 (2015).HHG SpectraHarmonic OrderH20H21H22H23H24H19H18H17What about circu

13、larly polarized attosecond pulses? We somehow need to suppress the harmonics with right/left helicity. We do it through phase matching.x-ray beamNeed phase matching to generate bright x-rays beamsPhase matching - the emission from all atoms interfere constructively! Pump laser Nonlinear mediumPhase

14、(momentum) matchingBright x-ray beamArmstrong et al., PR 127, 1918 (1962)Phase mismatchzPhase matchedHarmonic fieldNon phase matchedLC=/kHelicity-dependent phase matchingIf are phase matched, the are notLeft rotating HHGRight rotating HHGHHG orderTime fs170 asFourier transform of the measured harmon

15、ic spectrum corresponds to train of pulses with duration down to 170 attoseconds and elliptical polarization.Different degree of circular polarization of the pulses may stress the peak contrast in chiral experimentsHHG order60%80%60%60%60%Time domain - chiral attosecond pulsesSummaryFull control over polarization of HHGFirst experiments using circularly-polarized HHGConservation of spin angular momentum in HHG Phase matching of circularly polarized harmonicsTowards circularly polarized attosecond pulses