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Spin-polarization using ns~fs laser pulses Takashi Nakajima Institute of Advanced Energy

Spin-polarization using ns~fs laser pulses Takashi Nakajima Institute of Advanced Energy Kyoto University nakajima@iae.kyoto-u.ac.jp. Three kinds of spin: spin of electron 　 →　 spin-polarized electron electron-spin of ion → electron spin-polarized ions

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Spin-polarization using ns~fs laser pulses Takashi Nakajima Institute of Advanced Energy

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  1. Spin-polarization using ns~fs laser pulses Takashi Nakajima Institute of Advanced Energy Kyoto University nakajima@iae.kyoto-u.ac.jp

  2. Three kinds of spin: spin of electron　 →　spin-polarized electron electron-spin of ion → electron spin-polarized ions （nuclear spin 　　 →　polarized ion） (under progress) our work Europhys.Lett. 57, 25 (2002) Phys.Rev.A 68, 013413 (2003) Appl.Phys. Lett. 84, 3786 (2004) J.Chem.Phys. 117, 2112 (2002) Appl.Phys.Lett. 83, 2103 (2003) J.Chem.Phys. 120, 1806 (2004) Spin-polarized source Introduction – why spin polarization? Aim：　Develop new (and hopefully simple) method(s) to control spin-degree of freedom by purely optical method (but without optical pumping)

  3. Applications of spin-polarized species Spin-dependence of various quantities, f, provides more information on the dynamics If averaged over spin, subtle spin-dependent effects are easily smeared out Spin-polarized electrons ★high energy physics ★atomic and molecular processes ★surface physics, semiconductor physics Electron spin-polarized ions ★surface physics ★atomic and molecular processes Nuclear-spin-polarized (doped) atom ★ nuclear physics

  4. Outline • Electron spin-polarization upon photoionization of rare gas atoms • by UV~VUV pulse • 2. Simultaneous production of spin-polarized electrons/ions • with ns pulses • 3. Ultrafast spin polarization • 4. Summary

  5. Outline • Electron spin-polarization upon photoionization of rare gas atoms • by UV~VUV pulse • 2. Simultaneous production of spin-polarized electrons/ions • with ns pulses • 3. Ultrafast spin polarization • 4. Summary

  6. ★ resonance on 5p1/2P = - 60% ★ resonance on 5p3/2 P = + 70% ω 5p3/2 5p1/2 ω 5s1/2 Rb Polarized electrons via 2-photon ionization of alkali-metal atom (Rb) ★ far off-resonance (it is as if there were no fine structure) → P = 0% ★ between 5p1/2 and 5p1/2, P~100% due to two-path interference 2-photon ionization of Rb p3/2 circular light 237cm-1 p1/2 circular light Lambropoulos, Phys.Rev.Lett. 30, 413 (1973)

  7. 2-photon ionization of Xe Xe+ 5p5[2P1/2] Xe+ 5p5[2P3/2] ω circular light 5p5[2P1/2]6s (J=1) 9000cm-1 40 times larger splitting than Rb 5p state 5p5 [2P3/2]6s (J=1) ω circular light 5p6 (J=0) Xe Polarized electrons via 2-photon ionization of rare-gas atom (Xe) ★Technically, rare gas atoms are much more convenient than alkalis ★ Hopefully, similar behavior to that of the Rb atom, but we must solve a multichannel problem for Xe: p5 [2P1/2] 6s(J=1) = Σp5[2P1/2]ns1/2 + p5[2P3/2]ns1/2 + p5[2P1/2]nd3/2 + p5[2P3/2]nd3/2 + p5[2P3/2]nd5/2

  8. 9.2 eV photon (134 nm or THG of SHG of 800nm) σ(2) ~10-49 cm4.s 2-photon ionization of Xe Xe+ Xe Nakajima and Lambropoulos, Europhys.Lett. 57, 25 (2002)

  9. ~100% spin-polarization 1.5x1012 electrons/pulse 500 fs, 1mJ pulse focus to d=150μm , L=1cm 1 Torr Xe gas 4.8 eV photon (THG of 775nm) σ(3) ~10-81 cm6.s2 3-photon ionization of Xe Xe+ Xe Nakajima and Lambropoulos, Europhys.Lett. 57, 25 (2002)

  10. Electron spin-polarization upon photoionization of rare gas atoms • by UV~VUV pulse • 2. Simultaneous production of spin-polarized electrons/ions • with ns pulses • 3. Ultrafast spin polarization • 4. Summary

  11. example) Sr (5s5p 3P1) +  → Sr+ (5s ) + e- photoelectron spin angular momentum orbital angular momentum electron electron ejection electron e- e- ion e- e- spin angular momentum orbital angular momentum (3) spin of electron Sr2+ spin-orbit interaction (2) orbital momentum of electron ns pulse for ionization dipole interaction (1) angular momentum of photon No guarantee that both electrons and ions are spin-polarized Careful choice of the scheme is necessary nearly pure single LS coupling description ion core to be Sr+ (5s) requirements Nakajima and Yonekura, J. Chem. Phys.117, 2112 (2002) Production of spin-polarized electrons/ions – Dual spin-polarized source

  12. for a singlet state ∴ Level scheme Triplet state must be used!

  13. Vacuum chamber pulse timing Probe laser YAG laser 421nm (1x10-5Pa) ablation 5ns 308nm Sr disk Pump laser ∥ ⊥ 15ns Ionization laser Ionization laser Ablation laser 15ns Probe laser 1064nm 50ms trigger LIF signal delay Pump laser Monochro- mator 689nm trigger boxcar gate Box-car integrator Computer PMT Experimental setup ns pulses are used for ablation, excitation, ionization, and probe (detection)

  14. Sr+2P1/2 Sr+2P1/2 probe laser left-circular probe laser right-circular LIF Sr+2S1/2 Sr+2S1/2 m = -1/2 m = -1/2 m = +1/2 m = +1/2 where ILC: LIF by RC probe laser IRC: LIF by LC probe laser Polarization Optical detection for spin-polarization of Sr+ (52S1/2) ion Use of laser-Induced fluorescence (LIF) example）　if Sr+ (52S1/2) is 100% spin-polarized, LIF signal detected ! No LIF signal

  15. Polarization Spin-polarization of Sr+ ions determined from the LIF signal 1.0 0.8 LIF intensity (arb.units) 0.6 0.4 0.2 0 Right-Circular Left-Circular Probe laser polarization Agree well with our theoretical prediction (60%) (Nakajima and Yonekura, J. Chem. Phys.117, 2112 (2002)) Nakajima et al., Appl. Phys. Lett.83, 2103 (2003) Yonekura et al., J. Chem. Phys.120, 1806 (2004)

  16. LIF intensity (arb. units) spin-polarization (%) tunable Ionization laser Sr 5s6p 3P1 pump laser 295 nm Sr 5s21S 0 Detuning of the ionization laser (cm-1) spin-polarization：78% 1-orderof magnitude improvement of ionization efficiency Spin-polarization by the tunable ionization laser For better efficiency and spin-polarization, tune the laser to an autoionization resonance probe laser 421 nm Sr 4d5d 3S 1 autoionization resonance 640 nm Matsuo et al., (under preparation for submission)

  17. Electron spin-polarization upon photoionization of rare gas atoms • by UV~VUV pulse • 2. Simultaneous production of spin-polarized electrons/ions • with ns pulses • 3. Ultrafast spin polarization • 4. Summary

  18. Depicting the above scheme with magnetic sublevels explicitly, 4p 2P3/2 4p 2P1/2 4s 2S1/2 Mj= -1/2 Mj=+1/2 Spin-polarization using short laser pulses ―one-electron system example） K atom Coherent excitation of fine structure by ultrafast (broadband) lasers LS-coupled basis probe spin-orbit coupling time ~ Δ-1 ∴If pulse duration τ<< Δ-1 , the system does not see spin-orbit interaction during the pump pulse→ LS-uncoupled basis Δt 4p 2P3/2 Δ 4p 2P1/2 pump 4s 2S1/2 Two paths are independent Bouchene et al,J.Phys. B34, 1497 (2001)

  19. MJ= +1/2　→ MJ= +3/2transition MJ= -1/2　→ MJ= +1/2transition |B-> = | 1 , 1 , 1/2 , -1/2 > |D-> = | 1 , 0 , 1/2 , +1/2 > |B+> = | 1 , 1 , 1/2 , +1/2 > P P P 4p 2P3/2 spin-orbit interaction 4p 2P1/2 pump pump S S 4s 2S1/2 | L=0 , ML=0 , S=1/2 , MS= -1/2 > | L=0 , ML=0 , S=1/2 , MS= +1/2 > Mj= -1/2 Mj=+1/2 LS-coupled basis vs. LS-uncoupled basis for a one-electron system LS-coupled basis LS-uncoupled basis

  20. Representative result for a one-electron system For K 4p1/2 and 4p3/2, Δ=57.7 cm-1 ( = 7.15 meV) Δ-1=580 fs Δ-1 Bouchene et al,J.Phys. B34, 1497 (2001)

  21. example) Mg atom Coherent excitation of fine structure manifolds ultrafast pulse Δ spin-orbit coupling timeτ=Δ-1 ultrafast pulse Mg 3s3d 3D1,2 τ= 1.2 ns Ca 4s4d 3D1,2 τ= 9.0 ps Sr 5s5d 3D1,2 τ= 2.2 ps ns pulse Nakajima, Appl. Phys. Lett.84, 3786 (2004) Spin-polarization using short laser pulses ― two-electron system Advantages of two-electron system over a one-electron system: （１） spin-polarization of ion is easy to monitor by optical method (LIF) （２） spin-flip (change of polarity) can take place （３） Influence of hyperfine structure is much smaller

  22. Physical mechanism (a) coherent excitation by pump laser (in LS-coupled basis) ΔE state-flipping after the pump pulse LS-uncoupled basis (↑,↑), (↑,↓), etc. change basis (c) probe laser after some delay to pick up particular spin state state-flipping (LS-coupled basis) ⇔ spin-flipping (LS-uncouplede basis) ultrafast spin polarization ! Physical mechanism of polarizing a two-electron system (b) LS-coupled basis3s3d 3D1 & 3s3d 3D2

  23. dipole moment Photoelectron yield with ↑or↓ spin Photoelectron yield with↑ or ↓ spin As we expect, photoelectron yield into different spin states has different dependence on time delay

  24. ΔE Ionization cross section from Mg 3s3d 3D Consider two extreme cases： |<3sεp |D| 3s3d>| >> |<3sεf |D| 3s3d> | |<3sεp |D| 3s3d>| << |<3sεf |D| 3s3d> | Either case can be realized by the proper choice of the probe photon energy probe photon energy (eV) Time delay .vs. photoelectron yield and spin-polarization Photoelectron / photoion yield Degree of spin-polarization

  25. Mg atom probe laser photon energy = 4.03 eV probe laser photon energy = 4.47 eV |<3sεf |D| 3s3d>| >> |<3sεp |D| 3s3d> | |<3sεp |D| 3s3d>| >> |<3sεf |D| 3s3d> | spin↑ spin↓ change of delay leads to the change of spin-polarity ! Nakajima, Appl. Phys. Lett.84, 3786 (2004) Representative results for a two-electron system

  26. At ωprobe= 4.47 eV At ωprobe= 4.03 eV Dependence of spin-polarization on laser polarization Since spin-polarization is based on the momentum transfer from photons to electrons, thedynamics of spin-polarizationdepends on thelaser polarization pump: linear probe: linear pump: linear probe: r-circular pump: linear probe: l-circular probe probe probe pump pump pump excitation

  27. Summary ○ Discussed three different schemes to polarize spin of photoelectron spin of valence electron ○ Alkaline-earth atoms are conveniently used for the proof-of-principle experiment easy to optically analyzespin of the valence electron of photoions ○ Our methods are purely optical by pulsed (ns~fs) lasers no optical pumping no spin-exchange collision upon photoionization

  28. Yukari Matsuo (RIKEN) Tohru Kobayashi (RIKEN) Proof-of-principle experiment Financial support Ministry of Education and Science Grant-in-Aid for Basic Research (C) (year 2002-2004) Priority Research Area (year 2002- ) Basic Research (A) (year 2005- ) Casio Foundation Sumitomo Foundation Collaborators

  29. Comparison with experimental data （for 1-photon ionization of Xe) Phys. Rev. A 58, 1589 (1998) (Heinzmann’s group) Xe 7d’ experiment our theory Xe 9s’ our theory experiment

  30. 1. Electron spin-polarization of rare gas atoms by UV~VUV pulse 2. Simultaneous production of spin-polarized electrons/ions with ns pulses 3. Ultrafast spin polarization within transition rate approximation 4. Summary beyond transition rate approximation

  31. Ultrafast spin polarization beyond transition rate approximation (1) Time-dependent Schrödinger equation probe Δ pump excitation 1-photon Rabi frequency 2-photon Rabi frequency (complex） laser detuning of state Ionization width for state Stark shift for state

  32. Ultrafast spin polarization beyond transition rate approximation (2) dipole moment Spin-polarized electron yield spin-polarization

  33. Intensity-dependent spin-polarization (1) ωprobe=4.01 eV , Ipump=105 W/cm2 τpump=τprobe=10 ps no dependence on Iprobe saturation quantum beat

  34. Intensity-dependent spin-polarization (2) ωprobe=4.46 eV , Ipump=105 W/cm2 τpump=τprobe=10 ps Why this happens?

  35. depend on Iprobe Origin of intensity dependence Why spin-polarization exhibits dependence on Iprobe？ Spin-polarized electron yield

  36. pump pulse at t=0 (ps) probe pulse at t=500 (ps) Time evolution of for ωprobe=4.01eV

  37. pump pulse at t=0 (ps) probe pulse at t=500 (ps) Time evolution of for ωprobe=4.46eV rapid decrease of u2 by the probe pulse

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