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UIC

Ultrafast Electron Sources for Diffraction and Microscopy Applications UCLA Workshop, December 12-14, 2012. UIC. m * : A Route to Ultra-bright Photocathodes. W. Andreas Schroeder Joel A. Berger and Ben L. Rickman Physics Department, University of Illinois at Chicago.

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UIC

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  1. Ultrafast Electron Sources for Diffraction and Microscopy Applications UCLA Workshop, December 12-14, 2012 UIC m*: A Route to Ultra-bright Photocathodes W. Andreas Schroeder Joel A. Berger and Ben L. Rickman Physics Department, University of Illinois at Chicago Department of Energy, NNSA DE-FG52-09NA29451 Department of Education, GAANN Fellowship DED P200A070409

  2. Outline UIC • Experiment: Direct transverse rms momentum pT measurement •  Two-photon thermionic emission (2ωTE) from Au (2ħω < ) • GaSb and InSb photocathodes •  Excited state thermionic emission (ESTE); ħω <  •  Electron effective mass (m*) effects … • Metal photocathodes (Ag, Ta, Mo, and W) •  Single-photon photoemission (1ωPE); ħω >  •  More evidence of m* effects … • Simulation of photoemission (m*, g(E), T(p1,p2)) •  Agreement with standard expressions of pT for m* = m0 •  Significant reduction of pT for m* < m0

  3. Brightness: Transverse Emittance UIC • Measure of transverse electron beam (or pulse) quality: • … a conserved quantity in a ‘perfect’ system. • ‘Short-pulse’ Child’s Law: x0 ≈ 0.5mm for N = 108 •  Reduce pT • Standard theoretical expressions: •  Single-photon photoemission: •  Thermionic emission: D.H. Dowell & J.F. Schmerge, Phys. Rev. ST – Acc. & Beams 12(2009) 074201 K.L. Jensen et al., J. Appl. Phys. 107 (2010) 014903

  4. Experiment UIC • 2W, 250fs, 63MHz , diode- • pumped Yb:KGW laser •  1W, ~200fs at 523nm •  ~4ps at 261nm (ħω = 4.75eV) • Electron detector at back focal • plane of lens system •  Direct measurement of • ΔpT distribution

  5. Analytical Gaussian (AG) model UIC − Extended AG model simulation • Fourier plane • beam size • independent • of x0 • Agreement with • experiment • indicates minimal • aberrations DC photo-gun pT0 ½pT0 Lenses Detector J.A. Berger & W.A. Schroeder, J. Appl. Phys. 108 (2010) 124905

  6. 2ħω thermionic emission (2ωTE) UIC – ħω = 2.37eV and Au = 5.1eV • EXPECT: • Isotropic rms momentum pT • I2Laser dependence of emission • Increasing pT with ILaser •  Heating of Fermi electron gas e- 0.35eV ~35meV Au ħ EDC 8kV/cm ħ F Au Vacuum Thermionic emission of tail of two-photon excited Fermi electron distribution

  7. 2ωTE: Au results UIC – 300nm Au film on Si wafer substrate Au ħω = 2.37eV • Nonlinear I2 • electron yield •  2ω process • Zero free parameter • AG model fit to data: • Laser heating of • Fermi electron gas •  • … as m ≈ m0 in Au I2

  8. GaSb and InSb photoemission? UIC – ‘Real space’ picture: ħωLaser = 4.75eV (261nm) InSb GaSb GaSb InSb Electron yield, Y Expect minimal (if any) single-photon photoemission: ħωeff ≤ 0 … Schottky barrier suppression ~35meV at 8kV/cm ħω (eV) ħωLaser ħωLaser G.W. Gobeli & F.G. Allen, Phys. Rev. 137 (1965) A245

  9. GaSb and InSb results UIC − Strong electron emission with ~4ps, 261nm pulses • p-polarized UV • radiation incident • at 60º: •  GaSb ≈ 4x10-6 •  InSb ≈ 7x10-6 InSb GaSb GaSb

  10. GaSb band structure UIC – Vacuum level at eff = 4.84eV above bulk VB maximum eff • Strong absorption at 261nm: •  = 1.44x106cm-1 •  -1 ≈ 7nm • … ‘metal-like’ • -valley transitions from VB • (HH, LH, and SO bands) to • upper 8 conduction band εF J.R. Chelikowsky & M.L. Cohen, Phys. Rev. B 14 (1976) 556 D.E. Aspnes & A.A. Studna, Phys. Rev. B 27 (1983) 985

  11. ESTE in GaSb UIC − -valley absorption at ħω = 4.75eV E 8  Eelectron τdecay CB 7 Eg/ ħω Eg • Initially; exp[-/(kBTe)] ≈ 0.06 •  Excited state thermionic emission • Cooling rate of ~1,600K/ps • by LO phonon emission • AND possible fast decay via 7 band •  No electron emission latency k HH LH SO

  12. pT for GaSb UIC − Analysis of Fourier plane momentum distribution Fit to AG model simulation using gives mT ≈ 360m0 (i) For m = m0 with T = 360K: exp[-/(kBT)] ~ 10-15 … no emission !! (ii) For m = m* ≈ 0.3m0 with T = 1,200K: exp[-/(kBT)] ≈ 5x10-5 … reasonable for TE (c.f. GaSb ≈ 4x10-6) 480(±50)μm (HWe-1M) 

  13. m* dependence of pT UIC − Quantum mechanics: Potential step p2 e- Cathode e- p2 p//  p1 p// p1 Cathode Vacuum Vacuum Momentum parallel to interface is conserved AND for emission;  An implicit m* dependence for pT

  14. 1ωPE: Ag photocathode UIC − Fourier plane data vs. AG model simulation ħω = 4.75eV (261nm) Spot size (mm) Ag E = ħωeff (eV)

  15. 1ωPE: Metals UIC − Ag, Ta, Mo, and W ħω = 4.75eV (261nm) Spot size (mm) Ag Ta W Mo E = ħωeff (eV)

  16. pT and m* UIC − Effective mass in metal photocathodes: dH-vA, CR, optical, … Cu Ag W Mo Ta Mg H.J. Qian et al., Phys. Rev. ST – Acc. & Beams 15(2012) 040102 X.J. Wang et al., Proceedings of LINAC2002, Gyeongju, Korea.

  17. Photoemission Simulation UIC − Ag photocathode (eff = 4.52eV, ħω = 4.75eV, F = 5.5eV, Te = 300K) Transverse momentum distribution (Fourier plane) m* = m0 0.8 0.6 0.4 0.2 0.0 pz ((m0.eV)) -1.0 -0.5 0.0 0.5 1.0 pT ((m0.eV)) -1.0 -0.5 0.0 0.5 1.0 pT ((m0.eV))

  18. Photoemission Simulation UIC − ‘Light Fermion’ Ag photocathode (eff = 4.52eV, ħω = 4.75eV, F = 5.5eV, Te = 300K) m* = 0.3m0 Transverse momentum distribution (Fourier plane) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 m*  m0 max. = sin-1 ≈ 33 pz ((m0.eV)) -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 pT ((m0.eV)) -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 pT ((m0.eV))

  19. pT and m* UIC − Effective mass in metal photocathodes: dH-vA, CR, optical, … Cu Te ? Simulation (Te =0) Ag W Mo Oxide? Ta Mg H.J. Qian et al., Phys. Rev. ST – Acc. & Beams 15(2012) 040102 X.J. Wang et al., Proceedings of LINAC2002, Gyeongju, Korea.

  20. Summary UIC m* • Mean square transverse momentum: • … where M = min (m*, m0) • PLUS: small emission efficiency enhancement for m* < m0 •  A route to high brightness, planar photocathodes

  21. UIC Thank you!

  22. NEA GaAs UIC − Cesiated NEA GaAs photocathode (GaAs-CsO) m* = 0.067m0 1.8 1.6 1.4 1.2 1.0 0.8 ≈ 15 pz ((m0.eV)) -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 pT ((m0.eV)) Zhi Liu et al., J. Vac. Sci. Tech. B 23 (2005) 2758

  23. m*: Emission efficiency UIC − Quantum mechanics: Potential step e- • Barrier transmission: • |T |2 ≈ 1 for p1 ≈ p2 • i.e., for m*E1 ≈ m0E2 • … only possible for m* < m0  Cathode Vacuum

  24. m*: Emission efficiency UIC − Quantum mechanics: Potential step e- • Barrier transmission: • |T |2 ≈ 1 for p1 ≈ p2 • i.e., for m*E1 ≈ m0E2 • … only possible for m* < m0  = 4.5eV m* = 0.1m0 |T|2 m* = m0  Cathode Vacuum m* = 10m0 E = ħω (eV)  Emission efficiency enhancement for m* < m0

  25. UIC

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