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Advances in Ultrafast Electron Sources for Diffraction and Microscopy Applications

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The Ultrafast Electron Sources Workshop at UCLA (Dec 12-14, 2012) focused on pioneering techniques to generate ultra-bright photocathodes relevant for electron diffraction and microscopy. This workshop addressed direct transverse rms momentum measurements, two-photon thermionic emission from materials such as Au, GaSb, and InSb, and highlighted the impact of electron effective mass on photoemission processes. Researchers from UIC discussed experimental setups and simulations, demonstrating significant advancements in electron beam quality and thermal emission techniques in next-generation photonic applications.

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Advances in Ultrafast Electron Sources for Diffraction and Microscopy Applications

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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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