UIC

1. Photocathode Physics for Photoinjectors (P3) 2012 Workshop UIC Electron Effective Mass: Emittance and Efficiency W. Andreas Schroeder Physics Department, University of Illinois at Chicago Department of Energy, NNSA DE-FG52-09NA29451 National Science Foundation DMR-0619573

2. Acknowledgements UIC Senior personnel: N.D. Browning (PNNL, UC-Davis) N.J. Zaluzec (ANL) W.A. Schroeder (UIC) D.J. Miller (ANL) J.C.H. Spence (ASU) J. Hiller (ANL) A.W. Nicholls (UIC) J. Power (ANL) J.E. Evans (PNNL) M.L. Taheri (Drexel U.) P. Abellan (PNNL) Grad. Students: J.A. Berger (UIC) J. Hogan (UIC) D.J. Masiel (UC-Davis) B.L. Rickman (UIC) T. Veccione (ASU) T. Li (UIC) Department of Energy, Office of Science DE-AC02-06CH11357 Department of Education, GAANN Fellowship DED P200A070409

3. Outline UIC • Dynamic transmission electron microscopy (DTEM) •  Time-resolved diffraction and imaging examples •  Electron source requirements for ultrafast (sub-ns) regime •  Spatial emittance reduction • Experiment: Direct transverse rms momentum 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 >  •  Preliminary results analysis (incl. Cu and Mg)

4. The LLNL DTEM UIC • Retro-fitted standard JOEL 2000FX EM column • Separate drive (~10ns, 1064nm) and photocathode (30ns, 211nm) lasers • 107–108 electrons/pulse delivered to sample (beam clean-up looses >90%) • Single-shot and multi-shot capability • “Every electron” camera Laser-driven photoelectron cathode Cathode-drive laser Hydro-drive laser Sample location Electrostatic beam shifter δt ~10ns δx ~10nm δx.δt ~ 100 nm.ns CCD camera system M.R. Armstrong et al., Ultramicroscopy107 (2007) 356

5. LLNL DTEM: Diffraction UIC Time-resolved diffraction patterns of melting and re-crystallization of Ti: hcp → liquid → bcc → hcp Experimental conditions: Single-shot exposures ~10ns laser pulse ~106 electrons/pulse AND Imaging of Fourier plane onto CCD detector T. LaGrange et al., Appl. Phys. Lett. 89 (2006) 044105

6. LLNL DTEM: Reactive multilayer foils UIC ‒ Laser-initiated reaction in Al/Ni reactive multi-layer foil (RMLF) • Reaction front moving at 13m/s • Dendritic-type structures that cannot be found in • post-mortem TEM studies of quenched foils • − Dependent upon Al/Ni ratio J.S. Kim et al., Science321 (2008) 1472

7. Physical processes: δx.δt resolution UIC TU-Berlin UEM-2 Caltech LLNL UC-Davis Bio-DTEM PLUS: Structural dynamics of individual nano-particles …

8. Single-shot imaging: Rose criterion UIC 512x512 pixel images 104 electrons 105 electrons Original 106 electrons 107 electrons 107 electrons 106 electrons 107 electrons 108 electrons Rose criterion: Good image contrast requires >100 electrons/pixel  ~108 electrons/pulse detected for 1k1k CCD T. LaGrange et al., Appl. Phys. Lett. 89 (2006) 044105

9. Single-shot imaging: DTEM vs. UEM UIC TOF in DC photogun,  0.1−1ns UEM DTEM TU-Berlin UEM-2 Caltech LLNL UC-Davis Bio-DTEM

10. Spatial imaging resolution in EM UIC − No space-charge effects and perfect electron optics  Transverse beam emittance, , conserved Δθ Δx • Imaging resolution: • − detect all electrons on NpixelNpixel CCD Δx0Δθ0 … f1/Δx is f# of lens • NOTE: • For 1k1k CCD with Δx0Δθ0  1mm.mrad x ≥ 1nm  f#  10-20nm … as typically f# > 10 (c.f. LLNL nanosecond DTEM) f1 f1 5mm f1 f1.Δθ Objective lens system Specimen f2 f2 f2 f2λe/a Imaging Optics 1k1k CCD

11. Pulsed electron source UIC − Emittance reduction in ultrafast regime (planar photocathodes): N ≈ 108 electrons/pulse • Short-pulse Child’s Law: • … limit on charge extraction •  x0 ≈ 0.5-1.0mm • … c.f. laser beam focusing: • x0 ~ 10μm • … f = lens focal length • D = beam diameter Surface charge density EDC ESC Electron pulse Image charge Laser pulse Photocathode  Reduce pT

12. Transverse rms momentum ΔpT UIC – Standard theoretical expressions • Single-photon photoemission: • … reduce E = ħω  eff to enhance photocathode brightness • BUT: • Thermionic emission: • … reduce T to improve electron source brightness 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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) 

22. 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

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

26. 1ωPE: Metals UIC − Ag, Ta, Mo, and W ħω = 4.75eV (261nm) Spot size (mm) Ag Ta W Mo E = ħωeff (eV)

27. Metal photocathodes UIC − Metals (Ag, Ta, Mo, and W) and GaSb ħω = 4.75eV (261nm) m* = 2m0 m* = m0 1ωPE: metals ≈ 10-5 - 10-6 Spot size (mm) m* = m0/2 Ag Ta Mo W GaSb ESTE: GaSb ≈ 4x10-6 E = ħωeff (eV)

28. Summary UIC m* • Thermionic emission : •  Photoemission : ? • Emission efficiency enhancement for m* < m0 •  A route to high brightness, planar photocathodes

29. Preliminary analysis UIC − Effective mass in metal photocathodes: dH-vA, CR, optical, … Cu Surface roughness? 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.

30. UIC Thank you!