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 Ultrafast Electron Sources for Diffraction and Microscopy Applications UCLA Workshop, December 12-14, 2012 Department of Energy, NNSA DE-FG52-09NA29451 Department of Education, GAANN Fellowship DED P200A070409

UIC Outline  Experiment: Direct transverse rms momentum  p T 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(p 1,p 2 ))  Agreement with standard expressions of  p T for m* = m 0  Significant reduction of  p T for m* < m 0

Brightness: Transverse Emittance UIC D.H. Dowell & J.F. Schmerge, Phys. Rev. ST – Acc. & Beams 12 (2009) K.L. Jensen et al., J. Appl. Phys. 107 (2010)  Measure of transverse electron beam (or pulse) quality: … a conserved quantity in a ‘perfect’ system.  ‘Short-pulse’ Child’s Law:  x 0 ≈ 0.5mm for N = 10 8  Reduce  p T Standard theoretical expressions:  Single-photon photoemission:  Thermionic emission:

 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 Δp T distribution UIC Experiment

UIC Analytical Gaussian (AG) model − Extended AG model simulation J.A. Berger & W.A. Schroeder, J. Appl. Phys. 108 (2010) Detector Lenses DC photo-gun pT0pT0 ½pT0½pT0  Fourier plane beam size independent of  x 0  Agreement with experiment indicates minimal aberrations

UIC 2ħω thermionic emission (2ωTE) – ħω = 2.37eV and  Au = 5.1eV FF ħħ ħħ  Au 0.35eV E DC  8kV/cm e-e- Au Vacuum ~35meV EXPECT:  Isotropic rms momentum  p T  I 2 Laser dependence of emission  Increasing  p T with I Laser  Heating of Fermi electron gas Thermionic emission of tail of two-photon excited Fermi electron distribution

2ωTE: Au results UIC – 300nm Au film on Si wafer substrate Au ħω = 2.37eV I2I2  Nonlinear I 2 electron yield  2ω process  Zero free parameter AG model fit to data: Laser heating of Fermi electron gas  … as m ≈ m 0 in Au

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

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

GaSb band structure UIC J.R. Chelikowsky & M.L. Cohen, Phys. Rev. B 14 (1976) 556 D.E. Aspnes & A.A. Studna, Phys. Rev. B 27 (1983) 985 – Vacuum level at  eff = 4.84eV above bulk VB maximum  Strong absorption at 261nm:  = 1.44x10 6 cm -1   -1 ≈ 7nm … ‘metal-like’   -valley transitions from VB (HH, LH, and SO bands) to upper  8 conduction band  eff εFεF

UIC ESTE in GaSb −  -valley absorption at ħω = 4.75eV 88 77 CB HH LH SO EgEg Eg/Eg/  E k ħωħω GaSb properties Eg/Eg/ 3.85eV  0.99eV Initial excess E electron  T e ~0.35eV 4,200K ħω LO 29meV τ LO ~200fs m*(  8 ) ~0.3m 0  Initially; exp[-  /(k B T e )] ≈ 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 τ decay E electron

UIC  p T for GaSb − Analysis of Fourier plane momentum distribution Fit to AG model simulation using gives mT ≈ 360m 0 (i) For m = m 0 with T = 360K: exp[-  /(k B T)] ~ … no emission !! (ii) For m = m* ≈ 0.3m 0 with T = 1,200K: exp[-  /(k B T)] ≈ 5x10 -5 … reasonable for TE (c.f.  GaSb ≈ 4x10 -6 ) 480(±50)μm (HWe -1 M) 

UIC m* dependence of  p T − Quantum mechanics: Potential step Momentum parallel to interface is conserved AND for emission;  An implicit m* dependence for  p T  e-e- CathodeVacuum p2p2 p1p1 p // p1p1 p2p2 Cathode Vacuum e-e-

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

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

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

UIC Photoemission Simulation − Ag photocathode (  eff = 4.52eV, ħω = 4.75eV,  F = 5.5eV, T e = 300K) p z (  ( m 0.eV)) p T (  (m 0.eV)) m* = m 0 p T (  (m 0.eV)) Transverse momentum distribution (Fourier plane)

UIC Photoemission Simulation − ‘Light Fermion’ Ag photocathode (  eff = 4.52eV, ħω = 4.75eV,  F = 5.5eV, T e = 300K) p z (  ( m 0.eV)) p T (  (m 0.eV)) m* = 0.3m  max. = sin -1 ≈ 33  m*  m 0 p T (  (m 0.eV)) Transverse momentum distribution (Fourier plane)

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

UIC Summary m*m*  Mean square transverse momentum: … where M = min (m*, m 0 )  PLUS: small emission efficiency enhancement for m* < m 0  A route to high brightness, planar photocathodes

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UIC NEA GaAs Zhi Liu et al., J. Vac. Sci. Tech. B 23 (2005) 2758 − Cesiated NEA GaAs photocathode (GaAs-CsO) m* = 0.067m 0 p T (  (m 0.eV)) ≈ 15  p z (  ( m 0.eV))

UIC m*: Emission efficiency − Quantum mechanics: Potential step  e-e- CathodeVacuum Barrier transmission:  |T | 2 ≈ 1 for p 1 ≈ p 2 i.e., for m*E 1 ≈ m 0 E 2 … only possible for m* < m 0

UIC m*: Emission efficiency − Quantum mechanics: Potential step  Emission efficiency enhancement for m* < m 0  e-e- CathodeVacuum Barrier transmission:  |T | 2 ≈ 1 for p 1 ≈ p 2 i.e., for m*E 1 ≈ m 0 E 2 … only possible for m* < m 0 |T|2|T|2  E = ħω   (eV) m* = 10m 0 m* = m 0 m* = 0.1m 0  = 4.5eV

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