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
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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
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 FellowshipDED P200A070409
UICOutline
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
Brightness: Transverse Emittance UIC
D.H. Dowell & J.F. Schmerge, Phys. Rev. ST – Acc. & Beams 12 (2009) 074201K.L. Jensen et al., J. Appl. Phys. 107 (2010) 014903
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: Tmkp BT 3
)( effT
mp
TxT pxmc
kxmc
.122
DCE
Nq
0
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
UICExperiment
UICAnalytical Gaussian (AG) model− Extended AG model simulation
J.A. Berger & W.A. Schroeder, J. Appl. Phys. 108 (2010) 124905
pT0
½pT0
Fourier plane beam size independent of x0
Agreement with experiment indicates minimal aberrations
UIC2ħω thermionic emission (2ωTE)– ħω = 2.37eV and Au = 5.1eV
F
ħ
ħ
Au
0.35eV
EDC 8kV/cm
e-
Au Vacuum
~35meV
EXPECT:
Isotropic rms momentum pT
I2Laser dependence of emission
Increasing pT with ILaser
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
Auħω = 2.37eV
I2
Nonlinear I2 electron yield 2ω process
Zero free parameter AG model fit to data: Laser heating of Fermi electron gas