Transcript PPT

Ultrafast Electron Sources for Diffraction and Microscopy Applications
UCLA Workshop, December 12-14, 2012
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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
Outline
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 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
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 Measure of transverse electron beam (or pulse) quality:
T 

mc
x 2 k x2 
1
x.pT
mc
… a conserved quantity in a ‘perfect’ system.
 ‘Short-pulse’ Child’s Law: x0 
Nq
≈ 0.5mm for N = 108
 0 E DC
 Reduce pT
Standard theoretical expressions:
 Single-photon photoemission: pT 
m(  eff )
3
 Thermionic emission: pT  mkBT
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
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Experiment
 2W, 250fs, 63MHz , diodepumped 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
Analytical Gaussian (AG) model
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DC photo-gun
− Extended AG model simulation
 Fourier plane
beam size
independent
of x0
pT0
 Agreement with
experiment
indicates minimal
aberrations
Detector
Lenses
½pT0
J.A. Berger & W.A. Schroeder, J. Appl. Phys. 108 (2010) 124905
2ħω thermionic emission (2ωTE)
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– ħω = 2.37eV and Au = 5.1eV
EXPECT:
e0.35eV
Au
 Isotropic rms momentum pT
~35meV
ħ
ħ
EDC  8kV/cm
F
Au
Vacuum
Thermionic emission of tail
of two-photon excited Fermi
electron distribution
 I2Laser dependence of emission
 Increasing pT with ILaser
 Heating of Fermi electron gas
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2ωTE: Au results
– 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
I2
 pT 
m0 k BTe
… as m ≈ m0 in Au
GaSb and InSb photoemission?
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– ‘Real space’ picture: ħωLaser = 4.75eV (261nm)
Electron yield, Y
GaSb
GaSb
InSb
InSb
Expect minimal (if any) singlephoton photoemission:
ħωLaser
ħωLaser
ħω (eV)
G.W. Gobeli & F.G. Allen, Phys. Rev. 137 (1965) A245
ħω  eff ≤ 0
… Schottky barrier suppression
~35meV at 8kV/cm
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GaSb and InSb results
− Strong electron emission with ~4ps, 261nm pulses
 p-polarized UV
radiation incident
at 60º:
InSb
 GaSb ≈ 4x10-6
 InSb ≈ 7x10-6
GaSb
GaSb
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GaSb band structure
– Vacuum level at eff = 4.84eV above bulk VB maximum
 Strong absorption at 261nm:
eff
 = 1.44x106cm-1
 -1 ≈ 7nm
εF
… ‘metal-like’
 -valley transitions from VB
(HH, LH, and SO bands) to
upper 8 conduction band
J.R. Chelikowsky & M.L. Cohen, Phys. Rev. B 14 (1976) 556
D.E. Aspnes & A.A. Studna, Phys. Rev. B 27 (1983) 985
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ESTE in GaSb
− -valley absorption at ħω = 4.75eV
E
8

Eelectron
CB
Eg
/
τdecay
7
ħω
k
LH
SO
Eg/
3.85eV

0.99eV
Initial excess Eelectron
 Te
~0.35eV
4,200K
ħωLO
29meV
τLO
~200fs
m*(8)
~0.3m0
 Initially; exp[-/(kBTe)] ≈ 0.06
Eg
HH
GaSb properties
 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
pT for GaSb
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− Analysis of Fourier plane momentum distribution
Fit to AG model simulation using
pT  mkBT gives
mT ≈ 360m0
(i) For m = m0 with T = 360K:
exp[-/(kBT)] ~ 10-15
… no emission !!
480(±50)μm
(HWe-1M)
(ii) For m = m* ≈ 0.3m0 with
T = 1,200K:
exp[-/(kBT)] ≈ 5x10-5
… reasonable for TE
(c.f. GaSb ≈ 4x10-6)
 pT  m * k BTe
m* dependence of pT
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− Quantum mechanics: Potential step
e-
p2
Cathode
p 22
E2 
2m0
p12
E1 
2m *

p2
p//
p1
e-
p//
p1
Cathode
Vacuum
Vacuum
Momentum parallel to interface is conserved
AND for emission; p//max  2m * ( E1  )
 An implicit m* dependence for pT
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1ωPE: Ag photocathode
− Fourier plane data vs. AG model simulation
Spot size (mm)
ħω = 4.75eV
(261nm)
Ag
E = ħω  eff (eV)
pT 
m0 (  eff )
3
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1ωPE: Metals
− Ag, Ta, Mo, and W
Spot size (mm)
ħω = 4.75eV
(261nm)
pT 
Ag
Ta
W
Mo
E = ħω  eff (eV)
m0 (  eff )
3
pT and m*
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− Effective mass in metal photocathodes: dH-vA, CR, optical, …
Cu
pT ,expt.
m0 (   eff )
3
Mo
Mg
Ag
W
Ta
m*
m0
H.J. Qian et al., Phys. Rev. ST – Acc. & Beams 15 (2012) 040102
X.J. Wang et al., Proceedings of LINAC2002, Gyeongju, Korea.
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Photoemission Simulation
− Ag photocathode (eff = 4.52eV, ħω = 4.75eV, F = 5.5eV, Te = 300K)
m* = m0
Transverse momentum distribution
(Fourier plane)
0.8
1.0
pz ((m0.eV))
0.8
0.6
0.6
0.4
0.4
0.2
0.0
1.0
-1.0
0.2
0.5
0.0
-0.5
0.5
0.0
0.5
pT ((m0.eV))
pT ,sim.
0.0
-1.0
-0.5
0.0
pT ((m0.eV))
0.5
1.0
m0 (   eff )
3
 1.06
1.0
1.0
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Photoemission Simulation
− ‘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
pz ((m0.eV))
1.0
max. =
1.0
m* ≈ 33
 m0
sin-1
0.8
0.6
0.8
0.4
0.6
0.2
0.0
0.4
0.6
-0.6
0.4
-0.4
0.2
-0.2
0.0
0.0
0.2
0.2
0.4
0.4
pT ((m0.eV))
0.2
pT ,sim.
0.0
-0.6
-0.4
-0.2
0.0
pT ((m0.eV))
0.2
0.4
0.6
m0 (   eff )
3
 0.64 
m*
m0
0.6
0.6
pT and m*
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− Effective mass in metal photocathodes: dH-vA, CR, optical, …
Cu
pT ,expt.
m0 (   eff )
Te ?
3
Mo
Simulation
(Te =0)
Ag
W
Oxide?
Mg
Ta
m*
m0
H.J. Qian et al., Phys. Rev. ST – Acc. & Beams 15 (2012) 040102
X.J. Wang et al., Proceedings of LINAC2002, Gyeongju, Korea.
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Summary
m*
 Mean square transverse momentum:
pT 2 
M (   eff )
3
 3k BTe
1 
    eff





2
… where M = min (m*, m0)
 PLUS: small emission efficiency enhancement for m* < m0
 A route to high brightness, planar photocathodes
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Thank you!
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NEA GaAs
− Cesiated NEA GaAs photocathode (GaAs-CsO)
1.8
m* = 0.067m0
≈ 15
 max.  sin 1
m*
 15
m0
pz ((m0.eV))
1.6
1.4
1.2
1.0
0.8
-0.3
-0.2 -0.1
0.0
0.1
pT ((m0.eV))
Zhi Liu et al., J. Vac. Sci. Tech. B 23 (2005) 2758
0.2
0.3
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m*: Emission efficiency
− Quantum mechanics: Potential step
ep 22
E2 
2m0
p12
E1 
2m *

Barrier transmission:
T
2
 p  p2 
2

 1  R  1   1
 p1  p 2 
 |T |2 ≈ 1 for p1 ≈ p2
i.e., for m*E1 ≈ m0E2
Cathode
Vacuum
… only possible for m* < m0
2
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m*: Emission efficiency
− Quantum mechanics: Potential step
 = 4.5eV
Barrier
transmission:
eE2 
p12
E1 
2m *
p 22
2m0

|T|2
T
2
m* = 0.1m0
 p  p2 
2

 1  R  1   1
 p1  p 2 
2
 |T |2 ≈ 1 for p1 ≈ p2
i.e., for m*E1 ≈ m0E2
Cathode
Vacuum
m* = m0
m* = 10m0
… only possible for m* < m0
E = ħω   (eV)
 Emission efficiency enhancement for m* < m0
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