uic m* : a route to ultra-bright photocathodes w. andreas schroeder joel a. berger and ben l....
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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
… as m ≈ m0 in Au
eBT Tkmp 0
GaSb and InSb photoemission? UIC
G.W. Gobeli & F.G. Allen, Phys. Rev. 137 (1965) A245
– ‘Real space’ picture: ħωLaser = 4.75eV (261nm)
Ele
ctro
n yi
eld,
Y
ħωLaser ħωLaserħω (eV)
GaSb InSb
InSbGaSb
Expect minimal (if any) single-photon photoemission:
ħω eff ≤ 0
… Schottky barrier suppression ~35meV at 8kV/cm
UICGaSb 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
GaSb band structure UIC
J.R. Chelikowsky & M.L. Cohen, Phys. Rev. B 14 (1976) 556D.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:
= 1.44x106cm-1
-1 ≈ 7nm
… ‘metal-like’
-valley transitions from VB (HH, LH, and SO bands) to upper 8 conduction band
eff
εF
UICESTE in GaSb− -valley absorption at ħω = 4.75eV
8
7CB
HHLH
SO
Eg
Eg/
E
k
ħω
GaSb properties
Eg/ 3.85eV
0.99eV
Initial excess Eelectron
Te
~0.35eV4,200K
ħωLO 29meV
τLO ~200fs
m*(8) ~0.3m0
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
τdecay
Eelectron
UICpT for GaSb− 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)
Tmkp BT
480(±50)μm(HWe-1M)
eBT Tkmp *
UICm* dependence of pT
− Quantum mechanics: Potential step
Momentum parallel to interface is conserved
AND for emission;
An implicit m* dependence for pT
)(*2 1max// Emp
e-
Cathode Vacuum
*2
21
1 m
pE
0
22
2 2m
pE
p2
p1p//
p1
p//p2
Cathode
Vacuum
e-
UIC1ωPE: Ag photocathode− Fourier plane data vs. AG model simulation
3
)(0 effT
mp
E = ħω eff (eV)
Ag
ħω = 4.75eV(261nm)
UIC1ωPE: Metals− Ag, Ta, Mo, and W
3
)(0 effT
mp
E = ħω eff (eV)
Mo
Ta
W
Ag
ħω = 4.75eV(261nm)
UICpT and m*− Effective mass in metal photocathodes: dH-vA, CR, optical, …
H.J. Qian et al., Phys. Rev. ST – Acc. & Beams 15 (2012) 040102X.J. Wang et al., Proceedings of LINAC2002, Gyeongju, Korea.
AgW
Ta
Mo
Cu
Mg
0
*
m
m
3
)(0
.,
eff
T
m
p
expt
UICPhotoemission Simulation− Ag photocathode (eff = 4.52eV, ħω = 4.75eV, F = 5.5eV, Te = 300K)
pT ((m0.eV))
-1.0 -0.5 0.0 0.5 1.0
0.8
0.6
0.4
0.2
0.0
m* = m0
1.0 0.5 0.0 0.5 1.0
0.0
0.2
0.4
0.6
0.8
1.0
pT ((m0.eV))
-1.0 -0.5 0.0 0.5 1.0
Transverse momentum distribution (Fourier plane)
06.1
3
)(0
.,
eff
T
m
p
sim
UICPhotoemission Simulation− ‘Light Fermion’ Ag photocathode (eff = 4.52eV, ħω = 4.75eV, F = 5.5eV, Te = 300K)
pT ((m0.eV))
m* = 0.3m0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
max. = sin-1 ≈ 33
m* m0
0.6 0.4 0.2 0.0 0.2 0.4 0.6
0.0
0.2
0.4
0.6
0.8
1.0
pT ((m0.eV))
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
Transverse momentum distribution (Fourier plane)
00
., *64.0
3
)( m
m
m
p
eff
T
sim
UICpT and m*− Effective mass in metal photocathodes: dH-vA, CR, optical, …
H.J. Qian et al., Phys. Rev. ST – Acc. & Beams 15 (2012) 040102X.J. Wang et al., Proceedings of LINAC2002, Gyeongju, Korea.
AgW
Ta
Mo
Cu
Mg
0
*
m
m
3
)(0
.,
eff
T
m
p
expt
Oxide?
Te ?
Simulation(Te =0)
UICSummary
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
2
2 31
3
)(
eff
eBeffT
TkMp
UIC
Thank you!
UICNEA GaAs
Zhi Liu et al., J. Vac. Sci. Tech. B 23 (2005) 2758
− Cesiated NEA GaAs photocathode (GaAs-CsO)
m* = 0.067m0
15*
sin0
1.max m
m
pT ((m0.eV))
1.8
1.6
1.4
1.2
1.0
0.8
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
≈ 15
UICm*: Emission efficiency− Quantum mechanics: Potential step
e-
Cathode Vacuum
*2
21
1 m
pE
0
22
2 2m
pE
Barrier transmission:
|T |2 ≈ 1 for p1 ≈ p2
i.e., for m*E1 ≈ m0E2
… only possible for m* < m0
2
21
212211
pp
ppRT
UICm*: Emission efficiency− Quantum mechanics: Potential step
Emission efficiency enhancement for m* < m0
e-
Cathode Vacuum
*2
21
1 m
pE
0
22
2 2m
pE
Barrier transmission:
|T |2 ≈ 1 for p1 ≈ p2
i.e., for m*E1 ≈ m0E2
… only possible for m* < m0
2
21
212211
pp
ppRT
|T|2
E = ħω (eV)
m* = 10m0
m* = m0
m* = 0.1m0
= 4.5eV
UIC