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