UIC

m*: A Route to Ultra-bright Photocathodes

W. Andreas SchroederJoel 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

DCENq

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)

Elec

tron

yiel

d, 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 mp

E

0

22

2 2mp

E

p2

p1p//

p1

p//p2

Cathode

Vacuum

e-

UIC1ωPE: Ag photocathode− Fourier plane data vs. AG model simulation

3)(0 eff

Tm

p

E = ħω eff (eV)

Ag

ħω = 4.75eV(261nm)

UIC1ωPE: Metals− Ag, Ta, Mo, and W

3)(0 eff

Tm

p

E = ħω eff (eV)

Mo

TaW

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

*mm

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 1.00.0

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 0.60.0

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

mm

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

*mm

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 mp

E

0

22

2 2mp

E

Barrier transmission:

|T |2 ≈ 1 for p1 ≈ p2

i.e., for m*E1 ≈ m0E2

… only possible for m* < m0

2

21

2122 11

pppp

RT

UICm*: Emission efficiency− Quantum mechanics: Potential step

Emission efficiency enhancement for m* < m0

e-

Cathode Vacuum

*2

21

1 mp

E

0

22

2 2mp

E

Barrier transmission:

|T |2 ≈ 1 for p1 ≈ p2

i.e., for m*E1 ≈ m0E2

… only possible for m* < m0

2

21

2122 11

pppp

RT|T|2

E = ħω (eV)

m* = 10m0

m* = m0

m* = 0.1m0

= 4.5eV

UIC