![Page 1: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/1.jpg)
Special Topics Presentation on Electron Sources – Part 2
Joe Grames, Jefferson LabOctober 22, 2012
Old Dominion University, Fall 2012
(with many thanks for slides borrowed fromCarlos-Hernandez Garcia, Matt Poelker, Riad Suleiman)
![Page 2: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/2.jpg)
What about the probing with spin ?
Target
You measure
an experimental
asymmetry
![Page 3: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/3.jpg)
• Electron motion in the electric field of nucleus results in magnetic field in electron
rest frame, , if is nucleus-electron separation, then and EVc
B 1 r
r
r
ZeE
3
Lmcr
Zevr
cr
ZeB
33
More probable for red “spin down”
electron to scatter to the “right”
More probable for blue “spin up”
electron to scatter to the “left”
Mott Scattering (an asymmetry example)
![Page 4: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/4.jpg)
Vr
L L Vr
L L
• Interaction of this magnetic field with electron (spin) magnetic moment
introduces a term in the scattering potential,
• Presence of spin-orbit term in scattering potential introduces spin
dependence in scattering cross section which could be detected as a
left/right count rate asymmetry
LV sso
SLrcm
Ze
r
ZeVVV soc
322
2
2
)(
![Page 5: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/5.jpg)
Mott Cross Section
]ˆ)(1)[()( nPSI
“normal” to the scattering plane
Sign depends on
scattering to the
“left” or right”
…thus the likelihood (probability) to scatter into an
angle is correlated with the spin orientation
![Page 6: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/6.jpg)
Target
You measure
an experimental
asymmetry
]ˆ)(1)[()( nPSI
![Page 7: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/7.jpg)
But, real effects in the real world….
Target
e
I
M
NdR beam
foil
Afoilfoil)()(
![Page 8: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/8.jpg)
Target Target
Reverse the beam polarization….
![Page 9: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/9.jpg)
Parity Violation Experiments at CEBAFExperiment Energy
(GeV)
I
(µA)
Target Apv
(ppb)
Maximum
Charge
Asym
(ppb)
Maximum
Position
Diff
(nm)
Maximum
Angle Diff
(nrad)
Maximum
Size Diff
(δσ/σ)
HAPPEx-II
(Achieved)
3.0 55 1H
(20 cm)
1400 400 1 0.2 Was not
specified
HAPPEx-III
(Achieved)
3.484 100 1H
(25 cm)
16900 200±100 3±3 0.5±0.1 10-3
PREx 1.063 70 208Pb
(0.5 mm)
500 100±10 2±1 0.3±0.1 10-4
QWeak 1.162 180 1H
(35 cm)
234 100±10 2±1 30±3 10-4
Møller 11.0 75 1H
(150 cm)
35.6 10±10 0.5±0.5 0.05±0.05 10-4
PV experiments motivate polarized e-source R&D
![Page 10: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/10.jpg)
Electron Bunch Spin & Polarization
0% Polarization
50% Polarization
v
v
People with very different opinions
Light: a preference for the electric field vector to be oriented a certain way
Electrons: a preference for electrons to spin in one direction
![Page 11: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/11.jpg)
Self-Polarization: Sokolov-Ternov Effect• Electrons (positrons) self-polarize in storage rings due to spin-flip
synchrotron radiation: one spin state dominates the other• First observed @ VEPP-2 Ring at Budker Institute, Novosibirsk, 1971• Requires spin rotation (Siberian Snake) to create longitudinal
polarization at target & depolarizing resonances must be avoided• Process is slow, need storage lifetime to be longer than self-
polarization time & happy to end up with ~70% polarization
~ 1 hour at HERA
![Page 12: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/12.jpg)
What if you need a Direct Source of Polarized Electrons?
![Page 13: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/13.jpg)
GaAs….the method that caught on
![Page 14: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/14.jpg)
Three step photoemission in NEA-GaAs(Spicer model, 1958)*
energy
With Cs+O2 layer
semiconductor
EF
Evacuum
Evacuum ~ 4 eV
χeff ≈ - 0.3 eV
emissiondiffusion
relaxation
conduction band
minimum
valence band
maximum
vacuum
recombination
electron
Photo-excitation
1
23
* Still not fully understood
![Page 15: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/15.jpg)
GaAs Energy Levels
• Energy versus momentum• GaAs is a “Direct” transition semiconductor• Valence band P-state split due to spin-orbit coupling• mj quantum numbers describe electron’s spin and orbital angular momentum• Quantum mechanical selection rules dictate mj=+/-1 for absorption of circularly
polarized light• Clebsch-Gordon coefficients indicate the relative likelihood of transitions between
states
First proposed by Garwin, Pierce, Siegmann and Lampel and Weisbuch
![Page 16: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/16.jpg)
Pierce-Meier Apparatus
![Page 17: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/17.jpg)
First Observation of Polarization
Pierce and Meier, Phys. Rev. B, 13, 5484 (1976)
• Maximum polarization not 50%
• Note interesting non-zero polarization sub-peaks at 3.0eV and 3.2eV
• Flip the sign of polarization by flipping the polarity of the light
![Page 18: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/18.jpg)
First High Voltage GaAs Photogun
Started with a thermionic gun housing?
![Page 19: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/19.jpg)
First GaAs Photoinjector• Built for SLAC parity-violation experiment E122• Polarized electrons accelerated December, 1977• E122 announces parity violation June, 1978 - an important
verification of the Standard Model
![Page 20: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/20.jpg)
Pablo Saez, PhD Thesis, SLAC Report 501, 1997
Typical bulk GaAs Result
•QE at bandgap (i.e., where you get highest polarization) can be 1% or more
![Page 21: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/21.jpg)
Depolarization Mechanisms• BAP Process: the exchange
interaction between electrons and holes (after G. L. Bir, A. G. Aronovand G. E. Picus)
• DP Process: the dynamic narrowing of the magnetic resonance in spin–orbit split–off conduction bands (after M. I. Dyakonov and V. I. Perel)
• EY process in which the spin–orbit interaction generates non–pure spin states in the conduction band (after R. J. Elliot and Y. Yafet)
• Radiation Trapping, where recombination radiation is re-absorbed producing unpolarizedphotoemission
Time scales for these depolarization processes are roughly equal to the lifetime of the electron in the conduction band, ~ 200ps. Therefore, it is very important to get the polarized electrons out of the material as quickly as possible
-
-
-
χNEA ~ 0.2 eV
GaAs Vacuum
Light
![Page 22: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/22.jpg)
What limits polarization?
Maruyama et al., Appl. Phys. Lett., 55, 1686 (1989)
Polarization lost as electrons diffuse to the surface: thin samples provide higher polarization, at expense of QE
G. Fishman and G. Lampel, Phys Rev. B16, 820 (1977)
Absorption depth ~ 1um in GaAs
![Page 23: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/23.jpg)
PhD thesis, Paul Zorabedian, SLAC Report 248, 1982
Breaking the 50% barrier
Electron polarization inferred from photoluminescence measurements
Compress the GaAs crystal in hydraulic press!Hard to keep the GaAs sample from shattering
![Page 24: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/24.jpg)
Eliminate degeneracy of P3/2 state via “Interface Stress Method”
Image from Pablo Saez, PhD Thesis, Stanford University, SLAC Report 501, 1997
![Page 25: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/25.jpg)
Lattice mismatch provides stress
Pablo Saez, PhD Thesis, SLAC Report 501, 1997
• The band gap of the substrate layer must be larger than surface layer
• Lattice constants must differ enough to introduce suitable strain
• Adjust lattice constant of substrate by varying concentration of third element
1% lattice mismatch provides equivalent force as hydraulic press!
![Page 26: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/26.jpg)
Strained-layer GaAs
Zn dopant
~5.1018
(cm-3)
625 μm
2.5 μm
2.5 μm
100 nm
GaAs1-x Px (0<x<0.29)
GaAs
p-type GaAs
substrate
GaAs buffer
GaAs0.71 P0.29
• MOCVD-grown epitaxial spin-polarizer wafer
• Polarization ~ 75% at ~ 850nm• QE ~ 0.1%• Available from Bandwidth
Semiconductor• 3” dia. wafer ~ 10k$• Developed via DOE-SBIR
programManufactured by Bandwidth Semiconductor
![Page 27: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/27.jpg)
First Strained GaAs Result
Maruyama et.al., Phys. Rev. Lett., 66, 2376 (1991)
InxGa1-xAs grown on GaAs substrate (x = 0.13)
0.1um thick surface layer 1.14um thick surface layer
![Page 28: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/28.jpg)
Getting the Recipe Right
• Choice of Surface layer• Choice of Substrate layer• Tensile vs compressive strain?• What is correct lattice
mismatch?• How thick to make the active
layer?
![Page 29: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/29.jpg)
Getting the Recipe Right
• Thickness can be 10x greater than tc
• Band splitting needs to be > 30 meV
Aoyagi, Nakanishi, et.al., Division of Physics Nagoya University Tech Note 93-14
![Page 30: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/30.jpg)
Higher Polarizations Followed
Maruyama et al., Phys. Rev. B., 46, 4261 (1991)
GaAs grown on top of GaAs1-xPx substrateGaAs thickness ~ 0.1 um and x = 0.29, lattice mismatch ~ 1%
This became the standard SPIN Polarizer wafer sold by SPIRE, now Bandwidth Semiconductor
![Page 31: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/31.jpg)
Strained-layer GaAs
Zn dopant
~5.1018
(cm-3)
625 μm
2.5 μm
2.5 μm
100 nm
GaAs1-x Px (0<x<0.29)
GaAs
p-type GaAs
substrate
GaAs buffer
GaAs0.71 P0.29
• MOCVD-grown epitaxial spin-polarizer wafer
• Polarization ~ 75% at ~ 850nm• QE ~ 0.1%• Available from Bandwidth
Semiconductor• 3” dia. wafer ~ 10k$• Developed via DOE-SBIR
programManufactured by Bandwidth Semiconductor
![Page 32: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/32.jpg)
Higher P, Higher QE?
• Problem: Strained layers start relaxing
beyond thickness ~10nm. Strained layer
practical limit ~100nm
Strain relaxation Lower polarization
Thin layer Lower QE
• So how to get Higher Polarization and
Higher QE?
• Solution: Use many thin strained layers –
Strained Superlattice Photocathode…
![Page 33: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/33.jpg)
Strained Superlattice Photocathode
eg., GaAs/GaAsPstrained superlattice
GaAs substrate
Strain-relaxed GaAsP buffer layer
It is important that electrons are excitedONLY FROM HEAVY-HOLE MINI-BAND
×
NEA surface
Conduction mini-band
Heavy hole mini-band
Light hole mini-band
Slide courtesy Toru Ujihara, PESP 2008
Electrons tunnel through very thin buffer layers!!
![Page 34: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/34.jpg)
Getting the Recipe Right
From Aaron Moy, SVT Assoc and SLAC, PESP2002
Strained GaAs
GaAsP
Strained GaAs
GaAsP
Strained GaAs
GaAsP 30 A
30 A
GaAs Substrate
GaAs(1-x)Px Graded Layer
GaAs0.64P0.36
Buffer
Active Region
2.5 m
2.5 m
1000 A
Strained Superlattice Photocathode
x 16 pair
Notice more [P] → more strain, more P3/2 state splitting, higher Pol
![Page 35: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/35.jpg)
Higher Polarization AND Higher QE
100
80
60
40
20
0
Po
lari
za
tio
n (
%)
780760740720700680660640
Wavelength (nm)
0.01
0.1
1
QE
(%)
SVT-3984
SVT-3682
D. Luh et al, SLAC, PESP2002
• MBE-grown epitaxial spin-polarizer wafer
• Pol ~ 85% at ~ 780nm• QE ~ 1%• Available from SVT
Associates• 2” dia. wafer ~ 10k$• Developed via DOE-
SBIR program
![Page 36: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/36.jpg)
Significant FOM Improvement
This means it takes less time to do an experiment with same level of statistical accuracy
![Page 37: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/37.jpg)
III-V Compound Semiconductors
III IV V VI VII VIII
Still Tweaking the Recipe
Courtesy Aaron Moy of SVT Associates
Still looking for combinations that
provideHigher Polarization,
Higher QE, more rugged lifetime
![Page 38: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/38.jpg)
DBR – Equipped CrystalFor instance, talk by L. Gerchikov, St. Petersburg, at PESP 2007
![Page 39: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/39.jpg)
Resonant enhancement of QE
550 600 650 700 750 800 850 900 9500
2
4
6
8
10
0
20
40
60
80Q
E E
nch
an
ce
me
nt
Wavelength, nm
QE enchancement
SPTU data
P-4, SL QT 1890 non DBR P-2, SL QT 1890 DBR
Pola
rization,
%
Accepted for publication at Semiconductors, 2008
Leonid Gerchikov, PESP2008
![Page 40: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/40.jpg)
Internal Gradient Strained-Superlattice
• Photocathode active layers with
internal accelerating field
• Internal field enhances electron
emission for higher QE
• Less transport time also reduces
depolarization mechanisms
• Gradient created by varied alloy
composition or dopant profile
Courtesy Aaron Moy of SVT Associates
![Page 41: Special Topics Presentation on Electron Sourcescasa.jlab.org/publications/viewgraphs/USPAS2011/Grames... · 2012. 10. 31. · Special Topics Presentation on Electron Sources –Part](https://reader034.vdocuments.us/reader034/viewer/2022052015/602ca5d255fab8296605c515/html5/thumbnails/41.jpg)
Internal Gradient GaAs/AlGaAs SLs
• Polarization decreased
as aluminum gradient
increased
• Due to less low LH-HH
splitting at low aluminum %
• QE increased 25% due to
internal gradient field
• Peak polarization of 70 %
at 740 nm, shorter than
875 nm of GaAs
Courtesy Aaron Moy of SVT Associates