New Photocathode Materials
for Electron-ion-colliders
1
Zhaozhu Li, Kaida Yang, Jose M. Riso
2
2
1
and R. Ale Lukaszew
1 Department of Physics, College of William and Mary
2 Department of Applied Science, College of William and Mary
1
Acknowledgements
College of William and
Mary
Jefferson Lab
Dr Matt Peolker
Dr Marcy Stuzman
Professor R. A. Lukaszew
Dr Jose Riso
Kaida Yang
Doug Berringer
Funding
Department of Energy
Award #
DE-SC0008546
Principal Investigator
R. A. Lukaszew
2
Outline
• INTRODUCTION:
About the Goal and Photocathodes
• APPROACHES:
To Find A Metal-based Photocathodes Able to Sustain High
Currents
• REALIZATION:
Schematic Design and Experiment Setup
• ON THE WAY:
Premilinary Results and Future Plan
3
Goal
Electron Ion Collider
(EIC)
eRHIC: 50mA
polarized e-beam
robust metal-based
photocathode
+
large
currents
eRHIC and MEIC:
100mA unpolarized ebeam
spin-polarized
currents
1 http://www.bnl.gov/cad/eRhic/
2 http://www.jlab.org/conferences/qcd2012/talks/wednesday/Pawel%20NadelTuronski.pdf
Fig 11
Fig 2 2
4
Semiconductor Photocathodes
Polarized e-beam:
Strained Superlattice
GaAs/GaAsP
Polarization 90%
Quantum Efficiency 1%
Pressure ~ E-10 torr
Sensitive to contamination
Life time ~ hours or days
Response time ~ 10s picosecs
Unpolarized e-beam:
Many options
Multi-alkali photocathodes
GaAs, etc
Quantum Efficiency ≦10%
More stable to environment
contamination
Life time ~ years
Response time ~ picosecs
5
Metal-based Photocathodes
QE: much lower than that of semiconductor photocathodes
High reflectivity
step A
4/9/2015
Short escape
depth
High number of
scattering events
High Work
Function
step B
A: Surface Plasmon Resonance(SPR)
• SPR: Electrons oscillates coherently on a metal boundary
• Excitation: satisfying dispersion relationship
w 1 2
(
)
c 1   2
• We need to enhance the wave vector to
k spp 
excite the surface plasmon resonance
Fig 3 1
• Grating method to excite SPR
na sin( )  m

d

 mr na2
 mr  na2
1 A. Hibbins, "Grating Coupling of Surface Plasmon Polaritons at
Visible and Microwave Frequencies", phd thesis
Fig 4 1
7
B: Additional layer to lower the work function
MgO
Metal
Substrate
Theoretical Prediction
8
B: Additional layer to lower the work function
MgO
Metal
Substrate
Theoretical Prediction
Fig562 2
Fig
Fig 4 1
1 L. Giordano et al, Phs Rev B 73, 045414 (2005)
2 T Konig et, al,J. Phys. Chem. C 2009, 113, 11301
9
AFM characterization a Ag/MgO sample
20
Z[nm]
15
10
5
400nm
0
0
0.5
1
1.5
X[µm]
This sample gives closest SPR measurement to the predicted angle.
4/9/2015
2
SPR measurements
Ag30nm_CD
MgO10s_Ag30nm_CD
MgO20s_Ag30nm_CD
MgO20s_Ag30nm_CD
MgO40s_Ag30nm_CD
10
Rpp(arbitrary unit)
8
6
4
2
0
0
20
40
60
Angel(degree)
4/9/2015
80
100
SPR angle The 1st 20s MgO shows two
Ag30nm_CD
MgO10s_Ag30nm_CD
MgO20s_Ag30nm_CD
MgO20s_Ag30nm_CD
MgO40s_Ag30nm_CD
Rpp(arbitrary unit)
2
40
Angel(degree)
Ag ~ 41.5
degree
4/9/2015
MgO/Ag ~
48.8 degree
flat dips in SPR figure
between 43 to 47 as shown
in purple. The 2nd 20sMgO
sample also shows two dips
but the flat region from 1st
sample is more likely to be
one time occasion since the
other results seem to have
the same tendency.
The results for different
sputtering time of MgO up to
40s show a very similar SPR
angle~ 48.8 degree.(The
total internal reflection angle
has been adjusted to be the
same position for different
measurements.) However,
the Rpp reaches to a low
level region~less than 1.5V
from 43.5 to 55.5 degree
Schematic Design
2
1 Transport Fork
2 A New Arm: Manipulator
3 Faraday Cup
3
1
4
4 Sample holder and sample
5 Laser light
5
6 Additional fork to help
transport the sample
Sample preparation in-situ under
ultra high vacuum ~ E-9 torr
6
Loadlock Overview
13
Simulation
• Under two excitation methods: k vector to excite to be the same
Mathematic program
to simulate SPR
Find resonance
angle
na Sin grating  m
4/9/2015

d
 k sp
Calculate the SPR angle
under grating scheme
k sp 
w
Sin prism
c
Experiment Setup
15
Experiment Setup
Grounding
Keithley
Picoammeter
16
Experiment Setup
Ceramic Isolated
with Chamber
17
Experiment Setup
Grounded
18
Experiment Setup
19
Experiment Setup
20
Preliminary results
•Aspects of our setup
have been tested using
the photocathode
experimental system
at JLab
• Current very small ~
E-2 picoA
•We just finish setting
up this week!
21
Fine tuning photocurrent measurement
Blocking spurious light: The current increases from 0.083pA to ~0.089pA
~10 degrees
~60 degrees
~80 degrees
Rotating polarization with respect to pattern on sample: The current
decreases to 0.085 pA and again goes up to ~0.087pA
4/9/2015
Conclusions and Future Plans
• We use SPR and MgO thin film coating in our
experimental approach to achieve suitable metalbased photocathodes.
• The results are still very preliminary and further
improvements and calibration will be conducted.
• We will try more energetic photons for efficient
photocathode excitation (e.g. blue, at 400nm has an
energy of 3.1 eV compared to the ~0.8 eV in IR light).
For that we will use a tighter pattern for the
diffraction grating (going from CD to bluray DVD).
We will update our simulations to this new geometry
to establish the thickness so that the SPR can be
excited at 45 degrees incidence.
23
Polarized current?
• Our ultimate goal is to deposit a magnetic
material such as "silmanal", which is a silver
alloy with Mn and Al. This belongs to the socalled "Heussler alloys“ known for their high
degree of polarization
• Silmanal is magnetic and therefore it can be
used to spin-polarize the photo-electrons. The
major constituent of the alloy is silver. Hence
our preliminary studies on Ag photocathodes.
4/9/2015