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New Photocathode Materials for Electron-ion-colliders

Zhaozhu Li, Kaida Yang, Jose M.

Riso and R. Ale Lukaszew1 Department of Physics, College of William and Mary

2 Department of Applied Science, College of William and

Mary

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2 2

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Acknowledgements

College of William and Mary

Professor R. A. LukaszewDr Jose RisoKaida Yang

Doug Berringer

Jefferson Lab

Dr Matt PeolkerDr Marcy Stuzman

Funding

Department of Energy

Award #DE-SC0008546

Principal InvestigatorR. A. Lukaszew

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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

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Goal

robust metal-based photocathode

large currents

spin-polarizedcurrents

Electron Ion Collider(EIC)

eRHIC: 50mA polarized e-beam

eRHIC and MEIC: 100mA unpolarized e-beam

Fig 11 Fig 22

1 http://www.bnl.gov/cad/eRhic/2 http://www.jlab.org/conferences/qcd2012/talks/wednesday/Pawel%20Nadel-Turonski.pdf

+

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Semiconductor Photocathodes

Strained Superlattice GaAs/GaAsP

Polarization 90%

Quantum Efficiency 1%

Polarized e-beam:

Many optionsMulti-alkali photocathodes

GaAs, etc

Unpolarized e-beam:

Quantum Efficiency 10%≦

Pressure ~ E-10 torrSensitive to contamination

Life time ~ hours or daysResponse time ~ 10s picosecs

More stable to environment contamination

Life time ~ yearsResponse time ~ picosecs

04/21/23

Metal-based PhotocathodesQE: much lower than that of semiconductor photocathodes

High reflectivity Short escape depth

High Work Function

High number of scattering events step A step Bstep B

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Surface Plasmon Resonance(SPR) A:

• SPR: Electrons oscillates coherently on a metal boundary• Excitation: satisfying dispersion relationship

• We need to enhance the wave vector to excite the surface plasmon resonance

• Grating method to excite SPR

)(21

21

c

wkspp

2

2sin( ) mr a

amr a

nn m

d n

Fig 3 1

Fig 4 1

1 A. Hibbins, "Grating Coupling of Surface Plasmon Polaritons at Visible and Microwave Frequencies", phd thesis

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Additional layer to lower the work functionB:

Metal

Substrate

MgO

Theoretical Prediction

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Additional layer to lower the work functionB:

Metal

Substrate

MgO

Theoretical Prediction

Fig 4 1

Fig 6 2

1 L. Giordano et al, Phs Rev B 73, 045414 (2005)2 T Konig et, al,J. Phys. Chem. C 2009, 113, 11301

Fig 52

AFM characterization a Ag/MgO sample

04/21/23

400nm 21.510.50

20

15

10

5

0

X[µm]Z[nm]

This sample gives closest SPR measurement to the predicted angle.

SPR measurements

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0 20 40 60 80 100

0

2

4

6

8

10

Rp

p(a

rbitr

ary

un

it)

Angel(degree)

Ag30nm_CD MgO10s_Ag30nm_CD MgO20s_Ag30nm_CD MgO20s_Ag30nm_CD MgO40s_Ag30nm_CD

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SPR angle

40

2

Rpp

(arb

itrar

y un

it)

Angel(degree)

Ag30nm_CD MgO10s_Ag30nm_CD MgO20s_Ag30nm_CD MgO20s_Ag30nm_CD MgO40s_Ag30nm_CD

Ag ~ 41.5 degree

MgO/Ag ~ 48.8 degree

The 1st 20s MgO shows two 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

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Schematic Design

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1 Transport Fork

2 A New Arm: Manipulator

3 Faraday Cup

4 Sample holder and sample

5 Laser light

6 Additional fork to help transport the sample

Sample preparation in-situ under ultra high vacuum ~ E-9 torr

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Loadlock Overview

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Simulation• Under two excitation methods: k vector to excite to be the same

prismsp Sinc

wk

Mathematic program to simulate SPR

Calculate the SPR angleunder grating scheme

Find resonance angle

spgratinga kd

mSinn

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Experiment Setup

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Experiment Setup

Grounding

Keithley Picoammeter

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Experiment Setup

Ceramic Isolated with Chamber

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Experiment Setup

Grounded

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Experiment Setup

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Experiment Setup

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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!

04/21/23

Fine tuning photocurrent measurement

Blocking spurious light: The current increases from 0.083pA to ~0.089pA

Rotating polarization with respect to pattern on sample: The current decreases to 0.085 pA and again goes up to ~0.087pA

~10 degrees ~60 degrees ~80 degrees

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Conclusions and Future Plans

• We use SPR and MgO thin film coating in our experimental approach to achieve suitable metal-based 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.

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 so-called "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.

04/21/23