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New Photocathode Materials for Electron-ion-colliders. Zhaozhu Li, Kaida Yang, Jose M. Riso and R. Ale Lukaszew 1 Department of Physics, College of William and Mary 2 Department of Applied Science, College of William and Mary. Acknowledgements. College of William and Mary

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New photocathode materials for electron ion colliders

New Photocathode Materials for Electron-ion-colliders

Zhaozhu Li, Kaida Yang, Jose M. Riso and R. Ale Lukaszew

1 Department of Physics, College of William and Mary

2 Department of Applied Science, College of William and Mary


College of William and Mary

Professor R. A. Lukaszew

Dr Jose Riso

Kaida Yang

Doug Berringer

Jefferson Lab

Dr Matt Peolker

Dr Marcy Stuzman


Department of Energy

Award #


Principal Investigator

R. A. Lukaszew



About the Goal and Photocathodes


To Find A Metal-based Photocathodes Able to Sustain High Currents


Schematic Design and Experiment Setup


Premilinary Results and Future Plan


Electron Ion Collider


robust metal-based





eRHIC and MEIC: 100mA unpolarized e-beam

eRHIC: 50mA polarized e-beam





Fig 2

Fig 1

Semiconductor photocathodes
Semiconductor Photocathodes

Polarized e-beam:

Unpolarized e-beam:

Strained Superlattice


Many options

Multi-alkali photocathodes

GaAs, etc

Polarization 90%

Quantum Efficiency 1%

Quantum Efficiency ≦10%

Pressure ~ E-10 torr

Sensitive to contamination

Life time ~ hours or days

Response time ~ 10s picosecs

More stable to environment contamination

Life time ~ years

Response time ~ picosecs

Metal based photocathodes
Metal-based Photocathodes

QE: much lower than that of semiconductor photocathodes

High reflectivity

Short escape


High Work


High number of scattering events

step A

step B

step B

Surface plasmon resonance spr
SurfacePlasmon Resonance(SPR)


  • 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

Fig 3

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

Fig 4

Additional layer to lower the work function
Additional layer to lower the work function





Theoretical Prediction

Additional layer to lower the work function1
Additional layer to lower the work function





Theoretical Prediction

Fig 6

Fig 5

1 L. Giordano et al, Phs Rev B 73, 045414 (2005)

2 T Konig et, al,J. Phys. Chem. C 2009, 113, 11301

Fig 4

Afm characterization a ag mgo sample
AFM characterization a Ag/MgO sample

This sample gives closest SPR measurement to the predicted angle.

SPR angle

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

MgO/Ag ~ 48.8 degree

Ag ~ 41.5 degree

S chematic design
Schematic Design


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






Loadlock Overview


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

Mathematic program

to simulate SPR

Find resonance angle

Calculate the SPR angle

under grating scheme

Experiment setup1
Experiment Setup


Keithley Picoammeter

Experiment setup2
Experiment Setup

Ceramic Isolated with Chamber

Preliminary results
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!

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 decreasesto 0.085 pA and again goes upto ~0.087pA

Conclusions and future plans
Conclusions and Future Plans

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

The resultsare 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
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.