New Photocathode Materials for Electron-ion-colliders

1. 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

2. Acknowledgements College of William and Mary Professor R. A. Lukaszew Dr Jose Riso Kaida Yang Doug Berringer Jefferson Lab Dr Matt Peolker Dr Marcy Stuzman Funding Department of Energy Award # DE-SC0008546 Principal Investigator R. A. Lukaszew

3. 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

4. Goal Electron Ion Collider (EIC) robust metal-based photocathode large currents + eRHIC and MEIC: 100mA unpolarized e-beam eRHIC: 50mA polarized e-beam spin-polarized currents 1 http://www.bnl.gov/cad/eRhic/ 2 http://www.jlab.org/conferences/qcd2012/talks/wednesday/Pawel%20Nadel-Turonski.pdf Fig 2 Fig 1

5. Semiconductor Photocathodes Polarized e-beam: Unpolarized e-beam: Strained Superlattice GaAs/GaAsP 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

6. Metal-based Photocathodes QE: much lower than that of semiconductor photocathodes High reflectivity Short escape depth High Work Function High number of scattering events step A step B step B

7. SurfacePlasmon 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 Fig 3 1 A. Hibbins, "Grating Coupling of Surface Plasmon Polaritons at Visible and Microwave Frequencies", phd thesis Fig 4

8. Additional layer to lower the work function B: MgO Metal Substrate Theoretical Prediction

9. Additional layer to lower the work function B: MgO Metal Substrate 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

10. AFM characterization a Ag/MgO sample This sample gives closest SPR measurement to the predicted angle.

11. SPR measurements

12. 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

13. Schematic Design 2 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 3 1 4 5 6 Loadlock Overview

14. Simulation • 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

15. Experiment Setup

16. Experiment Setup Grounding Keithley Picoammeter

17. Experiment Setup Ceramic Isolated with Chamber

18. Experiment Setup Grounded

19. Experiment Setup

20. Experiment Setup

21. 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!

22. 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

23. 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.

24. 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.