1. Real-time Ellipsometry on Cesium-Telluride Photocathode Formation
Martijn Tesselaar & Peter van der Slot CARE07

2. Contents Introduction Electron Accelerator Photoelectric Effect
Ellipsometry on Cs2Te Photocathodes
Photocathode preparation
Rotating Compensator Ellipsometry
RCE measurement results
Conclusions

3. Electron Accelerator Applications
External Beam Radiotherapy
Synchrotron radiation
Electron collider experiments
Free Electron Laser

4. Linear Accelerator
Laser pulse on photocathode => short electron bunch
Radio Frequency Electromagnetic waves accelerate the bunch
Magnets are used for confinement

5. Photoelectric Effect Kinetic energy
Quantum Efficiency = Number of electrons emitted per photon

6. Cs2Te Photocathode Preparation
Substrate at 120°C
Deposit Tellurium by Physical Vapor Deposition (PVD) for about 30 minutes
Deposit Cesium by PVD until cathode is completed
Cs and Te mixing produces multiple CsxTey layers

7. Quantum Efficiency
Photocathode irradiated by UV lamp during deposition
Photocurrent measured using picoamperemeter
Photocathode considered finished at maximum QE
Start Cesium Deposition

8. Ellipsometry on Cs2Te Photocathodes
Ellipsometer
To study the deposition process
Optical method: photocathode stays inside, measurement device outside
Real-time measurements register steps in the deposition process
Preparation Chamber

9. Reflection from thin film structures
Path length difference:
Resulting in phase difference: 2 1 A B C D n2 n1 d
Noodzaak voor dunne films anders overlappen de diverse bundels natuurlijk niet meer. Link van dit effect naar variabel beta in de formule op volgende sheet; misschien wat te ingewikkeld om te onthouden.
Fresnel Reflection Coefficients give change in amplitude and phase determined by film thickness d, refractive index n and absorption coefficient  of the thin film material

10. Sample & Polarization
Sample optical properties contained in the ellipsometric quantities  and :
 and  also depend on film thickness, refractive index and absorption coefficient

11. Rotating Compensator Ellipsometry
Compensator (QWP) rotates continuously
Sample properties influence reflected beam characteristics
Reflected beam characteristics influence intensity after analyzer
Correlation between compensator angle and detector signal gives information about sample properties
HeNe laser
Faraday Isolator
HWP
Polarizer
QWP
Analyzer BS D1
Window
Sample
Copper mirror

12. Quarter Waveplate Rotation

13. Stokes Vector Beam characteristics: Intensity Polarization angle a b a2 a1 x y
Beam characteristics:
Intensity
Polarization angle
Polarization ellipticity
Polarization rotation direction (CW or CCW)
These 4 characteristics may be represented in a 4 element vector called the Stokes vector:

14. Mueller Matrix
Each optical element may be represented by a 4x4 matrix called a Mueller matrix, for example for a sample with properties  and :
So that the exiting Stokes vector is:
For a quarter wave plate (with vertical fast axis) : And a rotation matrix:
The total Mueller matrix of the system with two reflections and without window is found as:

15. Psi-Delta Calculation
S1 in the outgoing stokes vector is the intensity after the Analyzer
It is a Fourier series of the compensator angle C
Fitting to measurement data gives Fourier coefficients An and Bn
 and  are derived from An and Bn by calculations depending on the setup used
C (°)
Intensity after Analyzer as a function of compensator angle C

16. Ellipsometry Measurements 1
Arrows indicate corresponding points in time

17. Ellipsometry Measurements 2
Arrows indicate corresponding points in time

18. Ellipsometry Measurement 3
Calculation of ,  values for double reflection from sample (without taking into account the window) results in complex values
As an illustration of what ,  values could be the graphs below are calculated using an assumed single reflection from sample

19. Conclusions
Rotating Compensator Ellipsometry is a feasible method for studying photocathode growth
Different preparation conditions result in different measured Fourier coefficients
Ellipsometry results remain difficult to interpret

20. Questions?

21. Interferometric Ellipsometry
Photo-cathode
Copper mirror
P-polarized
S-polarized
D1 & D3 measuring amplitude of reflected beam
D2 & D4 measuring interference between reflected beam and reference beam, giving information about the phase of the reflected beam.

22. Interferometric Ellipsometry: Results
A+B
A-B
Vibrations make interference signal unstable.
0.25 Hz vibration of the vacuum chamber relative to the ellipsometer
75 Hz vibration of the photocathode relative to the ellipsometer
Solution to the 75 Hz vibration requires rigid mounting of the subtrate to the actuator
Conclusion: interferometric ellipsometry in this case seems impractical, requires major changes to the deposition system to work.

23. Multiphoton Photoelectron Emission
Energy per photon for 800nm:
Work Function  for Copper is 4.6eV
3 photons required for MPPE of 1 photoelectron
Probability for this is highest for short pulse, small focus, high intensity e-  h h h

24. MPPE Experimental Setup
Laser parameters =800 nm, E=2 mJ, = 30 fs, frep= 10 Hz spot size 0.2 x 0.4 mm
Photocathode at -90 V
Neutral Density filters used to vary pulse energy
Photocathode
Wire Mesh & 2
Channeltron
70°
Laser beam
Vacuum Chamber

25. Charge Measurement
Expected Charge emission from cathode is Q1 pC, 1 ps
Wire Mesh used as charge collector
Capacitor smears out charge over about 200 s
Oscilloscope set to trigger on glitches t>100 s V>1 mV V
10pF
1M
-90V
Oscilloscope
Cathode
Wire Mesh
Measurement circuit used in the experiment

26. Charge Emission Measurements
Pulse energy
Pulse energy
Emitted charge Q varies greatly with time
End result: Q1pC locally after 100s, Q<16pC uniformly after 3 positions
Heating to 500°C for 6 hours restores photocathode

27. Comparison with Other Research
Our results
Brogle et al. use =650 nm, = 500 fs, 3mm spotsize, Vanode= +5kV.
We use =800nm, = 30fs, 0.2 x 0.4 mm spot size, Vcathode= -90 V
from Brogle, R. et al. 1995

28. Ablation of Copper
Hashida et al. use =800nm, = 70fs and experiment in air
We use F=0.74 J/cm2 in vacuum so ablation will probably take place
from Hashida et al. (2002)

29. Part 1:
Ellipsometry and Quantum Efficiency Measurement on Cesium-Telluride thin film photocathodes irradiated by a 259nm (Ultraviolet) Hg lamp

30. Part 2:
Charge Emission Measurement from a Copper Photocathode after incidence with a 800nm (Infrared) femtosecond laserpulse