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

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

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

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

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

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

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

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

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

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

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

Quarter Waveplate Rotation

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:

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:

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

Ellipsometry Measurements 1 Arrows indicate corresponding points in time

Ellipsometry Measurements 2 Arrows indicate corresponding points in time

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

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

Questions?

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.

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.

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

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 1 & 2 Channeltron 70° Laser beam Vacuum Chamber

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

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

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

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)

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

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