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Published byDerrick Lee Modified over 9 years ago
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Real-time Ellipsometry on Cesium-Telluride Photocathode Formation
Martijn Tesselaar & Peter van der Slot CARE07
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Contents Introduction Electron Accelerator Photoelectric Effect
Ellipsometry on Cs2Te Photocathodes Photocathode preparation Rotating Compensator Ellipsometry RCE measurement results Conclusions
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Electron Accelerator Applications
External Beam Radiotherapy Synchrotron radiation Electron collider experiments Free Electron Laser
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Linear Accelerator Laser pulse on photocathode => short electron bunch Radio Frequency Electromagnetic waves accelerate the bunch Magnets are used for confinement
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Photoelectric Effect Kinetic energy
Quantum Efficiency = Number of electrons emitted per photon
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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
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Quantum Efficiency Photocathode irradiated by UV lamp during deposition Photocurrent measured using picoamperemeter Photocathode considered finished at maximum QE Start Cesium Deposition
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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
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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
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Sample & Polarization Sample optical properties contained in the ellipsometric quantities and : and also depend on film thickness, refractive index and absorption coefficient
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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
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Quarter Waveplate Rotation
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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:
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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:
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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
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Ellipsometry Measurements 1
Arrows indicate corresponding points in time
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Ellipsometry Measurements 2
Arrows indicate corresponding points in time
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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
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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
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Questions?
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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.
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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.
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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
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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
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Charge Measurement Expected Charge emission from cathode is Q1 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
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Charge Emission Measurements
Pulse energy Pulse energy Emitted charge Q varies greatly with time End result: Q1pC locally after 100s, Q<16pC uniformly after 3 positions Heating to 500°C for 6 hours restores photocathode
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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
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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)
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Part 1: Ellipsometry and Quantum Efficiency Measurement on Cesium-Telluride thin film photocathodes irradiated by a 259nm (Ultraviolet) Hg lamp
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Part 2: Charge Emission Measurement from a Copper Photocathode after incidence with a 800nm (Infrared) femtosecond laserpulse
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