KS Sullivan amp NI Agladze Short electron bunches are needed for dense collisions in particle accelerators How to measure the shape of a short electron bunch Use the crosscorrelation between coherent THz produced by the bunch together with narrowband incoherent visibleUV radiation ID: 728895
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Slide1
Selection of SiC for the electro-optic measurement of short electron bunches
K.S. Sullivan & N.I. Agladze
Short electron bunches are needed for dense collisions in particle accelerators.
How to measure the shape of a short electron bunch?
Use the cross-correlation between coherent THz produced by the bunch together with narrow-band incoherent visible/UV radiation.Slide2
Electro-optic crystals
http://dev.fiber-sensors.com/wp-content/uploads/2010/08/electro-optic_example-01.png
Material-specific properties
Electro-optic effect on polarized lightSlide3
Single shot capability
Resolution determined by the EO crystal dispersion
Cross-correlation of coherent and incoherent radiation in EO medium
THz coherent pulse
Incoherent pulse
Cross-correlation
Non-collinear propagation enables a delay dependence
Advantages
CRYSTAL
DETECTORSlide4
Cross-correlation: principle experiment
SourceSlide5
Zinc Telluride (ZnTe)
High electro-optic coefficient
Useful frequency range limited by low vibrational mode (190 cm
-1
compared to GaP’s 366 or SiC’s 794)
Dispersion due to TO resonance
http://refractiveindex.info/figures/figures_RI/n_CRYSTALS_ZnTe_HO.pngSlide6
Silicon Carbide (SiC)
Comparable electro-optic coefficient to ZnTe
Higher TO resonance permits larger frequency rangeSlide7
Polytype Choice
http://japantechniche.com/wp-content/uploads/2009/12/sdk-sic-mosfet.jpg
Cubic SiC
Hexagonal SiC
Pure
Expensive
Subject to free carriers
Readily availableSlide8
6H Considerations
Free carriers or doping
Metallic behavior
Electro-optic coefficient’s angular dependence
http://metallurgyfordummies.com/wp-content/uploads/2011/04/doping-semiconductor.jpgSlide9
6H Transmission
Increase in transmission toward Brewster angle
Lacks metallic free carriers
Unexpected feature at ~110 wavenumbersSlide10
6H Absorption Coefficient
Use transmission relation to plot absorption coefficient,
α
Ideally zero
Notable frequency dependence
Unknown feature possibly due to fold-back or material defectsSlide11
Focus on 3C
Unlike 6H, 3C does not require calculation of an angle to maximize the electro-optic coefficient
Cubic/Zinc-blende structure similar to ZnTe and GaP
Necessary to calculate electro-optic response
http://upload.wikimedia.org/wikipedia/commons/4/4f/SiC3Cstructure.jpgSlide12
Electro-optic Response
Transmission coefficient based on refractive index
Integral uses frequency, thickness, phase velocity of THz radiation, and group velocity at optical frequency
Shape of resulting function comes primarily from the mismatch between phase and group velocitySlide13
Dielectric Model
Because of the electro-optic response function’s reliance on
phase and group velocities, we need a model of the dielectric function from the UV to the THz.Slide14
Comparative Responses
GaP shown at optical group velocity at 8352 cm
-1
ZnTe at 12500 cm
-1
SiC at 37495 cm
-1
Cut-off frequency set at 4 THzSlide15
Electro-optic Performance
Previous approach masks full electro-optic properties
Transmission, crystal thickness, and electro-optic coefficient all important
Figure of merit proportional to the polarization rotation produced by the THz field
r (10
-12
m/V)
d (microns)
Figure of merit (r
×
d)
GaP
1
1800
1800
ZnTe
4
185
740
SiC
2.7
4950
13365Slide16
Alternate Comparison
Material group velocity maintained by choosing the optimal visible/UV frequency
Figure of merit held at 500 for each material
Note SiC covers a larger rangeSlide17
Results and Further Research
6H unsuited for measurement of bunch length
3C seems promising due to a larger broad-band capability than both ZnTe and GaP
Idealized electro-optic response analysis of SiC shows significant improvement over similar crystals at optimal optical frequenciesSlide18
Acknowledgements
Al Sievers and Nick Agladze
CLASSE
National Science Foundation