resonant faraday rotation in a hot lithium vapor scott waitukaitis university of arizona, department...
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Resonant Faraday Rotation in a Hot Lithium Vapor
Scott Waitukaitis
University of Arizona, Department of PhysicsMay 2007
Overview
• Highly frequency dependent
• Can be enhanced near resonances
How does this work?
• Linearly polarized light is a superposition of equal parts RCP and LCP
• RCP and LCP have different indices
• Resulting rotation proportional to difference in indices, i.e.
)()( LR nn
The role of B: the Zeeman effect
• RCP light causes • LCP light causes• Via Zeeman effect, degeneracy in is lifted so that
1 JJ mm1 JJ mm
JmRL ,0,0
Frequency dependence
20
20
0
)4/()(1)(
n )()( LR nn
Fine structure of lithium
-3/2
-1/2
1/2 3/2 -3/2
-1/2
-1/2
-1/2
-1/2
1/2
1/2
1/2
1/2
1/2-1/2 3/2
670.9761 nm
670.9510 nm
670.9785 nm
670.9919 nm
2/322 P
2/122 P
2/122 S
• Wavelength range ~0.04 nm• Frequency range ~30 GHz
Added complexities
• Natural linewidth ~6 MHz
• Observed width ~3000 MHz
• Broadening mechanisms– Doppler broadening– Power broadening– Pressure broadening
What do we need to observe Faraday rotation?
• A laser that can be tuned over a 0.04 nm range around 670 nm
• A lithium vapor
• A way to infer rotation has occured
Diode laser basics
~0.5 cm
• ~ 670 nm• Wavelength is modulated via
current adjustment– As wavelength changes so does
output power
Laser output
• Mode profile governed by boundary conditions of lasing medium
• At a given temperature, lasing occurs where product of profiles is highest
• Both mode and gain profile change with temperature
• Dominant wavelength bounces from one mode to the next
Typical laser trace
Piezo driven external cavity
• Piezos driven by function generator and control circuit
• Able to adjust plate offset
• Able to adjust amplitude of plate oscillations
Heat pipe oven
• ~ 3 cm in diameter• ~ 30 cm in length• ~ 650-700 K
Experimental setup
Data vs. model
• Most general features of data mimicked by model– Sign
– Order of magnitude
• Model predicts more active features
Possible causes of discrepancy
• Hyperfine splitting in the ground state (~800 MHz)
• Saturation effects due to high intensity of laser beam
• Off-axis B field
• Large laser line width
Acknowledgments
I’d like to thank Dr. Cronin for giving me the
opportunity to work with him. I’d also like to
thank Dr. Bickel for his advice along the way.
My gratitude also goes out to Tori Carr, Yancey
Sechrest, Vincent Lonij, Ben McMorran, John
Perrault for their help and support as well.