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.