Spin-Polarizing 3He at 8atm with a frequency narrowed

diode laser

C.W. Arnold, T.V. Daniels, A.H. Couture, T.B. Clegg

UNC / TUNL

Outline

• General Overview

• Goals

• Our System

• Results

General Overview

• For experiments in which spin polarized 3He is needed, lasers tuned to circularly polarized 795 nm light are used to optically pump Rb atoms into states that exchange spin with 3He nuclei through collisions.

Polarization

• Definition for spin ½ systems:

NN

NNP

Polarization

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

50% 55% 60% 65% 70% 75% 80% 85% 90% 95% 100%

Nup/Ntot

Po

lari

zati

on

NN

NNP

Optical Pumping

Optical Pumping

RCP light

Source: http://physics.nist.gov/Divisions/Div846/Gp3/Helium/production/SpinEx.html

Spin Exchange

3He

Rb

3He

Fermi-contact hyperfine interaction

Rb

• Works best for I = ½ noble gases (3He and 129Xe).

• Takes hours for 3He.

I · S

I

S

laser light

The build up of nuclear spin

polarization in the gas ensemble is

simply described by )1()( / sutsat ePtP

The saturation polarization will be proportional to the amount of

laser power available in the region of D1 absorption of Rb.

Therefore one desires a laser with high power and a very

narrow linewidth in the region of absorption.

Goals

• To increase polarization of 3He target nuclei

• To develop a versatile and easily transportable system

Our System

LaserLaser

Top view

The Laser

LaserLaser

These diodes put out 50 watts of laser

power at the source, and we get about

30 – 36 watts of laser power into our

system after losses.

Diode Lasers

• In semiconductor crystals the atomic spacing is very low.– Wave functions of electrons start to

overlap– Energy levels split satisfying the Pauli

exclusion principle– Energy level spacing ~10-18 eV

• The nearly continuous levels form “bands”

* from Fundamentals of Semiconductors

Diode Lasers

• “Impurity Recombinations” from Conduction Band to Valence Band

• Large Linewidths ~3nm• 1nm corresponds to 475GHz for our

setup;• 3nm ~1400 GHz• At modest pressures the acceptance

linewidth is ~40GHz• A lot of power wasted...or worse.

c

f 2

c

f

GHzf 475

*from Elementary Solid State Physics

The Laser

• What is “smile”?– Displacement of a particular diode from

the mean position of the array of diodes.

– Causes linewidth broadening due to how the way light is fedback into the diode.

– We want “smile” to be as little as possible.

LaserLaser

Lenses

Laser

“4x afocal Telescope”

Cylindrical lenses

f1 f1 + f2 f2

Grating

ma mi )sin(sin

Laser

The Grating Equation:

Groove spacingIncident Angle

Diffracted angle of mth orderOrder of

Diffraction

Wavelength

Littrow Mounting:

i

i

a sin2

1

So for a grating with 2400 lines/mm and a λ=795nm we find that our θi= 72.5

Θiφ1 Θi

The Grating Helps us tune our

laser to the desired output

frequency and provides the

desired narrowness of the output

light.

External Optical Cavity

Lasers Overview

Stimulated Emission– Excited atoms are triggered into

emission by the presence of photons of the proper frequency

– Stimulated Emission Photons have the same phase, direction and polarization of the stimulating photon

Lens-Grating System

LaserΘiφ1 Θi

The Lenses with the grating Help to reduce

the effects of SMILE

2

2

0

M0

*From B. Chann, I. Nelson & T.G. Walker

Laser lenses & Grating

79

4.5

79

4.6

79

4.7

79

4.8

79

4.9

79

5.0

Thus, we stimulate the emission of the desired

wavelength!

Wave Plates

2

Laser

4

To reduce excessive feedback!

To change linearly polarized light to circularly polarized light

A wave plates performance depends on

the angle between the E field of the

polarized light and the fast axis of the wave

plate. It effects a 2θ rotation of the E field

where θ is the angle between the E field

and the fast axis.

Our mirrored grating preferentially diffracts

light with E field in one orientation and

simply reflects light with E field 90o

to the

first orientation. Thus we can essentially

rotate the plane of polarization of our laser

to control the amount of feedback we

need.•

Img from Optics, Eugene Hecht & Alfred Zajac 1976

Wave Plates

Wave Plates

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

1.20E-03

1.40E-03

1.60E-03

791 792 793 794 795 796 797 798

Series1

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

1.20E-03

1.40E-03

1.60E-03

791 792 793 794 795 796 797 798

Series1

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

1.20E-03

1.40E-03

1.60E-03

791 792 793 794 795 796 797 798

Series1

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

1.20E-03

1.40E-03

1.60E-03

791 792 793 794 795 796 797 798

Series1

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

1.20E-03

1.40E-03

1.60E-03

791 792 793 794 795 796 797 798

Series1

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

1.20E-03

1.40E-03

1.60E-03

791 792 793 794 795 796 797 798

Series1

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

1.20E-03

1.40E-03

1.60E-03

791 792 793 794 795 796 797 798

Series1

Wave Plates

Peak Amplitude vs.Half Wave Plate Setting

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

1.20E-03

1.40E-03

1.60E-03

0 5 10 15 20 25 30 35 40 45 50

Half Wave Plate Setting (deg)

Series1

FWHM vs.Half Wave Plate Setting

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 5 10 15 20 25 30 35 40 45 50

1/2 Wave plate setting

Series1

Mirrors

Laser

Mirrors to steer the light to where we need it to go.

Our System

Results

3He

3He

3He

3He

3He

3He

3He

3He

3He

3He

Polarization Measurement

3He

NMR Coil

Polarization Measurement

Results

•We measure polarization with an NMR coil.

•3He Polarization NMR signal strength measured in mV

•After the cells and the NMR coil cooled The new laser

polarization read 3600mV and the old laser read 3000mV.

This represents a 20% increase in polarization from the old

laser

~30 W narrowed laser vs. ~60 watt non-narrowed laser

Results

~0.3 nm linewidth

~2nm linewidth

Summary

System is versatile and portable– Has been coupled to two different

setups

• Laser linewidth narrowed by ~ order of magnitude

• Observed 20% increase in polarization

Sources• http://science.howstuffworks.com/laser.htm• http://hyperphysics.phy-astr.gsu.edu/HBASE/hph.htm• http://physics.nist.gov/Divisions/Div846/Gp3/Helium/production/SpinEx.html• Tunable Lasers Handbook, F.J. Duarte Ch. 8, 1995• Polarized Light Production and Use, William A. Schurcliff• Optics, Eugene Hecht & Alfred Zajac 1976• Fundamentals of Semiconductor Lasers, Takahiro Numai, 2004• Elementary Solid State Physics, M. Ali Omar, 1993• High power diode lasers: Fundamentals Technology and applications, R. Diehl ,2000• Using Diode Lasers for Atomic Physics, Carl E. Weiman & Leo Hollberg, Rev. Sci.

Instrum.Vol 62, No.1 1991• Narrowing the Laser Diode Array, Xing Zong, Duke Physics • Frequency-Narrowed External Cavity Diode Laser Array Bar, B. Chann, I. Nelson, &

T.G. Walker (April 4, 1999)• Spin Exchange optical pumping of nobel-gas nuclei, Thad G. Walker& William,

Happer, Reviews of Modern Physics, Vol 69, No.2, April 1997• Spin-Exchange optical pumping using a frequency narrowed high power diode laser ,

I.A. Nelson, B. Chann, T.G. Walker, Applied Physics Letters, Vol 76, No.11, March 13, 2000.

• Private Communications with Alex Couture, Tom Clegg, Brian Collins, Bastian Driehuys

Acknowledgements

• Thanks to Tom Clegg, Tim Daniels, Alex Couture, Bastian Driehuys, Stephen Daigle, UNC Professors, UNC & TUNL machine shops

Thank You

Wave Plates

Narrowed Output

*Applied Physics Letters Vol. 76,No. 11

Lasers Overview

• Population Inversion

• Stimulated Emission– Excited atoms are triggered into

emission by the presence of photons of the proper frequency

– Stimulated Emission Photons have the same phase, direction and polarization of the stimulating photon

Goals

• To frequency narrow our laser output

*Applied Physics Letters Vol. 76,No. 11

Lasers Overview

• Laser: Light Amplification by Stimulated Emission of Radiation.

• Atoms– Absorb energy – electrons transition to an

excited state– Electrons return to lower state – Can release

energy in the form of a photon

Summary

• We will have a small, relatively light-weight, portable laser system

• We will be able to achieve higher polarizations of 3He than we can with the laser we have been using (40-50% up from ~25%)

• We spent a relatively small amount of money to achieve this

Applications

• Spin Exchange Optical Pumping– Tim’s Experiment– n+3He Experiment– Photodissociation of 3He at HIGS– Any experiment where you want Highly

spin polarized 3He

The Laser

LaserLaser

“Smile”

Concerns

• Losses– Try to minimize the number of things the

laser light has to interact with– Anti-reflection coatings on lenses– Compensate for SMILE

• Safety– Blindness– Fire

Lasers Overview

Wave Plates

2

Laser

4

To reduce excessive feedback!

To change linearly polarized light to circularly polarized light

Lenses