Spin-Polarizing He at 8atm

woundcallousSemiconductor

Nov 1, 2013 (3 years and 7 months ago)

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Spin
-
Polarizing
3
He 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
3
He is needed, lasers tuned
to circularly polarized 795 nm light
are used to optically pump Rb atoms
into states that exchange spin with
3
He nuclei through collisions.

Polarization


Definition for spin ½ systems:

Optical Pumping


Optical Pumping

RCP light

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

Spin Exchange

3
He

Rb

3
He

Fermi
-
contact

hyperfine interaction

Rb



Works best for
I

= ½ noble gases (
3
He and
129
Xe).



Takes hours for
3
He.



I


S

I

S

laser
light

The build up of nuclear spin
polarization in the gas ensemble
is simply described by

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
3
He target
nuclei


To develop a versatile and easily
transportable system

Our System

Laser

Laser

Top view

The Laser

Laser

Laser

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.



*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.

Laser

Laser

Lenses

Laser

“4x afocal Telescope”

Cylindrical lenses

f
1

f
1

+ f
2

f
2

Grating

Laser

The Grating Equation:

Groove spacing

Incident Angle

Diffracted angle
of m
th

order

Order of
Diffraction

Wavelength

Littrow Mounting:

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

Θ
i

φ
1

Θ
i

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

Laser

Θ
i

φ
1

Θ
i

The Lenses with the grating Help to
reduce the effects of SMILE

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

Laser lenses & Grating

794.5

794.6

794.7

794.8

794.9

795.0

Thus, we stimulate the emission of the desired
wavelength!

Wave Plates

Laser

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 2
θ

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 90
o

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

Wave Plates

Mirrors

Laser

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

Our System

Results


3
He

3
He

3
He

3
He

3
He

3
He

3
He

3
He

3
He

3
He

Polarization Measurement

3
He

NMR Coil

Polarization Measurement

Results



We measure polarization with an NMR coil.


3
He 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:
L
ight
A
mplification by
S
timulated
E
mission of
R
adiation.


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
3
He 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+
3
He Experiment


Photodissociation of
3
He at HIGS


Any experiment where you want Highly
spin polarized
3
He

The Laser

Laser

Laser

“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

Laser

To reduce excessive feedback!


To change linearly
polarized light to circularly
polarized light

Lenses