Transcript Tunable External-Cavity High

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:
Polarization
100.00%
N  N
NP
N
P
N N
N  N
90.00%
80.00%




Polarization
70.00%
60.00%
50.00%
40.00%
30.00%
20.00%
10.00%
0.00%
50%
55%
60%
65%
70%
75%
Nup/Ntot
80%
85%
90%
95%
100%
Optical Pumping
Optical Pumping
RCP light
Source: http://physics.nist.gov/Divisions/Div846/Gp3/Helium/production/SpinEx.html
Spin Exchange
The
build up of nuclear spin
Rb
He
polarization in the gas ensemble
is simply described by
3
laser
light
P(t )  Psat (1  e
I·S
 t /  su
)
Fermi-contact
hyperfine interaction
The saturation polarization will be proportional to the
amount of laser power available in the region of D1
Rb absorption of Rb. Therefore one desires a laser with
high power and a very narrow linewidth in the region
of absorption.
S
• Works best for I = ½ noble gases (3He and 129Xe).
• Takes hours for 3He.
I
3He
Goals
• To increase polarization of 3He target
nuclei
• To develop a versatile and easily
transportable system
Our System
Top view
Laser
The 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
c

c


• 1nmfcorresponds
to
475GHz
for our

f 
2
setup; 

• 3nm  ~1400 GHz
pressures
f  475GHz
• At modest
the acceptance
linewidth is ~40GHz
• A lot of power wasted...or worse.
*from Elementary Solid State Physics
The Laser
• What is “smile”?
Laser
– 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.
Lenses
“4x afocal Telescope”
Laser
Cylindrical lenses
f1
f1 + f2
f2
Grating
So for a grating with 2400
lines/mm and a λ=795nm
we find that our θi= 72.5
External Optical Cavity
Laser
φ1
Θi
Θi
The Grating
Littrow
Mounting:
Equation:
The
a(sin
mGrating
 Helps us tune our
i1 sin m ) 
i 
laser to the desired output
frequency
and provides the
Diffracted
angle
Wavelength
iof m Order
of
th order
desired
narrowness of the
Diffraction
output light.
  2a sin 
Groove spacing
Incident Angle
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



0
2
2
 
*From B. Chann, I. Nelson & T.G. Walker
Laser
φ1
The Lenses with the grating Help to
reduce the effects of SMILE
Θi
Θi
0
M
Laser lenses & Grating
795.0
794.9
794.8
794.7
794.6
794.5
Thus, we stimulate the emission of the desired
wavelength!
Wave Plates

2
To reduce excessive feedback!
Laser
Our mirrored grating preferentially
Adiffracts
wave plates
performance
light with
E field in depends
one
on
the
angle
between
the
E
fieldlight
of
orientation ando simply reflects
the
lighttoand
axis of
withpolarized
E field 90
the the
firstfast
orientation.
the
wave
It effects rotate
a 2θ the
Thus
we plate.
can essentially
rotation
the E field where
θ is the
plane ofofpolarization
of our laser
to
angle
between
the
E
field
and
the
control the amount of feedback we
fast
axis.
need.

4
To change linearly
polarized light to circularly
polarized light
•Img from Optics, Eugene Hecht & Alfred Zajac 1976
Wave Plates
Wave Plates
1.60E-03
1.40E-03
1.20E-03
1.00E-03
8.00E-04
Series1
6.00E-04
4.00E-04
2.00E-04
0.00E+00
791
792
793
794
795
796
797
797
798
Wave Plates
Mirrors
Laser
Mirrors to steer the light to where we
need it to go.
Our System
Results
Polarization Measurement
3
3
He
3
He
3
He
3
He
3
He
3
He
3
He
3
He
He
3
He
Polarization Measurement
3
NMR Coil
He
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
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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 lightweight, portable laser system
• We will be able to achieve higher
polarizations of 3He than we can with
the laser we have been using (4050% 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
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

2
To reduce excessive feedback!
Laser

4
To change linearly
polarized light to circularly
polarized light
Lenses