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LASER COOLING
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Contents
Introduction.
Laser Cooling.
Type and wavelength of laser.
Why the laser is used her rather than the ordinary light?
Doppler Effect.
The absorption-emission mechanism.
Laser beams arrangement.
Can we cool atoms to temperature that is lower than the
Doppler temperature? Conclusion.
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Lasers were invented about 50 years ago. They are able to . .
Introduction
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Two Nobel prizes (in 1997 & 2001) in physics were related to the use of laser for
Introduction
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Atoms are always moving in random directions with velocity around 1400 km/hr
at room temperature
Introduction
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High T Large v
v
vrms = (3RT/M)1/2
R is the gas constant M is the molar mass T is the temperature
Low T Small v
v
Introduction
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Studying fast atom
Introduction
difficult
minimize the atoms' temperature
we need to
reduce the velocity
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At low temperature
Introduction
Bonds more complicated
Study is still difficult
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We need a mechanism that gives temperature near the zero Kelvin & keep the density low.
Introduction
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Introduction
Sun
Transformation to heat
Absorption
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Involves a series of absorptions and emissions
Laser Cooling
a decrease in the atoms' velocity
Laser cooling is a technique that uses light to cool atoms to a very low temperature. The simplest form of laser cooling is the so called Optical Molasses
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The type and wavelength are chosen such that they suit the atoms which will be cooled
Type and Wavelength of Laser
Na vapor
Rb vapor
Cs vapor
l = 589.0 nm
l = 780.0 nm
l = 852 .0 nm
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In this application we need light of certain wavelength (frequency).
Why the laser is used her rather
than the ordinary light?
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Doppler Effect
∆E Photon
Atom
E = hn = ∆E
Stationary
atom
Moving
atom
Photon
Atom
E = hno
ν = νo (1+vi/c)1/2 / (1-vi/c)1/2
∆E vi
What about a moving atom???
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∆E Photon
Atom
Absorption
Recoil velocity
Spontaneous Emission
random direction ∆t ~10ns
Recoil
E = hno
The absorption-emission mechanism
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The absorption-emission mechanism
What is the effect of the recoil velocities due to the emissions?
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The absorption-emission mechanism
After a few absorptions v decreases So, atom "sees" the incoming photon with a frequency less than when the it was moving fast and it reaches the lowest temperature
using Doppler effect.
Photon
Atom
E = hno
ν = νo (1+vf/c)1/2 / (1-vf/c)1/2
∆E vf
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Laser beams arrangement
optical molasses
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Zeeman effect (Zeeman tuned cooling)
Can we cool atoms to temperature that
is lower than the Doppler temperature?
We can do that using either
Dye laser beam (chirped cooling)
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Using Zeeman effect (Zeeman tuned cooling)
ml
1
0
-1
Level p splits into 3 states with same energy in absence of B
S
p
E
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Zeeman effect
With B
ΔE = ml (e h/4 p m) B
S
E
1
0
-1
S
P(1,-1,0)
E
ΔE
Without B
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Magnetic trap
ΔE = ml (e h/4 p m) B
atomslaserB= min.
z =L
B= max.
z =0
atomslaserB= min.
z =L
B= max.
z =0
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Magneto-optical trapping (MOT)
Magnetic trap & optical molasses "magneto-optical trapping."
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Magneto-optical trapping (MOT)
A collection of sodium atoms ( yellow dot in middle of picture ) trapped in a MOT.
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Using dye laser beam (chirped cooling).
The dye lasers give a wide rang of frequencies and are tunable. So, we can increase the frequency as the atom's velocity decreases. This will keep the
atom in resonance with the laser
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Conclusion
lasers not only used for cutting, welding or heating. lasers can be used to cool atoms Doppler effect in collaboration with a magnetic trap or dye laser beam can cool the atoms to micro-kelvin. Further cooling may be achieved using the so-called evaporative cooling
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