3. contents
History
What is laser
Difference between laser and ordinary light
What is cooling
What is laser cooling
Different methods of laser cooling
Atom trapping by magneto optical method
Dopper effect
Limitations to laser cooling
Some questions have to be answered
4. Hansch and A.L Schawlow were
the first scientist that give the
first purposal to cool the neutral
atoms in propagating laser
beam in 1975
Details about history;
5. The noble prize
in physics was
awarded in 1997
to steven chu,
claude cohen and
William Philips for
the development
of methods to
cool and trap
atoms with laser
light
6. What is laser?
Ans.
Laser is an acronym which is stands for light
amplification by stimulated emission of
radiation
laser is a device in which light is amplified and
monochromatic and focused light is produced
of single wavelength or single frequency
7. Laser having optical region in electromagnetic region there
frequencies corresponding wavelengts are tens power 11 to 17
and microwave region to near soft x-rays
Their energy pulses are tens power 4 and having duration tens
power -15 seconds
They are used in drilling, cutting,detached the retinas of eyes
Lasers are used in low and high power appliances
8. Difference between laser and
ordinary light
Both ordinary light and laser light are electromagnetic waves.
Both travel with the velocity of light in vacuum.
Ordinary light is divergent
incoherent
laser light is highly directional
coherent
monochromatic
Ordinary light is a mixture of electromagnetic
9. Ruby laser
Maiman was the first man who
invented the first laser
A ruby laser is a solid-state laser that
uses a synthetic ruby crystal a ruby
lasers produce pulses of coherent
visible light at a wavelength of 694.3
nm, which is a deep red color. Typical
ruby laser pulse lengths are on the
order of a millisecond.
10. He-Ne laser
First helium neon laser was
operated at the 1.15 micrometer in
near infrared
Which was invented by Ali javan and
Donald R.
Further more researchers found
632.8 nm red line which made the
helium neon laser most possible
12. But
Laser are used to make thing hot
Such as laser guns
Laser shooting
Laser cutting
And laser can be used to burn the
hair off.
13. Before going to working of laser cooling
we have introduce the what is cooling
14. What is cooling process?
According to kinetic theory of gas molecules to which
temperature is directly proportional to the kinetic energy of gas
molecules so basically cooling is something which happens due
to decrease in kinetic energy of molecules in the presence of
gases
If we take an example of our every day life if we boil the water
the kinetic energy of water molecules increases as a result
temperature also increases
And we use this concept in our presentation
15.
16.
17. We use a laser to slow the molecules down
It’s a very strange thing that we are trying to decrease the energy
by adding some energy
For example we are trying to cool the hot water by adding a hot
water in it
And we are trying to blow the candle by using flame thrower
18. Key to laser
cooling
Key to laser cooling is that is that if an atom is moving in one direction
and absorb a photon from opposite direction which absorbed by atom
and it slows down
Laser cooling requires a specific wavelength
Q. Once atoms are slowed down and keep getting hit by more photons
they will speed up in opposite direction?
Ans. This is real genious of laser cooling we have to make sure the laser
wavelength should be tuned to be longer than the absorbtion
wavelength of still particle if it is not moving
20. Laser cooling refers to number of techniques in which
atomic and molecular samples are cooled down to
near absolute zero temperature.
Laser cooling techniques rely on the facts that when an
object absorb and re-emits the photon particularly of
light its momentum changes
Laser cooling
21. Methods of
laser cooling
Doppler cooling
Sisyphus cooling
Resolved side band cooling
Raman side band cooling
Velocity selective coherent
population trapping
Gray molasses
Cavity mediated cooling
Zeeman slower
Eit cooling (electromagnetic induced
transparency)
22. If an stationary atom sees the laser
neither red nor blue shifted and does
not absorb the photon
An atom is moving away from laser
sees it red shifted and does not absorb
the photon
An atom is moving towards photon
shifted and absorb the laser photon
sees it blue slowing down
The photon excites the atom moving an
electron to higher state
Laser cooling
process
23.
24. If an atom is moving towards any of laser that particular
beam would appear slightly higher frequency (Doppler
effect)
The laser light is tuned to be a frequency with an energy
just below the energy of an electronic transition of an
atom to excite the atoms to higher state
The atom will always scatter more photons from the laser
beam pointing opposite to their direction of motion
25. The small increase in frequency makes the frequency of laser
exactly equal to needed one for atomic excitation.The photons
are thus absorbed and atom excited
The atom recoil back due to momemtum provided by photons
and reemits a new photon in random direction coming back to
its ground state
26. Thus a atom is slowly down and thus kinetic energy of
system and thus temperature goes down
Kinetic energy is equal to half mass velocity square and
temperature is directly proportional to square root of
velocity
This makes temperature to milli kelvin
28. Magneto-optical trap
A magnetic trap is an apparatus which uses a magnetic
field to trap neutral particles with magnetic moments.The
magnetic trap (as a way of trapping very cold atoms) was
first proposed by David E. Pritchard.
29. Process of magnetic optical trap
A magneto-optical trap (abbreviated MOT) is an apparatus
that produces laser cooling with magneto-optical trapping in
order to produce samples of cold, trapped, neutral atoms at
temperatures as low as several microkelvins which is
approximately equal to absolute zero
30. By combining the small momentum of a single photon
with a velocity and spatially dependent absorption cross
section and a large number of absorption.
31.
32.
33. spontaneous emission cycles, atoms with initial velocities of
hundreds of metres per second can be slowed to tens of
centimetres per second.
Although charged particles can be trapped using a Penning trap or
a Paul trap using a combination of electric and magnetic fields,
those traps do not work for neutral atoms.
34. Atomic structure
necessary for magneto-
optical trapping
As a thermal atom at room temperature has many thousands of times the
momentum of a single photon, the cooling of an atom must involve many
absorption-spontaneous emission cycles, with the atom losing up to ħk of
momenta each cycle .
Because of this, if an atom is to be laser cooled, it must possess a specific
energy level structure known as a closed optical loop.
35. Doppler cooling
The relativistic Doppler effect is change in frequency of light
(wavelength) caused by the relative motion of source and
observer.
36.
37. Process of
Doppler cooling
Doppler cooling involves a light whose frequency is slightly tuned below
the electronic transition in an atom
Because light is detuned to red that is at lower frequency of transition the
atom will absorb more photons if they towards the light source due to
Doppler effect.
Thus if one applies the light from two opposite directions atom will always
absorb more photons from the laser beam pointing opposite to their
directions of motion.
38. In each absorption event atom losses a momentum equal to
momentum of photon if an atom is an excited state.
if the atom which now in the excited state emit a photon
spontaneously it will be kiked by the same amount of momentum but
in a random direction.
The result of absorption and emission process a reduced speed of
atoms provide it initial speed is larger than the recoil velocity from
scattering a single photon.
39. If the absorption and emission are repeated many times the mean
velocity and therefore the kinetic energy of the atom will be reduced
since the temperature of an ensemble of atom is measure of the random.
40. On the other hand, the mean squared velocity.And is not zero in the
random process, and thus heat is supplied to the atom.
At equilibrium, the heating and cooling rates are equal, which sets a
limit on the amount by which the atom can be cooled.
As the transitions used for Doppler cooling have broad natural
linewidths.
41. Some
problems of
laser cooling
Minimum temperature;
The atoms perform a random walk in momentum space with steps
equal to photon momentum due to spontaneous emission and photon
absorption.This constitute a heating effect which conteracts the cooling
process and impose a limit on the amount by which atoms can be
cooled
42. Maximum
concentration
The concentration must be minimul to prevent the absorption of photons
into gas in the form of heat .
This absorption happens when two atoms collide with each other while
one of them having excited electron.
Then there is a possibility of excited electron dropping back to ground
state with its extra energy liberated in additional kinetic energy to the
colliding atoms which heat the atoms
43. Apllications of
laser cooling
Construction of atomic clocks
Atom laser
In the development of instrument for atomic optics
Gps system is working on the basis of the laser cooling
44.
45.
46. Some questions have to be answered
Q.no.1 usually light incident on any material just increases its temperature but in this
case lasers are cooling down ?
Ans.the basic theory is used behind this is that photon bounce back from the atom for
example If we have a big rubber ball rolling we can hit it with small rubber ball and
can slow down it by transf the momentum and in this case laser light is small ball and
atom is a big rubber ball which is bouncing the photons from the surface of atom
As above example have not any relation with original one so in actual mean we are
going to understand by diagramme
47.
48. Q.no.2.The next thing is that atom require a photon of
particular wavelength or how it is going excite the atom?
Ans.This can be done by adjusting frequency as our
requirement.even the variation in the ground state can effect
the frequency of emitted light
49. Q.no.3.The laser slowing down fast
moving atoms but it should also
increase speed of slower moving
atom why it is not doing so?
Ans.The most difficult one question
is that but answer is Doppler effect
as frequency changes with the
change with the change in position
of light source and atom neglects
the most of the frequency.
50. Q.no.4.What about atoms which
are moving in random direction?
Ans.The laser from different
direction is used so we use six
lasers or 3 laser in front of
mirrors
51. Q.no.5.what about the atoms which are
moving in different directions they are not
controlled as they are moving in different
directions they can hit the walls and can
attain the heat?
Ans.they use strong magnets to control it
as atoms are tiny magnet they respond to
it get magnetic field and they are attracted
towards the centre.
52. Q.no.6.What happens now because atoms are in between themselves only
hit each other and get heated?
Ans.to avoid this scientists use the machines having vacuum pump so there
are no excess air atoms also the amount of atom taken in the container is
very low is it would not allow the atoms to hit each other.
