3/13/2012
LASER
Announcements
• Homework #8 posted and due on Friday at 5 pm
• Lab #7 next week on Lenses (read ahead since not covered in
lecture yet)
 remember to do the prelab in advance
• Reading Bloomfield sections 14.3 (LASERS), 15.1 (Lenses)
• Exam #2 in class next week (Thursday, March 22).
Review in class next Tuesday. Also, no lab next week so
Tuesday + Wednesday lab times for review.
No homework due that Friday (March 23) before spring break!
• Problem Solving Sessions Thursday 2-4 pm, Friday 1-3 pm
How does a LASER work?
Everything to know about interaction of light and atoms,
in 3 easy steps.
LASER-- Light Amplification by Stimulated Emission of Radiation
photon
Repeated cloning of photons to produce LOTS of identical
photons of light.
Excited atom
atom
1. Absorption
of light
e
Requirements:
e
1. Stimulated emission (always have)
2. Population inversion of bunch of atoms (tricky)
3. Optical feedback (mirror)
2. Spontaneous
emission of
light
e
e
3. Stimulated emission of light. First realized by A. Einstein
An atom has an energy separation between ground and first
excited state equal to the energy of a green photon.
A green photon hits atom already in the first excited state.
3. Stimulated emission of light. First realized by A. Einstein
Photon hits atom already in higher energy level. What happens?
a.
b.
c.
d.
gets absorbed by the atom, electron goes to even higher energy level.
goes right on through, atom does not notice,
original photon continues and second identical one comes out of atom.
original photon scatters off in random direction.
What happens?
a. gets absorbed by the atom, electron goes to even
higher energy level.
b. goes right on through, atom does not notice,
c. original photon continues and second identical one
comes out of atom.
d. original photon scatters off in random direction.
• Cloning photons.
• Amplifying amount
of light x 2.
e
e
See PhET Simulation
What happens when we increase the rate of photons hitting atom?
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3/13/2012
Stimulated Emission Question
Atom already in excited state
What happens when photon travels through discharge tube…..
Chance of stimulated emission of excited atom
EXACTLY the same as chance of absorption by
ground state atom.
Equal chance bottom atom will absorb photon
leaving no photon, as top atom will jump down
and so have two photons.
Atom already in excited state
What will come out on the right? Think before you pick…
a. 1 photon, b. 2 photons, c. 3 photons, d. 4 photons, e. 8
c. 3 photons: Double at first atom, then both hit second but atom only
has enough energy to give off one more photon. Remember, photon
energy E = hc/ must match difference in energy levels for electron.
e
atom 1
glass tube full of atoms (discharge lamp)
?
e
atom 2
glass tube full of atoms (discharge lamp)
b. less come out right and some atoms inside get excited.
How many photons come out at right?
How many photons will come out at right compared to number in on left?
a. more, b. less,
c. same number,
d. none will come out.
To amplify number of photons going through the atoms
one needs
a.
b.
c.
d.
more atoms in lower energy level,
half in lower, half upper,
more in upper energy level,
a sufficient number in upper level and it does not
matter how many are in the lower.
• For each experiment, a matter of chance / statistics /
probability
• But most of the time less photons come out because more
atoms are initially in lower level than upper level
To amplify number of photons going through the atoms I need more
atoms in upper energy level than in lower.
- Lower level atoms eat photons up,
- Upper level atoms clone photons
-Equal prob. for absorption/stimulated emission so amplification is
proportinal to Nupper-Nlower.
Population Inversion
• Get photon amplification if more atoms in upper level than lower
• This condition is very unnatural
Nupper > Nlower  photon amplification
How do we achieve this population inversion of atoms?
Nupper < Nlower  photon loss
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3/13/2012
How do we get population inversion?
How do we get population inversion?
Try shining lots of light on atoms.
Try shining lots of light on atoms.
If we turn the power of the light way up, we will get:
a. population inversion with all atoms excited to upper
level,
b. no atoms in upper level,
c. half the atoms in upper level.
Check the PHeT Simulation….
If we turn light way up we will get:
a. population inversion with all atoms excited to upper
level,
b. no atoms in upper level,
c. half the atoms in upper level.
- just as easy to excite (absorb)
- as to de-excite (stimulated emission)
So this 2 energy level system won’t work……………..
Level structure for population inversion
• For population inversion, want:
- atoms to pile up in upper LASER level,
- keep lower LASER level nearly empty
• Need at least 3 (ideally 4) energy levels involved.
• Need rate into upper laser level faster than rate out (sp. emission).
2
Pumping
(absorption or
electron
collisions)
Quick (collision or sp. emission)
3 Upper laser
level
Level structure for population inversion
What color light could come out on 3 to 4 transition?
a. green, b. blue, c. red , d. a and b., e. a and c
 Has to be lower energy than the green needed for 1 to 2
 One reason why it is easier to make longer wavelength lasers
2
Pumping
(absorption or
electron
collisions)
4 Lower laser
e
Quick (collision or sp. emission)
3 Upper laser
level
4 Lower laser
level
level
e
Quick (collision or sp. emission)
1
“pumping process to produce population inversion”
Making a LASER beam
Now we understand how to clone photons:
- Population inversion in the LASER medium
- Stimulated emission
Where does the first photon come from?
- Spontaneous emission from one of the excited atoms
Beam
direction
Quick (collision or sp. emission)
1
“pumping process to produce population inversion”
Making a LASER beam
Beam direction
Need to select a spontaneous photon travelling in
desired beam direction to seed the beam
How?
- Put mirrors on either end of tube
- Photon travelling EXACTLY along tube axis bounces back and
forth. ‘Snowballs’ more and more identical photons with each pass
- Other photons bounce out of side of tube
Will cloning the photon above contribute to the LASER beam?
a) Yes
b) No
Photon is not heading in beam direction !
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3/13/2012
Making a LASER beam
Putting it all together – He-Ne LASER
Beam direction
How do we get the light out as a beam (like a LASER pointer)?
Make one mirror a little leaky (only partially reflective)
Think about a two-way mirror
+
1000s of V
• Electrons accelerated from cathode to anode
• Electrons collide with and excite Helium (He) atoms
• Excited He atoms collide with and excite Neon (Ne) atoms
• Creates population inversion in the Ne atoms
• Spontaneously emitted photon that matches mirror conditions cloned
into powerful beam of identical red photons bouncing between mirrors
• One ‘leaky’ mirror allows a small amount of light out as laser beam
Just like discharge tube with mirrors on the end
What else can you make a LASER out of?
(other LASER media)
Look at output of HeNe laser (reflected) through diffraction grating.
What do you expect to see?
a) A continuous spectrum (like that from the lightbulb)
b) Several discreet colors in each order
c) One color in each order
d) No deflected light
•
•
- LED (P-N junction) with mirrors on either end
- Upper LASER state – electrons in p-type conduction band
- Lower LASER state – electrons in p-type valance band
All photons in LASER beam have identical energy (color)
Conduction
band
Look at HeNe tube through diffraction grating.
What do you expect to see?
a) A continuous spectrum (like that from the lightbulb)
b) Several discreet colors in each order
c) One color in each order
d) No deflected light
• Spontaneously emitted photons from He and Ne
• Not travelling in correct direction to be amplified
• Just like light from a He-Ne discharge lamp
LASER Summary
Why is LASER light special
- Made from identical photons (color, direction…)
- Very orderly and hence easy to control (focusing, parallel
beam etc)
Gas discharge lasers (HeNe, CO2)
Diode lasers
Valance
band
P type
N type
+ V •
•
Solid state lasers (Ruby, Neodinium YAG)
Dye lasers (organic dye molecules in solution)
National Ignition Facility
https://lasers.llnl.gov/about/nif/about.php
Light interacting with matter
- Absorption
- Stimulated emission
- Spontaneous emission
How does a LASER work?
- LASER medium with population inversion (tricky to achieve)
- Mirrors on either end (select and recycle photons)
- Single photon of correct  and direction will oscillate and be
amplified (cloned).
- Leaky mirror allows beam out.
4