PHYS 2053: Homework IX
First Draft
Justin Courtright
Thomas Riley
Jessica Belzer
1. Laser: Pumping a two state system
a. The emissions appear to be spontaneous, but “stimulated” by the incident light
(the emissions change with change in wavelength). There does not seem to be
much of a pattern (in time) in the emission. The time during which the electron is
in an excited state (n=2) varies, and only one electron moves between the two
levels.
b. The emitted photons become “fewer and farther between” when the intensity of
the light source is changed to a very low level. Otherwise, these photons
demonstrate the same characteristics as in the state used for question a.
c. The photon emissions occur much more rapidly when the intensity of the light
source is changed to a very high level. Excitation and “decay” both happen at a
much higher rate than previously. Other than this factor, the same characteristics
are present (i.e. one electron moves between the levels, no real pattern in time of
the emissions, and the electron moves to the n=2 excited state).
d. When the intensity is at a medium level and the lifetime is decreased to 50%,
photons are emitted visibly (as opposed to previously when they were not visible
within the simulation). These visible emissions are scattered in a random
direction, and do not occur each time the electron “decays” down from the n=2
state. Note: even though the previous simulation states did not show visible
emissions, the movement of the electron between the energy levels was observed
on the right side of the simulation experiment.
e. When the intensity is at a medium level, but the lifetime is at 100%, no visible
photons are emitted, but all other aspects appear the same (i.e. one electron moves
between the levels, no real pattern in time of the emissions, and the electron
moves to the n=2 excited state).
f. When the light is tuned to a color that has a lower energy (“dark red”) than the
preset (red) color, the following is observed. The electron of the atom is not being
excited to a higher (n=2) state. Instead, it just remains in the ground state. The
distance between the “potential” movement (the distance between the n=1 and
n=2 states) of the electron within the shell becomes shorter.
g. When the light is tuned to a color that has a higher energy (“dark blue”) than the
preset (red) color, the same occurrences as in part f are observed, with the
exception that the distance between the “potential” movement (the distance
between the n=1 and n=2 states) of the electron within the shell becomes longer.
h. Summary: In this part of the simulation, we discovered that visible photons are
not always emitted to reflect the movement of the electron between the excited
state and ground state. The number of photons emitted is directly correlated to the
intensity of the incident light. Lastly, the lifetime of the electron shells affects the
“decay time” of the electrons within the excited state, and the visibility of the
photon within the simulation.
2. Laser: Pumping a three state system
a. When the change is made to a three state system, electrons can be pumped to a
higher energy level with yellow, green and blue light in addition to red light.
Note: Perhaps these lights may all be distilled to just green light (as this is a fairly
common laser and the other light colors surround green), but further research on
the subject is needed to draw a conclusion.
b. The characteristics observed about the emission events on a per-color basis are as
follows:
Red
 All emissions are red (one color)
 Many of the emitted electrons are
given off in pairs
 Photons are scattered at random
 Many electrons are at an elevated
state (n=2) at a time, and multiple
electrons move between the two
levels at any given time
Yellow
 All emissions are yellow (one
color)
 About 50% of the emitted
electrons are given off in pairs
 Photons are scattered at random
 Photons are emitted less often
(visibly) in comparison to red
light
 Many electrons are at an elevated
state at any time, and multiple
move between the levels at any
given time
Green
 All emissions are green (one
color)
 Many fewer emissions occur (in
comparison to the red light case)
 Many electrons are at an elevated
state (n=2) at a time, and multiple
move between the levels at any
given time
Blue
 All emissions are blue (one
color)
 Very few visible photons are
emitted
 Photons are scattered at random,
and often they are released in
pairs
 Many electrons are at an elevated
(n=2) state at a time, and multiple
move between the levels at any
given time
Note: all of the photons seem to be spontaneously emitted (for visible emissions),
but are “stimulated” to an excited state by the incident light source. Also, cases
where a photon stimulates an atom to emit a second photon are seen as described
where the atom releases pairs of photons.
c. When the “display photons emitted from upper state” option is selected, these
photons represent the transition from the n=3 to the n=1 state. Photons from 31
are green light, while photons from 21 are red. The region of the spectrum to
which they belong is visible light.
d. When the lifetime for the n=3 level is increased, photons are emitted less often,
but in greater bursts. This is due to the resulting slowed decay from the n=3 level.
e. When the lifetime for the n=2 level is increased, photons are emitted less often.
The number of electrons in this level increases as directly correlated with lifetime.
This is because the decay from this level is slowed. The most electrons of the 3
levels are in the n=2 level. (The n=3 level had a lifetime of 50%).
f. Summary: Increased lifetime at a given level causes a slowed decay from that
level. The visible emissions appear spontaneous in nature, but cases where a
photon stimulates an atom to emit a second photon are seen as described where
the atom releases pairs of photons. We also noted that generally as the wavelength
of light decreases, photons are emitted less often.
3. Making a laser with a three state system
a. Conditions tha make a good laser include:
i. The use of mirrors, with a reflectivity of about 98% (90+%, but less than
100%)
ii. A mid-length lifetime for a “medium” state (ex. N=2 in a 3 state system)
iii. The use of red or green light
b. Note: I used green light in my observations
4. A small helium-neon laser produces a light beam with an average power of 3.5mW and a
diameter of 2.4 mm.
a. Note: Please see attached calculations
b. “The 633 nm line was found to have the highest gain in the visible spectrum,
making this the wavelength of choice for most HeNe lasers.” – Source:
Wikipedia: Helium-Neon Laser http://en.wikipedia.org/wiki/Helium-neon_laser
(Note: this information was cross referenced with other sources)