Transcript Slide 1 - SUNY Oswego

Set-up for Levy Flight of Photons
in Resonant Atomic Vapor
Danielle Citro (SUNY Oswego), Adailton Feliciano (UFPB), Martine
Chevrollier (UFPB), Marcos Oria (UFPB)
Objective
Introduction
Light propagation in resonant atomic vapors have been shown to not follow Gaussian
(normal) statistics. Instead, during the multiple scattering process, it is better described by
superdiffusion. The small probability that a photon will be scattered far from resonance leads
to the occurrence of rare but very long flights, called Lévy flights, which change the whole
dynamic of the system.
Send a laser through a heated Rubidium cell at the resonance frequency and collect the
fluorescence light with a fiber. The light collected should have a Doppler shape.
Theory and Methods
Theory:
Pictures:
Set-up:
Figure 6: Shows the set-up of
the current supply used to
heat the cell. We kept track of
the current used to heat the
bottom of the cell. The current
used was around 1.65
amperes.
Gaussian statistics is able to accurately describe many systems in
nature, yet light propagation in resonant atomic vapors in not one of
them. During normal diffusion, steps occur in random directions with
slightly varying lengths around the average value. However, due to
Lévy flights, Superdiffusion is a better statistical description of
resonant atomic vapor systems. Lévy flights are “abnormally long
steps interrupting a sequence of apparently regular jumps1.” These
Lévy flights are rare, but have a huge impact on the whole system.
Lévy flights come from the small but finite probability that a photon will
be scattered with a frequency far from resonance during frequency
redistribution.
Figure 1a: Shows the graph of a
.
Doppler (Gaussian) spectral distribution.
Figure1b: Shows the graph of a
Lorentzian distribution. Note how the
wings are larger for the Lorentzian
distribution, leading to long steps (Lévy Figure 1b
Figure 1a
flights) in a resonant vapor.
Figure 5
Radiation Trapping:
Figure 5:
Shows the top
view of the set
up. The red
line follows
the path of the
laser.
Radiation trapping is “the phenomenon of resonant multi-scattering of
light in atomic vapours1.” Particles get repeatedly deflected by other
particles during multiple scattering. When an incident photon enters a
medium, it can be absorbed and re-emitted many times throughout its
travel through the medium.
During radiation trapping, the incident photons are first absorbed by the
atoms in the gas medium at a frequency (v) that is close to the atomic
transition of (vo). Next, when the excited atoms return to ground state,
they emit a photon that can then be absorbed by another atom. This
absorption occurs when the incident photon frequency is at resonance,
with the resonance of Rubidium being 780.241 nm. Depending on the
absorption spectral shape, the absorption of the photon is greater at the
center of resonance.
Doppler (Gaussian) and Lorentzian Lineshapes:
“Doppler broadening is due to the distribution of atomic velocities, which
each have a Doppler shift with respect to an observer2.” Cold, resonant
atomic vapors have more of a Lorentzian shape than a Doppler. In a
thermal vapor, the Doppler effect changes the shape to a Doppler one.
To increase the probability of a large amount of scatterings, the density
may be increased. One way to increase the density is to increase the
temperature of the cell reservoir. The desired shape of the
fluorescence light is the Gaussian (Doppler) shape, similar to the
adsorption spectral shape of atoms in the vapor.
Methods:
Send the laser through the Rubidium cell at a slight angle as to not
collect the resonant laser with the lens as it exits the cell. By slightly
increasing the temperature of the cell, you increase the atomic density,
increasing the fluorescence and the absorption of the resonant beam.
After sending the laser through, collect only the fluorescent light using
a two lens system and a fiber that is sent to a photo-detector.
Figure 3: Shows the layout of the experiment.
The laser was sent through an optical isolator so
there would be no reflection of light back to the
laser. After a lens and a few mirrors, the laser
was sent through the heated Rubidium cell at an
angle. A two-lens system at the exit of the cell
collected the fluorescent light where it would be
sent through the fiber and finally into the photodetector.
Figure 4
Figure 4: Shows the fluorescence in a long cell
with the incident photons having a Dopplerbroadened distribution1. The brighter the spot,
the higher the fluorescence intensity. For this
project a shorter cell was used.
Results:
Figure 2: Shows the results printed from the
oscilloscope. The green line on the graph shows
the intensity of the fluorescent light. When the
laser is off resonance, the oscilloscope
Figure 2
should not read anything (right).
References
Conclusion
Now that the fluorescent light has been collected, the line
shape of the light can be tested to see if it has a Doppler
line shape. Once tested and confirmed, part two of this
project can be started. The fluorescent light will be sent
through another cell where that light will be scattered and
measured to observe the distribution of the steps length for
a thermal vapor excited by a Gaussian spectrum excitation.
Figure 3
1
Martine Chevrollier (2012): Radiation trapping and Lévy
flights in atomic vapours: an introductory review,
Contemporary Physics,
DOI:10.1080/00107514.2012.684481
2 "Spectroscopic Lineshapes." Stage 2 Chemistry Social
Relevance Projects.. N.p., n.d. Web. 24 Jul 2012.
<http://www.chemistry.adelaide.edu.au/external/socrel/content/lineshap.htm
Figure 6
Figure 7: Shows the side
angle of the exit side of the
cell. The cell is heated to
between 105oC and 115oC so
that the atomic density is
increased and the resonant
laser beam is totally absorbed.
Figure 7
Figure 8: Shows the side view
of the cell and the two lens
system The first lens that the
light enters has a focal length
of 5cm and the second lens
has a focal length of 1cm.
The light then proceeds to the
fiber.
Figure 8
Figure 9: Shows the fiber used
to collect the fluorescent light
and send it to the photodetector. After confirming that
the light entering the photodetector is all fluorescent light,
the fiber will be used to send
that light through another cell.
Figure 9
B
Figure 10: Shows the controls
for the laser and the photodetector. The laser controls
are (A), where the top controls
the temperature and the
bottom controls the current.
A
By changing both of these, the
frequency of the laser can be
Figure 10
set. The Oscilloscope is (B).
This controls the screen for the input from the photo-detector.
Here the fluorescence intensity can be seen when the laser
frequency is scanned around the resonance.
Future Steps:
After collecting the Fluorescence of Rubidium, the next step
is to use the light from the fiber which already has a Doppler
shape to measure the distribution of the steps length for a
thermal vapor excited by a Gaussian spectrum excitation.
Acknowledgements