Download THE FARADAY AND KERR EFFECTS The Faraday and Kerr Effects

1 THE FARADAY AND KERR EFFECTS
The Faraday and Kerr Effects
Zoë Little, Klaudia Kozek, and Julian Marrero
SVSM UNCC
2010
2 THE FARADAY AND KERR EFFECTS
Abstract
Though discovered over a century ago, the Faraday and Kerr Effects of polarized light are two of
the most instrumental components of optical communication technology today. The nature of
these properties is the manner in which the polarization of light is affected by an electromagnetic
field; the Faraday Effect concerns light transmitted through a magnetic field where as the Kerr
effect concerns light reflected off a magnetized surface. These two effects have many practical
applications in fiber optics and data storage as well as research into the future of optical
technology.
3 THE FARADAY AND KERR EFFECTS
Introduction
Michael Faraday and the Reverend John Kerr are renowned physicists who were active
throughout the 1800s. In 1845, in attempt to determine whether the polarity of light was
affected by strong electric fields when passed through a transparent insulator, Faraday failed to
detect any changes and turned his efforts toward magnetic fields. In due course, Faraday
successfully found that a small change in polarity did occur in this situation; this is now known
as the Faraday Effect. In 1875, thirty years later, Kerr decided to take up Faraday’s unfinished
experiment, trying to prove that an electric field could also affect the polarization of passing
through a transparent material. After using an assortment of mediums, including liquids and
solids such as glass, he demonstrated the electro-optic effect.
Michael Faraday was an English chemist and physicist. Though Faraday had little
formal education especially in higher mathematics, he is considered one of the most influential
experimentalists in history (2010, Faraday Effect). Faraday was an accomplished chemist,
popularizing terms such as “anode,” “cathode,” “electrode,” and “ion.” He also discovered
benzene, invented a precursor to the Bunsen burner, and created the system of oxidation numbers
we use today. However, Faraday’s most imperative work was in the field of electromagnetism.
He invented the first simple electric motor as well as conducting many experiments that made
electricity a viable technology. The Faraday Effect was discovered in 1845 (Mansuripur).
The Reverend John Kerr was a Scottish physicist. His most essential work was in the
field of electro-optics. In 1875, Kerr discovered that double refraction occurs in solid and liquid
dielectrics in an electrostatic field. Double refraction is when a ray of light breaks apart into two
waves and creates a double image. The most applicable discovery that Kerr proposed was the
Magneto-Optic Kerr Effect (MOKE) was in 1875 (7 June 2010. John Kerr (Physicist)).
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Light is electromagnetic radiation that varies in wavelength ranging from around 4,000
(violet) to around 7,700 (red) angstroms. This kind of light may be apparent to the average
unaided human eye. Light vibrates at different frequencies and travels at different speeds.
When light in which electromagnetic vibrations oscillate repeatedly in multiple directions, the
light is then considered a non-polarized light. Natural light is made up of photons that scatter in
Fig. 1
a random pattern, travelling in
many different directions; rather,
the light is not correlated.
However, certain materials can be
used to filter or align the light in
one direction. The resulting light
is called polarized light.
In Fig. 1, the beam of light is
shown with two distinct wave directions. It passes through the first polarizing filter which only
allows the horizontally polarized light through while blocking the other. Then, upon passing
through a vertically polarized filter, all the light is blocked. Polarized lenses are used in
sunglasses and cameras to cut glare and sharpen the image. Polarized light can also be used to
read stored data on optical drives.
5 THE FAR
RADAY AN
ND KERR EFFECTS
E
T Faraday Effect was
The
Fig. 2
discovereed in 1845. When lightt
is transm
mitted through
h a strong
electromagnetic field
d, a change inn
polarization proportio
onal to the
strength of
o the field can
c be
observedd; this is the Faraday
F
Effect (ββ=VBd). In Fig. 2, β is
the anglee of rotation (in radians),,
B is the magnetic
m
flux density (inn teslas) in thhe direction of propagatiion, d is the length of thee
path (in meters)
m
wherre the interacction betweeen the magneetic field andd the light occcurs, and V is
Fig. 3
the Verdet
V
Consstant (radianns per tesla per
p meter), a term
thatt describes thhe strength of
o the Faradaay Effect in a
matterial (Ghoshh, Atkinson). As the ligght passes
throough the maggnetized areaa, its polarizzation twists..
Thiss has many uses
u in comm
munications because it
allows for the crreation of ann optical diodde or opticall
T
isolator (Fig. 3) (24 June 20010. Optical Isolator). This
device, constructed
c
from
f
a Faradday rotator, liike the one pictured
p
in Fig.
F 2, only allows
a
light to
t
pass throough in one direction,
d
muuch like the diodes foundd in electricaal circuits annd the humann
body. This
T preventss optical feeddback and is especially practical
p
in lasers and opptical
communiication techn
nology. Thee Faraday Efffect also appplies to spinttronics reseaarch becausee it
can be ussed to determ
mine the spinn of electronns in semiconnductors (5 July
J
2010, Faraday Effecct).
6 THE FARADAY AND KERR EFFECTS
The Kerr Effect (Fig. 4), though
very similar in nature to its
predecessor, the Faraday Effect, was
discovered over thirty years later in
1875. Rather than concerning light
transmitted through a strong magnetic
field, it concerns light reflected off a
Fig. 4
magnetized surface. When light hits a
regular metallic surface, it reflects back with no change in polarization. However, this is not the
case when the surface is subjected to a magnetic field. When the Kerr Effect is observed, the
Fig. 5
light undergoes both a change in
polarization and reflected
intensity (Walker). The main
application of the Kerr Effect is
in magneto-optical (MO) drives
(Fig. 5). These use flat, circular
discs that can encode binary
data. When a laser beam is
directed at a specific point on
the optical disc, it reflects back with a different polarity; this change can be interpreted by the
computer as a zero or a one. If the laser head does not touch the disc, the spot represents a “0”,
and the spots where the disc has been heated up and magnetically written will translate into a
“1”. Optical discs are used for many consumer products such as CD-ROMs, DVDs, and modern
7 THE FARADAY AND KERR EFFECTS
video game discs. MO discs are
Fig. 6
capable of offering high capacity
and moderately inexpensive media
as top archival properties, usually
being rated with an average life of
30 years. MO drives last far longer
than any magnetic media.
Magneto-optical Kerr microscopy (Fig. 6) is another use of this effect (Walker). This device can
help identify and characterize the magnetic properties of different materials by observing and
analyzing the strength of the magneto-optical effect. This method is especially effective because
of the speed and clarity in which the effects can be observed. Furthermore, samples of material
are not damaged by the process (McCord, Brandow). The Kerr effect can also be used to create
high-speed shutters (Lux).
Conclusion
The Faraday Effect is a property of light. When light passes through a strong
electromagnetic field, its polarization changes depending on the strength of the field. The
Magneto-Optic Kerr Effect is a property of light where light is reflected off a strongly
magnetized surface that causes a change in both the polarization and reflected intensity of the
light.
The Faraday Effect has been studied in an attempt to try to apply it to optical
communications. By enhancing Faraday’s effect, it has been proven that optical resonance lines
can be applied to optical communication. A resonance line is the line of longest wavelength
8 THE FARADAY AND KERR EFFECTS
associated with a transition between the ground state and excited state. To scramble and
unscramble transmitted messages, an optical communication system was designed and
effectively tested. It used the improved Faraday Effect at low fields to produce polarization
modulation and high dispersion of the enhanced effect at high fields (Bomke). Such discoveries
will allow optical communication to improve and will give scientists a step forward into the
future of optical technology.
9 THE FARADAY AND KERR EFFECTS
Sources
McCord, J. (2009). Magneto-optical microscopy. Journal of Applied Physics, 105. Retrieved from
http://esm.neel.cnrs.fr/2005-constanta/abs/mccord-abs.pdf
Brandow, A.; Geiler, A.; Head, P.; Loura, R.; Marvin, H.; Zartarian, M. (2005). Magneto-Optical Kerr
Effect Microscope. Northeastern University Electrical and Computer Engineering Department.
Retrieved from
http://www.ece.neu.edu/faculty/dimarzio/capstone/samples/Final%20Capstone%20Report.pdf
Walker, C. and Morton, S. (2006). MOKE - Magneto-Optic Kerr Effect, SMOKE - Surface MagnetoOptic Kerr Effect. Surface Science Techniques. Retrieved from http://www.uksaf.org/tech/moke.html
Mansuripur, M. (Nov 1999). The Faraday Effect. Optics & Photonics News, 10. Retrieved from
http://www.mmresearch.com/articles/article3/
Lux, J., (1998). Electro-optical measurements (Kerr, Pockels, and Faraday). High Voltage Experimenter’s
Handbook. Retrieved from http://home.earthlink.net/~jimlux/hv/eo.htm
Ghosh, A.; Hill, W.; Fischer, P. (2007). Observation of the Faraday effect via beam deflection in a
longitudinal magnetic field. PHYSICAL REVIEW A 76. Retrieved from
http://www.rowland.harvard.edu/rjf/fischer/images/PRA_76_055402.pdf
Atkinson, R. (Fall 2001). Magnetism in a New Light. PEM Applications News for Users of Photoelastic
Modulators. Retrieved from www.hindsinstruments.com/wp-content/uploads/pem-10-MOKE.pdf
Bomke, H. and Harmatz, M. (1997). Enhanced Faraday effect and its application to optical
communication. Applied Optics, 16.
Retrieved from http://www.opticsinfobase.org/abstract.cfm?URI=ao-16-3-751
(15 July 2010). Michael Faraday. Retrieved from http://en.wikipedia.org/wiki/Michael_Faraday
(2010). Faraday Effect. Retrieved from http://science.jrank.org/pages/2661/Faraday-Effect.html
(5 July 2010). Faraday Effect. Retrieved from http://en.wikipedia.org/wiki/Faraday_effect
(24 June 2010) Optical Isolator. Retrieved from http://en.wikipedia.org/wiki/Optical_isolator
(7 June 2010) John Kerr (Physicist). Retrieved from http://en.wikipedia.org/wiki/John_Kerr_(physicist)
Images
Fig 1: http://light.physics.auth.gr/images/enc/pol.gif
Fig. 2: http://fr.academic.ru/pictures/frwiki/52/400px-Faraday_effect.svg.png
Fig. 3: http://image.tradevv.com/2009/08/10/violinjay_478505_600/optical-isolator-mi-1210cl-r.jpg
Fig. 4: http://lasuam.fmc.uam.es/lasuam/img/glossary/moke1.gif
Fig. 5: http://www.cdrinfo.com/Sections/Articles/Sources/0/1_3GB%20Fujitsu%20MagnetoOptical%20MCE%203130%20AP%20and%20SS/Images/Disk%20Layers.gif
Fig. 6: http://www.spring8.or.jp/wkg/BL43IR/instrument/img/BL43IR_opt1.jpg