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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)). 4 THE FARADAY AND KERR EFFECTS 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 FARADAY AND KERR EFFECTS The Faraday Effect was Fig. 2 discovered in 1845. When light is transmitted through a strong electromagnetic field, a change in polarization proportional to the strength of the field can be observed; this is the Faraday Effect (β=VBd). In Fig. 2, β is the angle of rotation (in radians), B is the magnetic flux density (in teslas) in the direction of propagation, d is the length of the path (in meters) where the interaction between the magnetic field and the light occurs, and V is the Verdet Constant (radians per tesla per meter), a term Fig. 3 that describes the strength of the Faraday Effect in a material (Ghosh, Atkinson). As the light passes through the magnetized area, its polarization twists. This has many uses in communications because it allows for the creation of an optical diode or optical isolator (Fig. 3) (24 June 2010. Optical Isolator). This device, constructed from a Faraday rotator, like the one pictured in Fig. 2, only allows light to pass through in one direction, much like the diodes found in electrical circuits and the human body. This prevents optical feedback and is especially practical in lasers and optical communication technology. The Faraday Effect also applies to spintronics research because it can be used to determine the spin of electrons in semiconductors (5 July 2010, Faraday Effect). 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 Magneto- Optic 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).
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