Optics Faraday

Circular birefringence in the presence of high linear birefringence?

Could someone with a good understanding of optics, fibers, and polarization, please attempt to answer the follow question:

In a highly linear birefringent medium, such as a polarization-maintaining fiber, what happens when circular birefringence is introduced - for example by a magnetic field?

I know that in a medium with no linear birefringence, the circular birefringence would give rise to Faraday rotation, and I can see that in the highly linear birefringent medium Faraday rotation cannot occur.

However, can the circular birefringence still exist in the medium?

If a linear polarisation is guided along hibi PM fiber, do the component circular polarizations still experience different refractive indices, as they would in an ideal medium with circular birefringence?

Thanks.

Well, I would say no.

Remember "linear polarization" and "circular polarization" are two basis for the same polarization state. If you have a material with linear birefringence, it is simply convenient to write all your polarization states in the basis of two orthogonal linear polarizations, because those are, er, "speed eigenstates", i.e. you can find two linear polarizations that have definite phase speed, whereas this is not true for circular polarizations.

And the same holds for a circularly birefringent medium. Circular polarizations happen to be two orthogonal states which have definite phase speed, and the linear polarizations have no such definite phase speed.

If you have both linear birefringence and circular birefringence, then neither will be true: i.e. no particular linear polarization will have definite phase speed, and no particular circular polarization will have a definite phase speed. My gut feeling is that you might be able to find some two orthogonal elliptical polarizations which does have definite phase speed, so if you prepare a light as a superposition of those two polarizations, then you will see the polarization of light oscillate between the two as it propagates through the fiber, but, well, it does sound like a complicated problem. If you want to go through the formalism, it looks like Huard's Polarization of Light does cover it. On the Amazon page http://www.amazon.com/gp/product/0471965367?ie=UTF8&tag=byunkyuparksp-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=0471965367 search inside the book and search for "phase-shifter rotator" ("rotator" is his term for circularly birefringent material) and the summary of the result is on pages 116 and 117.

I prefer to think about it from, well, physical point of view. Polarization-maintaining fibers are made with highly linearly birefringent material *because* when the polarization of light gets misaligned from the optical axis, the linear birefringence tends to rotate the light (with addition of some ellipticity) back along the optical axis. So, if you have a slight circular birefringence, as that circular birefringence tries to rotate the linear polarization of input light away from the optical axis, the linear birefringence pulls it back, like a tug-of-war.

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Spintronics In Computing

Spintronics in computing

Spintronics  is an emerging technology that exploits the intrinsic spin of the electron and its associated magnetic moment.

In 1980s the experiments on spin-dependent electron transport phenomena in solid-state devices donerevealed the research field of spintronics.It  includes the observation of spin-polarized electron injection from a ferromagnetic metal to a normal metal.

 Electrons are spin-1/2 fermions .Fermion constitute a two-state system with spin "up" and spin "down". A current of spin-polarized electrons comprising more of one spin species—up or down ,is the primary requirement of spintronic devices .SO we require the  devices spin injectors and a separate system that is sensitive to the spin polarization of the electrons (spin detector). Manipulation of the electron spin during transport between injector and detector (especially in semiconductors) via spin precession can be accomplished using real external magnetic fields or effective fields caused by spin-orbit interaction

Spin pol arization in non-magnetic materials can be achieved either through the Zeeman effect in large magnetic fields and low temperatures, or by non-equilibrium methods. In the latter case, the non-equilibrium polarization will decay over a timescale called the "spin lifetime". Spin lifetimes of conduction electrons in metals are relatively short (<1nsec) but in semiconductors the lifetimes can be very long (µsec at low temperatures), especially when the electrons are isolated in local trapping potentials .

Metals-based spintronic devices

The simplest method of generating a spin-polarised current in a metal is to pass the current through a ferromagnetic material. This effect is useful in  a giant magnetoresistance (GMR) device. A GMR device consists of at least two layers of ferromagnetic materials separated by a spacer layer. When the two magnetization vectors of the ferromagnetic layers are aligned, the electrical resistance will be lower (so a higher current flows at constant voltage) than if the ferromagnetic layers are anti-aligned. This constitutes a magnetic field sensor.ie  the device can acts as inverter with logic 0  or logic1.

Two variants of GMR have been applied in devices:

(1) current-in-plane (CIP), where the electric current flows parallel to the layers and

 (2) current-perpendicular-to-plane (CPP), where the electric current flows in a direction perpendicular to the layers.

Other metals-based spintronics devices:

1. Tunnel Magnetoresistance (TMR):In this  device CPP transport is achieved by using quantum-mechanical tunneling of electrons through a thin insulator separating ferromagnetic layers.
2. Spin Torque Transfer,:In this a current of spin-polarized electrons is used to control the magnetization direction of ferromagnetic electrodes in the device.

Semiconductor-based spintronic devices

The spin-polarized electrons are generated via optical orientation using circularly-polarized photons at the bandgap energy incident on semiconductors with appreciable spin-orbit interaction (like GaAs and ZnSe). Although electrical spin injection can be achieved in metallic systems by simply passing a current through a ferromagnet, the large impedance mismatch between ferromagnetic metals and semiconductors prevented efficient injection across metal-semiconductor interfaces. A solution to this problem is to use ferromagnetic semiconductor sources (like manganese-doped gallium arsenide GaMnAs), increasing the interface resistance with a tunnel barrier, or using hot-electron injection.

Spin detection in semiconductors is another challenge, which has been met with the following techniques:

1. Faraday/Kerr rotation of transmitted/reflected photons
2. Circular polarization analysis of electroluminescence
3. Nonlocal spin valve (adapted from Johnson and Silsbee's work with metals
4. Ballistic spin filtering

The latter technique was used to overcome the lack of spin-orbit interaction and materials issues to achieve spin transport in silicon, the most important semiconductor for electronics.

Because external magnetic fields can cause large Hall effects and magnetoresistance in semiconductors , the only conclusive evidence of spin transport in semiconductors is demonstration of spin precession and dephasing in a magnetic field non-collinear to the injected spin orientation. This is called the Hanle effect.

Applications spintronic devices

1. The storage density of hard drives is rapidly increasing along an exponential growth curve, in part because spintronics-enabled devices like GMR and TMR sensors have increased the sensitivity of the read head which measures the magnetic state of small magnetic domains (bits) on the spinning platter. The doubling period for the areal density of information storage is twelve months, much shorter than Moore's Law, which observes that the number of transistors that can cheaply be incorporated in an integrated circuit doubles every two years.
2. Racetrack memory

1. MRAM, or magnetic random access memory, uses a grid of magnetic storage elements called magnetic tunnel junctions (MTJ's). MRAM is nonvolatile so information is stored even when power is turned off, potentially providing instant-on computing. Motorola has developed a 1st generation 256 kb MRAM based on a single magnetic tunnel junction and a single transistor and which has a read/write cycle of under 50 nanoseconds .

          Racetrack memory, encodes information in the direction of magnetization between      

             domain  walls of a ferromagnetic metal wire.

       4.Advantages of semiconductor-based spintronics applications are potentially lower power use and a smaller footprint than electrical devices used for information processing Also, applications such as semiconductor lasers using spin-polarized electrical injection have shown threshold current reduction and controllable circularly polarized coherent light output. Future applications may include a spin-based having advantages over MOSFET devices such as steeper sub-threshold slope.

References

^ IBM RD 50-1 | Spintronics—A retrospective and perspective

^ Physics Profile: "Stu Wolf: True D! Hollywood Story"

^ http://prola.aps.org/pdf/PRL/v55/i17/p1790_1

^ Phys. Rev. Lett. 61 (1988): M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, and J. Chazelas - Giant Magnetoresistanc...

^ http://prola.aps.org/pdf/PRB/v39/i7/p4828_1

^ PII: 0370-1573(94)90105-8

^ http://www.sciencedirect.com/science/article/B6TVM-46R3N46-10D/2/90703cfc684b0679356dce9a76b2e942

^ Cookies Required

^ http://www.sigmaaldrich.com/materials-science/alternative-energy-materials/magnetic-materials/tutorial/spintronics.html

^ http://www.everspin.com/technology.html

^ The Emergence of Practical MRAM http://www.crocus-technology.com/pdf/BH GSA Article.pdf

^ http://www.eetimes.com/news/latest/showArticle.jhtml?articleID=218000269

^ Phys. Rev. B 62 (2000): B. T. Jonker, Y. D. Park, B. R. Bennett, H. D. Cheong, G. Kioseoglou, and A. Petrou - Robust electrical spin injection

^ Cookies Required

^ Phys. Rev. Lett. 90 (2003): X. Jiang, R. Wang, S. van Dijken, R. Shelby, R. Macfarlane, G. S. Solomon, J. Harris, and S. S. Parkin - Optical Detection of Hot-Electron

^ Phys. Rev. Lett. 80 (1998): J. M. Kikkawa and D. D. Awschalom - Resonant Spin Amplification in

^ Polarized optical emission due to decay or recombination of spin-polarized injected carriers - US Patent 5874749

^ Electrical detection of spin transport in lateral ferromagnet-semiconductor devices : Abstract : Nature Physics

^ Electronic measurement and control of spin transport in silicon : Abstract : Nature

^ Access : : Nature

^ Access : : Nature

^ Cookies Required

Further reading

"Introduction to Spintronics". Marc Cahay, Supriyo Bandyopadhyay, CRC Press, ISBN 0-8493-3133-1

Ultrafast Manipulation of Electron Spin Coherence. J. A. Gupta, R. Knobel, N. Samarth and D. D. Awschalom in Science, Vol. 292, pages 2458-2461; June 29, 2001.

Spintronics: A Spin-Based Electronics Vision for the Future. S. A. Wolf et al., Science 294, 1488-1495 (2001)

How to Create a Spin Current. P. Sharma, Science 307, 531-533 (2005)

Search Google Scholar for highly cited articles with query: spintronics OR magnetoelectronics OR "spin based electronics"

"Electron Manipulation and Spin Current". D. Grinevich. 3rd Edition, 2003.*

Semiconductor Spintronics. J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano, and I. Žutić, Acta Phys. Slovaca 57, 565-907 (2007)

Spintronics: Fundamentals and Applications. I. Žutić, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323-410 (2004)

External links

"Spintronics". Scientific American. June 2002. http://www.sciam.com/article.cfm?articleID=0007A735-759A-1CDD-B4A8809EC588EEDF. 

RaceTrack:InformationWeek (April 11, 2008)

IBM (2003)

Wired: update on MRAMs, 2003 Jul

Spintronics research targets GaAs.

Spintronics at Indian Institute of Science, Bangalore, India

Spintronics at SUNY Albany's College of Nanoscale Science and Engineering

Spintronics information community site

IBM to use 'spintronics' to increase computer memory capacity (April 12, 2008)

Semiconductor spintronics lab at University of Maryland

Spintronics Tutorial

 

RABIYA TANVEER.                                                               

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