27 November, 2009

1905: Annus Mirabilus - Photo electric effect

1905 was a great year for physics, in this year a 24 year old patent examiner in Bern Switzerland published 4 fundamental papers in physics in 4 disparate areas of the field. The topics included special relativity, the relationship between energy and matter, brownian motion and the subject of this post, the photo electric effect.

The photo electric effect paper by Einstein was probably the most practical paper next to the brownian motion paper in that it provided an answer to a long standing problem in electromagnetic theory at the time that had stood as an embarrassment to particle physics. This embarrasment was a legacy of the work of James Clerk Maxwell and his fundamental equations of electromagnetism, by using a continuous wave analog to describe the energy of propagating fields Maxwell was able to do the astonishing, he explained the riddle that was the relationship between electricity and magnetism in clear mathematical terms and he was able to show how light must be itself an electromagnetic wave by showing that all such waves are limited by the speed of light "c" or roughly 186,000 miles per second.

File:Electromagneticwave3D.gif

 The use of continuous waves to describe particles however led to serious difficulties when attempting to calculate the energy radiating from theoretical systems known as 'black bodies". Black body radiation could easily be approximated by taking an ingot of steel with a hole bored inside, according to Maxwell's electromagnetic theory this hole should be generating an infinite set of frequencies of light, but if there are infinite frequencies being generated then there must be infinite energy and experimentation clearly showed that such bodies had limited fixed energy release patterns so something was wrong.



Near the end of the 19th century a brilliant physicist Max Planck theorized the possibility that the infinite energy black body problem could be solved IF the energy of light eminating from the material was discrete in some way or "quantized". However he was unable to form a mechanism to describe the smooth transition from a continuous field and quanta of light or photons as they came to be called. At least this was the case until Einstein's arrival, Einstein took the job at Bern to work on his ideas in physics (but also because he had trouble finding a teaching position!)...his main aim was to answer the question he claimed plagued him from the time he was 17 and asked what it would be like to ride a beam of light, what would he see? He answered the question also in 1905 in his special relativity paper which I'll talk about in another post. Along the way he apparently amused himself by solving a couple of other huge problems in physics at the time, one of which was the black body problem. To be clear Einstein wasn't directly trying to solve the black body problem, he was trying to explain why it was that a metal surface shined with light would have ejected electrons or an induced current. Somehow the energy of the light was being absorbed by the metal material, some bouncing out as electrons and others forming a current flow. Einstein's solution involved using Planck's idea of quanta and tying it to the constant that Planck discovered (symbolically represented as h in physics) that would govern energy release in particle form devised the famous equation E = hv which in semiconductor physics circles is more important than F= ma or E = mc^2 , the reason is that this equation enabled theoretical results using a slight modification to Maxwell's wave formulation to match experimental results in the black body problem.

If Energy could only be released in packets of "hv" in size the infinite energy problem would go away, this would be so if the material of the black body and all material that radiate is restricted to "hv" units of radiative absorption. This victory on the part of Einstein could be said to be his most fruitful in a practical sense as it spawned more real technology than any of his other work, including Special and General Relativity. A minor caveat though, is that the discovery and explanation for quantization that emerged from Einstein's paper went on to describe a related by different phenomena the photo voltaic effect...the difference being subtle...the photo voltaic effect results as the liberated electrons in the material are free to flow, the bonding energy for ejecting the electrons of the photo electric effect are different from the conduction flow energy required to induce the photo voltaic effect. The latter phenomena is what led to later inventions that will be discussed below.


So surfaces emit energy in "hv" units of energy, so what?

The significance of Einstein's equation toward the electron ejection and current flow problems and the black body problem enabled a great embarrassment of EM theory to be over come but beside that what has come from the realization? A first practical utilization of the awareness that light could induce current flow came when Shockley and Bardeen invented the transistor in 1947, the transistor was realized by mating two different materials called semiconductors at a junction, one material was an electron donor , with free electrons ready to give ...the other material was an electron receiver, with free "holes" (or open valence shells) for accepting electrons...when mated in the double junction fashion used by Shockley and Bardeen it was hoped that a "bias" or current applied between two junctions could modulate a much larger current...effectively amplifying the smaller current.



It worked, This success ushered in the age of semiconductor electronics that accelerated in the 50's with the transistor radio and other devices and took off in the 60's and 70's. However a side effect of mating electron rich and electron poor junctions was what happened as electrons jumped the gap of what is called the "depletion region" between the two materials, in accordance with Einstein's relation for quantized photons on surfaces when current flowed through the junctions photons (in the infrared range at the time) would be produced. This single junction effect had actually been discovered decades before but was not exploited until...

LED's 

Technically a single junction device is a diode, a device known to electronics and electrical power generation for over 130 years (before semiconductors) that only allows current flow in one direction...but it's optical properties were unexplored for decades.



 The fact that diodes were creating light (infrared) when run in specific operating regimes was not seen as valuable. Thus the reverse of the photo-voltaic effect hibernated, flowing current through materials in just the right manner could liberate photons of a specific frequency 'v'...in transistor design this is actually a bad thing as it is energy that leaves the circuit and does not aid in the amplification of the bias signal which is the desired result...a way to use this reverse photo electric effect was realized shortly after Einstein's paper was written in 1907, the light produced was infra red...it took almost 60 years to produce visible light LED's and another 15 years to reduce costs for them to be included into practical devices. If you've seen an 8 segment display from old calculators you were looking at a bunch of early LED's. Today, LED lighting is every where and has diversified from the early red LED's to colors across the visible spectrum including combined options to create white light. Many cities have begun replacing their old bulb based street lights by packages of highly efficient and color pure LED lights, these packages are significantly more power efficient and thus will save cities millions in energy costs , they are also environmentally friendly as energy not used means less carbon emissions.



The LED based street lights are also noticeably more color pure and brighter from much further distances potentially allowing for a reduction in accidents. As a replacement for incandescent and flourescent bulbs LED lighting promises incredibly energy efficient and color pure light options without the environmental potential hazards associated with older technologies. In the most recent iterations of the technology rather than use semiconductors to produce the light, researchers are using bio molecules that change shape and release visible light photons, these new LED's called O (organic) LED's for their use of these biomolecules are poised to revolutize display technologies from hand held phones to large screen TV's.



OLED's can be even more efficient than regular LED's can be embedded in flexible or transparent membranes or surfaces and can produce color ranges not possible with CRT (cathode ray tube) or plasma or LCD based technologies.

Image Sensors

So a direct result of Einstein's explanation of the photo electric effect in the reverse case is clear and ever present but what of the original mode of light driving current in a metal? This aspect of the paper was explored in the mid 60's by engineers in Bell Labs who were on a quest to find a display technology for what was believed to be the pending big business of video phones. The investigation of a grid of photo diodes used to capture light in wells and induce individual currents which can then be used to infer the luminous intensity of the impinged light, sufficiently large arrays of these diodes could then be used as a sensor array for optical uses and the CCD or charge coupled device was born. The technology changed hands and advanced as the array densities improved and the methods for reading and processing the signals generated in the thousands and then millions of wells were refined, soon the CCD was being used in high end video cameras to capture light using a trichromatic process and then synthesize a full color image. It wasn't until the mid 90's that the technology really took off in production as the unique nature of the circuitry for CCD's that made them expensive became less of an issue. CCD's were used by Kodak to replace chemical film and the digital camera revolution was born, Japanese companies picked up on the power of the sensor technology and devoted large amounts of R&D to create more advanced sensors , Nikon, Canon , Sony and others joined the fray and led to the growth of the digital photography market that has occurred in the last15 years. A parallel development for the last 10 years has been the ability to retool existing MOS (metal oxide silicon) based production plants to produce sensors , these C(complementary) MOS sensors have recently matched and exceeded the former advantages of CCD's for most applications but at the much reduced production cost allowing such sensors to be placed in cell phones, web cams and home surveillance cameras in amazing numbers.



Solar Cells

An obvious use for exploiting the photo voltaic effect came up as semiconductor materials continued to fall in price and in conjunction with their use as receptor wells for creating image sensors and that is to collect photons from the sun and use them to generate electricity. The requirements for solar cells to function rely less on the type of energy input (you ideally want to take in all photon energy) so materials that allow the surface to conduct current at as many frequencies as possible will induce the largest current flow.



The original solar cells were very inefficient at liberating this current flow but were good enough to enable the creation of solar powered calculators by the late 70's, the models today can easily perform vast computation using the residual light of incandescent bulbs indoors. However, until recently they remained sensitive only to specific wavelengths of incoming light making them inefficient for power generation devices for large objects...recent advances in nano-materials though have opened the possibility for gathering across the optical spectrum to create solar cells that can convert nearly 50% of the incoming photon energy into conduction electrons. This technology promises to revolutionize how we generate power world wide in the next decade.



As a recognition of his explanation of light quanta and the work that fell out of it (I covered only the practical devices the ideas (quantization) has even wider ranging influence on quantum mechanics itself but I won't go into that here) Einstein won the Nobel prize in Physics (the only one he ever won amazingly) in 1923.

Quite a trip isn't it? In just over 100 years, Einstein's diversion has flowered into a multi-billion dollar industry for creating light and for capturing it, think about that the next time you are stopped at a traffic light or are taking a snapshot with your camera phone. Take a moment to say thanks to Einstein.



Links:
http://en.wikipedia.org/wiki/Black-body_radiation

http://www.embd.be/quantummechanics/black_body_radiation.html

http://en.wikipedia.org/wiki/Photo_electric_effect

http://www.youtube.com/watch?v=0qKrOF-gJZ4

http://en.wikipedia.org/wiki/Diode

http://en.wikipedia.org/wiki/Light-emitting_diode

http://en.wikipedia.org/wiki/Transistor

http://en.wikipedia.org/wiki/OLED

http://en.wikipedia.org/wiki/Charge-coupled_device

http://en.wikipedia.org/wiki/Active_pixel_sensor

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