An opaque medium only passes a small percentage of the light that strikes its surface. How much light is passed through it depends on its opacity. Light can be shone through an eggshell, for example, but the light that leaves the other side is scattered and uneven. Even laser light shone through an opaque substance becomes scattered and loses coherence. If the characteristics of the resulting scattered light waves can be reverse-programmed into a laser beam before it passes through the opaque substance than the beam that exits should be coherent.
That is the reasoning behind an experiment conducted by researchers I.M. Vellekoop and A.P. Mosk of the University of Twente in the Netherlands. After first decoding the scattering characteristics of the emerging light, they placed a programmed phase modulator in the laser beam before the light passed through the medium. The waves exiting the phase modulator perfectly matched the scattering sample such that all the phase irregularities created by the opaque medium were effectively cancelled out.
The opaque substance was then found to act as a lens, focusing the beam into a narrow region. This phenomenon is probably attributable to Kerr-induced self-focusing. The beam still contains non-linear components of the refractive index of the material and where this component is positive, as it is in most materials, the refractive index becomes larger in the areas where the intensity is higher, usually at the center of the beam. This causes the beam to collapse in on itself. Self-focusing occurs where the radiating power is greater than the critical power, a constant of the medium and, as was demonstrated, the angle of focus is indeed a function of the material.
Tuesday, January 22, 2008
More fun with light
Monday, January 21, 2008
Squeezing light into narrow spaces
The spectrum of visible light restricts what we are able to see to those frequencies that fall in the narrow range between ultra-violet and the infrared with a small overlap into the two extremes. Objects that are smaller than the smallest wavelength of visible light can not be seen by human eyes, nor can they be resolved by conventional microscopes that focus light waves.
Similarly, the diameter of an optical fiber cable needs to be at least one-half of the wavelength of the light wave which is transmitted through it. This places a constraint on their application in micro-circuits, since that diameter is much larger than the dimensions of the conducting strips in the smallest electronic devices.
Research in the 1980's uncovered an interesting phenomenon. It was found that when light was directed at the space between a conducting material such as a metal and a non-conducting material or dielectric such as glass a resonant oscillating wave was generated by the electrons on the metal's surface, much like the ripples on the surface of a pond when a pebble is dropped into it. The frequency of the oscillations matched the frequency of the electromagnetic field outside the metal. The metal conductor's surface oscillations have been given the term surface plasmons, and the field of study is called plasmonics.
Using empirical data, researchers have now engineered a metal-dielectric interface which generates surface oscillations at the same frequency as the outside electromagnetic field but with a much shorter wavelength. In fact, Hideki Miyazaki of the National Institute for Materials Science in Japan has squeezed red light with a normal wavelength of 651 nanometers into a plasmon slot waveguide that is only three nanometers thick and 55 nanometers wide. The effective wavelength of the surface plasmon travelling through the gap was 51 nanometers.
Using fabrication techniques similar to those used to manufacture integrated circuits it is possible to mass-produce nano-scale plasmonic devices. There is a tremendous advantage to incorporating plasmonics into micro-electronic circuitry. The frequency of an optical signal is more than 400,000 Gigahertz, and such a circuit could carry huge amounts of data for a given instant. Furthermore, such a circuit might be even simpler overall, since there would be little to none of the stray capacitance or inductive effects which must be compensated for in conventional micro-circuitry. Harry A. Atwater and his group at the California Institute of Technology and others have laid the groundwork for and have developed primitive versions of the plasmonic transistor. More research in this area is needed to improve their performance but the future of micro-photoelectronics looks promising.
Friday, January 18, 2008
An illumination on the subject of light
The study of light has been a favorite preoccupation of scientists for as long as there has been science. They still refer to "the speed of light". I prefer to be more precise in my description, and refer to it as "the rate of atomic interaction" or "the rate of propagation of the electromagnetic waves that fall in the visible range of the optical spectrum".
Oddly enough, there are still physicists who believe that photon particles scoot across the Universe forever or until their progress is impeded, either through absorption or transformation. Presumably, our eyes qualify as photon absorbers. The photons that originate in stars a billion light years away are said to have been travelling across the intervening space all that time until they hit our retinas. Photons are said to be the source of light.
Clarification is in order. Energy which is released or absorbed by one of an atom's electron shells in which the electron undergoes a transitional change whereby it drops to or is raised to another energy level is called photonic energy. This energy is absorbed or released as discrete quanta. This was first noticed by Max Planck, who is essentially the unwitting founder of quantum physics. In short, he witnessed energy released as pulses of a constant frequency from a black body radiator, or cavity resonator, a small hollowed out perfectly symmetrical sphere with a tiny hole punched in it. Albert Einstein extended Planck's observation to include energy absorption of discrete quanta, and the photon was born.
Albert Einstein won his Nobel prize not for his theory of relativity, but for his explanation of the photo-electric effect. When ultraviolet light is directed at a sodium block in a vacuum jar, a galvanometer measures electric current. He reasoned that a photon originating from a source of ultra-violet radiation had enough energy to knock the single electron out of the outermost orbital of a sodium atom. In fact, he didn't need the ultraviolet light, because he would have observed the same effect if he had simply heated the sodium block, and heat originates at the other end of the spectrum in the infra-red range. So, what was actually happening?
Obviously the sodium atom has a very weak hold on that single electron in its outermost orbital. That's true. And apparently all that is required to knock it loose is sufficient agitation, regardless of the source. And in that description lies the clue to how we see.
At the event horizon of a black hole, the point of no return, particles are rushing in toward the center with such violence that electrons are literally stripped from the nuclei of their atoms. That is why we do not see light emanating from a black hole, and scientists describe this by saying that not even light can escape the intense gravitational pull of a black hole. But in fact, we need stable electron orbitals around the atoms in a medium in order for the range of frequencies that we can detect as light to be propagated as EM waves. We can still detect X-rays emanating from a black hole, but they originate from deep within the atom. Think of the principle of like polar repulsion. All electrons have an electromagnetic charge, and if you bring an atom adjacent to another atom a little closer to it than their normal stable relationship in a medium would normally allow then you will begin to push the other atom away. If you set up an oscillating behaviour in the atoms in a medium then they will all respond by oscillating at the same frequencies. In effect, the outer electron shells of the atoms in a medium are modulated with the surface characteristics of the atoms of all the objects within our field of view so long as there is a source of photonic energy stimulating their oscillations. Distance is of no consequence, because there is no loss of energy in this process and it can go on forever.
A research physicist by the name of Philip Bucksbaum at Michigan University has been conducting experiments on the electron to determine if the theory that an infinite amount of information can be stored on an electron can be proved, and he has succeeded in proving this to his satisfaction. That makes sense, really, because if you place yourself in an infinitely large room with an infinite number of shelves lining the wall on which are placed an infinite number of knick-knacks you would see all those objects, your range and field of vision permitting. The electrons of the atoms in the medium are modulated with all the surface characteristics of all the objects in that room.
What is more impressive and wondrous is that when those EM modulations strike the cones and rods on our retinas, our brains decipher the photo-electric information and convert the intensity to degrees of brightness and the modulated EM waves to the characteristic hues and tones and textures via the optical nerves. We actually see with our brains. Our retinas are really only photosensitive EM wave detectors. Photons do not illuminate the world for us to see. That explains nothing.
In the realm of EM radiation, a fundamental frequency has a virtually infinite range of harmonics and subharmonics. The combined oscillations resulting from the atomic interaction of many components in a medium which permits propagation produces a wide range of these summed harmonics and subharmonics and we detect those that fall in the range to which our cones and rods are tuned. Our visible world is an interpretation of our brains.
