The future of photographic optics and sensors is going to be profoundly affected by advances in materials science that most photographers have never even heard of.
Broadly defined, metamaterials are substances whose properties depend on their microstructure more than their chemical composition and that exhibit novel properties not usually found in nature. Most of the definitions I've seen don't include photonic crystals, although I must confess that I don't understand why. Photonic crystals are also materials that have novel properties due to their structure.
The most notable thing about photonic crystals is that they can be what amounts to an optical semiconductor. Electronic semiconductors are valuable because they only allow electrons to have certain energies; between those allowable energy bands are what are called bandgaps. An electron cannot exist inside the material in that bandgap. The way an electron gets from one conduction band to another is by absorbing or emitting a photon that changes its energy by enough to let it jump across the bandgap. You can see where that leads: photodetectors, LEDs and laser diodes. By adding small amounts of contaminants (a.k.a. dopants) to a semiconductor, one can tailor the energy levels and bandgaps to one's needs, yielding the whole array of solid-state electronics and integrated circuits that exist today.
Photonic crystals can play the same tricks with photons. Within them, certain energy levels (a.k.a. wavelengths, a.k.a. colors) are forbidden while others are not. A natural example of a photonic crystal is opal; it's not chemistry but the geometric packing of silica spheres in bulk opal that cause the brilliant flashes of color that we see. Those packed spheres will only allow certain energy levels to exist. The phenomenon is very different from diffraction; when you tilt an opal, you don't see a continuous change in color as you would with a diffraction grating, instead you see it flash brilliantly when the angles are just right to reflect light at an allowable energy level into your eyes.
Artificial photonic crystals, like semiconductors, can be made to do all sorts of interesting things. You can make photonic crystals that act like diodes: Light can pass through them in one direction but not in the other. True one-way mirrors. You can make converters: if a photon of forbidden energy attempts to pass through the crystal, it's converted to a photon of a different energy.
Applications in sensor design are immediately apparent. Near-perfect anti-reflection coatings are one possibility. Another is much better color filters for the sensor. For example, digital cameras don't do well with blue light for two reasons only one of which is that today's blue filters aren't anywhere near perfect. The other is that silicon is relatively insensitive to blue light. Consequently, digital cameras don't have anywhere near perfect efficiency in this part of the spectrum.
One can imagine a photonic crystal blue filter. It blocks red and green light entirely. It doesn't allow blue light to passthrough, either, but it does convert blue photons to, oh say, infrared photons. Silicon is very responsive to infrared photons; the same sensor would have much more sensitivity with this filter than with a conventional one. A filter might even convert a blue photon into two infrared photons, thereby doubling the signal in the sensor and improving the signal-to-noise ratio in the final output.
This is relatively normal stuff compared to metamaterials. They just plain break the rules, at least as we thought we understood them. When modern metamaterials were proposed about 10 years ago, the mathematics of them allowed for such outrageous possibilities that many physicists fairly felt that they would turn out to be a theoretical concept that could never be realized in actuality. Well, that turned out to not be the case. We can make metamaterials, and they have proven themselves to be extremely weird.
The most important weird property for photographers is that metamaterials can have refractive indices of less than one. Nothing "normal" does that; a pure vacuum has a refractive index of one and all ordinary materials and gasses have refractive indices higher than one. Not only can metamaterials get below one, they can even go negative. The holy Grail has been a refractive index of less than –1 at optical wavelengths, broadband, and we're closing in on that.
What do such negative indices allow that you didn't have before? Superlenses and hyperlenses: optics that perform better than what conventional optical theory allows in a perfect lens. You think the Raleigh diffraction limit is the ultimate resolving limit for a perfect lens? Not for a superlens; it can resolve much better than that theoretical boundary. Do you believe that you can't build a lens that can resolve details smaller than a half wavelength of light? Hyperlenses blow right past that.
These technologies are working their way out of the laboratory into scientific (translated: expensive!) instruments right now. They will migrate down to the photographic equipment that we use. Unless, of course, something even better comes along in the meantime.
Timeframe? My guess is during the next decade; I'd be surprised to see much appearance of these materials in our toys before 2020, but I'd be utterly amazed if it was after 2030.
CteinPhysicist/photographer Ctein's regular column on TOP is published on Wednesdays.
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Original contents copyright 2011 by Michael C. Johnston and/or the bylined author. All Rights Reserved.