Evolution and the Retina - an Experiment
February 6th, 2006 | No Comments »I believe that it is possible to recreate in a silicon nanostructure the evolutionary processes that led to the human retina. I propose a singular experiment that I believe will start along the road to substantiating this claim.
We should begin by viewing the eye as an optical device that is based on the fundamental principles of physics and not from the currently fashionable view of biology or genetics. In essence, the physical laws of the refraction of light came first followed by the direct biological implementation (using the principles of molecular self-organization) of tuned antenna structures at the proper locations defined by this light refraction to appropriately detect radiation and ultimately form an image. This is the fundamental nature of the eye from which all else follows.
The retina as shown in this work is a diffractive surface located at the Fourier (or focal) plane of the eye structured to detect light exactly as dictated by light refraction within the eye. Nothing more!…individual light detection centers do not detect the many hues of “color” but are precisely tuned to three primary (RGB) wavelengths that form the short and long wavelength endpoints and precise middle of the visible band and from which color is subsequently synthesized.
I will not go into detail here recounting the number of recent physical discoveries that are important in substantiating the above assertion. Among these are our emerging knowledge of the nanostructural world, the recent finding that light travels outside of lightguides when the dimensions of the guide are of the order of the wavelength of light, increasing recognition as we enter the nanometer spatial domain of the reality of optical wavelength antennas, …..and more.
Crucial to the possiblity of fabricating an analogue of the retina in silicon was the surprising discovery in the solid state field in the 1990’s of a new visible light interactive silicon surface nanostructure. This discovery has been termed “porous silicon” (PSi) and this structure strangely resembles, as to high aspect ratio, the receptors of the human retina. I have experimentally demonstrated that the wavelength of light emitted by porous silicon is correllated with the spatial nanostructure employing the same principles that are proposed in this work as being operative in the human eye.
Now, another physics principle that was crucial in the evolution of the eye is the finding that a light frequency (or wavelength) incident onto a surface results in a lateral (i.e., orthogonal) spatial reflection of that frequency or wavelength onto that surface…the formation of a “grating” whose period is equal to the incident wavelength. I have mentoned this in the section “A Brief History” at the top of this webpage. The references to this work from that section are included below.
In our experimental work etching porous silicon surfaces we demonstrated that this effect is operative, i.e., that impressing a specific light wavelength onto the silicon surface during the porous silicon formation process resulted in light emisson at that wavelength (it is my recollection that this has subsequently been verified by other groups). Our work used narrow light wavelengths (specifically,green)….but….what if the broader solar spectrum were made incident onto…..or focused onto by a converging lens….the silicon surface during the PSi etching process? The silicon surface would be located at the focal (or Fourier) plane of the lens.
It is my belief that the result would be light emission exhibiting a lateral spatial reflection of the solar spectrum……i.e., a “rainbow”…..and that this is the basic physics principle involved in the evoluton of the retina of the eye.
I propose that an experiment be tried incidenting an (attenuated) solar spectrum focused using a converging lens onto a silicon surface during the PSi electro-etching process.
This would be the first step in replicating the human retina in silicon.
References:
1 ) K. Dworschak, J.E. Sipe, H.M. van Driel, “Transitions Between Ordered and Disordered Solid-Melt Patterns Formed on Silicon by Continuous Laser Beams: Competition Between Electrodynamics and Thermodynamics”, Ref. unknown.
2 ) Jeff F. Young, J.E. Sipe, and H. M. van Driel, “Laser Induced Periodic Surface Structure. III Fluence Regimes, the role of feedback and details of the induced topography in germanium”, Physical Review B, Vol. 30, No.4, 15 August 1984
3 ) Siegrist,M., Kaech,G., Kneubuhl, F.F., “Formation of Periodic Wave Structures on the Dry Surfaces of a Solid by TEA-CO2 Laser Pulses”, App. Phys, Vol.2, 45, 1973
GCH
