Diffractive optics: new tricks from an old subject
Oct 11, 2006
3:30PM to 4:30PM
Date(s) - 11/10/2006
3:30 pm - 4:30 pm
Title: Diffractive optics: new tricks from an old subject
Speaker: Dr. David Tanner
Institute: University of Florida
Location: ABB 102
Electromagnetic diffraction has been known and understood for more than 200 years. Despite this, new phenomena related to diffraction have been observed even in the past 5 years. Three examples will be discussed. First is the enhanced transmission by subwavelength hole arrays in metal films. Until recently, most people believed that light would not be transmitted by such a perforated a metal film. But, in fact, the transmission is observed to be very high at certain wavelengths, more even than the geometric optics prediction: the open area fraction of holes. (This is itself much above a more realistic estimate based on the diffracted intensity through the holes.) The spectra can be explained by interference of diffractive and resonant scattering. The resonant contribution comes from electromagnetic modes trapped in the film vicinity. Surface plasmons (an alternative explanation) play a minimal role in the enhanced transmission.
Second, a brief look will be taken at the transmission of a single hole in a metal film. The hole is much smaller than the optical wavelength, and surrounded by a corrugated (bullseye) pattern on the film surface. The transmission of such an object has been observed to be collimated in the forward direction, again much more than the computed diffracted transmission of the hole. Our experiments show that the corrugation is the source of the transmitted intensity, and that the hole can be detrimental to the effect.
Finally, the use of arrays of holes as a “photon sieve” to make images will be described. The sieve, like the Fresnel lens, has its largest holes near the center, smallest ones near the edge, and can be used to produce interesting photographs.