Extraordinary transmission of light and other interesting optical phenomena have been observed when light is incident on arrays of apertures in metal films. These phenomena are the result of the interaction between incident photons and excited surface plasmons with wavenumbers that are constrained by the geometry and periodicity of the metal structure. Our interest is in the use of these structures to make novel chemical and biological sensors and incorporating them into lab on a chip type system.

One issue in developing subwavelength aperture arrays is the cost of fabrication. Focus Ion beam (FIB) Milling and electron beam lithography are expensive serial fabrication methods.  An alternative technique is to use the self assembly of nanospheres to form the necessary nanoscale pattern. In this study, the fabrication and optical properties of subwavelength aperture arrays fabricated by FIB Milling are compared with subwavelength aperture arrays that are produced using the directed assembly of microspheres.

The basic patterning sequence was to deposit the nanospheres in a close packed single crystal structure, with the periodicity of the structure determined by the diameter of the spheres. The diameter of the apertures could then be engineered by reactive ion etching the spheres. A gold layer was then evaporated on the spheres and exposed substrate. The final step consisted of removing the spheres using a solvent.

The transmission spectrum was then obtained for samples prepared by both FIB and self assembly methods, with surface plasmon effects observed in both types of samples. Varying the polarization of incident light allowed investigation of the anisotropy of the arrays. As a test device for chemical sensing using a subwavelength array was also placed in a microfluidic channel.

Figure 1. A subwavelength aperture array fabricated using the sefl assembly of nanospheres.