Thin films can comprise nanometre-thick layers that are much thinner than the wavelength of light. It was long assumed that the optical properties do not depend on the composition and ordering of these individual layers. This is because the light wave does not ‘feel’ the layers one by one, but rather interacts with the film as a whole – the ‘effective-medium’ approach.

Break down of the ‘effective medium’

Nevertheless, research shows that it is possible to design a multilayer structure where the refractive indices of the layers and surrounding media are carefully matched, so that the effective-medium approach breaks down. Here, incident light in a narrow range of angles, close to the angle of total internal reflection, is reflected in a different way. The effect depends on layer ordering or nanometre-scale variations in layer thickness, and provides a means to probe these minute compositional details remotely using an optical beam.

High sensitivity

Theoretical calculations show that although this effective medium breakdown is unexpected it exists for both semi-infinite and finite multilayer structures. The recipe for a multilayer structure that maximises this effect is also identified. The breakdown effect is very sensitive to parameters like the incident angle and underlying refractive index, which opens up many routes for sensing and monitoring applications e.g. in microfluidic channels.

Research is underway to test the discovered effect experimentally using nanometre-thick dielectric multilayers fabricated by atomic layer deposition.

More information can be found in the journal Nanotechnology 26 184001. The article forms part of the Nanotechnology Focus Collection on Nanophotonics to celebrate the International Year Of Light 2015.

Further reading

Near-field transducers for next-generation hard-drive technology (Mar 2015)
Heat-assisted magnetic recording doubles hard disk drive capacity (June 2014)
Controlling emission from one dimensional photonic crystals for improved detection (Feb 2014)