Nanostructured Thin Films as Optical Waveguides and Sensors

Aaron K.H. Lau, Dong-Ha Kim, Wolfgang Knoll

Thin films with nanostructures are being actively pursued for a wide range of applications. For example, self-segregating, amphiphilic di-block copolymers (BCP) can self-assemble to form well-defined, periodic nanoscale morphologies. In the case of polystyrene-block-poly(methyl methacrylate), the cylindrical phase is formed at a volume ratio of 70:30 PS-b-PMMA (arrays of PMMA cylinders formed within a PS matrix.) Furthermore, a nanoporous structure may also be obtained by selective dissolution of PMMA by UV exposure followed by acetic acid rinsing. (Figure 1.) The resulting array of high aspect ratio cylindrical pores in a PS thin film may be used for template growth of nanotubes or nanorods, and the surface area increase due to the generation of internal porosity increases the number of sites available for surface specific processes.

Figure 1. AFM images and schematic of nanoporous thin film formation by selective dissolution of PMMA. The initial structure was a spin-coated film of 70:30 PS-b-PMMA copolymer formed after annealing at 180oC for 48h. PMMA homopolymer was added to promote formation of the cylindrical phase [1]. The centre-to-centre pore separation is 40 nm.

We have recently been exploring these composite thin films as optical waveguides. The individual nano-domains are sufficiently small to exhibit low scattering at visible or longer wavelengths. Moreover, the criteria for exciting waveguide modes are highly specific, and any thickness or composition changes to the film bulk and on either the internal or external surfaces, can be readily observed as shifts in waveguiding behaviour. Furthermore, these changes can be quantified by effective medium theory. Thus the nanostructured block copolymer thin film is at the same time a platform for surface processes and a sensor.

Optical Waveguide Spectroscopy (OWS)

In the Kretschmann configuration (figure 2) an incident laser beam is prism coupled into a thin film as waveguide modes if the film is sufficiently thick: typically > 300 nm.

(Anisotropic) dielectric constants and film thickness are independently determined from angle positions of the modes.

Figure 2.Kretschmann configuration as applied to block copolymer thin film waveguide

Monitoring Nanostructure Processing

As example, we followed the pore generation process described in figure 1 [2].

Figure 3. Reflectivity vs. angle scan of PS-b-PMMA thin film before (pink) and after (green) PMMA selective removal.