Nanopore-Based Optofluidic Devices for Single Molecule Sensing - Nanofluidics: Nanoscience and Nanotechnology - page 147

7.3 DESIGN RULES USING REAL METALS

7.3.1 Material Selection

Use of a metal layer allows suppression of light propagation through a solid-state
membrane and acts as the cladding of the waveguide. As described in Figure 7.2 a thin
metal film can be deposited prior to pore formation. Dielectric materials such as silicon
dioxide and silicon nitride are commonly used as free-standing membranes but are
optically transparent. Accordingly, it is essential that the metallic layer is thick enough to
efficiently block incident light whilst not appreciably extending the nanopore length. In the
waveguide theory described previously the metal cladding is considered to be a perfect
conductor and the thickness of the layer has no effect on the screening efficiency. In
contrast, real metals have a complex dielectric constant which can be expressed as

~

) 2

ε = ε ' + i ε " = n + ik (7.10)

r

where n is the refractive index and k is the extinction coefficient of the material. The
dielectric constant depends on the incident wavelength as do n and k . The variation of n
and k as a function of wavelength is shown in Figure 7.6. Figure 7.6(b) shows that metals
possess large extinction coefficients and are able to efficiently attenuate visible light.

(a)

(b)

Figure 7.6 (a) Variation of refractive index as a function of wavelength and (b) variation of extinction

coefficient as a function of wavelength for a selection of metals.

When detecting translocation events the signal-to-noise ratio can be maximized by
ensuring that all background fluorescence from the bulk sample is removed. Accordingly,
the metallic layer is crucial in efficiently screening incident light at the wavelength of
interest. Two figures of merit are useful when selecting an appropriate metal for such a
purpose. First, the reflectivity which is defined as the ratio of the intensity of the radiation
reflected to the intensity of the incident radiation. Second, the transmission which is
defined as the fraction of incident radiation that passes through the membrane. The
reflectivity R at normal incidence for any absorbing media can be expressed as

2

2

( n n ) + k

R =

1 2 2

(7.11)

2

2

( n + n ) + k

1 2 2