Metamaterials have demonstrated their unusual optical properties, ranging from negative refractive index to optical nonlinearity. Real-time adjustment of these properties becomes critical for the next generation of ‘smart’ optical devices. In this thesis, different methods for actively tuning the structure and output of metamaterials are demonstrated. These metamaterials consist of metallic nanoparticles (NPs) at solid-liquid interfaces (SLIs) and liquid-liquid interfaces (LLIs).
Electrically, the assembly of gold NPs at a SLI is reversibly controlled by changing the potential of the solid substrate. As the interparticle distance is adjusted, the corresponding reflectance dip is modulated over the wavelength range 530–580 nm, with intensity changing from 93% to 1%. Moreover, the resulting ‘hot spots’ between adjacent NPs, and between the NP array and the metallic substrate, bring about a tunable surface-enhanced-Raman-scattering (SERS) enhancement of up to ~105 times.
Thermal tunablity was achieved by constructing a nanoplasmonic optical switch, with poly(N-isopropylacrylamide) functionalized NPs on a metallic substrate. Within a 20–50 °C temperature window, the interparticle spacing and the separation between the NP array and the substrate can be simultaneously tuned in nanoscale.
Chemical bonding between the ligands on NPs and the heavy metal ions is also found to be able to adjust the interparticle distance of the NP array at LLI. For a glutathione functionalized NP array at LLI, an increase in the concentration of lead ions caused the reflectance peak to surge in intensity and to shift towards the red in wavelength. The fast dynamics (<1 min) and the low detection limit (ppb level) render this system an ideal candidate for the label-free detection of heavy metals.
These proof-of principle methods demonstrated how electric, thermal and chemical effects could influence the nanoscale arrangement of NP assembly at SLI or LLI, which in return reforms the macroscale optical properties of these interfaces.