Fabry-Pérot nanocavities controlled by Casimir forces in electrolyte solutions

We propose a design for tuning the resonant spectra of Fabry-Pérot nanocavities mediated by the Casimir force. The system involves a suspended gold nanoplate approaching a dielectric-coated gold substrate in a univalent electrolyte solution. The gold nanoplate can be stably suspended due to the delicate balance between repulsive and attractive components of the Casimir forces. In an electrolyte solution, the presence of ionic-charge fluctuations can partially or totally screen the thermal $n=0$ Matsubara term, resulting in strongly modified interactions. As a result, the separation between the gold nanoplate and the substrate experiences a significant modulation in response to variations in salt concentration. Under proper conditions, we find that the modulation of the Casimir force would strongly shift the resonances of Fabry-Pérot nanocavities at the optical frequencies, when the Debye length of the electrolyte decreases from 1000 nm to 10 nm. Finally, the temperature dependence of the thermal Casimir force would provide an additional modulation of Fabry-Pérot nanocavity resonances for their eventual fine tuning. These results open up a promising venue for general tuning of the optical resonances with potential applications in reconfigurable microfluidic nanophotonics.