Temperature-controlled photonic nanojet via VO coating.

In this work, a numerical investigation of how temperature can tune the FWHM and working distance (WD) of a photonic nanojet (PNJ) is conducted. Vanadium oxide (VO), a phase change material, is coated onto the top half-surface of a glass microsphere and illuminated with incident light at a wavelength of 800 nm. As VO changes from semiconducting to metallic phase, the refractive index of the VO layer changes at its transition temperature of 68°C. It is found that a coating of 75 nm on a 5.0 μm diameter microsphere with a refractive index of 1.50 is the most optimal, as it tunes the FWHM the greatest while remaining thin enough to have a high transmission. When temperature is raised from 20°C to 90°C, the FWHM varies from 0.43 to 0.37 μm, corresponding to a 14.0% change. The WD varies from 0.29 to 0.20 μm, corresponding to a 31.0% change. Tunable PNJs have potential applications in tunable nanolithography and imaging.