Vanadium dioxide-sapphire metasurfaces for actively tunable surface phonon polariton resonances at mid-infrared*

Vanadium dioxide (VO2), a reversible insulator-to-metal phase transition (IMT) material, provides an actively tunable platform for photonics devices because the IMT makes a drastic change in VO2 optical properties. VO2 film grows well on a sapphire substrate because of the small lattice mismatch, a good quality VO2 film on sapphire has used many photonics applications for electrically and thermally tunable plasmonic devices at near- and mid-infrared. Beyond plasmonics, recent studies show phonon polaritons are more efficient ingredients in mid-infrared photonics. VO2 film on sapphire will be an attractive platform to develop phonon polaritonic metasurfaces because sapphire supports phonon polaritons at its mid-infrared Reststrahlen band (10 - 20 microns). Here, we numerically simulate and experimentally demonstrate thermally tunable surface phonon polaritonic devices based on VO2 film on sapphire working at sapphire's Reststrahlen band.  The device consists of 40 nm thick gold grating (periodicity = 900 nm, gap = 100 nm) on 100 nm thick VO2 on sapphire. The cavity resonance is observed near 770 cm-1 and shows redshifts as temperature increases. We further studied different grating periods and gaps to observe the cavity resonance tuning and its dynamic redshift by heat.