Molybdenum trioxide thin films for infrared Photonics

Alpha-phase molybdenum trioxide (-MoO3) has attracted in-creasing attention in recent years due to its exciting physical properties which offer new opportunities for realizing Photonic devices with enhanced or novel functionalities. In fact, besides its strong anisotropy related to optical phonons, which allows polarization control and rotation, MoO3 supports strong field lo-calization by the excitation of surface waves called surface phonon polaritons (SPhPs), achieved through the coupling of the electromagnetic field with lattice vibrations. In this work we report on fabrication and structural, morphological, and optical characterization of polycrystalline -MoO3 thin films synthesized by pulsed laser deposition at 400 °C. Raman spectroscopy measurements re-vealed the polycrystalline single α-phase nature of the deposited MoO3 films, while scanning electron microscope analysis showed a random grain distribu-tion, which prevents optical anisotropy at normal incidence. Moreover, optical measurements in the mid-infrared showed an enhanced polarization-tuneable reflection peak at 1005 cm−1 both in amplitude and frequency, and a polariza-tion-independent perfect absorption behaviour at 979 cm−1, when 15° angular incidence is used. These interesting spectral properties, not least having pre-served the typical strong dispersion related to the phononic response of α-MoO3 flakes, indicate that pulsed laser deposition is a valuable technique to obtain large area and good quality α-MoO3 thin films for infrared Photonic applica-tions, including mid-infrared metamaterial devices for sensing applications.