Photo-Induced typography with vanadium dioxide thin films
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The optical, structural, and electrical properties of Vanadium Dioxide (VO2) thin films can be dynamically modified by an external stimulus such as temperature, optical excitation or an electrical current or voltage. VO2 exhibits a first-order reversible metal-insulator phase transition (MIT) which is typically accompanied by almost five orders of magnitude change in electrical conductivity. The ability to trigger the phase transition via optical excitation allows for producing reconfigurable patterns (typography) on the surface of the VO2 films. In this work, rewritable patterns were generated at the optical communication wavelength using a combination of pump-probe technique, dual-scanning mirrors, and an infrared (IR) camera. The sample used in these experiments is composed of VO2 ~150 nm thick films deposited on both sides of c-plane-oriented sapphire substrates which was fabricated using the sputtering deposition technique The sample was illuminated by an infrared probe light source emitting at ~1560 nm wavelength. A high-power near-IR laser was incident on the scanning mirrors and then deflected towards the VO2 sample’s surface. The high power laser optically triggers the VO2 MIT and the scanned region became opaque to the IR irradiation. The patterns generated at the sample surface were imaged using an IR camera. Clear and high contrast images with different shapes and sizes were obtained with the proposed approach. The characteristics of the generated patterns were controlled by the vibration amplitude and frequency of the scanning mirrors. As a proof of concept we demonstrated optically-induced Lissajous-like figures with different shapes, intersecting lines, and amplitudes. We anticipate that the developed method can be prospectively used to realize reconfigurable Fresnel lenses, spatial light modulators, optical equalizers, reconfigurable metamaterials, spatial light modulators, variable attenuators, and tunable frequency selective filters, operating in the near-IR. In particular, VO2 is very attractive to realize devices operating at the telecommunication wavelengths.