The technique of cathodic ac dual magnetron sputtering along with the high energy ionic bombardment is known to yield high-quality thin films in terms of their uniformity and high density. This technique has been applied for the first time to achieve thermochromic VO2 thin films that show a high optical and electrical contrast between normal and switched states. In this two-step process Vanadium metal films were deposited and subsequently oxidized in optimum conditions to achieve stoichiometric VO2 films. Typical films switched between more than 40 % to less than 5 % transmission in the infrared region while undergoing an electrical sheet resistance change between 1 × 10(5) and 1 × 10(2) Ω/cm(2). The application potential of such VO2 films in integrated optics is deemed high.

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Sputtered vanadium-rich films were subjected to rapid thermal annealing-cooling (RTAC) in air to produce vanadium dioxide (VO2) thin films with thermochromic switching behavior. High heating and cooling rates in the thermal oxidation process provided an increased ability to control the film's microstructure. X-ray diffraction patterns of the films revealed less intense VO2 peaks compared to traditional polycrystalline samples fabricated with a standard (slower) cooling time. Such films also exhibit a high optical switching reflectance contrast, unlike the traditional polycrystalline VO2 thin films, which show a more pronounced transmittance switching. The authors find that the RTAC process stabilizes the VO2 (M2) metastable phase, enabling a rutile-semiconductor phase transition (R-M2), followed by a semiconductor–semiconductor phase transition (M2-M1).

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We report laser beam focusing by a flat, homogeneous film with a thickness of less than 100 nm. The effect relies on refractive index changes occurring in vanadium dioxide as it undergoes a phase transition from insulator to metal. Phase front curvature is achieved by means of temperature gradients, and adjustable focal lengths from infinity to 30 cm are attained. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4903887] Thin flat lenses have found applications in areas such as integrated optics and satellite optical instruments, where lightweight and small-volume optical components are desired, as well as x-ray optics, where standard lens materials are ineffective. While the traditional approach to creating thin lenses is based on the Fresnel zone plate design, there has been a resurgence of activity exploiting effects of nanoplasmonics and metamaterials. For example, a quasiperiodic arrangement of sub-wavelength holes on a thin gold film was found to focus light to sub-wavelength spatial resolution. 1-3 Similarly, light focusing by ultrathin lenses was demonstrated by exploiting the properties of nano-antennas etched on a surface, 4,5 thus forming a type of optically functionalized metasurface. 6,7 In this paper, we report yet an alternative method for focusing light with a thin layer of material. There are three key differences that distinguish this work from previous studies:(1) it exploits a different mechanism, namely, the large refractive index change occurring during a phase transition from insulator-to-metal, (2) the focusing capability is dynamic, and can be adjusted in real time, and (3) the material used is uniform and does not require prior physical patterning. In the latter aspect, it is analogous to gradient-index (GRIN) and other such thin lenses, [8][9][10][11][12] with the exception that its index of refraction is not spatially profiled beforehand.When light interacts with a layer of vanadium dioxide (VO 2 ), it acquires a phase shift which depends on the state of the material. In the temperature range where VO 2 undergoes a transition from insulator to metal (roughly 50-80 C), the optical phase shift is approximately linear with temperature, as was shown in a previous study. 13 Figure 1 illustrates the concept. Figure 2 presents ellipsometry measurements showing the change in refractive index as VO 2 undergoes a phase transition. Given the complex refractive indices, Fresnel reflection coefficients at the air/VO 2 and VO 2 /substrate interfaces introduce phase shifts of 0.51 rad and 0.75 rad, respectively, during the phase transition of the material. Meanwhile, the round-trip propagation phase in the layer varies by À1.0 rad. From this we conclude that both propagation and reflection phases are responsible for the observed phase shift.From Figure 1(b), we see that between a temperature T A (the onset of the phase transition) and a temperature T B (the end of the phase transition), the optical phase shift from VO 2 changes by some amount D/. As a result, by mean of a temperature gradien...