Search

Optical effects are measured during the phase transition of vanadium dioxide (VO 2 ) exposed to friction, thereby opening new possibilities for friction measurements and detection. Exposure to periodic cycles of kinetic friction yield measurable, repeatable and reversible changes in optical properties of the films. Changes in reflectance are detected with thermal flux intensities as low as 1 mW/cm 2 . Characteristic response and recovery times are studied and modelled, as well as the effect of operating temperature, friction speed, pressure, friction coefficient and the thermal properties of the substrate.

Vanadium dioxide (VO2) undergoing phase transition is known alters the polarization state of light in reflection owing to large changes in complex refractive indices. While this effect is promising for optical modulation applications, the usual VO2 films on dielectric substrates tend to offer limited tunability for polarization modulation. In this paper, we show that metallic under-layers greatly enhance the performance by widening the spectral range and include visible wavelengths, by increasing the polarization modulation amplitude, and by widening the range of workable incidence angles. The imaginary part of the refractive index in the metallic layer is found to increase the relative phase shifts between s- and p-components of polarization as well as increasing the reflectance.

Photonic crystals are periodically arranged nanoparticles of various materials that exhibit interesting light propagation properties. The incoming light confronts a periodic variation of high and low refractive index and this contrast can lead to a photonic bandgap (PBG) at certain wavelengths. Chromogenics is the area of study of materials and thin films whose optical properties can be changed reversibly under the application of various external forces. Some transition metal oxides (TMO) exhibit very efficient chromogenic properties. In this chapter a detailed discussion of the TMO thin films–based chromogenic photonic crystals is presented. The fundamental properties underlying the chromogenic (photochromic, thermochromic, and electrochromic) phenomenon occurring in the well-known TMO thin films such as tungsten oxide (WO3), vanadium oxides (VO2 and V2O5), and molybdenum oxide (MoO3) are discussed. The possibility to interactively control the PBG is very interesting from both physics and application point of view.

Wavelength and polarization-dependent phase shifts on light interacting with a phase change material can lead to coloration shifts. By exploiting polarization selective effects, the range of attainable colors during switching of a phase-change layer is widened. Theoretical and experimental results are shown for the case of VO2, a reversible phase change material that can be excited thermally, electrically or optically.

We report on the possible use of vanadium dioxide to produce ultrathin (

It is generally assumed that changes in optical properties of vanadium dioxide, such as transmittance and reflectance, during phase transition originate from modified refractive indices and interference effects. In this study, we show that up to 50% of losses during phase transition is in fact created by enhanced optical scattering. From simultaneous measurements of diffuse and specular transmission and reflection, a detailed account of scattering losses is obtained. Since optical scattering only increases in the temperature range corresponding to the phase transition, it suggests the growth of metallic islands from nucleation sites may be the underlining scattering mechanism. Optical scattering offers an alternative method to study this important material and could have consequences for some applications such as optical switches and waveguides.

Thin layers of phase change materials are investigated theoretically and experimentally as way to modulate the properties of light interacting with silicon at a planar interface. By inducing a phase transition in vanadium dioxide films, the resulting changes in refractive index modulate the injection of light into silicon and the ejection out of it. Internal reflection of light propagating inside silicon causes the polarization state to be altered during the phase transition. Possible applications to silicon photonics are discussed.

The polarization of light interacting with a layer of vanadium dioxide (VO2) undergoing phase transition is known to change due to large changes in the material’s refractive index. While this effect is promising for optical modulation applications, the usual VO2 films on dielectric substrates tend to offer limited tenability in terms of wavelength and angle of incidence. In this paper, we show that metallic under-layers greatly enhance the performance by widening the spectral range to include visible wavelengths, by increasing the polarization modulation amplitude, and by widening the range of workable incidence angles. The imaginary part of the refractive index in the metallic layer is found to increase the relative phase shifts between s- and p-components of polarization as well as increasing the reflectance.

We investigate the theoretical limits of using diffraction of a single laser beam by a patterned surface as a way to measure surface deformation caused by stress or thermal expansion. Applying Gaussian beam optics to a lens-grating system, we identify the relevant parameters of the grating, the laser beam and the diffraction order and other conditions to the sensitivity limits. Theory suggests that sensitivity increases linearly with the diffraction order, regardless of the diffracted angle, and that relative deformation of the order of 10 −6 should be detectable with common optical components and laser beams. This corresponds to thermal expansion coefficients as low as 10 −8 °C −1 measured over 100 °C and stresses as low as 10 −6 . The proposed technique would be applicable to transparent or opaque samples and very small sample volumes.

Under certain conditions, films made of vanadium dioxide exhibit wavelengths at which transmittance or reflectance do not change as the material undergoes insulator to metal phase transition, in spite of refractive index changes on the order of unity. Exploiting this effect, we demonstrate control of optical phase at 800 nm in transmission and at 1310 nm in reflection. With a 68 nm film, the optical phase is adjusted while leaving all other properties of light unchanged, including amplitude, polarization and frequency. The phase change per unit of propagated distance is Δk=107 rad/m, orders of magnitude higher than typically obtained with electro-optic effects. We discuss potential application to nano-sized phase devices or thin film lenses.

We report the theory and experiment of how an ultrathin (

Vanadium dioxide, a material used in many filtering and imaging applications in the near-infrared, is shown to possess the capability of modifying the polarization of visible and infrared light.

The polarization state of light beams reflecting off thin layers of vanadium dioxide is measured as the material undergoes an insulator-metal phase transition by optical excitation. Under certain conditions, films as thin as 50 nm are shown to rotate linear polarization by 90° in reflection on the nanosecond and potentially shorter timescales. Possible applications for high-contrast optical modulation over a wide spectral range and high bandwidths are discussed.

We demonstrate laser beam focusing at 1300 nm with a flat lens with a thickness of less than 100 nm. The lensing exploits large refractive index changes occurring in vanadium dioxide (VO2) during a phase transition from insulator to metal. The focal length is 35 cm (with much shorter theoretical limits) and is adjustable in real time. Unlike traditional methods used to fabricate flat lenses, this one does not require surface patterning. A theoretical model of the effect will be presented, and the possibilities and limits will be discussed.

Large refractive index changes during phase transition in vanadium dioxide are exploited to control the phase of a light at 1300 nm over distances of less than 100 nm without altering other properties of light.

We demonstrate light polarization control over a broad spectral range by a uniform layer of vanadium dioxide as it undergoes a phase transition from insulator to metal. Changes in refractive indices create unequal phase shifts on s- and p-polarization components of incident light, and rotation of linear polarization shows intensity modulation by a factor of 103 when transmitted through polarizers. This makes possible polarization rotation devices as thin as 50 nm that would be activated thermally, optically or electrically.