Controlling the electromagnetic properties of materials, going beyond the limit that is attainable with naturally existing substances, has become a reality with the advent of metamaterials. The range of various structured artificial 'atoms' has promised a vast variety of otherwise unexpected physical phenomena, among which the experimental realization of a negative refractive index has been one of the main foci thus far. Expanding the refractive index into a high positive regime will complete the spectrum of achievable refractive index and provide more design flexibility for transformation optics. Naturally existing transparent materials possess small positive indices of refraction, except for a few semiconductors and insulators, such as lead sulphide or strontium titanate, that exhibit a rather high peak refractive index at mid- and far-infrared frequencies. Previous approaches using metamaterials were not successful in realizing broadband high refractive indices. A broadband high-refractive-index metamaterial structure was theoretically investigated only recently, but the proposed structure does not lend itself to easy implementation. Here we demonstrate that a broadband, extremely high index of refraction can be realized from large-area, free-standing, flexible terahertz metamaterials composed of strongly coupled unit cells. By drastically increasing the effective permittivity through strong capacitive coupling and decreasing the diamagnetic response with a thin metallic structure in the unit cell, a peak refractive index of 38.6 along with a low-frequency quasi-static value of over 20 were experimentally realized for a single-layer terahertz metamaterial, while maintaining low losses. As a natural extension of these single-layer metamaterials, we fabricated quasi-three-dimensional high-refractive-index metamaterials, and obtained a maximum bulk refractive index of 33.2 along with a value of around 8 at the quasi-static limit.

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An abrupt first-order metal-insulator transition (MIT) as a current jump has been observed by applying a dc electric field to Mott insulator VO2-based two-terminal devices. The size of the jumps was measured to be asymmetrical depending on the direction of the applied voltage due to heating effects. The structure of VO2 is investigated by micro-Raman scattering experiments. An analysis of the Raman-active Ag modes at 195 and 222cm−1, explained by pairing and tilting of V cations, and 622cm−1, shows that the modes below a low compliance (restricted) current do not change when the MIT occurs, whereas a structural phase transition above the low compliance current is found to occur secondarily, due to heating effects in the device induced by the MIT. The MIT has applications in the development of high-speed and high-gain switching devices.

In femtosecond pump-probe measurements, the appearance of coherent phonon oscillations at 4.5 and 6.0 THz indicating the rutile metal phase of VO2 does not occur simultaneously with the first-order metal-insulator transition (MIT) near 68 degrees C. The monoclinic and correlated metal (MCM) phase between the MIT and the structural phase transition (SPT) is generated by a photoassisted hole excitation, which is evidence of the Mott transition. The SPT between the MCM phase and the rutile metal phase occurs due to subsequent Joule heating. The MCM phase can be regarded as an intermediate nonequilibrium state.

An abrupt metal-insulator transition (MIT) was observed in VO2 thin films during the application of a switching voltage pulse to two-terminal devices. Any switching pulse over a threshold voltage for the MIT of 7.1 V enabled the device material to transform efficiently from an insulator to a metal. The characteristics of the transformation were analyzed by considering both the delay time and rise time of the measured current response. The extrapolated switching time of the MIT decreased down to 9 ns as the external load resistance decreased to zero. Observation of the intrinsic switching time of the MIT in the correlated oxide films is impossible because of the inhomogeneity of the material; both the metallic state and an insulating state co-exist in the measurement volume. This indicates that the intrinsic switching time is in the order of less than a nanosecond. The high switching speed might arise from a strong correlation effect (Coulomb repulsion) between the electrons in the material.Vanadium dioxide, VO 2 , possesses a first-order metalinsulator transition (MIT) making it an attractive material for switching devices.1-3 The MIT occurs near 68 • C and is accompanied by a structural phase transition. Various transition properties have been studied such as crystal structure and other physical quantities. In particular, aspects of the transition have been examined during thermal and optical inducements.3-7 It is also known that a negative differential resistance (NDR) is observed when the current-voltage characteristic of this material is controlled by a static current. 8,9 Such an experiment allows the measurement of the MIT with respect to temperature. The current-controlled NDR has been widely investigated for the various compounds of vanadium oxide. 8-10The resistance of the systems abruptly changes at the transition point in contrast to the NDR properties of a conventional semiconductor system. The MIT behavior controlled by a static current is extremely stable and reversible.9,10 Up until now, controversy over the mechanism of this transition has existed. It is not known whether the transition is due to thermal or electronic effects.Recent research favors the electronic model. Observations of the sample stability and the current injected to initiate the MIT support this view.9,10 We have reported a stable MIT induced by a constant applied electric field in highly oriented VO 2 films and revealed the mechanism of the MIT to be based upon electron-electron correlation using a Raman study of the planar devices.11 The transition speed of the MIT in VO 2 films has been reported to be below a picosecond through the use of ultrafast optical techniques.12 If the field-induced MIT occurs quickly enough to apply to high speed devices and shows a reproducible behavior, there are various applications for VO 2 in switching devices such as electrical switches, modulators, and electro-optical devices. Therefore, it is very important to observe the time dependence of the fieldinduced MIT and also to study the transient...

Highly oriented VO 2 thin films were grown on sapphire substrates by the sol-gel method that includes a low pressure annealing in an oxygen atmosphere. This reduction process effectively promotes the formation of the VO 2 phase over a relatively wide range of pressures below 100 mTorr and temperatures above 400 • C. X-ray diffraction analysis showed that as-deposited films

As the demand for flexible, rollable, and foldable displays grows, various state-of-the-art component technologies, including thin-film transistors (TFTs), electrodes, thin-film encapsulations (TFEs), and touch screen panels, have been developed based on organic light-emitting diodes (OLEDs) with flexible organic layers. Developing highly reliable flexible OLEDs is essential to realize flexible displays, but the flexible encapsulation technology still has technical difficulties and issues to be addressed. This review covers the recent developments in encapsulation technologies, particularly their material and structural designs, for highly reliable, flexible OLEDs. The solution concepts for the existing technical hurdles in flexible encapsulations are addressed. Among the various advanced flexible encapsulation technologies developed so far, neutral-axis engineering with a thin metal layer and a crack arrester is introduced.

VO 2 thin films were successfully grown on sapphire and SiO 2 /Si substrates by the sol-gel process. The VO 2 phase was well formed during simplified low pressure annealing in oxygen. The films prepared on sapphire directly crystallized to the VO 2 phase without passing through intermediate phases with increasing the annealing temperature, resulting in highly [010]oriented films on Al 2 O 3 (10 " 1 10) substrate. In contrast, the polycrystal films grown on SiO 2 /Si reached the final VO 2 phase with passing through several phases. Mixed phases existed at the interface between the film and substrate. For the films on sapphire, the phases with low-valent vanadium appeared drastically at the interface region along the depth, whereas the phase of the polycrystal film slowly changed into a low valence state at the initial stage of the interface and then returned to a high value. And both films showed an abrupt change in resistivity at the different transition temperature. The property of the interface may affect the crystallization and the metal-insulator transition of the films.