ABSTRACT
Perfect light absorption in the visible and near-infrared (NIR) was demonstrated using metamaterials, plasmonic nanostructures, and thin films. Thin film absorbers offer a simple and low-cost design as they can be produced on large areas and without lithography. Light is strongly absorbed in thin film metal-dielectric-metal (MDM) cavities at their resonance frequencies. However, a major drawback of MDM absorbers is their strong resonance iridescence, i.e., angle dependence. Here, we solve the iridescence problem by achieving angle-insensitive narrowband perfect and near-perfect light absorption. In particular, we show analytically that using a high-index dielectric in MDM cavities is sufficient to achieve angle-insensitive cavity resonance. We demonstrate experimentally angle-insensitive perfect and near-perfect absorbers in the NIR and visible regimes up to ±60°. By overcoming the iridescence problem, we open the door for practical applications of MDM absorbers at optical frequencies.
ABSTRACT
Inherent absorptive losses affect the performance of all plasmonic devices, limiting their fascinating applications in the visible range. Here, we report on the enhanced optical transparency obtained as a result of the broadband mitigation of optical losses in nanocomposite polymeric films, embedding core-shell quantum dots (CdSe@ZnS QDs) and gold nanoparticles (Au-NPs). Exciton-plasmon coupling enables non-radiative energy transfer processes from QDs to metal NPs, resulting in gain induced transparency of the hybrid flexible systems. Experimental evidences, such as fluorescence quenching and modifications of fluorescence lifetimes confirm the presence of this strong coupling between plexcitonic elements. Measures performed by means of an ultra-fast broadband pump-probe setup demonstrate loss compensation of gold NPs dispersed in plastic network in presence of gain. Furthermore, we compare two films containing different concentrations of gold NPs and same amount of QDs, to investigate the role of acceptor concentration (Au-NPs) in order to promote an effective and efficient energy transfer mechanism. Gain induced transparency in bulk systems represents a promising path towards the realization of loss compensated plasmonic devices.
ABSTRACT
Hyperbolic Metamaterials are artificially engineered materials whose optical properties can be specifically tailored to manifest an extremely high level of anisotropy. Due to this remarkable anisotropy they represent a unique opportunity to realize effective bulk meta-structure with extraordinary optical properties in the visible range. A simultaneous dielectric singularity in the in plane permittivity, with respect to the propagation direction, has to lead to a complete sign inversion of the same permittivity for that specific visible frequency. Such a drastic phase change has been theoretically highlighted in the past as the major challenge to be overcome in order to unlock many remarkable optical properties not present artificial optical systems. In this paper we experimentally demonstrate the realization of a metal-dielectric multilayer structure showing an inversion point of coexisting anisotropies at a specified wavelength in the visible range, rising from the particular design and fabrication process. Theoretical models and numerical simulations are in very good agreement with experimental data. Ellipsometrical experiments and optical modeling demonstrate the drastic type I/type II transition. Supercollimation effect has been achieved at the inversion point of the coexisting extreme anisotropies, whereas at the epsilon near zero and pole frequency the perfect lens behavior has been observed.
ABSTRACT
Nonlinear straining and random sweeping spatiotemporal decorrelation properties, originally introduced as the main processes for turbulent fluctuations decorrelation in usual fluid flows, have been observed experimentally in anisotropic electroconvective turbulence generated in a nematic liquid crystal under the action of an external oscillating electric field. A transition between both processes occurs when the instability is driven toward states of increasing complexity, thus showing that decorrelation mechanisms in turbulent media are more universal than naively expected. A model for both decorrelation mechanisms is introduced, its comparison with experimental results providing an estimate of the characteristic sweeping velocity.
ABSTRACT
This experimental work is aimed to investigate the thermal behavior of random laser action in dye doped nematic liquid crystals. The study evidenced an important temperature dependence of the random lasing characteristics in the nematic phase and in close proximity of the nematicisotropic (N-I) phase transition. A lowering of the laser emission intensity as the temperature increases is strictly related to the shift of the lasing threshold as function of the temperature even though the pump energy is kept fixed. The optical losses increasing owing to the thermal fluctuation enhanced scattering drive the input-output smoother behavior until the system stops to lase, because below threshold. The unexpected reoccurrence of random lasing at higher temperature, in proximity of N-I transition is found to be related to a different scattering mechanism, the micro-droplets nucleation and critical opalescence.
ABSTRACT
The first observation of random laser action in a partially ordered, optically anisotropic nematic liquid crystal with long-range dielectric tensor fluctuations is reported. Above a given pump power the fluorescence curve collapses and the typical narrowing and explosion effect leads to discrete sharp peaks. The unexpected surviving of interference effects in recurrent multiple scattering provide the required optical feedback for lasing in nematics. Coherent backscattering of light waves in orientationally ordered nematic liquid crystals manifests a weak localization of light which strongly supports diffusive laser action in presence of gain medium. Intensity fluctuations of the speckle-like emission pattern indicate the typical spatio-temporal randomness of diffusive laser emission. A comparison of the laser action is reported for systems with different order degree: fully disordered semiconductor powders, self-ordered cholesterics and partially ordered nematic liquid crystals.
ABSTRACT
The influence of the bias-voltage on the anisotropic part of the nematic surface energy is analyzed. The experimental data show a strong dependence of the anchoring strength on the bias-voltage when the electrodes of the nematic cell are covered with WO3. The observed dependence can be interpreted taking into account the ions dissolved in the liquid crystal. We propose a model in which the effect of the bias-voltage is to collect the ions near the electrodes, in a surface layer whose thickness is of the order of the Debye's screening length. The surplus of electric field due to this ions confinement gives rise to an electrostatic contribution to the total energy that can be considered as a surface energy. The proposed model is in good agreement with the experimental data. The model is used to interpret the observed independence of the anchoring strength on the bias-voltage when the (indium-tin-oxide) electrode is covered with a film of polyimide, or it is without any covering. The influence of a charge emission from the electrodes under the bias voltage on the anchoring energy is also analyzed. Possible applications of the observed phenomenon are discussed.
ABSTRACT
A liquid crystal (LC) cell has been made by using a standard sandwich configuration with one of the indium tin oxide (ITO) electrodes covered by a thin layer of tungsten trioxide (WO3). In this kind of cell optical polarization switching (observed under a crossed polarizer microscope) occurs for only one of the two polarities of an ac external applied electric field, while in the usual liquid crystal cells the electro-optic response does not depend on the sign of the field. The inhibiting switching configuration corresponds to anodic polarization of the tungsten trioxide film in which the deintercalation of cations occurs. Here we present the time behavior of charge and discharge for both the anodic and cathodic currents. A model based on charge carrier exchange between the ITO-WO3 and WO3-LC interfaces and also electrochemical processes is reported. Our model is also capable of explaining the electric and electro-optic asymmetric responses of the cell. Numerical calculations confirm the model.
ABSTRACT
An interesting application of the fast ion transport properties of tungsten trioxide is presented, when it is inserted as an electrode in nematic liquid crystal (NLC) cells. In a standard sandwichlike cell the nematic liquid crystal, confined between two transparent plane electrodes of purely electronic conductors [indium tin oxide (ITO)], undergoes a molecular reorientation under the action of an external electric field E. This electrically controlled birefringence (electro-optical switching) is proportional to E2, thus polarity insensitive [L. M. Blinov and V. G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials (Springer-Verlag, New York, 1994)]. When a thin film of tungsten trioxide is deposited by magnetron sputtering onto one of the transparent ITO electrodes, and a NLC cell is assembled with such asymmetry, the electro-optical response becomes polarity sensitive [G. Strangi et al., Appl. Phys. Lett. 74, 534 (1999)]. The analysis of this response suggests the occurrence of a reverse internal electric field, associated with the ionic diffusion process of protons always present in these sputtered WO3 films [E. Cazzanelli et al., Electrochim. Acta 44, 3101 (1999)]. By using an opportune voltage waveform it is possible to evaluate such an internal field. Impedance and cyclic voltammetry measurements were carried out on these cells, comparing "as-deposited" and "annealed" tungsten trioxide electrodes. These studies confirm that an important ionic diffusion process is involved in the establishment of an internal electric field, which modifies the electro-optical response of the nematic liquid crystal cell.
ABSTRACT
This work was aimed at the photopolarimetric characterization of the transition between two dynamic scattering modes that take place in a planarly aligned nematic liquid crystal sample, under the effect of an external low-frequency electric field. The time evolution of the degree of polarization and the behavior of the radiation entropy of the transmitted light allow us to interpret the transition between two turbulent states, or dynamic scattering modes, as a decay from a two-dimensional (2D) to a (3D) turbulence.
ABSTRACT
In this paper, a study of the electrically driven turbulent-turbulent transition in a homeotropically oriented nematic sample is reported. The transition has the characteristics of a nucleation process, and its threshold has been experimentally determined. The nucleation rate and the growth velocity of the new turbulent nuclei are also reported.