Plasmonics in Metal Insulator Cavities

Subwavelength multilayer metal-insulator nanostructures with tuneable resonances have been widely used for various applications in optoelectronics and photonics, due to their unique dispersion relation of the dielectric permittivity. In this thesis, we firstly studied the optical properties and the resonance modes of the metal/insulator/metal (MIM) metamaterial system by spectroscopic ellipsometry and COMSOL Multiphysics calculations based on finite element methods. Our calculation results show that MIM systems with vertical or lateral gratings can both support the multiple cavity modes that form the epsilon-near-zero (ENZ) resonance with an effective dielectric constant close to zero. Their large local density states are beneficial to the Purcell effect enhancement of the spontaneous emission. Moreover, the low-energy multi-cavity modes can be adjusted in the visible range via tuning the insulator thickness. The difference is that the MIM system with lateral grating leads to uncoupled multiple ENZ resonance, while the vertical grating MIM structure owns strongly coupled modes which form ENZ bands. To demonstrate the usefulness of the emission enhancement of MIM structures in practical applications, multilayer metal-insulator nanostructures are adopted to improve the spontaneous emission of the emitter. Herein, we explored the effects of interface modifications on the overall performance in perovskite LEDs. Firstly, we designed and optimized the flat perovskite LED (PeLED) through systematic analysis of the power loss channels based on the optical mode. All the theoretical analysis is carried out through finite element simulations. Under the assumption of efficient photon generation in the emitting layer with an internal quantum yield of 0.9, the effect of the dipole orientation is analyzed and then thickness of the charge injection and emitter layer was optimized. Finally, we tuned the transparent electrode thickness to get the maximum value of the external quantum efficiency. Moreover, we further studied the influence of interface modifications happening at the electron-transport interface on the whole performance of perovskite-based flat PeLEDs. Particularly, we explored the integrating of photonic structure, while keeping the optical property of the emitting material. Interesting, our calculations reveal that the specially designed nanopatterning can promote to improve the Purcell factor and the outcoupling efficiency, thereby enhance the external quantum efficiency, related to the nanopattern-free PeLED configuration. In particular, an average enhancement around 100% for the external quantum efficiency was achieved, and thus improving the radiative emission of the PeLED devices. These findings indicated that using morphological patterning to enhance LED performance is realistic method, similar to other light emission technologies. Finally, a nanoscale optical pressure/temperature nano sensor based on gap-plasmonic nanostructure, composing of the MIM nanopillar arrays covered by a metallic film, is proposed. The gap plasmon frequency is highly sensitive to the distance of the pillars to the Ag film, which allows optical sensing of pressure/ambient temperature/ refractive-index by variation in the colour of the device

Design and modelling of anisotropic thin film light-emitting devices with the plane wave expansion method

In most emitting materials the dipole moments are distributed in an isotropic way, and light is emitted in all directions (modified by interference in the OLED layers). But the radiation of each individual dipole is directed around the equatorial plane of the dipole moment. By analyzing the photoluminescent decay times in different microcavities, a phosphorescent emitter with an anisotropic distribution of 80% in-plane dipoles is demonstrated. Simulations show that if all dipoles are arranged in-plane with the OLED layers the outcoupling efficiency to air would improve from 20% to 30%. Anisotropic emitters can be combined with known outcoupling techniques like microlens foils to provide even higher outcoupling efficiencies. With a fully oriented emitter nearly 70% of the light is reaches the OLED substrate where it is available for extraction by outcoupling foils. When organic laser dye molecules are dissolved into the CLC, a laser can be made. A CLC film supports a resonant standing wave for wavelengths at the edge of the bandgap. When the dye molecules are pumped above the lasing threshold by a pulsed shorter wavelength pump laser, longer wavelength laser light is emitted perpendicular to the CLC film. The thickness of the CLC film is around 10μm. Laser emission has been shown across the entire visible spectrum employing different dye molecules. The emission wavelength can also be tuned across ranges of about 50nm in different ways. However CLC lasers are still hindered by high lasing thresholds and low output power, as well as bleaching of the dye molecules. In this work the light emitting properties of CLC films are simulated with the anisotropic plane wave expansion method both for spontaneous and stimulated emission. The measured emitted spectrum at different angles and polarization are accurately modelled by the plane wave expansion. A model for estimating the gain threshold and laser wavelength of CLC films was developed and verified by experiment. Optical amplification is treated by introducing gain terms to the refractive index. This model may prove a valuable tool to design more advanced liquid crystal lasers

Advances in small lasers

M.T.H was supported by an Australian Research council Future Fellowship research grant for this work. M.C.G. is grateful to the Scottish Funding Council (via SUPA) for financial support.Small lasers have dimensions or modes sizes close to or smaller than the wavelength of emitted light. In recent years there has been significant progress towards reducing the size and improving the characteristics of these devices. This work has been led primarily by the innovative use of new materials and cavity designs. This Review summarizes some of the latest developments, particularly in metallic and plasmonic lasers, improvements in small dielectric lasers, and the emerging area of small bio-compatible or bio-derived lasers. We examine the different approaches employed to reduce size and how they result in significant differences in the final device, particularly between metal- and dielectric-cavity lasers. We also present potential applications for the various forms of small lasers, and indicate where further developments are required.PostprintPeer reviewe

Interactions between excitation and extraction modes in an organic-based plasmon-emitting diode

International audienceThis study demonstrates the feasibility of enhancing an organic-based plasmon-emitting diode on the directional light beaming efficiency by near-field surface plasmon polaritons (SPPs) in both metal grating and polymer grating nanostructures. The interaction between organic/metal and PR/metal interfaces to cause SPPs can facilitate specific directional emission. Directional emission properties give rise to a spectral band-gap response enhancement. Our results also verify that efficient surface plasmon grating coupled emissions (SPGCEs) can improve directionality under index-mediated tuning. Experimental results indicate SP decoupling emission in the visible light. The subsequent emission intensity can increase by up to 3.5 times. Moreover, a narrow FWHM of approximately 60 nm in a defined direction is achieved, and an SP coupling rate is approximately 80% on the metal grating structure. The proposed method is highly promising for use as an active plasmonic emitter and discoloration biosensors with enhanced SPPs resonance energy, owing to interactions with the organic/metal nanostructur

Distributed feedback lasers based on perylenediimide dyes for label-free refractive index sensing

The refractive index sensing capabilitity of distributed feedback (DFB) lasers based on highly photostable (under ambient conditions) perylenediimide-based active films, are reported. The sensor bulk sensitivity is determined from changes in the laser emission wavelength upon exposure to different liquids. The role of the active film thickness (hf) on the sensor sensitivity and on the laser parameters is studied. Sensors based on very thin films (hf = 160 nm) show the highest sensitivities, but their laser thresholds are relatively high and their operational durabilities moderate. The use of thicker films (hf = 850 nm) allows reducing the laser threshold and increasing the durability by two orders of magnitude. In this case, a higher sensitivity is achieved when the sensor operates at the wavelength corresponding to the first-order TE1 mode, instead of at the TE0 one. Finally, it is also shown that the inclusion of a high refractive index TiO2 layer on top of the sensor structure improves the sensitivity by around two times.This work was supported by the Spanish Government (MINECO) and the European Community (FEDER) through grant no. MAT-2011–28167-C02. This work was partially funded by the Basque Government within the framework of the Etortek Program (Grant No. IE13-360). M. Morales-Vidal has been partly supported by a MINECO FPI fellowship (no. BES-2009-020747)

Top-Emitting OLEDs: Improvement of the Light Extraction Efficiency and Optimization of Microcavity Effects for White Emission

In the last decades, investigations of organic light-emitting diodes (OLEDs) have tackled several key challenges of this lighting technology and have brought the electron to photon conversion efficiency close to unity. However, currently only 20% to 30% of the photons can typically be extracted from OLED structures, as total internal reflection traps the major amount of the generated light inside the devices. This work focuses on the optimization of the optical properties of top-emitting OLEDs, in which the emission is directed away from the substrate. In this case, opaque materials, e.g. a metal foil or a display backplane can be used as substrate as well. Even though top-emitting OLEDs are often preferred for applications such as displays, two main challenges remain: the application of light extraction structures and the deposition of highly transparent materials as top electrode, without harming the organic layers below. Both issues are addressed in this work. First, top-emitting OLEDs are deposited on top of periodically corrugated light outcoupling structures, in order to extract internally trapped light modes by Bragg scattering and to investigate the basic scattering mechanisms in these devices. It is shown for the first time that the electrical performance is maintained in corrugated top-emitting OLEDs deposited on top of light extraction structures. Furthermore, as no adverse effects to the internal quantum efficiency have been observed, the additional emission from previously trapped light modes directly increases the device efficiency. It has been proven that the spectral emission of corrugated OLEDs is determined by the interference of all light modes inside the air light-cone, including the observation of destructive interference and anti-crossing phenomena. The formation of a coherently coupled mode pair of the initial radiative cavity mode and a Bragg scattered mode has been first observed, when grating structures with an aspect ratio > 0.2 are applied. There, the radiative cavity mode partially vanishes. The observation and analysis of such new emission phenomena in corrugated top-emitting OLEDs has been essential in obtaining a detailed insight on fundamental scattering processes as well as for the optimization and control of the spectral emission by light extraction structures. Second, the adverse impact of using only moderately transparent silver electrodes in white top-emitting OLEDs has been compensated improving the metal film morphology, as the organic materials often prevent a replacement by state-of-the-art electrodes, like Indium-tin-oxide (ITO). A high surface energy Au wetting layer, also in combination with MoO3, deposited underneath the Ag leads to smooth, homogeneous, and closed films. This allows to decrease the silver thickness from the state-of-the-art 15 nm to 3 nm, which has the advantage of increasing the transmittance significantly while maintaining a high conductivity. Thereby, a transmittance comparable to the ITO benchmark has been reached in the wavelength regime of the emitters. White top-emitting OLEDs using the wetting layer electrodes outperform state-of-the art top-emitting devices with neat Ag top electrodes, by improving the angular colorstability, the color rendering, and the device efficiency, further reaching sightly improved characteristics compared to references with ITO bottom electrode. The enormous potential of wetting layer metal electrodes in improving the performance of OLEDs has been further validated in inverted top-emitting devices, which are preferred for display applications, as well as transparent OLEDs, in which the brittle ITO electrode is replaced by a wetting layer electrode. Combining both concepts, wetting layer electrodes and light extraction structures, allows for the optimization of the grating-OLED system. The impact of destructive mode interference has been reduced and thus the efficiency increased by a decrease of the top electrode thickness, which would have not been achieved without a wetting layer. The optimization of corrugated white top-emitting OLEDs with a top electrode of only 2 nm gold and 7 nm silver on top of a grating with depth of 150 nm and period of 0.8 µm have yielded a reliable device performance and increased efficiency by a factor of 1.85 compared to a planar reference (5.0% to 9.1% EQE at 1000 cd/m2). This enhancement is comparable to common light extraction structures, such as half-sphere lenses or microlens foils, which are typically restricted to bottom-emitting devices. Overall, the deposition of top-emitting OLEDs on top of light extraction structures finally allow for an efficient extraction of internally trapped light modes from these devices, while maintaining a high device yield. Finally, the investigations have resulted in a significant efficiency improvement of top-emitting OLEDs and the compensation of drawbacks (optimization of the white light emission and the extraction of internal light modes) in comparison to the bottom-emitting devices. The investigated concepts are beneficial for OLEDs in general, since the replacement of the brittle ITO electrodes and the fabrication of roll-to-roll processing compatible light extraction structures are also desirable for bottom-emitting, or transparent OLEDs

Towards label-free biosensors based on localized surface plasmon resonance

Medical diagnostics is in constant search of new tools and devices able to provide in short time, accurate and versatile tests performed on patients. Nanotechnology has contributed largely in developing biosensors of smaller size at a lower cost by using a minimal amount of sample. Biosensors aim to monitor and diagnosticate “in situ” the patient status and the diseases caused by alteration of the body metabolism by, for example, the detection of gene mutations, alteration of gene expression or alteration of proteins. The aim of this work is the development of biosensors that satisfy the requirements which are critical for applications. A biosensor must be i) easy to use, ii) economically convenient, and therefore preferentially label free, iii) highly sensitive, iv) reversible, v) and suitable for Point of Care Testing, that is to be used ”in situ” on the patient. We have focused on biosensors based on the optical properties of nanostructured metals as Au or Ag, in particular by using on Localized Surface Plasmon Resonance (LSPR) spectroscopy. Nanostructured metals under irradiation of electromagnetic wave (as light) exhibit intense absorption bands as results of the localized electronic charges of the metal surface coming into resonance with the incident energy. According to the Mie’s theory, the LSPR absorption band feature changes when the refractive index of the media surrounding the metal nanostructures is varied. Of particular interest for our purpose are the possible changes of the LSPR band features taking place under molecular interactions occurring at the nanostructures surfaces: the shift of LSPR bands is the “transducer” of molecular interactions. These changes can be easily detected by conventional UV-Vis spectroscopy, in transmittance mode. While a large number of studies have been carried out on monodisperse nanoparticles suspended in solution, gold nanoparticles (NPs) deposited on a transparent surface open the possibility to fabricate biosensor based on multiplex array platforms. Nonetheless, one of the major problems in using these plasmonic materials for biosensing purpose is related to the stability of the metal NPs in different solvents and in particular in aqueous solutions. In this study we demonstrate i) the possibility to achieve highly stable NPs by simple thermal evaporation of Au on a substrate commercially available, the Fluorine Tin Oxide (FTO) (Chapter 2); ii) a reproducible variation of the LSPR bands under formation of organic selfassembled monolayers (SAMs), iii) reversible changes in the features of the LSPR bands, (Chapter 3), iv) a specific and reproducible LSPR band changes under molecular interactions occurring at NPs surfaces, as DNA hybridization (Chapter 4). This work demonstrates that the plasmonic material based on Au NPs deposited on FTO surfaces represents a convenient platform for biosensors because of i) inexpensive fabrication, ii) stability of this material in various solvent, including water, of, iii) the easy way to detect the molecular interaction, and iv) the good sensitivity to molecular interactions

Light Matter Interaction in Epsilon Near Zero Metal/Insulator Layered Nanocavities Thesis

Light-matter interaction has been a widely investigated phenomena enlarging the area of nanophotonics beyond the limit. This stand out to be the back bone for future generation optical devices. Light confinement and propagation in a small volume gives rise to several rich optical properties. This can be realized in different type of nanostructured materials. Metal(M)/Insulator(I) multilayer nanocavities are highly versatile systems for light confinement and wave guiding at nanoscale. Their physical behavior is discussed successfully by electromagnetic theory. However, it is still obscured about the nature of cavity modes in layered metal/insulator nanocavities. The reason why such cavity mode can be excited without having any momentum matching technique are yet to be investigated. We start with a quantum treatment of the MIM as a double barrier quantum well where the resonant modes are assisted by tunneling of photons. The lossless characteristics of these modes with zero wavevector condition are inherent to the epsilon-nearzero (ENZ) band. We further investigated the coupling between epsilon near zero assisted volume plasmons in MIMIM nanocavities where one MIM cavity placed above the other. Strong coupling has been demonstrated in this system by an anticrossing of the ENZ modes in the individual cavities, where the splitting depends strongly on the thickness of the central metal layer. The properties of ENZ bulk plasmon modes for MIM and MIMIM systems are exploited to achieve both enhancement of spontaneous emission and decay rate of the perovskite nanocrystal film placed on the top of the nanocavity. However, the enhancement is within the limit of weak coupling regime. In order to achieve strong coupling between ENZ mode of cavity and emission mode of the fluorophore, one need to embed the fluorophore inside the cavity. But it has been realized that in such a case, long term stability of fluorophore by retaining its original optical properties are primary challenges. We studied the optical properties of nanocrystal layer that were overcoated with alumina by atomic layer deposition. This enabled us to effectively embed the NCs inside the dielectric layers of planar MIM and MIMIM nanocavities