GENERATION OF SEQUENCES OF STRONG ELECTRIC MONOPULSES IN NITRIDE FILMS

This paper presents theoretical investigation of the excitation of the sequences of strong nonlinear monopulses of space charge waves from input small envelope pulses with microwave carrier frequencies due to the negative differential conductivity in n-GaN and n-InN films. The stable numerical algorithms have been used for nonlinear 3D simulations. The sequences of the monopulses of the strong electric field of 3 – 10 ps durations each can be excited. The bias electric field should be chosen slightly higher than the threshold values for observing the negative differential conductivity. The doping levels should be moderate 1016 –1017 cm-3in the films of £ 2 mm thicknesses. The input microwave carrier frequencies of the exciting pulses of small amplitudes are up to 30 GHz in n-GaN films, whereas in n-InN films they are lower, up to 20 GHz. The sequences of the electric monopulses of high peak values are excited both in the uniform nitride films and in films with non-uniform conductivity. These nonlinear monopulses in the films differ from the domains of strong electric fields in the bulk semiconductors. In the films with non-uniform doping the nonlinear pulses are excited due to the inhomogeneity of the electric field near the input end of the film and the output nonlinear pulses are rather domains.

Theory of Optical Nonlocality in Polar Dielectrics

Sub-wavelength confinement of mid-infrared light can be achieved exploiting the metal-like optical response of polar dielectric crystals in their Reststrahlen spectral region, where they support evanescent modes termed surface phonon polaritons. In the past few years the investigation of phonon polaritons localised in nanoresonators and layered heterostructures has enjoyed remarkable success, highlighting them as a promising platform for mid-infrared nanophotonic applications. Here we prove that the standard local dielectric description of phonon polaritons in nanometric objects fails due to the nonlocal nature of the phonon response and we develop the corresponding nonlocal theory. Application of our general theory to both dielectric nanospheres and thin films demonstrates that polar dielectrics exhibit a rich nonlocal phenomenology, qualitatively different from the one of plasmonic systems, due to the negative dispersion of phononic optical modes.Comment: 13 pages, 6 figure

Nonlinear optical functionalities of VO2- and GaN-based nanocomposites

This thesis presents fundamental research and concepts for active photonic elements operating in the telecom wavelength regime. The aim of the study is to determine the characteristics of the investigated nanostructures and to evaluate the implementation of the proposed materials in potential optical devices. In the first part of this thesis the optical properties as well as the photonic application of vanadium dioxide (VO2) nanocrystals (NCs) are studied. VO2 exhibits an easily accessible insulator-to-metal phase transition (IMT) near ambient temperatures. Upon excitation it undergoes an atomic rearrangement that is accompanied by a substantial modification of the complex dielectric function. When VO2 undergoes the IMT, the near-infrared transmission peaks of a moderate-finesse etalon containing a sub-wavelength layer of VO2 NCs are found to markedly shift in their spectral position and peak transmissivity. Both heat deposition and optical excitation permit to actively control the etalon’s functionality. Much less is known about the nonlinear optical properties of VO2 beyond the established IMT. To this end the nonlinear optical response of a thin film of VO2 NCs is investigated with open aperture z-scans involving femtosecond near-infrared pulses. A pronounced saturable absorption on the short-wave side of the resonance as well as a marked reverse saturable absorption in the telecom window are observed. The results hold promise for the use of VO2 nanocrystals as a saturable absorber, e.g., to mode-locked near-infrared lasers. In the second part a semiconductor heterostructure based on hexagonal ultranarrow GaN/AlN multi-quantum wells (MQWs) is investigated. The tailored inter-miniband (IMB) transition is characterized in terms of its linear as well as ultrafast nonlinear optical properties using the established pump-probe scheme. In line with theoretical predictions for LO-phonon scattering, a fast relaxation is found for resonant IMB excitation. In stark contrast, significantly larger relaxation times are observed for photon energies addressing the above barrier continuum. The last section reports on a new type of nonlinear metasurface taking advantage of these telecom-range IMB transitions. The heterostructure is functionalized with an array of plasmonic antennas featuring cross-polarized resonances at these near-infrared wavelengths and their second harmonic. This kind of nonlinear metasurface allows for substantial second harmonic generation at normal incidence which is completely absent for an antenna array without the heterostructure underneath

Nonlinear acousto-magneto-plasmonics

We review the recent progress in experimental and theoretical research of interactions between the acoustic, magnetic and plasmonic transients in hybrid metal-ferromagnet multilayer structures excited by ultrashort laser pulses. The main focus is on understanding the nonlinear aspects of the acoustic dynamics in materials as well as the peculiarities in the nonlinear optical and magneto-optical response. For example, the nonlinear optical detection is illustrated in details by probing the static magneto-optical second harmonic generation in gold-cobalt-silver trilayer structures in Kretschmann geometry. Furthermore, we show experimentally how the nonlinear reshaping of giant ultrashort acoustic pulses propagating in gold can be quantified by time-resolved plasmonic interferometry and how these ultrashort optical pulses dynamically modulate the optical nonlinearities. The effective medium approximation for the optical properties of hybrid multilayers facilitates the understanding of novel optical detection techniques. In the discussion we highlight recent works on the nonlinear magneto-elastic interactions, and strain-induced effects in semiconductor quantum dots.Comment: 30 pages, 12 figures, to be published as a Topical Review in the Journal of Optic

Nonlinear nanophotonic devices in the Ultraviolet to Visible wavelength range

Although the first lasers invented operated in the visible, the first on-chip devices were optimized for near-infrared (IR) performance driven by demand in telecommunications. However, as the applications of integrated photonics has broadened, the wavelength demand has as well, and we are now returning to the visible (Vis) and pushing into the ultraviolet (UV). This shift has required innovations in device design and in materials as well as leveraging nonlinear behavior to reach these wavelengths. This review discusses the key nonlinear phenomena that can be used as well as presents several emerging material systems and devices that have reached the UV-Vis wavelength range.Comment: 58 pages, 10 figure

Application of Graphene within Optoelectronic Devices and Transistors

Scientists are always yearning for new and exciting ways to unlock graphene's true potential. However, recent reports suggest this two-dimensional material may harbor some unique properties, making it a viable candidate for use in optoelectronic and semiconducting devices. Whereas on one hand, graphene is highly transparent due to its atomic thickness, the material does exhibit a strong interaction with photons. This has clear advantages over existing materials used in photonic devices such as Indium-based compounds. Moreover, the material can be used to 'trap' light and alter the incident wavelength, forming the basis of the plasmonic devices. We also highlight upon graphene's nonlinear optical response to an applied electric field, and the phenomenon of saturable absorption. Within the context of logical devices, graphene has no discernible band-gap. Therefore, generating one will be of utmost importance. Amongst many others, some existing methods to open this band-gap include chemical doping, deformation of the honeycomb structure, or the use of carbon nanotubes (CNTs). We shall also discuss various designs of transistors, including those which incorporate CNTs, and others which exploit the idea of quantum tunneling. A key advantage of the CNT transistor is that ballistic transport occurs throughout the CNT channel, with short channel effects being minimized. We shall also discuss recent developments of the graphene tunneling transistor, with emphasis being placed upon its operational mechanism. Finally, we provide perspective for incorporating graphene within high frequency devices, which do not require a pre-defined band-gap.Comment: Due to be published in "Current Topics in Applied Spectroscopy and the Science of Nanomaterials" - Springer (Fall 2014). (17 pages, 19 figures

Advanced Applications in Nanophotonics

Nanophotonics is a fast-growing area of both scientific significance and practical value for applications. Nanophotonics studies the interaction between light and electronic systems in nanomaterials and nanostructures as well as the behavior of light in nanometer scales. It covers many hot topics such as plasmonics, two-dimensional materials, and silicon photonics. Increasing attention is given to the area and nanophotonics is expected to have significant impact on future technology advances. This thesis work focuses on three aspects of nanophotonics. The first aspect is in exploring the nonlocal effect and surface correction for nanometer-length-scale plasmonic structures. Plasmonics is the study of the interaction between electromagnetic fields and free electrons in a metal. It exploits the unique optical properties of metallic nanostructures to enable routing and manipulation of light at the nanoscale, where nonlocal effect becomes important. Here we introduce a new surface hydrodynamic model for plasmon propagation at interfaces, which incorporates both nonlocality and surface contributions. This surface correction is calculated via a discontinuity in the normal component of the electric displacement in conjunction with Feibelman's d-parameters, thus enabling rapid numerical calculation of nanostructures without requiring a full quantum calculation because of its large computational requirement. We examine numerical calculations of surface plasmon polaritons propagation at a single interface structure, and then for a more complex thin-film structures. The second aspect is investigating the third-harmonic generation in thick multilayer graphene. Graphene is the first two-dimensional material to be discovered and has attracted much interest because of its remarkable two-dimensional electronic, optical, mechanical, and thermal properties. Multilayer graphene, can be seen as stacking of monolayer graphene, and it offers an array of properties that are of interest for optical physics and devices. We describe the layer-dependent for third-harmonic generation in thick multilayer graphene on quartz substrate. The third harmonic signal of multilayer graphene exhibits a complex dependence on its layer number showing that the optimal third harmonic signal at 24 layers, in good agreement with two theoretical models. The third aspect is an exploration in silicon photonics of design and demonstration of a differential phase shift keying demodulator based on coherent perfect absorption effect. Silicon photonics is considered a potential future communication system mainly due to its compact footprint, dense integration, and compatibility with mature silicon integrated circuit manufacturing. Differential phase shift keying based system offers advantages, e.g., dispersion tolerance, improved sensitivity, and does not require coherent detection. Coherent perfect absorption uses a ring resonator works for the critical coupling condition at resonance frequency. This work shows a new compact demodulator circuit can be integrated in all optical-system