Planar metal plasmon waveguides: frequency-dependent dispersion, propagation, localization, and loss beyond the free electron model

A numerical analysis of surface plasmon dispersion, propagation, and localization on smooth lossy films is presented. Particular attention is given to determining wavelength-dependent behavior of thin Ag slab waveguides embedded in a symmetric SiO2 environment. Rather than considering Ag as a damped free electron gas, the metal is defined by the experimentally determined optical constants of Johnson and Christy and Palik. As in free electron gas models, analytic dispersion results indicate a splitting of plasmon modes—corresponding to symmetric and antisymmetric field distributions—as film thickness is decreased below 50 nm. However, unlike free electron gas models, the surface plasmon wave vector remains finite at resonance with the antisymmetric-field plasmon converging to a pure photon mode for very thin films. In addition, allowed excitation modes are found to exist between the bound and radiative branches of the dispersion curve. The propagation characteristics of all modes are determined, and for thin films (depending upon electric field symmetry), propagation distances range from microns to centimeters in the near infrared. Propagation distances are correlated with both the field decay (skin depth) and energy density distribution in the metal and surrounding dielectric. While the energy density of most long-range surface plasmons exhibits a broad spatial extent with limited confinement in the waveguide, it is found that high-field confinement does not necessarily limit propagation. In fact, enhanced propagation is observed for silver films at ultraviolet wavelengths despite strong field localization in the metal. The surface plasmon characteristics described in this paper provide a numerical springboard for engineering nanoscale metal plasmon waveguides, and the results may provide a new avenue for integrated optoelectronic applications

Compacte vlakke-golfgeleiderkoppelingen in silicium-op-isolator Compact planar waveguide spot-size converters in silicon-on-insulator

An optical chip is a planar component in which a light signal is guided by optical waveguides and in which light can be processed in various ways and for various purposes. Examples are splitting a light beam into its separate wavelengths, absorption of the light for sensor applications, modulating a high-content data signal onto a light beam, etc., all offering the possibility for cheaper and better optical systems. In these chips often connections occur between waveguides with different widths. Traditionally these connections are realized by long coupling sections having a linear or parabolic shape when viewed from above. If we want to shorten these coupling sections to gain precious chip surface area, innovative shapes should be applied. A general shape for an innovative coupler is proposed in this work and is optimized in various ways leading to a good coupling efficiency. In these simulations an in-house eigenmode expansion method, CAMFR, is used both for two- and three-dimensional waveguide calculations. At the optimization side, a successive line minimization and a standard genetic algorithm are successfully applied. Different initial assumptions for the shapes of these couplers can, after optimization, lead to similar structures, confirming the value of the applied methods. After simulation, a selection of optimized structures is realized in the material system silicon-on-insulator. For this manufacturing, standard deep-UV lithography with an illumination wavelength of 248 nm, normally used in the creation of CMOS chips, is applied. Optical measurements on these optical coupling structures confirm the calculated coupling efficiencies and lead us to the conclusion that couplers between planar optical waveguides with a different width can be shortened if, instead of the conventional linear or parabolic shapes, more complex structures are applied

Advancements in nuclear waste assay

The research described in this thesis is directed at advancing the state of the practice of the non-destructive gamma-ray assay of nuclear waste containers. A number of potentially accuracy-limiting issues were identified and addressed, resulting in new developments which were implemented on an instrument prior to entering it into service. A set of Pu reference sources used for experimental data have been studied to determine the internal composition (density and fill height) of the sources to assist with validation of a point kernel model. This model has been used to observe the behaviour of gamma-rays in lumps of fissile material from plutonium over the mass range 0.001g to 350g, for a number of densities corresponding to Pu, PuO$_2$ and PuF$_3$. Established lump corrections have been analysed and have been found to produce large over- and under-corrected results for the range of masses. Due to the inadequacies of current techniques, a new Pu self-absorption correction method has been developed using the data from numerical simulations, allowing nature to reveal the correlations rather than traditional approaches based upon approximate models. For a 25g 1cm-high Pu flat-plate of density 15g.cm$^{-3}$, the developed Pu correction produces a result of (24.9 ± 8.8)g compared to (19.5 ± 0.9)g for the Fleissner 2-line method, and (14.7 ± 0.4)g for the Infinite Energy Extrapolation method. The developed Pu correction method has been extended to the application of uranium lumps in waste matrices, provided the enrichment of the sample is known or may be determined via sophisticated isotopic analysis methods such as MGAU or FRAM. The U self-absorption correction method has been found to produce results within 30% of the true mass of the sample for the lumps studied. An analysis of ‘real drum’ effects has been performed, including the revisiting of the Total Measurement Uncertainty (incorporating the uncertainty components of the new Pu and U self-absorption corrections) and results from known sources placed in artificial inhomogeneous waste matrices assayed inside a Canberra Auto Q2 system

Development of a new silicon pixel detector with 10 ps time resolution for high luminosity future experiments

In this thesis work the results of the characterizations of the 3D-trench TimeSPOT silicon sensors with minimum ionizing particles are described. Such devices have been developed to satisfy the requirements of the LHCb VErtex LOcator detector for the Upgrade II, planned to face the high luminosity conditions. These requirements are a time resolution better than 50 ps per hit, a spatial resolution of the order of 10–20 μm and a radiation hardness up to 6·10^16 1 MeV n_eq/cm^2. In this thesis the characterizations performed both in the laboratory with a 90Sr source and in several test beam campaigns are illustrated. The main measurements done are the time resolution and the detection efficiency before and after the irradiation up to 2.5·10^16 1 MeV n_eq/cm^2

Modelización y producción de elementos fotónicos en superficie y volumen por estructuración de dieléctricos con pulsos láser ultracortos

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Óptica, leída el 04/11/2016Photonics was born with the advent of lasers. Lasers provided the kind of light required for the development of active and passive photonics devices. Ultrafast pulsed lasers provided, in addition, enough optical power to study the interaction of light with matter beyond the linear regime. Non-linear optical phenomena are the base for the development of non-linear laser processing of transparent materials (dielectrics), a laser processing approach that has unique characteristics in terms of resolution, versatility and 3D-processing capabilities for the production of photonic devices. Fs-laser writing has been indeed successfully applied to the production of a large variety of high performance passive and active devices in 2D and 3D configuration. In spite of this success, sub-surface fs-laser writing has several limitations that hamper its widespread application for the production of photonics devices. The first one is the limited refractive index contrast (Δn) that can be achieved using conventional modification mechanisms (i.e. local glass matrix densification, or polarizability changes, generation of point defects or changes in the glass network configuration). With a few exceptions, the maximum index contrast achievable is normally below 10-2, which limits the performance of the produced devices. Also, the presence of optical nonlinearities is inherent to the propagation of fs-laser pulses inside dielectrics. When the material shows a high non-linear refractive index, as it happens in crystalline materials or glasses with a high linear refractive index, the distortion of the desired energy deposition profile severely affects the control over the morphology of the structure. Finally, fs-laser processing can be similarly used for micro-structuring applications at the surface of dielectrics in order to produce active and passive surface waveguides. However, scattering losses at the channel walls produced by fs-laser ablation usually lead to high losses that strongly limit the performance of fs-laser structured surface waveguide-based devices...La fotónica surge con la aparición de los láseres, fuentes de luz imprescindibles para el desarrollo de los dispositivos fotónicos activos y pasivos. Junto a ello, el desarrollo de los láseres de pulsos ultracortos ha posibilitado el estudio de la interacción luz-materia más allá del régimen lineal. Los fenómenos de interacción no-lineal son la base del procesado por láser de materiales dieléctricos. Esta técnica tiene numerosas ventajas (elevada resolución y versatilidad, capacidad de procesado tridimensional, reducción de productos contaminantes e instalaciones de elevado coste) para la producción de dispositivos fotónicos. El procesado por láser de pulsos ultracortos (PLPU) se ha aplicado en numerosas ocasiones para fabricar dispositivos fotónicos de altas prestaciones. Sin embargo, a pesar de sus numerosas ventajas, el procesado de materiales por láser no está exento de dificultades. Una es el limitado contraste de índice de refracción (Δn) que es posible generar usando los mecanismos de modificación convencionales (densificación, cambios de polarizabilidad o estructura, etc.), generalmente menor que 10-2. Ello limita las prestaciones de los dispositivos generados. Por otra parte, los fenómenos de propagación no lineal tienen un gran impacto en el perfil de depósito de energía (PDE) del haz bajo la superficie del material, distorsionándolo y dificultando enormemente encontrar las condiciones de procesado adecuadas en proceso bajo superficie. Por último, la elevada rugosidad producida en las estructuras generadas en superficie por irradiación con pulsos ultracortos genera niveles de pérdidas que merman sustancialmente la eficacia de los dispositivos producidos...Depto. de ÓpticaFac. de Ciencias FísicasTRUEunpu

Nanoscale local modification of PMMA refractive index by tip-enhanced femtosecond pulsed laser irradiation

Investigation techniques based on tip-enhanced optical effects, capable to yield spatial resolutions down to nanometers level, have enabled a wide palette of important discoveries over the past twenty years. Recently, their underlying optical setups are beginning to emerge as useful tools to modify and manipulate matter with nanoscale spatial resolution. We try to contribute to these efforts by reporting a method that we found viable to modify the surface refractive index of polymethyl methacrylate (PMMA), an acrylic polymer material. The changes in the refractive index are accomplished by focusing a femtosecond pulsed near-infrared laser beam on the apex of a metalized nano-sized tip, traditionally used in scanning probe microscopy (SPM) applications. The adopted illumination strategy yields circular-shaped modifications of the refractive index occurring at the surface of the PMMA sample, exhibiting a lateral size <200 nm, under 790 nm illumination, representing a four-fold increase in precision compared to the current state-of-the-art. The light intensity enhancement effects taking place at the tip apex makes possible achieving refractive index changes at low laser pulse energies (<0.5 nJ), which represents two orders of magnitude advantage over the current state-of-the art. The presented nanoimprinting method is very flexible, as it can be used with different power levels and can potentially be operated with other materials. Besides enabling modifications of the refractive index with high lateral resolution, this method can pave the way towards other important applications such the fabrication of photonic crystal lattices or surface waveguides