Diffractive sidewall grating coupler: towards 2D free-space optics on chip.

Silicon photonics has been the subject of intense research efforts. In order to implement complex integrated silicon photonic devices and systems, a wide range of robust building blocks is needed. Waveguide couplers are fundamental devices in integrated optics, enabling different functionalities such as power dividers, spot-size converters, coherent hybrids and fiber-chip coupling interfaces, to name a few. In this work we propose a new type of nanophotonic coupler based on sidewall grating (SIGRA) concept. SIGRAs have been used in the Bragg regime, for filtering applications, as well as in the sub-wavelength regime in multimode interference (MMI) couplers. However, the use of SIGRAs in the radiation regime has been very limited. Specifically, a coarse wavelength division multiplexer was proposed and experimentally validated. In this work we study the use of SIGRAs in the diffractive regime as a mean to couple the light between a silicon wire waveguide mode and a continuum of slab waveguide modes. We also propose an original technique for designing SIGRA based couplers, enabling the synthesis of arbitrary radiation field profile by Floquet- Bloch analysis of individual diffracting elements while substantially alleviating computational load. Results are further validated by 3D FDTD simulations which confirm that the radiated field profile closely matches the target design field.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

On the Core of Routing Games with Revenues

Traveling salesman problems with revenues form a generalization of traveling salesman problems.Here, next to travel costs an explicit revenue is generated by visiting a city.We analyze routing problems with revenues, where a predetermined route on all cities determines the tours along subgroups.Corresponding routing games with revenues are analyzed.It is shown that these games have a nonempty core and a complete description of the core is provided.

Zigzag domain wall mediated reversal in antiferromagnetically coupled layers

The Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling between two magnetic layers leads to many important technological applications. Here, the interaction between changing antiferromagnetic RKKY coupling and domain structure is studied in a sample consisting of two 5 nm thick CoFeB layers separated by a wedge of Cu up to 4 nm thick. Magnetic reversal occurs via the propagation of a zigzag domain wall front along the wedge. The modification of domain patterns created in the reversal of a coupled layers in the presence of antiferromagnetic RKKY coupling and coupling gradients is demonstrated. Firstly, the coupling leads to a smaller amplitude of the zigzag wall, which is aligned perpendicular to the easy axis, followed by elongation of the walls at higher coupling strength. The antiferromagnetic RKKY coupling, while not strong enough to cause antiparallel alignment of the layers, is argued to lead to coupling between the spins in the domain walls in the two layers, lowering their energy and driving the reversal behavior of the film

Curved waveguide grating demultiplexer (CWG) with a flattened response via bimodal output waveguides

We demonstrate a compact wavelength demultiplexer for the silicon-on-insulator platform based on the curved waveguide grating (CWG) architecture. The proposed device uses bi- modal output waveguides to achieve a low-loss flattened spectral response. The device shows insertion loss as low as 1.2 dB and crosstalk below -20 dB.Universidad de Málaga; Ministerio de Educación, Cultura y Deporte (MECD) (FPU16/03401), Ministerio de Ciencia, Innovación y Universidades (MCIU) (PID2019-106747RBI00), Consejería de Economía, Conocimiento, Empresas y Universidad (CECEU) (UMA18-FEDERJA-219, P18-RT1453, P18-RT-793). Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Analysis of Process Configurations for CO2 Capture by Precipitating Amino Acid Solvents

Precipitating amino acid solvents are an alternative to conventional amine scrubbing for CO2 capture from flue gas. Process operation with these solvents leads to the formation of precipitates during absorption that need to be re-dissolved prior to desorption of CO2. The process configuration is crucial for the successful application of these solvents. Different process configurations have been analyzed in this work, including a full analysis of the baseline operating conditions (based on potassium taurate), the addition of lean vapor compression, multiple absorber feeds, and the use of different amino acids as alternative solvents to the baseline based on potassium taurate. The analysis is carried out with an equilibrium model of the process that approximates the thermodynamics of the solvents considered. The results show that the precipitating amino acid solvents can reduce the reboiler duty needed to regenerate the solvent with respect to a conventional MEA process. However, this reduction is accompanied by an expenditure in lower grade energy needed to dissolve the precipitates. To successfully implement these processes into power plants, an internal recycle of the rich stream is necessary. This configuration, known as DECAB Plus, can lower the overall energy use of the capture process, which includes the energy needed to regenerate the solvent, the energy needed to dissolve the precipitates, and the energy needed to compress the CO2 to 110 bar. With respect to the energy efficiency, the DECAB Plus with lean vapor compression configuration is the best configuration based on potassium taurate, which reduces the reboiler duty for regeneration by 45% with respect to conventional MEA. Retrofitting this process into a coal fired power plant will result in overall energy savings of 15% with respect to the conventional MEA process, including compression of the CO2 stream to 110 bar. Potassium alanate was found to reduce the energy use with respect to potassium taurate under similar process configurations. Therefore, the investigation of potassium alanate in a DECAB Plus configuration is highly recommended, since it can reduce the energy requirements of the best process configuration based on potassium taurat

Subwavelength gratings for sensing and polarization management

Sub-wavelength grating (SWGs) structures are becoming important building blocks in planar waveguide photonic devices [1]. SWG structures have been successfully applied in the design of a range of devices with remarkable performance by using refractive index engineering and dispersion engineering techniques [2]. In this work we explore two new promising applications of these structures, namely in evanescent field waveguide sensing and polarization management. For the evanescent waveguide sensing devices, we show that sub- wavelength patterning of silicon wires can be used to control the delocalization of the waveguide mode and therefore enhance both bulk and surface sensitivities (Fig. 1). We will also discuss the implementation of subwavelength structures in efficient polarization splitter and rotator (PSR) devices [3]. PSR devices based on asymmetrical directional couplers typically exhibit stringent fabrication tolerances. We show that by implementing SWG structures in PSR design both the effective mode index and its derivatives with respect to critical dimensions can be controlled, which significantly improves tolerance to fabrication errors (Fig. 2).Universidad de Málaga, Campus de Excelencia Internacional Andalucía Tech

Silicon-on-insulator polarization controller with relaxed fabrication tolerances

Polarization control is essential in applications ranging from optical communications to interferometric sensors. The implementation of in- tegrated polarization controllers is challenging as they require polariza- tion rotating waveguides with stringent fabrication tolerances. Here, we present a fully integrated polarization controller scheme that signi cantly relaxes the requirements on the rotating waveguides, alleviating fabri- cation tolerances. We analytically establish a technology-independent, easily measurable tolerance condition for the rotating waveguides. Po- larization control in the presence of waveguide width errors of 25% is shown through full vectorial simulation.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Dynamical mean field theory for transition temperature and optics of CMR manganites

A tight binding parametrization of local spin density functional band theory is combined with a dynamical mean field treatment of correlations to obtain a theory of the magnetic transition temperature, optical conductivity and T=0 spinwave stiffness of a minimal model for the pseudocubic metallic $CMR$ manganites such a $La_{1-X}Sr_{x}MnO_{3}$. The results indicate that previous estimates of $T_{c}$ obtained by one of us (Phys. Rev. \textbf{B61} 10738-49 (2000)) are in error, that in fact the materials are characterized by Hunds coupling $J\approx 1.5eV$, and that magnetic-order driven changes in the kinetic energy may not be the cause of the observed 'colossal' magnetoresistive and multiphase behavior in the manganites, raising questions about our present understanding of these materials.Comment: Published version; 10 pages, 9 figure

Geometric phases for generalized squeezed coherent states

A simple technique is used to obtain a general formula for the Berry phase (and the corresponding Hannay angle) for an arbitrary Hamiltonian with an equally-spaced spectrum and appropriate ladder operators connecting the eigenstates. The formalism is first applied to a general deformation of the oscillator involving both squeezing and displacement. Earlier results are shown to emerge as special cases. The analysis is then extended to multiphoton squeezed coherent states and the corresponding anholonomies deduced.Comment: 15 page