Multipole expansion of strongly focussed laser beams

Multipole expansion of an incident radiation field - that is, representation of the fields as sums of vector spherical wavefunctions - is essential for theoretical light scattering methods such as the T-matrix method and generalised Lorenz-Mie theory (GLMT). In general, it is theoretically straightforward to find a vector spherical wavefunction representation of an arbitrary radiation field. For example, a simple formula results in the useful case of an incident plane wave. Laser beams present some difficulties. These problems are not a result of any deficiency in the basic process of spherical wavefunction expansion, but are due to the fact that laser beams, in their standard representations, are not radiation fields, but only approximations of radiation fields. This results from the standard laser beam representations being solutions to the paraxial scalar wave equation. We present an efficient method for determining the multipole representation of an arbitrary focussed beam.Comment: 13 pages, 7 figure

Calculation of the T-matrix: general considerations and application of the point-matching method

The T-matrix method is widely used for the calculation of scattering by particles of sizes on the order of the illuminating wavelength. Although the extended boundary condition method (EBCM) is the most commonly used technique for calculating the T-matrix, a variety of methods can be used. We consider some general principles of calculating T-matrices, and apply the point-matching method to calculate the T-matrix for particles devoid of symmetry. This method avoids the time-consuming surface integrals required by the EBCM.Comment: 10 pages. 2 figures, 1 tabl

Microwave pyrolysis of biomass in a rotatory kiln reactor: deep characterization and comparative analysis of pyrolytic liquids products

[EN] The pursuit of sustainable relationship between the production and consumption of energy has accelerated the research into new fuels alternatives, and mainly focused on new technologies for biomass based fuels. Microwave pyrolysis of biomass is a relatively new process which has been long recognized to provide better quality bio-products in shorter reaction time due to the direct sample heating and the particular heating profile resulting from the interaction of biomass with the electric field component of an electromagnetic wave [1,2]. During the course of this research, flax shives were pyrolysed using a rotatory kiln reactor inside a microwave single mode cavity using a range of power between 100 and 200 watts, to reach a temperature range between 450 °C and 650°C. The liquid bio-oil samples recovered in each case were analyzed though gas chromatography-mass spectrometry (GC-MS) and gas chromatography-flame ionization detection (GC-FID) to identify and quantify the different molecules presents and paying a particular attention to the BTX’s concentration. More than two hundred compounds were identified and grouped into families such as carboxylic acids, alcools, sugars for a deep analysis of the results. The effect of the operating conditions on the proportion of gas, liquid and char produced were studied as well as some properties of the pyrolysis products. In most cases, carboxylic acids were the dominating chemical group present. It was also noticed that the increase of temperature enhanced the carboxylic acids production and diminished the production of other groups, as sugars. Finally, pyrolysis oils were produced in higher quantities by microwaves than in a classical oven and showed a different composition. The examination of the pyrolytic liquid products from different biomass components helped to determine the provenance of each molecule family. On the operational side, the rotatory kiln reactor provided a fast and homogeneous heating profile inside the reactor, desired for fast pyrolysis. The high temperature was maintained without making hot spots during the reaction time. The microwave irradiation setup consisted in a single-mode cavity, a system of plungers, incident and reflected power monitors, an isolator and a 2.45 GHz continuous microwave generator with a power upper limit of 2000 watts. The plunger system was calibrated to maintain a range of reflective wave between 5 and 15%, taking advantage of a minimum of 85 percent of the applied power. In conclusion, the developed microwave pyrolysis process gives a clear way to produce an exploitable bio-oil with enhanced properties. References Beneroso, D., Monti, T., Kostas, E., Robinson, J., CEJ, 2017.,316, 481- 498. Autunes E., Jacob M., Brodie, G., Schneider, A., JAAP, 2018,129, 93-100.Ubiera, L.; Polaert, I.; Abdelouahed, L.; Taouk, B. (2019). Microwave pyrolysis of biomass in a rotatory kiln reactor: deep characterization and comparative analysis of pyrolytic liquids products. En AMPERE 2019. 17th International Conference on Microwave and High Frequency Heating. Editorial Universitat Politècnica de València. 194-202. https://doi.org/10.4995/AMPERE2019.2019.9807OCS19420

Dynamic Compact Thermal Model with Neural Networks for Radar Applications

This article deals with the creation of a compact thermal model. In this aim, we apply some well-known methods such as FEM model reduction and identification of RC networks. To go further than already existing approaches, we also introduce the use of artificial neural networks (ANNs) to cope with nonlinearities which may appear in thermal phenomenons. A new hybrid model, trying to gather the advantages of ANNs and RC networks, is applied on a simple thermal problem. The need of samples will also lead us to carry out, in parallel, the FEM model reduction. The reduced FEM model will then be used to generate the required databases and validate the compact model results

Integral localized approximation description of ordinary Bessel beams and application to optical trapping forces

Ordinary Bessel beams are described in terms of the generalized Lorenz-Mie theory (GLMT) by adopting, for what is to our knowledge the first time in the literature, the integral localized approximation for computing their beam shape coefficients (BSCs) in the expansion of the electromagnetic fields. Numerical results reveal that the beam shape coefficients calculated in this way can adequately describe a zero-order Bessel beam with insignificant difference when compared to other relative time-consuming methods involving numerical integration over the spherical coordinates of the GLMT coordinate system, or quadratures. We show that this fast and efficient new numerical description of zero-order Bessel beams can be used with advantage, for example, in the analysis of optical forces in optical trapping systems for arbitrary optical regimes

Rotational dynamics of optically trapped polymeric nanofibers

The optical trapping of polymeric nanofibers and the characterization of the rotational dynamics are reported. A strategy to apply a torque to a polymer nanofiber, by tilting the trapped fibers using a symmetrical linear polarized Gaussian beam is demonstrated. Rotation frequencies up to 10 Hz are measured, depending on the trapping power, the fiber length and the tilt angle. A comparison of the experimental rotation frequencies in the different trapping configurations with calculations based on optical trapping and rotation of linear nanostructures through a T-Matrix formalism, accurately reproduce the measured data, providing a comprehensive description of the trapping and rotation dynamics.Comment: (21 pages, 5 figures

Fundamentals of negative refractive index optical trapping: forces and radiation pressures exerted by focused Gaussian beams using the generalized Lorenz-Mie theory

Based on the generalized Lorenz-Mie theory (GLMT), this paper reveals, for the first time in the literature, the principal characteristics of the optical forces and radiation pressure cross-sections exerted on homogeneous, linear, isotropic and spherical hypothetical negative refractive index (NRI) particles under the influence of focused Gaussian beams in the Mie regime. Starting with ray optics considerations, the analysis is then extended through calculating the Mie coefficients and the beam-shape coefficients for incident focused Gaussian beams. Results reveal new and interesting trapping properties which are not observed for commonly positive refractive index particles and, in this way, new potential applications in biomedical optics can be devised

Spin angular momentum transfer from TEM00 focused Gaussian beams to negative refractive index spherical particles

We investigate optical torques over absorbent negative refractive index spherical scatterers under the influence of linear and circularly polarized TEM00 focused Gaussian beams, in the framework of the generalized Lorenz-Mie theory with the integral localized approximation. The fundamental differences between optical torques due to spin angular momentum transfer in positive and negative refractive index optical trapping are outlined, revealing the effect of the Mie scattering coefficients in one of the most fundamental properties in optical trapping systems

Furfural Synthesis from d-Xylose in the Presence of Sodium Chloride: Microwave versus Conventional Heating

We investigate the existence of specific/nonthermal microwave effects for the dehydration reaction of xylose to furfural in the presence of NaCl. Such effects are reported for sugars dehydration reactions in several literature reports. To this end, we adopted three approaches that compare microwave-assisted experiments with a) conventional heating experiments from the literature; b) simulated conventional heating experiments using microwave-irradiated silicon carbide (SiC) vials; and at c) different power levels but the same temperature by using forced cooling. No significant differences in the reaction kinetics are observed using any of these methods. However, microwave heating still proves advantageous as it requires 30 % less forward power compared to conventional heating (SiC vial) to achieve the same furfural yield at a laboratory scale