The project deals with optical properties of droplets and particles of “complex” shapes, and whose sizes are beyond a few visible light wavelengths. In this domain, rigorous theories and existing numerical methods cannot be applied to accurately describe light scattering phenomena. We propose a new methodology to calculate the interaction of light with such objects, in parallel to a series of experimental tests for validation. The project includes novel applications in optical metrology, and in optical trapping and manipulation of non spherical particles.

Complex-shaped particles (CSP)  are everywhere present in fluid mechanics problems (multiphase flows, sprays, aerosols..), chemical engineering and life science. Attempts to characterize particles in flows exploit their far-field light scattering properties (laser diffractometers, phase Doppler interferometers and particle imaging techniques are standard). Light scattering is also the source of the radiation pressure acting on a particle in a laser beam. The involved forces and torques make possible laser trapping (optical tweezers) and contact-less manipulation, a technique of ever growing importance in biophysics and micro technologies.

Modeling the interaction of light with particles is essential. Many theories and models (scattering, absorption, and radiation pressure) have been developed accordingly. Rigorous methods are limited (for theoretical or numerical reasons) to simple shape particles, i.e. spheres and cylinders, and then cannot deal with CSPs. Different numerical methods such as T-matrix, DDA , MoM and FDTD allow calculating the scattering properties of arbitrarily shaped particles, but their applicability is limited to sizes not more than a few tens of wavelengths, even with supercomputers. There is currently no accurate method to predict the scattering properties of CSPs of sizes larger than a few tens of microns! This is the crux of AMO-COPS project: developing a novel model for large CSPs.

Ray tracing, or geometrical optics, is flexible in terms of particle shapes. However ray models completely or partially neglect wave effects in general and contributions of high order rays. Recently, Partner 1 has successfully introduced wave properties in the ray model and developed a mathematical formalism that allows describing wave front curvatures and phase shifts due to focal lines. This approach, called “Vectorial Complex Ray Model” (VCRM), permits to compute precisely the scattering of a wave by large CSPs of smooth surfaces. VCRM has been applied to 2D scattering of ellipsoidal particles and elliptical cylinders. As a further contribution, the promoters of this project have proposed methods to include forward diffraction by Heisenberg’s uncertainty principle and near-critical-angle scattering effects in the model. The central goal of the project is to offer a generalized version of these works, to be cast into a general “Ray Theory of Wave” (RTW).

Objectives of the project are:  (1) Extension of VCRM to 3D CSPs and for various shaped beams; (2) Modeling of wave effects; (3) Prediction of radiation pressure forces and torques for CFPs; (4) Theoretical and experimental validation of RTW; (5) Application to optical characterization instruments and experimental tests on sprays and bubbly flows; (6) Manipulation and trapping of non spherical particles.

The deliverables of AMO-COPS project will be computation software for prediction of optical forces, scattering properties of CFPs, and simulation of experimental characterization tools. We also anticipate providing various original theoretical and experimental results. PhD students will be trained to research throughout the project. Special attention will be paid to valorization through publications, software licenses and patents. Looking forward to the future, we expect potential applications of RTW well beyond the particular systems to be investigated in this 4-year program.