Gérard Gouesbet
email: gouesbet@coria.fr

Fabrice Onofri

Maciej Kolwas

Daniel Jakubczyk

Preface. Laser-light and Interactions with Particles (LIP), 2020

Keywords: T-matrix methods, Optical Particle Characterization, Laser beams, Beam shape coe¢ cients, Laser beam scattering and propagation, Arbitrary shaped particles

. The 2020 conference was scheduled to be held in Warsaw, Poland, but has been postponed to 2022 due to the Covid pandemic. It has been decided, nevertheless, to publish another virtual issue of JQSRT to limit the e¤ects of the impact of the pandemic and stick to a tradition as much as possible.

LIP conferences were in the …liation of a series of conferences which started under the name of "Optical Particle Sizing, OPS, theory and practice" in Rouen,

France, 1987, essentially devoted to optical measurements, using laser sources, of sizes of particles embedded in ‡ows. Later on, the development of the …eld required some extensions toward the measurement and/or the characterization of other particle properties, such as shapes, temperatures, refractive indices or chemical compositions, so that the OPS conference was extended to an OPC (Optical Particle Characterization) conference.

After Rouen conference in 1987, OPS and afterward OPC conferences were held in Phoenix, USA, 1990 (organized by D. Hirleman), Yokohama, Japan, 1993 (M. Maeda), Nürnberg, Germany, 1995 (F. Durst), Minneapolis, USA, 1998 (A. Naqwi), Brighton, England, 2001 (A. Jones), Kyoto, Japan, 2004 (M. Itoh), and Graz, Austria, 2007 (O. Glätter). The delay between two successive conferences has usually been three years, but for one case, namely Nürnberg, 1995, adjusted to make the conference coinciding with Partec 1995.

Further developments such as involving morphology-dependent resonances, mechanical e¤ects of lights, the use of di¤erent kinds of structured beam shapes including laser pulses, the development of new experimental techniques for optical particle characterization, and of theoretical approaches to deal with the scattering of arbitrary shaped beams by arbitrary shaped particles, required a new extension of the conference scope. Hence, it has been decided to reformat the conference and to start a new series, actually an OPC follow-up, the 9th of the series, in Rouen, 2012, under the title "Laser-light and Interactions with Particles", to be abbreviated as LIP2012, and chaired by G. Gouesbet and G. Gréhan who actually started the series 25 years ago. LIP2012 has been followed by LIP2014, in Marseille, France, under the chair of F.R.A. Onofri and B. Stout and by LIP2016, in Xi'An, China, under the chair of Yiping Han and Lixin Guo. In 2018, the 12th "Laser-light and Interactions with Particles" conference (LIP2018) took place at the Texas A&M University, College Station, USA, under the chairs of P. Yang, G. Kattawar and E. Fry, from 4th to 9th March 2018 jointly with the "Electromagnetic and Light Scattering by nonspherical particles" conference ELSXVII, see [START_REF] Yang | Preface: Electromagnetic and light scattering by nonspherical particles XVII[END_REF]. The present preface for the 2020 "noshown" conference compiles and reviews 23 peer-reviewed papers, providing a signi…cant sample of the issues usually discussed during LIP-conferences. Authors and readers are kindly asked to forgive the authors of the present preface for unavoidable biases resulting from their particular limited expertise. Papers are essentially ordered from more fundamental topics to more applied topics, following the organization of recent reviews, e.g. the contents of [START_REF] Gouesbet | T-matrix methods for electromagnetic structured beams: A commented reference database for the period 2014-2018[END_REF].

We begin with a paper devoted to the study of photonic wheels carried out in the framework of generalized Lorenz-Mie theory, i.e. GLMT in short, by Orlov and Berskys [START_REF] Orlov | Generalized Lorenz-Mie theory of photonic wheels[END_REF]. We recall that the GLMT stricto sensu deals with the interaction between an arbitrary shaped beam and a homogeneous sphere characterized by its diameter and its complex refractive index [START_REF] Gouesbet | Combustion measurements[END_REF], [START_REF] Gouesbet | Generalized Lorenz-Mie theories, second edition[END_REF] and references therein dating back to [START_REF] Gouesbet | Sur la généralisation de la théorie de Lorenz-Mie[END_REF]. The expression GLMT is used as well, in a generic way, for theories which describe the interaction between arbitrary shaped beams and regular particles which allow one to solve Maxwell's equations by using the method of separation of variables. In [START_REF] Orlov | Generalized Lorenz-Mie theory of photonic wheels[END_REF], the author deals with a GLMT for illuminating electromagnetic …elds which carry both longitudinal and transverse angular momentum, called photonic wheels, applied to the interaction with gold and silicon spheres. They also employ a multiple scattering method to simulate a chiral cluster of nanoparticles -speci…cally a chiral assembly of three particles -in the focal plane of a photonic wheel. It is worthwhile to mention that the authors correctly state that the GLMT is a genuine T-matrix method, avoiding the misleading identi…cation between T-matrix and EBCM (Extended Boundary Condition Method). We also have to recall that, in GLMT approaches, the illuminating beam is encoded in a set of coe¢ cients, named beam shape coe¢ cients (BSCs in short), usually denoted g m n;T M and g m n;T E , in which T M stands for "Transverse Magnetic" and T E for "Transverse Electric", with n ranging from 1 to 1, and m from ( n) to (+n). Equivalently, the authors deal with BSCs denoted A mn and B mn which are evaluated by using a quadrature method under the form of one-dimensional integrals.

Next paper by Zhang et al. [START_REF] Zhang | Scattering of a non-paraxial Bessel pincer light-sheet by a dielectric sphere of arbitrary size[END_REF] deals as well with GLMT stricto sensu , i.e. scatterers are (in particular) spherical, but the illumination is achieved by using non-paraxial Bessel pincer light-sheet depending on two parameters, namely the order l of the Bessel beam and a scaling parameter 0 , and having autofocusing and self-bending properties. The electric …eld of the beam is expanded using an angular spectrum of decomposition (ASD, see subsection 2.6 of [START_REF] Van De Hulst | Essay: A review on generalized Lorenz-Mie theories with wow stories and epistemological discussion[END_REF]) and the BSCs are expressed in terms of one-dimensional integrals. Numerical results, obtained by using a Python program, concern the distribution of …eld intensities (internal, near-surface …eld intensities, intensities of the incident light, of the scattered light, and of the total -incident plus scattered -light) and the e¢ ciency factors (for extinction and scattering and, by substraction for absorption), paying attention to the in ‡uence of the beam parameters and of the size parameter of the scatterer.

Next paper by Shi et al. [START_REF] Shi | Scattering of a higher-order Bessel Gaussian beam by a spherical marine aerosol[END_REF] deals as well with the GLMT but the illuminating beams are high-order vector Bessel Gaussian beams (which are nondi¤racting beams) and the scatterers are formed by a spherical marine aerosol, sensitive to the humidity of the surrounding medium, characterized by its diameter and complex refractive index, therefore satisfying the conditions of applicability of the GLMT stricto sensu. Linear (x and y), circular (left and right), azimuthal and radial polarizations are considered. It must be recalled that high-order Bessel beams are not physical because they carry an in…nite amount of energy and that Bessel Gaussian beams (or Bessel-Gauss beams) are Bessel beams apodized by a Gaussian envelope, therefore satisfying the constraint of …nite energy. BSCs are evaluated by using an ASD in terms of one-dimensional integrals. The e¤ects of various parameters (of the incident beam, and of the aerosol) on the normalized di¤erential scattering cross-sections (DSCS) are investigated. From the results obtained, it is concluded that it is not feasible to retrieve the relative humidity of aerosol environment by using the di¤erences of normalized DSCS for various polarization states, although DSCS may be highly in ‡uenced by the parameters of the incident beam and of the illuminated aerosol.

The paper by Cha…q and Belafhal [START_REF] Cha…q | Optical Fourier transform of pseudo-nondi¤racting beams[END_REF] deals as well with nondi¤racting beams made physical by using a two-dimensional Gaussian envelope, named pseudo-nondi¤racting beams (pNDBs) because, in contrast with the case of the unphysical Bessel beams, they are actually di¤racting. The aim of the paper is to calculate the Fourier transform of pNDBs in the focal plane of a converging thin lens, dealing with the electric …elds of the beams expressed using cylindrical coordinates (the Fourier transform is actually expressed as the electric …eld in the focal plane of the thin lens). The propagation in free space, after the focal plane, of the beams so generated is studied using the Huygens-Fresnel di¤raction integral, and illustrated numerically. Various kinds of pNDBs, known as Cosine-Gauss beams, Bessel-Gauss beams, Mathieu-Gauss beams, Parabolic-Gauss beams and Lommel-Gauss beams, are investigated under paraxial approximations. It is observed that the nondi¤racting property of the original beams is conserved in the …rst stage of propagation, and afterward dramatically a¤ected at the end of propagation because of the presence of the Gaussian envelope which represents a di¤raction beam (i.e. characterized by a beam waist radius).

The evaluation of BSCs is a major ingredient of the use of any GLMT and a strong e¤ort has been devoted to the development of various techniques to the purpose of such evaluations, e.g. subsection 2.5 in a recent review [START_REF] Van De Hulst | Essay: A review on generalized Lorenz-Mie theories with wow stories and epistemological discussion[END_REF]. The most popular method may have been the use of localized approximations (with several variants), see review in [START_REF] Gouesbet | Generalized Lorenz-Mie theories and description of electromagnetic arbitrary shaped beams: localized approximations and localized beam models, a review[END_REF], and complements in [START_REF] Wang | Note on the use of localized beam models for light scattering theories in spherical coordinates[END_REF], [START_REF] Gouesbet | Second modi…ed localized approximation for use in generalized Lorenz-Mie theories and other theories revisited[END_REF], [START_REF] Gouesbet | Comments on localized and integral localized approximations in spherical coordinates[END_REF]. A justi…cation of the use of localized approximations for "arbitrary shaped beam" illumination has been demonstrated in 1999 [START_REF] Gouesbet | Validity of the localized approximation for arbitrary shaped beams in generalized Lorenz-Mie theory for spheres[END_REF]. However, it has been demonstrated as well that this justi…cation does not hold for two classes of beams (i) "axionic" beams whose description contains a propagation term of the form exp(ikz cos ), in which is the axicon angle, in contrast with the more "usual" form exp(ikz), such as Bessel beams, e.g. [START_REF] Gouesbet | On the validity of localized approximations for Bessel beams: all N-Bessel beams are identically equal to zero[END_REF], [START_REF] Ambrosio | Assessing the validity of the localized approximation for discrete superposition of Bessel beams[END_REF] and references therein and (ii) helical beams possessing a topological charge, such as Laguerre-Gauss beams, e.g. [START_REF] Gouesbet | On the validity of the use of a localized approximation for helical beams. I. Formal aspects[END_REF], [START_REF] Ambrosio | On the validity of the use of a localized approximation for helical beams. II. Numerical aspects[END_REF], [START_REF] Ambrosio | On localized approximations for Laguerre-Gauss beams focused by a lens[END_REF]. As a result, another technique known as the …nite series technique, invented in 1988 [START_REF] Gouesbet | Computations of the g n coe¢cients in the generalized Lorenz-Mie theory using three di¤erent methods[END_REF], [START_REF] Gouesbet | Expressions to compute the coe¢ cients g m n in the generalized Lorenz-Mie theory, using …nite series[END_REF] and given up the same year due to the success of localized approximations techniques, was born again in 2019 in the case of Laguerre-Gauss beams freely propagating [START_REF] Gouesbet | Finite series expressions to evaluate the beam shape coe¢ cients of a Laguerre-Gauss beam freely propagating[END_REF], or focused by a lens [START_REF] Gouesbet | Finite series expressions to evaluate the beam shape coe¢ cients of a Laguerre-Gauss beam focused by a lens in an on-axis con…guration[END_REF]. The paper by Votto et al. summarized in the present preface [START_REF] Votto | Finite series algorithm design for lens-focused Laguerre-Gauss beams in the generalized Lorenz-Mie theory[END_REF] implements the …nite series technique for Laguerre-Gauss beams focused by a lens, complements another paper [START_REF] Votto | Evaluation of beam shape coe¢ cients of paraxial Laguerre-Gauss beam freely propagating by using three remodeling methods[END_REF] devoted to the case of Laguerre-Gauss beams freely propagating, and constitutes some kind of conclusion in the series told in the present paragraph.

The previous papers of the special issue have been dealing with scatterers being spheres or assemblies of spheres, more generally with the treatment of Maxwell's equations in spherical coordinates. With a paper by Chen et al. [START_REF] Chen | Electromagnetic beam propagating through a gyrotropic anisotropic circular cylinder[END_REF], the special issue is pursued with the use of cylindrical coordinates for a study devoted to the electromagnetic beam propagation through a gyrotropic anisotropic circular cylinder. The theoretical approach relies on the expansion of the scattered and internal electromagnetic …elds in terms of appropriate cylindrical wave functions and the use of boundary conditions, with a theoretical structure akin to the one of a GLMT for cylinders. However, as stated by the authors, they used a projection method which, "compared to the GLMT for cylinders", relies on the knowledge of "an explicit description of the incident EM beam, either in terms of coordinates or expansions over CVWFs" (Cylindrical Vector Wave Functions), meaning that the evaluation of BSCs (cylindrical BSCs) is not necessarily required. If the incident …elds are expanded over CVWFs, the projection method takes the form of a GLMT for cylinders. Otherwise, the projection method is a semi-analytical solution in which integrals related to the incident …elds have to be evaluated numerically. Various kinds of illuminating beams are considered, namely Gaussian beam in the mode T EM 00 , Hermite-Gaussian beam and zeroth-order Bessel beam, and the wave/particle interaction is described in terms of normalized …eld intensity distributions. In particular, in the case of an incident Gaussian beam, the …elds are described by using a thirdorder Davis beam (which does not exactly satisfy Maxwell's equations), e.g. [START_REF] Davis | Theory of electromagnetic beams[END_REF], [START_REF] Barton | Fifth-order corrected electromagnetic …eld components for fundamental Gaussian beams[END_REF], [START_REF] Gouesbet | Partial wave representations of laser beams for use in light scattering calculations[END_REF], agreeing well however with the description in terms of a localized beam (which does exactly satisfy Maxwell's equations). Propagation inside the gyrotropic anisotropic circular cylinder leads to a distortion of the beam and to a splitting e¤ect due to the birefringence property of a gyrotropic medium.

The next paper by Hricha et al. [START_REF] Hricha | Propagation properties of vortex cosine-hyperbolic-Gaussian beams in strongly nonlocal nonlinear media[END_REF] deals as well with propagation properties in "exotic" media, here in strongly nonlocal nonlinear media (SNNM). The propagating beam is a vortex cosine-hyperbolic-Gaussian (vChGB) beam which is a beam carrying angular momentum, with a spiraling wavefront implying a dark hollow pro…le associated to a topological charge. The authors took advantage of the fact that the characteristic length of the response function in a SNNM is very large compared to the beam width and the wave equation, which is a nonlocal nonlinear Schrödinger equation, can be simpli…ed to a linear model which is afterward reduced to the paraxial Helmholtz equation. The propagation formula of the vChGB, the beam width and the curvature radius of the wavefront are obtained in closed-form. These beam characteristics evolve periodically during propagation with a period which depends on the initial beam power. Depending on this power, the beam can steadily propagate in SNNM or adopt a soliton-like behavior.

Sekulic et al. [START_REF] Sekulic | T-matrix method for calculation of second-harmonic generation in terms of clusters of spherical particles[END_REF] deals as well with nonlinear e¤ects, numerically studying second-harmonic generation from clusters of arbitrarily distributed homogeneous centrosymmetric and isotropic spherical nanoparticles. They used a T-matrix method in which the electromagnetic …elds at the fundamental and second-harmonic frequencies are expanded in series of vector spherical wave functions (VSWFs) and the single sphere T-matrix entries are computed by imposing …eld boundary conditions at the surface of the particles. The illuminating beam is a monochromatic electromagnetic plane wave (so that the BSCs are "trivial") and the nonlinear polarization density responsible for the secondharmonic generation is governed by a third-rank nonlinear susceptibility tensor. The T-matrix method used, which allows one to relate the incident and scattered electromagnetic …elds, is speci…cally the Extended Boundary Condition Method, EBCM in short (recall however that EBCM is not the only T-matrix method available, e.g. [START_REF] Gouesbet | T-matrix formulation and generalized Lorenz-Mie theories in spherical coordinates[END_REF], [START_REF] Gouesbet | On the electromagnetic scattering of arbitrary shaped beams by arbitrary shaped particles: A review[END_REF]). Numerical examples to compute the scattered …eld and the scattering and absorption cross-sections, both at the fundamental frequency and at the second harmonic frequency, are provided for several generic examples (with nanospheres made of gold, or silicon), and the numerical results obtained are validated by comparing with those of a commercial software based on the …nite-element method.

With Zhang et al. [START_REF] Zhang | Scattering of evanescent wave generated by total re ‡ection[END_REF], we consider the scattering of evanescent waves which are generated by total re ‡ection as an electromagnetic wave propagates from an optically denser medium to an optically thinner one. The optically thin medium is then the support of an evanescent wave which is scattered by a particle located in this medium. The scattering is described by using an exact semi-analytical method in which the scattered and internal …elds are expanded in terms of VSWFs, using a projection technique already mentioned above in the paper by Chen et al. [START_REF] Chen | Electromagnetic beam propagating through a gyrotropic anisotropic circular cylinder[END_REF], requiring the evaluation of surface integrals. A speci…c di¢ culty in the problem at stake is that the …eld scattered by the particle will be re ‡ected by the interface between the dense and the thin media, requiring to take account for the scattered-re ‡ected …elds. The theoretical approach is applied to discuss the far …eld scattering of various kinds of scatterers, expressed by using the di¤erential scattering cross-section (DSCS), namely a polystyrene sphere, or a polystyrene prolate spheroid, and a polystyrene circular cylinder of …nite length, deposited at the interface, as the sum of two contributions, one from the scattered …eld and the other from the scattered-re ‡ected …eld. It is found that, contrary to the evanescent wave that decays fast, the presence of a particle can transfer a portion of the electromagnetic energy to the radiation zone, so that the particle actually functions as an antenna. It is also found that the spherical particle radiates more energy than the circular cylinder, and the circular cylinder more energy than the prolate spheroid.

Beside analytical (e.g. GLMTs) and semi-analytical methods (e.g. EBCM, projection method), light scattering may be handled as well by using numerical methods, for instance …nite-di¤erence time-domain (FDTD) method and …niteelement method (FEM) which are alternative methods to solve Maxwell's equations. In FDTD, Maxwell's equations are discretized with the computational domain being meshed by a cubic grid, and the method can been implemented by using a commercial software package. Both FDTD method and FEM have been used by Geints [START_REF] Geints | Numerical study of photothermal e¤ect in core-shell microcapsules[END_REF] to carry out a numerical study of photothermal e¤ect in core-shell microcapsules. More speci…cally, the author presented a systematic study of optical absorption and laser-induced heating of dielectric microcapsules with water …lling and light absorbing polymer shell, such particles simulating transport cargo microcontainers which are the basis of modern drug delivering systems which can signi…cantly increase therapeutic e¤ects while minimizing side e¤ects. The numerical calculations of optical …eld spatial distribution inside and near the microcapsules are carried out, and the temporal dynamics of temperature pro…les of core-shell microparticles of various morphological types (spheres, ellipsoids, boxes, cubes, cylinders) and microcapsule clusters in di¤erent con…gurations is addressed, for a near-IR radiation at = 800 nm (which is the fundamental harmonic of a titanium-sapphire laser) radiation. Depending on the situations investigated, the illumination was the one of a linearly polarized plane wave or of a pulsed optical radiation.

Core-shell microstructures, more speci…cally core-shell magneto-plasmonic nanostructures incorporating Ni, Ag, Fe and Au chemical elements, with in mind the same applications to drug delivery than in the aforementioned paper, are studied by Bhatia et al. [START_REF] Bhatia | Optical absorption analysis of core-shell type Ni@Ag and NiFe@Ag/Au magneto-plasmonic nanostructures[END_REF]. They however use another numerical method which is the discrete dipole approximation (DDA), in which the particle is represented by an assembly of discrete coupled (interacting) electric-dipoles located on a cubic lattice spanning the particle volume. Spherical and prolate geometries are investigated in order to characterize the behavior of localized surface plasmon resonance (LSPR). It is found that this behavior strongly depends on the geometry of the nanoparticle, aspect ratio (when relevant), core sizes and shell thickness. Furthermore, there is a high tunability of LSPR peak position as well as larger enhancement in the absorption e¢ ciency when the spherical nanoparticle is stretched to prolate core-shell con…guration. The publicly available code DDSCAT written in FORTRAN 90 is used for DDA calculations, and calculations are carried out in the range from 300 to 1100 nm.

DDA is used as well in a work by Argentin et al. [START_REF] Argentin | Assessing the limits of Rayleigh-Debye-Gans theory: Phasor analysis of a bisphere[END_REF], relying -as in the paper above -on a publicly available code DDSCAT, to investigate and understand empirically observed deviations between accurate calculations of forward scattering and absorption cross-sections (using DDA) with those predicted by the Rayleigh-Debye-Gans (RDG) theory. This is carried out by investigating the internal electric …eld within bi-spherical nanoparticles under various con…gurations, illuminated by an incoming linearly polarized plane wave, allowing one to emphasize that the internal …elds may exhibit signi…cantly di¤erent amplitudes from one monomer to the other, in contrast with the usual RDG-assumption that internal …elds in monomers are uniform. The analysis is carried out by using the phasor concept relating contributions to the far-…eld scattered wave from local internal …eld at a volume element in a particle, leading possibly to semigraphical representations, allowing one to understand how particle morphologies in ‡uence the scattering behavior, using an in-house Python code post-processing the data retrieved from preliminary DDSCAT runs. The technique used aims to the visualization of slices through the particles in order to reveal which parts of them cause under-or over-estimation of the e¤ective scattering with respect to the idealistic RDG-scattering.

Among the various applications of light scattering theories, the study and use of mechanical e¤ects of light o¤ers a large panorama of opportunities made famous by Arthur Ashkin (2018 Nobel prize of physics). Since his large body of works, in particular concerning the invention of optical tweezers -see [START_REF] Ashkin | Optical trapping and manipulations of neutral particles using lasers: A reprint volume with commentaries[END_REF] for a volume of reprints -, a huge number of papers has been devoted to mechanical e¤ects of light, in particular for optical levitation, trapping and manipulation of particles. A review with 284 references has recently been devoted to the applications of GLMTs to mechanical e¤ects of light [START_REF] Gouesbet | Generalized Lorenz-Mie theories and mechanical e¤ects of laser light, on the occasion of Arthur Ashkin's receipt ot the 2018 Nobel prize in physics for his pioneering work in optical levitation and manipulation: A review[END_REF]. However, up to recently, there has not been any systematic study of optical forces using GLMT in the Rayleigh regime, a particular appealing situation because the choice of working in the limit of point-like particles makes the formalism simpli…ed to easily handled expressions, allowing one to interpret the physical mechanisms at hand.

A …rst article in this framework has been devoted to the study of longitudinal optical forces exerted by o¤-axis Bessel beams in the Rayleigh regime of GLMT [START_REF] Gouesbet | Gradient, scattering and other kinds of longitudinal optical forces exerted by o¤-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory[END_REF]. The present special issue has been the opportunity to present a similar study, but devoted to transverse optical forces rather than to longitudinal optical forces [START_REF] Gouesbet | Axicon optical forces and other kinds of transverse optical forces exerted by o¤-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory[END_REF]. An important question has then been raised to know whether the Rayleigh limit of GLMT does identify or not with the dipole theory of forces (more speci…cally with the Rayleigh limit of the dipole theory of forces). Relying on numerical studies [START_REF] Ambrosio | On longitudinal radiation pressure crosssections in the generalized Lorenz-Mie theory and their numerical relationship with the dipole theory of forces[END_REF], [START_REF] Ambrosio | On transverse radiation pressure crosssections in the generalized Lorenz-Mie theory and their numerical relationships with the dipole theory of forces[END_REF] and on formal studies [START_REF] Ambrosio | On the Rayleigh limit of the generalized Lorenz-Mie theory and its formal indenti…cation with the dipole theory of forces. I. The longitudinal case[END_REF], [START_REF] Ambrosio | On the Raileigh limit of the generalized Lorenz-Mie theory and its formal identi…cation with the dipole theory of forces. II. The transverse case[END_REF], the answer has been found to be positive. However, the dipole theory of forces expresses forces in terms of the total electric …eld E. Conversely, the Rayleigh limit of GLMT demonstrates that all the partial waves of the electric …eld, from n = 3 to 1 actually have a null contribution, in such a way that only partial waves of order 1 and 2 do contribute to optical forces. These forces (catalogued as gradient, scattering and non-standard forces) can then be expressed in terms of a few BSCs g m n;T M and g m n;T E with n = 1 and 2: Therefore, the Rayleigh limit of GLMT, although equivalent to the Rayleigh limit of the dipole theory of forces, nevertheless expresses a "reduction" of the dipole theory of forces to a few relevant contributions. This approach has been applied to on-axis Bessel beams [START_REF] Gouesbet | Optical forces exerted by on-axis Bessel beams on Rayleigh particles in the framework of the generalized Lorenz-Mie theory[END_REF] and, more generally, to on-axis non dark axisymmetric beams of the …rst kind [START_REF] Gouesbet | Rayleigh limit of generalized Lorenz-Mie theory for on-axis beams and its relationship with the dipole theory of forces. Part I: Non-dark axisymmetric beams of the …rst kind, with the example of Gaussian breams[END_REF].

With a paper by Yang et al. [START_REF] Yang | Trapping two types of Rayleigh particles simultaneously by a focused rotational elliptical Laguerre-Gaussian correlated Schell-model beam[END_REF], we go on with optical forces exerted on Rayleigh particles, with gradient and scattering forces considered in the framework of the classical Rayleigh limit of the dipole theory of forces. The illuminating beam is a focused rotational elliptical Laguerre-Gaussian correlated Schell-model beam. Such a beam is a Laguerre-Gauss Schell model beam (which exhibits a spatial coherence which is only partial) extended by introducing elliptical symmetry into the degree of coherence. It is used to trap small dielectric particles of di¤erent refractive indices (speci…cally glass spheres and bubbles of diameter 50 nm, in water) and it is found that, for a judicious choice of the beam parameters, the particles with higher relative refractive index can be trapped along an elliptical ring centered at the focus, while, at the same time, the particles with lower refractive index are trapped at the focal point. Relying on the comparison between gradient, scattering, Brownian and gravity forces acting on the particles, an analysis of the trapping stability is carried out.

The next paper by Ambrosio [START_REF] Ambrosio | Photophoresis in the slip- ‡ow and free molecular regimes for arbitrary-index particles[END_REF] deals with the examination of other kinds of forces, which can have an amplitude orders of magnitude greater than gradient and scattering optical forces, namely photophoresis forces exerted on low-loss particles of the order of a few micrometers under plane wave or uniform illumination. Such thermal forces, which can be attractive or repulsive, arise from the uneven heating of the particle surface under illumination over a hemisphere. Di¤erent regimes may be considered (i) a free molecular regime when the Knudsen number Kn>>1, i.e. when the particle is much smaller than the mean free path of the gas molecules, so that photophoretic forces can be investigated using kinetic theory of gases, (ii) a slip- ‡ow regime for Kn<1, and (iii) a continuous regime when Kn<<1. The applied force then depends on the asymmetry factor which involves an integration of the normalized absorbed light intensity over the volume of the particle. The author then considered particles with arbitrary index of refraction and analytically investigated photophoretic forces in terms of the asymmetry factor, for both slip ‡ow and free molecular regimes. He made use of the Mie theory for lossy and homogeneous spheres with arbitrary size parameters, and obtained the photophoretic forces as analytic closed-form solutions in the case of low-loss spherical particles under plane wave illumination. Examples of the behavior of the asymmetric factor for micro-spheres with distinct materials are provided and discussed, for all optical regimes, i.e. for Rayleigh, Mie and geometrical regimes. The author states that he is planning to extend this work to the case of non-uniform illumination by using GLMT instead of the classical Mie theory.

An application of optical tweezers used by Donato et al. [START_REF] Donato | Improved backscattering detection in photonic force microscopy near dielectric surfaces with cylindrical vector beams[END_REF] was devoted to photonic force microscopy (PFM) where an optically trapped probe in liquid is scanned over a sample (possibly a three-dimensional environment) to collect both morphology and forces sensing detection through its thermal ‡uctuations, i.e. by recording an interference pattern which ‡uctuates due to the ‡uctuations of the position of the particle induced by Brownian motion. In the present case, to avoid the requirement of using a transparent sample chamber, the authors relied on a backscattering con…guration in which the sample is illuminated by light and the back pattern is detected by a quadrant photodiode (QVD) whose voltage signals are converted in 3D particle displacement in the trap. Noisy modulations, due to the interference of backscattered and back-re ‡ected light (from nearby chamber surfaces), detrimental for the experimental procedure, are limited or even completely removed by the use of cylindrical vector beams (CVBs) when compared to the use of Gaussian beams, depending on whether radially or azimuthal CVBs are used. The apparatus allows one to measure trap spring constants and particle ‡uctuations. Calculations of light scattered by the trapped particle in forward and backward directions, as well as the optical forces for particles of any shape and composition, are obtained by T-matrix methods (e.g. EBCM) which, as mentioned by the authors, reduce to GLMT for spherical particles. The incident focused illuminating …elds are expanded using an ASD and each plane wave can in turn be expanded into multipoles having their own BSCs (this is indeed one of the ways to use ASD to deal with scattering, see subsection 2.6 of [START_REF] Van De Hulst | Essay: A review on generalized Lorenz-Mie theories with wow stories and epistemological discussion[END_REF]).

Optical trapping has indeed become a famous …eld of research with many applications. It is however worth recalling that another active …eld of research concerns trapping by acoustical waves, a …eld which presents many analogies with the one of electromagnetic trapping, if only because the theoretical description of the set-up may require the use of "acoustical BSCs". Such analogies are exhibited in a review by Thomas et al. [START_REF] Thomas | Acoustical and optical radiation pressures and the development of single beam acoustical tweezers[END_REF] who very comprehensively discussed acoustical and optical radiation pressures and the development of single beam acoustical tweezers, with a strong emphasis on the mutual enrichment which arose from a long common history between electromagnetic and acoustical scattering.

However, this should not lead us to forget a more classical method in which particles are trapped in a quadrupole ion trap in high vacuum. Such a trapping technique has been used by Coppock et al. [START_REF] Coppock | High temperature measurements of levitated Au nanospheres derived from Au suspensions[END_REF] for preparing highly charged Au nanospheres, and heat them beyond the melting point. The charge and mass of the nanospheres can then be determined from the e¤ect of discrete discharging on their charge to mass ratio. Also, nanosphere temperatures can be estimated by mass loss from Au thermal evaporation. It is furthermore possible to observe that contaminants in the vacuum can have a profound e¤ect on the thermal and optical properties of the nanospheres near their melting point. Oxygen can be used to prevent such contaminations, likely due to carbon, to avoid to spoil the accuracy of measurements.

For another study involving a non-optical trapping/levitating system, we may consider trapping in a 3D electrodynamic trap, allowing one to levitate a charged particle with a combination of AC and DC electric …elds, which has been used by Kolwas et al. [61] to study the evolution of mass, surface layer composition and light scattering of evaporating single microdroplets of sodium dodecyl sulphate/diethylene glycol mixture illuminated by laser beams. Such droplets exhibit a surfactant surface monolayer which is compressed during evaporation due to the shrinking of surface area, leading to a possible transition from a simple monolayer, named Langmuir monolayer, to a behavior in which the saturated monolayer must fold/buckle or even break above a critical concentration, therefore generating a transition from a 2D monolayer to a 3D structure. Surface area evolution are obtained from measurements of the droplet mass with electrostatic weighting, while morphology/structure dependent resonances (known as MDRs or WGMs) manifested in the intensity of scattered light allows one to examine the evolution of the microdroplet surface smoothness. Mie theory is used to calibrate radii of droplets at the beginning of evolution with the aid of Mie Scattering Look-up Table Method (MSLTM).

Light extinction spectroscopy (LES) is another optical characterization technique to measure the particle size distributions (PSDs) and concentrations of spherical to complex shaped aggregates or irregular nano-to microparticles, e.g. [START_REF] Su | Optimization of regularization parameters of inversion in particle sizing using light extinction method[END_REF], [START_REF] Onofri | On the optical characterisation of nanoparticle and their aggregates in plasma systems[END_REF], [START_REF] Onofri | Sizing highly-ordered buckyball-shaped aggregates of colloidal nanoparticles by light extinction spectroscopy[END_REF], [START_REF] Barbosa | An introduction to light extinction spectrometry as a diagnostic for dust particle characterization in dusty plasmas[END_REF], [START_REF] Ceolato | Aerosol light extinction and backscattering: A review with a lidar perspective[END_REF]. It is used by Erenen et al. [START_REF] Eneren | Light extinction spectroscopy applied to polystryrene colloids: Sensitivity to complex refractive index uncertainties and to noise[END_REF] to measure PSDs, more speci…cally concentration and volumetric PSDs of colloids and nano ‡uids, with a particular attention paid to the issues of sensitivity to complex refractive index uncertainties and to noise. The LES technique analyzes the variations in the transmittance of a collimated poly-chromatic light beam passing through a medium containing an ensemble of particles. For a given wavelength of the poly-chromatic illuminating beam, and under some restrictions such as the absence of dependent scattering -produced by near-…eld optical interactions between particles -or of multiple scattering e¤ects, the light extinction for homogeneous, isotropic, non-magnetic spheres, is analytically modeled by the Mie theory in an expression incorporating a normalized PSD function in number, from which it is possible to retrieve a PSD in weight if the particle density is known. This requires a careful inversion procedure which has been selected as being the Tikhonov regularization procedure also known as penalized least square method. Experiments are carried out using polystyrene latex beads commonly employed for the calibration of light extinction based instruments to generate mono-modal or bi-modal colloidal suspensions in water. These beads are illuminated by a highly stabilized UV light source and with the transmitted light being recorded by a spectrometer highly sensitive in the UV range and o¤ering a high signal-to-noise ration, before being fed to a laptop for data inver-sion (these LES experiments are complemented by Dynamic Light Scattering experiments).

Next, although Gaussian beams are the most familiar kinds of beams used in measurements, we have above met papers showing that alternative kinds of beams can be very bene…cial for measurements. The paper by Zhao et al. [START_REF] Zhao | Sub-di¤raction-limit realization and micro-displacement measurements via complex Gaussiancorrelated beam[END_REF] used complex Gaussian-correlated beam to overcome the Rayleigh di¤raction limit (i.e. realizing a sub-di¤raction resolution) and achieve micro-displacement measurements. It is a fact that overcoming the Rayleigh di¤raction limit is a somewhat counter-intuitive but exciting …eld in which the modulation of the properties of illumination sources may be e¢ ciently introduced, in particular the use of partially coherent beam, e.g. Laguerre-Gaussian correlated Schellmodel beams, such as already encountered in the present preface when dealing with [START_REF] Yan | Transverse trapping forces of focused Gaussian beam on ellipsoidal particles[END_REF]. The present paper deals with an improved choice of partially coherent beams named complex Gaussian-correlated beams, which allowed an optimum resolution distance equal to 0.05 times the Rayleigh di¤raction limit when applied to the separation of images of four pinholes in the detection plane. Displacement measurements (requiring calibration) are carried out by using two symmetrical pinholes in the object plane, consistently with the fact that a displacement measurement can be considered as a measurement of two endpoints.

Partially coherent beams as discussed above can furthermore be useful for studying laser propagation in turbulent atmospheres as discussed by Liu et al. [START_REF] Liu | Propagation properties of Laguerre-Gaussian Schell-model beams with a twist phase[END_REF] who deal with the propagation of twisted Laguerre-Gaussian Schell model. Analytical expressions for the cross-spectral density function propagating in free space and in turbulence are derived based on the extended Huygens-Fresnel integral, and the evolution of the spectral density as well of the orbital angular momentum (OAM) are discussed. A choice of parameters allows one to enhance the OAM and, simultaneously, to improve the resistance of the beam to its propagation in a turbulent medium. A purpose of such a work is in particular to increase the capacity of free-space communications.

The propagation of beams in a turbulent atmosphere is also studied, using the Rytov Approximation Theory, by Li et al. [START_REF] Li | Speckle characteristics of vortex beams scattered from rough targets in turbulent atmosphere[END_REF], but using a more conventional Laguerre-Gauss (vortex) beam. More speci…cally, the authors studied the echo speckle characteristics of the light scattering from rough targets in turbulence and compared these speckle characteristics with those produced by Gaussian beams. It was found that, in the weak turbulence regions, the target characteristics are the main in ‡uencing factors of the speckle …elds while, in the strong turbulence regions, the main in ‡uencing factors are the turbulence e¤ects. Furthermore, the e¤ects of these in ‡uencing factors are observed to be less than those of the Gaussian beams. It is concluded that such studies can potentially provide theoretical and technical guidances for applications to laser target detection and recognition, related to speckle measurements processes in space.

Concerning speci…cally weak turbulence regions, Gökçe et al. [START_REF] Gökçe | Adaptive optics e¤ect in performance of BPSK-SIM oceanic optical wireless communication systems with aperture averaging in weak turbulence[END_REF] noticed that turbulence-induced wavefront deformations cause the irradiance of an optical signal to ‡uctuate resulting in a serious degradation of the bit-error rate (BER) performance of optical wireless communication systems. They therefore investigated an adaptive optics aiming to compensate for the wavefront aberrations in order to reduce the ‡ucutations in the received intensity (known as scintillations), knowing that the use of adaptive optics may improve the BER by a few orders of magnitude. They used a still more conventional laser than in the previous paper, namely a Gaussian beam, and studied its propagation in a communication channel which is a weak oceanic turbulence, considering both scintillations and the beam spreading e¤ects causing an additional power loss in the receiver. The adaptive optics technique employed for their study in ocdeanic optical wireless communication (OOWC) takes the form of a sum of spatial …lters. It is found, in particular, that the e¤ective bene…cial e¤ect on BER of the adaptive optics used increases when the turbulence level decreases, and that it becomes more e¢ cient with an enlarged receiver aperture diameter.

Acknowledgments

We would like to thank the authors as well as the anonymous reviewers who have contributed by their work to this …fth special issue on "Laser-light and Interactions with Particles". Our thanks also go to the JQSRT editorial board, Gomathy Tamiselvan, Lisa Li and José Stoop, for the quality of their work and their unwavering support, and obviously to the editors-in-chief, namely Michael I. Mischchenko who have initiated this special issue (see commemoration …le in https://www.giss.nasa.gov/research/news/20200727/, and on line memorial on https://www.youtube.com/watch?v=ag8bts9o_gA), and Pinar Mengüç who have continued the action taken by Mike.

We are now looking forward to the next edition, namely the 13th "Laserlight and Interactions with Particles, Optical Particle Characterization follow-up (LIP2022)", to be organized by the authors of the present preface, in Warsaw, Poland, on August 21-26th, see at www.lip-conferences. Since Science also required technical and …nancial resources, our …nal acknowledgments go to the institutions who are sponsoring LIP-conferences, and most notably to the French National Research Agency [grant number ANR-13-BS090008-02 (AMO-COPS)].

Scienti…c

Committee. W. Bachalo, Artium technologies Inc., CA, Sunnyvale, USA. X.C. Cai, University of Shanghai for Science and Technology, Shanghai, China. Y.P. Han, School of Science, Xidian University, Xi'an, China. G. Gouesbet, CORIA, National Institute of Applied Science, Rouen, France. G. Gréhan, CORIA, National Center for Scienti…c Research, Rouen, France. J.A. Lock, Cleveland State University, Cleveland, OH, USA. O. Maragò, IPCF, National Research Council, Messina, Italy. P.L. Marston, Depart. Physics & Astronomy, Washington State University, Pullman, WA, USA.