Invited Speakers

Matthew Berg, Kansas State University, USA.

Imaging Aerosol Particles with Digital Holography

Light-scattering techniques are commonly used to characterize micron and submicron particulate matter, such as aerosols and colloids, in a rapid and contact-free way. A typical objective is to infer the size and shape of particles from light-scattering patterns. While this approach can be useful for simple shapes, difficulties arise for large or irregularly shaped particles. An alternative well-suited for many types of particles is digital holography. Here, a particle is illuminated by a beam and the interference pattern produced by unscattered and scattered light is recorded on a sensor. The pattern is the hologram, and from it, an image of the particle can be unambiguously reconstructed revealing particle size and shape. If the holography is carried out with red, green, and blue lasers simultaneously, color images of the particles are possible as well, which can aid particle-material classification. We have applied the method to a variety of aerosols, in both the laboratory and the outdoor environment with a drone-based instrument. The talk will give an overview of our work in this area and describe how we are advancing the capabilities of digital holography in ongoing research.

Lei Bi, Zhejiang University, China.

Light Scattering by Non-spherical and Inhomogeneous Particles: Advances in Computational Methods and Their Applications in Atmospheric Radiation and Remote Sensing

The study of light scattering by non-spherical and inhomogeneous particles is essential for advancing both atmospheric science and remote sensing. This talk presents research activities focused on investigating the optical properties of atmospheric ice clouds and aerosol particles, including dust, black carbon, and sea salt, which are often non-spherical and exhibit inhomogeneous structures. The work has involved the development and application of advanced computational methods, such as the invariant embedding T-matrix (IITM) technique, physical-geometric optics approaches, and the integration of Debye series into T-matrix frameworks to model scattering processes. Additionally, machine learning techniques have been employed to enhance computational efficiency and accuracy in predicting optical properties of these particles. These methodologies address critical challenges in atmospheric science and remote sensing studies by enabling more accurate modeling of particle optics and radiative transfer. The research provides new insights into how the morphological properties of particles influence their optical characteristics, with significant implications for weather and climate simulations, radar observation modeling, and satellite-based data interpretation. Furthermore, this talk briefly overviews a comprehensive database and related public research tools that facilitate applications of particle scattering in weather-climate models, remote sensing, and data assimilation studies.

Kyle Daun, University of Waterloo, Canada.

Laser-induced incandescence on metal nanoparticles

While laser-induced incandescence is mainly used as a tool for characterizing soot in combustion-related applications, it is increasingly applied to other gas-particle systems, including metal nanoparticles. These measurements may be used to develop and control gas-phase synthesis processes for producing industrial quantities of metal nanoparticles, explore laser-nanoparticle and gas-surface interactions, and characterise the thermophysical properties of metals at extreme temperatures. However, LII signals from metal nanoparticles often have features that cannot be explained using conventional models, which undermine the reliability of quantities derived from LII data analysis. Although these features have been attributed to non-incandescent laser-induced emissions that contaminate the LII signal, recent work suggests the inapplicability of standard assumptions and simplifications used to derive the measurement model to be a more likely culprit, including the misapplication of the Rayleigh approximation and laser-induced changes to the particle morphology. While these results apply to metal nanoparticles, they inspire a re-assessment as to whether similar assumptions and simplifications could also explain commonly observed artifacts in LII data collected on soot.

Nikolai Khlebtsov, Russian Academy of Sciences.

Fabrication, characterization, and biomedical application of gold and hybrid plasmonic nanoparticles

 Recent advances in the fabrication and biomolecular functionalization of plasmonic gold and composite nanoparticles have led to a dramatic expansion of their potential biomedical applications [1] (simultaneously with growing biosafety concerns [2]), including biosensors, bioimaging, photothermal therapy, and delivery of target molecules through cellular nanoparticle uptake [3]. Multifunctional nanocomposites that combine therapeutic, diagnostic, and sensing modalities in a single nanostructure are widely used in a new field of nanobiotechnology called theranostics. This key lecture is divided into three parts. First, we present a Gallery of plasmonic nanoparticles and nanocomposites currently fabricated at the Lab of Nanobiotechnology at IBPPM RAS. The Gallery consists of the 20 most popular particle shapes and structures. It covers UV-vis-NIR localized plasmon resonances (LPR) from 400 to 1100 nm that can be tuned for a desired laser wavelength, tissue transparency window, maximal absorption or elastic light scattering, strong surface-enhanced Raman scattering (SERS), or bright test line color in the lateral flow immunoassay (LFIA). To exemplify advanced fabrication possibility, we demonstrate (i) TEM images of gold particles with an almost ideal spherical shape, (ii) fine LPR tuning (within several nanometers) for gold nanorods, (iii) gap-enhanced Raman tags (GERTs) and (iv) fabrication of dried powder of PEG-coated gold nanorods. In GERTs, Raman molecules are protected from desorption and subjected to a strongly enhanced electromagnetic field in the gap. Their SERS response does not depend on the environmental conditions and aggregation. The AuNR@PEG powders can be obtained with a standard lyophilization method, stored in powder form indefinitely, and transformed to water colloid within several seconds. 

         Further, in the second part, we consider two examples of optical characterization of plasmonic nanomaterials. First, we show how uncritical routine use of the dynamic light scattering method can lead to artifacts in the measured size distributions of common citrate colloidal gold nanoparticles. Absorption at 400 nm is a convenient, cheap, and fast method for in situ determination of Au concentration in colloids, even in the presence of Au(+3) ions and other interference factors. We demonstrate the universality of the UV-vis absorption method with six experimental and theoretical models, including nanospheres, nanosphere clusters, nanorods, nanostars, thin nanotriangles, and nanoplates. From theoretical simulations, we derived a straightforward and helpful relation [Au⁰](mM) = 0.44×A₄₀₀, confirmed by correlation of UV-vis spectroscopy, adsorption atomic spectroscopy, and ICP-MS data for 34 experimental samples examined. 

         To illustrate biomedical applications, we selected three examples. First, we consider gold nanoisland films as reproducible substrates for the detection of fungicides. Then we consider a plasmonic optoporation system that can be used for precisely controlled, high-performance laser transfection compatible with broad types of cells ("easy-to-transfect" and "hard-to-transfect cells) and delivered objects of interest such as small molecular dyes propidium iodide, control plasmids, 3-10 kb; and fluorophore-labeled dextranes,10-100 kDa. Finally, we discuss the application of multifunctional nanocomposites based on fluorescent Au nanoclusters (25-30 Au atoms) stabilized by BSA and functionalized with targeting antibodies and photodynamic dye as well as glutathione-stabilized fluorescent atomic nanoclusters for detection of bacterial biofilms.

Fengshan Liu, National Research Council, Canada.

Absorption properties and the mechanism of laser-induced graphitization of carbonaceous nanoparticles

Carbonaceous nanoparticles have drawn increasing research attention in the last few decades. On one hand, they are a particulate pollutant and cause various health issues and a major climate forcer due to their strong light absorption ability in the visible spectral range. On the other hand, they have been widely used as valuable nanomaterials in electronics, chemical industry, and battery. These particles possess different physical, chemical, and optical properties, depending strongly on their compositions and the internal structure. It has been known that their internal structure can be altered by pulsed lasers. Pulsed laser-induced incandescence (pLII) has been widely used as a powerful diagnostic

technique for in-situ probing of various properties of carbonaceous nanoparticles, such as the volume fraction, a mean particle size, and specific surface area of flame-generated soot and carbon black particles through time-resolved signal detection. Several phenomena related to the optical properties of carbonaceous nanoparticles, such as the strong size-dependent mass absorption coefficient (MAC), the strong laser fluence dependence of pLII measured soot volume fraction, and the mechanism of laser-induced graphitization of young soot, remain unresolved. The optical properties of carbonaceous nanoparticles depend on the so-called degree of maturity, which is a term to describe the level of graphitization, and are also subject to quantum confinement when the particle size is less tan about 20 nm.

It has been well-known that at sufficiently high fluences, a single laser pulse of either 532 nm or 1064 nm can strongly modify the internal structure and cause the formation of hollow interior and ribbon-like graphitic layers at the periphery of primary soot particles without or with mass loss, depending on the laser fluence. However, the mechanisms and time scale over which the laser-induced graphitization occurs have not been adequately understood. Although thermal annealing has been commonly accepted as the mechanism responsible for laser-induced graphitization of soot, photon-induced graphitization has also been proposed to play a role in pulsed laser graphitization of soot and other materials. The majority of experimental studies of pulsed laser irradiation effects on soot particles provided no temporal information, which is critically important to assess the role of phone-induced graphitization. The failure of thermal annealing models in the prediction of double-pulse LII experiments of young soot provides indirect evidence to support the notion that thermal annealing is likely not the dominant mechanism of pulsed laser-induced graphitization of young soot. Laser-induced graphitization of carbonaceous nanoparticles remains largely unexplored.

Physical processes affecting the size dependent absorption properties and laser-induced graphitization of carbonaceous nanoparticles are discussed. These processes include the quantum confinement, thermodynamics, and the role and mechanism of photon-induced graphitization in pLII measurements of young soot. Relevant literature results of experimental and numerical studies are reviewed and some new results will also be presented to offer new insights into the currently unresolved phenomena in laser diagnostics of carbonaceous nanoparticles.

Jiangqi Shen, University of Shanghai for Science and Technology, China.

Evaluation of the beam shape coefficients based on a scalar description: comparison in remodeling effects

The evaluation of the beam shape coefficients (BSCs) for the structured beam is crucial in light-scattering based applications. When the light beam is arbitrarily located, the formulation of the BSCs is quite complicated, requiring heavily algebraic work and careful coding in numerical calculation. The task may become simple, when the relation between the electromagnetic (EM) field and a scalar potential function is used. The spherical wave expansion of the EM field can be replaced with that of the potential function so that the EM-BSCs can be obtained as a superposition of the scalar BSCs. This reduces the analytical deduction and the numerical calculation by nearly half. While dealing with the spherical wave expansion of the scalar potential function, the translation addition theorem may be employed and hence only the scalar BSCs for a light beam which is centered at the origin of the coordinate system is required. This further simplifies the formulation of the BSCs greatly and speeds up the numerical calculation dramatically. In formulating the scalar BSCs for the beam centered at the coordinates, all the methods in the framework of the generalized Lorenz-Mie theory, including the quadrature, the localized approximation, the finite series technique, the radial quadrature, and the angular spectrum decomposition method, may be easily used. The different methods remodel the light beam in their own way. In this presentation, we shall make a comparison between the methods, together with some discussion on the modeling effects.