Expansion of arbitrary electromagnetic fields in terms of vector spherical wave functions

Annular optical trapping of metallic nanoparticles using the azimuthally polarized beam

The unique physical and chemical properties make metallic nanoparticles promising for broad applications in many fields. Exploring the dynamics of metallic nanoparticles in optical traps is crucial for exploiting optical tweezers to advance the applications of metallic particles. In this paper, we present a detailed study of the annular optical trapping of gold nanoparticles with azimuthal polarization. Theoretical analysis based on the T-matrix method shows that the gold nanoparticles experience optical forces pointing to the equilibrium position along the radial direction, while there is no force along the azimuthal direction at this equilibrium position. Therefore, a tightly focused azimuthally polarized beam captures gold nanoparticles in an annular region. Experimental measurements of the motion trajectory of the confined gold nanoparticles reveal a donut profile consistent with the theoretical predictions. Our work reported in this paper is expected to deepen our understanding of the interactions between metallic nanoparticles and light and promote the application of metallic nanoparticles.

Intensity patterns of a focused electromagnetic spherical wave with aberration

The laser pulse focused by a relativistic flying parabolic mirror can exceed the laser intensity focused by conventional physical focusing optics. Depending on the Lorentz γ-factor, the focal length of the relativistic flying mirror in the boosted frame of reference becomes much shorter than the incident beam size. The 4π-spherical focusing scheme is applied to describe such a focused field configuration. In this paper, a theoretical formalism has been developed to describe the field configuration focused by the 4π-spherical focusing scheme with an arbitrary phase error of an incident electromagnetic wave. The focused field configuration is described by the linear combination of the product of the spherical Bessel function and the spherical harmonics, resulting in the same expression as the multipole radiation. The mathematical expression showing the focused field for the femtosecond laser pulse, as well as the continuous wave, has been derived for the application to the femtosecond high-power laser. We show the three-dimensional intensity distribution near focus for the 4π-spherically focused electromagnetic field with phase error.

Optical trapping and light scattering in atmospheric aerosol science

Aerosol particles are ubiquitous in the atmosphere, and currently contribute a large uncertainty to climate models. Part of the endeavour to reduce this uncertainty takes the form of improving our understanding of aerosol at the microphysical level, thus enabling chemical and physical processes to be more accurately represented in larger scale models. In addition to modeling efforts, there is a need to develop new instruments and methodologies to interrogate the physicochemical properties of aerosol. This perspective presents the development, theory, and application of optical trapping, a powerful tool for single particle investigations of aerosol. After providing an overview of the role of aerosol in Earth's atmosphere and the microphysics of these particles, we present a brief history of optical trapping and a more detailed look at its application to aerosol particles. We also compare optical trapping to other single particle techniques. Understanding the interaction of light with single particles is essential for interpreting experimental measurements. In the final part of this perspective, we provide the relevant formalism for understanding both elastic and inelastic light scattering for single particles. The developments discussed here go beyond Mie theory and include both how particle and beam shape affect spectra. Throughout the entirety of this work, we highlight numerous references and examples, mostly from the last decade, of the application of optical trapping to systems that are relevant to the atmospheric aerosol.

Optical trapping force on a stratified chiral particle by high-order Bessel beam

We examined optical trapping force (TF) exerted on non-uniform chiral stratified spheres by a high-order Bessel beam (HOBB). Present theories were proven to be valid by comparison with the existing reference. Numerical simulations considering the effects of various parameters on TF are displayed in detail. The results show that different chirality distributions in stratified chiral sphere will affect significantly the trapping characteristics, and a stable three-dimensional capture can be realized only by selecting the appropriate parameters of incident beam and particles. The theoretical investigations may provide an analytical method to help understand the interaction of light with more complex stratified chiral cells and thus become an encouraging approach to better design an optical manipulation system.

Light Interaction with Cluster Chiral Nanostructures by High-Order Bessel Beam

Interactions between cluster chiral nanoparticles and a high-order Bessel beam (HOBB) with arbitrary illuminations are investigated. The generalized Lorenz–Mie theory (GLMT) is applied to derive the expansions of HOBB. Based on the additional theorem, multiple scattering results of cluster chiral nanoparticles are obtained by taking into account the tangential continuous boundary conditions. The present theory and codes proved to be effective when confronted with the simulations obtained from the Computer Simulation Technology (CST) software. Numerical results concerning the effects of beam order, beam conical angle, incident angles, beam polarization state, the chirality, and the material loss on the scattering of various types of aggregated chiral particles are displayed in detail, including the linearly chiral sphere chain, the chiral cube array, and the complex models composed of aggregated chiral spheres. This study may provide critical support to analytically understand the optical scattering characteristics with aggregated chiral particles of complex shapes, and may find important applications in manipulating collective chiral particles.

Size-dependent optical forces on dielectric microspheres in hollow core photonic crystal fibers

Optical forces on microspheres inside hollow core photonic crystal fibers (HC-PCFs) are often predicted using a ray optics model, which constrains its validity based on wavelength and microsphere sizes. Here, we introduce a rigorous treatment of the electromagnetic forces based on the Lorenz-Mie theory, which involves analytical determination of beam shape coefficients for the optical modes of a HC-PCF. The method is more practicable than numerical approaches and, in contrast with ray optics models, it is not limited by system size parameters. Time of flight measurements of microspheres flying through the HC-PCF lead to results consistent with the Lorenz-Mie predictions.

Lineshape study of optical force spectra on resonant structures

Understanding the frequency spectrum of the optical force is important for controlling and manipulating micro- and nano-scale objects using light. Spectral resonances of these objects can significantly influence the optical force spectrum. In this paper, we develop a theoretical formalism based on the temporal coupled-mode theory that analytically describes the lineshapes of force spectra and their dependencies on resonant scatterers for arbitrary incident wavefronts. We obtain closed-form formulae and discuss the conditions for achieving symmetric as well as asymmetric lineshapes, pertaining, respectively, to a Lorentzian and Fano resonance. The relevance of formalism as a design tool is exemplified for a conceptual scheme of the size-sorting mechanism of small particles, which plays a role in biomedical diagnosis.

Electromagnetic energy in multilayered spherical particles

We obtain exact analytic expressions for (i) the electromagnetic energy radial density within and outside a multilayered sphere and (ii) the total electromagnetic energy stored within its core and each of its shells. Explicit expressions for the special cases of lossless core and shell are also provided. The general solution is based on the compact recursive transfer-matrix method, and its validity includes also magnetic media. The theory is illustrated on examples of electric field enhancement within various metallo-dielectric silica-gold multilayered spheres. The user-friendly MATLAB code, which includes the theoretical treatment, is available as a supplement to the paper.

Calculation of light scattering of an elliptical Gaussian beam by a spherical particle

The use of an elliptical Gaussian beam (EGB) for applications relying on light scattering depends much on the ability to evaluate the beam shape coefficients (BSC) effectively and accurately. Based on the angular spectrum decomposition (ASD) of the radial components of the beam field, we present the formulations of the BSCs for the EGB. Numerical calculations of the BSCs, the beam reconstruction, and light scattering are performed. The fidelity of the reconstructed field to the given one is discussed so as to examine the BSC calculation. A comparison of the ASD method with the quadrature and localized approximation methods leads to the conclusion that the ASD method is much faster than the quadrature method, and it is very powerful for acquiring reliable and accurate results of BSCs, even for far off-axis locations and high ellipticities of the EGB.