Superhybrid Mode-Enhanced Optical Torques on Mie-Resonant Particles.

Circularly polarized light carries spin angular momentum, so it can exert an optical torque on the polarization-anisotropic particle by the spin momentum transfer. Here, we show that giant positive and negative optical torques on Mie-resonant (gain) particles arise from the emergence of superhybrid modes with magnetic multipoles and electric toroidal moments, excited by linearly polarized beams. Anomalous positive and negative torques on particles (doped with judicious amount of dye molecules) are over 800 and 200 times larger than the ordinary lossy counterparts, respectively. Meanwhile, a rotational motor can be configured by switching the s- and p-polarized beams, exhibiting opposite optical torques. These giant and reversed optical torques are unveiled for the first time in the scattering spectrum, paving another avenue toward exploring unprecedented physics of hybrid and superhybrid multipoles in metaoptics and optical manipulations.

Core-shellchiralpolymeric-metallic particles obtained in a single step by concurrentlight induced processes.

Core-shell architecture enables to impart unique customized properties to microparticles, through the proper selection of composition and aggregation state of the inner and outer materials. Here, the synthesis of microparticles with a chiral dielectric core and a metallic shell of gold nanoparticles is demonstrated. The chiral core is obtained by UV induced polymerization of the self-organized droplets of a cholesteric reactive mesogen in a chloroauric acid aqueous solution. Gold nanoparticles precipitation contemporarily occurs upon UV irradiation, covering the microparticles surface. Electron microscopy and optical spectroscopy investigations give evidence that the degree of coverage of the core by gold nanoparticles, with size less than 100 nm, depends on the chloroauric acid concentration, while their aggregation is influenced by the polymeric surface morphology. The optical properties of the chiral microparticles are modified by the gold shell. Specifically, gold coating of dye doped chiral microparticles, working as Bragg onion resonators, clearly improves the stability of omnidirectional microlasers. The proposed strategy, due to the flexibility of the chiral material and of the method, opens a route toward fabrication of microdevices with wide control over light manipulation, in term of intensity, polarization, generation.

Correction: Collective motion of chiral Brownian particles controlled by a circularly-polarized laser beam.

Correction for 'Collective motion of chiral Brownian particles controlled by a circularly-polarized laser beam' by Raúl Josué Hernández et al., Soft Matter, 2020, 16, 7704-7714, DOI: .

Collective motion of chiral Brownian particles controlled by a circularly-polarized laser beam.

We demonstrate the emergence of circular collective motion in a system of spherical light-propelled Brownian particles. Light-propulsion occurs as consequence of the coupling between the chirality of polymeric particles - left (L)- or right (R)-type - and the circularly-polarized light that irradiates them. Irradiation with light that has the same helicity as the particle material leads to a circular cooperative vortical motion between the chiral Brownian particles. In contrast, opposite circular-polarization does not induce such coupling among the particles but only affects their Brownian motion. The mean angular momentum of each particle has a value and sign that distinguishes between chiral activity dynamics and typical Brownian motion. These outcomes have relevant implications for chiral separation technologies and provide new strategies for optical torque tunability in mesoscopic optical array systems, micro- and nanofabrication of light-activated engines with selective control and collective motion.

Chirality-assisted lateral momentum transfer for bidirectional enantioselective separation.

Lateral optical forces induced by linearly polarized laser beams have been predicted to deflect dipolar particles with opposite chiralities toward opposite transversal directions. These "chirality-dependent" forces can offer new possibilities for passive all-optical enantioselective sorting of chiral particles, which is essential to the nanoscience and drug industries. However, previous chiral sorting experiments focused on large particles with diameters in the geometrical-optics regime. Here, we demonstrate, for the first time, the robust sorting of Mie (size ~ wavelength) chiral particles with different handedness at an air-water interface using optical lateral forces induced by a single linearly polarized laser beam. The nontrivial physical interactions underlying these chirality-dependent forces distinctly differ from those predicted for dipolar or geometrical-optics particles. The lateral forces emerge from a complex interplay between the light polarization, lateral momentum enhancement, and out-of-plane light refraction at the particle-water interface. The sign of the lateral force could be reversed by changing the particle size, incident angle, and polarization of the obliquely incident light.

Chiral particles in the dual-beam optical trap.

We investigate the dynamics of chiral microparticles in a dual-beam optical trap. The chiral particles have the structure of spherical chiral microresonators, with a reflectance deriving from the supramolecular helicoidal arrangement. Due to the strong asymmetric response of the particles to light with a specific helicity and wavelength, their trapping position and rotational frequency can be controlled by proper combination of the polarization state of the two light beams. Here symmetric and asymmetric polarization configurations of dual- interfering beam traps have been investigated. Based on the polarization controlled asymmetric transmission of the chiral particles, a tunable wash-board potential is created enabling the control of the trapping position along the beams axis. Asymmetric configurations display polarization controlled rotation of the trapped particles. Optical binding of rotating particles exhibits a complex dynamics.

Attractive-repulsive dynamics on light-responsive chiral microparticles induced by polarized tweezers.

Multifunctional colloidal micro and nano-particles with controlled architectures have very promising properties for applications in bio and nanotechnologies. Here we report on the unique dichotomous dynamical behaviour of chiral spherical microparticles, either fluid or solid, manipulated by polarized optical tweezers. The particles are created using a reactive mesogen mixed with a chiral dopant to form cholesteric liquid crystal droplets in water emulsion. The photopolymerization enables the chiral supramolecular configurations to be frozen in solid particles. Different internal architectures in the supramolecular structures, guided by the interfacial chemistry, enable optically isotropic or anisotropic spherical objects to be obtained. For particles having radial configuration of the cholesteric helices, we show that light can exert either a repulsive or attractive force depending on the handedness of its circular polarization, due to the unique selective reflection property of the cholesteric phase. On the other hand, very exotic dynamics is observed in the case of anisotropic chiral particles. Depending on the light handedness, they behave like Janus spherical particles with dissimilar optical properties, meaning that the surface of the dielectric particles is partly transparent and partly reflecting. We foresee interesting potential applications in micro and optofluidics, microphotonics and materials science.

Chiral self-assembled solid microspheres: a novel multifunctional microphotonic device.

Solid chiral microspheres with unique and multifunctional optical properties are produced from cholesteric liquid crystal-water emulsions using photopolymerization processes. These self-organizing microspheres exhibit different internal configurations of helicoidal structures with radial, conical or cylindrical geometries, depending on the physicochemical characteristics of the precursor liquid crystal emulsion.

Method for artifact-free circular dichroism measurements based on polarization grating.

We propose a simple method to perform real-time measurements of circular dichroism (CD), which suppresses the artifacts introduced by anisotropic samples and nonideal optical elements in conventional spectrometers. A single polarization holographic grating is adopted, whose first orders of diffraction have amplitudes that are proportional to the right and left circular polarization component of the input light. We demonstrate that, exploiting unpolarized white light and the intrinsic spectral selectivity of the grating, the true CD spectrum is evaluated in parallel in the spectral range of interest from the intensities of the two diffraction orders, I(+1) and I(-1).

Spectrograph based on a single diffractive element for real-time measurement of circular dichroism.

In this study, a novel and simple diffractive spectrographic method for real-time measurements of circular dichroism (CD) is considered from a theoretical and experimental approach. A demonstrator prototype of the CD spectrograph has been developed and its performance has been compared with a commercial phase-modulation CD spectrometer. The main element of the device is a polarization holographic grating, recorded in a thin photosensitive organic film, by two interfering opposite circularly polarized beams. A peculiarity of this grating is that the amplitude of the +1 (-1) order of diffraction is proportional to the right (left) circular polarization component of the incoming beam. Here we demonstrate that the CD spectrum of a specimen can be easily evaluated from the intensities of the diffracted beams. A white light beam passing through the specimen is diffracted from the grating and the intensities of the +/-1 orders of diffraction are measured. Due to the spectral selectivity of the grating, the CD at each wavelength can be evaluated at the same time using two linear array detectors.

Photopolarimeter based on two gratings recorded in thin organic films.

A photopolarimeter based on two different kinds of diffraction gratings (a two-grating photopolarimeter) has been developed for real-time measurements of the four elements of the Stokes vector. The main elements of the device are a pure polarization grating and an ordinary transmission grating, both recorded by means of holographic techniques in thin films of organic materials. The first one consists of a diffraction grating recorded by two interfering opposite circularly polarized beams in a Langmuir-Blodgett film of an azo-compound material. The second component is a grating recorded by two interfering parallel circularly polarized beams in a thin film of a photosensitive polymer. Both gratings offer long time stability and good diffraction efficiency. Four photodiodes collect the first-order diffracted beams from these gratings, the output signals of which are read through an analog-to-digital converter by a PC. The optical alignment of the device is easy and the calibration is realized in a one-step procedure.