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.

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.

Photopatterning of Azobenzene-Containing Liquid Crystalline Triblock Copolymers: Light-Induced Anisotropy and Photostabilization.

Photochromic liquid crystalline block copolymers (PLCBCs) are currently playing a significant role as light-responsive materials because of their light controllable features over multiple length scales. Herein, a study of the photoinduced optical anisotropy derived by the combination of orientation phenomena at molecular and supramolecular levels in a novel kind of side-chain PLCBCs with mesogenic phenyl benzoate groups and pyridine units that is hydrogen bonded with azobenzene-containing phenol is reported. Based on the polymeric architectures and composition, the supramolecular configuration self-organizes in different microphases that affect the material response to the external stimuli. Simple, 1D, polarization holograms are recorded to evaluate the photoinduced birefringence. The first step, light patterning, involves the orientation of the azobenzene units and precedes a thermal treatment that amplifies the induced anisotropy through the cooperative orientation of the mesogenic units. By selective extraction, the azobenzene units can be removed, making the material transparent to the visible light. Excellent photostability of the material birefringence is obtained, whose final value is strongly affected by the block copolymer's architecture. The versatility in the molecular design, the fine control of the photoinduced features by external parameters, and, finally, the possibility to achieve photostability make these materials of great potential for developing optical and photonic devices.

Mixed emulsion of liquid crystal microresonators: towards white laser systems.

Microdroplet systems have attracted great interest because of their wide range of applications, easiness in processing and handling, feasibility in developing miniaturized devices with high performances and large flexibility. In this study, a stable emulsion based on different dye-doped chiral liquid crystal droplets has been engineered in order to achieve simultaneous omnidirectional lasing at different wavelengths. To obtain the mixed emulsion of dye doped Bragg onion-type microresonators the twofold action, as a surfactant and a droplet stabilizer, of the polyvinyl alcohol dissolved in water has been exploited. Multiple wavelengths lasing in all directions around the mixed emulsion is demonstrated. By water evaporation, a plastic sheet including different types of chiral droplets is also obtained, retaining all the emission characteristic of the precursor emulsion. A relevant feature is the large flexibility of the preparation method that enables an easy and full control of the lasing spectrum addressing white laser systems. However, the simplicity of the procedure based on a single-step process as well as the high stability of the mixed emulsion is a relevant result, envisaging strong potentiality for developing easy and friendly technologies useful in the field of identification, sensing, imaging, coating and lab-on-a-chip architectures.

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.

Liquid crystal microlens arrays recorded by polarization holography.

We report the characterization of diffractive microlens arrays (MAs) using a polarization holographic approach assisted by a spatial light modulator (SLM), in a nematic liquid crystal (NLC) cell. The MAs were recorded in the photoaligning substrates of the cell and then replicated in the NLC bulk, through the surface interactions. The transparency of the NLC on a wide range of wavelengths and the ability to tune its optical birefringence, through an external voltage, allowed us to create MAs with high efficiency. We have presented the results obtained for diverse MAs configurations, composed by spherical and cylindrical microlenses and characterized by different focal lengths. The efficiency reaches a value of 90%, at a wavelength of 633 nm.

Polarization holograms allow highly efficient generation of complex light beams.

We report a viable method to generate complex beams, such as the non-diffracting Bessel and Weber beams, which relies on the encoding of amplitude information, in addition to phase and polarization, using polarization holography. The holograms are recorded in polarization sensitive films by the interference of a reference plane wave with a tailored complex beam, having orthogonal circular polarizations. The high efficiency, the intrinsic achromaticity and the simplicity of use of the polarization holograms make them competitive with respect to existing methods and attractive for several applications. Theoretical analysis, based on the Jones formalism, and experimental results are shown.

Pure two-dimensional polarization patterns for holographic recording.

Two-dimensional (2D) polarization patterns are achieved by the interference of two pairs of beams with perpendicular planes of incidence and orthogonal polarizations (i.e. linear or circular). In both cases, imposing a phase shift of π/2 between consecutive beams contains the amplitude modulation of the optical field in the superposition region and, thus, pure 2D polarization patterns are created. The recording of these interference fields in a polarization-sensitive material, namely an amorphous azopolymer, creates reconfigurable 2D periodic microstructures with peculiar diffraction properties.

Single-step polarization holographic method for programmable microlens arrays.

Optical microsystems have become important tools for imaging, optofluidics, and sensor applications. Here we show a versatile method to create microlens arrays (MAs) exploiting spatial-light-modulator-assisted polarization holography, which enables an efficiency of diffraction up to 100%. We demonstrate the large flexibility of the proposed approach by codifying mixed MAs, i.e., composed of spherical and cylindrical lenses with different focal lengths, either positive or negative. Reconfigurable MAs with 70% total diffraction efficiency have been recorded on a photosensitive polymer that exhibits linear photoinduced birefringence with long time stability as well as optical and thermal reversibility. The good quality of the MA has been shown by a digital holographic test.

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.

Polarization gradient: exploring an original route for optical trapping and manipulation.

We report a study of the capabilities of an optical tweezer based on polarization gradient. We use a light polarization pattern that is able to simultaneously exert forces and torques in opposite directions depending on the particle's position. It allows to perform oscillatory displacements and control the sense of rotation of several particles inside a uniformly illuminated region. Unconventional trapping of spinning particles in circularly polarized fringes has been observed, which suggests the involvement of hydrodynamic forces.

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).

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.

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.

Polarization holograms in a bifunctional amorphous polymer exhibiting equal values of photoinduced linear and circular birefringences.

Light-controlled molecular alignment is a flexible and useful strategy introducing novelty in the fields of mechanics, self-organized structuring, mass transport, optics, and photonics and addressing the development of smart optical devices. Azobenzene-containing polymers are well-known photocontrollable materials with large and reversible photoinduced optical anisotropies. The vectorial holography applied to these materials enables peculiar optical devices whose properties strongly depend on the relative values of the photoinduced birefringences. Here is reported a polarization holographic recording based on the interference of two waves with orthogonal linear polarization on a bifunctional amorphous polymer that, exceptionally, exhibits equal values of linear and circular birefringence. The peculiar photoresponse of the material coupled with the holographic technique demonstrates an optical device capable of decomposing the light into a set of orthogonally polarized linear components. The holographic structures are theoretically described by the Jones matrices method and experimentally investigated.

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.

Polarization holographic recording in amorphous polymer with photoinduced linear and circular birefringence.

Polarization grating recording in an amorphous and nonchiral azo copolymer has been investigated. The reported study shows that the amorphous polymeric film undergoes a light-guided inhomogeneous supramolecular modification as a consequence of the illumination with proper polarized light patterns, acquiring new functionalities. Both linear and circular, spatially modulated, photoinduced birefringences occur, attaining their peak values in the linearly and circularly polarized regions of the light pattern, respectively. The photoinduced anisotropic structures strongly affect the polarization state of the light propagating through them, and the characterization of their optical diffraction enables measurement of the amplitude of the linear and circular birefringences. The recorded gratings show long-time stability and full reconfigurability functional to the multiple holographic recording.

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.