Enantioselective manipulation of single chiral nanoparticles using optical tweezers

Enantioselective transport of chiral spheres using focused femtosecond laser pulses

Optical tweezers are commonly used for manipulating chiral particles by tailoring the properties of the electromagnetic field or of the particles themselves. Non-linearity provides additional degree of freedom to control the manipulation by changing the trapping conditions. In this work, we leverage the nonlinear optical properties of a medium by illuminating it with a circularly polarized laser pulse, enabling single particle enantioselection for the chiral spheres immersed in it. By adjusting the power of the laser pulses, we demonstrate stable trapping of chiral spheres with one handedness near the focal region, while spheres with the opposite handedness are repelled. This enables the chiral resolution of racemic mixtures. Additionally, we perturbed the stable equilibrium position of the trap by driving the sample stage, leading to the emergence of a new stable equilibrium position achieved under the action of the Stokes force. Here we show that the chirality of each individually trapped particle can also be characterized by the rotation of the equilibrium position. Since the power of the laser pulses can be experimentally controlled, this scheme is practical to perform enantioselection, chiral characterization, and chiral resolution of a single chiral sphere with arbitrarily small chirality parameters.

Ultrafast chirality: the road to efficient chiral measurements

Today we are witnessing the electric-dipole revolution in chiral measurements. Here we reflect on its lessons and outcomes, such as the perspective on chiral measurements using the complementary principles of "chiral reagent" and "chiral observer", the hierarchy of scalar, vectorial and tensorial enantio-sensitive observables, the new properties of the chiro-optical response in the ultrafast and non-linear domains, and the geometrical magnetism associated with the chiral response in photoionization. The electric-dipole revolution is a landmark event. It has opened routes to extremely efficient enantio-discrimination with a family of new methods. These methods are governed by the same principles but work in vastly different regimes - from microwaves to optical light; they address all molecular degrees of freedom - electronic, vibrational and rotational, and use flexible detection schemes, i.e. detecting photons or electrons, making them applicable to different chiral phases, from gases to liquids to amorphous solids. The electric-dipole revolution has also enabled enantio-sensitive manipulation of chiral molecules with light. This manipulation includes exciting and controlling ultrafast helical currents in vibronic states of chiral molecules, enantio-sensitive control of populations in electronic, vibronic and rotational molecular states, and opens the way to efficient enantio-separation and enantio-sensitive trapping of chiral molecules. The word "perspective" has two meanings: an "outlook" and a "point of view". In this perspective article, we have tried to cover both meanings.

Implementation of flexible virtual microchannels based on optically induced dielectrophoresis

Micro-nano particle manipulation methods in liquid environments have been widely used in the fields such as medicine, biology and material science. Nevertheless, the methods usually rely on pre-prepared physical microfluidic channels. In this work, virtual electrodes based on the optically induced dielectrophoresis (ODEP) method were used as virtual microchannels instead of traditional physical microfluidic channels. Virtual microchannels with different shapes were implemented by the designs of projected light patterns, which made the virtual microchannels have great flexibility and controllability. The theory of ODEP was verified by simulation and analysis of electric field distributions. The relationship between the manipulation force and the alternating current (AC) voltage or the AC frequency exerted on the cells was assessed. The experimental results indicated that the manipulation force was increased with the increase of the AC voltage, and it was reduced with the increase of the AC frequency. Moreover, different virtual microchannels were designed to carry out the transportation, aggregation and sorting of yeast cells and rat basophilic leukemia cells (RBL-2H3 cells) and the survival rate of the cells was evaluated. This work shows that the virtual microchannels can be flexibly realized by ODEP in liquid environments.

Precise measurement of trapping and manipulation properties of focused fractional vortex beams

Fractional vortex beams (FVBs) were believed to be hard to rotate microparticles at a half-integer topological charge due to the unique radial opening (low-intensity gap) in their intensity ring. However, recent research discovered more symmetric intensity structures with less intensity inhomogeneity of practical FVBs at the focal plane. Here, we experimentally demonstrated the manipulation of trapped microparticles and precisely measured their rotation periods at the focal plane of practical FVBs by using a high-speed camera. We verified that the measured orbital angular momentum (OAM) derived from the collective microparticle rotation is roughly proportional to the fractional OAM of practical FVBs. Furthermore, we also experimentally obtained the trapped microparticles' power spectra under the illumination of FVBs, from which we achieved the average trap stiffness to evaluate the two-dimensional trapping strength of the practical focused FVB intensity ring. Our results provide a new insight and an efficient tool on finely trapping and rotating microparticles and bio-cells by using fractional vortex beams.

Selective trapping of chiral nanoparticles via vector Lissajous beams

We report selective trapping of chiral nanoparticles via vector Lissajous beams. Local optical chirality densities appear in these beams by properly choosing the values of two parameters (p,q) that determine the polarization vectors of light. For a particular set of parameter (p,q) = (2,1) which is found preferable for the selective trapping, the resulting vector beam has two dominant intensity spots with opposite chirality. In the transverse plane, one spot traps a chiral particle while the other one repels the same particle under appropriate conditions, which can be reversed for a particle of opposite chirality. Various chiral parameters and radii of a particle are considered for analyzing this selective trapping effect. The longitudinal forces that are found non-conservative are also discussed. The achieved functionality of identifying and separating different chiral particles may find applications in enantiomer separation and drug delivery in pharmaceutics.

Tunable Anomalous Scattering and Negative Asymmetry Parameter in a Gain-Functionalized Low Refractive Index Sphere

Usually, low refractive index passive spheres exhibit strong forward scattering and a positive asymmetry parameter due to weak interference between the electric and magnetic scattering channels. In this work, we investigate, analytically and numerically, the forward scattering of light by a gain-functionalized low refractive index dielectric sphere. It is shown that by tuning the optical gain one can optimize the interference, which provides a novel paradigm to achieve the zero forward scattering and negative asymmetry parameter even for a low refractive index sphere. As a result, a low-density collection of such identical back scatterers provides an anomalous regime, where the scattering mean free path and extinction mean free path are greater than the transport mean free path. Furthermore, we also provide the numerical guideline to achieve the larger extinction mean free path without achieving preferential back-scattering.

Optical separation and discrimination of chiral particles by vector beams with orbital angular momentum

Chirality describes a reduced symmetry and abounds in nature. The handedness-dependent response usually occurs only when a chiral object interacts with another chiral entity. Light carrying orbital angular momentum (OAM) is inherently chiral due to the helical wave front. Here, we put forward a scheme that enables optical separation and simultaneous discrimination of single chiral particles using focused vector beams with OAM. Such focused vector vortex beams carrying radial-splitting optical chirality can selectively trap one enantiomer inside or outside the intensity maxima depending on the sign of the OAM. The particles with different chirality parameters can be trapped on different orbits and experience enhanced orbital motion. Moreover, the magnitude of OAM as well as the size of particle plays an important role in the chiral separation and discrimination. In addition to particle manipulation, the discussion of OAM in chiral light-matter interactions has potential application in, for example, optical enantioseparation or chiral detection.

Plasmonic elliptical nanoholes for chiroptical analysis and enantioselective optical trapping

A simple yet effective achiral platform using elliptical nanoholes for chiroptical analysis is demonstrated. Under linearly polarized excitation, an elliptical nanohole in a thin gold film can generate a localized chiral optical field for chiroptical analysis and simultaneously serve as a near-field optical trap to capture dielectric and plasmonic nanospheres. In particular, the trapping potential is enantioselective for dielectric nanospheres, i.e., the hole traps or repels the dielectric nanoparticles depending on the sample chirality. For plasmonic nanospheres, the trapping potential well is much deeper than that for dielectric particles, rendering the enantioselectivity less pronounced. This platform is suitable for chiral analysis with nanoparticle-based solid-state extraction and pre-concentration. Compared to plasmonic chiroptical sensing using chiral structures or circularly polarized light, elliptical nanoholes are a simple and effective platform, which is expected to have a relatively low background because chiroptical noise from the structure or chiral species outside the nanohole is minimized. The use of linearly polarized excitation also makes the platform easily compatible with a commercial optical microscope.

Enantioselection and chiral sorting of single microspheres using optical pulling forces

We put forward a novel, twofold scheme that enables, at the same time, all-optical enantioselection and sorting of single multipolar chiral microspheres based on optical pulling forces exerted by two non-collinear, non-structured, circularly polarized light sources. Our chiral resolution method can be externally controlled by varying the angle between their incident wavevectors, allowing for fine-tuning of the range of chiral indices for enantioselection. Enantioselectivity is achieved by choosing angles such that only particles with the same handedness of the light sources are pulled. This proposal allows one to achieve all-optical sorting of chiral microspheres with arbitrarily small chiral parameters, thus outperforming current optical methods.

Theory of optical tweezing of dielectric microspheres in chiral host media and its applications

We report for the first time the theory of optical tweezers of spherical dielectric particles embedded in a chiral medium. We develop a partial-wave (Mie) expansion to calculate the optical force acting on a dielectric microsphere illuminated by a circularly-polarized, highly focused laser beam. When choosing a polarization with the same handedness of the medium, the axial trap stability is improved, thus allowing for tweezing of high-refractive-index particles. When the particle is displaced off-axis by an external force, its equilibrium position is rotated around the optical axis by the mechanical effect of an optical torque. Both the optical torque and the angle of rotation are greatly enhanced in the presence of a chiral host medium when considering radii a few times larger than the wavelength. In this range, the angle of rotation depends strongly on the microsphere radius and the chirality parameter of the host medium, opening the way for a quantitative characterization of both parameters. Measurable angles are predicted even in the case of naturally occurring chiral solutes, allowing for a novel all-optical method to locally probe the chiral response at the nanoscale.

Generation of controllable chiral optical fields by vector beams

Chirality is common in nature, describing not only the geometrical property of a three-dimensional object, but also an intrinsic feature of an optical field. Chiral optical fields are attracting increasing attention due to their potential applications in chiral light-matter interaction. Here we demonstrate a strategy to realize a controllable chiral optical field by tightly focusing two tailored vector beams in a 4π optical microscopic system. By modulating the wavefronts of the incident vector beams with appropriately designed phase masks, a chiral optical field with multiple spots carrying switchable handedness or controllable chirality can be generated. The location, the number and the handedness of such chiral spots can be arbitrarily adjusted depending on the actual application requirements. In addition to trapping and manipulating multiple particles, this controllable chiral optical field may find applications in enantioselective separation, chiral detection and chiral sensing at the nanoscale.