Chiral objects are abundant in nature, and although the enantiomers have almost identical physical properties apart from their handedness, they can exhibit significantly different chemical properties and biological functions. This underscores the importance of sorting chiral substances. In this Letter, we demonstrate that chirality-sorting optical force pairs can be inversely generated in a tightly focused Gaussian beam by tailoring the input polarization state. We provide a detailed method for constructing the polarization state of the incident light to create the desired chiral optical field that generates the chirality-sorting optical force pairs. These force pairs precisely trap two opposite enantiomers at distinct predetermined positions within the same equilibrium plane, enabling their simultaneous identification and separation. Notably, the trapping positions and separation distances can be freely adjusted by altering the incident polarization parameters.@en

Since the invention of optical tweezers, optical manipulation has advanced significantly in many applications, including atomic physics, biochemistry and soft matter physics. Here, we propose a method to trap metallic particles with adjustable trapping range in the transverse plane with the help of customized field. By tailoring the polarization state of the incident field, the focal field with elongation in the direction perpendicular to optical axis can be turned in the 4π focusing system. As a result, optical trapping with tunable trapping range is possible when the metallic particle is interacted with such customized field.

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Nonzero transverse spin density, which describes phenomenon in which the electromagnetic fields of localized light spin in a plane containing its wavenumber vector, has gained enormous interest recently because of its useful applications like spin-direction coupling and routing. In this Letter, using the Richards–Wolf vectorial method for standard full Poincaré beams, we present an analytical model for the high-numerical-aperture focusing system to calculate all components of the electric and magnetic field strength vectors as well as spin density and Poynting vector. The role and contribution of the optical degrees of freedom including ellipticity, handedness, and orientation when the transverse spin density is present, are revealed based on this analytical model. Ellipticity affects the localization and magnitude of the transverse spin density for both transverse and longitudinal components. In contrast, handedness only affects the longitudinal component whereas orientation only affects the transverse component. Furthermore, the energy flux in the focal plane are also studied in detail for the standard full Poincaré beams. These findings may be help in spin-controlled directive coupling and optical tweezers.

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Carrying orbital angular momentum per photon, the optical vortex has elicited widespread interest. Here, we demonstrate that dual coaxial longitudinal polarization vortices can appear upon a nonparaxial propagation of a tightly focused Pancharatnam-Berry tailored Laguerre-Gaussian beam. Most importantly, it is capable of accessing arbitrary independent topological charges for both vortices, as well as predesigned tunable spacing distances between them.

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Nonzero transverse energy flow, which describes phenomenon in which the energy flux of localized light propagates in a plane perpendicular to the optical axis, has attracted enormous interest recently due to its useful application in micromanipulation. We show that the appearance of transverse energy flow in the focal plane of an aplanatic high numerical aperture focusing system is possible. We demonstrate our approach by specially tailoring the input state of polarization. Calculations reveal that number of transverse energy flow rings is controllable and depend on azimuthal index of the input field, thereby giving rise to tunable manipulating locations in optical trapping.

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As an intrinsic attribute of light, the spin angular momentum (SAM) of photons has aroused considerable attention because of the fascinating properties emerging from light–matter interactions. We show that a diffraction-limited focal field with a steerable photonic spin structure in three dimensions can be produced under a 4π microscopic system. This is achieved by focusing two counter-propagating configurable vector beams produced in the coherent superposition of three different beams with x-polarization, y-polarization, and radial-polarization. By altering the amplitude factors of these resultant beams, the ratios between the three mutually orthogonal polarized components can be freely tuned within the focal plane, thereby allowing dynamic control over the spin orientation and ellipticity of the tightly focused optical field. The results demonstrated in this paper may find applications in spin-controlled nanophotonics.

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With dual two-dimensional Airy-like waveforms, we demonstrate the creation of highly confined electromagnetic fields in the transverse plane and circular or elliptical propagation trajectories in the longitudinal plane by using specially designed Pancharatnam-Berry (PB) phases. Applying the Richards and Wolf vectorial diffraction methods, the explicit expressions are obtained to calculate the strength vectors and energy flux of the three-dimensional electromagnetic fields. Calculations reveal that the nanointerferometric structures of such highly confined fields highly depend on the indexes γ₁ and γ₂ determining the PB phase, thereby enabling the engineering of highly confined fields with tunable size, spacing, and propagation trajectories.

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We show that elongating a tightly focused field in the direction perpendicular to the optical axis is possible. We demonstrate our approach by specially shaping the Pancharatnam–Berry (PB) phase. Moreover, the analytical formulae required to calculate the strength vectors and energy flux of the three-dimensional electromagnetic fields near the focus of an aplanatic optical system are derived using the Richards and Wolf vectorial diffraction methods. Calculations reveal that the transverse enhancement is controllable and depend on the phase index in the PB phase, thereby giving rise to a focus with tunable length and subwavelength width in the focal plane.

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Optical angular momenta (AM) have attracted tremendous research interest in recent years. In this paper we theoretically investigate the electromagnetic field and angular momentum properties of tightly focused arbitrary cylindrical vortex vector (CVV) input beams. An absorptive particle is placed in focused CVV fields to analyze the optical torques. The spin-orbit motions of the particle can be predicted and controlled when the influences of different parameters, such as the topological charge, the polarization and the initial phases, are taken into account. These findings will be helpful in optical beam shaping, optical spin-orbit interaction and practical optical manipulation.

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Using the Richards and Wolf formulas for an arbitrary cylindrical vector (CV) beam, we obtain explicit expressions for all components of the electric and magnetic field strength vectors near the focus, as well as expression for the energy flux in an aplanatic optical system. Based on such analytical models, it reveals that the intensity pattern of the magnetic field at the focus can be tailored by appropriately adjusting the initial phase, peak-centered, doughnut, and flat-topped magnetic fields can be achieved using this method. For the energy flows, in contrast, they are almost the same for arbitrary CV beams, which exhibit hollow shaped patterns for both the transverse and longitudinal components. Unlike the longitudinal component, however, the hollow shaped pattern of the transverse component is separated into two regions, arriving from the null transverse energy flow in the focal plane. Besides, the directions of the transverse energy flow are reversed between these two regions, which are directed inwardly and outwardly along the radial direction, respectively.

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We propose a simple method to control the move and elongation of focus along the optical axis in a high-numerical aperture focusing system. By introducing the optical degree of freedom of polarization in the radial direction, a tunable focal shift and elongation of focus are achieved simply by tailoring the polarization index and the topological charge of vortex phase, without the need of additional modulations of amplitude or diffractive optical elements. These findings may be of help in the applications like optical micro-manipulation, laser processing, and imaging.

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The tip-enhanced Raman spectroscopy (TERS) provides a non-destructive and label-free molecular detection approach with high sensitivity due to the hotspot excited at tip apex, which depends crucially on the tip parameters. Here, we employed an Au cladded atomic force microscopy probe to investigate enhancement effect of the Au claddings, both in theories and experiments. The simulation optimized a highest enhancement with Au cladding thickness of 50 nm under given experimental condition, being consistent with the experimental results. It provides a practical guide to optimize the probes to improve the TERS sensitivity and exert potentials in further TERS applications.

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