Tightly focused optical field with controllable photonic spin orientation

Dynamic control of magnetization spot arrays with three-dimensional orientations

We report a new paradigm for achieving magnetization spot arrays with controllable three-dimensional (3D) orientations. Toward this aim, we subtly design a tailored incident beam containing three parts and further demonstrate that the designed incident beam is phase-modulated radial polarization. Based on the raytracing model under tight focusing condition and the inverse Faraday effect on the magneto-optic (MO) film, the magnetization field components along the y-axis and z-axis directions are generated through the focus. In particular, we are able to garner orientation-tunable 3D magnetization under different numerical apertures of the focusing objectives by adjusting the ratios between the three parts of incident beam. Apart from a single magnetization spot, magnetization spot arrays capable of dynamically controlling 3D orientation in each spot can also be achieved by multi-zone plate (MZP) phase filter. Such a robust magnetization pattern is attributed to not only the constructive interferences of three orthogonal focal field components, but also the position translation of each magnetization spot resulting from shifting phase of the MZP phase filter. It is expected that the research outcomes can be beneficial to spintronics, magnetic encryption and multi-value MO parallelized storage.

Tightly focused light field with controllable pure transverse polarization state at the focus

We report on a facile and flexible scheme for producing the controllable pure transverse polarization state at the focus within a tightly focused field. Toward this aim, a special type of hybrid vector beam exhibiting unusual "8-type" mapping tracks of azimuthal polarization states on the Poincaré sphere is employed. Due to the peculiar polarization structures, at the focus, there is only the transverse component, while the longitudinal component is zero for any 8-type vector beam. More strikingly, the transverse polarization state at the focus is exactly the same as that of the cross point of the 8-type mapping track. Benefiting from this appealing polarization relationship, an arbitrary transverse polarization state can be easily achieved at the focus via altering the mapping track of incident vector beams. These results may have potential applications in nano and spin photonics.

Enantioselective optical trapping of chiral nanoparticles using a transverse optical needle field with a transverse spin

Since the fundamental building blocks of life are built of chiral amino acids and chiral sugar, enantiomer separation is of great interest in plenty of chemical syntheses. Light-chiral material interaction leads to a unique chiral optical force, which possesses opposite directions for specimens with different handedness. However, usually the enantioselective sorting is challenging in optical tweezers due to the dominating achiral force. In this work, we propose an optical technique to sort chiral specimens by use of a transverse optical needle field with a transverse spin (TONFTS), which is constructed through reversing the radiation patterns from an array of paired orthogonal electric dipoles located in the focal plane of a 4Pi microscopy and experimentally generated with a home-built vectorial optical field generator. It is demonstrated that the transverse component of the photonic spin gives rise to the chiral optical force perpendicular to the direction of the light's propagation, while the transverse achiral gradient force would be dramatically diminished by the uniform intensity profile of the optical needle field. Consequently, chiral nanoparticles with different handedness would be laterally sorted by the TONFTS and trapped at different locations along the optical needle field, providing a feasible route toward all-optical enantiopure chemical syntheses and enantiomer separations in pharmaceuticals.

Fully controlled photonic spin in highly confined optical field

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.

Three-dimensional magnetization needle arrays with controllable orientation

Based on the Richards-Wolf vectorial diffraction theory and inverse Faraday effect, we first propose a scheme to generate three-dimensional magnetization needle (MN) arrays with arbitrary orientation for each individual needle and controllable spatial position and number by reversing the electric dipole array radiation. To achieve this, each unit of the electric dipole array has two electric dipoles with orthogonal oscillation directions and quadrature phase and is located mirror-symmetric with respect to the focal plane of the high numerical aperture lens. Uniformly distributed MNs with a subwavelength lateral size of 0.44λ and a longitudinal depth of 5.36λ with four different orientations are obtained by optimized arrangement for 2N (here, N=2) units of the electric dipole array. The corresponding purity of MNs is also discussed in detail. Furthermore, two combinations of MN arrays with orthogonal orientation are emphatically exploited in the hybrid bit-patterned media recording. The results illustrate the richness of the proposed methods to locally control the particular orientation properties of the MN and find many potential applications in multichannel/multilayer magneto-optical storage, information security, and spintronics.

All-optical generation of magnetization with arbitrary three-dimensional orientations

In this Letter, all-optical generation of magnetization with arbitrary three-dimensional (3D) orientations is numerically demonstrated through the inverse Faraday effect (IFE) by using a reversing calculation method. The IFE-induced magnetization with an expected 3D orientation is initially conceived by coherently configuring two orthogonally arranged electric dipoles with a phase difference of π/2 in the focal region of a to-be-determined incident light field. Based on the dipole antenna theory, this required incident light field can be deduced analytically according to the orientations of the electric dipoles. By utilizing this field as illumination and reversing the field propagation, magnetization with the expected orientation can be obtained in the focal region through the IFE. Moreover, this method showcases a high magnetization orientation purity (greater than 93%) within the focal volume defined by the full width at half maximum when the numerical aperture of the focal lens is 0.95. This result demonstrates extended flexibility of magnetization manipulations in an all-optical fashion and possesses great potential in spintronics and all-optical magnetic recording.

Arbitrarily spin-orientated and super-resolved focal spot

In this Letter, we propose a facile approach for achieving a robust focal spot bearing both super-resolution and arbitrary spin orientation. Toward this aim, we meticulously devise a structured incident light consisting of three sorts of beams, which can be produced definitely by the superposition of a radially polarized beam and an azimuthally polarized beam. Based on the vectorial diffraction integral and spin density theory, such newly configurable beams are tightly focused and isotropically interfered in a 4π microscopic configuration to create three polarized field components perpendicular to each other beyond the diffraction limit, thus enabling us to yield a super-resolved focal spot possessing spatial spin axis. By further willfully adjusting the amplitude factors of the reconstituent fields, the photonic spin direction can be freely tunable. The demonstrated results in this Letter may hold great potential for the spin photonics.

Vectorial optical fields: recent advances and future prospects

Driven by their potential applications, vectorial optical fields with spatially inhomogeneous states of polarization within the cross section have drawn significant attention recently. This work intends to review some of the latest development of this rapidly growing field of optics and offer a general overview of the current status of this field in a few areas. Mathematical descriptions of generalized vectorial optical fields are provided along with several special examples. A time-reversal methodology for the creation of a wide variety of exotic optical focal fields with prescribed characteristics within the focal volume is presented. Recently developed methods for the generation of vectorial optical fields that utilize fiber lasers, digital lasers, vectorial optical field generator, metasurfaces or photoalignment liquid crystals are summarized. The interactions of these vectorial optical fields with various micro- and nano-structures are presented and the prospects of their potential applications are discussed. The connection of vectorial optical fields with higher dimensionality in quantum information is summarized.