Spatiotemporal coupling induced controllable orientation of photonic orbital angular momentum at subwavelength scale.

Recently, the emergence of transverse orbital angular momentum (OAM) as a novel characteristic of light has captured substantial attention, and the significance of adjustable OAM orientation has been underscored due to its pivotal role in the interaction between light and matter. In this work, we introduce a novel approach to manipulate the orientation of photonic OAM at subwavelength scales, leveraging spatiotemporal coupling. By tightly focusing a wavepacket containing dual spatiotemporal vortices and a spatial vortex through a high numerical aperture lens, the emergence of intricate coupling phenomena leads to entangled and intricately twisted vortex tunnels. As a consequence, the orientation of spatial OAM deviates from the conventional light axis. Through theoretical scrutiny, we unveil that the orientation of photonic OAM within the focal field is contingent upon the signs of the topological charges in both spatiotemporal and spatial domains. Additionally, the absolute values of these charges govern the precise orientation of OAM within their respective quadrants. Moreover, augmenting the pulse width of the incident light engenders a more pronounced deflection angle of photonic OAM. By astutely manipulating these physical parameters, unparalleled control over the spatial orientation of OAM becomes achievable. The augmented optical degrees of freedom introduced by this study hold considerable potential across diverse domains, including optical tweezers, spin-orbit angular momentum coupling, and quantum communication.

Degeneration of topological corner, hinge, and surface states in three-dimensional photonic crystals.

The third-order topological insulators based on three-dimensional (3D) photonic crystals (PCs) have hardly been achieved because the nontrivial bandgap in 3D PCs is difficult to form. In this Letter, we elaborately construct 3D Su-Schrieffer-Heeger lattice in which the periodic modulation of refractive index is uniform in three axis directions. The high-order topological PCs are characterized by the nontrivial bulk polarizations and the mirror eigenvalues. Such a structure can achieve topological 1-codimensional surface states, 2-codimensional hinge states, and 3-codimensional corner states. More importantly, it is found for the first time, to the best of our knowledge, that the topological states exhibit a degeneration behavior, i. e., the corner, and hinge state, or corner and surface states coexist at nearly the same frequency, but maintain their own mode properties. The multiple topological states in 3D PCs as well as the degeneration of topological states will open a new window for the study of topological photonics.

Higher-order hybrid topological bound states in a non-Hermitian system.

Higher-order topological states, such as the corner and pseudo-hinge states, have been discovered in both Hermitian and non-Hermitian systems. These states have inherent high-quality factors that make them useful in the application of photonic devices. In this work, we design a non-Hermiticity solely induced Su-Schrieffer-Heeger (SSH) lattice and demonstrate the existence of diverse higher-order topological bound states in the continuum (BICs). In particular, we first uncover some hybrid topological states that occur in the form of BICs in the non-Hermitian system. Furthermore, these hybrid states with an amplified and localized field have been demonstrated to excite nonlinear harmonic generation with high efficiency. The appearance of these topological bound states will advance the study of the interplay of topology, BICs, and non-Hermitian optics.

Degenerate and non-degenerate all-optical switches using violet phosphorus nanosheets.

As an allotrope of phosphorus, layered violet phosphorus (VP) has a wide range of applications in electronics, photonics, and optoelectronics. However, its nonlinear optical properties remain to be explored. In this work, we prepare and characterize VP nanosheets (VP Ns), investigate their spatial self-phase modulation (SSPM) effects, and develop them in all-optical switching applications. The ring forming time of SSPM and the third-order nonlinear susceptibility of monolayer VP Ns were found to be about 0.4 s and 10-9 esu, respectively. The mechanism of SSPM formed by coherent light-VP Ns interaction is analyzed. Using the superior coherence electronic nonlinearity of VP Ns, we realize degenerate and non-degenerate all-optical switches based on the SSPM effect. It is demonstrated that the performance of all-optical switching can be controlled by adjusting the intensity of the control beam and/or the wavelength of the signal beam. The results will help us to better design and realize non-degenerate nonlinear photonic devices based on two-dimensional nanomaterials.

Unidirectional spintronic terahertz emitters with high efficiency.

Due to the high performance and low cost, spintronic terahertz emitters (STEs) have been a hot topic in the field of terahertz sources. However, most of the research focuses on the THz generation process and little attention has been paid to the control and modulation of the THz wave generated by the STE. In this Letter, a unidirectional spintronic terahertz emitter (USTE) integrating a common STE with a metal grating is proposed to manipulate the THz emission process. The dyadic Green's function method and finite element method are adopted to survey the characteristics of the USTE. Simulations show that the metal grating not only has a transmission larger than 97% in the optical band, but also has a higher reflectivity larger than 99% in the THz band. As a result, the USTE has a unidirectional THz emission along the direction of the pump beam with a larger than 4-fold enhancement in intensity. Moreover, the USTE has the capability of tuning the central frequency and THz wave steering by adjusting the distance and angle between the STE and the metal grating. We believe that this USTE can be used in THz wireless communications and holographic imaging, especially in the field of THz bio-sensing, which needs some resonance frequencies to sense.

Nanoscale chiral imaging under complex optical field excitation with controllable oriented chiral dipole moment.

Since chirality is a fundamental building block of nature, the identification of the chiral specimen's structure is of great interest, especially in applications involving the modification and utilization of proteins. In this work, by exploiting photoinduced force exerted on an achiral tip placed in the vicinity of a reciprocal chiral sample, a novel technique is proposed to detect the sample's chirality in nanoscale spatial resolution. Under separate excitation of focal field carrying chiral dipole moment with opposite handedness, there is a differential optical force ΔF exerted on the tip apex, which is connected to the enantiomer type and quasi-linearly depends on specific component of the sample's chirality parameter. With the help of time-reversal approach, we prove that the required excitation can be derived by radiation fields from the superposition of parallel electric and magnetic dipoles. Through adjusting the orientation of the chiral dipole moment, all the diagonal components of the sample's chirality can be exclusively retrieved. In addition, the sensitivity of the proposed technique is demonstrated to enantiospecify nanoscale chiral samples with chirality parameter on the order of 0.001. The proposed technique may open new avenue for wide applications in biomedicine, material science and pharmaceutics.

Numerical modeling for the characteristics study of a focusing ultrashort spatiotemporal optical vortex.

Spatiotemporal (ST) wave packet carrying pure transverse orbital angular moment (OAM) with subwavelength spatial size has attracted increasing attentions in recent years, which can be obtained by tightly focusing a linear superposition of ST vortices with different topological charges. In this work, numerical models are proposed to explore the impact of the pulse width of the ST vortex on the characteristics of its focal field. We demonstrate that the rigorous model for calculating the focused ST wave packet is essential for ultrashort optical pulse, while the simplified model has the advantage of high efficiency but can only provide credible results when the pulse width of the illumination is long enough. Specifically, when the pulse width decreases from 100 fs to 5 fs, the accuracy of the simplified model would decrease significantly from 99% to 65.5%. In addition, it is found that the pulse duration would still lead to the collapse of transverse OAM structure near the focus of a high numerical aperture lens, even though the ST astigmatism has already been corrected. To analyze the physical mechanism behind this distortion, Levenberg-Marquardt algorithm is adopted to retrieve the OAM distribution of the focal field. It is shown that the contributions from undesired OAM modes would become nontrivial for short pulse width, leading to the formation of the focal field with hybrid OAM structures. These findings provide insight for the focusing and propagation studies of ultrashort ST wave packets, which could have wide potential applications in microscopy, optical trapping, laser machining, nonlinear light-matter interactions, etc.

Understanding of transverse spin angular momentum in tightly focused linearly polarized vortex beams.

Spin angular momentum (SAM) is widely used in spin-dependent unidirectional optical interfaces, optical manipulation, integrated optical signal processing, laser structuring and other fields, but its physical mechanism has not been fully understood so far. In this work, we investigate the three-dimensional (3D) SAM in tightly focused x-polarized first-order vortex beams from the perspectives of light field itself, phase distribution, and focusing propagation. It is shown that the distribution of three orthogonal components of SAM at the focal plane has pseudo two-fold rotational symmetry, because the cycloidal rotation of the electric field of the tightly focused vortex beam is opposite. The 3D SAM distribution in the focal region is visualized by mapping the 3D distribution of state of polarization (SoP). In addition, a principle experimental method for identifying the transverse SAM by using the direction of particle's rotation axis in optical tweezers is proposed.

Synthesis of multiple longitudinal polarization vortex structures and its application in sorting chiral nanoparticles.

As the essential properties of organisms, detection and characterization of chirality are of supreme importance in physiology and pharmacology. In this work, we propose an optical technique to sort chiral materials by use of longitudinal polarization vortex (LPV) structures, which is generated with tightly focusing Pancharatnam-Berry tailored Laguerre-Gaussian beam. The nonparaxial propagation of the focusing field leads to the creation of multiple pairs of dual LPV structures with arbitrary topological charge and location, which can be independently controlled by the spatial phase modulation applied on the illumination. More importantly, the opposite spin angular momentums carried by each pair of dual foci lead to different energy flow directions, making it suitable to sort nanoparticles by their handedness. In addition, the LPV structures would also bring different dynamic behaviors to the enantiomers, providing a feasible route toward all-optical enantiopure chemical syntheses and enantiomer separations in pharmaceuticals.

Angular momentum separation in focused fractional vector beams for optical manipulation.

The generation, propagation, and applications of different types of integer vector beams have been extensively investigated. However, little attention focuses on the photophysical and photomechanical properties of the fractional vector beam (FVB). Herein, we theoretically and experimentally investigate the spin angular momentum (SAM) separation and propagation characteristics of weakly focused FVBs. It is demonstrated that such a beam carrying no SAM leads to both the transverse separation of SAM and the special intensity patterns in the focal region. Furthermore, we study the intensity, SAM, and orbital angular momentum (OAM) distributions of the tightly focused FVBs. It is shown that both three-dimensional SAM and OAM are spatially separated in the focal region of tightly focused FVBs. We investigate the optical forces, spin torques, and orbital torques on a dielectric Rayleigh particle produced by the focused FVBs. The results reveal that asymmetrical spinning and orbiting motions of optically trapped particles can be realized by manipulating FVBs.

Phase-change metasurface with tunable and switchable circular dichroism.

Metasurfaces with tunable/switchable circular dichroism (CD) response have great potential to serve as important elements for plenty of advanced applications. In this work, we proposed a novel metasurface absorber integrated with periodic ${{\rm Ge}_2}{{\rm Sb}_2}{{\rm Te}_5}$ (GST) resonators and numerically demonstrated its capability in reconfiguration of the CD effect. Due to the strong chiral plasmonic resonance, a strong CD of about 0.75 can be achieved in a prescribed spectrum. Additionally, the phase transition of GST resonators enables the quasi-linearly modification of CD strength in a broad range (from 0.03 to 0.75). Furthermore, reversible chirality of the metasurface absorber can be realized by controlling the states of the left- and right-hand GST resonators separately, enabling the CD signal to be readily switched between on-, off-, and reverse-state.

Nonlinear accelerated orbiting motions of optical trapped particles through two-photon absorption.

Vortex beams carrying optical angular momentum (AM) could drive the orbital motion of a small particle around the optical axis. In general, the orbital rotation speed of trapped particles increases linearly with the increasing laser power. Beyond the linear optics regime, in this work, we investigate both the optical force and torque on a two-photon absorbing Rayleigh particle produced by the tightly focused femtosecond-pulsed circularly polarized vortex beam. Different from the trapping dynamics of particles without two-photon absorption (TPA), it is shown that the orbital motion of trapped particles with TPA accelerates nonlinearly as the laser power increases. Moreover, the orbital motion acceleration of trapped particles is proportional to the TPA coefficient. The corresponding underlying mechanism is discussed in detail. Our results may find interesting applications in the characterization of the optical nonlinearity of a single nanoparticle, and AM manipulation and particle transportation in the nonlinear optics regime.

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 controllable three-dimensional light-induced longitudinal magnetization using a single objective lens.

With features of fast and energy-efficient data writing, all-optical helicity-dependent switching (AO-HDS) has emerged as a competitive technology to satisfy the demand for the next-generation volume data storage. Unfortunately, to switch the magnetizations in different positions of the magnetic-optic film, the laser beam, the objective lens, or the magnetic recording film should be moved, limiting the advantage of AO-HDS in fast data writing. To achieve on-the-fly magnetization switching, the induced magnetization should be fully controllable. In this Letter, by focusing an azimuthally polarized vortex beam (APVB) and introducing an additional phase, a feasible strategy constructing subwavelength light-induced pure longitudinal multi-magnetization spots is proposed. In addition, the position of the multi-magnetization spots can be dynamically controlled. The distributions of the focused APVBs with different orbital angular momentum, and the induced magnetizations are surveyed. We believe that this is a practical and flexible three-dimensional magnetic recording technique with dynamic control of the recording position.

Tailoring the magnetic field induced by the first higher order mode of an optical fiber.

In this paper, according to the inverse Faraday effect (IFE), the amplitude, phase, polarization and field distribution of the first higher order mode of an optical fiber are tailored carefully, and a magnetic field with arbitrary orientation is generated in the focal region. Compared with traditional strategies to generate a magnetic field with arbitrary orientation, where the configurations are complicated and the components employed for the system are costly, the first higher order mode of a fiber, which has two lobes with opposite instantaneous electric fields, draws more attention for generating a magnetic field with arbitrary orientation. We believe that such an arbitrary orientation state of magnetic field can be applied in the field of confocal and magnetic resonance microscopy and spin dynamics, especially for the use of optical magnetic recording, where laser pulses are used to trigger the magnetization switching.

Optical Trapping with Focused Surface Waves.

Near-field optical trapping can be realized with focused evanescent waves that are excited at the water-glass interface due to the total internal reflection, or with focused plasmonic waves excited on the water-gold interface. Herein, the performance of these two kinds of near-field optical trapping techniques is compared using the same optical microscope configuration. Experimental results show that only a single-micron polystyrene bead can be trapped by the focused evanescent waves, whereas many beads are simultaneously attracted to the center of the excited region by focused plasmonic waves. This difference in trapping behavior is analyzed from the electric field intensity distributions of these two kinds of focused surface waves and the difference in trapping behavior is attributed to photothermal effects due to the light absorption by the gold film.

Anisotropic nonlinear Kerr media: Z-scan characterization and interaction with hybridly polarized beams.

The interaction of intense laser with matter gives rise to a variety of novel nonlinear optical effects, reflects the nonlinear optical property of a material, and modulates the light propagation behavior. Herein, we investigate anisotropic Kerr nonlinearities induced by both scalar and vectorial optical fields. Firstly, we present the anisotropic third-order nonlinear refraction indexes related to left-hand and right-hand components, which depend on the ellipticity, the dichroism coefficient, the anisotropy coefficient, as well as the crystal orientation angle. Secondly, we develop the elliptically polarized light Z-scan technique for characterizing third-order nonlinear susceptibility tensor in anisotropic nonlinear Kerr media, which is demonstrated experimentally. Lastly, with the known nonlinear optical parameters, we numerically study both the vectorial self-diffraction behaviors and spin angular momentum (SAM) characteristics of hybridly polarized beams induced by an anisotropic Kerr nonlinearity. It is shown that the anisotropic Kerr nonlinearity offers a new approach to manipulate the polarization-structured light field, which has potential applications in SAM manipulation, three-dimensional crystal orientation, and polarization-sensitive detection and sensing.

Focus shaping and optical manipulation using highly focused second-order full Poincaré beam.

Generation of vectorial optical fields with arbitrary polarization distribution is of great interest in plenty of applications. In this work, we propose and experimentally demonstrate the generation of a second-order full Poincaré (FP) beam and its application in two-dimensional (2D) flattop beam shaping with spatially variant polarization under a high numerical aperture focusing condition. In addition, the force mechanism of the focal field with 2D flattop beam profile is numerically studied, demonstrating the feasibility to trap a dielectric Rayleigh particle in three-dimensional space. The results show that the additional degree of freedom provided by the FP beam allows one to control the spatial structure of polarization, to engineer the focusing field, and to tailor the optical force exerted on a dielectric Rayleigh particle. The findings reported in the work may find useful applications in laser micromachining, optical trapping, and optical assembly.

Theoretical investigation on asymmetrical spinning and orbiting motions of particles in a tightly focused power-exponent azimuthal-variant vector field.

We generate a new kind of azimuthal-variant vector field with a distribution of states of polarization (SoPs) described by the square of the azimuthal angle. Owing to asymmetrical SoPs distribution of this localized linearly polarized vector field, the tightly focused field exhibits a double half-moon shaped pattern with the localized elliptical polarization in the cross section of field at the focal plane. Moreover, we study the three-dimensional distributions of spin and orbital linear and angular momenta in the focal region. We numerically investigate the gradient force, radiation force, spin torque, and orbital torque on a dielectric Rayleigh particle produced by the tightly focused vector field. It is found that asymmetrical spinning and orbiting motions of trapped Rayleigh particles can be realized by the use of a tight vector field with power-exponent azimuthal-variant SoPs.

Publisher Correction: Trapping and manipulation of nanoparticles using multifocal optical vortex metalens.

A correction to this article has been published and is linked from the HTML version of this paper. The error has been fixed in the paper.