Radially-polarized Beam Research Articles

Anapole states and transverse displacement sensing based on the interaction between cylindrical vector beams and Au core-Si shell nanodisks.

Precise optical control at the nanoscale is crucial for advancing photonic devices and sensing technologies. Herein, we theoretically introduce what we believe to be a novel approach for nano-optical manipulation, employing Au core-Si shell nanodisks interacting with tightly focused cylindrical vector beams to achieve electric and magnetic anapole states. Our investigations unveil that the interplay between individual nanodisks and radially polarized beams (RPBs) located in the center of RPBs yields a position-dependent electric anapole state. Conversely, under illumination by azimuthally polarized beams (APBs), the electric anapole state exhibits independence from the nanodisk's positioning and is accompanied by significant magnetic dipole excitations. Furthermore, the interaction between APBs and nanodisk multimers enables the formation of a magnetic anapole state, marking an advancement in nano-optical control. This study further explores the application of the position-dependent electric anapole state for nanoscale transverse displacement sensing, which allows for precise determination of the nanodisk's position within a plane. These findings not only facilitate versatile control over anapole states but also set a foundation for integrated displacement sensing technologies on-chip.

Generation of Tightly Focused Cylindrical Vector Beams with Dual-Channel Transmissive Metasurfaces

Tightly focusing cylindrical vector beams (CVBs) are widely used in diverse applications from particle trapping to high-resolution microscopy. Recently, planar metasurfaces have been proposed as promising alternatives to traditional bulky optical devices for tightly focused CVB generation. However, few metasurfaces allow the generation and independent phase control of different CVBs. In this paper, cascaded dual-channel transmissive metasurfaces that can be used to simultaneously generate different CVBs and independently control their phases are designed. The polarization and phase of the transmitted waves are controlled independently via only two geometric parameters. Under $x$- and $y$-polarized wave incidences, the radially polarized beam (RPB) and azimuthally polarized beam (APB) are generated by a first metasurface. In addition, a second metasurface is used to produce different focal fields in different focal planes under RPB and APB excitations. In such a proposed approach, the phase responses of the RPB and APB are distinct, which is significantly different from existing designs. A regular polygonlike focus and a lateral displacement of the focal spot are, respectively, realized in different focal planes at 7.8 GHz through different channels. Our approach improves the flexibility of focusing, allowing the generation of focal arrays at different locations within the same focal plane. Since the electromagnetic energy can be confined to a small specific zone in the microwave band, the proposed method can provide the basis for a number of important applications in the medical domain, such as the accurate hyperthermia treatment of large area tumors.

Plasmonic metafiber for all-fiber Q-switched cylindrical vector lasers

Abstract Metafibers, by integrating metasurface at the optical fiber tip, are emerging as the significant optical coupling platforms for nanophotonics and fiber-optic communities. Here, we propose a plasmonic metafiber for converting the fundamental mode to first-order mode in fiber, and as proof of device performance, demonstrate an all-fiber Q-switched cylindrical vector laser using the metafiber. Based on polarization-dependent plasmonic resonance, a polarization-independent mode conversion metasurface is designed theoretically and numerically, fabricated directly on fiber facet, and packaged as an all-fiber component with efficiency up to 21% at 1550-nm band. Using the metafiber in an all-fiber laser, Q-switched azimuthally polarized beam (APB) and radially polarized beam (RPB) are delivered at wavelength of 1548.5 nm with pulse durations from ∼7 to ∼2 μs when pump power increases from 30 to 120 mW. The mode purities of the APB and RPB are 86.5% and 90.7%, respectively. This work outlines a new strategy to integrate metasurfaces into “all-in-fiber” systems and offers a reliable route to construct next-generation laser sources, such as all-fiber ultrafast structured lasers.

Selective triggering in-plane and out-of-plane dipolar modes of hexagonal Au nanoplate with the polarization of excitation beam

The dipolar responses of a single hexagonal Au nanoplate are investigated under the illuminations of linearly polarized beam and tightly focused radially polarized beam (RPB). It is found from the scattering spectra that the in-plane and out-of-plane electric dipole modes can be selectively triggered with a linearly polarized beam and tightly focused RPB, respectively. The features of these two dipolar modes are further confirmed in terms of electrical field and charge maps by the finite-difference time-domain simulation. Additionally, using the multipole expansion method, the existence of the out-of-plane dipole mode is further verified by the fact that the z-component of electric dipole response has a dominant contribution to the scattered power. Moreover, by combining the back focal plane imaging technique with the simulation, the appearance of in-plane and out-of-plane dipoles in the scattering pattern are clearly discerned. Our results provide an efficient method for selectively exciting the in-plane and out-of-plane dipolar modes of the nanoplate. We envision that the ease of tuning the dipolar momentum may facilitate the enhancement of the interaction between the plasmon and emitters at single-particle level.

Generation of broadband terahertz focused vector beam using multifunctional metasurface

Focused vector beams are special beams with spatially inhomogeneous polarization distribution, which have broad application prospects in photonics applications such as high-resolution imaging, communications, and optical sensing. Recently, multifunctional metasurfaces for generating focused vector beams have attracted great interest due to their highly integratable, miniaturized, and compact properties. However, in terahertz (THz) range, related research remains largely unexplored. In this paper, a multifunctional broadband metasurface with an all-dielectric structure is proposed for generating focused vector beams in the terahertz region, enabling efficient linear polarization conversion and complete phase control of the transmitted field. Without the help of a polarization diversity scheme, this planar focused vector beam metasurface (FVBM) can simultaneously convert x- and y-polarized incident beams into focused radially polarized (RP) and azimuthally polarized (AP) beams, respectively. The simulation results show that the proposed metasurface exhibits excellent average conversion efficiencies (CEs) higher than 85.47% and 80.94% in the frequency range of 1.7 THz - 2.0 THz under linear x-polarized and y-polarized excitation, respectively. At 1.85 THz, the proposed metasurface can focus RP and AP beams with focusing efficiencies of 70.25% and 69.88%, respectively. The proposed metasurface has potential applications in integrating multiple functionalities into single, compact and ultrathin terahertz devices.

Self-focusing propagation characteristics of a radially-polarized beam in nonlinear media.

In this study, an analytical formula for the self-focusing length of a radially polarized beam (RPB) is first derived, which has a similar behavior to the semi-empirical Marburger formula of a Gaussian beam, and is beneficial to quantitatively and qualitatively analyze practical experimental scenarios. However, the relation of the self-focusing length with the states of polarization (SoPs) was evaluated, and it was found that RPB with spatially inhomogeneous SoP at the field cross-section can retain a further self-focusing length compared to a beam with a spatially homogeneous one. The influence of the topological charge on the self-focusing length is explored, which shows that RPB with a low topological charge can achieve a high-power density at a relatively further receiver plane. Therefore, it is demonstrated that the RPB as a laser source not only extends the self-focusing length, but also improves the power density of the target. With the help of RPB, it is possible to realize a controllable self-focusing length and a high target optical power density, which may have potential applications in fine optical manipulation, optical communication, high-power long-range laser atmospheric propagation, and related areas.

High-resolution dark-field confocal microscopy based on radially polarized illumination.

Dark-field confocal microscopy (DFCM) facilitates the 3D detection and localization of surface and subsurface defects in high-precision optical components. The spatial resolution of conventional DFCM is commonly undermined owing to complementary aperture detection. We employed a radially polarized (RP) beam for illumination in DFCM. The RP beam creates a sub-diffraction-sized longitudinal optical component after being focused and effectively enhances the lateral resolution by 30.33% from 610 nm to 425 nm. The resolution improvement was verified by imaging a 2D sample containing sparsely distributed gold nanorods along with a 3D neodymium glass containing surface and subsurface defects.

Sharper photonic nanojets generated by microspheres under higher-order radially polarized beam illumination.

Photonic nanojets (PNJs) generated from a single microsphere illuminated by higher-order radially polarized (RP) beams are investigated. The effects of the size parameters of higher-order RP beams, the refractive index, and radius of the dielectric microsphere on the full width at half-maximum and peak intensity of the PNJ are numerically discussed and qualitatively interpreted. The results show that the minimal width of the PNJ can be obtained by optimally adjusting the size parameter. The PNJ beam waist becomes gradually narrower with increasing the radial mode number. As compared to the case of plane wave illumination, sharper PNJs are more easily generated when irradiated by a higher-order RP beam, even for microspheres with lower refractive indices or larger radii. Our findings can promote potential applications of PNJs in a variety of fields including super-resolution microscopy, nanolithography, and optical data storage.

Tailoring multi-singularity structure induced by a focused radially polarized beam

A structured optical field with controllable three-dimensional intensity and multiple polarization singularities is demonstrated by utilizing a combination of a radially polarized (RP) beam, a designed phase mask, and a high numerical aperture lens. Owing to the tight focusing property of RP beams as well as the interference of multiple linearly polarized non-coplanar plane waves, various lattice-like optical structures can emerge at the focal plane with multiple structured singularities in the transverse plane and optical needle array along with propagation. Compared with recently proposed phase and polarization engineering methods with spatial light modulators, the method presented here is convenient and flexible, and can easily realize the generation of V-point and C-point lattices. More importantly, a structured longitudinal field, namely, an optical needle array, with steerable positive and reverse energy flows may be extensively applied in multi-particle acceleration and trapping, optical microscopes, and second-harmonic generation.

Numerical study on the tight focusing of radially polarized beams with polarization-insensitive metalenses

Radially polarized beam (RPB) could be focused to a tighter spot than a spatially homogeneous polarized beam with a high numerical aperture (NA) objective lens. However, the traditional objective lens suffers from high cost and bulky size. In this paper, polarization-insensitive nanopillar metalenses were designed to execute tight focusing without the requirement of the incident beams to be specific polarized states. It was found that the focal spot of RPB was potentially smaller than that of a linear polarized beam because of the longitudinal component. Diffraction-limited focusing of RPB was achieved by a high NA polarization-insensitive metalens through an annular aperture illumination. This study may contribute to high-resolution photonics and optoelectronic applications.

Exploring the focal field of radially polarized beam through electron scattering

An electron scattering scheme is proposed to explore and quantify the focal field of radially polarized beam (RPB). Based on the Richard–Wolf method we deduce the description of the RPB's focal field, analyze the interaction and motion of the electrons in the focal field of the RPB, using both electrodynamical and pondermotive method. Our results show that the geometrical and field information of RPB focus are included in the electron scattering patterns which can be measured easily by experiment. Then we develop and demonstrate an experimental scheme to probe the RPB focal field by electron beam scattering, whose results agree well with the theoretical conclusions. Our scheme is also available for analyzing other cylindrically polarized beams.

Nanosecond Yb-doped fiber MOPA system based on an LCP vortex half-waveplate with a 20 W radially polarized output

In this paper, we demonstrate the generation of a radially polarized beam (RPB) with a maximum average output power of 20 W by our self-developed nanosecond pulsed ytterbium (Yb)-doped fiber MOPA system. The method of spatial phase conversion was adopted for the system, and a liquid crystal polymer (LCP) vortex half-waveplate (LCP-WPV) was utilized to realize efficient conversion of the linearly polarized LP01 mode to the radially polarized TM01 mode. A narrow-linewidth, linearly polarized fundamental mode with an average power of 20.5 W was produced in the multistage all-fiber MOPA system consisting of a continuous-wave (CW) single-frequency fiber laser seed, an electro-optical intensity modulator, two-stage core-pumped amplifiers and three-stage cladding-pumped amplifiers. Afterwards, an RPB output with a 20 W maximum average output power, a 7.5 ns pulse width and a 10 kHz repetition rate was finally obtained after the LCP-WPV was adopted as the spatial mode converter. Then, the mode purity of the RPB was measured to be approximately 88.5 % with the PBS (polarization beam splitter) method. This work provides a reference for the generation of cylindrical vector beam sources with the advantages of high efficiency and high mode purity.

Reversible optical binding force in a plasmonic heterodimer under radially polarized beam illumination

We investigated the optical binding force in a plasmonic heterodimer structure consisting of two nano-disks. It is found that when illuminated by a tightly focused radially polarized beam (RPB), the plasmon modes of the two nano-disks are strongly hybridized, forming bonding/antibonding modes. An interesting observation of this setup is that the direction of the optical binding force can be controlled by changing the wavelength of illumination, the location of the dimer, the diameter of the nano-disks, and the dimer gap size. Further analysis yields that the inhomogeneous polarization state of RPB can be utilized to readily control the bonding type of plasmon modes and distribute the underlying local field confined in the gap (the periphery) of the dimer, leading to a positive (negative) optical binding force. Our findings provide a clear strategy to engineer optical binding forces via changes in device geometry and its illumination profile. Thus, we envision a significant role for our device in emerging nanophotonics structures.

径向偏振矢量光束在大气湍流下的传输分析

径向偏振矢量光束在大气湍流下的传输分析

Nonlinear propagation characteristics of a radially polarized beam in a uniaxially aligned dye-doped liquid crystal

The nonlinear propagation characteristics of a radially polarized (RP) beam in uniaxially aligned dye-doped liquid crystal (DDLC) are investigated in experiments and theory. The photothermal effect produces changes in the refractive index of the nonlinear polarization-sensitive medium, in both ordinary and extraordinary directions. The changes promote a self-modulation of the polarization pattern in the incident RP beam as it propagates through the DDLC. The experimental results are explained from theory by modeling the change in the refractive index of the medium. Our results may be applied to spatial polarization modulation, by which an axially symmetric polarized beam is converted into another structured light, such as the full Poincare beam.

Low-cost and highly accessible technology based on radially polarized beam-excited plasmonic microfiber for label-free Raman detection

A low-cost and highly accessible method for high sensitivity label-free Raman spectroscopy based on a cylindrical metalized microfiber internally excited via a radially polarized beam (RPB) has been developed. A silver (Ag)-pyramid coated microfiber was designed to be matched with the RPB. Simulation results show that the enhancement of the electric-field intensity of the cylindrically metalized microfiber excited by the RPB can reach a factor of 2.5 × 103, which is much higher than that of linearly polarized beam (LPB) excitation. In experiments, the RPB is directly generated in the optical fiber by converting the LPB to an RPB using electrically controllable, low cost acoustic-optical technology. An Ag-pyramid coated microfiber with ∼9.6 µm diameter was prepared using a simple Ag-mirror reaction. The Raman intensity for malachite green (MG) detected by the Ag-pyramid coated microfiber under RPB excitation was 5 times stronger than that under LPB excitation. The Raman sensitivity for MG in the case of RPB excitation was as low as 10−10 M, a concentration level which was not detectable for LPB excitation. The sensor system gave high repeatability and reliability and should find application in high sensitivity label-free detection in optical sensing with the advantages of low-cost and ease of operation.

Characterizing localized surface plasmon resonances using focused radially polarized beam.

We demonstrate a scheme to characterize the localized surface plasmon resonances (LSPRs) of an individual metallic nanorod by employing a focused radially polarized beam (RPB) illumination under normal incidence. The focused RPB has a unique three-dimensional electric field polarization distribution in the focal plane, which can effectively and selectively excite the dipole and multipole plasmon resonances in a metallic nanorod by just moving the nanorod within the focal plane. This performance can be attributed to the mode matching between the excitation electric field of the incident RPB and the LSPRs in a metallic nanorod. Emphatically, in contrast to the commonly used oblique incidence illumination with the linearly polarized light, our proposed scheme is based on the normally incident light illumination and compatible with conventional optical microscopy, which is more scalable for spectroscopic characterization of individual nanostructures.

High focusing efficiency in subdiffraction focusing metalens

Abstract Vector beams with phase modulation in a high numerical aperture system are able to break through the diffraction limit. However, the implementation of such a device requires a combination of several discrete bulky optical elements, increasing its complexity and possibility of the optical loss. Dielectric metalens, an ultrathin and planar nanostructure, has a potential to replace bulky optical elements, but its optimization with full-wave simulations is time-consuming. In this paper, an accurate and efficient theoretical model of planar metalens is developed. Based on this model, a twofold optimization scheme is proposed for optimizing the phase profile of metalenses so as to achieve subdiffraction focusing with high focusing efficiency. Then, a metalens that enables to simultaneously generate radially polarized beam (RPB) and modulate its phase under the incidence of x-polarized light with the wavelength of 532 nm is designed. Full-wave simulations show that the designed metalens of NA = 0.95 can achieve subdiffraction focusing (FWHM = 0.429λ) with high transmission efficiency (77.6%) and focusing efficiency (17.2%). Additionally, superoscillation phenomenon is found, leading to a compromise between the subdiffraction spot and high efficiency. The proposed method may provide an accurate and efficient way to achieve sub-wavelength imaging with the expected performances, which shows a potential application in super-resolution imaging.

Resonant harmonic generation in AlGaAs nanoantennas probed by cylindrical vector beams.

We investigate second- and third-harmonic generation from individual AlGaAs nanoantennas using far-field mapping with radially- and azimuthally-polarized cylindrical vector beams. Due to the unique polarization structure of these beams, we are able to determine the crystal orientation of the nanoantenna in a single scanning map. Our method thus provides a novel and versatile optical tool to study the crystal properties of semiconductor nanoantennas. We also demonstrate the influence of cylindrical vector beam excitation on the resonant enhancement of second- and third-harmonic generation driven by electric and magnetic anapole-like modes, despite falling in the strong absorption regime of AlGaAs. In particular, we observe a greater nonlinear conversion efficiency from a single nanoantenna excited with a radially-polarized beam as compared to an azimuthally polarized cylindrical vector beam. The fundamental field of the radially-polarized beam strongly couples to the multipoles increasing the near-field enhancement of the nanoantenna. Our work introduces new ways to study individual nanostructures and to tailor the efficiencies of nonlinear phenomena at the nanoscale using non-conventional optical techniques.

Nanoscale Polarization Manipulation and Encryption Based on Dielectric Metasurfaces

AbstractManipulating the polarization of light is highly desired for versatile applications ranging from super resolution, optical trapping, to particle acceleration. The enormous freedom in metasurface design motivates the implementation of polarization control in ultrathin and compact optical systems. However, the majority of proposed strategies based on metasurfaces have demonstrated only a spatially homogeneous polarization generation, while less attention has been devoted to spatially variant inhomogeneous vector beams. Here, a novel method for generating arbitrary radial and azimuthal polarization beams with high efficiencies of up to 80% is demonstrated by utilizing transmission‐type dielectric metasurfaces. Polarization conversion metasurfaces are suitable candidates for the implementation of polarization encryption, which is demonstrated by encoding a hidden image into the spatial polarization distribution. In addition, it is shown that the image pattern can be modified by appropriate polarization selection of the transmitted light. Such a method may provide a practical technique for a variety of applications such as imaging, encryption, and anticounterfeiting.