Efficient optical trapping with cylindrical vector beams

Optical trapping of multiple particles based on a rotationally-symmetric power-exponent-phase vortex beam

In this paper, the optical trapping of multiple particles based on a rotationally-symmetric power-exponent-phase vortex beam (RSPEPVB) was introduced and demonstrated. Based on the theories of tight focusing and optical force, the optical force model of RSPEPVB was established to analyze the optical trapping force of tightly focused RSPEPVB. Then, an experimental setup of optical tweezer, by utilizing the RSPEPVB, was built to demonstrate that the optical tweezer of RSPEPVBs can achieve the optical trapping of multiple particles, and the number of captured particles is equal to the topological charge l of RSPEPVB, which shows that the RSPEPVBs can achieve multi-particles trapping with controllable number. Moreover, compared to vortex beam, the captured particles by RSPEPVB will not rotate around the circular light intensity distribution. The results will provide a new option for optical trapping of multiple particles in biomedicine, laser cooling and so on.

Influences of salinity and temperature on propagation of radially polarized rotationally-symmetric power-exponent-phase vortex beams in oceanic turbulence

In this paper, the propagation properties of radially polarized rotationally-symmetric power-exponent-phase vortex beams (RP-RSPEPVBs) in oceanic turbulence were theoretically and experimentally studied. Based on the extended Huygens-Fresnel diffraction integral and vector beams theories, the theoretical propagation model of RP-RSPEPVBs in the oceanic turbulence was established. Then, the numerical simulations were carried out to study the influences of the propagation distance z, the rate of dissipation of turbulence kinetic energy per unit mass of fluid ε, the temperature-salinity contribution ratio ω, and the dissipation rate of the mean-squared temperature χT on the optical intensity, spectral degree of polarization (DOP) and spectral degree of coherence (DOC) of RP-RSPEPVBs. Further, an experiment setup was demonstrated to confirm the influences of salinity and temperature on propagation of RP-RSPEPVBs in oceanic turbulence. The results showed that increasing salinity, propagation distance, and turbulence intensity, will result in beam diffusion and intensity reduction of the RP-RSPEPVBs, as well as depolarization and decoherence. Contrarily, high temperature mitigated the intensity loss of the RP-RSPEPVBs and the spectral DOP and spectral DOC increased when the turbulence tends to be dominated by temperature. As a vector beam, the RP-RSPEPVB shows well anti-turbulence interference characteristics, which provides a new choice for optical underwater communication and imaging.

Optical trapping and fluorescence control with vectorial structured light

Here we functionalized micro-scaled polymer beads with nano-scaled quantum dots and demonstrate optical trapping and tweezing, with in-situ fluorescence measurement, in an all-digital all-optical configuration. We outline the chemistry required to facilitate this, from deactivating the optical trapping environment to size, adhesion and agglomeration control. We introduce a novel holographic optical trapping set-up that leverages on vectorially structured light, allowing for the delivery of tuneable forms of light from purely scalar to purely vector, including propagation invariant flat-top beams for uniform illumination and tailored intensity gradient landscapes. Finally, we show how this has the potential to quench bleaching in a single wavelength trap by linear (spatial mode) rather than non-linear effects, advancing the nascent field of optics for chemistry.

All-fiber bidirectional mode-locked laser to generate cylindrical vector beam asynchronous noise-like pulses

We proposed and demonstrated a bidirectional mode-locked fiber laser to generate cylindrical vector beam (CVB) asynchronous pulses based on a graded index multimode fiber. A homemade fused taper two-mode fiber optical coupler (TMF-OC) is employed as a mode converter. The central wavelength for clockwise (CW) pulses can be tuned from 1030.32 nm to 1041.04 nm due to the filtering effect based on multimode interference, that of counterclockwise (CCW) pulses is from 1030.81 nm to 1039.28 nm. When the central wavelengths are 1033.22 nm and 1032.71 nm for CW direction and CCW direction respectively, CVB asynchronous noise-like pulses with a repetition rate difference of ∼436.9 Hz can be obtained. The purity of CVB in CW direction and CCW direction is 95.7% and 93.4% respectively. This bidirectional mode-locked fiber laser with CVB output can be better applied to laser gyroscopes, asynchronous sampling, and dual-comb technique, and impel the interdisciplinary studies in the future.

Generation of cylindrical vector beams in a linear cavity mode-locked fiber laser based on nonlinear multimode interference

In this paper, a linear cavity mode-locked pulsed fiber laser generating cylindrical vector beams (CVBs) is proposed and demonstrated based on a nonlinear multimode interference. A homemade long-period fiber grating with a broad bandwidth of 121 nm is used as a mode converter inside the cavity. The saturable absorber was formed by single-mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) structure. By controlling the pump power, the operation states are switchable among continuous-wave, Q-switched mode-locked (QML), and mode-locked regimes. The repetition rate of the QML CVB pulse envelope varies from 57.4 kHz to 102.7 kHz at the pump range of 118 to 285 mW. When increasing pump power to 380 mW, mode-locked CVB pulse repetition rate of 3.592 MHz, and pulse duration of 4.62 ns are achieved. In addition, the maximum single-pulse envelope energy can reach 510 nJ, and 142 mW average-power CVBs with a slope efficiency of as high as 20.2% can be obtained. Moreover, azimuthally and radially polarized beams can be obtained with mode purity over 95% in different operating regimes. The proposed fiber laser has a simple structure, and the operation is controllable in both temporal and spatial domains, which presents a flexible pulsed CVB source for application of laser processing, time or mode division multiplexing system, and spatiotemporal nonlinear optics.

Focusing characteristics of cylindrical vector beams through a multi-focal all-dielectric grating lens

A novel, to the best of our knowledge, type of multi-focal all-dielectric grating lens is proposed in this work, and focusing characteristics of cylindrical vector beams through the lens are investigated in detail. Based on the negative refraction mechanism of negative-first-order diffraction and Fermat's principle, a multi-focal lens is designed. By analyzing the diffraction effect of the grating, the essential factor that affects the focus quality is found. Through a two-step optimization process, secondary foci and the focal displacement of primary foci caused by high-order diffractions are overcome, and the quality of the focal field is significantly improved. This work provides a reference for micro-lens design for focus modulation, and the research results also have potential applications in the fields of light-field manipulation and optical tweezers.

Propagation factor of partially coherent radially polarized vortex beams in anisotropic turbulent atmosphere

We skillfully combined the cosine theorem with the second moment theory and the Wigner distribution function and derived the analytical expressions of the propagation factor (M²-factor) of a partially coherent radially polarized vortex beam (PCRPVB) in atmospheric turbulence. Then, we comparatively studied the propagation factors of a PCRPVB and a partially coherent electromagnetic vortex beam (PCEVB) in atmospheric turbulence. The results show that a PCRPVB has a smaller value of a relative M²-factor than a PCEVB, which means that a PCRPVB has a stronger ability to resist atmospheric turbulence than a PCEVB. To confirm our theoretical studies, the hyperbolic fitting method is combined with the random phase screen (RPS) to simulate the M²-factor of a PCRPVB and a PCEVB through atmospheric turbulence. The study results indicate that the theoretical values agree well with the simulated values. Our results may find applications in free-space optical communications and remote sensing.

Microsphere-coupled optical tweezers

In this Letter, we study the optical trapping of particles in a focal spot engineered by a combination of a dielectric microsphere and the conventional optical tweezers setup. The dielectric microsphere is placed in the laser path before the focal spot, and its impact on the trapping stiffness is theoretically and experimentally studied in detail. The suggested method for considering the geometric phase shift, due to the presence of the microsphere, into the Debye diffraction integral shows a good agreement with the experiment. We stably trap particles as small as 350 nm in the microsphere-coupled optical trap using a low numerical aperture objective (NA=0.7), while in a conventional optical tweezers setup, it requires at least an NA=1.1. Moreover, the results show that choosing an appropriate microsphere and depth of trapping can enhance the trapping efficiency in the axial direction by a factor of ∼3.8.

Generation of Multiple Vector Optical Bottle Beams

We propose binary diffractive optical elements, combining several axicons of different types (axis-symmetrical and spiral), for the generation of a 3D intensity distribution in the form of multiple vector optical ‘bottle’ beams, which can be tailored by a change in the polarization state of the illumination radiation. The spatial dynamics of the obtained intensity distribution with different polarization states (circular and cylindrical of various orders) were investigated in paraxial mode numerically and experimentally. The designed binary axicons were manufactured using the e-beam lithography technique. The proposed combinations of optical elements can be used for the generation of vector optical traps in the field of laser trapping and manipulation, as well as for performing the spatial transformation of the polarization state of laser radiation, which is crucial in the field of laser-matter interaction for the generation of special morphologies of laser-induced periodic surface structures.

Compensation-free high-dimensional free-space optical communication using turbulence-resilient vector beams

Free-space optical communication is a promising means to establish versatile, secure and high-bandwidth communication between mobile nodes for many critical applications. While the spatial modes of light offer a degree of freedom to increase the information capacity of an optical link, atmospheric turbulence can introduce severe distortion to the spatial modes and lead to data degradation. Here, we demonstrate experimentally a vector-beam-based, turbulence-resilient communication protocol, namely spatial polarization differential phase shift keying (SPDPSK), that can reliably transmit high-dimensional information through a turbulent channel without the need of any adaptive optics for beam compensation. In a proof-of-principle experiment with a controllable turbulence cell, we measure a channel capacity of 4.84 bits per pulse using 34 vector modes through a turbulent channel with a scintillation index of 1.09, and 4.02 bits per pulse using 18 vector modes through even stronger turbulence corresponding to a scintillation index of 1.54.

Resistance to turbulence is an ongoing challenge for point-to-point freespace communications. Here the authors present a protocol for encoding a large amount of information in vector beams that are transmittable through a moderately strong turbulent channel without adaptive beam compensation.

Optimized anti-reflection core-shell microspheres for enhanced optical trapping by structured light beams

In this paper, we study the optical trapping of anti-reflection core-shell microspheres by regular Gaussian beam and several structured beams including radially polarized Gaussian, petal, and hard-aperture-truncated circular Airy beams. We show that using an appropriate anti-reflection core-shell microsphere for the optical trapping by several structured light beams can dramatically enhance the strength of the trap compared to the trapping by the common Gaussian beam. The optimal core-shell thickness ratio that minimizes the scattering force is obtained for polystyrene-silica and anatase-amorphous titania microspheres, such that the core-shells act as anti-reflection coated microspheres. We show that the trapping strength of the anti-reflection coated microparticles trapped by the common Gaussian beam is enhanced up to 2-fold compared to that of trapped uncoated microparticles, while the trapping of anti-reflection coated microparticles, by the radially polarized beam, is strengthened up to 4-fold in comparison to that of the trapped uncoated microparticles by the Gaussian beam. Our results indicate that for anatase-amorphous titania microparticles highest trap strength is obtained by radially polarized beam, while for the polystyrene-silica microparticles, the strongest trapping is achieved by the petal beam.

Detection of degenerate Stokes index states

Stokes phase is the phase difference between orthogonal component states in the decomposition of any polarization state. Phase singularities in the Stokes phase distribution are Stokes singularities of an inhomogeneous polarization distribution. Under circular decomposition, Stokes phase distribution \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(\phi _{12})$$\end{document}(ϕ12) represents polarization azimuth \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(\gamma )$$\end{document}(γ) distribution and the singularities present in it are polarization singularities. Therefore, the charge of the Stokes vortices depicted as Stokes index \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sigma _{12}$$\end{document}σ12 is an important parameter associated with the polarization singularity. The Hybrid order Poincaré sphere (HyOPS)/Higher order Poincaré sphere (HOPS) beams, all having same Stokes index, contain a Stokes singularity at the center of the beam as these beams are constructed by vortex superposition. These beams, being superposition of orthogonal orbital angular momentum (OAM) states in orthogonal spin angular momentum (SAM) states can offer great multiplexing capabilities in communication. In this article, we identify these degenerate Stokes index states and discuss the ways and means of lifting this degeneracy. Otherwise, there are limitations on intensity based detection techniques, where demultiplexing or segregation of different HOPS/HyOPS beams is warranted. The method adduced here uses the diffraction of these beams through an equilateral triangular aperture in combination with polarization transformation as a probe to lift the Stokes index/Stokes phase degeneracy. Successively, the novelty of the detection scheme is discussed in the context of beams with alike polarization distributions where even the technique of Stokes polarimetry fails to predict the OAM and SAM content of the beam.

Gold cauldrons as efficient candidates for plasmonic tweezers

In this report, gold cauldrons are proposed and proved as efficient candidates for plasmonic tweezers. Gold cauldrons benefit from high field localization in the vicinity of their apertures, leading to particle trapping by a reasonably low power source. The plasmonic trapping capability of a single gold cauldron and a cauldrons cluster are studied by investigating the plasmon-induced variations of the optical trap stiffness in a conventional optical tweezers configuration. This study shows that the localized plasmonic fields and the consequent plasmonic forces lead to enhanced trap stiffness in the vicinity of the cauldrons. This observation is pronounced for the cauldrons cluster, due to the additive plasmonic fields of the neighboring cauldrons. Strong direct plasmonic tweezing by the gold cauldrons cluster is also investigated and confirmed by our simulations and experimental results. In addition to the presented plasmonic trapping behavior, gold cauldrons benefit from a low cost and simple fabrication process with acceptable controllability over the structural average dimensions and plasmonic behavior, making them attractive for emerging lab-on-a-chip optophoresis applications.

Variable transformation of singular cylindrical vector beams using anisotropic crystals

We demonstrated and investigated, both theoretically and experimentally, the transformation of cylindrical vector beams with an embedded phase singularity under the condition of focusing perpendicularly to the axis of the anisotropic calcite crystal. Theoretical and numerical analysis, performed on the basis of decomposing the light field into a set of plane waves for an anisotropic medium, allowed us to study the dependence of the structural transformation of the initial laser beam on the polarisation and phase state in detail. The proposed approach allows one to perform the visual recognition of cylindrically-polarised vector beams of various orders and can be used for the demultiplexing of information channels in the case of polarisation-division multiplexing. The experimentally-obtained results agree with the theoretical findings and demonstrate the reliability of the approach.

High-power femtosecond cylindrical vector beam optical parametric oscillator

We report on high-power femtosecond cylindrical vector beam (CVB) generation from a Gaussian-pumped optical parametric oscillator (OPO). By introducing a half waveplate and a vortex half-wave plate of m = 1 to realize intracavity polarization modulation to the resonant Gaussian signal, the OPO could deliver broadband signal beam in CVB profile, i.e., radially and azimuthally polarized beam profile. The central wavelength of the generated CVB signals can be tuned continuously from 1405 to 1601 nm, while the corresponding pulse durations are all around 150 fs. A maximum average output power of 614 mW at 1505 nm is obtained. Moreover, our OPO cavity design can be extended to generate high order CVB by simply changing the vortex half-wave plate with different orders. Such a high-power CVB OPO configuration has the advantages of flexible control and wide tuning range, making it a practical tool for applications in super-resolution imaging, optical communication and quantum correlations.