Counter-propagating Bessel Beams Research Articles

Scattering characteristics of dual counter-propagation high-order circularly symmetric Bessel beam by a multilayer chiral sphere

The interaction between dual counter-propagating high-order circularly symmetric Bessel beams (CSBBs) and multi-layered chiral particles is investigated. Within the framework of generalized Lorenz-Mie theory (GLMT), the distribution characteristics of the superposition of two beams are studied based on the vector superposition theorem. The near-field, internal field, and far-field radar cross section (RCS) of the dual-layered chiral sphere illuminated by dual CSBBs are obtained according to the boundary conditions. By comparing the results of RCS with those from the references for two cases: first, when CSBBs degenerate into plane waves incident on multi-layered chiral sphere, and second, when Bessel beams illuminate isotropic multi-layer sphere degenerated by chiral multi-layered sphere, the effectiveness of the principle and program exhibited here is confirmed. The impact of various parameters on the distribution of the superposed field, the near-field, internal field, and far-field RCS of the particles are analyzed in detail, such as the order, half-cone angle, incidence angle, and polarization angle. The research results in this paper provide theoretical assistance for understanding the interaction between dual CSBBs and non-uniform chiral multi-layered particles and have important value in realizing optical manipulation of complex chiral layered particles.

Fully controllable multichannel waveguides induced by counterpropagating Bessel beams

We theoretically analyze the waveguiding structures photo-induced by two incoherent counter-propagating Bessel beams (BBs) in a biased photorefractive crystal. We demonstrate that the cross-coupling of two BBs enables adressable channels and tunability of the forming guiding structures. The truncation parameter of the BBs, their Bessel orders and the misalignment between the two beams are all key parameters for tailoring the characteristics of the photo-induced waveguides such as the number of outputs, the output intensity levels and the distance between each output channel. Accordingly, we optimized the different parameters for designing not only a fully tunable Y-coupler but also optical splitters with up to five outputs and even more complex star couplers for all-optical interconnect applications. Finally, we report on the stability behavior of the photo-induced platform. The stability threshold depends on the nonlinearity parameter beyond which the beams display time-periodic, quasi-periodic and turbulent dynamics where spatially localized instabilities can be observed. All these results suggest more opportunities for fully controllable complex waveguiding structures and new all-optical solutions for active components in optical telecommunication and innovative ways of performing optical computing based on spatiotemporal chaos.

Creation of rotating spiral structures using interfering Bessel beams for optical manipulation

A simple and effective optical tweezer setup which uses the interference of two counter propagating Bessel beams is proposed and investigated. The potential energy of a Rayleigh particle in the generated interference patterns is numerically evaluated by writing a MATLAB code. A rigorous analytical treatment is also performed for verification of the numerical results. The results show that the generated patterns have a spiral structure. It is also found that these spiral patterns and consequently the trapped particles can be rotated about the axis of the patterns by introducing a phase difference between the two beams. The suggested setup is hoped to provide an effective way for guiding particles along the spiral trajectories, making a laser-driven micro- and nanomotor, measuring the torsional properties of biological molecules, and separating particles from a liquid mixture according to their mass, size, or refractive index.

Giant resonant enhancement of optical binding of dielectric particles

Optical coupling of two identical dielectric particles gives rise to bonding and anti-bonding resonances. The latter is featured by significant narrowing of the resonant width and strong enhancement of the $Q$ factor for the high index micron size particles in subwavelength range. We consider particles shaped as spheres and disks under coaxial illumination of dual incoherent counter propagating Bessel beams. In the case of spheres we derive analytical expressions for the optical binding (OB) force which decreases as $1/L^2$ for large distance $L$ between the spheres and displays two periods of oscillations. For close distances the OB force enormously increases in the resonant regime. The case of two coaxial disks owing to variation of the distance between disks and aspect ratio of each disk is featured by extremal enhancement of the $Q$ factor compared to the case of two spheres. In that case we demonstrate unprecedent enhancement of the OB force up to several decades of nano Newtons. We show that the magnitude and sign of the OB force strongly depend on the longitudinal wave vector of the Bessel beams.

Active-quadrupole interaction of Bessel beams and their influence on atoms

This paper explains how the optical quadrupole interaction between a two-level neutral atom and two counter-propagating non-diffracting Bessel beams can exhibit interesting rotational characteristics. It shows that, in this case, the atom dynamics are determined by a radiation pressure force as well as a gradient quadrupole force. Under appropriate circumstances, neutral atoms are able to rotate with zig-zag trajectories in specific spaces inside annular shapes produced by quadrupole potential. This occurs especially when Bessel beam.

Modeling of optical binding of submicron aerosol particles in counterpropagating Bessel beams

We theoretically investigate the interparticle force between a pair of spherical aerosol nanoparticles in a dual counterpropagating Bessel beam configuration. We study the dependence of optical binding in the aerosol phase on the wavelength of the electromagnetic radiation, the particle radius, and the refractive index, including the cases of weak, moderate, and strong light absorption by the particles. We also investigate the relation between optical binding and the time-averaged intensity of the incident and scattered light. Our results show that optical binding in the aerosol phase depends strongly on the specific values of these parameters. This explains some of the difficulties associated with optical binding experiments with aerosol nanoparticles.

Dynamics of an optically bound structure made of particles of unequal sizes

This theoretical study based on the coupled dipoles model focuses on the dynamics of two optically bound dielectric spheres of unequal sizes confined in counter-propagating incoherent Bessel beams. We analyzed the relative motion of the particles with respect to each other and defined conditions where they form a stable optically bound structure (OBS). We also investigated the motion of the center of mass of the OBS and found that its direction depends on the particle separation in the structure. Besides the optical interaction between objects, we also considered a hydrodynamic coupling in order to obtain more precise results for moving an OBS.

Stability of aerosol droplets in Bessel beam optical traps under constant and pulsed external forces.

We report on the dynamics of aerosol droplets in optical traps under the influence of additional constant and pulsed external forces. Experimental results are compared with simulations of the three-dimensional droplet dynamics for two types of optical traps, the counter-propagating Bessel beam (CPBB) trap and the quadruple Bessel beam (QBB) trap. Under the influence of a constant gas flow (constant external force), the QBB trap is found to be more stable compared with the CPBB trap. By contrast, under pulsed laser excitation with laser pulse durations of nanoseconds (pulsed external force), the type of trap is of minor importance for the droplet stability. It typically needs pulsed laser forces that are several orders of magnitude higher than the optical forces to induce escape of the droplet from the trap. If the droplet strongly absorbs the pulsed laser light, these escape forces can be strongly reduced. The lower stability of absorbing droplets is a result of secondary thermal processes that cause droplet escape.

Low-temperature Bessel beam trap for single submicrometer aerosol particle studies.

We report on a new instrument for single aerosol particle studies at low temperatures that combines an optical trap consisting of two counter-propagating Bessel beams (CPBBs) and temperature control down to 223 K (-50 °C). The apparatus is capable of capturing and stably trapping individual submicrometer- to micrometer-sized aerosol particles for up to several hours. First results from studies of hexadecane, dodecane, and water aerosols reveal that we can trap and freeze supercooled droplets ranging in size from ~450 nm to 5500 nm (radius). We have conducted homogeneous and heterogeneous freezing experiments, freezing-melting cycles, and evaporation studies. To our knowledge, this is the first reported observation of the freezing process for levitated single submicrometer-sized droplets in air using optical trapping techniques. These results show that a temperature-controlled CPBB trap is an attractive new method for studying phase transitions of individual submicrometer aerosol particles.

Dynamics of submicron aerosol droplets in a robust optical trap formed by multiple Bessel beams

In this paper, we model the three-dimensional escape dynamics of single submicron-sized aerosol droplets in optical multiple Bessel beam traps. Trapping in counter-propagating Bessel beams (CPBBs) is compared with a newly proposed quadruple Bessel beam (QBB) trap, which consists of two perpendicularly arranged CPBB traps. Calculations are performed for perfectly and imperfectly aligned traps. Mie-theory and finite-difference time-domain methods are used to calculate the optical forces. The droplet escape kinetics are obtained from the solution of the Langevin equation using a Verlet algorithm. Provided the traps are perfectly aligned, the calculations indicate very long lifetimes for droplets trapped either in the CPBB or in the QBB trap. However, minor misalignments that are hard to control experimentally already severely diminish the stability of the CPBB trap. By contrast, such minor misalignments hardly affect the extended droplet lifetimes in a QBB trap. The QBB trap is found to be a stable, robust optical trap, which should enable the experimental investigation of submicron droplets with radii down to 100 nm. Optical binding between two droplets and its potential role in preventing coagulation when loading a CPBB trap is briefly addressed.

Size-dependent position of a single aerosol droplet in a Bessel beam trap

The equilibrium position of an aerosol droplet trapped in a counter-propagating Bessel beam and gas flow is studied both experimentally and theoretically. This provides an aerosol analogue to the separation of particles of differing size and refractive index in counter-propagating laser beam and liquid flow, referred to as optical chromatography. Using the model system of a pure glycerol droplet it is found that, as evaporation takes place and the size of the droplet decreases, the size-dependent equilibrium position does not change in a simple monotonic fashion. Instead, the position of the droplet is greatly affected by the excitation of whispering gallery modes. This leads to sharp peaks in the equilibrium position curve, not unlike those that occur in single particle spectroscopy. The conditions necessary to excite whispering gallery modes are thoroughly investigated.

Timescales of water transport in viscous aerosol: measurements on sub-micron particles and dependence on conditioning history

Evaporation studies of single aqueous sucrose aerosol particles as a function of relative humidity (RH) are presented for coarse and fine mode particles down into the submicron size range (600 nm < r < 3.0 μm). These sucrose particles serve as a proxy for biogenic secondary organic aerosols that have been shown to exist, under ambient conditions, in an ultraviscous glassy state, which can affect the kinetics of water mass transport within the bulk phase and hinder particle response to changes in the gas phase water content. A counter-propagating Bessel beams (CPBBs) optical trapping setup is employed to monitor the real-time change in the particle radius with RH decreasing from 75% to 5%. The slow-down of the size change upon each RH step and the deviation from the theoretical equilibrium hygroscopic growth curve indicate the onset of glassy behavior in the RH range of 10-40%. Size-dependent effects were not observed within the uncertainty of the measurements. The influence of the drying time below the glass transition RH on the timescale of subsequent water condensation and re-equilibration for sucrose particles is explored by optical tweezers measurements of micron-sized particles (3 μm < r < 6 μm). The timescale for water condensation and re-equilibration is shown to increase with increasing drying time, i.e. the time over which a viscous particle is dried below 5% RH. These studies demonstrate the importance of the history of the particle conditioning on subsequent water condensation and re-equilibration dynamics of ultraviscous and glassy aerosol particles.

Atom dynamics in two counter-propagating evanescent Bessel beams

It is shown that total internal reflection of two counter-propagating Bessel beams with a phase singularity can generate evanescent light that displays a rotational character. This effect retains the phase singularity of input light, as well reflects associated orbital angular momentum. The product optical system gives rise to optical forces that can trap lateral in-plane motion of such atoms, thus acting as a trap. We have explained basic properties of this processes and discuss how they can be used to influence atoms localized in vicinity of the surface.

Bessel standing wave technique for optical induction of complex refractive lattice structures in photorefractive materials

A novel counter-propagating Bessel beam technique is suggested and realized for optical formation of complex two-dimensional (2D) refractive lattice structures in photorefractive materials. The 2D complex lattices optically induced by the suggested method are a combination of annular and planar gratings with refractive index modulation in the radial direction in the transverse plane and the in axial direction along the optical beam propagation. The recording of the lattices is performed by 532 nm and 633 nm laser beams. The counter-propagating beam geometry builds up the Bessel standing wave with periodic annular structure in each anti-node. The refractive lattices are recorded in Z- and Y-cut 2 mm thick photorefractive lithium niobate crystals singly doped by Fe and doubly doped by Fe and Cu. The readout of recorded lattices is performed by Gaussian red and green probe laser beams. The direct observation of recorded lattices by optical phase microscope is also performed. The formed photonic structures have a period of 10 ± 1 m in the radial direction and a half-wavelength standing wave period of 266 nm or 316.5 nm in the axial direction. The diffraction efficiency of the photonic lattices reaches up to 25%.

Longitudinal optical trapping and sizing of aerosol droplets

We present evidence that aerosol droplets, approximately 1-2microm in diameter, can be optically bound over a 4mm distance within a volume formed by the overlap of the central cores and rings of two counterpropagating Bessel beams. The sizes of the individual polydisperse aerosol particles can be estimated from the angular variation of the elastic light scattering. Scattered light from the two orthogonally polarized trapping beams and from a Gaussian probe beam of different wavelength can be used to provide independent estimations of size. The coalescence of two droplets was observed and characterized.

Acoustic radiation force on a sphere in standing and quasi-standing zero-order Bessel beam tweezers

Starting from the exact acoustic scattering from a sphere immersed in an ideal fluid and centered along the propagation axis of a standing or quasi-standing zero-order Bessel beam, explicit partial-wave representations for the radiation force are derived. A standing or a quasi-standing acoustic field is the result of propagating two equal or unequal amplitude zero-order Bessel beams, respectively, along the same axis but in opposite sense. The Bessel beam is characterized by the half-cone angle β of its plane wave components, such that β = 0 represents a plane wave. It is assumed here that the half-cone angle β for each of the counter-propagating acoustic Bessel beams is equal. Fluid, elastic and viscoelastic spheres immersed in water are treated as examples. Results indicate the capability of manipulating spherical targets based on their mechanical and acoustical properties. This condition provides an impetus for further designing acoustic tweezers operating with standing or quasi-standing Bessel acoustic waves. Potential applications include particle manipulation in micro-fluidic lab-on-chips as well as in reduced gravity environments.

Resonantly enhanced multiphoton ionization of xenon in Bessel beams.

Resonantly enhanced multiphoton ionization of xenon near the three-photon 6s resonance has been studied under the excitation by the Bessel beams. It has been shown that the ionization spectra are significantly different in the cases of the Bessel and the Gaussian laser beams. This difference is caused by an internal third-harmonic field, which in the Bessel beam is generated in a wide spectral range due to the self-phase-matching mechanism. The cancellation of the 6s resonance in the Bessel beam has been demonstrated. Similar to the case of the Gaussian beams the canceled resonance appears in the spectra under the excitation by counterpropagating Bessel beams. The origin of some features in the ionization spectra is discussed. \textcopyright{} 1996 The American Physical Society.