Optical Gradient Force Research Articles

Optical bistability with a repulsive optical force in coupled silicon photonic crystal membranes

We demonstrate actuation of a silicon photonic crystal membrane with a repulsive optical gradient force. The extent of the static actuation is extracted by examining the optical bistability as a combination of the optomechanical, thermo-optic, and photo-thermo-mechanical effects using coupled-mode theory. Device behavior is dominated by a repulsive optical force which results in displacements of ≈1 nm/mW. By employing an extended guided resonance which effectively eliminates multi-photon thermal and electronic nonlinearities, our silicon-based device provides a simple, non-intrusive solution to extending the actuation range of micro-electromechanical devices.

Ultrafast Spinning of Gold Nanoparticles in Water Using Circularly Polarized Light

Controlling the position and movement of small objects with light is an appealing way to manipulate delicate samples, such as living cells or nanoparticles. It is well-known that optical gradient and radiation pressure forces caused by a focused laser beam enables trapping and manipulation of objects with strength that is dependent on the particle's optical properties. Furthermore, by utilizing transfer of photon spin angular momentum, it is also possible to set objects into rotational motion simply by targeting them with a beam of circularly polarized light. Here we show that this effect can set ∼200 nm radii gold particles trapped in water in 2D by a laser tweezers into rotation at frequencies that reach several kilohertz, much higher than any previously reported light driven rotation of a microscopic object. We derive a theory for the fluctuations in light scattering from a rotating particle, and we argue that the high rotation frequencies observed experimentally is the combined result of favorable optical particle properties and a low local viscosity due to substantial heating of the particles surface layer. The high rotation speed suggests possible applications in nanofluidics, optical sensing, and microtooling of soft matter.

Optical trapping forces depending on size of dielectric polystyrene microspheres

A single-beam gradient force trap or an optical laser tweezer has been a powerful tool to accurately measure pico-newton to femto-newton forces in a variety of fields, such as biology, colloid/polymer physics, and rheology. When an appropriate refractive index mismatch between an object and a medium is provided, the laser beam can hold the object of which its size ranges from a few nanometers to tens of micrometer scale. The optical trapping mechanism depends on the relative length scale between an object and the wavelength of a light. In the Rayleigh regime where the diameter of the object is smaller than the wavelength of the light ( ), the trap stiffness increases with the particle size, as the number of rays incident on the particle increases. On the contrary, all rays hit an object in the ray optics regime ( ) such that Ashkin expected the trapping force to be independent of its size without quantitative calculations. Similarly, Wright et al. predicted that the size dependence on the axial and radial trapping forces are negligible in the ray optics regime, based on calculating the dimensionless efficiency of the optical trapping force for a single ray. Using the generalized LorenzMie theory, Lock also suggested that the trapping force in the axial direction does not change with the particle size. However, Simmons et al. reported the experimental results in which the trap stiffness for dielectric particles decreases as the size increases when the particle size is larger than the wavelength of a light. More recently, Mazolli et al. used an explicit partial-wave representation to extend the force expression from the Rayleigh regime (i.e., electromagnetic force model) to the GOA. In the ray optics regime, they reported that the trap stiffness is inversely proportional to the particle radius. Tlusty et al. also calculated the trapping force of highly localized fields on dielectric particles in which the trapping force in the ray optics regime decreases as their size increases. In this communication, we aim to address such contradiction, regarding the dependence of the optical trapping force on the particle size in the ray optics regime. Based on the GOA, we analytically calculate the optical trapping force with varying the particle size. Then, we compare the analytical calculations to the drag experiments in which a particle trapped by the laser tweezers is subjected to the Stokes drag force. We also provide a schematic explanation, based on drawing the ray propagation and finding the magnitude of the momentum change. Before we report our results, experimental materials and method are briefly described. We use polystyrene (PS, Interfacial Dynamics Corporation) latex particles with different radii, a=1.4, 1.7, and 3.7 μm. The refractive index of these particles is np=1.57. In order to obtain the trap stiffness (κt), which is the ratio of the Stokes drag force (Fs=6πaηu) to the lateral displacement (∆y) from the center of the particle, it is held in the aqueous phase (refractive index, nm=1.326) by the laser tweezers, and subjected to drag forces by translating the microscope stage at several constant velocities between u=3 and 7 μm/s. The viscosity of the aqueous solution is η=1 cP. We use particle tracking to measure the particle displacement during applying the Stokes flow around the particle. A movie captured on a CCD camera (Hitachi KP-M1) is saved at a frame rate of 29.97 frames/s. The digitized image sequences are imported to image processing software (ImageJ) to find the particle displacement (∆y) from the equilibrium position in the optical trap. The laser tweezer apparatus is constructed around the inverted microscope. The laser is generated by a 4-Watt CW Nd:YAG laser (λ0=1,064 nm, Coherent Compass 1064-400M). A water immersion objective (63×, NA 1.2, Zeiss C-Apochromat) is used to provide a high convergence angle (φmax), which is a maximum half angle of the conelike laser beam, given by φmax=sin(NA/nm)≈64.5. We refer the reader to Pantina and Furst for a more detailed description of the laser tweezer setup. In order to quantitatively calculate the optical trapping force in the ray optics regime, we consider the first reflection and the successive refractions of a single incident ray with power (P) hitting a dielectric spherical particle, the dimensionless efficiency (Q) of the trapping force is given by,

Enhanced optical forces in integrated hybrid plasmonic waveguides

We demonstrate gradient optical forces in metal-dielectric hybrid plasmonic waveguides (HPWG) for the first time. The magnitude of optical force is quantified through excitation of the nanomechanical vibration of the suspended waveguides. Integrated Mach-Zehnder interferometry is utilized to transduce the mechanical motion and characterize the propagation loss of the HPWG. Compared with theory, the experimental results have confirmed the optical force enhancement, but also suggested a significantly higher optical loss in HPWG. The excessive loss is attributed to metal surface roughness and other non-idealities in the device fabrication process.

Diamond-integrated optomechanical circuits

Diamond offers unique material advantages for the realization of micro- and nanomechanical resonators because of its high Young's modulus, compatibility with harsh environments and superior thermal properties. At the same time, the wide electronic bandgap of 5.45 eV makes diamond a suitable material for integrated optics because of broadband transparency and the absence of free-carrier absorption commonly encountered in silicon photonics. Here we take advantage of both to engineer full-scale optomechanical circuits in diamond thin films. We show that polycrystalline diamond films fabricated by chemical vapour deposition provide a convenient wafer-scale substrate for the realization of high-quality nanophotonic devices. Using free-standing nanomechanical resonators embedded in on-chip Mach-Zehnder interferometers, we demonstrate efficient optomechanical transduction via gradient optical forces. Fabricated diamond resonators reproducibly show high mechanical quality factors up to 11,200. Our low cost, wideband, carrier-free photonic circuits hold promise for all-optical sensing and optomechanical signal processing at ultra-high frequencies.

Focusing properties of hyperbolic-cosine-Gaussian beams combining a helical axicon

Focus shape plays an important role in many optical systems, and has attracted much attention. In this article, the focusing properties of hyperbolic-cosine-Gaussian (cosh-Gaussian) beams through a helical axicon were investigated theoretically. Results show that intensity distribution in focal region can be altered considerably by the decentered parameter of cosh-Gaussian beam, axicon parameter and topological charge of the helical axicon. Many novel focal pattern may occur, including annular focus, rectangular focal shape, multiple-peak focal pattern. And in order to show the possible applications of these alter focal pattern, some optical gradient force distributions were also calculated and illuminated.

Optical manipulation of gold nanoparticles using an optical nanofiber

Gold nanoparticles are gaining increasing attention due to their biological and medical applications. In this letter, we experimentally demonstrate the optical manipulation of 250-nm-diameter gold nanoparticles along an optical nanofiber (550 nm in diameter) injected by an 808-nm laser light. The nanoparticles situated in the evanescent optical field are trapped by optical gradient force and move along the direction of light propagation due to optical scattering force. The velocities reach as high as 132 μm/s at an optical power of 80 mW.

Enhancing Optical Gradient Forces with Metamaterials

We demonstrate how the optical gradient force between two waveguides can be enhanced using transformation optics. A thin layer of double-negative or single-negative metamaterial can shrink the interwaveguide distance perceived by light, resulting in a more than tenfold enhancement of the optical force. This process is remarkably robust to the dissipative loss normally observed in metamaterials. Our results provide an alternative way to boost optical gradient forces in nanophotonic actuation systems and may be combined with existing resonator-based enhancement methods to produce optical forces with an unprecedented amplitude.

A nanoelectromechanical systems optical switch driven by optical gradient force

A nanoelectromechanical systems (NEMS) optical switch driven by optical gradient force is demonstrated. The switch consists of a free-standing ring resonator and two bus waveguides. When the free-standing ring is bended by the optical gradient force, the output transmission signal is tuned from the on-state to the off-state of the switch. The NEMS optical switch shows a switching time of 43.5 ns and a switching contrast of more than 25 dB. With its chip-scale integrability, the optical switch has potential applications in signal processing and high-speed optical communication networks.

Optical gradient force of cosh-Gaussian with sine-azimuthal and half-space phase modulation

The optical gradient force distributions in focal plane of cosh-Gaussian beams with sine-azimuthal variation wavefront and half space phase modulation were investigated. Results show that optical gradient force distributions can be affected considerably by the phase retardation of half-space phase modulation, of a phase parameter that indicates the phase change frequency on an increasing azimuthal angle, and beam parameters in cosh terms of the incident beams. Many gradient force patterns occur, including cross-shape, multiple optical trap arrays, multiple-trap wheel, and many kinds of gradient force lines and curves. Symmetry of the whole gradient force pattern can also be altered remarkably. Above results may find wide applications in optical trapping systems.

Light-induced phase transitions of colloidal monolayers with crystalline order

We experimentally study the phase behavior of a charge-stabilized two-dimensional colloidal crystal which is subjected to a one-dimensional periodic light field. Such light fields are created by a scanned optical line tweezer which allows the variation of the periodicity without optical realignments. In order to realize a wide range of line spacings relative to the lattice constant, we use a suspension of silica particles in bromobenzene. This colloidal system has a Debye screening length of about 4.6 μm which results in the formation of crystals with lattice constants up to 20 μm. Because the refractive index of bromobenzene is larger than that of the colloids, optical gradient forces lead to the attraction of particles at regions where the intensity is smallest. Depending on the depth and periodicity of the optical potential, we observe the light-induced assembly of colloids into triangular, rhombic and square phases.

Microfluidic cell sorter for use in developing red fluorescent proteins with improved photostability.

This paper presents a novel microfluidic cytometer for mammalian cells that rapidly measures the irreversible photobleaching of red fluorescent proteins expressed within each cell and achieves high purity (>99%) selection of individual cells based on these measurements. The selection is achieved by using sub-millisecond timed control of a piezo-tilt mirror to steer a focused 1064-nm laser spot for optical gradient force switching following analysis of the fluorescence signals from passage of the cell through a series of 532-nm laser beams. In transit through each beam, the fluorescent proteins within the cell undergo conversion to dark states, but the microfluidic chip enables the cell to pass sufficiently slowly that recovery from reversible dark states occurs between beams, thereby enabling irreversible photobleaching to be quantified separately from the reversible dark-state conversion. The microfluidic platform achieves sorting of samples down to sub-millilitre volumes with minimal loss, wherein collected cells remain alive and can subsequently proliferate. The instrument provides a unique first tool for rapid selection of individual mammalian cells on the merits of photostability and is likely to form the basis of subsequent lab-on-a-chip platforms that combine photobleaching with other spectroscopic measurements for on-going research to develop advanced red fluorescent proteins by screening of genetic libraries.

Plasmonic Nanostructures as Accelerators for Nanoparticles: Optical Nanocannon

We suggest a model of an optical structure that allows to accelerate nanoparticles to velocities on the order of tens of centimeters per second using low-intensity external optical fields. The nano-accelerator system employs metallic V-grooves which concentrate the electric field in the vicinity of their bottoms and creates large optical gradient forces for the nanoparticles in that groove. The conditions are found when this optical force tends to eject particles away from the groove.

Focus shaping of linearly polarized Lorentz beam with sine-azimuthal variation wavefront

Focal shaping plays an important role in many optical focusing systems. In this paper, the focusing properties of linearly polarized Lorentz beam with sine-azimuthal variation wavefront were investigated. Simulation results show that the focal pattern can be altered considerably by the beam parameters and the phase parameter that indicates the phase change frequency on increasing azimuthal angle. And some novel focus shape may appear, including multiple-peak focal pattern, wheel focal pattern. And some optical gradient force distributions were also given to show that the focusing properties of this kind of Lorentz beams may be used to construct tunable optical traps, which may pave the way for application of laser diode in tweezers technique.

Broadband opto-mechanical phase shifter for photonic integrated circuits

A broadband opto-mechanical phase shifter for photonic integrated circuits is proposed and numerically investigated. The structure consists of a mode-carrying waveguide and a deformable non-mode-carrying nanostring, which are parallel with each other. Utilizing the optical gradient force between them, the nanostring can be deflected. Thus the effective refractive indices of the waveguide is changed with the deformation, and further causes a phase shift. The phase shift under different geometry sizes, launched powers and boundary conditions are calculated and the dynamical properties as well as the thermal noise's effect are also discussed. It is demonstrated that a $\pi$ phase shift can be realized with only about 0.64 mW launched power and 50 $\mu m$ long nanostring. The proposed phase shifter may find potential usage in future investigation of photonic integrated circuits.

Gold nanorod-induced localized surface plasmon for microparticle aggregation

We investigate a technique for microparticle aggregation via optical fiber-excited localized surface plasmon resonance of gold nanorods, which were deposited on the designated surface of a tapered optical fiber (TOF). Through the enhanced photothermal effect, a microbubble forms on the TOF at an input optical power of 14 mW. Marangoni convection transports the microparticles dispersed in water to the bubble, where they accumulate on the gold nanorod surface because of strong optical gradient force.

Force-induced optical nonlinearity and Kerr-like coefficient in opto-mechanical ring resonators

This paper demonstrates the optical nonlinearity in opto-mechanical ring resonators that consist of a bus waveguide and two ring resonators, which is induced by the optical gradient force and characterized by the Kerr-like coefficient. Each ring resonator has a free-hanging arc that is perpendicularly deformable by an optical gradient force and subsequently this deformation changes the effective refractive index (ERI) of the ring resonator. The change of the ERI induces optical nonlinearity into the system, which is described by an equivalent Kerr coefficient (Kerr-like coefficient). Based on the experimental results, the Kerr-like coefficient of the ring resonator system falls in the range from 7.64 × 10(-12) to 2.01 × 10(-10) m(2)W(-1), which is at least 6-order higher than the silicon's Kerr coefficient. The dramatically improved optical nonlinearity in the opto-mechanical ring resonators promises potential applications in low power optical signal processing, modulation and bio-sensing.

Optical Manipulation in Liquid Crystals: Effects of Tight Focusing on Nonlinear Optical Reorientation

We report a detailed analysis of the problem of nonlinear optical reorientation induced in a nematic liquid crystal by a tightly focused Gaussian beam under experimental conditions that prevent the effect of conventional trapping originated by optical gradient forces. Novel features arise under these conditions: no threshold effect; reduced reorientation if compared the one induced by a paraxial Gaussian beam, orientational singularity at the focal waist.

Microfluidic sorting with a moving array of optical traps

Optical sorting was demonstrated by selective trapping of a set of microspheres (having specific size or composition) from a flowing mixture and guiding these in the desired direction by a moving array of optical traps. The approach exploits the fact that whereas the fluid drag force varies linearly with particle size, the optical gradient force has a more complex dependence on the particle size and also on its optical properties. Therefore, the ratio of these two forces is unique for different types of flowing particles. Selective trapping of a particular type of particles can thus be achieved by ensuring that the ratio between fluid drag and optical gradient force on these is below unity whereas for others it exceeds unity. Thereafter, the trapped particles can be sorted using a motion of the trapping sites towards the output. Because in this method the trapping force seen by the selected fraction of particles can be suitably higher than the fluid drag force, the particles can be captured and sorted from a fast fluid flow (about 150 μm/s). Therefore, even when using a dilute particle suspension, where the colloidal trafficking issues are naturally minimized, due to high flow rate a good throughput (about 30 particles/s) can be obtained. Experiments were performed to demonstrate sorting between silica spheres of different sizes (2, 3, and 5 μm) and between 3 μm size silica and polystyrene spheres.

Optical forces in plasmonic nanoantennas

The optical forces acting on nanoparticles in V-shaped plasmonic resonators with a high local-field gain have been investigated. Two versions are considered, which make it possible to implement either attractive or repulsive gradient optical forces. A plasmonic resonator is proposed, which can focus 350-nm radiation and implement a repulsive gradient force. It has been shown for the first time that a perturbation induced by a nanoparticle redistributes the field in the resonator so that additional intensity peaks arise in both versions to hold the nanoparticle in the resonator by forming an optical trap.