Optical Gradient Force Research Articles

Characterization of the near-field and convectional transport behavior of micro and nanoparticles in nanoscale plasmonic optical lattices.

Here, we report the characterization of the transport of micro- and nanospheres in a simple two-dimensional square nanoscale plasmonic optical lattice. The optical potential was created by exciting plasmon resonance by way of illuminating an array of gold nanodiscs with a loosely focused Gaussian beam. This optical potential produced both in-lattice particle transport behavior, which was due to near-field optical gradient forces, and high-velocity (∼μm/s) out-of-lattice particle transport. As a comparison, the natural convection velocity field from a delocalized temperature profile produced by the photothermal heating of the nanoplasmonic array was computed in numerical simulations. This work elucidates the role of photothermal effects on micro- and nanoparticle transport in plasmonic optical lattices.

Lateral shearing optical gradient force in coupled nanobeam photonic crystal cavities

We report the experimental observation of lateral shearing optical gradient forces in nanoelectromechanical systems (NEMS) controlled dual-coupled photonic crystal (PhC) nanobeam cavities. With an on-chip integrated NEMS actuator, the coupled cavities can be mechanically reconfigured in the lateral direction while maintaining a constant coupling gap. Shearing optical gradient forces are generated when the two cavity centers are laterally displaced. In our experiments, positive and negative lateral shearing optical forces of 0.42 nN and 0.29 nN are observed with different pumping modes. This study may broaden the potential applications of the optical gradient force in nanophotonic devices and benefit the future nanooptoelectromechanical systems.

An opto-mechanical coupled-ring reflector driven by optical force for lasing wavelength control

In this paper, an opto-mechanical coupled-ring reflector driven by optical gradient force is applied in an external-cavity tunable laser. A pair of mutually coupled ring resonators with a free-standing arc serves as a movable reflector. It obtains a 13.3-nm wavelength tuning range based on an opto-mechanical lasing-wavelength tuning coefficient of 127 GHz/nm. The potential applications include optical network, on-chip optical trapping, sensing, and biology detection.

Dynamic analysis of hyperbolic waveguide resonator driven by optical gradient force

Abstract. As a unique type of driving force, the transverse optical gradient force has been extensively studiedand applied in the nanowaveguides resonator. Recently, it is demonstrated that the optical forces in slot wave-guides of hyperbolic metamaterials can be over two orders of magnitude stronger than that in conventionaldielectric slot waveguides. To investigate the nonlinear dynamic characteristic of hyperbolic waveguide reso-nator driven by optical gradient force, a continuum elastic model of the optoresonator is presented and analyti-cally solved using the methods of Rayleigh–Ritz and multiple scales. The results show that the optical force isstrengthened with the increase of the filling ratio of silver in the hyperbolic waveguide. The resonance frequencybecomes greater with the increase of the filling ratio of silver no matter what the geometric parameters andphysical property parameters are. However, the steady maximum vibration amplitude becomes smaller, andthe degree of system stiffness softening also reduces.

Plasmon-assisted trapping of nanoparticles using a silver-nanowire-embedded PMMA nanofiber.

The integration of surface plasmon with waveguide is a strategy for lab-on-a-chip compatible optical trapping. Here, we report a method for trapping of nanoparticles using a silver nanowire (AgNW) embedded poly(methyl methacrylate) (PMMA) nanofiber with the assistance of surface plasmon polaritons (SPPs). The nanoparticles (polystyrene, 700 nm diameter) are transported along the nanofiber and ultimately trapped at the AgNW embedded region because of the enhanced optical gradient force towards the nanofiber exerted on the nanoparticles and optical potential well generated by the excitation of SPPs. The low optical power requirement and the easy fabrication of the AgNW-embedded nanofiber with broad range of wavelength for SPPs are advantageous to the applications in optofluidics and plasmofluidics.

Optical gradient force of linearly polarized sine-azimuthal Lorentz beam with one on-axis optical vortex

Optical gradient force of linearly polarized sine-azimuthal Lorentz beam with one on-axis optical vortex was investigated theoretically in this article, and calculation results show that the effect of charge number of optical vortex on optical gradient force pattern is very considerable, and force evolution principle differs for different beam parameter and phase parameter that indicates the sine phase change frequency on increasing azimuthal angle. Some novel gradient force patterns appear, such as, force rings, swallow shape, force ripples, multiple force lines. In addition, the whole force pattern is symmetric about one axis when phase parameter is odd number, while, whole force pattern is symmetric about two orthogonal axes under condition that phase parameter is even number. Introduction of charge number adds one freedom for controlling optical gradient force distribution.

Influence of optical forces on nonlinear optical frequency conversion in nanoscale waveguide devices.

We theoretically investigate the influence of optical gradient forces on nonlinear frequency conversion in a typical nanoscale optomechanical system, which consists of two parallel, suspended waveguides. The waveguides deform with the input power and the phase-matching wavelength changes along the waveguides. Utilizing the spread of deformations collectively allows phase matching over a wider range of pump wavelengths. The third harmonic phase-matching wavelength shift can be as large as 3.6 nm/mW when the waveguide length is 100 μm and the initial gap is 150 nm. It is analogous to chirping the poling period of quasi-phase-matched devices to extend their bandwidths, and allows broad third harmonic to be generated for uses such as biological spectroscopy. Finally, we discuss the conversion efficiency and the optimal phase-matching wavelength with a single-frequency pump.

Systematic analysis of optical gradient force in photonic crystal nanobeam cavities.

In this work, we provide systematic analysis of the optical force for a particle trapped in the waveguide/cavity/waveguide system. The theoretical analysis shows that the optical trapping force is proportional to QT1/2/V for the particle at a fixed position. We provide numerical proof for the proposed principles and systematically optimize the design recipe to investigate optical tapping force of photonic crystal (PhC) nanobeam cavities. The numerical results are in accordance with theoretical analysis and show that the maximum optical trapping force for a particle occurs at the transmission of cavity around 0.25 and the maxima are almost the same for different PhC cavities.

Optical tweezing by photomigration.

Photomigration in azo polymers is an area of research that witnessed intensive studies owing to its potential in optical manipulation, e.g., optical tweezing, the physical mechanism of which remains unsolved since its discovery about two decades ago. In this paper, a detailed theoretical study that reproduces the phenomena associated with photomigration is presented, including the physical models and the associated master equations. Polarization effects are discussed and analytical solutions are given to describe the steady-state and the dynamics of photomigration. Such a theory leads to new theoretical experiments relating material properties to light action. A photoisomerization force which is described by a spring-type model is introduced. This force is derived from a harmonic light potential that moves the azo polymer. This force is parenting to optical tweezers, but it is quite different in the sense that it requires photoisomerization to occur. The azo polymer's motion is governed by four competing forces: the photoisomerization force, and the restoring optical gradient and elastic forces, as well as the random forces due to spontaneous diffusion.

Self-propelled round-trip motion of Janus particles in static line optical tweezers.

Controlled propulsion of microparticles and micromachines in fluids could revolutionize many aspects of technology, such as biomedicine, microfluidics, micro-mechanics, optomechanics, and cell biology. We report the self-propelled cyclic round-trip motion of metallo-dielectric Janus particles in static line optical tweezers (LOT). The Janus particle is a 5 μm-diameter polystyrene sphere half-coated with 3 nanometer thick gold film. Both experiment and theory show that this cyclic translational and rotational motion is a consequence of the collective and fine action of the gold-face orientation dependent propulsion optical force, the gradient optical force, and the spontaneous symmetry breaking induced optical torque in different regions of the LOT. This study indicates a novel way to propel and manipulate the mechanical motion of microscopic motors and machines wirelessly in fluid, air, or vacuum environments using a static optical field with a smartly designed non-uniform intensity profile allowing fully controlled momentum and angular momentum exchange between light and the particle.

Nanostructured grating patterns over a large area fabricated by optically directed assembly.

Optical trapping and manipulation of nanoparticles (NPs) have been widely used in nanotechnology and biology. Here, we demonstrate an optically directed assembly (ODA) route for bottom-up fabrication of stable nanostructured grating patterns in solution using laser standing evanescent wave (LSEW) fields. The control mechanism is the intriguing cooperative action of the periodically line-centered attractive optical gradient force and the near field dipolar coupling force induced by LSEW, which leads to assembly of the colloidal silver NPs into robust grating patterns within minutes. The anisotropic polarization nature of the grating patterns was studied further by examining the morphology correlation of the surface-enhanced Raman scattering (SERS)-based signal amplification. We show the LSEW ODA method can optimize and stabilize the strongest dipolar coupling style among the NPs during pattern assembly. These results advance the further understanding of ODA of colloid NPs and might have many potential applications in SERS, catalysis, nanophotonics and nano-fabrication.

A silicon-nanowire memory driven by optical gradient force induced bistability

In this paper, a bistable optical-driven silicon-nanowire memory is demonstrated, which employs ring resonator to generate optical gradient force over a doubly clamped silicon-nanowire. Two stable deformation positions of a doubly clamped silicon-nanowire represent two memory states (“0” and “1”) and can be set/reset by modulating the light intensity (<3 mW) based on the optical force induced bistability. The time response of the optical-driven memory is less than 250 ns. It has applications in the fields of all optical communication, quantum computing, and optomechanical circuits.

Transverse Chiral Optical Forces by Chiral Surface Plasmon Polaritons

Recently the new concepts of transverse spin angular momentum and Belinfante spin momentum of evanescent waves have drawn considerable attention. Here, we investigate these novel physical properties of electromagnetic fields in the context of locally excited surface plasmon polaritons. We demonstrate, both analytically and numerically, that locally excited surface plasmon polaritons possess transverse spin angular momentum and Belinfante momentum with rich and non-trivial characteristics. We also show that the transverse spin angular momentum of locally excited surface plasmon polaritons leads to the emergence of transverse chiral forces in opposite directions for chiral objects of different handedness. The magnitude of such a transverse force is comparable to the optical gradient force and scattering forces. This finding may pave the way for realization of optical separation of chiral biomolecules.

Silver Nanoparticles - Trapping and Manipulation

Silver nanoparticles with different radii were synthesized using silver nitrate solution added with sodium borohydride solution at different ratios. The synthesized silver nanoparticles of radii 25.3 nm, 31 nm, 33.6 nm and 37.1 nm were characterized by transmission electron microscopy (TEM) and UV-vis absorption spectroscopy. The synthesized silver nanoparticles exhibit spherical morphology for all radii. The silver nanoparticles exhibit the plasmon resonance band between 390 nm – 400 nm. The optical trapping effect of Gaussian beam acting on a silver nanoparticle in Rayleigh regime was studied. The optical scattering and optical gradient forces were calculated for silver nanoparticles of different radii.

Trapping and Manipulation of Copper Nanoparticles in Rayleigh Regime

The copper nanoparticles were synthesized by green technology using leaf extracts of Ocimum sanctum. The synthesized copper nanoparticles were confirmed by the change of colour after the addition of leaf extract into the copper sulfate solution. The synthesized copper was characterized by X-ray Diffraction (XRD), transmission electron microscopy (TEM) and UV-vis absorption spectroscopy. The copper nanoparticles are crystallized with FCC structure. The synthesized copper nanoparticles exhibit spherical morphology with average particle size of 20 nm. The copper nanoparticle exhibits absorption broad band between 550 nm – 575 nm. The optical trapping effect of Gaussian beam acting on a copper nanoparticle in Rayleigh regime was studied. The optical scattering and optical gradient forces were calculated for 20 nm copper particle.

Optical manipulation in optofluidic microbubble resonators

An optical manipulation system based on optofluidic microbubble resonators (MBR) is proposed. As the high-Q whispering gallery modes (WGMs) are excited in an MBR, the buildup of the field intensity inside the resonator is large enough to trap nanoscale particles. The optical gradient forces generated by the WGMs with different radial orders are investigated numerically. The negative effect of the resonance detuning induced by the particles is taken into account to investigate the optical gradient forces exerting on the particles. By the stability analysis, the WGMs with high radial orders show a better trapping stability under Brownian motion since most of the optical fields reside within the water core.

Movement of a millimeter-sized oil drop pushed by optical force**Project supported by the National Natural Science Foundation of China (Grant Nos. 90921009 and 11274401).

We show experimentally that when an unfocused continuous wave (CW) laser beam is obliquely incident onto the surface of a millimeter-sized mineral oil drop on sucrose solution, it will exert a pushing force on the oil drop, making it move forwards along the surface of the sucrose solution. However, after a period of time, the oil drop stops moving. This can be explained as the phenomenon caused by the change of Abraham momentum, the optical gradient force, and friction together.

Optical regulation of cell chain.

Formation of cell chains is a straightforward and efficient method to study the cell interaction. By regulating the contact sequence and interaction distance, the influence of different extracellular cues on the cell interaction can be investigated. However, it faces great challenges in stable retaining and precise regulation of cell chain, especially in cell culture with relatively low cell concentration. Here we demonstrated an optical method to realize the precise regulation of cell chain, including removing or adding a single cell, adjusting interaction distance, and changing cell contact sequence. After injecting a 980-nm wavelength laser beam into a tapered optical fiber probe (FP), a cell chain of Escherichia colis (E. colis) is formed under the optical gradient force. By manipulating another FP close to the cell chain, a targeted E. coli cell can be trapped by the FP and removed from the chain. Further, the targeted cell can be added back to the chain at different positions to change the cell contact sequence. The experiments were interpreted by numerical simulations and the impact of cell sizes and shapes on this method was analyzed.

Optical nonreciprocity in asymmetric optomechanical couplers.

We propose an all-optical integrated nonreciprocal device on the optomechanical platform with a large nonreciprocal bandwidth and low operating power. The device is based on an asymmetric silicon coupler consisting of two branches. One of them is a conventional strip waveguide fixed on the substrate, and the other is a freestanding nanostring suspended above a groove in the substrate. When light is launched into the coupler, the optical gradient force between the freestanding nanostring and the underlying substrate leads to the deflection of the nanostring, and finally results in destruction of the initial phase-matching condition between the two branches. The suspended branch would achieve distinct deflections when light is incident from different ports. The simulation results show a nonreciprocal bandwidth of 13.1 nm with operating power of 390 μW. With the advantages of simple structure, low power consumption and large operating bandwidth, our work provides a promising solution for on-chip passive nonreciprocal device.

A nano-silicon-photonic switch driven by an optical gradient force

Using light to dynamically and stably redirect the flow of another beam of light is a long-term goal for photonic-integrated circuits. However, it is challenging to realize a practically all-optical switching device in silicon owing to its weak optical nonlinearity. Major published work on all-optical switches were using single-photon absorption and two-photon absorption, which requires ultrahigh switching energy. This paper presents a nano-silicon-photonic all-optical switch driven by an optical gradient force, in which a fast switching speed with low power consumption is obtained. Each switching element is composed of a waveguide crossing connection and a micro-ring resonator. The ring resonator is side-coupled to a double-etched waveguide crossing, while the micro-ring resonator is partially released from the substrate and becomes free-standing. When the “drop” port is in “OFF” state, the wavelength of the signal light from the “input” port does not satisfy the resonant condition in the micro-ring. Therefore, light is mainly transmitted to the "thru" port without control light. When a control light is loaded to the “add” port, of which the wavelength satisfies the resonance condition in the micro-ring, a strong optical gradient force is generated by the induced evanescent optical field. The freestanding arc of the ring is then bent down to the substrate, leading to a cavity resonance wavelength shift. As a result, the signal light is diverted to the “drop” port and the corresponding transmission state is switched to the “ON” state. The optical switch is fabricated by nano-photonic fabrication processes using standard silicon-on-insulator (SOI) wafer. The waveguide structures have a width of 450 nm and a height of 220 nm for a single mode transmission; the outer radius of the ring in the switching element is 15 μm; the coupling gap between the ring and the nano-waveguide is 200 nm; the system is fabricated through two-step lithography and plasma dry etching processes while the free-standing arc is released by undercutting the buried oxide layer. #br#A switching time of 180 ns(rise) and 170 ns (fall) is experimentally demonstrated, which is much faster than that of conventional optical switches. The present optical switch can reach a high extinction ratio (10.67 dB) and a low crosstalk (-11.01 dB). In addition, the proposed switch has the advantages of compact size and low power consumption. Potential applications of this optical switch include photonic integrated circuits, signal processing, and high speed optical communication networks.