Optical Gradient Force Research Articles

Controllable transport of nanoparticles along waveguides by spin-orbit coupling of light.

Waveguide optical tweezers can capture and transport nanoparticles, and have important applications in biology, physics, and materials science. However, traditional waveguide optical tweezers need to couple incident light into one end of the waveguide, which causes problems such as difficulty in alignment and low efficiency. Here, we propose a new type of waveguide optical tweezers based on spin-orbit coupling of light. Under the effect of spin-orbit coupling between the waveguide and nearby particles illuminated by a circularly polarized light, the particles experience a lateral recoil force and a strong optical gradient force, which make particles in a large area to be trapped near the waveguide and then transmitted along the waveguide, avoiding the coupling of light into one end of the waveguide. We further demonstrate that the particles can be transmitted along a curved waveguide and even rotated along a ring-shaped waveguide, and its transmission direction can be simply switched by adjusting the spin polarization of incident light. This work has significance in the research of optical on-chip nano-tweezers.

Bending behavior and its effect on switching performance of an all-optical switch

The all-optical switch is one of the key elements of all-optical high speed information networks and computing systems. Their bending behavior, actuated by optical gradient forces, dramatically influences their switching performance and is therefore the research topic of this paper. The coupled behavior between optical gradient forces and the bending deflection is taken into account in the present model. The effects of the effective refractive index and the stretching induced by bending are included and their influences on the resonance wavelength shift of the signal light are discussed. Results show that the resonance wavelength shift increases almost linearly at first, and then increases suddenly as the circulating power increases. The resonance wavelength shift increases as the central angle of the suspended part increases, while it decreases with the initial gap. The present theoretical model may be helpful when designing all-optical switches.

Focusing and propagation properties of Bessel-Gaussian beam with a power-order mixing helical-conical phase wavefront.

Based on the vector diffraction theory, this paper investigates the focusing and propagation characteristics of a Bessel-Gaussian beam with a power-order mixing helical-conical wavefront. It discusses the focused light intensity, optical gradient force distribution, and propagation characteristics under different parameters in detail. The results show that the topological charge number L can finely adjust the opening of the spiral-like curve at the focal plane. The power order n can adjust the energy distribution in the focus area. By increasing the eccentricity parameters K, the position of the maximum intensity spot will move along with the y coordinate axis. When the beam parameter β increases, the number of intensity peaks on the focal plane increases. As the propagation distance z increases, the pattern of intensity distribution will change from an ellipse to "doughnut-like." Then a circular dark core will appear in the center of the light, and the propagation mode will gradually become an optical trap. Some optical gradient force distributions also are investigated to illuminate the applications of these alterable focal patterns.

Optical Trapping of Photosoftened Solid Polymers

We present evidence of optical trapping of solid polymer films by a tightly focused near-infrared (NIR) laser beam (785 nm) when the polymer is softened by green laser (532 nm) irradiation, which is uniformly distributed over the film surface. While the whole film is softened by the green laser, only the part which is subjected to the NIR laser is trapped. The film, which is transparent in the NIR range and strongly absorbs the visible light, is deposited on a glass substrate. The NIR laser is focused at the sample plane and attracts the polymer to the laser focus, by the optical gradient force, during the green light irradiation, indicating that molecular mobility is strongly reduced and the optical gradient force of the NIR laser is inefficient in moving the polymer below the glass-transition temperature of the polymer without photosoftening. The observations are consistent with the theory of photochemical tweezing, which rests on the assumption that photosensitive polymers, containing azo dyes, are softened, i.e., molecular mobility is enhanced, by photoisomerization of the azo dyes during green light irradiation, and are moved in gradients of light fields. We also found that this two-color laser tweezing scheme is sensitive to light polarization, owing to the polarization sensitivity of the azo dye chromophores.

Efficient actuation design for optomechanical sensors.

For any nanomechanical device intended for sensing applications, actuation is an important consideration. Many different actuation mechanisms have been used, including self-oscillation, piezoelectric shakers, capacitive excitation, and optically pumping via the optical gradient force. Despite the relatively frequent use of optical pumping, the limits of optical actuation with a pump laser have not been fully explored. We provide a practical framework for designing optical cavities and optomechanical systems to maximize the efficiency of the optical pumping technique. The effects of coherent backscattering on detection and actuation are included. We verify our results experimentally and show good agreement between the model and experiment. Our model for efficient actuation will be a useful resource for the future design of optomechanical cavities for sensor and other high-amplitude applications.

Formation mechanism of the nanostructure in laser streaming phenomenon.

Laser streaming is a phenomenon in which liquid streaming is driven directly from the laser through an in situ fabricated nanostructure. In this study, liquid streaming of a gold nanoparticle suspension driven by a pulsed laser was studied using a high-speed camera. The laser streaming formation time, streaming velocity, and relative energy conversion efficiency of laser streaming was measured for different nanoparticle concentrations, focal lens position, laser powers, and laser repetition rates. In addition to the laser intensity, which played a significant role in the formation process of laser streaming, the optical gradient force was found to be an important approach involved in the transport and provision of nanoparticles during the formation of laser streaming. This finding facilitated a better understanding of the formation mechanism of laser streaming and demonstrated the possibilities of a new potential laser etching technique based on nanosecond lasers and nanoparticle suspensions. This result can also expand the application of laser streaming in microfluids and other fields that require lasers to move macroscopic objects at relatively high speeds.

Axicon optical forces and other kinds of transverse optical forces exerted by off-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory

In two recent papers, it has been demonstrated that, beside usual scattering and optical forces, new optical forces and terms are exhibited when a Rayleigh particle is illuminated by an off-axis Bessel beam. Namely, (i) axicon optical forces are associated with scattering optical forces and (ii) additional axicon terms are associated with gradient optical forces. These extra-axicon forces and terms are zero when the axicon angle is zero and/or (most often) when an on-axis configuration is considered rather than an off-axis configuration. This study was devoted to longitudinal forces. The present paper is devoted to transverse forces and demonstrates the existence of newly additional axicon forces associated to both the usual scattering and gradient forces. Again, these new extra-axicon forces are zero when the axicon angle is zero and/or when an on-axis configuration is considered rather than an off-axis configuration.

Optical Trapping of Polystyrene Nanoparticles on Black Silicon: Implications for Trapping and Studying Bacteria and Viruses

The optical trapping of polymer beads of different diameters d of 100, 200, 500, 1000, and 3000 nm on planar (F-Si) and nanostructured (BSi) crystalline silicon was investigated at laser wavelengths λ = 808 and 1064 nm, respectively. We demonstrate that BSi surfaces can enhance the optical gradient force required to grip nanoparticles (100 nm) in contrast to F-Si surfaces, significantly changing the trapping behavior. Thus, different modes of optical tweezing were characterized by modification of Si surface topography (nanostructuring), wavelength λ, laser irradiation intensity I, and irradiation area. Specifically, we present four separate modes of optical tweezing using planar and nanostructured Si for (i) a single particle trapped by tightly and loosely focused irradiation on F-Si, (ii) a large number of 3D assembled beads trapped by tightly focused irradiation on BSi, (iii) a small number of particles trapped on F-Si, and (iv) a large number of particles trapped on BSi, resulting in a 2D self-ordered assembly. The mechanisms of the optical manipulation of particles in the range 100–1000 nm were discussed with implications for the trapping of bacteria or viruses by using nanostructured semiconductor assisted optical tweezing (NASSCA/OT) and their response to high mechanical shear forces at locally changed temperatures on nanostructured surfaces.

Axicon terms associated with gradient optical forces in generalized Lorenz-Mie theory

In a recent paper devoted to the study of longitudinal optical forces exerted by off-axis Bessel beams in the Rayleigh regime in generalized Lorenz-Mie theory, new kind of optical forces are discovered. They are different than the well established scattering and gradient forces, and called axicon forces. In the present paper, within the same framework, we demonstrate the existence of a new kind of axicon terms, associated with gradient forces, which are zero both when the axicon angle is zero and when an on-axis configuration is considered.

Distance-controllable and direction-steerable opto-conveyor for targeting delivery

Opto-conveyors have attracted widespread interest in various fields because of their non-invasive and non-contact delivery of micro/nanoparticles. However, the flexible control of the delivery distance and the dynamic steering of the delivery direction, although very desirable in all-optical manipulation, have not yet been achieved by opto-conveyors. Here, using a simple and cost-effective scheme of an elliptically focused laser beam obliquely irradiated on a substrate, a direction-steerable and distance-controllable opto-conveyor for the targeting delivery of microparticles is implemented. Theoretically, in the proposed scheme of the opto-conveyor, the transverse and longitudinal resultant forces of the optical gradient force and the optical scattering force result in the transverse confinement and the longitudinal transportation of microparticles, respectively. In this study, it is experimentally shown that the proposed opto-conveyor is capable of realizing the targeting delivery for microparticles. Additionally, the delivery distance of microparticles can be flexibly and precisely controlled by simply adjusting the irradiation time. By simply rotating the cylindrical lens, the proposed opto-conveyor is capable of steering the delivery direction flexibly within a large range of azimuthal angles, from − 75 ° to 75°. This study also successfully demonstrated the real-time dynamic steering of the delivery direction from − 45 ° to 45° with the dynamical rotation of the cylindrical lens. Owing to its simplicity, flexibility, and controllability, the proposed method is capable of creating new opportunities in bioassays as well as in drug delivery.

All-Silicon Energy-Efficient Optical Diode Using Opto-Mechanical Microring Resonators

By utilizing two cascaded all-silicon opto-mechanical microring resonators (MRRs), an energy-efficient optical diode with high nonreciprocal transmission ratios (NTRs) is proposed and experimentally realized. The optical diode is composed of an all-pass opto-mechanical MRR and an add-drop opto-mechanical MRR. Due to the largely enhanced interaction between the photons and the suspended structure, the opto-mechanical effect can be dramatically improved. With injecting low optical powers, the optical gradient force can be effectively aroused in the opto-mechanical MRRs, which would arise nano-meter scale waveguide deformations and the significant spectrum red-shifts of the rings. The opto-mechanical effect would cause different red-shifts of the two MRR resonances in the forward and backward transmissions, which contributes to realizing the nonreciprocal transmissions. The experimental results show that with −4.2 dBm power consumption, the optical diode can achieve high NTRs approach 41.8 dB. Due to the dominant advantages of complementary metal oxide semiconductor (COMS) compatibility, high NTRs (41.8 dB), low power consumption (−4.2 dBm) and compact size (0.015 mm2), the device has remarkable applications in on-chip signal processing systems.

Focusing and propagation characteristics of radially polarized helical-conical Airy beams.

This paper uses a spiral cone wavefront to modulate a radially polarized Airy beam. Based on the vector diffraction theory, the focusing and propagation characteristics of a radially polarized spiral cone Airy beam are studied. In order to conduct in-depth analysis on the unique properties of radial polarized spiral cone-shaped Airy beams, the focused light intensity, optical gradient force distribution, and propagation characteristics under different spiral cone parameters were simulated. The results show that the parameters of the helical-conical have a significant effect on the focusing characteristics of the radially polarized helical-conical Airy beam. The radial polarization of the radially polarized helical-conical Airy beam shows a lateral stretching trend. During propagation, focused light field distribution gradually becomes an optical trap. The light trap-like mode displayed after Airy beam modulation can be applied to optical tweezers operation, optical capture, and other fields.

Efficient Molecular Aggregation of Rhodamine 6G and Pseudoisocyanine by Light-Induced Force

A highly efficient light-induced aggregation of porphyrin molecules in solution was recently reported for 4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid (TPPS). Here, we demonstrate that rhodamine 6G (R6G) and pseudoisocyanine (PIC) also show efficient light-induced aggregation in unsaturated aqueous solution, being detected with a multichannel lock-in amplifier as the absorbance decrease/increase in the monomers/aggregates, induced by a laser at 633 nm, which is non-resonant off their main monomer absorption wavelengths. The light-induced aggregation states are H-aggregates that are hardly formed in the thermal equilibrium. The similar absorbance changes are absent in the monomer molecules fixed in polyvinyl alcohol (PVA) films. The aggregation efficiency defined as ( Δ A / A ) / ( U t r a p / k B T ) , where A is the absorbance of the monomers, Δ A is the absorbance increase in the aggregates, U t r a p is the optical gradient force potential, and k B T is the thermal energy at room temperature, is approximately 100 for R6G and 500 for PIC, which are much smaller than that of TPPS.

Probing optical mode hybridization in an integrated graphene nano-optomechanical system

We propose a scheme for sensitive local monitoring of mode hybridization in vertically asymmetric waveguides with a nano-optomechanical probe based on graphene. Extracting local information about mode hybridization is challenging using intensity measurements at the output or scanning optical probes over the waveguide. Transferring the information about the guided field profiles into the mechanical mode of graphene (with ultra-low-force sensitivity) using the optical gradient force allows for sensitive probing of the mode hybridization. In our proposed scheme, we estimate that a 100% change in the TE fraction of the fundamental quasi-TM waveguide mode would cause a change in the vibration amplitude of graphene on the order of 1000 pm. The limit of detection of the TE fraction is approximately 0.001. The change in the TE fraction due to index perturbations in the core and cladding can also be used for index sensing with responsivity on the order of 1000 pm change in vibration amplitude per refractive index unit and a limit of detection of 2 × 10 − 4 refractive index units. This work provides novel methods for applications in optomechanical modulation and sensing.

Focus properties of cosh-Gaussian beams with the power-exponent-phase vortex.

Vector diffraction theory is used to investigate the focusing properties of cosh-Gaussian beams with the power-exponent-phase vortex. The effects of the decentered parameter, the power order, and the topological charge on the normalized intensity distribution are examined. Results show that intensity distribution in the focal region can be altered significantly by the topological charge, the power order, and the decentered parameter. The pattern of the optical intensity slowly enlarges with the increase of topological charge. The strongest intensity part of the annulus rotates by the changing topological charge. As the power order increases, the intensity distribution is more concentrated. As the decentered parameter increases, there occur multiple relatively strong intensity peaks, and the entire focus pattern extends outward. Some optical gradient force distributions are investigated to illuminate the applications of these alterable foci patterns.

Design considerations for effective thermal management in mobile nanotweezers

Controlled manipulation of nanoscale objects in fluids is relevant to both fundamental studies and technological advances in nanotechnology. While standard techniques of nanomanipulation, such as optical and plasmonic tweezers have limitations in simultaneous trapping and transport of nanoscale cargo, magnetically driven plasmonic nanorobots under optical illumination provide a promising solution. These so called mobile nanotweezers (MNT) use strongly localized electromagnetic field near plasmonic nanostructures to trap objects with high efficiency and can simultaneously be driven by magnetic fields to selectively trap, transport and release colloidal cargo. Upon illumination, apart from strong optical gradient forces due to local electric field enhancement, additional fluidic forces arise due to the heat generated by absorption of light. Here, we present a method to understand and engineer thermally induced fluidic forces in mobile nanotweezers. The temperature enhancement and associated thermofluidic forces are studied as a function of MNT geometry. We also discuss illumination at wavelengths slightly detuned from plasmon resonance frequency, which produces sufficient field enhancement with negligible generation of heat, and therefore much reduced thermophoretic and convective forces. This allowed us to engineer thermoplasmonic forces in MNTs for enhanced trapping performance and diverse applications.

Metasurface-Based Wide-Angle Beam Steering for Optical Trapping

Metasurfaces become increasingly important for wavefront shaping and beam steering with high efficiency. We propose a wide-angle beam steering device based on all-dielectric metasurfaces consisting of two symmetrical blazed gratings for optical trapping of particles, which can convert the input Gaussian beam to a special beam with intensity gradient along the propagation direction. Two major types of metasurfaces (Pancharatnam-Berry phase and nanoposts) are compared, and dramatic difference is found between them, especially at oblique incidence. We show that the Pancharatnam-Berry phase-based metasurface is more tolerant to the incident angle and that significant optical gradient force can be generated over a large angle range of -50° to 50°, and the maximum attractive force is up to tens of pN/W, with a trapping range of 14 gym for particles of 2.5 μym in diameter. The proposed scheme exhibits great potential to trap a large fraction of particles floating in a microfluidic channel when the beam is dynamically steered.

X-ray diffraction measurement of a single nanometre-sized particle levitated in air by anoptical-trap sample holder.

A single-beam optical-trap sample holder for X-ray diffraction measurements with synchrotron radiation has been developed. The sample holder was used to obtain an X-ray diffraction image of a single ZnO particle levitated in air, without mechanical contact, by the optical gradient force exerted by a focused laser beam. The diffraction image showed a Debye ring pattern, which was similar to a powder diffraction pattern of an assemblage of ZnO particles. While the ZnO particle is held by the optical trap in air, it rotates irregularly. Therefore, the Debye ring pattern of the ZnO particle can be clearly obtained even if the ZnO particle is a single grain. Lattice parameters and crystallite size of the single ZnO particle were determined simultaneously. The lattice parameters were determined to be a = 3.2505 ± 0.0005 Å and c = 5.207 ± 0.006 Å, which are consistent with those of the assemblage of ZnO particles. The crystallite size determined by the Scherrer method was 193.4 ± 26.2 nm.

Elucidating optical field directed hierarchical self-assembly of homogenous versus heterogeneous nanoclusters with femtosecond optical tweezers.

Insights into the morphology of nanoclusters would facilitate the design of nano-devices with improved optical, electrical, and magnetic responses. We have utilized optical gradient forces for the directed self-assembly of colloidal clusters using high-repetition-rate femtosecond laser pulses to delineate their structure and dynamics. We have ratified our experiments with theoretical models derived from the Langevin equation and defined the valid ranges of applicability. Our femtosecond optical tweezer-based technique characterizes the in-situ formation of hierarchical self-assembled clusters of homomers as well as heteromers by analyzing the back focal plane displacement signal. This technique is able to efficiently distinguish between nano-particles in heterogeneous clusters and is in accordance with our theory. Herein, we report results from our technique, and also develop a model to describe the mechanism of such processes where corner frequency changes. We show how the corner frequency changes enables us to recognize the structure and dynamics of the coagulation of colloidal homogeneous and heterogeneous clusters in condensed media over a broad range of nanoparticle sizes. The methods described here are advantageous, as the backscatter position-sensitive detection probes the in-situ self-assembly process while other light scattering approaches are leveraged for the characterization of isolated clusters.

Non-Photochemical Pulsed-Laser-Induced Nucleation in a Continuous-Wave-Laser-Induced Phase-Separated Solution Droplet of Aqueous Glycine Formed by Optical Gradient Forces

A centimeter-sized, laser-induced phase-separated (LIPS) solution droplet, which was formed by tightly focusing a continuous-wave near-infrared laser beam at the glass/solution interface of a milli...