Kerker‐Type Intensity‐Gradient Force of Light

Xiaohao Xu Manuel Nieto‐Vesperinas Cheng‐Wei Qiu Xiaoshuai Liu Dongliang Gao Yao Zhang Baojun Li

The intensity gradient of light represents the most important property for optical tweezers to manipulate small particles, which is known to produce a conservative optical force that is either attractive or repulsive. Here, it is shown that Kerker interference, the interplay between electric and magnetic dipoles induced in nanoparticles, permits the intensity gradient to exert a nonconservative optical force, in the case of a standard optical trap created with linearly or elliptically polarized Gaussian beams. The Kerker‐type intensity‐gradient force has an “anisotropic” directionality, and it tends to repel particles away from the beam axis. Such repulsive effects can greatly sensitize the particle trapping behavior of optical tweezers to the particle size. Utilizing these peculiar properties, all‐optical sorting of Si nanoparticles is theoretically demonstrated, with tunable size‐selection criterion and accuracy.

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Strategies for Optical Trapping in Biological Samples: Aiming at Microrobotic Surgeons

Ada‐Ioana Bunea, Jesper Glückstad

Optical trapping and manipulation of objects down to the Ångstrom level has revolutionized research at the smallest scales in all natural sciences. The flexibility of optical trapping methods facilitates real‐time monitoring of the dynamics of biological processes in model systems and even in living cells. Different optical trapping and manipulation approaches allow displacement of nanostructures with subnanometer precision and force measurements with femtonewton precision. Due to inherent constraints of optical methods, most optical trapping experiments are performed in water or simple aqueous solutions. However, in recent years, there is an ever‐growing interest of shifting from simple aqueous media towards more biologically‐relevant media. Precise optical trapping and manipulation, combined with state‐of‐the‐art microfabrication, will enable the development of microrobotic “surgeons” with tremendous potential for biomedical and microengineering applications. This review introduces the basics of optical trapping and discusses its applications for biological samples, with focus on trapping in biological media and strategies for overcoming the challenges of optical manipulation in complex environments as a stepping‐stone for microrobotic “surgeons.”

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Counter‐Propagating Optical Trapping of Resonant Nanoparticles Using a Uniaxial Crystal

Pawel Karpinski Steven Jones Daniel Andrén Mikael Käll

Laser tweezing of optically resonant nanostructures, such as plasmonic nanoparticles and high‐index dielectric nanoresonators, is extremely challenging because the enhanced light–matter interaction usually amplifies radiation pressure to an extent where conventional single beam gradient trapping in three dimensions becomes impossible. Such particles are therefore typically trapped off resonance or in two dimensions only. To extend the application potential of optical tweezers to the resonant case, focus splitting inside a uniaxial birefringent crystal and reflection from a mirror is used to develop a counter‐propagating beam configuration based on a single microscope objective. The setup allows one to trap and rapidly rotate resonant gold nanorods in water far from any interface, thereby opening a range of possibilities for novel studies of resonantly enhanced optical forces and interactions in uniform environments.

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On-chip laser processing for the development of multifunctional microfluidic chips

Huan Wang, Yong-Lai Zhang, Wei Wang, Hong Ding, Hong-Bo Sun

In the development of microfluidic chips, conventional 2D processing technologies contribute to the manufacturing of basic microchannel networks. Nevertheless, in the pursuit of versatile microfluidic chips, flexible integration of multifunctional components within a tiny chip is still challenging because a chip containing micro-channels is a non-flat substrate. Recently, on-chip laser processing (OCLP) technology has emerged as an appealing alternative to achieve chip functionalization through in situ fabrication of 3D microstructures. Here, the recent development of OCLP-enabled multifunctional microfluidic chips, including several accessible photochemical/photophysical schemes, and photosensitive materials permiting OCLP, is reviewed. To demonstrate the capability of OCLP technology, a series of typical micro-components fabricated using OCLP are introduced. The prospects and current challenges of this field are discussed.

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Lab on Fiber Technology for biological sensing applications

Patrizio Vaiano, Benito Carotenuto, Marco Pisco, Armando Ricciardi, Giuseppe Quero, Marco Consales, Alessio Crescitelli, Emanuela Esposito, Andrea Cusano

This review presents an overview of “Lab on Fiber” technologies and devices with special focus on the design and development of advanced fiber optic nanoprobes for biological applications. Depending on the specific location where functional materials at micro and nanoscale are integrated, “Lab on Fiber Technology” is classified into three main paradigms: Lab on Tip (where functional materials are integrated onto the optical fiber tip), Lab around Fiber (where functional materials are integrated on the outer surface of optical fibers), and Lab in Fiber (where functional materials are integrated within the holey structure of specialty optical fibers).
This work reviews the strategies, the main achievements and related devices developed in the “Lab on Fiber” roadmap, discussing perspectives and challenges that lie ahead, with special focus on biological sensing applications.

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Conical refraction: fundamentals and applications

Alex Turpin, Yury V. Loiko, Todor K. Kalkandjiev, Jordi Mompart

In 1832 Hamilton predicted that a collimated light beam propagating through a biaxial crystal parallel to one of its optical axes refracts as a slanted cone within the crystal and emerges as a hollow light cylinder, this optical effect being named as conical refraction (CR). The diffractive solution of CR presented by Belsky and Khapalyuk in 1978 and the corresponding re-formulation carried out by Berry in 2004 rekindled this old and almost forgotten phenomenon. In this article, we review the CR phenomenon following different approaches that allow understanding light propagation in biaxial crystals, including the case of multiple crystals in cascade. We then focus on the description of the singular properties of the CR beams, presenting some examples such as optical bottle beams and beams carrying orbital angular momentum. All these features are used to introduce some of the most appealing applications of CR in the fields of optical trapping, free-space optical communications, polarization metrology, super-resolution imaging, two-photon polymerization, and lasers.

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Surface plasmon polariton assisted optical pulling force

Mihail I. Petrov, Sergey V. Sukhov, Andrey A. Bogdanov, Alexander S. Shalin and Aristide Dogariu

We demonstrate both analytically and numerically the existence of optical pulling forces acting on particles located near plasmonic interfaces. Two main factors contribute to the appearance of this negative recoil force. The interference between the incident and reflected waves induces a rotating dipole with an asymmetric scattering pattern, while the directional excitation of surface plasmon polaritons (SPPs) enhances the linear momentum of scattered light. The strongly asymmetric SPP excitation is determined by spin–orbit coupling of the rotating dipole and surface plasmon polariton. As a result of the total momentum conservation, the force acting on the particle points in a direction opposite to the incident wave propagation. We derive analytical expressions for the force acting on dipolar particles placed in the proximity of plasmonic surfaces. Analytical expressions for this pulling force are derived within the dipole approximation and are in excellent agreement with results of electromagnetic numerical calculations. The forces acting on larger particles are analyzed numerically, beyond the dipole approximation.

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Bacteria-based branched structures for bionanophotonics

Hongbao Xin, Yuchao Li and Baojun Li

Branched photonic structures have served as paramount important components for nanophotonic integration and circuitry. However, these structures are generally constructed with photonic and plasmonic nanowires, which are nonbiomaterials and often need to be specially engineered to interface with cells and biological system. For bionanophotonics, photonic components assembled with self-adaptive biomaterials are highly desirable to be directly interfaced with the dynamic biological system. In this work, branched structures for bionanophotonics assembled with natural living biomaterials, i.e., nanorod-shaped Escherichia coli bacteria are reported. The E. coli cells were orderly trapped using a specially desired tapered optical fiber, forming structures with different branches and lengths. Light-propagation performances along these branched structures were investigated, and the robustness property of the structures were demonstrated. The results show that the bacteria-based branched structures provide different promising self-sustainable and evolvable components, such as multidirectional waveguides and beam splitters, for bionanophotonics by connecting the biological and optical worlds with a seamless interface.

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Optical trapping and manipulation of micrometer and submicrometer particles

Mark Daly, Marios Sergides and Síle Nic Chormaic
Subwavelength features in conjunction with light-guiding structures have gained significant interest in recent decades due to their wide range of applications to particle and atom trapping. Lately, the focus of particle trapping has shifted from the microscale to the nanoscale. This few orders of magnitude change is driven, in part, by the needs of life scientists who wish to better manipulate smaller biological samples. Devices with subwavelength features are excellent platforms for shaping local electric fields for this purpose. A major factor that inhibits the manipulation of submicrometer particles is the diffraction-limited spot size of free-space laser beams. As a result, technologies that can circumvent this limit are highly desirable. This review covers some of the more significant advances in the field, from the earliest attempts at trapping using focused Gaussian beams, to more sophisticated hybrid plasmonic/metamaterial structures. In particular, examples of emerging optical trapping configurations are presented.

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Spectrally resolved resonant propulsion of dielectric microspheres

Yangcheng Li, Alexey V. Maslov, Nicholaos I. Limberopoulos, Augustine M. Urbas and Vasily N. Astratov 

Use of resonant light forces opens up a unique approach to high-volume sorting of microspherical resonators with much higher uniformity of resonances compared to that in coupled-cavity structures obtained by the best semiconductor technologies. In this work, the spectral response of the propulsion forces exerted on polystyrene microspheres near tapered microfibers is directly observed. The measurements are based on the control of the detuning between the tunable laser and internal resonances in each sphere with accuracy higher than the width of the resonances. The measured spectral shape of the propulsion forces correlates well with the whispering-gallery mode resonances in the microspheres. The existence of a stable radial trap for the microspheres propelled along the taper is demonstrated. The giant force peaks observed for 20-μm spheres are found to be in a good agreement with a model calculation demonstrating an efficient use of the light momentum for propelling the microspheres.

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Unveiling the correlation between non-diffracting tractor beam and its singularity in Poynting vector

Dongliang Gao, Andrey Novitsky, Tianhang Zhang, Fook Chiong Cheong, Lei Gao, Chwee Teck Lim, Boris Luk'yanchuk and Cheng-Wei Qiu

This paper investigates the singular optics of nonparaxial light beams in the near field when the light behaves as a tractor beam. New insights into the optical pulling force, which is usually represented by integrating the stress tensor at a black box enclosing the object, are interpreted by the optical singularity of the Poynting vector. The negative nonconservative pulling force originates from the transfer of the azimuthal Poynting vector to the longitudinal component partly owing to the presence of a scatterer. The separatrice pattern and singularity shifts of the Poynting vector unanimously exhibit a differentiable near-field distribution in the presence of optical pulling force. A new method is established to calculate the near-field optical force using the differential Poynting vector in the far field. The results obtained provide a clear physical interpretation of the light–matter interaction and manifest the significance of singular optics in manipulating objects.

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Nano-opto-mechanical effects in plasmonic waveguides

Alexander S. Shalin, Pavel Ginzburg, Pavel A. Belov, Yuri S. Kivshar, Anatoly V. Zayats

In order to achieve interaction between light beams, a mediating material object is required. Nonlinear materials are commonly used for this purpose. Here a new approach to control light with light, based on a nano-opto-mechanical system integrated in a plasmonic waveguide is proposed. Optomechanics of a free-floating resonant nanoparticle in a subwavelength plasmonic V-groove waveguide is studied. It is shown that nanoparticle auto-oscillations in the waveguide induced by a control light result in the periodic modulation of a transmitted plasmonic signal. The modulation depth of 10% per single nanoparticle of 25 nm diameter with the clock frequencies of tens of MHz and the record low energy-per-bit energies of 10−18 J is observed. The frequency of auto-oscillations depends on the intensity of the continuous control light. The efficient modulation and deep-subwavelength dimensions make this nano-optomechanical system of significant interest for opto-electronic and opto-fluidic technologies.

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Optical formation and manipulation of particle and cell patterns using a tapered optical fiber

Hongbao Xin, Rui Xu, Baojun Li
A method for optical formation and controllable manipulation of particle and cell patterns using a tapered optical fiber is demonstrated. With a laser beam at 980-nm wavelength launched into the fiber, different sized silica particles were formed into particle patterns (both one-dimensional chains and two-dimensional arrays) with different particle numbers by optical binding. The formed particle patterns can be controllably manipulated in three dimensions. Using yeast cells as an example, it was demonstrated that the method is applicable for the formation of biological cell patterns, without damage to the yeast cell viability. This method provides a new facile way for biophotonic and biological researches with particles and cells in a highly organized manner.
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Particle sorting using a subwavelength optical fiber

Yao Zhang, Baojun Li

Size-based particle sorting using a subwavelength optical fiber was demonstrated with 600-nm and 1-μm sizes of polystyrene particles. Optical forces acting on the particles were calculated based on three-dimensional finite-difference time-domain simulations at wavelengths of 808, 1047, and 1310 nm propagating in a subwavelength optical fiber with diameter of 800 nm. Calculations indicate that by launching two counterpropagating laser beams at different wavelengths into the fiber, the directions of the resultant optical scattering forces acting on the two particle sizes can be opposite along the fiber, which leads to a countertransport of the particles. To verify the theoretical prediction, experiments were performed using the 800-nm fiber to sort the two particle sizes. The results show that with two counterpropagating beams at 808 and 1310 nm, a continuous particle sorting was achieved. Measured particle velocities were in agreement with the theoretical calculations.

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Advanced optical trapping by complex beam shaping

Mike Woerdemann, Christina Alpmann, Michael Esseling, Cornelia Denz

Optical tweezers, a simple and robust implementation of optical micromanipulation technologies, have become a standard tool in biological, medical and physics research laboratories. Recently, with the utilization of holographic beam shaping techniques, more sophisticated trapping configurations have been realized to overcome current challenges in applications. Holographically generated higher-order light modes, for example, can induce highly structured and ordered three-dimensional optical potential landscapes with promising applications in optically guided assembly, transfer of orbital angular momentum, or acceleration of particles along defined trajectories. The non-diffracting property of particular light modes enables the optical manipulation in multiple planes or the creation of axially extended particle structures. Alongside with these concepts which rely on direct interaction of the light field with particles, two promising adjacent approaches tackle fundamental limitations by utilizing non-optical forces which are, however, induced by optical light fields. Optoelectronic tweezers take advantage of dielectrophoretic forces for adaptive and flexible, massively parallel trapping. Photophoretic trapping makes use of thermal forces and by this means is perfectly suited for trapping absorbing particles. Hence the possibility to tailor light fields holographically, combined with the complementary dielectrophoretic and photophoretic trapping provides a holistic approach to the majority of optical micromanipulation scenarios.
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Gearing up for optical microrobotics: micromanipulation and actuation of synthetic microstructures by optical forces

Darwin Palima, Jesper Glückstad

Optics is usually integrated into robotics as part of intelligent vision systems. At the microscale, however, optical forces can cause significant acceleration and so optical trapping and optical manipulation can enable the noncontact actuation of microcomponents. Microbeads are ubiquitous optically actuated structures, from Ashkin's pioneering experiments with polystyrene beads to contemporary functionalized beads for biophotonics. However, micro- and nanofabrication technologies are yielding a host of novel synthetic structures that promise alternative functionalities and new exciting applications. Recent works on the actuation of synthetic microstructures using optical trapping and optical manipulation are examined in this review. Extending the optical actuation down to the nanoscale is also presented, which can involve either direct manipulation of nanostructures or structure-mediated approaches where the nanostructures form part of larger structures that are suitable for interfacing with diffraction-limited optical fields.
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Novel optical techniques for measurements of light extinction, scattering and absorption by single aerosol particles

R.E.H. Miles, A.E. Carruthers, J.P. Reid

Aerosol particles play important roles in a broad range of scientific disciplines, from atmospheric chemistry and physics, to the delivery of fuels for combustion and drugs to the lungs, and extending to industrial processes such as spray drying. Measurements of the light extinction, scattering and absorption by ensembles of aerosol particles can be used to non-intrusively characterise aerosol particle samples. However, such measurements often lead to ambiguity in interpreting the properties and processes occurring on individual particles. In this review, recent developments in the use of laser based techniques to isolate and manipulate single particles and to characterise them will be highlighted. In particular, the use of cavity ring down spectroscopy, Bessel beams and optical tweezers for investigating light extinction, scattering and absorption, respectively, will be considered. The prospects for using optical techniques to interrogate the fundamental processes occurring in aerosol at the single particle level are discussed.

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Optical traps for single molecule biophysics: a primer

T.T. Perkins

Optical trapping experiments of different complexities are making a significant impact in biology. This review seeks to highlight design choices for scientists entering the field or building new instruments and to discuss making calibrated measurements with optical traps. For specificity, this review focuses on nucleic acid-based assays, but the discussion reflects the general experimental design considerations of developing a biological assay and an optical trap to measure it.

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