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Advanced Fiber Materials

, Volume 1, Issue 2, pp 83–100 | Cite as

Optical Trapping and Manipulation Using Optical Fibers

  • Yuanhao Lou
  • Dan Wu
  • Yuanjie PangEmail author
Review

Abstract

An optical trap forms a restoring optical force field to immobilize and manipulate tiny objects. A fiber optical trap is capable of establishing the restoring optical force field using one or a few pieces of optical fiber, and it greatly simplifies the optical setup by removing bulky optical components, such as microscope objectives from the working space. It also inherits other major advantages of optical fibers: flexible in shape, robust against disturbance, and highly integrative with fiber-optic systems and on-chip devices. This review will begin with a concise introduction on the principle of optical trapping techniques, followed by a comprehensive discussion on different types of fiber optical traps, including their structures, functionalities and associated fabrication techniques. A brief outlook to the future development and potential applications of fiber optical traps is given at the end.

Keywords

Fiber optical traps Optical trapping Optical manipulation Optical fiber 

Introduction

Optical trapping is a technique capable of immobilizing and manipulating small objects in three dimensions using optical forces. It has been applied in numerous fields, including force transducer [1, 2, 3, 4, 5], optical spectroscopy [6, 7, 8, 9, 10] and optical assembly [11, 12, 13, 14, 15, 16]. In particular, due to its remarkable capability of immobilizing micro- and nanoparticles in three dimensions in a gentle, low-damage manner, the optical trapping technique has facilitated the revolutionary advance in single particle and single molecule biology [17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29].

The key to realize optical trapping of a particle is to establish a restoring optical force field in space. In 1970, Arthur Ashkin firstly demonstrated a “radiation pressure” trap based on the balance of scattering forces from two counter-propagating laser beams [30]. Later in 1986, Ashkin developed a gradient optical trap—better known as a single beam optical tweezers, requiring only one, highly focused laser beam [31]. In conventional optical trapping setups, bulky optical elements are usually required for forming a tight focus or coupling to the micro-/nanophotonic structures. Fiber-optic systems can provide a flexible, low-complexity alternative for optical trapping. Fiber-based implementations of the two above-mentioned basic forms of optical traps have been developed in 1993 (fiber dual-beam trap, equivalent to the “radiation pressure” trap) [32] and 1997 (single fiber gradient trap) [33]. Since then, researchers have been increasingly using optical fibers in the construction of optical traps. Fiber optical traps (FOTs) quickly recruit innovations emmerging in the research field of optical trapping, many novel functionalities originally designed for the conventional trapping setup, including angular tweezing [34], plasmonic trapping of nanoparticles [35, 36] and holographic micromanipulation [37], have been replicated using FOTs. Moreover, FOTs have implemented functionalities that are not directly available in conventional setups. For example, a hollow-core photonic crystal fiber (HC-PCF) trap allows for a “flying particle” in its hollow core to be transferred over a distance of meters (extendable to kilometers in theory), making it a strong candidate for distributive sensing. Such long-distance particle delivery capability is unfeasible in conventional optical traps.

This review summarizes the fiber-based optical traps with different structures and functions. The working principle of optical traps is introduced first, and we present how FOTs are implemented. We then provide a thorough review of different FOT structures, including fiber dual-beam traps, single fiber tweezers, fiber holographic tweezers, plasmonic FOTs, and HC-PCF traps. Fiber-optic traps based on photothermally induced forces will be also introduced. We discuss in details design principles associated with different functionalities and we review advanced fabrication techniques used in building the special fiber structures. We provide a brief perspective for FOTs at the end of this review.

Principle of optical traps

In this chapter, we first briefly introduce the origin of optical forces, followed by the basic arrangements of laser beams to form a restoring force field required for trapping, and how these arrangements can be implemented using optical-fiber-based setups.

The scattering and the gradient forces

Optical forces are usually at pico- or femto-Newton level with a typical input power ranging from few to tens of milliwatts. This force is traditionally decomposed into two components: Scattering force, a “repulsive” force along the propagating direction of light, and gradient force, an “attractive” force along the gradient of the optical intensity. In general, both types of forces exist in an optical trap, and the restoring force field is the result of the balance of the two.

We first make a clarification: the following formulations of optical forces are the most appropriate for Rayleigh particles—those with diameters dp ≤ 0.1λ which can be approximated by electric dipoles. Nevertheless, the optical restoring force fields for larger trapped particles (as in many literatures reviewed later) have qualitatively similar mechanisms, and the precise formulation essentially involves integrations of the scattering and gradient forces exerted on the volume elements of the larger particles. Readers interested in a full derivation of optical forces are directed to two comprehensive review articles [38, 39].

The scattering force can be thought of as a photon “fire hose” pushing the particle away in the direction of light propagation [38]. For a Rayleigh sphere of diameter dp, this force can be formulated as:
$$F_{\text{scatt}} = \frac{{I_{ 0} \sigma n_{\text{m}} }}{c}$$
(1)
and the scattering cross-section σ of a Rayleigh particle is given as:
$$\sigma = \frac{{2\pi^{5} d_{\text{p}}^{6} }}{{3\lambda^{4} }}\left( {\frac{{n_{\text{p}}^{2} - n_{\text{m}}^{2} }}{{n_{\text{p}}^{2} + 2n_{\text{m}}^{2} }}} \right)$$
(2)
where I0 is the intensity of incident laser, np and nm are the refractive index of the particle and the surrounding medium respectively, c is the speed of light in vacuum and λ is the wavelength of trapping laser.
In addition to the scattering force, a polarized particle in an inhomogeneous electric field experiences a gradient force, which is proportional to the optical intensity gradient:
$$F_{\text{grad}} = \frac{2\pi \alpha }{{cn_{\text{m}}^{2} }}\nabla I_{ 0}$$
(3)
where
$$\alpha = \frac{{n_{\text{m}}^{2} d_{\text{p}}^{3} }}{8}\left( {\frac{{n_{\text{p}}^{2} - n_{\text{m}}^{2} }}{{n_{\text{p}}^{2} + 2n_{\text{m}}^{2} }}} \right)$$
(4)
is the polarizability of the particle.

Optically trapped particles are not absolutely still, and they undergo Brownian motion about the trapping center as governed by the Langevin equation [39, 113]. In the Langevin equation, the inertia term is usually neglected in a microfluidic environment (low Reynolds number regime); the damping term originates from the viscous drag governed by the Einstein relation; and the restoring force term originates from the optical forces.

Fiber-based implementations of restoring optical force fields

Ashkin came up with the first optical trapping idea, which he called it a “radiation pressure” trap [30]: it can be implemented using two counter-propagating, coaxially aligned laser beams as shown in Fig. 1a. In this setup, the balance of the scattering forces from both beams stabilizes the particle in the axial direction, and the gradient force keeps the particle on the beam axis. This is the first demonstration of the immobilization of a microparticle using light. In 1993, Constable et al. constructed a similar fiber-optic dual-beam trap using two aligned pieces of single mode fibers (SMFs), in which the two counter-propagating, slightly divergent laser beams from the cleaved end-faces of the two fibers forms a similar force field as in the lensed system (Fig. 1b) [32]. The details, as well as further developments, of fiber dual-beam traps will be further discussed in Sect. 3.1.
Fig. 1

Schematic setup for optical trapping. Black dotted circle indicates the equivalent trapping position. a, b Dual-beam radiation pressure trap. c, d Single-beam gradient optical tweezers

In 1987, Ashkin et al. constructed a second type of optical trap, known as a single-beam optical tweezers [31]: a diffraction-limited laser focal spot, created using a high numerical aperture (NA) microscope objective, constructs a local intensity maximum and subsequently a trapping center with all gradient force at all locations pointing to it (Fig. 1c). The gradient force pulls the particle toward laser focal point and stable trapping is achieved slightly down-stream of the focal point where scattering force is fully balanced.

Instead of using a high-NA objective, the tight focal spot required by the gradient optical trap can alternatively be formed using a piece of optical fiber with structured end-face, creating the single-fiber gradient optical trap (Fig. 1d) [33]. Since then, different structures including tapered lenses, spherical lenses, graded index fiber (GIF) lenses, total internal refraction (TIR) lenses are constructed on a single fiber tip to construct a gradient force trap (more details given in Sect. 3.2).

Fiber optical traps

Optical fiber provides an inexpensive, robust, versatile and commercially mature platform for many applications. The combination of optical fiber with optical traps reduces the use of bulky optical instruments, providing a compact, flexible solution at the working area, thereby promoting fundamentally new applications. In this chapter, various FOTs associated with their applications are discussed.

Dual-beam FOTs

In 1993, Constable et al. demonstrate the first FOT utilizing two pieces of SMFs [32]. This dual-beam fiber optical trap is a much more concise implementation of A. Ashkin’s dual beam radiation pressure trap (Fig. 2a). The fiber-based beam delivery provides a larger lateral space for other optical or mechanical probe to investigate trapped objects [40] and possibility to integrate with on-chip device (Fig. 2b, c) [41, 42, 43].
Fig. 2

Optical traps and rotators based on dual-beam fiber-optic systems. a A 100-micron polymer sphere trapped in a fiber optical light force trap, viewed from below. b Sealed optical trapping chip and c cross-section of the microchannel at the height of the fiber grooves. d Trapping and rotation of a human smooth muscle cell at the center of two transversely offset fibers. e Spinning of particles realized by rotating fibers through a rotation mount (RM). f Schematic of the basic setup of optical rotation perpendicular to the optical axis. g Rotational control of modes through an optical fiber with a spatial light modulator. The image sequences show mode orientations of the laser beam emitted from the optical fiber for an  LP11 mode (top row) and an LP21 mode (bottom row) and callouts show the corresponding binary phase-modulating holograms displayed on the SLM. h Array of equidistant spacing beads with a diameter of 1 μm. Figures reproduced permission from: a Ref. [40], © 2006 OSA; b, c Ref. [42], © 2015 OSA; d Ref. [47], © 2012 OSA; e Ref. [48], © 2008 OSA; f, g Ref. [49], © 2014 Springer Nature; h Ref. [52], © 2018 OSA

The principle behind stable trapping in a fiber dual-beam trap has been explained previously: it is a result of the balance of scattering force in the axial direction, and the centripetal gradient force in the radial direction. Slightly unbalancing of the optical force will result in particle motion. When the ratio of optical power from two fibers is adjusted, the particle will be pushed to the side with a weaker optical power until a new balance position is found [44].

Spinning of the trapped particle can be achieved in two ways. When there is a transverse offset of the two fibers, the non-collinear scattering force forms a torque which spins the particle around an axis perpendicular to the beam axis (Fig. 2d) [45, 46, 47].

Particle spinning around an axis parallel to the beam axis requires the particle to be anisotropic. Note that the same requirement applies for an angular optical tweezers [34]. The anisotropic particle tends to align its long optical axis to the intensity lobes of the fiber modes. This requires an axially asymmetrical mode supported by a multimode fiber (MMF). Spinning of the particle will be then realized by rotating the axially asymmetrical modes, in one of the two ways: 1. rotating the fiber itself; [48] or 2. rotating the mode through shaping wavefront using a spatial light modulator (SLM) [49].

Kreysing et al. reported an optical rotator (Fig. 2e), using a dual-mode fiber (DMF), which carries a high order linear polarization mode (LP11 mode). The rotation of specimens is realized by rotating the DMF and thereby the LP11 mode [48]. Alternatively, rotation of high-order modes can be realized by wavefront shaping using a SLM [49]. Mode orientations adjustment of the laser beam emitted from the few-mode fiber (Fig. 2f) was realized by applying different binary phase-modulating holograms displayed on the SLM (Fig. 2g). The structured and multi-material anisotropic dielectric particles can be fabricated using a fiber-drawing like method [50, 51]. This flexible and compact fiber-optic rotator robust against fiber bending can be used on practically any light microscope and promises to find applications in tomographic microscopy techniques.

The fiber dual-beam trap also allows for multiple trapping spots between the two fiber end-faces as a result of interactions between particles (Fig. 2h) [52, 53]. When multiple particles are trapped, misalignment between the two laser beams introduce unbalance in the optical forces, enabling oscillations and rotations of the trapped particle. Researchers claim that this functionality can be utilized as a storage of multiple samples.

Single fiber-based optical tweezers

A dual beam fiber optical trap relies on balancing the scattering force provided by the beams from two opposite fiber end-faces, and requires precise alignment between the two pieces of fibers. Alternatively, a single piece of fiber can also construct an FOT, and the method is to form a gradient trap on a tight focal spot near the fiber tip, analogous to the structure of a single beam optical tweezers. In this section, we will present different lens-like fibers with a high NA to focus laser beams for optical trapping.

Tapered fiber tips

A tapered fiber tip is the first fiber-based structure that was built to realize the high NA focusing required to form a single beam gradient trap (Fig. 3a), as demonstrated by Taguchi et al. (Fig. 3b) [33, 54]. A tapered fiber trap can achieve a tight enough focus so that the gradient force dominates in all directions for micron-sized particles. In this case, a pair of inclined fiber tapers (which may resemble a badly aligned fiber dual-beam trap) can also stably trap and levitate a particle, because the gradient force overwhelms the non-coaxial scattering force [55, 56, 57].
Fig. 3

Tapered fiber tips with various functions. a The intensity of the optical field emerging from the fiber probe numerically calculated using beam propagation method (BPM). b Photograph of yeast cell trapped near focal point. c Schematic of single-bacterium labeling. UCNPs I and II are connected to the ends of an E. coli bacterium, with all of the UCNPs and bacterium being trapped by the light from the tapered fiber. d Dark-field images of a labeled single bacteria. e Experimental set-up and working principle of the twin-core fiber optical tweezers. f Sequence of images showing the rotation of an 8-μm borosilicate microparticle by adjusting the polarization of the input signal. Figures reproduced permission from: a Ref. [61], © 2006 OSA; b Ref. [33], © 1997 IET; c, d Ref. [64], © 2017 WILEY; e Ref. [71], © 2008 OSA; f Ref. [72], © 2018 OSA

Fabrication of tapered fiber tips is usually done using two very simple and common techniques, i.e., the chemical etching method [58, 59, 60] or heating and drawing method [61, 62, 63, 64, 65]. With the advantages of simplicity in structure and ease in fabrication, tapered fiber tips have been widely applied in trapping and manipulating biological specimens [63, 64, 66, 67]. Further, single-bacterium labeling and analysis is realized by co-trapping of single up-conversion nanoparticles and bacteria together (Fig. 3c, d) [64].

Liu et al. realized axial trapping position adjustment, utilizing mode division multiplexing [68], based on the following fact: a lens-like fiber tip converges light propagating through different fiber modes to different focal points. For example, the LP00 and LP01 mode in a fiber will be focused to two different points on the fiber axis by a conic tip, and by combining the two modes with different power ratio, continuous modulation of the combined focus, thereby the trapping point, between the LP00 and LP01 foci, can be achieved.

Spinning of trapped particle around an axis parallel to optical axis can be realized through rotation of high-order mode, much similar to the mechanism in the fiber dual-beam traps. Strategies used in dual-beam fiber traps are still efficient: mechanical rotation of trapping fibers [69, 70], or mode modulation [71, 72]. Figure 3e shows a twin core fiber optical tweezers with an in-fiber integrated Mach–Zehnder interferometer (MZI). The output power of two trapping beams can be adjusted through bending the MZI. A resulting variation of output optical field distribution is used to control the orientation of trapped particles [71]. Besides, Velázquez-Benítez et al. demonstrated an optical rotator based on a photonic lantern spatial-mode multiplexer and a few-mode fiber [72]. Rotation of the particles are achieved by switching between degenerate LP modes, as well as rotating the polarization of input light (Fig. 3f).

FOTs with spherical lensed tips

A spherical lens attached onto the fiber tip is another method to achieve a high-NA focus required for gradient trapping. Yuan et al. and Oh et al. used a spherical lens on a fiber tip (Fig. 4a) to focus the output beam from an all-fiber Bessel beam generator for trapping [73, 74, 75, 76]. The resultant Bessel-beam benefits from its non-diffractive and self-reconstructive properties [77], enabling the formation of multiple trapping centers along the longitudinal direction (Fig. 4b).
Fig. 4

FOTs with converging spherical lenses. a Schematic diagram of the 3D dark traps using single optical fiber Bessel beam. b Image of three yeast cells axial traps achieved by the all-fiber Bessel single semi-ellipsoid fiber tips. c Optical microscope image of the probe bound with a 3-μm microlens through electrostatic attraction. d Numerical simulation and calculation of energy density distribution near the tips of a bare tapered fiber and one with a yeast cell bio-lens in the x–y plane. e Schematic illustration of selective trapping and detection of multiple particles using a parallel photonic nanojet array. Figures reproduced permission from: a Ref. [75], © 2018 OSA; b Ref. [76], © 2017 IEEE; c Ref. [80], © 2016 Springer Nature; d Ref. [81], © 2018 ACS; e Ref. [82], © 2016 ACS

A tighter focus that breaks the diffraction limit, called the “photonic nanojet”, can be realized using a microsphere or microcylinder with a high refractive index [78]. In addition to super-resolution optical microscopy [79], photonic nanojet also finds application in optical trapping for forming a high gradient force.

A simple photonic nanojet trapping system contains a tapered optical fiber tip and a dielectric polystyrene (PS) microsphere attached on the tip [80] (Fig. 4c). With the generated photonic nanojet, three-dimensional manipulations were achieved for a single 85-nm fluorescent PS nanoparticle as well as a plasmid DNA molecule. Further, a spherical yeast cell trapped by the fiber tip can also generate photonic nanojet (Fig. 4d) [81]. Since the nanojet “hot-spots” is a result of scattering by individual microspheres, multiple trapping points can be implemented using a single piece of fiber, simply by arraying the microspheres onto the fiber end-face (Fig. 4e) [82]. The produced parallel photonic nanojets can be used to trap cells with high  throughput and high selectivity. Interestingly, researchers found they can also help enhance the backscattering, fluorescence and Raman signal [83, 84, 85]. The strong confinement of trapping/excitation field provides new opportunities for trapping and in situ characterization of particles.

The transfer of trapping spots using the trapped particles

Generally, a rod-shaped particle in a single-fiber trap spontaneously aligns itself in a way shown in Fig. 5a (in this example a multiwalled carbon nanotube (MWCNT) is trapped: gradient force ensures stable trapping of the rod and scattering force exerts a torque on the rod to align its long axis with the optical axis [86].
Fig. 5

Optical assembly of one-dimensional waveguide constructed using sequentially trapped particles. a Schematic of the optical orientation and shifting of a single MWCNT. The inset schematically represents the structure of the MWCNT. b, c Schematic and image of E. coil based biophotonic waveguides. d Simulated energy density distribution for E. coli cell chains with different cell numbers (N). e Fluorescence and backscattering enhancement of E. coli chains with a yeast cell bio-microlens. f Schematic illustration of the bio-nanospear. g Point-excitation of GFP on the surface of a leukemia cell using the bio-nanospear. h Flexibility testing of bionanospear with a bending angle θ. Figures reproduced permission from: a Ref. [86], © 2014 Springer Nature; b, c Ref. [87], © 2013 ACS; d Ref. [88], © 2013 Springer Nature; e Ref. [94], © 2017 ACS; fh Ref. [81], © 2018 ACS

The alignment of rod-shaped particles in the FOT gives rise to a special and interesting phenomenon: the fiber modes, and therefore the trapping spot, can be transferred by the trapped particles, in such case the particles act as elongation of the fiber waveguide. Xin et al. constructed a bio-waveguide consisting of multiple E. coli and Chlorella cells [87, 88, 89, 90, 91, 92]. A rod-shaped E. coli cell was first trapped at the fiber tip, aligned with the optical axis, transferring the trapping spot to the end-tip of the cell, allowing for the trapping of a next cell with the same alignment. The result of this “chain-action” is the trapping of a linear cluster of cells, creating a so-called “bio-waveguide” (Fig. 5b–d).

The photonic nanojet can be transferred by the trapped particles, too [93]. A “living nanospear” of E. coli was constructed in this manner (Fig. 5e, f) [81, 94]. The super-focusing nanojet transferring in the nanospear can lead to an upconverting fluorescence enhancement by a factor of 102 in all E. coli cells [94]. Further, the living nanospear were used as a probe to scan fluorescently doped leukemia cells at subwavelength spatial resolution (Fig. 5g). The bio-friendliness of the living nanospear was also proven: it did not puncture into the cell membrane even when being bent by the cell (Fig. 5h) [81]. It is worth noting that near-infrared trapping beam is used for its low absorption by biological specimens, thus specialized infrared fibers [95] are probably required.

TIR structured fiber tips

TIR is another mechanism used in FOT to achieve a tight focus for gradient force trapping [96, 97, 98, 99, 100, 101, 102]. A trapezium-like slanted ring-trench is cut across the annular core of the fiber (Fig. 6a, b), so that the light guided through the annular core incidents at the interface beyond the critical angle and undergoes TIR towards a single focal point (Fig. 6c) [96, 97]. The structure has an equivalent NA > 1, which is sufficient for three-dimensional optical trapping. Alternatively, multi-core fibers with prism-shaped TIR mirrors can also achieve the diffraction-limited focal spot for trapping (Fig. 6d) [98]. Multiple trapping points have been established using a bundle of several fibers with TIR mirrors at the end-face (Fig. 6e) [96].
Fig. 6

Fiber-based total internal reflection lens for optical trapping. a, b SEM image of a trapezium-like slanted hole. c Cross section of the fiber tweezers and working principle. d 3D depiction of the propagation of the two converging beams exiting the machined diagonal cores. e Schematic representation of different functional fiber bundles. f Schematic of the optical gun based on a coaxial multi-core fiber. g Schematic of tweezers consisting four fibers with microprisms. Figures reproduced permission from: ac, e Ref. [96], © 2007 Springer Nature; d Ref. [98], © 2018 OSA; f Ref. [99], © 2017 ACS; g Ref. [102], © 2013 Springer Nature

The unique TIR ring focus mechanism provides an extra-stable trapping as compared to focal points formed by a taper or a spherical lens, because here the light component with a tilt-angle smaller than φ (refer to Fig. 6c) is missing, greatly reducing the scattering force in the axial direction. Interestingly, this missing component of the axial scattering force can be selectively added back with an extra center-core in the fiber, creating an “optical gun” (Fig. 6f). This mechanical milled optical gun can propel the small particle out of the trapping well at a high speed of tens of micrometers per second and reach a shooting range of hundreds of micrometers within only a few micrometers deviation from the axis [99].

Except for a mechanical milling method [99], several nanofabrication techniques are applied for manufacturing fiber lens at the fiber end-face. Liberale C. et al. fabricated shaped fiber end-face with different angles to adjust trapping position using focused ion beam (FIB) [96, 97]. Two-photon lithography (TPL) technology [100, 101] has also been applied to fabricate microprisms on the fiber end to focus trapping beams (Fig. 6g) [102]. Compared to FIB, TPL is reliable, fast and low cost and, owing to a computer-controlled piezoelectric positioning system, offers a great freedom in the fabrication of optical components with different shapes.

Graded-Index Lens

The self-focusing effect (Fig. 7a) of a graded-index lens (GRIN) or GIF has also been used for trapping (Fig. 7b, c) [103, 104, 105]. The self-focusing effect of a flat-ended GRIN is strong enough to achieve a focal spot required for trapping a 200 nm particle, but the focus and trapping capability (especially in the axial direction) of a GIF taper is stronger with the lensing effect of a taper incorporated [100, 101, 102, 106, 107].
Fig. 7

GRIN lens and GIF-based FOTs. a Periodic intensity distribution in GIF. b Schematic of optical tweezers based on GRIN lens with a flat end-face. c Image of the GRIN tweezers consisting of a 102-μm GRIN lens assembled with a 105-μm spacer and a piece of SMF (SM980). d Schematic setup of a GIF taper for optical trapping. e Principles of tunable trapping distance by mechanically adjusting the length of air cavity. Figures reproduced permission from: a, d Ref. [107], © 2014 OSA; b, c Ref. [104], © 2018 Springer Nature; e Ref. [107], © 2014 OSA

A simple method to create a GIF trap is to build a tapered end [100, 101, 102] to focus trapping beam outside the taper (Fig. 7d). The mismatch of diameter between GIF and the lead-in SMF is usually compensated by creating an air cavity, using a capillary [107, 108] or fiber connector [103], or splicing a glass spacer [104]. A trapping point with a distance of more than 100 μm away from the taper tip can be achieved using a GIF taper tweezers, and this distance can be changed through adjusting the length of air cavity (Fig. 7e) [108] or directly tuning the wavelength of incident laser due to the wavelength dependence of the GIF design parameters [105]. The tunable long working distance enables a more flexible and efficient manipulation of trapped particles without physical contact.

Holographic FOTs

Holographic optical tweezers [37, 109] is a technique to create multiple foci (trapping points) by patterning the laser beam using a SLM and it has realized applications such as particle assembly with nano-sized features and lab-on-a-chip actuations.

The holographic, multi-trapping-center micromanipulation can be transferred to the tip of an optical fiber (Fig. 8a). Since light propagating through a piece of MMF with a certain length is a linear (with a low optical power, which is the case for most optical trapping application) and deterministic process, any desired pattern of trapping spots at the fiber tip (Fig. 8b) is correspondent with a certain wavefront shaping at the input plane of the fiber, which again can be achieved using an SLM. Using this approach, multiple diffraction-limited focal spots can be created on the output plane outside the fiber end-face [110, 111]. Individual in-plane manipulation of as many as 16 particles were demonstrated by T. Čižmár et al.
Fig. 8

Holographic FOTs. a Schematic of fiber-based holographic optical tweezers. b 2-D optical manipulation of 16 PS micro-particles each with a 3-μm diameter. c, d Multiple traps created in three dimensions. e Relocation of the trap site along the lateral direction in increments of 20 nm and along the axial direction in increments of 50 nm. Figures reproduced permission from: a, b Ref. [110], © 2011 OSA; ce Ref. [112], © 2017 Springer Nature

Multiple particle manipulation in 3D requires a much larger number of modes to establish the trapping pattern (Fig. 8c, d), which must be supported by a piece of optical fiber with an NA of approximately 1, higher than most commercial MMF. Leite et al. fabricated an all-solid, step-index MMF with a NA = 0.96 at 1064 nm, and the method was to use two types of flint glass with different density to achieve a large core-cladding refractive index contrast [112]. This high-NA fiber successfully supported the complex modes required for out-of-plane, highly precise manipulation of multiple particles, with a lateral resolution of 5 nm and axial resolution of 25 nm in particle relocation (Fig. 8e).

Plasmonic FOTs

It is challenging to trap objects with sub-100-nm diameters using a dual-beam trap or a gradient optical trap, for the gradient force vanishes with the third power of the particle diameter, as shown in Eqs. 3 and 4 (the scattering force vanishes even faster as shown in Eq. 2 and is usually neglected in the nano-regime). Simply increasing the optical power is not always an option since a high optical power may damage the trapped object. Nanostructures have been developed to achieve focus beyond the diffraction limit, and the subsequent steeper field gradient and stronger optical force, by employing the near-field effects.

Surface plasmons (SPs) are near-field modes residing on the surface of metallic nanostructures. They are evanescent in one or more dimensions, creating steep gradients. SP is usually classified into two categories: SP polaritons (SPPs) propagate along a metal–dielectric interface; localized SP (LSP) resonates in the vicinity of subwavelength metal structures (particles or holes of various shapes), building up a tight spot beyond the diffraction limit and a great local field intensity. Plasmonic nanotweezers makes use of the unique properties of SP, allowing ultra-accurate positioning of single nanoobjects and opens unprecedented opportunities in many fields of science. Multiple review articles have given a thorough discussions on the subwavelength photonic nanotraps [36, 113]; here, we specifically focus on the subwavelength nanotraps that have been merged with optical fibers.

Nanoaperture FOT

A special type of metallic nanostructure used for optical trapping consists of a nanoaperture in a metal nanofilm [29, 114, 115, 116]. It is worth noting that a nanoaperture optical trap is “non-perturbative”, meaning that the particle itself has a strong influence on the local electric field and thereby has an active role in the trapping mechanism. In the subwavelength nanoaperture case, the transmitting power through the aperture vanishes as the 4th power of the aperture diameter. As a high-index particle enters the aperture, it effectively “dilates” the aperture via dielectric loading, thereby significantly enhancing the light transmission. The resultant photon momentum change reacts on the particle, forming a strong optical force. This so-called self-induced back-action (SIBA) optical trap allowed for an order-of-magnitude improved trapping efficiency over the usual perturbative approach [116].

Double-nanoholes-shaped or bowtie-shaped nanoapertures has been fabricated in a metal nanolayer coated on a fiber tip to achieve trapping of nanoparticles with diameters ranging from 20 to 50 nm [117, 118]. The basic method of fabricating an apertured FOT is straight-forward with a metal deposition step followed by a FIB aperture milling step (Fig. 9a–d) [117, 118, 119].
Fig. 9

SIBA FOTs. a Schematic of experimental configuration for three-dimensional manipulation of a single 50-nm PS bead. b SEM image of an 85-nm-gap BNA patterned at the extremity of a tapered optical fiber. c Fiber mount for processing the ends of fibers in the FIB. d SEM image of the active region of the fiber. e Schematics for template-stripped integration steps. Figures reproduced permission from: a, b Ref. [117], © 2014 Springer Nature; c, d Ref. [118], © 2014 OSA; e Ref. [120], © 2018 OSA

The trapping of even smaller particles requires better matching between the near-field focus (usually reside at the sharpest metal feature) and the particle to achieve a stronger SIBA effect. Due to the unavoidable oblique wall of a FIB-milled aperture, the sharpest metal feature is usually formed at the interface between the metal film and the fiber end-face, and steric hindrance from the fiber core prevents a perfect coincidence between the particle and the mode volume. Ehtaiba et al. provided a solution to make the sharp metal feature accessible by directly ‘gluing’ to an apertured metal film onto the end-face of an SMF. This way, the bowtie-nanoaperture (BNA)  is turned ‘bottom-up’, leaving its sharpest feature facing out, directly accessible to the nanoparticles (Fig. 9e) [120]. A single 20 nm PS nanosphere is successfully trapped using this fiber-optic SIBA nanotweezers at a low optical intensity about 0.5 mW/μm2.

Plasmonic Fresnel-Lens FOT

SP also provides support for super focusing “off-shore” from the metal surface [121]. In theory, a focus with an arbitrarily small waist diameter can be established (blue curve, Fig. 10a), given certain distribution at the source plane at a distance away from the focus (red curve, Fig. 10a). However, such source plane distribution, being the result of the backward propagation of the focal point, has to have spatial field variations of even higher pitch. SPP modes propagating in narrow metal gaps can provide such high-pitch spatial field variation (Fig. 10b), thereby supporting a super-focus beyond the diffraction limit at the focal plane.
Fig. 10

FOTs utilizing SP lens. a Source distribution required to achieve an extreme subwavelength Gaussian focus. b Schematic of waveguide array distribution required to achieve source distribution of a. c Schematic of a submicron-sized bead trapped by the subwavelength focus beyond the near field created by the fiber-based SP lens. Inset shows the close-up of the trap. d SEM image of SP lens on the fiber end. Figures reproduced permission from: a, b, Ref. [121], © 2009 APS; c, d Ref. [123], © 2013 OSA

The SPP “super-lens” has been brought to a fiber end-face to achieve a focal point beyond the diffraction limit, and subsequently a strong gradient force. Metallic Fresnel phase plates, fabricated on a cleaved fiber end using FIB milling (Fig. 10c), have been harnessed to achieve surperfocusing [122, 123]. The laser beam first gets coupled to SPP in the ring-slits at one side of the metal film, and gets coupled back to the dielectric medium at the other side, forming the source pattern at the metal surface and subsequently the super-focus at the focal plane. Theoretical analysis suggests that far-field superfocusing can be achieved through designing ring slits with various width to produce different phase decay [122] and the proposed structure can be easily fabricated using FIB milling (Fig. 10d). Similar to the situation in TIR focusing, here the scattering force is reduced due to skewed rays exiting from SP lens [123].

HC-PCF FOTs

HC-PCF is a fundamentally different type of optical waveguide. Instead of utilizing the TIF of light at the core-cladding interface, HC-PCF achieves light confinement and guiding by surrounding the light conduit with two-dimensional photonic bandgap or anti-resonant structures, forbidding light leakage into the cladding (Fig. 11a) [124, 125].
Fig. 11

Functional HC-PCF based optical traps for various applications. a SEM image of a HC-PCF (core diameter, 12 µm) with a superimposed near-field optical mode profile (at 1064 nm wavelength). b Image of a PS particle guided into HC-PCF. c Schematic of flying particle sensors for external electric field, temperature and radiation luminescence. d A series of snapshots of the bound-particle array during one resonant period. e Optomechanically coupled silica nanospike and a HC-PCF. Inset shows SEM image of the final section of the nanospike. Figures reproduced permission from: a, c Ref. [128], © 2015 Springer Nature; b, Ref. [127], © 2002 OSA; d, Ref. [136], © 2018 Springer Nature; e, Ref. [137], © 2016 OSA

These special HC-PCF allows for a special type of trapping, where the trapped particle resides inside the fiber core, and the conduit for particle transport coincides with the conduit for light guidance. Lateral confinement comes from the centripetal gradient force of a Bessel-shaped guided mode, and axial acceleration comes from the scattering force from the two counter-propagating modes from both ends of the fiber. Stable trapping in axial direction is possible by introducing interference between the counter-propagating beams [126].

A “flying particle” experiment was demonstrated using the HC-PCF, in which a microsphere was levitated within the hollow core (Fig. 11b), as well as being transported for a distance of 150 mm within the core [127]. The transport distance is only limited by the loss of the fiber, and as the loss was reduced from 5 to 0.08 dB/m, the particle delivery range was increased to meter-scale correspondingly [128]. The particle transportation distance already far-surpasses any other OT techniques. Given the recent progress on ultra-low-loss HC-PCF [129, 130], a kilometer-long distance of particle delivery is feasible.

The encapsulated, long-range transported “flying particle” trapped in a HC-PCF is a strong candidate for highly-sensitive multi-parameter distributed fiber sensors (Fig. 11c). Remote sensing of electric field, temperature and irradiation was demonstrated using charged particles and fluorescent particles respectively with a spatial resolution down to tens of microns [128, 131, 132]. In this scheme, the trapped particle responds locally to the sensing quantities when it is passing through the region of interest and, determined by the particle’s properties, it can react in different degrees of freedom including transverse motion, variations in speed, fluorescence radiation, etc. These effects can be probed by the intensity, frequency of the transmitted or scattered light.

The HC-PCF can also be filled with liquid and work as an ideal platform for optofluidic research. The perfect coincidence of the conduits for the light and for the fluid allows for applications including measurement, synthesis and biomedical research [133, 134, 135].

In addition, it was demonstrated that multiple particles can be trapped in one HC-PCF at the same time, where they self-arrange into a bound particle array along the axis of the fiber due to optical binding forces [136] (Fig. 11d). The entire levitated particle array can be moved to-and-fro along the HC-PCF without loss of binding effect. The inter-particle distance is found to be orders of magnitude longer than the trapping wavelength. Under low gas pressure, the dynamics of the bound particle array can be observed.

The optical force in a HC-PCF has also been used to achieve a self-alignment and self-stabilization of a mechanically compliant SMF “nanospike” inserted into the hollow core (Fig. 11e). The nanospike was fashioned by thermally tapering a SMF down to tip with dimensions smaller than the wavelength. The centripetal optical force stabilizes the nanospike precisely at the center of the hollow-core, allowing for nearly perfect light coupling between the SMF and HC-PCF [137, 138, 139]. What makes this work especially attractive is that the interaction between the nanospike and the HC-PCF is reported to be self-induced, very similar to the SIBA trap introduced previously. Any deviation of the nanospike from the center of the hollow-core detrimentally affects the coupling and the field distribution in the hollow core, and the back-action of the photon momentum change tends to pull the nanospike back to the center. Furthermore, when the hollow core is evacuated to a low pressure, the trapped nanospikes is able to resonate at a high Q-factor, makes the HC-PCF system a unique platform for optomechanical dynamics researches (Fig. 11d, e) [137, 140].

Photothermal Effects in FOT

Electromagnetic interaction is not the only source of force in an optical trap. Finite heating always takes place when dissipative materials are illuminated by the laser, and an uneven temperature distribution on the target particle or on the surrounding medium gives raise to thermally induced forces. Usually these are much weaker than the optical forces, but they can be intentionally enhanced to establish a photothermal trap [141, 142].

Fibers with different structures such as multicore fiber [143], fiber-ring [144], as well as tapered fiber tips [145, 146] have been designed for photothermal trapping. Fibers with end-face coated by graphene [147] and plasmonic random nanostructures [148] are also used for trapping live cells and colloidal particles. As compared to optical force fields, photothermal force fields usually have larger scales in space, it is therefore not suitable for single particle manipulation, but it can provide a higher throughput for trapping. A side-glow fiber with the leakage evanescent field was used for high throughput particle size sorting for its good selectivity in trapping particles with size of several micrometers (Fig. 12a) [149]. This thermal trap was also proposed to be a portable water cleaner: a fiber-ring photothermal trap is able to trap thousands of particles at the center of the ring, and it can be used as an efficient tool to clean the bacterium in the liquid (Fig. 12b) [144].
Fig. 12

Photothermal optical traps. a Schematic for particle separation in fluidic flow by an optical fiber. The inset shows the scanning electron microscope image of a 1.2-μm optical fiber. b Particles trapped in the region of fiber-ring. c, d Image of SMF end-faces before and after nanotubes have been deposited. e Schematic of the mode-locked erbium-doped fiber ring laser. Figures reproduced permission from: a Ref. [149], © 2012 OSA; b Ref. [144], © 2011 OSA; ce Ref. [150], © 2007 OSA

This technique was demonstrated for depositing carbon nanotubes on the ends of optical fibers to form saturable absorbers used for constructing ultrafast laser (Fig. 12c–e) [150]. Nicholson et al. demonstrated an erbium-doped fiber passive-modelocked laser with a spectral FWHM of 10.3 nm, and a pulse FWHM of 247 fs. An ytterbium-doped fiber modelocked laser generating 137 fs pulses from an oscillator-amplifier configuration is also realized.

Perspectives and Conclusions

Optical fibers have made great achievements in numerous fields including optical communication, sensing and imaging in the past few decades, and here we review its development in the field of optical trapping and manipulation. FOTs provide a more flexible, robust and reliable optical manipulation tool for many emerging fields at a low cost. We predict that the FOT technique will continue to make important contributions to the interdisciplinary fields such as biophotonics and quantum communications. Here we provide two examples of speculated future applications of FOTs.

In-Vivo Applications

If one is to name a single most important application of the optical trapping of micro-/nanoparticles, it will be in the field of biological studies. The optical trapping technique has allowed for a revolutionary development of single-molecule biology [17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29]. A better cell imaging technique was also developed with the help of optical manipulation, adding important parts to cell biology.

Conventional OT setups are inherently designed for in vitro studies. Only a handful of demonstrations of biological sample trapping are performed in vivo, but strong restrictions apply in the experimental environment—it has to be done at a very superficial location of a living subject, such as in the blood capillary in a mouse ear [151]. It is of great interest to biologists to be able to perform studies in vivo, where the overall effects of an experiment can be captured.

FOT is the most suitable candidate to realize in vivo trapping experiments using an endoscope-like setup. To build an FOT that is capable for efficient in vivo biological experiments, the authors foresee two major challenges to be overcome:
  1. 1.

    In-vivo sensing/imaging techniques. Data collection will be challenging since the conventional microscopy techniques become unavailable in vivo. Fiber-based endoscopic imaging techniques, such as fiber-optic fluorescence imaging, may be incorporated with FOTs and become a solution [152]. Nevertheless, the label-free sensing/imaging techniques, especially Raman spectroscopy [6, 7, 40], are demanded in in vivo studies. Current Raman enhancement technique such as surface-enhanced and tip-enhanced Raman scattering need to integrate with fiber-optic system and evaluate the biological toxicity due to the use of heavy metal.

     
  2. 2.

    Tether-free trapping techniques. Most single-molecule biology experiments rely on tethering of biological molecules, including protein, DNA and RNA, onto a microsphere [17, 18, 19, 20, 21, 22, 23, 24]. It is expected that sample preparation for the tethering is difficult, if not impossible, in the interior of a living subject. In addition, the microparticle may introduce hindrance to certain functional areas of the biomolecules. For these reasons, it will be beneficial for the endoscope to develop tether-free, direct manipulation and monitor methods for single molecules [153].

     

Single-photon sources

A single photon source (SPS), a basic element in quantum information science, is another potential application of FOT. The FOT may be able to simultaneously tackle two challenges in the development of SPS: (1) the sub-10 nm precision required in the fabrication, and (2) the all-fiber integration. The advantage of FOT in the all-fiber integration is largely apparent, here we mainly focus on how FOTs may be competent in SPS fabrication.

Plasmonic FOTs (Sect. 3.5) are capable of realizing ultra-precise position control for both the position and orientation of a single emitter in a nanoantenna. Plasmonic traps can have trapping area smaller than 10 nm in size [115], and the high-local-field excitation area coincides with the trapping area, making the trap bi-functional. The polarization of the emitter will also naturally align with the electric field in the trap (this feature of an optical trap gives rise to the angular optical tweezers [30]). Future work is needed to make the metallic nanostructures in the FOTs function as nanoantennas to achieve the required near-field/far-field coupling [154].

In addition to plasmonic FOTs, whispering-gallery mode (WGM) resonators based on HC-PCF traps combine small optical mode volumes with narrow resonance linewidths, making them great candidates for single photon sources. Zeltner et al. constructed a WGM microlaser consisting of a dye-doped particle optically trapped in the core of HC-PCF [132]. SPS is theoretically possible if the resonator can achieve a higher Q factor.

Conclusions

FOTs are slim, flexible, robust implementations of optical traps. The combination with different fabrication methods and special fibers paved the way to obtain more advanced manipulation features and even new functions. Fiber optical traps create a broader arena and necessitate discipline integration to explore future developments.

Notes

Acknowledgements

The authors acknowledge funds from National Natural Science Foundation of China (Grant number: 11874164), the Innovation Fund of Wuhan National Laboratory for Optoelectronics and 1000 Talent Youth Program. We also appreciate valuable discussions with Prof. Guangming Tao from Huazhong University of Science and Technology, Dr. Shangran Xie from Max Planck Institute for the Science of Light, Prof. Guanghui Wang from Nanjing University, Prof. Hongbao Xin and Prof. Baojun Li from Jinan University.

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Copyright information

© Donghua University, Shanghai, China 2019

Authors and Affiliations

  1. 1.School of Optical and Electronic Information and Wuhan National Laboratory for OptoelectronicsHuazhong University of Science and TechnologyWuhanChina

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