1. Introduction

Since its invention by Ashkin and associates [1], the optical tweezer has emerged as an important tool in micromanipulation of small particles. The single-beam optical tweezers bring revolutionary new opportunities for fundamental and applied research in the fields of atomic spectroscopy, cell biology, nano-bio photonics [2–4]. Non-diffracting single optical beam which propagates with a high degree of energy confinement near its axis is commonly used for optical tweezer. Among various types of non-diffracting optical beams Bessel beam [5, 6] and 3D bottle beam [7–9] are recommended effective for optical tweezers. The 3D bottle beam, also known as self-imaged bottle beam, has a unique intensity pattern with nearly zero intensity (dark trap) at a point on the optical axis surrounded by regions of high intensity in all spatial directions. Such bottle beam can suppress the thermal motion of micron-size and even sub-micron-size objects and spatially localize single or more particles. The other advantages of dark trap are their ability to trap dark-field seeking objects and the reduced light intensity to which the trapped object is exposed. A beam of this kind has applications in optical tweezers for trapping particles with low refractive indices [10–13].

Recent advances in nano-photonics and nanotechnology demands the development of nano-optical tweezers with added features. Optical tweezer is proven successful for manipulating objects larger than the wavelength of the optical source, but it faces difficulties at the nanoscale. The nanometer size specimen experiences larger Brownian motion, <x²> = kTt / (3πηa), x, k, T, t,η and a being mean square displacement, Boltzmann constant, absolute temperature, elapsed time, viscosity of the liquid and radius of the suspended particle respectively. Smaller particles also experience reduced restoring force and viscous dragging force [2]. Surface plasmon resonance based optical traps formed by nanostructured surface have been fabricated successfully [2, 14] to immobilize nanometer size specimen on a substrate, though plasmonic tweezer has drawback of excess local heating effect and complex fabrication methodology. The requirement of 3D manipulation of nano-objects demands a further step from nano-trapping towards nano-tweezing. The realization of a nano-optical tweezer based on a single optical fiber can provide a miniaturized and simple tool for 3D manipulation at nano-scale. As per our knowledge, neither the unique property of a 3D bottle beam has been exploited so far in optical fiber based tweezer nor the fiber based nano-tweezer has been explored for picking up nanoscale object. It is to be mentioned that the reports are available [15–17] on optical fiber based tweezers showing the manipulation of micron-size particle in aqueous medium. Also an optical fiber attached with a micro-sphere lens to create a tight spot of light has been reported [18] for trapping application. Further, as diffraction prevents confinement of propagating light, preparing optical element to prevent diffraction itself is a challenge. Therefore a simple solution for such an optical element should be very promising.

This letter presents the first experimental demonstration of nano-tweezing in 3D based on a single optical fiber nano-tip. The nano-tip works as an integrated 3D bottle beam generator, which generates beam of smaller spot size. Such beam should favour to trap individual nano-particle instead of cluster besides being benefited from the advantages of optical fiber. This specially fabricated axicon like nano-tip, generating 3D bottle beam, is not only capable of trapping and manipulating nanometer-sized objects in 3D, but can also pick such object from aqueous medium.

2. Design of fiber nano-tip based non-plasmonic tweezer and its optics

The tweezer is made of a novel ultrafine optical fiber nano-tip which is prepared by etching a photosensitive single-mode fiber in hydrofluoric acid (HF) in a hydrophobic polymer coated tube. The dimension of the probe is determined by the radius of the tube, surface tension meniscus around the fiber dipped in the acid and capillary ring cavity formed around the fiber core cladding boundary during the etching process. The detail of the novel fabrication technique is given in [19, 20]. The technique enables us to fabricate axicon like fiber tip with nano order aperture and changing wedge angle at the pit of the tip. Figure 1 is SEM image of one such fiber nano-tip. The concave curvature at the tip pit, Fig. 1 and Fig. 4, show gradually changing wedge angle starting from very small value. The curvature is determined by the relation of dh_c/dr∝-(1/r)² where h_c(∝1/rρg) is the height of the HF solution drawn in the capillary ring around the fiber core-cladding boundary due to capillary action during the etching process (r, ρ and g being half-width of capillary ring, density of HF solution and gravitational acceleration respectively). The wedge angle in such asymmetrically etched fiber core varies ideally from zero degree to higher values. The typical probe shown in Fig. 1 has dimension of ~3 micron base diameter, 3 micron height and small tip aperture.

 

Fig. 1 Axicon like optical fiber tip with variable wedge angle. The wedge angle γ ideally starts with very small value and changes smoothly at the pit of the probe with sudden jump to ~60°.

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Such axicon like optical element illuminated with a Gaussian beam can generate non-diffracting Bessel beams over a distance approximated as Z_max~ω₀/θ, ω₀ and θ being Gaussian beam radius and beam divergence at the tip of the axicon respectively. Using Snell’s law for refraction the beam divergence can be approximated as θ ~(n-1)γ where n and γ are the refractive index and the wedge angle of the axicon lens respectively. In theory, the electric field of a l^th order Bessel beam is given by [7]

From Fig. 1 it is observed that the fiber nano-tip resembles to a good quality axicon lens [6]. Its additional advantage is continuously changing wedge angle (γ) starting from a very small value. The changing wedge angle generates Bessel beams with different wave vectors k_r and k_z. The beams interfere in longitudinal direction, following Eq. (1b), resulting in a self-imaged 3D bottle beam [21] in the propagation direction (z-axis). In order to observe the free space propagation of the resultant beam, we place a tip, similar to Fig. 1, vertically up under an optical microscope (Ziess, Stemi 2000). The other end of the fiber is butt coupled to a HeNe (Melles Giot) laser source of wavelength 633 nm and power 35 mw. A CMOS camera, integrated with the optical microscope records the optical field pattern at different distances (i.e. at different z-values) from the fiber tip along the propagation axis (z-axis). The images are recorded by varying the observation points of the microscope objective with respect to its working distance Z₀ = 9 cm. The small values of γ favor longer working distance of the tweezer. The wedge angle can be varied depending upon the etching rate of the core material, concentration of the acid solution etc. Accordingly, the intensity pattern can also be modulated.

Figure 2 shows two-dimensional representation of 3D intensity distribution obtained at different distances Z₀ + ΔZ from the tip. The first row shows transverse field distribution at ΔZ = −3.4 mm, 3.8 mm and 4.6 mm respectively with central dark spot surrounded by bright rings. The second row shows transverse field distribution with bright central spot at ΔZ = −3.2 mm, 4.2 mm and 5.2 mm respectively.

 

Fig. 2 3D bottle beam, the first row showing central dark point surrounded by bright rings and the second row shows annular rings with bright central spot, the two set of figures correspond to destructive and constructive interference of Bessel beams generated from axicon like fiber tip with variable wedge angle. The attached line profile (not in scale) shows variation in corresponding transverse optical field distribution along an arbitrary line passing through the centre.

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The attached line profile shows variation in corresponding transverse optical field distribution along an arbitrary line passing through the centre. The two sets of figures correspond to destructive and constructive interference regime of bottle beams generated from the fiber nano-tip.

3. Experimental demonstration of the nano-tip as non-plasmonic tweezer

To demonstrate the probe’s capability in optical manipulation of submicron particles, we show trapping in transverse plane, in longitudinal direction and finally tweezing nano-particle from an aqueous solution. Figure 3(a) is the schematic of the experimental set up. A thin film of thickness ~200μm of silver nanoparticles (diameter ~60 nm) solution is formed on a thin (~15μm) cover glass slip which is placed above the tip. The other end of the fiber is connected to the laser source. The Microscope objective is focused on the cover glass slip to observe trapping of particles. Figure 3 presents the trapping of silver nano-particles around the rings of bottle beam in transverse plane. The particles are drawn to the ring’s circumference, as shown in Fig. 3(b) and 3(c). The population of the trapped particles in the outer ring is less as optical field in the outer rings are weaker compared to the inner rings. Two cases of transverse trappings performed with different probes and same concentration of Ag particles. Figure 3(d) is the grayscale representation of Fig. 3(b). Figure 3(e) presents the beam profile in an arbitrarily selected transverse plane to give an impression of correlation between optical field and trapping of particles. It is to be mentioned that ordinary microscope cannot image the silver nano particle in absence of laser. In presence of laser the scattering cross section of the particles becomes significantly large compared to its physical cross section making it visible through ordinary microscope as in Fig. 3 (b)-3(d).

 

Fig. 3 (a) Experimental set up for transverse trapping of nano-particles, (b)-(d) Arrays of silver (Ag) nanoparticles around the rings of Bessel beam in transverse plane taken with different probes and different concentrations of Ag particles, (e) 3D presentation of beam profile in an arbitrarily selected transverse plane, (f) Beam propagation images on the X–Z plane in a highly concentrated silica nano particle solution (g) Chain of silica Particles trapped in the beam path through a low concentration silica particle solution (the zoomed image and Media 1 is captured by Camera from the computer screen), tip is feed with HeNe laser at 633 nm.

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Fig. 4 SEM image of the fiber tip showing attached (a) ~160 nm silica nano-sphere (b) ~60 nm Ag nano-particle as obtained experimentally using fiber tip based optical tweezers.

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To show the trapping capability of the tip in longitudinal (3D) direction, the fiber tip is horizontally placed on the glass slide in the same experimental setup and few drops of solution of silica nanoparticles (160 nm) are added on the tip. Figure 3(f) shows the beam from the tip in longitudinal direction through the solution. A close observation shows central dark region (marked by an arrow in the fig.), which could be effect of 3D bottle beam in the solution. The Video link representing Fig. 3(g) shows suppressing of Brownian motion of the particles and trapping of the particles in the longitudinal interference pattern of the beam. The long chain of aligned silica nano particles reveals that the tip can generate non-diffracting beam over a long distance Z_max. As the central spot experiences diffraction free propagation, we do not observe the presence of outer rings in the solution in Fig. 3(g) as it experiences more diffraction compared to central one. The particles appear to be trapped at certain interval which may be due to the bottle beam effect.

For tweezing silica nano-particle, the same experimental setup is used but the fiber tip is moved vertically downward until it gently touches and dipped inside the solution. The other end of the fiber remains coupled to the same laser source. Subsequently the tip is slowly vertically removed from the solution. Finally the laser source is switched off and the tip is removed for SEM imaging. Similar experiment is repeated in a low concentration Ag nano-particle solution to pick Ag particle.

Figure 4 shows tweezing of single silica and silver particle from their respective aqueous solutions. The particle is trapped symmetrically on the nano-tip of apertures ~100 nm. The particles have dimension of ~160 nm and ~60 nm for silica and silver respectively. The wavelength used for trapping is λ = 633 nm. In the Rayleigh regime, λ>>r, the optical force $F\propto \alpha \nabla E^2$, α and E being polarizability of the particle and electric field respectively, traps the particle against the Brownian motion. The viscous dragging force$F=3\pi \eta av$acts as damping force, where η, a and v are viscosity of the liquid medium, diameter of the nano-sphere and flow velocity respectively. The particle remains attached on the tip permanently even after laser is switched off due to Van der Waals force between the two microscopic objects. The fact that the particle hangs on the tip end (not at side wall) and tweezing never happens in repeated experiments without laser assures the role of radiation pressure in the tweezing. Such tweezing technique may be exploited to prepare readymade TERS (Tip Enhanced Raman Scattering) probe.

4. Conclusion

In conclusion, we have developed a nano-optical tweezer based on an axicon type optical fiber nano-tip. The simple and cost-effective processing technique compared to microfabrication [21] makes the nano-tweezer very promising. It also possesses features of a good axicon lens with changing wedge angle, which supports Bessel beams with different longitudinal propagation constants. The Bessel beams interfere in longitudinal direction generating 3D bottle beam, which is exploited as nano-optical tweezer. Though nanometric tweezer has been theoretically proposed earlier [22], we are successful to make an optical nano tweezer with self-sufficient optical properties and experimentally demonstrate tweezer operation with the fiber nano-tip. The tip being made of dielectric material minimizes its interference when probing nano-materials. The nano-tweezer may find applications in cell biology, atomic spectroscopy, airborne particles trapping, small force measurement and other advanced research of handling nanoscale particles.