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Particles nanomanipulation by the enhanced evanescent field through a near-field scanning optical microscopy probe
Sensors and Actuators A 169 (2011) 171–177
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Particles nanomanipulation by the enhanced evanescent field through a near-field scanning optical microscopy probe B.H. Liu a,b,∗ , L.J. Yang b , Y. Wang b , J.L. Cui b a b
State Key Laboratory of Robotics and System, Harbin Institute of Technology, China School of Mechatronics Engineering, Harbin Institute of Technology, China
a r t i c l e
i n f o
Article history: Received 22 December 2010 Received in revised form 26 April 2011 Accepted 26 April 2011 Available online 5 May 2011 Keywords: Near-field optical trapping 3D FDTD Maxwell’s stress tensor Field enhancement Trapping force
a b s t r a c t A near-field scanning optical microscopy (NSOM) probe and a polarized semiconductor laser (808 nm, cw) were applied to push the trapping resolution down to 120 nm on near-field optical manipulation. A multi-circular shape with a minimum size of 400 nm consisting of 120 nm polystyrene particles can be obtained. They are at a resolution of d (d: NSOM probe tip diameter) and /7 (: laser wavelength), respectively. It is proved that sample concentration and laser power can affect feature size of trapping patterns. In this paper, the effect of trapping forces acted on a nanoparticle along three axis directions on trapping positions is studied, and different trapping positions are generated: the aperture edge in polarization direction and center surface of the probe tip. The result indicates that the single mode NSOM fiber probe is able to trap nanoparticles in a circular shape with lower laser intensity than that required by conventional optical tweezers. The simulated trapping positions around the probe tip based on the conservation law of momentum are found to agree well with experimental results. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently the field of optical trapping and fabrication is entering an exciting newphase due to the rapid development of near-field optical techniques. It offers a high resolution to overcome the optical diffraction limit [1], which is one of the main bottlenecks in optical applications. The technology using the evanescent wave generated around a scattering body has been receiving great attention. Based on field enhancement near the AFM tip and mechanical indentation of the tip under laser irradiation, Huang et al. investigated the nanopatterning of metallic layers on silicon substrates, which is beyond the diffraction limit of the laser wavelength [2,3]. Lu et al. further analyzed the nanofabrication by a laser-assisted scanning probe microscope operating in air ambient [4,5]. Furthermore, Gu et al. showed a single beam near-field laser trapping technique under focused evanescent wave illumination for optical stretching, folding and rotation of a single red blood cell [6]. Novotny et al. proposed a scheme for optical trapping at the nanometer scale based on the highly enhanced electric field close to a laser-illuminated metal tip [7]. By following the time evolution of the fluorescence, Kwak et al. demonstrated a technique for optical trapping of 200-nm fluorescent latex beads in water,
∗ Corresponding author at: School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, China. Tel.: +86 451 86413257; fax: +86 451 86402755. E-mail address: [email protected] (B.H. Liu). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.04.042
which is based on the intense near-field gradients around small apertures in a metal film [8]. Reece et al. demonstrated optical manipulation and large scale trapping of 500 nm particles arrays using cavity enhanced evanescent waves produced with a dielectric resonator [9]. Mellor et al. described the optical binding of sub-micron polystyrene spheres in the evanescent field of counterpropagating laser beams [10]. However, these techniques, although very promising, are delicate to implement for manipulating particles with diameters of tens of nanometers main for three reasons. Firstly, before the particle can be captured, Brownian fluctuation has a disruptive effect and the radiation pressure from the illuminating laser imparts momentum to the particle. Therefore, one has to capture a moving particle. Secondly, due to the inflexible configuration, manipulating techniques are mostly restricted to trap particles in a fixed position. Finally, it is difficult to find and position one particle at a few nanometers in size directly during the manipulation in water. Thus it requires new experimental approaches when dealing with neutral particles of tens of nanometers. The near-field scanning optical microscope (NSOM) uses the evanescent field confined at the tip of a metal-coated fiber probe to provide images of surfaces with high resolution [11]. Based on the integration of NSOM and femtosecond laser, Lin et al. used the high evanescent light across the fiber probe to produce near-field optical nanolithography on the UV photoresist [12]. The direct laser writing of lithography patterns with a feature width of 20 ± 5 nm was realized by employing the combined effects of an evanescent wave and an ultra-short laser pulse [13]. On the other hand, the NSOM probe
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Fig. 1. Schematic diagram of the experimental setup for nanoparticles trapping using an NSOM probe.
has now opened up a wide range of new applications in nanoparticle manipulation due to strong gradient forces associated with the enhanced evanescent field [14]. Using polarized laser beams in NSOM probes, nanoparticles can be trapped on the tip surface owing to the near-field enhancement at the tip. Furthermore, the trapped particles can be absorbed to the substrate in a circular shape. In this paper, the trapping force distributions near the fiber probe along three axis directions are described. The nanomanipulating details are analyzed at different probe-to-particle distances. Simulation of trapping positions and comparison of other forces versus trapping force based on the conservation law of momentum shows that the single mode NSOM probe is able to trap nanoparticles in a circular shape, which agrees well with the experimental results. The effects of sample concentration and laser power on trapping patterns observed by laser scanning confocal microscope (LSCM) are discussed. 2. Experimental setup The fiber probe used in the experiment is an NSOM fiber (NTMDT), which is operated in a wavelength ranging from 780 nm to 970 nm. It is made of a single mode tapered optical fiber with an aperture diameter of 400 nm by coating the metal film around the tip. Fig. 1 shows the experimental setup for the NSOM probe nanomanipulation. The optical source is a polarized cw laser (Lumics LU0808M100) operating at a wavelength of 808 nm. The semiconductor laser and a polarization-maintaining (PM) pigtail are connected directly by a FC-PC connector. The laser beam transmitted in the PM fiber firstly passes through a variable attenuator to permit continuous variation of the laser power, and a fiber polarization rotator (OZ Optics) to permit the rotation of the polarization, then passes through a fiber splitter (OZ Optics) with a ratio of 95:5 for the power detection. It is finally coupled into the core of an NSOM fiber with a maximum optical input power of 400 W using a fiber adapter. Some of the light traveling down the fiber core is reflected from the tapered metal-coated probe, then is directed by the fiber splitter to a power meter (Connet JW3203R) for the detection. The back-reflected signal at the detector consists of a small depolarized component due to the polarization scrambling nature of the fiber. This signal is optimized to ensure that the laser light is efficiently coupled into the fiber core and then directed all the ways to the fiber tip. In order to adjust the inserted position and angle of the fiber probe with a cone angle of 30◦ and a cone length of 200 m, the other end of the fiber adheres to a syringe needle which is attached to a three-dimensional nanopositioning
stage (PI P-517.3). In the sample cell, the output end of the fiber is placed inside the solution perpendicular to the glass coverslip. The sample solution contains 120 nm polystyrene particles (Structure Probe Inc.) with a refraction index of 1.56 and a density of 2.4 × 103 kg/m3 . An overhead laser scanning confocal microscope (OLYMPUS OLS3000) is used to observe the manipulation process in real time. The maximum magnification with 100× objective and 6× zoom is 14,400, corresponding to the minimum view field of 21 m × 21 m. The resolutions in xy directions and z direction are 120 nm and 10 nm, respectively. 3. Theoretical study Although there are many models developed up to now, physical properties of particles manipulation by evanescent wave including the trapping size, trapping position and comparison of other forces versus optical trapping force have not been solved yet. The forces acted on Rayleigh particles in the near-field have been treated using wave optics [15]. However, this method is no longer valid when the aperture diameter is much smaller than the wavelength, and the gradient force is not accurate due to the limitation of dipole approximation. Some realistic models about near-field distribution from optical fiber probe have been proposed, such as Multiple-Multipole method and Boundary Integral Equation method [16,17]. But the multipolar functions used in the basis set of the Multiple-Multipole method are in a short range that only their close neighborhood is affected. This method is better suited to account for localized geometries than the expansion in plane waves. In addition, the Boundary Integral Equation method is only restricted to twodimensional studies due to the constraints of time and memory. To illustrate the experimental results in Section 4, the combination of Maxwell’s stress tensor and three-dimensional finite difference time domain method (3D FDTD) is used for the calculation of trapping forces in this paper. Considering the electromagnetic field near the fiber probe with an aperture diameter of 200 nm, the electromagnetic force subjected to the charge can be expressed by Lorentz force formula. The momentum of the charge system changes under the influence of the force, and the electromagnetic field should also change accordingly the conservation law of momentum. Here the FDTD geometry is placed in a volume discretized in mesh cells of 320 × 315 × 210 with a space step of x = y = z = 3 nm, as shown in Fig. 2. The probe is laid out in yz coordinates and assumed to be placed in a medium with refractive index n1 = 1.2. It is assumed that the laser polarized along y-axis is launched into the fiber probe with a coupling
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Fig. 2. Schematic diagram of the geometry used in the calculation.
power of 400 W and at a wavelength of 808 nm. The direction and polarization of incident plane wave are indicated by the K and E vectors. A small particle with the radius of a = 10 nm, the density of = 2.4 × 103 kg/m3 and the refractive index of n3 = 1.8 is placed near the probe. Given the volume V near the optical fiber probe surrounded by an outer boundary S, the trapping force acted on the nanoparticle can be given by:
F=
↔ T · n dS,
f d = − V
(1)
S
↔ where T is Maxwell’s stress tensor, which can be expressed as: ↔ T =
ε0 εr E 2 + 0 r H 2 ↔ ıij − ε0 εr E i E j − 0 r H i H j 2
,
(2)
ij
where E and H are the vectors of electric field intensity and magnetic field intensity in free space, foot notes i and j represent the ↔ direction x, y or z, and ıij represents the unit stress tensor. ε0 and εr are the permittivity of vacuum and the relative permittivity of medium respectively, 0 and r are the permeability of vacuum and the relative permeability of medium respectively. Then the specific expressions of trapping forces along three axis directions are given by:
Fx =
− S
Fy =
− S
Fz =
− S
ε0 εr E 2 + 0 r H 2 ex − ε0 εr E 2x ex − 0 r H 2x ex 2
dS,
ε0 εr E 2 + 0 r H 2 ey − ε0 εr E 2y ey − 0 r H 2y ey 2 ε0 εr E 2 + 0 r H 2 ez − ε0 εr E 2z ez − 0 r H 2z ez 2
(3)
dS, (4)
dS.
(5)
In order to know where the particle is finally trapped, a series of calculations are made to obtain the trapping force distributions along three axes. Fig. 3(a) shows the trapping forces acted on a particle along z-axis direction with different radius at different heights. The particle moves along z-axis direction when the trapping force is positive. In contrast, the particle moves along −z-axis direction when the trapping force is negative. For the particle with relatively small radius, i.e., a = 10 nm and 50 nm, the trapping force near the probe tip is attractive, then it descends to zero and becomes repulsive due to the scattering force. Thus for a small particle, the global decay of the trapping force in the axial direction can form a deep
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one-dimensional trap in the near-field region. In z-axis direction, the particle tends to be attracted to the tip surface. When the particle radius is large enough, i.e., a = 100 nm, the trapping force acted on the particle is always a repulsive one. It indicates that the large particle cannot be trapped and tends to move away from the probe tip along z-axis direction. The trapping forces in y-axis and x-axis directions given to the particle with the radius of a = 10 nm at 0 nm, 10 nm and 70 nm heights above the aperture are plotted. In Fig. 3(b), the particle moves along y-axis direction when the trapping force is positive. In contrast, the particle moves along −y-axis direction when the trapping force is negative. The curve in z = −70 nm plane shows that the particle is pulled close to z-axis in the lateral direction. In z = −10 nm plane, the trapping forces near z-axis and the aperture are attractive, it means that three trapping positions appear in this plane. In is also found that two unstable trapping positions exist in the middle of the aperture. While in z = 0 nm plane, the trapping forces around z-axis and near the aperture are repulsive and attractive respectively, it means that the particle is only trapped to the aperture edge. In Fig. 3(c), the particle moves along −x-axis direction when the trapping force is negative. Because the trapping force along x-axis is always attractive and symmetric to the probe axis, the particle always tends to be attracted to z-axis along x-axis direction. According to the distributions of trapping forces, the particle placed near the aperture along y-axis direction tends to be dragged to the aperture edge immediately. While the particle placed at other positions around the aperture tends to be pulled close to z-axis in the lateral direction, and finally is attracted to the tip surface along z-axis direction. As shown in Fig. 4(a), the trap position for the fixed polarization of incident light has a quasi-line shape, which would be smoothed into a circular shape if the polarization direction is rotated around the symmetry axis. Due to the existence of birefringence in a single mode optical fiber with the variation of ambient temperature and stress, the polarization direction of incident laser changes randomly. When the polarized laser in a PM fiber coupling into the single mode NSOM fiber, the polarization of the light cannot be maintained any more, thus the trapping position occurs all around the aperture of the fiber probe. In the actual condition, the particles tend to be trapped in a circular shape, as shown in Fig. 4(b). In order to get the information of trapping strength, the role of forces other than the trapping force must be taken into account. During the manipulation the particle is mainly in five kinds of forces: Brownian motion, gravitational, capillary, van der Waals and optical trapping forces. The calculations show that the magnitude of the trapping forces is 10−13 N. For a particle with a radius of 10 nm immersed in water, it is found that the gravity is 9.8 × 10−20 N, according the gravitational force Fg = 4/3a3 g, where g = 10 m/s2 and = 2.4 × 103 kg/m3 . Since the trapping force acted on the particle is larger (by a factor 106 ) than the gravitational force, the effect of the gravity can be neglected. If there is water on the substrate, according to the capillary force Fc = 2a , where a is the radius of the particle and is the surface tension of water [18], one can get Fc = 4.5 × 10−12 N for a particle with a radius of 10 nm. The capillary force is about the same magnitude as the optical force, thus it is not easy for optical trapping on this substrate. However, it is beneficial to trap particles on the surface of water to the probe tip when the probe is placed in water. The Brownian motion is due to the thermal fluctuations. The fluctuation–dissipation theorem of Einstein states that the Brownian motion force is equal to 12a kb T, where is the viscosity of water, kb is the Boltzmann constant, and T is the temperature of water [19]. At room temperature, the Brownian motion force is 1.0 × 10−13 N, which is of the same magnitude as the trapping force. It suggests that the trapped particle is not affected greatly by Brownian motion with appropriate trapping parameters. The van der Waals force between two objects can be
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Fig. 3. Trapping forces acted on the particle: (a) along z-axis direction with different particle radius, (b) along y-axis direction and (c) along x-axis direction at different heights above the aperture plane.
Fig. 4. Schematic diagram of trapping positions using the metal-coated fiber probe: (a) in the ideal condition and (b) in the actual condition.
function of the size of the particle or the aperture. The strong capillary force is beneficial to trap particles on the surface of water when the fiber probe is placed in water. The trapping force depends on the diameter of the aperture much more than it depends on the size of the particle. By comparison of the other three forces versus optical trapping force, it is found that the trapping force obtained
Aperture diameter (nm) 200 300 400 500 600 700
Magnitude of forces (N)
described as a short range force, derived from the Lennard–Jones potential, in the form Fw = 0.167AS/h2 (0.25z6 /h6 − 1), where h is the distance between the two objects, A is the Hamaker constant, z corresponds to the separation of lowest energy between two objects and S is the Derjaguin geometrical factor related to the mutual curvature of the two objects [20]. When computing the van der Waals force between the particle and substrate surface one has Fw = 4.0 × 10−13 N. It also suggests that the trapped particle is not affected greatly by the van der Waals force with appropriate trapping parameters. In addition, it is necessary to consider the van der Waals force between the probe tip and particle when they are in contact. When the curvature of the tip is equal to 10 nm, one has Fw = 2.7 × 10−13 N with S = 5 nm and A = 109.5 zJ. It can be seen that the two van der Waals forces cancel each other partially when the particle is in contact with both the substrate and tip. Under the action of van der Waals force, the trapped particle tends to be released to the substrate when turning off the laser. In comparison with other forces acted on the particle with a diameter of 20 nm, it is concluded that the trapping force is not dominant among the five forces when the aperture diameter of the probe tip is 200 nm and the laser intensity is assumed to be 1040 W/mm2 . To further determine the trapped size of nanoparticles, Fig. 5 shows plots of five forces acted on the particle as a
10-10 10-12 10-14
Capillary force Actual trapping force Calculated trapping force Van der Waals force Brownian motion force
10-16 10-18
Gravitational force
10-20 20 40 60 80 100 120 140 Particle diameter (nm)
Fig. 5. Forces given to the particle located near the fiber probe.
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Fig. 6. The trapping images of nanoparticles using the NSOM probe: (a) before turning on the laser, (b) after turning on the laser and then (c) after turning off the laser.
with an aperture larger than 200 nm is dominant among the four forces of a particle smaller than 20 nm. It means that one can trap a particle smaller than 20 nm using the fiber probe with an aperture diameter larger than 200 nm. However, the aperture diameter cannot be too large, and the particle size cannot be too small, otherwise it is difficult to achieve the optical trapping in the near-field due to the weaker near-field intensity near the aperture than the far-field intensity and the difficulty of observing particles in real time. It should be noted that for the reduction of computation the calculation above is only limited to the tip region of fiber probe less than 1 m. According to tip-induced plasma oscillations, much stronger evanescent field occurs at the tip of fiber probe with longer tip height [21]. With an actual tip height of 200 m, the magnitude of the trapping force acted on the particle must be greater than 10−13 N. The actual trapping force acted on the particle is shown as a dashed line in Fig. 5. The dominant trapping force indicates that one can realize near-field optical trapping with much lower intensity (1040 W/mm2 ) than that required by classical fiber optical tweezers (105 W/mm2 ) [22]. In view of the incident wavelength of 808 nm and the LSCM resolution of 120 nm, the NSOM probe with a diameter of 400 nm is chosen to trap the particles with the size of 120 nm in experiments. 4. Results and discussion In the initial experiment, the sample solution is placed on the glass slide without any boundary constraint, whereas it is difficult to observe the particles due to the liquid flowing. In order to resolve the liquidity of the sample solution, a small space is fabricated using glass glue to get a more stable observing area. The specific packaging process of the sample cell is as follows: firstly, the glass glue is used to form a semi-closed chamber on the glass slide, which is sealed from the top with the glass coverslip, then a small amount of sample solution is sucked into the space between two slides by capillarity. Using the specific sample cell, the laser scanning confocal microscope can observe 120 nm polystyrene particles with a maximum magnification of 14,400. In order to get a stable trapping area in the sample cell that is suitable for observing, a small opening is processed at the edge of the glass slide. The optical fiber probe is directed to the sample cell from the open end and then adjusted to the bottom of the glass coverslip. However, it is not convenient to trap particles due to the thin solution in the sample cell. To prevent excessive evaporation of the sample solution at the opening, the height of the semi-enclosed chamber is set to be 1 mm. During the experiment, it is found that when the imaging cross-section of the laser scanning confocal microscope goes deep into the sample solution, the quality of observed image is significantly reduced. It shows that when the thickness of the glass coverslip is about 170 m, the thickness of the sample should be
less than 50 m. Although the thin glass coverslip (100 m) is used in the experiment, it is still difficult to observe the manipulation details in the sample solution clearly. Therefore, in order to overcome the problems of easy evaporation with thin sample solution and difficult observation with thick sample solution, it requires new experimental approaches. It is found that the laser scanning confocal microscope can observe the boundary of sample solution at the bottom of glass coverslip with the maximum magnification of 14,400. Using this phenomenon, the fiber probe can be directed into the boundary of sample solution at the bottom of glass coverslip for the nanomanipulation. Thus the trapping results can be reflected directly through observing the bottom of the glass coverslip. In the experiment, firstly the trapping of 120 nm polystyrene particles diluted by 10:1 in deionized water is carried out to explore the trapping ability of the system. The optical fiber probe is adjusted to the bottom of glass coverslip using the three-dimensional nanopositioning stage. The sample cell is placed on the stage of the confocal microscope, and the trapping image is collected by the image processing system. After the fiber probe moved to the view field of the microscope, the laser power is adjusted to make the trapping force enough for the trapping of nanoparticles. As shown in Fig. 6(a), the optical fiber probe placed in the bottom of the glass coverslip is ready for particles trapping. After turning on the laser for a while, it is found that the suspended polystyrene particles in the view field begin to move, and the particles in different locations move in different directions and speeds. When the measured power is adjusted to 20 W, the particles around the fiber probe are all directed to the probe tip, and finally the expected trapping result at the probe tip appears as shown in Fig. 6(b). From the analysis of forces given to the particle, the particles in the boundary of sample solution are easilier trapped to the probe tip under the action of capillary force and trapping force. It can be seen that some particles are quickly trapped to form the inner circle at the aperture edge by trapping forces. The minimum diameter of arranged particles is the same as the aperture diameter of 400 nm. Meanwhile, under the action of capillary forces, the particles near the probe tip are also trapped to form the outer circular distribution with a diameter of 3 m. Due to the action of the two forces, the particles are eventually trapped in multi-circular shape. Because the particle size has reached the observation limit of the microscope, the obtained images are a bit of a blur. Fig. 6(c) shows that the trapped particles are released to the bottom of glass coverslip due to the role of van der Waals force or hydrophobic interaction when turning off the laser. Some damage occurs in the arranged shape of particles owing to the disturbance of other forces. This experiment shows that strong field enhancements from light scattering at the probe tip can generate a trapping potential strong enough to trap nanoparticles.
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Fig. 7. The trapping images of polystyrene particles: (a) only under the action of capillary force and (b) under the action of capillary force and trapping force.
In order to further confirm that the particle can be easilier trapped under the action of capillary force and trapping force, the fiber probe is directed into the boundary of sample solution for the manipulation of nanoparticles. Fig. 7(a) shows the trapping details around the probe tip before turning on laser. It is found that the trapping phenomenon is not obvious when particles are only under the action of capillary force. Due to the lack of distance feedback between the fiber probe and the bottom of glass coverslip, the probe tip gradually becomes blunt that result in a wider trapping area of particles in the experimental process. With the same sample concentration, Fig. 7(b) gives the trapping image of polystyrene particles at a diameter of 120 nm after turning on laser. When the measured power is adjusted to 18 W, the particles are trapped under the action of capillary force and trapping force, forming a larger multi-circular shape at the probe tip. While the boundary solution gradually evaporates, the polystyrene particles around the fiber probe move to the probe tip due to the capillary force and trapping force. In the moving process, the nanoparticles collide with each other and then gradually merge into larger and heavier particles that are better for LSCM observing. Thus the trend of particles trapping at the probe tip can be reflected based on the percentage of trapped particles in the view field of the microscope. In order to get different patterns of particles trapping with the concentration of sample solution, Fig. 8 shows the effect of sample concentration on polystyrene particles trapping using the same measured power of 20 W. With the reduction of sample concentration, the trend of particles trapping at the tip of fiber probe slows down gradually. Meanwhile, the aggregate behaviour in the movement of particles also weakens. When the concentration drops to 0.3% W/V, there is no apparent trapping pattern near the probe tip due to the scarce polystyrene particles.
Fig. 9. Effect of laser power on polystyrene particles trapping using the fiber probe.
In order to avoid the damage to the fiber probe, the power coupled into the fiber is controlled in 400 W in experiment. After calibrating the power of the optical circuit, it is found that the measured power in the power meter should be less than 26.33 W. To get different patterns of particles trapping with the laser power, Fig. 9 gives the effect of laser power on polystyrene particles trapping with the same concentration of sample solution. It can be seen that the particles are trapped in an obvious multi-circular shape by capillary force and trapping force when the measured power is 21 W. As the laser power reduces, the trend of particles trapping toward the tip becomes weaker. When the measured power reduces to 13 W, the particles are hard to be trapped due to the weak laser power. Here the low trapping capacity of the fiber probe is clarified again when particles are only under the action of capillary force. 5. Conclusions
Fig. 8. Effect of sample concentration on polystyrene particles trapping using the fiber probe.
Theoretical and experimental results of manipulating nanoparticles by an enhanced evanescent field through an NSOM probe are obtained. Numerical results indicate that the fiber probe with an aperture diameter larger than 200 nm is able to trap nanoparticles smaller than 20 nm in a circular shape with lower laser intensity than that required by conventional optical tweezers. In our experiments, polystyrene particles at a diameter of 120 nm are trapped in a multi-circular shape using the NSOM probe with an aperture diameter of 400 nm. The particles can be easilier trapped under the action of capillary force and trapping force, which is proved by the theoretical simulation and experimental results. It is found that the sample concentration and laser power affect the pattern greatly. Although the nanomanipulation using the NSOM probe is still at an early stage, the realization of particles trapping in the range of tens
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of nanometers largely promotes the role of near-field manipulation at the nanometer scale. Acknowledgments This research is supported by the National Natural Science Foundation of China (Grant No. 90923041), State Key Laboratory of Robotics and System (Harbin Institute of Technology) (Grant No. SKLRS-2010-2 D-10), the Fundamental Research Funds for the Central Universities (Grant No. HIT.NSRIF.201171), and the 111 Project of China (Grant No. B07018). References [1] S. Davy, M. Spajer, Near field optics: snapshot of the field emitted by a nanosource using a photosensitive polymer, Appl. Phys. Lett. 69 (1996) 3306–3308. [2] M.H. Hong, S.M. Huang, B.S. Luk’yanchuk, T.C. Chong, Laser assisted surface nanopatterning, Sens. Actuators A 108 (2003) 69–74. [3] S.M. Huang, M.H. Hong, B.S. Luk’yanchuk, Y.F. Lu, W.D. Song, T.C. Chong, Sub50 nm nanopatterning of metallic layers by green pulsed laser combined with atomic force microscopy, J. Vac. Sci. Technol. B 20 (2002) 1118–1125. [4] Y.F. Lu, B. Hu, Z.H. Mai, W.J. Wang, W.K. Chim, T.C. Chong, Laser-scanning probe microscope based nanoprocessing of electronics materials, Jpn. J. Appl. Phys. 40 (2001) 4395–5398. [5] Z.H. Mai, Y.F. Lu, W.D. Song, W.K. Chim, Nano-modification on hydrogenpassivated Si surfaces by a laser-assisted scanning tunneling microscope operating in air, Appl. Surf. Sci. 154 (2000) 360–364. [6] M. Gu, S. Kuriakose, X.S. Gan, A single beam near-field laser trap for optical stretching, folding and rotation of erythrocytes, Opt. Exp. 15 (2007) 1369–1375. [7] L. Novotny, X.B. Randy, X.S. Xie, Theory of nanometric optical tweezers, Phys. Rev. Lett. 79 (1997) 645–648. [8] E.S. Kwak, T.D. Onuta, D. Amarie, R. Potyrailo, B. Stein, S.C. Jacobson, W.L. Schaich, B. Dragnea, Optical trapping with integrated near-field apertures, Phys. Chem. B 108 (2004) 13607–13612. [9] P.J. Reece, V.G. Chavez, K. Dholakia, Near-field optical micromanipulation with cavity enhanced evanescent waves, Appl. Phys. Lett. 88 (2006) 2211161–221116-3. [10] C.D. Mellor, C.D. Bain, J. Lekner, Pattern formation in evanescent wave optical traps, Proc. SPIE 5930 (2005), 59301C-1–59301C-10. [11] O.L. Mooren, E.S. Erickson, N.E. Dickenson, R.C. Dunn, Extending near-field scanning optical microscopy for biological studies, Technol. Rev. 8 (2006) 268–270. [12] Y. Lin, M.H. Hong, W.J. Wang, Z.B. Wang, G.X. Chen, Q. Xie, L.S. Tan, T.C. Chong, Surface nanostructuring by femtosecond laser irradiation through near-field scanning optical microscopy, Sens. Actuator A 133 (2007) 311–316. [13] Y. Lin, M.H. Hong, W.J. Wang, Y.Z. Law, T.C. Chong, Sub-30 nm lithography with near-field scanning optical microscope combined with femtosecond laser, Appl. Phys. A 80 (2005) 461–465. [14] G.P. Zhang, Z.R. Zhu, Y.P. Li, G. Xia, Q. Lin, Numerical analysis of near-field optical trapping using tapered fiber probe, Proc. SPIE 4082 (2000) 321–328. [15] K.Y. Wang, Z. Jin, W.H. Huang, The possibility of trapping and manipulating a nanometer scale particle by the SNOM tip, Opt. Commun. 149 (1998) 38–42.
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Biographies Bing Hui Liu was born in 1981, and he received the B.S. degree from the School of Mechanical Engineering and Automation, Jilin University in 2005 and the M.S. degree from the Department of Mechanical Manufacturing and Automation, Harbin Institute of Technology in 2007, respectively. He is currently a doctoral candidate in the Lab of Advanced Manufacturing Technology and Equipments, Harbin Institute of Technology. He is engaged in research on nanoscale material characterization by laser and CAD for near-field optical manipulation. Li Jun Yang was born in 1972, PhD, Associate Professor. He graduated from the Department of Mechanical Engineering, Liaoning Institute of Technology in 1995. In September 2000, he became a post-graduate student of Harbin Institute of Technology and got the Master’s degree in July 2002. He became a PhD student of Harbin Institute of Technology in 2002, and got his PhD degree in January 2007. His main research interest is laser processing technology, mainly including: laser micromachining technology and laser nanomanipulation technology. Yang Wang was born in 1960, PhD, Professor. He graduated from the Department of Mechanical Engineering, Harbin Institute of Technology in 1982, and became a teacher of Harbin Institute of Technology in the same year. In July 1985, he became a post-graduate student of Harbin Institute of Technology and got the Master’s degree in October 1988. He was promoted to Professor in 1995. He became a part-time PhD student of Harbin Institute of Technology in 1995, and got his PhD degree in December 1999. His main research interest is laser processing technology, mainly including: laser micromachining technology; laser heated bending and formation technology and laser nanomanipulation technology. Jian Lei Cui was born in 1984, and he received the B.S. degree from the School of Mechatronics Engineering, Naval Aeronautical Engineering Institute in 2007 and the M.S. degree from the School of Mechatronics Engineering, Wuhan University of Technology in 2010, respectively. He is currently a doctoral candidate in the Lab of Advanced Manufacturing Technology and Equipments, Harbin Institute of Technology. He is engaged in research on nanoscale material characterization by laser.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Particles nanomanipulation by the enhanced evanescent field through a near-field scanning optical microscopy probe B.H. Liu a,b,∗ , L.J. Yang b , Y. Wang b , J.L. Cui b a b
State Key Laboratory of Robotics and System, Harbin Institute of Technology, China School of Mechatronics Engineering, Harbin Institute of Technology, China
a r t i c l e
i n f o
Article history: Received 22 December 2010 Received in revised form 26 April 2011 Accepted 26 April 2011 Available online 5 May 2011 Keywords: Near-field optical trapping 3D FDTD Maxwell’s stress tensor Field enhancement Trapping force
a b s t r a c t A near-field scanning optical microscopy (NSOM) probe and a polarized semiconductor laser (808 nm, cw) were applied to push the trapping resolution down to 120 nm on near-field optical manipulation. A multi-circular shape with a minimum size of 400 nm consisting of 120 nm polystyrene particles can be obtained. They are at a resolution of d (d: NSOM probe tip diameter) and /7 (: laser wavelength), respectively. It is proved that sample concentration and laser power can affect feature size of trapping patterns. In this paper, the effect of trapping forces acted on a nanoparticle along three axis directions on trapping positions is studied, and different trapping positions are generated: the aperture edge in polarization direction and center surface of the probe tip. The result indicates that the single mode NSOM fiber probe is able to trap nanoparticles in a circular shape with lower laser intensity than that required by conventional optical tweezers. The simulated trapping positions around the probe tip based on the conservation law of momentum are found to agree well with experimental results. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently the field of optical trapping and fabrication is entering an exciting newphase due to the rapid development of near-field optical techniques. It offers a high resolution to overcome the optical diffraction limit [1], which is one of the main bottlenecks in optical applications. The technology using the evanescent wave generated around a scattering body has been receiving great attention. Based on field enhancement near the AFM tip and mechanical indentation of the tip under laser irradiation, Huang et al. investigated the nanopatterning of metallic layers on silicon substrates, which is beyond the diffraction limit of the laser wavelength [2,3]. Lu et al. further analyzed the nanofabrication by a laser-assisted scanning probe microscope operating in air ambient [4,5]. Furthermore, Gu et al. showed a single beam near-field laser trapping technique under focused evanescent wave illumination for optical stretching, folding and rotation of a single red blood cell [6]. Novotny et al. proposed a scheme for optical trapping at the nanometer scale based on the highly enhanced electric field close to a laser-illuminated metal tip [7]. By following the time evolution of the fluorescence, Kwak et al. demonstrated a technique for optical trapping of 200-nm fluorescent latex beads in water,
∗ Corresponding author at: School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, China. Tel.: +86 451 86413257; fax: +86 451 86402755. E-mail address: [email protected] (B.H. Liu). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.04.042
which is based on the intense near-field gradients around small apertures in a metal film [8]. Reece et al. demonstrated optical manipulation and large scale trapping of 500 nm particles arrays using cavity enhanced evanescent waves produced with a dielectric resonator [9]. Mellor et al. described the optical binding of sub-micron polystyrene spheres in the evanescent field of counterpropagating laser beams [10]. However, these techniques, although very promising, are delicate to implement for manipulating particles with diameters of tens of nanometers main for three reasons. Firstly, before the particle can be captured, Brownian fluctuation has a disruptive effect and the radiation pressure from the illuminating laser imparts momentum to the particle. Therefore, one has to capture a moving particle. Secondly, due to the inflexible configuration, manipulating techniques are mostly restricted to trap particles in a fixed position. Finally, it is difficult to find and position one particle at a few nanometers in size directly during the manipulation in water. Thus it requires new experimental approaches when dealing with neutral particles of tens of nanometers. The near-field scanning optical microscope (NSOM) uses the evanescent field confined at the tip of a metal-coated fiber probe to provide images of surfaces with high resolution [11]. Based on the integration of NSOM and femtosecond laser, Lin et al. used the high evanescent light across the fiber probe to produce near-field optical nanolithography on the UV photoresist [12]. The direct laser writing of lithography patterns with a feature width of 20 ± 5 nm was realized by employing the combined effects of an evanescent wave and an ultra-short laser pulse [13]. On the other hand, the NSOM probe
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Fig. 1. Schematic diagram of the experimental setup for nanoparticles trapping using an NSOM probe.
has now opened up a wide range of new applications in nanoparticle manipulation due to strong gradient forces associated with the enhanced evanescent field [14]. Using polarized laser beams in NSOM probes, nanoparticles can be trapped on the tip surface owing to the near-field enhancement at the tip. Furthermore, the trapped particles can be absorbed to the substrate in a circular shape. In this paper, the trapping force distributions near the fiber probe along three axis directions are described. The nanomanipulating details are analyzed at different probe-to-particle distances. Simulation of trapping positions and comparison of other forces versus trapping force based on the conservation law of momentum shows that the single mode NSOM probe is able to trap nanoparticles in a circular shape, which agrees well with the experimental results. The effects of sample concentration and laser power on trapping patterns observed by laser scanning confocal microscope (LSCM) are discussed. 2. Experimental setup The fiber probe used in the experiment is an NSOM fiber (NTMDT), which is operated in a wavelength ranging from 780 nm to 970 nm. It is made of a single mode tapered optical fiber with an aperture diameter of 400 nm by coating the metal film around the tip. Fig. 1 shows the experimental setup for the NSOM probe nanomanipulation. The optical source is a polarized cw laser (Lumics LU0808M100) operating at a wavelength of 808 nm. The semiconductor laser and a polarization-maintaining (PM) pigtail are connected directly by a FC-PC connector. The laser beam transmitted in the PM fiber firstly passes through a variable attenuator to permit continuous variation of the laser power, and a fiber polarization rotator (OZ Optics) to permit the rotation of the polarization, then passes through a fiber splitter (OZ Optics) with a ratio of 95:5 for the power detection. It is finally coupled into the core of an NSOM fiber with a maximum optical input power of 400 W using a fiber adapter. Some of the light traveling down the fiber core is reflected from the tapered metal-coated probe, then is directed by the fiber splitter to a power meter (Connet JW3203R) for the detection. The back-reflected signal at the detector consists of a small depolarized component due to the polarization scrambling nature of the fiber. This signal is optimized to ensure that the laser light is efficiently coupled into the fiber core and then directed all the ways to the fiber tip. In order to adjust the inserted position and angle of the fiber probe with a cone angle of 30◦ and a cone length of 200 m, the other end of the fiber adheres to a syringe needle which is attached to a three-dimensional nanopositioning
stage (PI P-517.3). In the sample cell, the output end of the fiber is placed inside the solution perpendicular to the glass coverslip. The sample solution contains 120 nm polystyrene particles (Structure Probe Inc.) with a refraction index of 1.56 and a density of 2.4 × 103 kg/m3 . An overhead laser scanning confocal microscope (OLYMPUS OLS3000) is used to observe the manipulation process in real time. The maximum magnification with 100× objective and 6× zoom is 14,400, corresponding to the minimum view field of 21 m × 21 m. The resolutions in xy directions and z direction are 120 nm and 10 nm, respectively. 3. Theoretical study Although there are many models developed up to now, physical properties of particles manipulation by evanescent wave including the trapping size, trapping position and comparison of other forces versus optical trapping force have not been solved yet. The forces acted on Rayleigh particles in the near-field have been treated using wave optics [15]. However, this method is no longer valid when the aperture diameter is much smaller than the wavelength, and the gradient force is not accurate due to the limitation of dipole approximation. Some realistic models about near-field distribution from optical fiber probe have been proposed, such as Multiple-Multipole method and Boundary Integral Equation method [16,17]. But the multipolar functions used in the basis set of the Multiple-Multipole method are in a short range that only their close neighborhood is affected. This method is better suited to account for localized geometries than the expansion in plane waves. In addition, the Boundary Integral Equation method is only restricted to twodimensional studies due to the constraints of time and memory. To illustrate the experimental results in Section 4, the combination of Maxwell’s stress tensor and three-dimensional finite difference time domain method (3D FDTD) is used for the calculation of trapping forces in this paper. Considering the electromagnetic field near the fiber probe with an aperture diameter of 200 nm, the electromagnetic force subjected to the charge can be expressed by Lorentz force formula. The momentum of the charge system changes under the influence of the force, and the electromagnetic field should also change accordingly the conservation law of momentum. Here the FDTD geometry is placed in a volume discretized in mesh cells of 320 × 315 × 210 with a space step of x = y = z = 3 nm, as shown in Fig. 2. The probe is laid out in yz coordinates and assumed to be placed in a medium with refractive index n1 = 1.2. It is assumed that the laser polarized along y-axis is launched into the fiber probe with a coupling
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Fig. 2. Schematic diagram of the geometry used in the calculation.
power of 400 W and at a wavelength of 808 nm. The direction and polarization of incident plane wave are indicated by the K and E vectors. A small particle with the radius of a = 10 nm, the density of = 2.4 × 103 kg/m3 and the refractive index of n3 = 1.8 is placed near the probe. Given the volume V near the optical fiber probe surrounded by an outer boundary S, the trapping force acted on the nanoparticle can be given by:
F=
↔ T · n dS,
f d = − V
(1)
S
↔ where T is Maxwell’s stress tensor, which can be expressed as: ↔ T =
ε0 εr E 2 + 0 r H 2 ↔ ıij − ε0 εr E i E j − 0 r H i H j 2
,
(2)
ij
where E and H are the vectors of electric field intensity and magnetic field intensity in free space, foot notes i and j represent the ↔ direction x, y or z, and ıij represents the unit stress tensor. ε0 and εr are the permittivity of vacuum and the relative permittivity of medium respectively, 0 and r are the permeability of vacuum and the relative permeability of medium respectively. Then the specific expressions of trapping forces along three axis directions are given by:
Fx =
− S
Fy =
− S
Fz =
− S
ε0 εr E 2 + 0 r H 2 ex − ε0 εr E 2x ex − 0 r H 2x ex 2
dS,
ε0 εr E 2 + 0 r H 2 ey − ε0 εr E 2y ey − 0 r H 2y ey 2 ε0 εr E 2 + 0 r H 2 ez − ε0 εr E 2z ez − 0 r H 2z ez 2
(3)
dS, (4)
dS.
(5)
In order to know where the particle is finally trapped, a series of calculations are made to obtain the trapping force distributions along three axes. Fig. 3(a) shows the trapping forces acted on a particle along z-axis direction with different radius at different heights. The particle moves along z-axis direction when the trapping force is positive. In contrast, the particle moves along −z-axis direction when the trapping force is negative. For the particle with relatively small radius, i.e., a = 10 nm and 50 nm, the trapping force near the probe tip is attractive, then it descends to zero and becomes repulsive due to the scattering force. Thus for a small particle, the global decay of the trapping force in the axial direction can form a deep
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one-dimensional trap in the near-field region. In z-axis direction, the particle tends to be attracted to the tip surface. When the particle radius is large enough, i.e., a = 100 nm, the trapping force acted on the particle is always a repulsive one. It indicates that the large particle cannot be trapped and tends to move away from the probe tip along z-axis direction. The trapping forces in y-axis and x-axis directions given to the particle with the radius of a = 10 nm at 0 nm, 10 nm and 70 nm heights above the aperture are plotted. In Fig. 3(b), the particle moves along y-axis direction when the trapping force is positive. In contrast, the particle moves along −y-axis direction when the trapping force is negative. The curve in z = −70 nm plane shows that the particle is pulled close to z-axis in the lateral direction. In z = −10 nm plane, the trapping forces near z-axis and the aperture are attractive, it means that three trapping positions appear in this plane. In is also found that two unstable trapping positions exist in the middle of the aperture. While in z = 0 nm plane, the trapping forces around z-axis and near the aperture are repulsive and attractive respectively, it means that the particle is only trapped to the aperture edge. In Fig. 3(c), the particle moves along −x-axis direction when the trapping force is negative. Because the trapping force along x-axis is always attractive and symmetric to the probe axis, the particle always tends to be attracted to z-axis along x-axis direction. According to the distributions of trapping forces, the particle placed near the aperture along y-axis direction tends to be dragged to the aperture edge immediately. While the particle placed at other positions around the aperture tends to be pulled close to z-axis in the lateral direction, and finally is attracted to the tip surface along z-axis direction. As shown in Fig. 4(a), the trap position for the fixed polarization of incident light has a quasi-line shape, which would be smoothed into a circular shape if the polarization direction is rotated around the symmetry axis. Due to the existence of birefringence in a single mode optical fiber with the variation of ambient temperature and stress, the polarization direction of incident laser changes randomly. When the polarized laser in a PM fiber coupling into the single mode NSOM fiber, the polarization of the light cannot be maintained any more, thus the trapping position occurs all around the aperture of the fiber probe. In the actual condition, the particles tend to be trapped in a circular shape, as shown in Fig. 4(b). In order to get the information of trapping strength, the role of forces other than the trapping force must be taken into account. During the manipulation the particle is mainly in five kinds of forces: Brownian motion, gravitational, capillary, van der Waals and optical trapping forces. The calculations show that the magnitude of the trapping forces is 10−13 N. For a particle with a radius of 10 nm immersed in water, it is found that the gravity is 9.8 × 10−20 N, according the gravitational force Fg = 4/3a3 g, where g = 10 m/s2 and = 2.4 × 103 kg/m3 . Since the trapping force acted on the particle is larger (by a factor 106 ) than the gravitational force, the effect of the gravity can be neglected. If there is water on the substrate, according to the capillary force Fc = 2a , where a is the radius of the particle and is the surface tension of water [18], one can get Fc = 4.5 × 10−12 N for a particle with a radius of 10 nm. The capillary force is about the same magnitude as the optical force, thus it is not easy for optical trapping on this substrate. However, it is beneficial to trap particles on the surface of water to the probe tip when the probe is placed in water. The Brownian motion is due to the thermal fluctuations. The fluctuation–dissipation theorem of Einstein states that the Brownian motion force is equal to 12a kb T, where is the viscosity of water, kb is the Boltzmann constant, and T is the temperature of water [19]. At room temperature, the Brownian motion force is 1.0 × 10−13 N, which is of the same magnitude as the trapping force. It suggests that the trapped particle is not affected greatly by Brownian motion with appropriate trapping parameters. The van der Waals force between two objects can be
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Fig. 3. Trapping forces acted on the particle: (a) along z-axis direction with different particle radius, (b) along y-axis direction and (c) along x-axis direction at different heights above the aperture plane.
Fig. 4. Schematic diagram of trapping positions using the metal-coated fiber probe: (a) in the ideal condition and (b) in the actual condition.
function of the size of the particle or the aperture. The strong capillary force is beneficial to trap particles on the surface of water when the fiber probe is placed in water. The trapping force depends on the diameter of the aperture much more than it depends on the size of the particle. By comparison of the other three forces versus optical trapping force, it is found that the trapping force obtained
Aperture diameter (nm) 200 300 400 500 600 700
Magnitude of forces (N)
described as a short range force, derived from the Lennard–Jones potential, in the form Fw = 0.167AS/h2 (0.25z6 /h6 − 1), where h is the distance between the two objects, A is the Hamaker constant, z corresponds to the separation of lowest energy between two objects and S is the Derjaguin geometrical factor related to the mutual curvature of the two objects [20]. When computing the van der Waals force between the particle and substrate surface one has Fw = 4.0 × 10−13 N. It also suggests that the trapped particle is not affected greatly by the van der Waals force with appropriate trapping parameters. In addition, it is necessary to consider the van der Waals force between the probe tip and particle when they are in contact. When the curvature of the tip is equal to 10 nm, one has Fw = 2.7 × 10−13 N with S = 5 nm and A = 109.5 zJ. It can be seen that the two van der Waals forces cancel each other partially when the particle is in contact with both the substrate and tip. Under the action of van der Waals force, the trapped particle tends to be released to the substrate when turning off the laser. In comparison with other forces acted on the particle with a diameter of 20 nm, it is concluded that the trapping force is not dominant among the five forces when the aperture diameter of the probe tip is 200 nm and the laser intensity is assumed to be 1040 W/mm2 . To further determine the trapped size of nanoparticles, Fig. 5 shows plots of five forces acted on the particle as a
10-10 10-12 10-14
Capillary force Actual trapping force Calculated trapping force Van der Waals force Brownian motion force
10-16 10-18
Gravitational force
10-20 20 40 60 80 100 120 140 Particle diameter (nm)
Fig. 5. Forces given to the particle located near the fiber probe.
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Fig. 6. The trapping images of nanoparticles using the NSOM probe: (a) before turning on the laser, (b) after turning on the laser and then (c) after turning off the laser.
with an aperture larger than 200 nm is dominant among the four forces of a particle smaller than 20 nm. It means that one can trap a particle smaller than 20 nm using the fiber probe with an aperture diameter larger than 200 nm. However, the aperture diameter cannot be too large, and the particle size cannot be too small, otherwise it is difficult to achieve the optical trapping in the near-field due to the weaker near-field intensity near the aperture than the far-field intensity and the difficulty of observing particles in real time. It should be noted that for the reduction of computation the calculation above is only limited to the tip region of fiber probe less than 1 m. According to tip-induced plasma oscillations, much stronger evanescent field occurs at the tip of fiber probe with longer tip height [21]. With an actual tip height of 200 m, the magnitude of the trapping force acted on the particle must be greater than 10−13 N. The actual trapping force acted on the particle is shown as a dashed line in Fig. 5. The dominant trapping force indicates that one can realize near-field optical trapping with much lower intensity (1040 W/mm2 ) than that required by classical fiber optical tweezers (105 W/mm2 ) [22]. In view of the incident wavelength of 808 nm and the LSCM resolution of 120 nm, the NSOM probe with a diameter of 400 nm is chosen to trap the particles with the size of 120 nm in experiments. 4. Results and discussion In the initial experiment, the sample solution is placed on the glass slide without any boundary constraint, whereas it is difficult to observe the particles due to the liquid flowing. In order to resolve the liquidity of the sample solution, a small space is fabricated using glass glue to get a more stable observing area. The specific packaging process of the sample cell is as follows: firstly, the glass glue is used to form a semi-closed chamber on the glass slide, which is sealed from the top with the glass coverslip, then a small amount of sample solution is sucked into the space between two slides by capillarity. Using the specific sample cell, the laser scanning confocal microscope can observe 120 nm polystyrene particles with a maximum magnification of 14,400. In order to get a stable trapping area in the sample cell that is suitable for observing, a small opening is processed at the edge of the glass slide. The optical fiber probe is directed to the sample cell from the open end and then adjusted to the bottom of the glass coverslip. However, it is not convenient to trap particles due to the thin solution in the sample cell. To prevent excessive evaporation of the sample solution at the opening, the height of the semi-enclosed chamber is set to be 1 mm. During the experiment, it is found that when the imaging cross-section of the laser scanning confocal microscope goes deep into the sample solution, the quality of observed image is significantly reduced. It shows that when the thickness of the glass coverslip is about 170 m, the thickness of the sample should be
less than 50 m. Although the thin glass coverslip (100 m) is used in the experiment, it is still difficult to observe the manipulation details in the sample solution clearly. Therefore, in order to overcome the problems of easy evaporation with thin sample solution and difficult observation with thick sample solution, it requires new experimental approaches. It is found that the laser scanning confocal microscope can observe the boundary of sample solution at the bottom of glass coverslip with the maximum magnification of 14,400. Using this phenomenon, the fiber probe can be directed into the boundary of sample solution at the bottom of glass coverslip for the nanomanipulation. Thus the trapping results can be reflected directly through observing the bottom of the glass coverslip. In the experiment, firstly the trapping of 120 nm polystyrene particles diluted by 10:1 in deionized water is carried out to explore the trapping ability of the system. The optical fiber probe is adjusted to the bottom of glass coverslip using the three-dimensional nanopositioning stage. The sample cell is placed on the stage of the confocal microscope, and the trapping image is collected by the image processing system. After the fiber probe moved to the view field of the microscope, the laser power is adjusted to make the trapping force enough for the trapping of nanoparticles. As shown in Fig. 6(a), the optical fiber probe placed in the bottom of the glass coverslip is ready for particles trapping. After turning on the laser for a while, it is found that the suspended polystyrene particles in the view field begin to move, and the particles in different locations move in different directions and speeds. When the measured power is adjusted to 20 W, the particles around the fiber probe are all directed to the probe tip, and finally the expected trapping result at the probe tip appears as shown in Fig. 6(b). From the analysis of forces given to the particle, the particles in the boundary of sample solution are easilier trapped to the probe tip under the action of capillary force and trapping force. It can be seen that some particles are quickly trapped to form the inner circle at the aperture edge by trapping forces. The minimum diameter of arranged particles is the same as the aperture diameter of 400 nm. Meanwhile, under the action of capillary forces, the particles near the probe tip are also trapped to form the outer circular distribution with a diameter of 3 m. Due to the action of the two forces, the particles are eventually trapped in multi-circular shape. Because the particle size has reached the observation limit of the microscope, the obtained images are a bit of a blur. Fig. 6(c) shows that the trapped particles are released to the bottom of glass coverslip due to the role of van der Waals force or hydrophobic interaction when turning off the laser. Some damage occurs in the arranged shape of particles owing to the disturbance of other forces. This experiment shows that strong field enhancements from light scattering at the probe tip can generate a trapping potential strong enough to trap nanoparticles.
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Fig. 7. The trapping images of polystyrene particles: (a) only under the action of capillary force and (b) under the action of capillary force and trapping force.
In order to further confirm that the particle can be easilier trapped under the action of capillary force and trapping force, the fiber probe is directed into the boundary of sample solution for the manipulation of nanoparticles. Fig. 7(a) shows the trapping details around the probe tip before turning on laser. It is found that the trapping phenomenon is not obvious when particles are only under the action of capillary force. Due to the lack of distance feedback between the fiber probe and the bottom of glass coverslip, the probe tip gradually becomes blunt that result in a wider trapping area of particles in the experimental process. With the same sample concentration, Fig. 7(b) gives the trapping image of polystyrene particles at a diameter of 120 nm after turning on laser. When the measured power is adjusted to 18 W, the particles are trapped under the action of capillary force and trapping force, forming a larger multi-circular shape at the probe tip. While the boundary solution gradually evaporates, the polystyrene particles around the fiber probe move to the probe tip due to the capillary force and trapping force. In the moving process, the nanoparticles collide with each other and then gradually merge into larger and heavier particles that are better for LSCM observing. Thus the trend of particles trapping at the probe tip can be reflected based on the percentage of trapped particles in the view field of the microscope. In order to get different patterns of particles trapping with the concentration of sample solution, Fig. 8 shows the effect of sample concentration on polystyrene particles trapping using the same measured power of 20 W. With the reduction of sample concentration, the trend of particles trapping at the tip of fiber probe slows down gradually. Meanwhile, the aggregate behaviour in the movement of particles also weakens. When the concentration drops to 0.3% W/V, there is no apparent trapping pattern near the probe tip due to the scarce polystyrene particles.
Fig. 9. Effect of laser power on polystyrene particles trapping using the fiber probe.
In order to avoid the damage to the fiber probe, the power coupled into the fiber is controlled in 400 W in experiment. After calibrating the power of the optical circuit, it is found that the measured power in the power meter should be less than 26.33 W. To get different patterns of particles trapping with the laser power, Fig. 9 gives the effect of laser power on polystyrene particles trapping with the same concentration of sample solution. It can be seen that the particles are trapped in an obvious multi-circular shape by capillary force and trapping force when the measured power is 21 W. As the laser power reduces, the trend of particles trapping toward the tip becomes weaker. When the measured power reduces to 13 W, the particles are hard to be trapped due to the weak laser power. Here the low trapping capacity of the fiber probe is clarified again when particles are only under the action of capillary force. 5. Conclusions
Fig. 8. Effect of sample concentration on polystyrene particles trapping using the fiber probe.
Theoretical and experimental results of manipulating nanoparticles by an enhanced evanescent field through an NSOM probe are obtained. Numerical results indicate that the fiber probe with an aperture diameter larger than 200 nm is able to trap nanoparticles smaller than 20 nm in a circular shape with lower laser intensity than that required by conventional optical tweezers. In our experiments, polystyrene particles at a diameter of 120 nm are trapped in a multi-circular shape using the NSOM probe with an aperture diameter of 400 nm. The particles can be easilier trapped under the action of capillary force and trapping force, which is proved by the theoretical simulation and experimental results. It is found that the sample concentration and laser power affect the pattern greatly. Although the nanomanipulation using the NSOM probe is still at an early stage, the realization of particles trapping in the range of tens
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of nanometers largely promotes the role of near-field manipulation at the nanometer scale. Acknowledgments This research is supported by the National Natural Science Foundation of China (Grant No. 90923041), State Key Laboratory of Robotics and System (Harbin Institute of Technology) (Grant No. SKLRS-2010-2 D-10), the Fundamental Research Funds for the Central Universities (Grant No. HIT.NSRIF.201171), and the 111 Project of China (Grant No. B07018). References [1] S. Davy, M. Spajer, Near field optics: snapshot of the field emitted by a nanosource using a photosensitive polymer, Appl. Phys. Lett. 69 (1996) 3306–3308. [2] M.H. Hong, S.M. Huang, B.S. Luk’yanchuk, T.C. Chong, Laser assisted surface nanopatterning, Sens. Actuators A 108 (2003) 69–74. [3] S.M. Huang, M.H. Hong, B.S. Luk’yanchuk, Y.F. Lu, W.D. Song, T.C. Chong, Sub50 nm nanopatterning of metallic layers by green pulsed laser combined with atomic force microscopy, J. Vac. Sci. Technol. B 20 (2002) 1118–1125. [4] Y.F. Lu, B. Hu, Z.H. Mai, W.J. Wang, W.K. Chim, T.C. Chong, Laser-scanning probe microscope based nanoprocessing of electronics materials, Jpn. J. Appl. Phys. 40 (2001) 4395–5398. [5] Z.H. Mai, Y.F. Lu, W.D. Song, W.K. Chim, Nano-modification on hydrogenpassivated Si surfaces by a laser-assisted scanning tunneling microscope operating in air, Appl. Surf. Sci. 154 (2000) 360–364. [6] M. Gu, S. Kuriakose, X.S. Gan, A single beam near-field laser trap for optical stretching, folding and rotation of erythrocytes, Opt. Exp. 15 (2007) 1369–1375. [7] L. Novotny, X.B. Randy, X.S. Xie, Theory of nanometric optical tweezers, Phys. Rev. Lett. 79 (1997) 645–648. [8] E.S. Kwak, T.D. Onuta, D. Amarie, R. Potyrailo, B. Stein, S.C. Jacobson, W.L. Schaich, B. Dragnea, Optical trapping with integrated near-field apertures, Phys. Chem. B 108 (2004) 13607–13612. [9] P.J. Reece, V.G. Chavez, K. Dholakia, Near-field optical micromanipulation with cavity enhanced evanescent waves, Appl. Phys. Lett. 88 (2006) 2211161–221116-3. [10] C.D. Mellor, C.D. Bain, J. Lekner, Pattern formation in evanescent wave optical traps, Proc. SPIE 5930 (2005), 59301C-1–59301C-10. [11] O.L. Mooren, E.S. Erickson, N.E. Dickenson, R.C. Dunn, Extending near-field scanning optical microscopy for biological studies, Technol. Rev. 8 (2006) 268–270. [12] Y. Lin, M.H. Hong, W.J. Wang, Z.B. Wang, G.X. Chen, Q. Xie, L.S. Tan, T.C. Chong, Surface nanostructuring by femtosecond laser irradiation through near-field scanning optical microscopy, Sens. Actuator A 133 (2007) 311–316. [13] Y. Lin, M.H. Hong, W.J. Wang, Y.Z. Law, T.C. Chong, Sub-30 nm lithography with near-field scanning optical microscope combined with femtosecond laser, Appl. Phys. A 80 (2005) 461–465. [14] G.P. Zhang, Z.R. Zhu, Y.P. Li, G. Xia, Q. Lin, Numerical analysis of near-field optical trapping using tapered fiber probe, Proc. SPIE 4082 (2000) 321–328. [15] K.Y. Wang, Z. Jin, W.H. Huang, The possibility of trapping and manipulating a nanometer scale particle by the SNOM tip, Opt. Commun. 149 (1998) 38–42.
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Biographies Bing Hui Liu was born in 1981, and he received the B.S. degree from the School of Mechanical Engineering and Automation, Jilin University in 2005 and the M.S. degree from the Department of Mechanical Manufacturing and Automation, Harbin Institute of Technology in 2007, respectively. He is currently a doctoral candidate in the Lab of Advanced Manufacturing Technology and Equipments, Harbin Institute of Technology. He is engaged in research on nanoscale material characterization by laser and CAD for near-field optical manipulation. Li Jun Yang was born in 1972, PhD, Associate Professor. He graduated from the Department of Mechanical Engineering, Liaoning Institute of Technology in 1995. In September 2000, he became a post-graduate student of Harbin Institute of Technology and got the Master’s degree in July 2002. He became a PhD student of Harbin Institute of Technology in 2002, and got his PhD degree in January 2007. His main research interest is laser processing technology, mainly including: laser micromachining technology and laser nanomanipulation technology. Yang Wang was born in 1960, PhD, Professor. He graduated from the Department of Mechanical Engineering, Harbin Institute of Technology in 1982, and became a teacher of Harbin Institute of Technology in the same year. In July 1985, he became a post-graduate student of Harbin Institute of Technology and got the Master’s degree in October 1988. He was promoted to Professor in 1995. He became a part-time PhD student of Harbin Institute of Technology in 1995, and got his PhD degree in December 1999. His main research interest is laser processing technology, mainly including: laser micromachining technology; laser heated bending and formation technology and laser nanomanipulation technology. Jian Lei Cui was born in 1984, and he received the B.S. degree from the School of Mechatronics Engineering, Naval Aeronautical Engineering Institute in 2007 and the M.S. degree from the School of Mechatronics Engineering, Wuhan University of Technology in 2010, respectively. He is currently a doctoral candidate in the Lab of Advanced Manufacturing Technology and Equipments, Harbin Institute of Technology. He is engaged in research on nanoscale material characterization by laser.