The possibility of trapping and manipulating a nanometer scale particle by the SNOM tip

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Abstract

Calculations show that the SNOM tip can be used to trap and move a nanometer scale particle. If the gradient force acting on the particle is positive, in the lateral direction, there exist a one-dimensional trap along the aperture edge and a two-dimensional trap along the z-axis. If the gradient force is negative, there is only a one-dimensional trap very close to the aperture. Generally, the force is large enough to push or pull the particle directly. On the other hand, we can generate a trap along the z-axis by adding an external field. The depth of the trap is large enough to suppress the thermal motion of the particle.

Introduction

In the last 20 years, laser control of the motion of neutral atoms 1, 2, such as laser cooling, laser acceleration of small particles, etc. has become one of the most interesting branches of atomic physics. At the same time, many new techniques have been proposed for optical trapping, levitation and manipulation of macroscopic particles [3]. The `optical tweezer' developed by Ashkin et al. [4] can directly observe and manipulate particles which size ranges from 1 to 100 μm. Thus, it can be used to trap and observe cells and viruses [5]. It has been widely investigated for biological researches.

Scanning tunneling microscope (STM) developed in the early 1980s expanded our sight to the atomic level, and led to the invention of a series of scanning probe techniques with ultra-high resolution, such as atomic force microscope (AFM), scanning near field optical microscope (SNOM), etc. The SNOM uses the evanescent field confined at the aperture, provides images of surfaces with a resolution beyond the classical diffraction limit.

Some researchers began to use the evanescent field for transportation and trapping of large particles and atoms 6, 7, 8, 9, 10. Jhe talked about the existence of an optical trap of atoms built by SNOM tip [11], and emphasized its potential application to crystal growth on the atomic scale and its theoretical value [12].

In this paper, we discuss theoretically the possibility of trapping and manipulation of nanometer scale particles by the strong gradient force generated by the evanescent field near the tip. Because this new configuration of optical trap depends entirely on the light field near the tip, first we investigate the field distribution by numerical simulation. Then we calculate the force acting on a particle and the corresponding potential surface with given parameters. Finally, the trap's strength, the possibility and method of preliminary manipulation are analyzed and directions for further research are proposed.

Section snippets

The light field distribution near the aperture

The model of the aperture is a simplified structure of a near-zone field transmitted through an electrically small aperture in a perfectly conducting plane screen [13], The analytic calculation of this problem was proposed by Bethe [14], and further developed by Bouwkamp [15] and Grobor et al. [16]. Grobor et al. brought about the concept of Fourier optics [17] and used cylindrical coordinates instead of Cartesian coordinates in their calculation. Their main results are that the components of

The analysis of forces and corresponding potential surface of a small particle in the near-field light

Ashkin has made a quantitative analysis of the forces exerted on Rayleigh particles in a light field [18]. Because the radius of the Rayleigh particle is far less than the wavelength, we can treat this problem using wave optics. Due to the induced polarization of the particle in the light field, the particle will undergo a gradient force, which is given byFgrad=−n3br32m2−1m2−2∇E2,in which r is the radius of the particle, nb is the refractive index of the surrounding medium, m is the effective

The trap's strength, the method of preliminary manipulation and others

In order to obtain the information of the trap's strength, we estimate the relative strength of three kinds of force – the gravitational force, the gradient force and the radiation force. In our treatment, we simplified the optical fiber aperture using a small circular aperture as described by Bethe [14]. Although there are some differences between them, this model should give some features that are common to the practical used tip aperture 20, 21. In order to obtain general conclusions, we

Conclusion and outlook

We have calculated the distribution of the evanescent light distribution near the SNOM tip. By analyzing the forces exerted on the nanometer size particle, we obtain its potential surface in such a field. Several possibilities of confinement and manipulation of the particle have been proposed. After evaluating the calculated results, we find that trapping and controlling are possible if we judiciously choose the experimental parameters.

In the present work, we assume that the particle is much

Acknowledgements

This research is supported by National Nature Science Foundation of China.

References (22)

  • H. Ito et al.

    Optics Comm.

    (1995)
  • W. Seifert et al.

    Optics Comm.

    (1994)
  • R.J. Cook et al.

    Optics Comm.

    (1982)
  • D.W. Pohl et al.

    Thin Solid Films

    (1996)
  • D.W. Pohl

    Thin Solid Films

    (1995)
  • S. Chu et al.

    J. Opt. Soc. Am. B

    (1989)
  • S. Chu

    Science

    (1991)
  • W. Cai

    Prog. Phys.

    (1997)
  • Y. Li, The Principles, Techniques and Applications of the Optical Tweezers, USTC press, Chinese ed.,...
  • A. Ashkin et al.

    Science

    (1987)
  • V.V. Kilmov et al.

    Optics Comm.

    (1995)

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