Focused-Laguerre-Gaussian 3D-Trapping and Spanner at Low Numerical Aperture

Pengcheng Peng1,a, Xintong Chen1,b, Weizhu Chen1,c, Mingyuan Xie1,2,d, Fuli Zhao1,e,* 1State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University,

Guangzhou, 510275, China 2School of Information Technology, Beijing Institute of Technology, Zhuhai,

Zhuhai, 519088, China a pengpch@mail2.sysu.edu.cn, b chxint@mail2.sysu.edu.cn, c chwzhu@mail2.sysu.edu.cn,

d xiemingy@mail2.sysu.edu.cn, e stszfl@mail.sysu.edu.cn *Corresponding author

Keywords: 3D-Trapping, Long Axial Trapping, Optical Spanner.

Abstract. Stable 3D micromanipulation by light requires that gradient force overcome axial scattering force introduced by an objective lens. Although high numerical aperture (NA) objective lenses in conventional optical tweezers could match the requirement, the dramatic limited axial working range and a narrow view field draw back the application seriously. With purpose to improve the application of micromanipulation, we succeed in the three dimensional (3D) trapping of polystyrene microspheres with a low-numerical-aperture (NA=0.40) objective releasing a long working distance (WD=5.89mm) by utilizing the Laguerre-Gaussian beams. A series of rotating manipulation through modulating the asymmetry of Laguerre-Gaussian beams are presented. This work offers an extended axial trapping range for 3D manipulation and a delicate hand-actuated rotating system for optical manipulation.

1. Introduction

Optical trapping [1] has been widely used in physics, biologyand other fields [2-6] with the development of laboratory techniques, but the trapping performances vary with different wave-front modulation methods [7]. Optical tweezers [1,8] with a single Gaussian beam for 3D trapping requires that the gradient force overcomes the scattering one, which needs a high numerical aperture microscope objective (typically NA>1) to satisfy. However, high NA brings a series of disadvantages such as large spherical aberrations, small field of view, high local heating of trapped sample and short working distance, crippling the potential for optical tweezers to perform relevant innovative biophysical measurements [9]. For the purpose of axial trapping with low NA objective lenses, different approaches have been demonstrated to date. Surface tension, as an additional force, was introduced to balance intensive scattering force [10,11]. Annular laser beam generated by two axicons was focused to get a long-distance axial trapping [12]. Single focused-Bessel light beam was employed to obtain 3D optical manipulation [13]. Laguerre-Gaussian (LG) beams was focused with a nonimmersion objective (NA=0.8) to achieve an axial trapping range of less than 300µm

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Journal of Engineering Mechanics and Machinery (2017) Vol. 2, Num. 1 Clausius Scientific Press, Canada

[14]. These contributions were significant, but more longer axial trapping range 3D trapping systems are needed for experiments, such as trapping objects at free liquid surface, reducing the influence of substrate while studying the nucleation of crystals and some other special processes. Our single optical trap of long axial trapping range and low-injury required power would be more convenient and flexible in some special biophysical experiments: the translocation of a protein, sensitivity study of protein-mediated DNA loops, single molecule fluorescence techniques [9,15,16]. In this letter, we demonstrate the stable 3D trapping and manipulation of polystyrene microspheres with diameter of 8m, using a single LG light beam focused through a microscope objective with low numerical aperture (NA=0.40), long working distance (WD=5.89mm) and low required trapping power (P=3.3mW).

2. Experiment

The experimental layout of our trapping system is shown in Figure 1 (a). A semiconductor laser (Excelsior-532-300-CDRH, Spectra-Physics, Japan) working at wavelength of 532nm is expanded by a telescope formed by Lens 1 and Lens 2, which holds a focal distance of 38.1mm and 200mm respectively. After beam expanding, the collimated beam passes through the spiral phase plate (SPP) to be transformed into the LG beam. Then LG beam is reflected by a dichroic mirror (short-pass 532nm) into the back aperture of a long working distance 20 objective (L Plan, NA 0.40, 5.889mm working distance, Daheng Optics Inc., China). The sample is mounted on a motorized XYZ stage (DRVER Model, New Focus Inc., USA) that can be driven manually or by a programmable software interface. A condenser was used to converge the red light produced by the illumination (GCI-0604 LED, wavelengh 621nm, Daheng Optics Inc., China) into sample cell. A USB CCD camera (MC20-C, 6.45×6.45µm pixel size, Micro-shot Technology Inc., China) is employed to record the trapping process. The CCD camera allows imaging speed as high as 15 fps at a resolution of 1360×1024 pixels. A long-pass filter (long-pass 550nm) is inserted in front of the CCD camera to block the trapping laser beam. The maximal laser power delivered on the sample is about 10mW. Polystyrene microspheres (8um in diameter, DAE Scientific Inc., China) immersed in water in a sample chamber are used for optical trapping and manipulation. In order to verify the reliability of our SPP, the interference pattern of LG beam (topological charge =2) and the plane wave as shown in Figure 1 (b), Figure 1 (c) shows the interference pattern of LG beam (topological charge =2) and the spherical beam.

To analyse trapping stiffness of our optical tweezers system, we calculate the escape force of 3D trapped beads based on Stokes’ law. Then, we considered a series of generalized LG laser beams by moving SPP perpendicular to the direction of beam propagation (laterally). In this situation, the output modulated LG beams lose axial symmetry, and we performed a set of rotation manipulation based on this precisely controlled asymmetry, proving that the rotation rate of polystyrene microspheres increases with increasing the asymmetry parameter in a certain range. Because of sufficient working distance and a large lateral stiffness, the particles can be fully controlled in three dimensions and, moreover, it can be moved across the entire sample cell at the trapped situation. Our work offers an advanced mechanical rotating system for optical manipulation and a long axial trapping range for 3D optical manipulation, a large field of view, low local heating of trapped samples, and reduced spherical aberrations compared with typical high NA 3D trapping systems. All measurements were carried out at room temperature.

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