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We propose an optical manipulation system by using a liquid crystal (LC) device with unique functions such as an anamorphic lens property, variable-focusing and deflection properties. The positions and rotation of optical trapped microscopic slender particles suspended in water can be controlled. The trapped particles can be aligned along the major axis of the elliptically shaped laser beam spot and the position of the particle can be controlled three-dimensionally.
©2008 Optical Society of America
1. Introduction
Optical micro-manipulation is widely used for manipulating glass spheres, biological samples such as viruses, bacteria and living cells by using single-beam gradient force and dual-beam traps. The optical manipulation system requires certain mechanical procedures for moving the focusing point of an objective lens and deflecting the laser beam by using mechanical movements such as Galvano mirror and piezo electric actuator devices. The manipulation is established as a powerful method for many applications of various fields of physics and biology. The optical pump and valve with colloidal particles in micro-fluidic channels have also been widely used by activating a focused laser beam [1, 2]. The polystyrene and silica spheres in two- and three-dimensional configurations can be controlled by the holographic optical tweezers created from a single laser beam with a computer-designed hologram of a single beam wavefront using liquid crystal (LC) spatial light modulators (SLMs) in real time [3]. The multiple optical traps and rotation of microscopic dielectric particles can also be controlled by using diffractive optical elements implemented on SLMs [4, 5]. The position controls of the trapped particles are realized by adjusting the computer-designed hologram of the SLMs. In our group, electro-optic liquid crystal (LC) devices such as LC lenses [6, 7], LC micro-lenses [8], LC gratings [9] have been studied and demonstrated. The focal length and focal length plane of the LC lens and LC micro-lens can be tuned by applying a voltage. We reported three-dimensional microscopic particle manipulation system by using an objective lens and an LC lens with variable focusing and beam deflection properties. The trapped particles such as polystyrene balls can be shifted in the longitudinal direction as well as the transverse direction without any use of mechanical parts [10].
In this paper, we present an optical manipulation for controlling the positions and rotations of the trapped slender particles by using an LC lens with eight-divided circularly hole-patterned electrodes. The LC device has a new function of an anamorphic lens property in addition to both the variable focusing and beam deflection properties.
2. Experimental Setup
Figure 1 shows the schematic diagram of the optical manipulation system with an LC device. The system consists of a TEM00 doubled-Nd:YVO4 laser source at a wavelength of 532 nm (Verdi, Coherent Japan Co.), a microscope system with a condenser lens (numerical aperture; NA=0.45) and an imaging system. The laser beam transmits through the collimated lens and the LC device with anamorphic lens property. The laser beam is highly focused on a sample stage which microscopic particles such as slender glass rod particles (length: 30~50 µm, diameter: 11 µm) or polymer ball particles (diameter: 5 µm) are suspended in water. The microscope images of the trapped particles by the focused laser beam can be monitored and taken by a CCD (a charge coupled device) array camera through an objective lens, interference and dichroic color filters. These filters are used in order to cut the highly focused laser light.
Figure 2 illustrates the device structure of an LC device for controlling the position and rotation of the microscopic particles. The LC material (ZLI-6080, Merck) with a positive dielectric anisotropy is injected between two glass substrates. The lower glass substrate is coated with a transparent indium tin oxide (ITO) film and its surface with the ITO faces the LC layer. The eight-divided circularly hole-patterned electrodes are fabricated by carrying out a chemical etching of the evaporated aluminum thin film coated on the outer surface of the upper substrate and the diameter of the circularly-hole pattern is 3.8 mm. The surfaces of the substrate which face the LC material are coated with polyimide (PI) film and are unidirectionally rubbed to align the LC directors. The top glass substrate with ITO film and the substrate with eight-divided circularly electrodes is attached by using a thin glass substrate of 100 µm. Each voltage of eight-divided electrodes V 1~V 8 and external electrode VITO can be applied independently by using nine power amplifiers connected to a computer controllable signal wave-generator (WE7000, Yokogawa co.).
3. Results and discussion
The transmission light images through the circularly hole-patterned region in the LC device are observed by a polarizing microscope with the CCD camera under crossed polarizers. The rubbing direction of the LC device is set to 45 degree to the polarization direction of the polarizers. Figures 3(a)~3(d) show the circular interference fringe patterns of the circularly hole-patterned region in the LC optical device when the applied voltages to the eight-divided electrodes are same values (V 1~V 8=55 V). Where the applied voltage to the external control electrode (V ITO) is gradually decreased from 56 V to 10 V. Since there is a phase difference of 2π between the neighboring fringes, the phase difference properties of exiting ray from hole-patterned region can be estimated and the phase difference profile varies with the applied voltage. Then the parabolic profile of the effective refractive index distribution is attained. The LC director distributions, that is the profile of phase retardation in the hole-patterned region can easily be arranged by the external voltage. The optical property seems to be a lenslike distribution of the refractive index and the wavefront passed through the LC device is converged. The focal length can be controlled from 10 cm to 120 cm by applying the control voltage to the external electrode as shown in Fig. 3.
Figures 4(a)~4(d) show the interference fringe pattern images when the applied voltages to eight-divided circularly hole-patterned electrodes are adjusted. As the center of the interference fringe is shifted to the opposite direction of the electrode where its applied voltage is lower than others, the center of the fringe pattern is moved from right side to left side. Otherwise, the location of the focal spot can be deflected from left side to right side as controlling the voltage between V 1 and V 5.
The elliptical interference fringe patterns of the hole-pattern region in the LC optical device are shown in Figs. 5(a)~5(d), where control voltage to the external electrode is 10 V. The applied voltage to the minor axis of the elliptical fringe pattern is higher than that to the major axis, and then the anamorphic lens property such as elliptical refractive index can be obtained. The direction of the major and minor axes in the fringe patterns can be rotated in a clockwise direction, where each applied voltage to eight-divided circularly hole-patterned electrodes is arranged to be V major=58 V and V minor=27 V corresponding to the major and minor axes of the elliptical interference fringe and V side=38 V to other directions. As the influence between the LC layer thickness of 110 µm, applied voltage and the viscosity of the LC material and so on, the major and minor axis in fringe pattern can be rotated at around 5~7 rpm. The positions of the laser spot and the elliptically shaped light intensity can be controlled sufficiently for the application in the optical manipulation system. If the number of the division of the hole-patterned electrode is above eight parts, the laser beam distribution can also be controlled more smoothly. But, switching the applied voltage to the divided electrodes becomes complicated. The improvement of its rotation speed is now in progress and the detail results will be published in elsewhere.
Figure 6 shows the position control of the trapped microscopic glass particle (diameter: 10 µmϕ, length: 40 µm) of about 6×10-3 mg. The output power of the laser source is around 0.4 W. The rubbing direction in the LC device is aligned along the polarization direction of the laser beam. The glass rod particle is trapped and the position of the trapped particle is moved about 90 µm along with the location of the focal spot as controlling the applied voltage to the divided-electrodes as shown in Fig. 4. The three-dimensional positions of the trapped particle can also be attained by deflecting and focusing the laser spot in x-y-z directions.
Figures 7(a)~7(l) show the microscope images of the trapped slender particle at diameter of 10 µmϕ and length of 20 µm (about 3×10-3 mg). The trapped particle can be rotated in clockwise direction by using the LC device with eight-divided circularly hole-patterned electrodes and external control electrode. Since the trapped slender particle aligns along the major axis of the elliptical intensity distribution at the focusing point, the particle can be rotated by controlling each applied voltage to the divided electrode and setting the major axis direction of the ellipse. The trapped particle can also be rotated in the anticlockwise direction. The Relationship between the rotation angle of the trapped particle with respect to the x axis and operating time is shown in Fig 8. The trapped particle can be rotated with an almost uniform speed. The rotation speed of the trapped particle can be improved by operating the optimum voltages applied to the divided-electrodes of the LC device.
Since the numerical aperture of the condenser lens (NA=0.45) used in our experiment is smaller than that of the high numerical aperture lens (NA>0.65 or higher) usually used in the optical manipulation system [1–5], the optical force for trapping the microscopic particles is also small. The optical trapping force depends on the numerical aperture of the condenser lens and the power of the laser beam. The power of the laser beam is about 0.4 W and a condenser lens (NA=0.45) is used in our experimental setup, then the microscopic particles such as slender glass rod particles can be trapped and controlled by deflecting the trapped laser beam and adjusting the laser spot profiles by using liquid crystal device with eight-divided circularly hole-patterned electrodes. The numerical analysis of the relationship between numerical aperture of the combination lenses (the LC lens and condenser lens), the light intensity of the laser beam and trapping force for the microscopic particles is now in progress.
Using our LC device with functions of rotatable elliptically shaped laser beam spot in addition to both the variable focal length lens properties and deflecting the laser beam spot, the positions of the trapped microscopic slender particles can be controlled in x-y-z directions and rotated. This LC device without any mechanical movements is more useful for optical manipulation system when it is compared with other manipulation system by using mechanical movement parts such as Galvano mirror and piezo electric actuator devices. Since the circularly and elliptically shaped wavefront of the transmitted laser beam through the LC lens can be controlled by applying the voltages to the circularly hole-patterned electrodes, the trapped slender particle aligns along the major axis of the elliptical light intensity distribution at the focusing point and the trapped particle can be rotated in clockwise or counterclockwise directions. On the other hand, using mechanical movement parts in the optical manipulation system, both edges of the microscopic slender particles can be trapped by laser beams of two and over, and then the trapped particle can be rotated. The particle can also be controlled by using diffractive optical elements implemented on SLMs, the shaped beams into patterns of two and over bright spots are used. Therefore it must be useful for our LC device to control the rotation of the trapped particles.
4. Summary
The optical manipulation system for controllable and rotatable trapping the micro-sized slender particles was proposed by using an LC device. The LC device with eight-divided circularly hole-patterned electrodes and external control electrode has three functions of a variable focal length, a beam steering and an elliptical light intensity distribution change. The positions of the trapped particles can be shifted by using the function of the beam steering and variable focusing of the LC device. The trapped particles can also be rotated in clockwise or anticlockwise directions by adjustable laser spot profile.
Acknowledgement
This research was partially supported by Grant-in-Aid for Scientific Research Young Scientists (B), 18760246 from the Ministry of Education, Science, Sports and Culture, 2007.
References and links
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