Micro particle launcher/cleaner based on optical trapping technology Zhihai Liu, Peibo Liang, Yu Zhang,* Yaxun Zhang, Enming Zhao, Jun Yang, Libo Yuan Key Lab of In-fiber Integrated Optics, Ministry Education of China, Harbin Engineering University, Harbin 150001, China *[email protected]

Abstract: Efficient and controllable launching function of an optical tweezers is a challenging task. We present and demonstrate a novel single fiber optical tweezers which can trap and launch (clean) a target polystyrene (PS) microsphere (diameter~10μm) with independent control by using two wavelengths beams: 980nm and 1480nm. We employ 980nm laser beam to trap the target PS microsphere by molding the fiber tip into a special tapered-shape; and we employ 1480nm laser beam to launch the trapped PS microsphere with a certain velocity by using the thermophoresis force generated from the thermal effect due to the high absorption of the 1480nm laser beams in water. When the launching force is smaller than the trapping force, the PS microsphere will be trapped near the fiber tip, and the launching force will blow away other PS microspheres in the workspace realizing the cleaning function; When the launching force is larger than the trapping force, the trapped PS microsphere will be launched away from the fiber tip with a certain velocity and towards a certain direction, realizing the launching function. The launching velocity, acceleration and the distance can be measured by detecting the interference signals generated from the PS microsphere surface and the fiber tip end-face. This PS microsphere launching and cleaning functions expanded new features of single fiber optical tweezers, providing for the possibility of more practical applications in the micro manipulation research fields. ©2015 Optical Society of America OCIS codes: (350.4855) Optical tweezers or optical manipulation; (130.3990) Micro-optical devices

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Received 22 Jan 2015; revised 5 Mar 2015; accepted 11 Mar 2015; published 27 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008650 | OPTICS EXPRESS 8650

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1. Introduction The optical tweezers developed by Ashkin provides a convenient way to manipulate small particles nondestructively in a liquid environment [1], and has been widely investigated for trapping and observing cells and viruses in biological researches [2]. It has proved useful not only for trapping particles, but also for assembling objects ranging from micro spheres to biological cells [3]. Compared with the objective based optical tweezers, which are bulky and expensive, fiber optical tweezers have many advantages. They are compact in size and do not require high NA objectives, which is compatible to be integrated on chip. Besides that, the focal length (usually about 10μm) of the fiber lens is smaller than that of an objective (usually on the scale of hundreds of micrometers), which bestows fiber optical tweezers the capability to trap particles in cloudy liquids (typical for many biological fluids, such as blood and bile). Therefore, from multi fibers single optical tweezers [4–9] to single fiber single optical tweezes [10–15], and then to single fiber multi optical tweezers [16, 17], there have developed lots kinds of fiber optical tweezers. Although then we can develop single fiber multi optical tweezers based on the micro-structured multi core fiber [17], the special multi-core fiber is hard to obtain and not convenient to popularize. Therefore the exploitation of the fiber optical trapping technology based on the normal single core fiber is strongly needed. In summary, people need use optical tweezers based on the normal single fiber to realize more manipulation functions: besides micro particle trapping, orientation and rotation, other manipulations, such as cell migration [18], actin movement measurement [19] and some others are demanded eagerly. Therefore we

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Received 22 Jan 2015; revised 5 Mar 2015; accepted 11 Mar 2015; published 27 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008650 | OPTICS EXPRESS 8651

propose and demonstrate a novel single fiber optical tweezers. Besides the normal optical trapping function, this single fiber optical tweezers also have two other functions: one is launching the target trapped polystyrene (PS) microsphere with a certain velocity towards a certain direction; and another one is cleaning the workspace keeping the target PS microsphere being trapped, which means when the optical tweezers trap a target micro particle, the optical tweezers also can blow away other irrelevant micro particles keeping the target micro particle trapped. The trapping force is realized by using the 980nm laser beam, and the launching (cleaning) force is realized by using the 1480nm laser beam. Although people often avoid using the 1480nm laser beam to manipulate the PS microsphere because of the high water absorption and thermal effects [20], here we just use this thermal effect to construct the optical launching and cleaning functions. Although this optical tweezers are likely to produce some heat or temperature increasing effects on the micro particles (which may be not as important as the biological cells or tissues), it provides for the possibility of more practical applications in the micro manipulation research fields. Compared with other micro particle launching approach [21], such as by using the mode multiplexing technology, this light-induced thermal launching approach is much more convenient to control and manipulate. According to the Fig. 1(a), we employ 980nm laser light source to trap a polystyrene microsphere (diameter ~10μm, index ~1.59, density ~1.05g/cm3), and 1480nm laser light source to launch the trapped polystyrene microsphere with a certain velocity, therefore the trapping force and the launching force can be controlled independently. When we only use the 980nm laser source, the PS microsphere will be trapped stably; When 1480nm light source works, the launching force generated from the thermal effect is smaller than the trapping force generated from the large gradient optical field distribution, the PS microsphere will keep being trapped near the fiber tip, and the launching force will blow away other PS microspheres in the working platform which are probably to disturb the optical tweezers manipulation [see Fig. 1(b)]; When the launching force is larger than the trapping force, the trapped PS microsphere will be launched away from the fiber tip with a certain velocity and towards a certain direction, realizing the launching function [see Fig. 1(c)]. The launching velocity and the distance can be measured by detecting the interference signals generated from the PS microsphere surface and the fiber end-face. (a)

980nm laser 1480nm laser, pp0

(b)

Target micro particle Other micro particle Optical trapping force

(c)

Optical launching force Optical cleaning region

Fig. 1. Schematic diagram of the dual-wavelength single fiber optical tweezers. (a) 980nm laser beam is used to generate optical trapping force, as labeled with the red arrow; (b) 1480nm laser beam is used to generate launching force, as labeled with the green arrow; when the power of 1480nm laser beam is smaller than p0, the launching force is relative small, which can blow other PS microsphere away keeping the target PS microsphere being trapped; (c) when the power of 1480nm laser beam is larger than p0, the launching force is relative large, which can launch the target PS microsphere away with a certain velocity.

2. Principle The total forces exerting on the PS microsphere contain optical trapping force and light-induced thermophoretic force. #232827 - $15.00 USD © 2015 OSA

Received 22 Jan 2015; revised 5 Mar 2015; accepted 11 Mar 2015; published 27 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008650 | OPTICS EXPRESS 8652

The optical trapping force contains two parts: one is the optical trapping force generated from the 980nm laser source, and the other one is the optical trapping force generated from the 1480nm laser source. The trapping force drives the PS microsphere moving towards the fiber tip (along the opposite direction of the beam propagation). Therefore the optical trapping force depends on both 980nm and 1480nm light source powers. The light-induced thermophoretic force [20, 22] generates from the temperature gradient distribution caused by the strong absorption of the 1480nm laser in water [23] (absorption coefficient α = 26.0cm−1 [24]). The temperature gradient of the water solution occurs, therefore a thermophoretic force generates caused by the heat convection. The thermophoretic force drives the PS microsphere moving far away from the fiber tip (along the beam propagation direction). Similarly, 980nm laser source, whose absorption by water is low (coefficient α = 0.46cm−1 [24]), the thermophoretic force is very small, which can be omitted. Therefore the thermophoretic force only depends on 1480nm light source power. But it is necessary to note that if we only use 1480nm light source, the thermophoretic force is larger than optical gradient (trapping) force generated from the 1480nm light source only, therefore the launching and clearing functions need to be realized by using both 980nm and 1480nm light source. Consequently when we use 980nm laser source only, there are only optical trapping force (along the opposite direction of the beam propagation) exerting on the PS microsphere, it means we realize the trapping function; when we turn on the 1480nm laser source (keeping the 980nm laser source being on work), there are optical trapping force (along the opposite direction of the beam propagation) and thermophoretic force (along the beam propagation direction) exerting on the PS microsphere. Therefore we adjust the power ratio η of 1480nm: 980nm light source, when 0η1(here η1 = 5/30 = 0.1667), keeping the power of 980nm light source is 30mW), the PS microsphere will be launched. Then we can monitor the interference signals and obtain the velocity, acceleration and distance information (see Fig. 7(a), (b) and (c)). The velocity adds rapidly at the initial moment, which means the PS #232827 - $15.00 USD © 2015 OSA

Received 22 Jan 2015; revised 5 Mar 2015; accepted 11 Mar 2015; published 27 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008650 | OPTICS EXPRESS 8657

microsphere moves away from the fiber tip with a large velocity at the initial moment, which is excited by the thermal convection generated from the absorption of 1480nm laser power by the water solution; and then the velocity decreases, which is effected by the viscosity resistance of the water solution.

Fig. 7. Testing and calculated (a) Displacement-time curve, (b) Velocity-time curve, and (c) Acceleration-time curve of the PS microsphere launching along the z axis. Here p1480 in the figures means the 1480nm light source power.

5. Conclusion In conclusion we have successfully fabricated and tested a novel dual-wavelength single fiber optical tweezers. The single fiber optical tweezers can trap and launch (clean) the PS microsphere by using 980nm and 1480nm beams respectively. The target PS microsphere trapping is demonstrated, and the other PS microspheres blown away is also demonstrated, besides that the target PS microsphere is launched away is also realized, and the launching distance, velocity and acceleration are measured by detecting the interference signals generated from the PS microsphere surface and the fiber end-face. This PS microsphere launching and cleaning functions expanded new features of single fiber optical tweezers, providing for the possibility of more practical applications in the micro manipulation research fields. It provides a new probably development direction for the optical tweezers technology applying on the micro manipulation research fields, and solve the optical tweezers multi functions integrated problems. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grants No. 11204047, 61227013, 61275087 and 61377085), and partially supported by the following grants: the 111 project (B13015), Research Fund for the Doctoral Program of Higher Education of China (Grants No. 20112304110017), Postdoctororal Science Foundation Fund of China (Grants No. 2014M550181), Post-Doctor Research Fund of Heilongjiang Province of China (Grants No. LBH Q10147), and Fundamental Research Funds for Harbin Engineering University of China.

#232827 - $15.00 USD © 2015 OSA

Received 22 Jan 2015; revised 5 Mar 2015; accepted 11 Mar 2015; published 27 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.008650 | OPTICS EXPRESS 8658

cleaner based on optical trapping technology.

Efficient and controllable launching function of an optical tweezers is a challenging task. We present and demonstrate a novel single fiber optical tw...
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