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
The optical tweezers developed by Ashkin provides a convenient way to manipulate small particles nondestructively in a liquid environment , and has been widely investigated for trapping and observing cells and viruses in biological researches . It has proved useful not only for trapping particles, but also for assembling objects ranging from micro spheres to biological cells .
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 , 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 , actin movement measurement  and some others are demanded eagerly. Therefore we 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 , 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 , 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.
The total forces exerting on the PS microsphere contain optical trapping force and light-induced thermophoretic force.
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  (absorption coefficient α = 26.0cm−1 ). 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 ), 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 0<η<η1, the magnitude of the thermophoretic force is smaller than the optical trapping force, (here means the magnitude of the thermophoretic force is smaller than the optical trapping force, in out experiment, the power of 980nm laser source is 30mW, the power of 1480nm laser source is <5mW) the optical tweezers realizes the clearing function; when the power ratio meets η>η1, magnitude of the thermophoretic force is larger than the optical trapping force, (here means the magnitude of the thermophoretic force is larger than the optical trapping force, in out experiment, the power of 980nm laser source is 30mW, the power of 1480nm laser source is >5mW), therefore optical tweezers realizes the launching function.
Optical Trapping Function
In order to construct the optical trapping force, a large enough gradient light field need to be built. Therefore a selective chemical etching procedure is developed for fabricating fiber tip. The chemical etching solution is composed of 48% hydrofluoric acid (HF), 40% ammonium fluoride (NH4F), and de-ionized distilled water. The volume ratio of the three solutions is 1.5:1:1 . In our investigation, we choose 1-Bromodecane as the protective liquid layer , which is very important for the cone angle of final fiber tip. After being etched in solution for 60mins at room temperature, the cone angle of cladding region is 22°, and the cone angle of core region is 45° as shown in Fig. 2(b); the difference between the cone angle in cladding and core region is caused by the difference of materials composed in cladding and core in fiber. The SEM (Scanning Electron Microscopy) images of the tapered fiber tip are shown in Fig. 2 with different magnifications.
To reveal the trapping force features in this tapered fiber tip, the 2-D FDTD algorithm  is used to simulate and calculate the trapping force (see Fig. 3). In the FDTD simulation, the light source wavelengths are 980nm and 1480nm; the diameter of fiber core is 3.7μm (here we use the conventional 980nm single mode fiber (ClearLite®980 Photonic Fibers, OFS)), the refractive index of the fiber core, PS microsphere, and the surrounding medium (water) are 1.4681, 1.40 and 1.33 respectively. The grid step is chosen as 0.05μm. According to the Fig. 3(a), in the range of 5~30μm, the axial trapping forces generated from the two wavelength laser beams are both less than zero, which means the direction of the trapping forces is opposite to the beam propagating direction, and the forces will pull the particle near to the fiber tip; therefore the PS microsphere will be trapped stably on the fiber tip. According to the Fig. 3(b), in the range of 0~10μm, the transverse trapping forces generated from the two wavelength laser beams are both less than zero, which means the direction of the trapping forces is appointing to the fiber main axis, and the forces will pull the particle near to the fiber tip; therefore the PS microsphere will be trapped stably on the fiber tip.
Optical launching (cleaning) functions
Optical Launching (Cleaning) Functions are realized by using the light-induced thermophoretic force, with which the particle is drifted induced by temperature gradients. In a thermal gradient, a PS microsphere attains a drift velocity , where the thermophoretic mobility DT is usually dubbed “thermal diffusion coefficient”. According to the reference , the sign of DT is negative here, and it means that the direction of temperature gradient is opposite to the particle drift velocity direction. Therefore if we obtain the temperature field distribution, we can obtain the velocity distribution and calculate the PS microsphere moving velocity along the z axis.
The output power energy distribution (see Fig. 4(a)) shows that the output light focuses just outside of the fiber tip, and then, the light diverges to a large area and becomes weaker gradually. Figure 4(b) provides the temperature and temperature gradient distribution simulated results. On the z axis, the direction of the temperature gradient is along -z axis. Therefore we can deduce that the thermophoretic driving velocity is along + z direction. It is why this fiber optical tweezers can launch the trapped PS microsphere moving along the z axis by using the 1480nm light source. According to the simulated results, on the condition of the input light power is 30mW, the temperature increase induced by the absorption of 1480nm laser is ~20K, and the response time is ~3ms, which means after 3ms, the temperature of the water near the focus point keeps ~316K.
The sketch diagram of the experiment setup is shown in Fig. 5. The 980nm laser light source (HoYatek, TPLM98-110) with power adjusting range of 0 to 110mW is employed to push and trap the PS microsphere. The 1480nm laser light source (HoYatek, TPLM148-90) with power adjusting range of 0 to 90mW is employed to launch the trapped PS microsphere. Besides that, the 1550nm laser light source (Golight, OS-ASE, center wavelength:1550nm, wavelength range: 1520~1560nm) is utilized to monitor and detect the interference signals generated from the temperature sensing cell. WDM (wavelength division multiplexing), fiber optical coupler and other normal devices are employed to realize the optical path connection.
According to the Fig. 5, we can control the 980nm and 1480 light source independently. When we need to trap the micro particle, we turn on the 980nm light source and keeping the power of 980nm light source to be ~30mW(in our experiment, the micro particle can be trapped stably when the 980nm light source power is ~30mW); when we need to clear the working space, we keep the 980nm light source power is on, and turn on the 1480nm light source modulating the power to be ~5mw, then the trapped micro particle is still trapped stably by the fiber probe while the other micro particles in the working space are blown away; when we need to launch the trapped micro particle, we turn off the 980nm light source, and simultaneously, turn on the 1480nm light source and modulate the power to be higher than 5mw, such as 10mw or 20mw, the higher the 1480nm light source power is, the larger the launching velocity and acceleration are.
In order to detect the launching velocity and acceleration of the launching particle, we employ the 1550nm ASE light source to monitor and calculate the interference signals information obtaining from the oscilloscope.
When the launching force is smaller than the trapping force, here means the 1480nm laser power is less than 5mW, the PS microsphere can be trapped stably on the fiber tip, while the 1480nm laser thermophoresis force blow the other PS microspheres away from the working platform (see Fig. 6(a) and 6(b)). When the launching force is larger than the trapping force, here means the 1480nm laser power is larger than 5mW, the trapped target PS microsphere can be launched away [see Fig. 6(c) and 6(d)]. The launching velocity can be measured by using the interference signals detection.
4. Results and discussion
The mechanics equation of the PS microsphere being launched in the axial optical field is:
Where , is the three dimensional coordinate presenting the placement of the PS microsphere; is the launching force, meets , here is the thermophoretic force, generated from the thermal convection in the solution (water), and this thermophoretic force is related with the temperature T in the solution, named ; is the optical trapping force generated from the 1480nm laser source, and the magnitude can be obtained from the simulated results in Fig. 3. is the viscous resistance, and η is the viscosity coefficient of water, which is also related with the temperature T, named η(T), therefore the viscous resistance is also related with the temperature T in the solution, named ; a (10μm) is the PS microsphere diameter. According to the calculated results in Fig. 4(b), the temperature is the function of , named . and are the velocity and the acceleration of the PS, which can be measured by using the optical interference method .
Here the velocity can be measured by detecting the backscattering interference information generating from the micro particle and the fiber end. Supposed that the propagating field of the reflecting beam from the fiber end can be expressed as
where E01 is the amplitude of the reflecting beam generating from the fiber end, then the propagating field of the reflecting beam generating from the particle can be expressed as
Therefore the interference light field should be
Then according to Eq. (5), we can measure the velocity by detecting the light intensity signals. The frequency of the interference signal pattern represents the velocity of the micro particle. It means the density of the interference fringes indicate the velocity of the micro particle: the denser the interference fringes are, the larger the micro particle velocity is; the sparser the interference fringes are, the less the micro particle velocity is. The sampling frequency of the oscilloscope is 5 × 105/s. We regard every 5 × 103 data, which represent 0.01s, as a time interval. The detecting light source wavelength is 1.55μm. Therefore the velocity can be expressed as:
where n is the quantity of the interference fringes during each 0.01s time interval. Therefore, the acceleration also can be obtained from the measure results.
Figure 7 provides the measuring and calculated results. The launching velocity, acceleration and distance are related with the 1480nm and 980nm laser power ratio η, the larger is η, the larger are the velocity, acceleration and distance. Once we adjust the power of 1480nm laser, whose power is larger than 5mW (when η>η1(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 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.
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.
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.
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