1. Introduction

With conventional single-beam optical tweezers [1], we can trap and move a dielectric particle or a biological cell [2]. Many approaches to further rotate or orient the trapped microparticle of special optical or geometrical characteristics have been developed, such as by use of a linearly polarized laser beam [3–12], an elliptically polarized laser beam [3,13–18], a costly phase-control spatial light modulator [19,20], a rotating interference pattern [21–23], a rotating aperture [24], one or two rotating cylindrical lenses [25–27], a rotating dual-mode fiber [28], two-beam tweezing [2,19,29–31], and so on [32–41]. In this report, to manually rotate a trapped and twisted red blood cell of asymmetric appearance around the optical axis, we propose and demonstrate another more simple, flexible, and non-interferometric method of reforming the original single-mode Gaussian laser beam into a dual-mode split beam with a pair of closely spaced light spots [19] by adding a glass coverslip into the system [42,43].

In a second experiment, we have found that a birefringent polystyrene micro strip particle trapped in a linearly polarized single-beam optical tweezers may self rotate at a varying speed along its structural principal axes [18], fast spin about the optical axis in a tilting pose [12], or precess like a gyroscope [17], due to the anisotropy in shape and polarizability [3–12]. The proposed dual-mode split-beam optical tweezers system is shown to be capable of stabilizing the optical trapping of the birefringent strip particle. Moreover, in addition to moving the microparticle, it also provides another dimension of orientational manipulation of the specific microparticle. It is a very simple, cheap and convenient multifunctional optical tweezers system based on the conventional linearly polarized single-beam optical tweezers.

2. The manually controlled rotation of a red blood cell

Figure 1 shows the experimental setup for the dual-mode split-beam optical tweezers. The output light of a continuous-wave linearly polarized fiber-coupled diode laser (wavelength ≈ 980 nm) is first delivered by an optical fiber and then collimated by an aspheric collimating lens when emerging from the fiber output. The output beam is expanded by a pair of lenses (L₁ and L₂) and directed into a polarizing beam splitter (PBS). We launch the reflected beam after the PBS into an immersion-oil 100X microscope objective lens to form a tightly focused beam which can trap a microparticle by the gradient field force [1,2]. A white light LED is used to illuminate inversely the microparticle sample and the microscope image is projected onto a CCD camera through a focusing lens (L₃) behind the PBS. A bandpass filter can be placed in front of the CCD camera to reduce the scattered laser light from the sample.

 

Fig. 1 The configuration of the experimental setup for the dual-mode split-beam optical tweezers. PC, personal computer; CCD, charge coupled device; LED, light emitting diode.

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Next, we insert a thin glass coverslip [42,43] which is installed in a rotary mount [Fig. 2(a) ] into half of the single-mode Gaussian laser beam [Figs. 2(b) and 2(e)] by shifting the three-dimensional translation stages. The effective optical path of the semi-beam passing through the coverslip can be tuned by horizontally tilting the coverslip [Fig. 2(d)] using the rotation stage beneath the rotary mount. When adjusting the phase difference between the field amplitudes of the two semi-beams to an odd multiple of π [Fig. 2(f)], we can obtain a closely spaced dual-mode split beam [Fig. 2(c)] (similar to a TEM₁₀ mode) at the laser focus [42,43], of which the image is projected onto another CCD camera by another lens along the direction of laser beam propagation. The reducing of the vertical beam size might be due to the optical power loss induced by the coverslip. Through manually rotating the rotary mount (by hand), we can rotate the coverslip situated therein and change the angular displacement (orientation) of the two light spots of the split beam. As a result, we can achieve manually rotating a trapped specific microparticle around the optical axis by this set of dual-mode split-beam optical tweezers. An automatically controlled rotary mount can be used instead to achieve precise and quick orientational manipulation of the optically trapped microparticle.

 

Fig. 2 (a) The photograph of a coverslip installed in a rotary mount which can be shifted by the rotation and the three-dimensional translation stages. The observed images of (b) the original single-mode Gaussian beam and (c) the dual-mode split beam at the laser focus when inserting a coverslip into half of the laser beam. (d) The top view of the laser beam and the coverslip. The field amplitude and the optical intensity distribution profiles in the lateral position of (e) a single-mode Gaussian beam and (f) a dual-mode split beam, respectively.

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We take the human red blood cells as the test biological microparticles. The blood cells are prepared in an isotonic buffer solution and placed in a sealed slide/coverslip cell which is positioned at the sample stage (Fig. 1). The original disk-like biconcave red blood cell in the optical trap may undergo folding and twisting due to polarization-induced optical forces [6,15,28,31,36,38]. The trapped red blood cell with a distorted shape will be aligned along the two light spots of the split beam. The consecutive observed images of a trapped and manually rotated red blood cell by the dual-mode split-beam optical tweezers are shown in Fig. 3 at an angular interval of 45 degrees under an incident laser power of 25 mW approximately.

 

Fig. 3 The manually controlled rotation of a red blood cell trapped by the dual-mode split-beam optical tweezers (Media 1) displayed at an angular interval of 45 degrees. The original disk-like biconcave red blood cell has a diameter of 7 μm approximately.

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We have successfully carried out the stable trapping and manually controlled rotation of a red blood cell which looks asymmetric due to optically induced twisting. This technique is also useful in the optical orientational manipulation of rod-like biological microscopic objects [2]. On the other hand, in the movie (Media 1) we can observe that a second blood cell initially in Brownian motion has entered the optical trap [Figs. 3(d)-(e)] which tweezes the single red blood cell solely. The induced optical loss results in a weaker twisting of the trapped red blood cell [Figs. 3(g)-(h)]; yet, it does not disturb the manually controlled rotation of the trapped red blood cell. Moreover, in our experiments, the self rotation of a red blood cell trapped in the original linearly polarized single-beam optical tweezers has not been observed, since a normal (healthy) red blood cell does not have a large enough anisotropy in polarizability and thus would not experience a net optically induced torque [6].

3. The self rotation and the manually controlled rotation of a birefringent polystyrene micro strip particle

In a second experiment, we use a linearly polarized green laser (wavelength = 532 nm) as the light source instead in the same experimental setup as Fig. 1 to perform the optical manipulation of a birefringent polystyrene micro strip particle. At first, the coverslip is removed off. In this conventional linearly polarized single-beam optical tweezers system, we have found that a trapped birefringent polystyrene micro strip particle may self rotate at a varying speed along its two structural principal axes (long and short axes) [Figs. 4(a)-(h) ]. It looks like a naturally occurring, optically driven micro rotor or motor [15,32,33,37]. The optically trapped birefringent polystyrene micro strip particle periodically accelerates and decelerates its rotational motion due to a varying spin angular momentum transfer from the linearly polarized laser light [18], which might be attributed to the anisotropy in shape and material birefringence [3–12]. We conjecture that the strip particle is composed of two or more deteriorated birefringent polystyrene microparticles with sizes about 3~10 μm. The resultant crossed and disordered birefringence principal axes of the compound strip particle induce the incessant rotational motion with the linearly polarized laser light. Besides, the high-speed spinning about the optical axis in a tilting pose [12], and the gyroscope-like precession [Figs. 4(i)-(l)] [17] can also be observed. The change of the self-rotation style may be initiated by blocking off the laser beam or shifting the particle away from the optical trap (using the translation stage) for a short duration [12]. The release, fall, and recapture of the birefringent strip particle contribute the transformation of the optically induced self rotation.

 

Fig. 4 The self rotations of a birefringent polystyrene micro strip particle trapped by the linearly polarized single-beam optical tweezers (a)-(h) along the two structural principal axes, respectively (Media 2). The other types of random and faster self rotations, such as the high-speed spinning and (i)-(l) the gyroscope-like precession, can also be observed (Media 3).

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Several research reports [3–5,8,31] have revealed that a birefringent microparticle trapped by a linearly polarized focused laser beam can be rotated manually by rotating the polarization plane of the linearly polarized laser beam with a half-wave plate under angular control. Nevertheless, in our experiments, as shown above, the birefringent micro strip particle cannot be stably trapped by the linearly polarized single-beam optical tweezers. However, in the same way, by inserting a coverslip into half of the laser beam and adjusting the phase difference between the two semi-beams [42,43], we can obtain a closely spaced dual-mode split beam at the laser focus, such that we can trap stably the birefringent micro strip particle, and we can also further rotate the birefringent micro strip particle (Fig. 5 ) by rotating the coverslip and changing the orientation of the two light spots of the split beam synchronously. Notice that the birefringent polystyrene micro strip particle in Fig. 4 and that in Fig. 5 are different in size. They are sought and targeted individually during the two separate experiments.

 

Fig. 5 The manually controlled rotation of a birefringent polystyrene micro strip particle (length ≈ 5 μm) trapped by the dual-mode split-beam optical tweezers (Media 4).

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We have successfully overcome the spontaneous self-rotation difficulty of a birefringent polystyrene micro strip particle trapped in a conventional linearly polarized single-beam optical tweezers system by adding a coverslip into the system solely. This expedient treatment can greatly improve the stability of a trapped birefringent microparticle and also provide another dimension of optical manipulation simultaneously.

4. An alternative way to split the laser beam

Another common way to divide the laser beam into two parts is to place a slim line block laterally at the beam middle. In another optical tweezers system constructed using an 830 nm diode laser as the trapping laser source, we have ever tried to put a slim copper wire at the halfway of the laser beam, transforming the single-mode Gaussian laser beam into a dual-spot split beam by the spatial notch filtering on the beam intensity profile. This kind of split-beam optical tweezers can also achieve the stable trapping and manually controlled rotation of a blood cell. Nevertheless, the line width of the copper wire must be restricted to an adequate small fraction of the laser beam width, inhibiting the convenience and applicability of this approach in performing the stable trapping and manually controlled rotation of a specific microparticle using the dual-spot split-beam optical tweezers. It reveals implicitly the advantage of generating a dual-spot split beam using the coverslip instead.

5. Conclusion

We have achieved the stable trapping and the manually controlled rotation of an optically trapped red blood cell or birefringent polystyrene micro strip particle around the optical axis using the dual-mode split-beam optical tweezers, which can be switched back easily to the conventional single-beam optical tweezers. Due to being a non-interferometric configuration, this set of easy-to-reform split-beam optical tweezers system exhibits high flexibility and feasibility in operation, and provides a much more simple, cheap and convenient solution for the stable trapping and the controlled orientational manipulation of an asymmetric or birefringent microparticle when using a linearly polarized trapping laser source.