The short working distance of microscope objectives has severely restricted the application of optical micromanipulation techniques at larger depths. We show the first use of fiber-optic tweezers toward controlled guidance of neuronal growth cones and stretching of neurons. Further, by mode locking, the fiber-optic tweezers beam was converted to fiber-optic scissors, enabling dissection of neuronal processes and thus allowing study of the subsequent response of neurons to localized injury. At high average powers, lysis of a three-dimensionally trapped cell was accomplished.
There has been considerable current interest in the rotational behavior of red blood cells (RBCs) in optical tweezers. However, the mechanism of rotation in polarized tweezers is still not well understood and conflicts exist in the understanding of this phenomenon. Therefore, we reexamined the underlying phenomenon by use of confocal fluorescence microscopy in combination with optical tweezers. Under different osmolarities of the buffer, the three-dimensionally reconstructed images showed that the trapped RBC maintains its shape and is oriented in the vertical direction. Using dual optical tweezers, the RBC could also be oriented three-dimensionally in a controlled manner. The mechanism of orientation and alignment of RBCs with the polarization of the tweezers' beam was attributed to its form-birefringence rather than optical birefringence.
There has been considerable current interest in rotational behavior of red blood cells (RBC) in optical tweezers.
However, the mechanism of rotation in polarized tweezers is still not well understood and there exists conflicts in the
understanding of this phenomenon. Therefore, we re-examined the underlying phenomenon by use of confocal
fluorescence microscopy. Under different osmolarities of the buffer, the three dimensionally reconstructed images
showed that the trapped RBC maintains its discotic shape and is oriented in vertical direction. Using dual optical
tweezers, the RBC could also be oriented three-dimensionally in a controlled manner. Since, no folding of the RBC was
observed under optical trapping beam, the rotational mechanism based on optical birefringence caused by folding of
RBC can be ruled out. The alignment of RBC with polarization of the tweezers beam can be attributed to its formbirefringence.
We also present the mechanism for possible rotational behavior of RBC in circularly polarized beam.
Trapping of microscopic objects using fiber optical traps is gaining considerable interest since it has the potential to
manipulate objects inside turbid medium such as tissue, thus removing the limitation of short working distance of the
conventional optical tweezers based on high numerical aperture microscope objective. Here, we show that scattering
force of an output beam from a single fiber can be reduced as compared to the axial gradient force when an axicon is
built on the tip of the fiber, thus enabling single beam fiber-optic tweezers. Trapping of wide range of objects in size
range of few hundreds of nanometers to tens of micrometers could thus be achieved. This fiber optic tweezers could be
easily maneuvered in all three directions by moving the mechanical manipulator holding the axicon tip fiber. Further,
chain of upto 40 particles could be trapped along the axial direction, which can be attributed as longitudinal optical
binding where each trapped object acts as lens to trap subsequent object near its focal point. Apart from miniaturization
capability, axicon tipped optical fiber can be used in multi-functional mode for cellular manipulation, as well as for two-photon
fluorescence excitation for biomedical diagnosis.
Recently, we have reported self-rotation of normal red blood cells (RBC), suspended in hypertonic buffer, and trapped in unpolarized laser tweezers. Here, we report use of such an optically driven RBC-motor for microfluidic applications such as pumping/centrifugation of fluids. Since the speed of rotation of the RBC-motor was found to vary with the power of the trapping beam, the flow rate could be controlled by controlling the laser power. In polarized optical tweezers, preferential alignment of trapped RBC was observed. The aligned RBC (simulating a disk) in isotonic buffer, could be rotated in a controlled manner for use as a microfluidic valve by rotation of the plane of polarization of the trapping beam. The thickness of the discotic RBC could be changed by changing the osmolarity of the solution and thus the alignment torque on the RBC due to the polarization of the trapping beam could be varied. Further, in polarized tweezers, the RBCs in hypertonic buffer showed rocking motion while being in rotation. Here, the RBC rotated over a finite angular range, stopped for some time at a particular angle, and then started rotating till it was back to the aligned position and this cycle was found repetitive. This can be attributed to the fact that though the RBCs were found to experience an alignment torque to align with plane of polarization of the tweezers due to its form birefringence, it was smaller in magnitude as compared to the rotational torque due to its structural asymmetry in hypertonic solution. Changes in the laser power caused a transition from/to rocking to/from motor behavior of the RBC in a linearly polarized tweezers. By changing the direction of polarization caused by rotation of an external half wave plate, the stopping angle of rocking could be changed. Further, RBCs suspended in intermediate hypertonic buffer and trapped with polarized tweezers showed fluttering about the vertical plane.
Trapping and manipulation of microscopic objects using fiber optical traps is gaining considerable interest, as these objects can be manipulated inside complex environments, thus removing the limitation of short working distance of the conventional optical tweezers. We show that an axicon like structure built on the tip of a single mode optical fiber produces a focused beam shape with a central hole, implying a very small fraction of the power traveling with rays nearly parallels to the optical axis. Interesting transportation behavior of polystyrene particles using the scattering forces from such an axicon tip fiber was observed. As the distance of the particle from the fiber tip increased, since almost no rays interact with the particle, the scattering forces decreased substantially. Therefore, velocity of the particle at different distances was found to depend much more critically on the particle size in contrast to the beam generated by the bare fiber. While the speed of transport could be increased linearly by increasing the laser power in both axicon tipped fiber and bare fiber, increased speed was observed for particles of larger sizes for both the fiber types. However, the fractional increase in speed for increased size of particles was found to be quite large for axicon tipped fiber as compared to the bare fiber. Use of the observed differences in speed of transportation of microscopic objects could be used to sort them based upon their size.
Since the low index particles are repelled away from the highest intensity point, trapping them optically requires either a rotating Gaussian beam or optical vortex beams focused by a high numerical microscope objective. However, the short working distance of these microscope objectives puts a limit on the depth at which these particles can be manipulated. Here, we show that axicon like structure built on tip of a single mode optical fiber produces a focused beam that is able to trap low index particles. In fact, in addition to transverse trapping inside the dark conical region surrounded by high intensity ring, axial trapping is possible by the balance of scattering force against the buoyancy of the particles. The low-index particle system consisted of an emulsion of water droplets in acetophenone. When the fiber was kept horizontal, the low index spheres moved away along the beam and thus could be transported
by influence of the scattering force. However in the vertical position (or at an angle) of the fiber, the particles could be trapped stably both in transverse and axial directions. Chain of such particles could also be trapped and transported together by translation of the fiber. Using escape force technique, transverse trapping force and thus efficiency for particle in Mie regime was measured. Details of these measurements and theory showed that trapping of Raleigh particle is possible with such axicon-tip fibers. This ability to manipulate low-index spheres inside complex condensed environments using such traps will throw new insights in the understanding of bubble-bubble and bubble-wall interactions, thus probing the physics behind sonoluminescence and exploring new applications in biology and medicine.