Objective Recently, optical tweezers and related optical manipulation technologies have been extensively studied. One important method to improve the capability of optical manipulation is to use structured beams with novel propagation properties to realize specific capture and manipulation of particles. Here, we propose a new type of multi-axis asymmetric exponential cone device composed of an asymmetric structure and a multi-axis structure for generating multi-axis asymmetric structured beams. These beams are proved to be effective to give the dielectric microspheres a unique ‘accelerating-decelerating-reaccelerating’ movement and provide new ideas and implementation methods for particle screening and transporting.
Methods Themulti-axis asymmetric exponential cone is designed with the geometrical optics theory and the diffractive optics theory. The modulating effects of such a device are produced by the different height distribution functions of its two asymmetric parts, which causes the redirection of the wave vector and influences the phase difference during propagation. With the three-dimensional finite-difference time-domain simulations, the electro-magnetic field distribution of the multi-axis asymmetric structured beam is obtained. Also, the energy flux distribution and the optical force distribution of dielectric microspheres are investigated based on the electro-magnetic field distributions of these beams. Moreover, the measurement of a focused multi-axis asymmetric structured beam and the manipulation of polystyrene fluorescent microspheres are conducted with our home-made experimental system to study the application potential in optical manipulation. The spatial light modulator and the phase mask of the device are employed to generate a multi-axis asymmetric structured beam in experiments.
Results and Discussions From the simulation results, the multi-axis asymmetric structured beam shows an alternation of multiple focal points and a ring-shaped focal pattern during propagation, possessing unique energy flow characteristics. There are two processes in which the beam energy flows to the geometric center, thus a ring-shaped equilibrium position is formed between the two energy convergences, as shown in Fig. 3. The optical force distribution demonstrates that the optical force on microspheres at each secondary focal point is basically uniform, indicating that the microspheres may move between different secondary focal points, as shown in Figs. 4 and 5. The capture of microparticles can be divided in two parts: one is from free space to the secondary focuses, and the other is from the secondary focus to the geometric center, indicating that the dielectric microspheres move in multi-stages under the effect of the focused multi-axis asymmetric structured beam. In experiment, when the focused multi-axis asymmetric structured beam is irradiated into the sample cell, the microsphere starts to accelerate toward the beam center, which is mainly caused by the focal gradient force. Then the speed of the microsphere moving toward the beam center decreases, and the distance between the microsphere and the beam center keeps stable for several seconds. This phenomenon confirms the equilibrium position found in our simulation results of energy flow and optical force. The small-angle rotation of the microsphere corresponds to the energy flow between different focal points. Eventually, the microsphere accelerates toward the beam center again, and leaves the focal plane driven by the scattering force. The whole process can be seen in Fig. 7. The experimental results show that multi-axis asymmetric structured beams cause the ‘accelerating-decelerating-reaccelerating’ movement of polystyrene microspheres during the trapping process, which is consistent with the simulated results, as shown in Fig. 8.
Conclusions In this paper, we propose a new type optical element, i.e. multi-axis asymmetric exponential cone, to generate multi-axis asymmetric structured beams, which can effectively capture and manipulate dielectric microspheres. Theoretical calculations, numerical simulations, and experimental measurement results show that the multi-axis asymmetric structured beam has unique propagation properties, including alternation of multiple focuses and ring-shaped focal patterns during propagation, equilibrium positions appearing in energy flow, and continuous light force on dielectric microspheres. In order to verify these propagation properties and explore the application potential, an independently designed experimental system for optical manipulation and monitoring is constructed using the focused multi-axis asymmetric structured beams to manipulate polystyrene fluorescent microspheres. Under the effect of the focused multi-axis asymmetric structured beam, the polystyrene fluorescent microspheres show a consistent ‘accelerating-decelerating-reaccelerating’ movement, and this movement is not affected by the initial motion direction of the microspheres, the initial speed, and other environmental factors. Different from the traditional optical tweezer technology that uses the gradient force of the focus to capture microspheres, the proposed method using the multi-axis asymmetric exponential cone to generate the multi-axis asymmetric structure of the beam provides a simple experimental implementation to give the dielectric microspheres a unique movement. This technology provides a new idea for applications in such as drug delivery, biological research, light-controlled screening, and light sensing.
