Localized hollow beam optical tweezers techniques have been widely used in life and nanosciences as physical tools to generate microforces without direct contact and realize the precise manipulation of microparticles. The expanding application needs have created more requirements for the optical tweezers technology, necessitating new optical field regulation techniques for producing various tunable optical traps. In conventional axial cone-lens optical systems, closed localized hollow beams can be generated to capture particles. However, once the localized hollow beams form, it is difficult for the particles to pass through the light wall into the light trap. Furthermore, the size cannot be freely regulated after the conventional localized hollow beams are produced. Therefore, this paper proposes the production of a localized hollow beam that freely opens and closes, allowing the particles to enter through the gap and controlling the particle capture and escape using the change in the gap. This is important for capturing particles and more conducive to such an operation.
We added a rectangular appendix in a conventional axial cone-lens optical system. We found that by modulating the incident circular Gaussian beam using the axial-cone optical appendix, the localized hollow beam generated after the axion-lens optical system could regulate and had opening properties. We simulated the distribution properties of the local rectangular Gaussian beam by simulating the beam passing through the axis cone and the beam after focusing on the incident lens. Next, we analyzed the causes and influences of localized hollow beam defects and experimentally verified the localized hollow beam from generation to closure. The particles were first analyzed using the beam properties in both longitudinal and transverse directions in localized hollow beams. The longitudinal gradient and scattering forces were calculated in the beam propagation direction, and gravity was claculated in the transverse direction to analyze the particle process from the gap into the stable confinement.
The incident circular Gaussian beam is modulated using a rectangular aperture, and the size and orientation of the gap are adjusted by changing the length-width ratio of the rectangular aperture. This ensures that the localized hollow beam generated after passing the axial cone-lensing optical system freely regulates the size. Furthermore, we increase the localized hollow beam light field gradient using a high numerical aperture lens, and analyze the gradient force, scattering force, and gravity of the particles in a liquid environment. In this paper, the process of particles from entering the bottle beam to stable trapping is recorded by analyzing the lateral and longitudinal forces (Fig.7).
We designed a size-tunable bottle beam. To be more specific, the size of the bottle beam can be changed by adjusting the distance between the axicon and the lens. Besides, the gap can be generated by adjusting the length-width ratio of the rectangular aperture. We use MATLAB to simulate the size change of the bottle beam in the adjustment and the Bessel beam change of the beam after the axicon. Furthermore, we demonstrated the process of the bottle beam from forming to closing with the help of simulation and experiments. Experimental results show that the simulation of gradient, scattering, and resultant forces in size-adjusted localized hollow beams creates a particle passage through the gap and controls particle capture and escape according to the change of the gap.
