Based on the light-matter interaction, optical tweezers generate strong force on micro and nano-sized particles by momentum transfer, which is non-contact and non-damage. In bioscience, optical tweezers have been applied to capture bacteria and non-invasive manipulation of organelles within a single living cell and become an effective way to detect and control micro- and nano-scale objects. However, optically capturing and manipulating individual particles smaller than the light wavelength remains a major challenge. To overcome the light diffraction limitation, the local surface plasmons (LSPs) in the metal nanostructure can effectively focus and confine the propagating light within the nanometer scale, with better spatial locality and higher local field intensity. A plasma well is generated by the coupling of electromagnetic waves at the interface of the metal-dielectric layer by surface plasmon excitation. This unique electromagnetic pattern can limit light beyond the diffraction limit, causing the electromagnetic field to decay exponentially from the metal-dielectric interface. These two properties are crucial for optical capture applications: the former significantly reduces the volume of the captured object, and the latter enhances the production of optical forces due to the field intensity gradient. Meanwhile, we study the distribution of the electric field and Poynting vector diagram under different polarization modes and the distribution of light force and potential well generated by the interaction between nanoparticles and the scattering near the field of coaxial structure. Finally, it provides a new way for capturing and manipulating micro and nano particles and other living cells in a low-concentration solution.

The coaxial structure consists of silicon concentric ring and a silver layer. The coaxial structure of the laser source illuminates vertically from the bottom, and the electric field distribution of online polarization, circular polarization, and different coaxial apertures under the light source and Poynting vector diagram are calculated by finite-difference time-domain (FDTD) method. Additionally, the Maxwell stress tensor method is adopted to calculate the light force generated by the interaction of dielectric particles with a 10 nm radius with the structure in free space. The optical trapping performance of the structure in two light modes is studied. The optical trapping force and potential well distribution of particles in the x-y and y-z planes under different light source modes are calculated. The force analysis of the nanoparticles shows that the positive force F_x and negative force F_-x in both the x-y and y-z planes cause the coaxial structure to produce the trapped particles in the center of the potential well.

The coaxial structure is coupled with the optical field to enhance transmission and local electromagnetic field (Fig. 1). The transmission characteristic curve shifts to the right as the height of the coaxial structure h increases. When h=150 nm, the transmission spectrum has two peaks at the wavelength of 540 nm and 750 nm. These peaks are transmitted by light waves into a coaxial aperture of finite thickness and are fully reflected, which brings the Fabry-Perot resonance mode. Under the action of the circularly polarized light field, the coaxial plasma structure generates a vortex light field of spin energy flow (Fig. 6). The resulting vortex field affects the spin angular momentum carried by the circularly polarized light and makes it pass through the coaxial aperture, and the orbital angular momentum is transmitted in the near field through the spin-orbit interaction of the electromagnetic field. The optical trapping force and potential well distribution of particles in the x-y and y-z planes under different light source modes are calculated respectively (Figs. 7 and 8). The dual trapping potential well is generated to expand the trapping region, overcome the Brownian diffusion of nanoparticles in a low-concentration solution, and improve the trapping efficiency, providing a new way for capturing and manipulating micro-nano particles and other living cells in a low-concentration solution.

The distribution of electric field and Poynting vector diagram under linear polarization and circular polarization light sources are calculated by the FDTD method, with the optical trapping performance of nanoparticles under two light modes. The results show that the transmission value reaches the maximum at 750 nm, and the depth of the potential well reaches 17kBT under the incident light intensity of 1 μW/μm². Meanwhile, the circularly polarized light forms a potential well depth of 8kBT vortex field above the structure, which overcomes the Brownian diffusion of nanoparticles in a low-concentration solution and improves the capture efficiency. The results can be employed both as optical tweezers for manipulating nanoparticles and as semiconductor structures for laser emission.