Technical Papers and Presentations

 Graphene, as a two-dimensional material with exceptional physical and chemical properties, exhibits highly tunable optical absorption within the visible spectrum. Its absorption is sensitive to polarization, the number of layers, and incident angles, making it an ideal candidate for dynamic photothermal systems [1]. Leveraging these distinctive characteristics, we simulated a switchable opto-thermoelectric tweezer system through comprehensive modulation of these properties, thereby applicable to ultralow-power particle manipulation.
 Here, a 3D full-domain simulation model is established in COMSOL Multiphysics®. In this numerical model, the Electromagnetic Waves, Frequency Domain (ewfd) and Heat Transfer in Solids and Fluids (ht) modules are coupled through the Electromagnetic Heating Multiphysics coupling to solve for the interaction between optical and thermal fields. To simplify the mesh generation process, a transition boundary condition is applied in the calculations to replace the extremely thin graphene layer. When a laser beam, with tunable polarization orientations (φ) and incident angles (θ), is focused onto the graphene surface, simulations reveal pronounced anisotropic light absorption in graphene. At an incident angle of 65°, the absorption rate for p-polarization light reaches 80%, generating a pronounced localized temperature gradient in the focal region, while s-polarization light remains nearly transparent. The underlying optical mechanism of graphene's selective absorption and resulting temperature gradient distribution can be employed to induce efficient trapping of particles. 
To quantitatively evaluate the trapping capability, the thermoelectric force on a gold nanoparticle above the graphene is calculated by integrating the tangential thermoelectric field over the particle surface, which arises from the temperature gradient [2]. To visualize the trapping capability enabled by the polarization-dependent thermoelectric force, the spatial potential energy is calculated by integrating the force field. The results indicate that, under different polarizations, the depth of the potential well formed by the thermoelectric potential differs significantly. Under p-polarization, a pronounced and localized potential well exceeding -120 kBT is generated (where kB is the Boltzmann constant and T = 300 K is the ambient temperature), which is much deeper than that formed under s-polarization [3]. The simulation deepens the understanding of graphene-based thermoelectric optical tweezers, and expands the application of low-power manipulation.