Theoretical Research On Photothermal Effect And Optofluidic Based On Micro-nano Chiral Structure

As an emerging interdisciplinary subject,optofluidics aim to use the power of light to dynamically control liquid flow.Due to its characteristics of contact-free,high resolution and flexible operation,it has shown application prospects in many fields such as physics,chemistry,biomedicine and so on.The photothermal effect of plasmonic nanostructures can stimulate the natural convection of fluids,making it a focus in the field of optofluidics.Specifically,the chiral structure has unique selective absorption properties for circularly polarized light,which shows stronger efficiency of the photothermal effect than that under linearly polarized light,and has attracted widespread research interest.However,the differential response of most chiral photothermal effects to circularly polarized light is not pronounced,and it takes a long time to reach a specific temperature rise.There are still certain limitations in the application of polarizationsensitive optofluids.To address this scientific issue and better utilize light for precise fluid manipulation,this dissertation investigates the photothermal effects and optofluidic control technology based on micro/nano chiral structures.Through the optimization of structural design,efficient differential absorption of LCP and RCP was achieved,enhancing the chirral photothermal effect.This dissertation explored fluid convection induced by chiral photothermal effects,theoretically discovered phenomena of polarization sensitive liquid surface deformation induced by circularly polarized light,and further expanded the applications of optofluidic technology.The major results are summarized below:1.A chiral metamaterial absorber was designed to achieve selective perfect absorption of circularly polarized light in a single band,significantly enhancing the chiral photothermal effect and thermally induced chiral fluid convection was observed theoretically.Surface plasmons can be excited when the light frequency matches the oscillation frequency of free electrons in the metal,causing the energy to be greatly localized in a very small volume near the metal surface and result in strong absorption.The designed right-handed absorber exhibits nearly 100% absorption of right-hand circularly polarized light(RCP)and almost totally reflect the left-hand circularly polarized light(LCP)at the resonance wavelength of 1310 nm.Consequently,under RCP incidence at the resonant wavelength,the temperature of the structure rises rapidly and far exceeds the results under LCP incidence,demonstrating the relationship between the photothermal effect and absorption rate.The natural convection caused by the temperature gradient was further analyzed,and different fluid velocity and asymmetric convection distribution under LCP and RCP illumination was observed,which proving that fluid flow can also have chiral characteristics.These results provide a new perspective for research into polarization-sensitive optofluidics.2.A chiral resonant metasurface was theoretically designed,achieving chiral photothermal effects and optofluidic control with dual-band circular polarizationselective absorption properties.Without altering its intrinsic chirality,the structure exhibited enhanced absorption for LCP and RCP in two bands,877 nm and 1045 nm.Therefore,a pair of circular dichroisms with values of approximately 0.6 and-0.6 was obtained at these two wavelengths.By analyzing the electromagnetic field distribution of the structure,it is found that the enhanced absorption at the short wavelength originates from quadrupole resonance,and the absorption at the long wavelength originates from the coupling mode of electric resonance and magnetic resonance.Further analysis found that the dual-band responses to circularly polarized light were not limited to far-field optical absorption,similar behaviors were also observed in the chiral photothermal effects and thermally induced fluid motion at the two characteristic wavelengths.Specifically,under LCP illumination at 877 nm,both the structure’s temperature rise and fluid velocity were higher than that under RCP illumination,while the opposite was true at 1045 nm.This work simulated dual-band chiral photothermal effect enhancement and expanded the application of optofluidic technology in multi-band manipulation.3.The photoinduced liquid surface asymmetric deformation based on chiral photothermal effect was discovered theoretically,further expanding the application research of chiral optofluidics.A pinwheel-shaped chiral metal nanostructure was designed and the height of water around the structure was limited.Under the illumination of circularly polarized light,the heat generated by the chiral resonator increases the temperature of the water above it,causing the liquid surface tension coefficient to decrease.Thus,a surface tension gradient appears at the interface between water and air,causing fluid flow driven by the Marangoni effect,resulting in deformation of the liquid surface.The deformation characteristics are consistent with the contour of the designed chiral structure.A theoretical model of multi-physics coupling was developed to explanation the kinetic mechanism of liquid surface deformation,demonstrating that liquid surface deformation at different depths can be obtained by adjusting the incident light intensity or the thickness of water.A fundamental understanding of light-induced asymmetric deformation of liquid surfaces will aid in expanding research in chiral-related disciplines.The use of asymmetric fluid deformation can be applied in the fabrication of functional and specific nanopatterned metasurfaces,opening new avenues for controlling fluid flow at the micro or nanoscale.