Polarization Effect In Light Scattering Of Nanostructures And Application In Nano-Polarimetry

The light scattering of plasmonic nanostructures,on one hand,is often associated with strong and rich polarization effects,such as optical rotation,circular dichroism,and enhancement of the optical chiral density,which are commonly known as optical activity;on the other hand,spin-orbit interaction in light-matter interaction leads to strong spin-dependent scattering.For the former,we will explore the general rules for optical activity in light scattering of plasmonic nanostructures.For the latter,we will use it to measure the local spin density.For the first case,these effects have many important applications from building nanoscale optical modulators to characterizing the handedness of trace amount of biomolecules,a crucial technique in chemistry,life sciences and medicine.However,to date,most of works were based on numerical simulations of a limited number of geometries.There is still lack of a general theoretical framework for the optical activity in the light scattering of plasmonic nanostructures,particularly for the relation between the optical activity,plasmon resonances,and symmetry.On the contrary,in the field of molecular light scattering,the theoretical framework has been well established.Inspired by this,here we study the general properties of the optical activity for plasmonic nanostructures using the eigenmode approach.In the first part of our work,we establish a theoretical framework for studying the optical activity of plasmonic nanostructures,which allows us to describe the optical rotational strength,for each plasmon resonance mode explicitly.Using the theory,sum rule,is derived for the light scattering of plasmonic nanostructures.In addition,relations between optical activity and symmetry/chirality of plasmonic nanostructures are revealed.Finally,how symmetry breaking gives rise to optical activity is investigated by introducing dissymmetric parts to a symmetric achiral nanostructure.A linear dependence of the rotational strength on the size of the dissymmetric parts is found in the perturbation regime,providing a simple guideline for tailoring the optical activity of nanostructures.We believe that this work not only reveals the underlying physical rules in the light scattering of nanostructures but also can help researchers engineer the optical properties of chiral plasmonic nanostructures,which have many important applications in the fields of spectroscopy,imaging,and sensingThe second case involves quantitative measurements of the spatial distribution of intensity and polarization of highly confined light,which are crucial for imaging and sensing.One example is superchiral fields in the vicinity of nanostructures,which hold the promise for ultrasensitive detection of chirality of biomolecules,a crucial factor in chemistry,biology and medicine.To obtain the detailed spatial information of the polarization of light.various techniques have been developed in the last couple of decades,including single molecule fluorescence mapping,anisotropic scattering of nanoprobes and near-field scanning optical microscopy,to name a few.With those techniques,components of electric and magnetic fields along different axes can be retrieved with subwavelength resolutionDespite the successes,the measurement of the local spin density of light remains a challenge because there is lack of mechanism for distinguishing the two spin states Today,the most effective way for local spin density measurement relies on the spin-orbit interaction in light scattering,i.e.,the spin-dependent scattering of subwavelength nanostructures.For example,by utilizing a spherical Au nanoparticle,the transverse spin density of a tightly focused beam was mapped with subwavelength resolution.On the other hand,the reported spin-orbit interaction based techniques require a perfect spherical nanoparticle as a probe for data retrieval,a high NA objective for signal collection and imaging detector for data recording.These requirements make them inapplicable in many scenarios,e.g.,the measurement of spin density on the surface of photonic devices.To tackle this issue,we propose a new approach for local spin density detection,which does not require well-defined probe,high NA objective,or image detector.In the second work,we report a scattering-type nano-polarimeter for measuring the local polarization,particularly the local spin density of light.This nano-polarimeter is based on the spin-orbital coupling effect in the light scattering,which leads to spin-dependent scattering patterns,allowing us to obtain the local polarization information using the spatial information of the scattered light.In addition,we developed the dark-spot elimination technique,which greatly simplifies the data retrieval process and enables a direct measurement of the spin density without acquiring the phase information.Moreover,the scattering-type nano-polarimeter is not limited to the spin density detection,and can also be used for measuring any given polarization component.Finally,it can be combined with the scanning probe microscope,providing a powerful tool for mapping the polarization state of the near-field components on photonics nanostructures.