NSF Award
1809410
The ability of metal nanoparticles to confine light to a very small volume (down to a few tens of nanometer) has been developed into novel miniaturized optical and electronic devices. However, the high level of losses associated with the noble metals and heating have always been a challenge limiting the efficiency of optical devices. In this regard, there is a "new kid on the block", namely electrodynamic "anapole" mode (i.e. "without poles" in Greek), that can overcome these issues by minimizing the radiative loss. This proposal plans to explore anapole mode associated with high-index dielectric nanosphere (Silicon nanoparticles) that can act as a radiationless source and confine energy efficiently by minimizing the radiative loss. The knowledge gained from our research can potentially be translated into prototypes that can be developed into novel optical and photonic devices, such as nano-lasers, broadband photo-detectors, sensors, etc. Our research will also enhance the undergraduate education by providing undergraduate participation in cutting-edge experimental research, integration of research into the curriculum, and networking opportunities with external collaborators and scientists, which will help motivate our students to choose a STEM career path, including students from under-represented community. <br/><br/><br/>Resonant optical excitation of dielectric particles offers unique opportunities for future optical and nanophotonic devices because of their reduced dissipative losses and large resonant enhancement of both electric and magnetic near-fields. In this regard, the discovery of the electrodynamic "anapole mode" as a non-radiating source in high index dielectric materials provides a unique playground to realize new nanophotonic devices. Under specific conditions, the superposition of internal modes (magnetic and toroidal) of high-index dielectric nanostructures can generate non-radiating states, called "anapoles", that are free from radiative loss. Even though the study of non-radiating objects has been part of fundamental physics for a long time, the dynamic anapole corresponding to the time-varying oscillating charge-current distributions in the optical frequencies was only experimentally demonstrated in 2015. Since spherical geometry is not suitable for excitation of the anapole mode under plane wave illumination, excitation of anapole mode in the demonstrated structure relied on the design of a highly specialized structure (Si nanodisk). However, in spite of constructing the nanodisk for the specific anapole condition, the nanodisk was unable to produce an "ideal" anapole mode.<br/>Here, instead of specifically designed structures, we propose to excite the anapole mode in isotropic nanosphere. Since plane wave illumination is not suitable for anapole mode excitation in a nanosphere, we will exploit the polarization symmetry of cylindrical vector beam to excite "ideal" anapole mode in isotropic nanosphere. More specifically, we will use the radial electric field distribution and absence of magnetic field in the focal plane of the radially polarized cylindrical vector beam to excite the ideal anapole mode. Since the nature of the excitation would be responsible for generating the anapole mode, our approach would provide a simple, straightforward alternate path to excite anapole mode that has been predicted to give rise to enhanced nonlinear effects, nanolasers, ideal magnetic scattering, as well as extremely high Q-factor and near-field enhancements.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
