Dielectric nanoantennas and metasurfaces for optical trapping

Abstract

Analysis of biological nanoparticles in medical sciences is very promising, as it can reveal groundbreaking information about disease mechanisms, potentially leading to innovative and more effective treatment strategies. The existing methods used for analyzing biological nanoparticles come with several limitations, involving extensive sample preparation, and giving limited information about the nanoparticles. Optical tweezers combined with Raman spectroscopy is an analytical technique that has opened new possibilities for chemical analysis of biological nanoparticles. Currently used Raman optical tweezers are restricted to capturing Raman spectra from a single or a few nanoparticles and a large number of measurements are necessary to obtain valid statistics, giving low throughput analysis. Thus, development of new techniques is necessary for high-throughput chemical analysis of biological nanoparticles. The presented thesis uses numerical simulation to investigate two types of dielectric nanostructures for on-chip optical trapping and Raman spectroscopy. The aim is to find a suitable structure for high throughput analysis of multiple extracellular vesicles (EVs), a type of biological nanoparticle released from various cells. The technique relies on using the near-visible laser to trap and excite Raman scattering from the EVs. First, a low-quality factor optical nanoantenna is investigated to find the most suitable material for optical trapping of EVs. The design parameters of the nanoantennas are optimized for maximum field enhancement. The optimized designs are then used for investigating optical trapping of various nanoparticles. The temperature increase around nanoantennas with absorbing dielectric materials and corresponding thermally induced flow are also presented. The nanoantennas can be used to trap quantum dots (QDs) and polystyrene (PS) beads up to a 40 nm diameter but are not found suitable for the trapping of EVs. Then, a silicon nitride metasurface with tilted bars is numerically investigated, which supports a high-quality factor quasi-bound state in the continuum (quasi-BIC). The field enhancement is very high at the quasi-BIC resonance. It gives a significant Raman enhancement at the excitation wavelength and can be used to trap EVs with a modest input power but the size of the EVs is limited to 70 nm in diameter. The influence of trapped EVs on the quasi-BIC and trapping potential is studied. Finally, a metasurface design is presented, which consists of two parallel bars and a cylindrical disk. This gives a larger tip-to-tip gap, which can be used to trap EVs with a diameter up to 200 nm. The influence of the ellipticity of the bars on fabrication tolerances is explored and found to decrease for lower ellipticity, i.e., more circular, bars.