Novel nanotweezers for high efficiency optical trapping at submicrometer scale

Trapping, manipulation, and sensing of particles at micro- and nanoscale have obtained a fundamental role for several applications and particularly in biomedical environments, such as oncology, proteomics and biology. Among life science application domain, the ability to trap matter at microscale, such as cells and bacteria, up to single molecule, protein and virus of few nanometers, allows to investigate many diseases (e.g. cancer early diagnosis, motility of bacteria and cells activity), and also the effects of new drugs on a single pathogen like a virus, prion or virion, in addition to the analysis of proteins interaction. Optically-based techniques have been largely used to realize devices for trapping, typically called “optical tweezers”, due to their several advantages compared to other trapping methods. Optical tweezers provide traps with high spatial resolution, and particularly contact-free manipulation, so obtaining a relevant decrease of risks of damage and disruption of the trapped matter. Such properties are very important for trapping of living matter in biomedical applications, in order to avoid changes of surface properties or alterations in the behavior of the target object, so affecting the results of the experiments. A limitation of bulk optical tweezers is related to low values of optical forces that can be exerted, which poses a challenge for the manipulation of life matter at the nanoscale. In the last years, nanotweezers based on different technologies have been proposed in order to improve the trapping efficiency. Higher values of optical forces have been obtained with integrated optical devices, and particularly photonic and plasmonic resonant cavities have demonstrated to enable trapping of living matter at the nanoscale, so interesting a strong research effort and becoming attractive for commercial devices. In this work, an integrated photonic/plasmonic cavity, based on a bowtie nanoantenna vertically coupled to a 1D PhC cavity, has been proposed as hybrid nanotweezer, aiming at enhancing light-matter interaction and improving the trapping efficiency. The cavity provides ultra-high Q/V values with optical forces in the pN range for dielectric particles smaller than 100 nm. Such performance corresponds to high stability of the optical traps of beads at the nanoscale with long trapping time, potentially including also biological matter. A good matching between simulated and experimental results of the hybrid cavity performance have been obtained. The suitability of the proposed device as an efficient hybrid nanotweezer has been demonstrated experimentally by the stable trapping condition of Au beads up to 200 nm observed for several minutes with sub-mW optical powers.