Self-induced Back-action Trapping Research Articles

Enabling Self-Induced Back-Action Trapping of Gold Nanoparticles in Metamaterial Plasmonic Tweezers.

The pursuit for efficient nanoparticle trapping with low powers has led to optical tweezers technology moving from the conventional free-space configuration to advanced plasmonic systems. However, trapping nanoparticles smaller than 10 nm still remains a challenge even for plasmonic tweezers. Proper nanocavity design and excitation has given rise to the self-induced back-action (SIBA) effect offering enhanced trap stiffness with decreased laser power. In this work, we investigate the SIBA effect in metamaterial tweezers and its synergy with the exhibited Fano resonance. We demonstrate stable trapping of 20 nm gold particles with trap stiffnesses as high as 4.18 ± 0.2 (fN/nm)/(mW/μm2) and very low excitation intensity. Simulations reveal the existence of two different groups of hotspots on the plasmonic array. The two hotspots exhibit tunable trap stiffnesses, a unique feature that can allow for sorting of particles and biological molecules based on their characteristics.

Optimization of metallic nanoapertures at short-wave infrared wavelengths for self-induced back-action trapping.

This paper presents simulation results for double nanohole and inverted bowtie nanoapertures optimized to resonate in the short-wave infrared regime (1050 nm and 1550 nm). These geometries have shown great promise for trapping nanoparticles with applications in optical engineering, physics, and biology. Using a finite element analysis tool, we found that the outline length for inverted bowtie nanoapertures in a 100 nm thick gold film with a 20 nm gap dimension having an optimized transmission resonance for 1050 nm and 1550 nm optical wavelengths is 106.5 nm and 188.5 nm, respectively. With the same gap size, the radii of the circles for the double nanohole nanoapertures are 72 nm and 128 nm. The near-field enhancements of the two structures are almost the same, while the double nanohole geometries have a 20% larger full width at half-maximum than the inverted bowtie. Next, by studying the effect of changing the inner radii of the inverted bowtie corners, we found that the difference between 2 nm and 6 nm corner radii can blue-shift the optical resonance by up to 45 nm. As a result of not having any inner corners, the double nanohole structure requires less precise fabrication and therefore could potentially have a higher successful yield of nanoapertures during the manufacturing process. Lastly, we will show experimental results that confirm the optical resonance of the nanoapertures at 1550 nm. These results will enable better performance and signal-to-noise ratio in nanoaperture trapping for the short-wave infrared wavelength regime.

All-dielectric structure for trapping nanoparticles via light funneling and nanofocusing

We propose a dielectric structure which focuses the laser light well beyond the diffraction limit and thus considerably enhances the exerted optical trapping force upon dielectric nanoparticles. Although the structure supports a Fabry-Perot resonance, it actually acts as a nanoantenna in that the role of the resonance is to funnel the laser light into the structure. In comparison with the lens illuminating the structure, the proposed structure offers roughly a 10000-fold enhancement in the trapping force - part of this enhancement comes from an 80-fold enhancement in the field intensity, and the remaining comes from a 130-fold enhancement in the normalized gradient of the field intensity (viz. the gradient of the field intensity divided by the field intensity). Also, the proposed structure offers roughly a 100-fold enhancement in the depth of the trapping potential. It is noteworthy that 'self-induced back-action trapping' (SIBA), which has recently been the focus of interest in the context of optical resonators, does not take place in the proposed resonator. In this paper, we also point out some misconceptions about SIBA together with some hitherto unappreciated subtleties of the dipole approximation.

Self-induced back-action optical trapping in nanophotonic systems

Optical trapping is an indispensable tool in physics and the life sciences. However, there is a clear trade off between the size of a particle to be trapped, its spatial confinement, and the intensities required. This is due to the decrease in optical response of smaller particles and the diffraction limit that governs the spatial variation of optical fields. It is thus highly desirable to find techniques that surpass these bounds. Recently, a number of experiments using nanophotonic cavities have observed a qualitatively different trapping mechanism described as ‘self-induced back-action trapping’ (SIBA). In these systems, the particle motion couples to the resonance frequency of the cavity, which results in a strong interplay between the intra-cavity field intensity and the forces exerted. Here, we provide a theoretical description that for the first time captures the remarkable range of consequences. In particular, we show that SIBA can be exploited to yield dynamic reshaping of trap potentials, strongly sub-wavelength trap features, and significant reduction of intensities seen by the particle, which should have important implications for future trapping technologies.