Materials for Dielectric Nanotweezers in the Near-Visible Region

Abstract

Nanotweezers made of dielectric nanoantennas have enabled optical trapping and manipulation of nanoscale objects. Commonly used dielectric materials with a high refractive index, notably silicon (Si), suffer from absorption at visible wavelengths and thus Joule heating of the structure and the trapped object. They are thus normally considered unsuitable for trapping applications in this wavelength range. In this work, dielectric materials with low absorption at 785 nm are studied to minimize Joule heating. We consider a bowtie configuration, which is simulated for four different dielectric materials and optimized for maximum electric field enhancement. The results show a significant electric field enhancement in the bowtie gap, which can act as a “hotspot” for optical trapping and enhancement of Raman scattering of a single nanosphere. We numerically investigate optical trapping of nanospheres and compare the results with an analytical expression. Three nanospheres with different refractive indices are used as examples. We further study the temperature increase and the thermally induced flow around the bowtie nanoantenna to understand if and how the trapping is perturbed by Joule heating. We find that nanoantennas made by Si on a Si substrate, although absorbing, can be used for trapping quantum dots (QDs) and polystyrene (PS) beads. The required input intensities for stable trapping are approximately 10 and 50 mW/μm2, for QDs and PS beads, respectively, inducing temperature increases of ∼1.4 K and ∼7 K, respectively. For transparent materials, the input intensity is the limiting factor, and gallium phosphide (GaP) requires the lowest input intensity due to its high refractive index. However, Si is a far more common and standard material and thus gives a significantly easier fabrication process.