Linear and nonlinear chiroptical response from individual 3D printed plasmonic and dielectric micro-helices.

Sub-wavelength chiral resonators formed from artificial structures exhibit exceedingly large chiroptical responses compared to those observed in natural media. Owing to resonant excitation, chiral near fields can be significantly enhanced for these resonators, holding great promise for developing enantioselective photonic components such as biochemical sensors based on circular dichroism (CD) and spin-dependent nonlinear imaging. In the present work, strong linear and nonlinear chiroptical responses (scattering CD > 0.15 and nonlinear differential CDs > 0.4) at visible and near infrared frequencies are reported for the first time for individual micrometer-scale plasmonic and dielectric helical structures. By leveraging dark-field spectroscopy and nonlinear optical microscopy, the circular-polarization-selective scattering behavior and nonlinear optical responses (e.g., second harmonic generation and two-photon photoluminescence) of 3D printed micro-helices with feature sizes comparable to the wavelength (total length is ∼5λ) are demonstrated. These micro-helices provide potential for readily accessible photonic platforms, facilitating an enantiomeric analysis of chiral materials. One such example is the opportunity to explore ultracompact photonic devices based on single, complex meta-atoms enabled by state-of-the-art 3D fabrication techniques.

Reconfigurable Positioning of Vertically-Oriented Nanowires Around Topographical Features in an AC Electric Field.

We report the effect of topographical features on gold nanowire assemblies in a vertically applied AC electric field. Nanowires 300 nm in diameter ×2.5 µm long, and coated with ∼30 nm silica shell, were assembled in aqueous solution between top and bottom electrodes, where the bottom electrode was patterned with cylindrical dielectric posts. Assemblies were monitored in real time using optical microscopy. Dielectrophoretic and electrohydrodynamic forces were manipulated through frequency and voltage variation, organizing nanowires parallel to the field lines, i.e., standing perpendicular to the substrate surface. Field gradients around the posts were simulated and assembly behavior was experimentally evaluated as a function of patterned feature diameter and spacing. The electric field gradient was highest around these topographic features, which resulted in accumulation of vertically oriented nanowires around the post perimeters when dielectrophoresis dominated (high AC frequency) or between the posts when electrohydrodynamics dominated (low AC frequency). This general type of reconfigurable assembly, coupled with judicious choice of nanowire and post materials/dimensions, could ultimately enable new types of optical materials capable of switching between two functional states by changing the applied field conditions.