Plasmonic nanostructures with spatial symmetry breaking have a variety of applications, from enhancing the enantioselective detection of chiral molecules to creating photonics devices such as circular polarizers. Compared to their molecular counterparts, engineered nanostructures exhibit orders of magnitude larger circular dichroism (CD) at optical frequencies. Although 3D nanostructures such as nanohelices have been reported with high CD at mid-IR frequencies, such high CDs have not yet been achieved at visible frequencies with decent efficiencies. Here, we propose a planar array of plasmonic ramp-shaped nanostructures with an azimuthally gradient depth that exhibits a giant CD and dissymmetry factor at visible frequencies. The structure is fabricated on a gold-coated glass slide using focused ion beam (FIB) with gradient intensity to induce the required gradient depth, hence, breaking symmetry. Optical experimental characterization in the reflection spectrum shows a CD up to 64% and a dissymmetry factor up to 1.13 at 678 nm, in a good agreement with numerical simulations. We envision our proposed structure together with the suggested fabrication method to inspire the design of novel optical devices such as nanoscale circular polarizers and a host of chiral molecules to improve enantioselectivity in the pharmaceutical industry.

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We introduce a microscopy technique that facilitates the prediction of spatial features of chirality of nanoscale samples by exploiting the photo-induced optical force exerted on an achiral tip in the vicinity of the test specimen. The tip-sample interactive system is illuminated by structured light to probe both the transverse and longitudinal (with respect to the beam propagation direction) components of the sample's magnetoelectric polarizability as the manifestation of its sense of handedness, i.e., chirality. We specifically prove that although circularly polarized waves are adequate to detect the transverse polarizability components of the sample, they are unable to probe the longitudinal component. To overcome this inadequacy and probe the longitudinal chirality, we propose a judiciously engineered combination of radially and azimuthally polarized beams, as optical vortices possessing pure longitudinal electric and magnetic field components along their vortex axis, respectively. The proposed technique may benefit branches of science like stereochemistry, biomedicine, physical and material science, and pharmaceutics. 1) LOCAL FIELDS AT THE TIP-APEX AND SAMPLE LOCATION, EXCITED BY AN ARPBWe prove that under ARPB excitation (a superposition of two coaxial beams: an APB and an RPB with proper phase shift  ) the local fields at the tip-apex and sample locations (both on the ARPB axis, see Fig. 1 of the paper) lack transverse components and we determine the longitudinal field components that include the near-field interaction. We consider the schematic of the problem in Fig. 1 of the manuscript and assume that the tip-apex and chiral sample are located at t z and s z , respectively, at a distance d from each other.

Recent work has shown that optical magnetism, generally considered a challenging light–matter interaction, can be significant at the nanoscale. In particular, the dielectric nanostructures that support magnetic Mie resonances are low-loss and versatile optical magnetic elements that can effectively manipulate the magnetic field of light. However, the narrow magnetic resonance band of dielectric Mie resonators is often overshadowed by the electric response, which prohibits the use of such nanoresonators as efficient magnetic nanoantennas. Here, we design and fabricate a silicon (Si) truncated cone magnetic Mie resonator at visible frequencies and excite the magnetic mode exclusively by a tightly focused azimuthally polarized beam. We use photoinduced force microscopy to experimentally characterize the local electric near-field distribution in the immediate vicinity of the Si truncated cone at the nanoscale and then create an analytical model of such structure that exhibits a matching electric field distribution. We use this model to interpret the PiFM measurement that visualizes the electric near-field profile of the Si truncated cone with a superior signal-to-noise ratio and infer the magnetic response of the Si truncated cone at the beam singularity. Finally, we perform a multipole analysis to quantitatively present the dominance of the magnetic dipole moment contribution compared to other multipole contributions into the total scattered power of the proposed structure. This work demonstrates the excellent efficiency and simplicity of our method of using Si truncated cone structure under APB illumination compared to other approaches to achieve dominant magnetic excitations.

We investigate second harmonic generation from anisotropic or longitudinal epsilon-near-zero materials. We find conversion efficiencies well above their isotropic counterparts thanks to additional field intensity enhancement provided by the anisotropy. At the same time anisotropic epsilon-near-zero materials are also less sensitive to material's losses compared to the isotropic ones. In turn, these improvements become pivotal for epsilon-near-zero materials that do not possess bulk dipole-allowed quadratic nonlinearities. We predict that second harmonic generation from a Dy:CdO/Si multilayer with longitudinal epsilon-near-zero properties can exceed the conversion efficiency of a homogeneous Dy:CdO slab of equivalent thickness by at least 20 times for almost any angle of incidence.