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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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 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.

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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.

Light-matter interactions enable the perception of specimen properties such as its shape and dimensions by measuring the subtle differences carried by an illuminating beam after interacting with the sample. However, major obstacles arise when the relevant properties of the specimen are weakly coupled to the incident beam, for example when measuring optical magnetism and chirality. To address this challenge we propose the idea of detecting such weakly-coupled properties of matter through the photo-induced force, aiming at developing photo-induced magnetic or chiral force microscopy. Here we review our pursuit consisting of the following steps: (1) Development of a theoretical blueprint of a magnetic nanoprobe to detect a magnetic dipole oscillating at an optical frequency when illuminated by an azimuthally polarized beam via the photo-induced magnetic force; (2) Conducting an experimental study using an azimuthally polarized beam to probe the near fields and axial magnetism of a Si disk magnetic nanoprobe, based on photo-induced force microscopy; (3) Extending the concept of force microscopy to probe chirality at the nanoscale, enabling enantiomeric detection of chiral molecules. Finally, we discuss difficulties and how they could be overcome, as well as our plans for future work.

We propose a novel high resolution microscopy technique for enantio-specific detection of chiral samples down to sub-100 nm size, based on force measurement. We delve into the differential photo-induced optical force F  exerted on an achiral probe in the vicinity of a chiral sample, when left and right circularly polarized beams separately excite the sample-probe interactive system. We analytically prove that F  is entangled with the enantiomer type of the sample enabling enantio-specific detection of chiral inclusions. Moreover, we demonstrate that F  is linearly dependent on both the chiral response of the sample and the electric response of the tip and is inversely related to the quartic power of probe-sample distance. We provide physical insight into the transfer of optical activity from the chiral sample to the achiral tip based on a rigorous analytical approach. We support our theoretical achievements by several numerical examples, highlighting the potential application of the derived analytic properties. Lastly, we demonstrate the sensitivity of our method to enantiospecify nanoscale chiral samples with chirality parameter in the order of 0.01, and discuss how this could be further improved.