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.

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.

A nanoscopy technique that can characterize light-matter interactions with ever increasing spatial resolution and signal-to-noise ratio (SNR) is desired for spectroscopy at molecular levels.Photoinduced force microscopy (PiFM) with Au-coated probe-tips has been demonstrated as an excellent solution for this purpose. However, its accuracy is limited by the asymmetric shape of the Au-coated tip resulting in tip-induced anisotropy. To overcome such deficiencies, we propose a Si tip-Au nanoparticle (NP) combination in PiFM. We map the near-field distribution of the Au NPs in various arrangements with an unprecedented SNR of up to 120, a more than 10-fold improvement compared to conventional optical near-field techniques, and a spatial resolution down to 5.8 nm, smaller than 1/100 of the wavelength, even surpassing the tip-curvature limitation. We also map the beam profile of an azimuthally polarized beam (APB) with an excellent symmetry. The proposed approach can lead to the promising single molecule spectroscopy.Recently the photoinduced force microscopy (PiFM) technique has been developed as a superior near-field optical imaging and spectroscopy technique with both high SNR and nanoscale spatial resolution based on a modified atomic force microscopy (AFM) system. 16 Compared to s-SNOM in which the excitation is in near field and the detection is in the far field, in PiFM both the excitation and detection take place in near field which effectively suppresses the background scattering photons from the far field. 17,18 As a result, PiFM has been widely used for stimulated Raman spectroscopy, 19,20 nanoscale mapping of tightly focused electromagnetic beams 21,22 and propagating surface plasmon polaritons, 23 enantioselectivity of chiral nanostructures, 24,25

Abstract:We demonstrate the measurement of laterally induced optical forces using an Atomic Force Microscope (AFM). The lateral electric field distribution between a gold coated AFM probe and a nano-aperture in a gold film is mapped by measuring the lateral optical force between the apex of the AFM probe and the nano-aperture. Torsional eigen-modes of an AFM cantilever probe were used to detect the laterally induced optical forces. We engineered the cantilever shape using a focused ion beam to enhance the torsional eigen-mode resonance. The measured lateral optical force agrees well with simulations. This technique can be extended to simultaneously detect both lateral and longitudinal optical forces at the nanoscale by using an AFM cantilever as a multi-channel detector. This will enable simultaneous Photon Induced Force Microscopy (PIFM) detection of molecular responses with different incident field polarizations. The technique can be implemented on both cantilever and tuning fork based AFM's. Lateral force AFM is a technique that is primarily used to differentiate nanoscale surface properties [1], [2].In Lateral force AFM, the frictional forces between the tip and sample creates a torsion of the cantilever which in turn is a function of the surface properties, and leads to chemical force microscopy [3], [4]. In this letter we demonstrate the detection of lateral optical forces using the torsion mode of cantilever thereby enhancing the capability of lateral force AFM to detect optical forces at the nanoscale. Photon Induced Force Microscopy (PIFM) is a promising new technique to study linear and non-linear optical properties measured only using photon induced forces [5] - [9]. PIFM uses an Atomic Force Microscope (AFM) to measure the optical forces between an optically induced dipole in the sample under measurement and another optically induced dipole formed at the tip of the gold coated AFM probe. Previously, PIFM was introduced to detect and image linear molecular resonances at nanometer level [5], [7], [8] and perform non-linear imaging and spectroscopy at the nanoscale [6], [9] as well as time-resolved imaging of non-linear optical properties [9] of molecules. Indeed, PIFM has been used to image molecular resonances over a wide range of wavelengths from the visible to mid-IR wavelength regimes [10]. In addition, optical forces between a gold coated AFM tip and its image on a glass substrate was used to image the focal field distributions of tightly focused laser beams with different polarizations [7]. The response of the tip to different polarizations was used to estimate the aspect ratio of the AFM probe tip making it a useful technique to estimate the quality of probes for sensitive experiments such as Tip Enhanced Raman Spectroscopy (TERS) [7]. The previous works used the AFM in the attractive mode to measure the component of optical force, along the tip axis.In this paper, we demonstrate the use of PIFM to measure the lateral optical force (perpendicular to the tip axis) between a gold coated AFM probe ...