Interferometric Evanescent Wave Excitation of a Nanoantenna for Ultrasensitive Displacement and Phase Metrology

Infinitesimal optical singularity ruler for three-dimensional picometric metrology

Optical metrology with picometer-scale precision in three-dimensional space is of considerable importance in modern physics and state of the art technology, optical interference is an effective method, but techniques with rapid spatial variation have the potential to enhance measurement precision, which will be required as measurement dimensions decrease. Here, the concept of the vanishingly small optical phase singularity ruler is introduced. Inspired by the well-known plumb-line technique used to locate the centroid, an analogous singularity line technique is proposed to locate the optical singularity with a precision of ~4.5 pm (~λ/140000) in the transverse direction and ~24.2 pm (~λ/26000) in the longitudinal direction. This precisely positioned singularity can serve as a ruler to detect displacement signals with an accuracy approaching ~60 pm.

Polarization-Dependent Forces and Torques at Resonance in a Microfiber-Microcavity System

Spin-orbit interactions in evanescent fields have recently attracted significant interest. In particular, the transfer of the Belinfante spin momentum perpendicular to the propagation direction generates polarization-dependent lateral forces on particles. However, it is still elusive as to how the polarization-dependent resonances of large particles synergize with the incident light's helicity and resultant lateral forces. Here, we investigate these polarization-dependent phenomena in a microfiber-microcavity system where whispering-gallery-mode resonances exist. This system allows for an intuitive understanding and unification of the polarization-dependent forces. Contrary to previous studies, the induced lateral forces at resonance are not proportional to the helicity of incident light. Instead, polarization-dependent coupling phases and resonance phases generate extra helicity contributions. We propose a generalized law for optical lateral forces and find the existence of optical lateral forces even when the helicity of incident light is zero. Our work provides new insights into these polarization-dependent phenomena and an opportunity to engineer polarization-controlled resonant optomechanical systems.

Optical Fingerprint of Flat Substrate Surface and Marker-Free Lateral Displacement Detection with Angstrom-Level Precision

We report that flat substrates such as glass coverslips with surface roughness well below 0.5 nm feature notable speckle patterns when observed with high-sensitivity interference microscopy. We uncover that these speckle patterns unambiguously originate from the subnanometer surface undulations, and develop an intuitive model to illustrate how subnanometer nonresonant dielectric features could generate pronounced interference contrast in the far field. We introduce the concept of optical fingerprint for the deterministic speckle pattern associated with a particular substrate surface area and intentionally enhance the speckle amplitudes for potential applications. We demonstrate such optical fingerprints can be leveraged for reproducible position identification and marker-free lateral displacement detection with an experimental precision of 0.22 nm. The reproducible position identification allows us to detect new nanoscopic features developed during laborious processes performed outside of the microscope. The demonstrated capability for ultrasensitive displacement detection may find applications in the semiconductor industry and superresolution optical microscopy.

Observation of high-order imaginary Poynting momentum optomechanics in structured light

Optical forces on small particles are conventionally produced from the intensity or phase gradient of light. Harnessing the imaginary Poynting momentum (IPM) of light to generate nontrivial forces would unlock the full potential of optical manipulation techniques, but so far, it is demonstrated only for dipolar magnetoelectric particles. Here, we show that the IPM can be coupled to the force via the interplay of multipoles higher than dipoles, giving rise to high-order IPM forces that can be exerted on a large variety of Mie particles. The high-order concept and theory can be extended to the well-known optical gradient force and radiation pressure, and may inspire new insights for studying the interaction of matter with other classic waves, such as acoustics.

The imaginary Poynting momentum (IPM) of light has been captivated as an unusual origin of optical forces. However, the IPM force is predicted only for dipolar magnetoelectric particles that are hardly used in optical manipulation experiments. Here, we report a whole family of high-order IPM forces for not only magnetoelectric but also generic Mie particles, assisted with their excited higher multipoles within. Such optomechanical manifestations derive from a nonlocal contribution of the IPM to the optical force, which can be remarkable even when the incident IPM is small. We observe the high-order optomechanics in a structured light beam, which, despite carrying no angular momentum, is able to set normal microparticles into continuous rotation. Our results provide unambiguous evidence of the ponderomotive nature of the IPM, expand the classification of optical forces, and open new possibilities for levitated optomechanics and micromanipulations.

Ultrasensitive and long-range transverse displacement metrology with polarization-encoded metasurface

A long-range, high-precision, and compact transverse displacement metrology method is of crucial importance in many research areas. Recent schemes using optical antennas are limited in efficiency and the range of measurement due to the small size of the antenna. Here, we demonstrated the first prototype polarization-encoded metasurface for ultrasensitive long-range transverse displacement metrology. The transverse displacement of the metasurface is encoded into the polarization direction of the outgoing light via the Pancharatnam-Berry phase, which can be read out directly according to the Malus law. We experimentally demonstrate nanometer displacement resolution with the uncertainty on the order of 100 picometers for a large measurement range of 200 micrometers with the total area of the metasurface being within 900 micrometers by 900 micrometers. The measurement range can be extended further using a larger metasurface. Our work opens new avenues of applying metasurfaces in the field of ultrasensitive optical transverse displacement metrology.

Label-Free Optical Analysis of Biomolecules in Solid-State Nanopores: Toward Single-Molecule Protein Sequencing

Sequence identification of peptides and proteins is central to proteomics. Protein sequencing is mainly conducted by insensitive mass spectroscopy because proteins cannot be amplified, which hampers applications such as single-cell proteomics and precision medicine. The commercial success of portable nanopore sequencers for single DNA molecules has inspired extensive research and development of single-molecule techniques for protein sequencing. Among them, three challenges remain: (1) discrimination of the 20 amino acids as building blocks of proteins; (2) unfolding proteins; and (3) controlling the motion of proteins with nonuniformly charged sequences. In this context, the emergence of label-free optical analysis techniques for single amino acids and peptides by solid-state nanopores shows promise for addressing the first challenge. In this Perspective, we first discuss the current challenges of single-molecule fluorescence detection and nanopore resistive pulse sensing in a protein sequencing. Then, label-free optical methods are described to show how they address the single-amino-acid identification within single peptides. They include localized surface plasmon resonance detection and surface-enhanced Raman spectroscopy on plasmonic nanopores. Notably, we report new data to show the ability of plasmon-enhanced Raman scattering to record and discriminate the 20 amino acids at a single-molecule level. In addition, we discuss briefly the manipulation of molecule translocation and liquid flow in plasmonic nanopores for controlling molecule movement to allow high-resolution reading of protein sequences. We envision that a combination of Raman spectroscopy with plasmonic nanopores can succeed in single-molecule protein sequencing in a label-free way.

Directional imbalance of Bloch surface waves for ultrasensitive displacement metrology

Precise position sensing and nanoscale optical rulers are important in many applications in nanometrology, gravitational wave detection and quantum technologies. Several implementations of such nanoscale displacement sensors have been recently developed based on interferometers, nanoantennas, optical field singularities and optical skyrmions. Here, we propose a method for ultrasensitive displacement measurements based on the directional imbalance of the excitation of Bloch surface waves by an asymmetric double slit, which have low propagation loss and provide high detected intensity. The directionality of excitation changes dramatically with a sub-nanometric displacement of the illuminating Gaussian beam across the slit and can be used for displacement and refractive index metrology. We demonstrate a theoretical intensity ratio of the BSW excitation in opposite directions exceeding 890, which provides a displacement sensitivity of up to 2.888 nm^-1 with a resolution below 0.5 nm over a 100 nm linearity range. Experimentally, a directional intensity ratio more than 90 has been achieved, with a displacement sensitivity of 0.122 nm^-1 over a 300 nm linearity range, resulting in a resolution below 8 nm for a 600 nm illumination wavelength. The proposed facile configuration may have potential applications in nanometrology and super-resolution microscopy.

Mapping the near-field spin angular momenta in the structured surface plasmon polariton field

Optical spin angular momenta in a confined electromagnetic field exhibit a remarkable difference with their free space counterparts; in particular, the optical transverse spin that is locked with the energy propagating direction lays the foundation for many intriguing physical effects such as unidirectional transportation, quantum spin Hall effects, photonic Skyrmions, etc. In order to investigate the underlying physics behind the spin-orbit interactions as well as to develop the optical spin-based applications, it is crucial to uncover the spin texture in a confined field, yet it faces challenges due to their chiral and near-field vectorial features. Here, we propose a scanning imaging technique which can map the near-field distributions of the optical spin angular momenta with an achiral dielectric nanosphere. The spin angular momentum component normal to the interface can be uncovered experimentally by employing the proposed scanning imaging technique and the three-dimensional spin vector can be reconstructed theoretically with the experimental results. The experiment is demonstrated on the example of surface plasmon polaritons excited with various vector vortex beams under a tight-focusing configuration, where the spin-orbit interaction emerges clearly. The proposed method, which can be utilized to reconstruct the photonic Skyrmion and other photonic topological structures, is straightforward and of high precision, and hence it is expected to be valuable for the study of near-field spin optics and topological photonics.

Asymmetric Excitation of Surface Plasmon Polaritons via Paired Slot Antennas for Angstrom Displacement Sensing

Optical antennas enable efficient coupling between propagating light and bonding electromagnetic waves like surface plasmon polaritons (SPPs). Under the illumination of inhomogeneous optical fields, propagating SPPs mediated by multimode antennas could be spatially asymmetric and the asymmetry strongly depends on the position of the antennas relative to the illumination field. Here we develop such asymmetric excitation of SPPs through illuminating a pair of slot antennas with the (1,0) mode Hermite-Gaussian beam. The physical scenario of the interaction between the illumination optical field and the paired slot antennas are elaborated by full-wave electromagnetic simulations. We also carry out experiments to monitor the asymmetric SPPs propagation with a back-focal plane imaging technique. By retrieving the asymmetric intensity ratio of the SPP pattern in the back-focal plane image, lateral displacement of the antennas down to angstrom level is demonstrated.

Selective magnetic responses of silicon nanoparticles modulated by waveguide structures

High-refractive-index nanoparticles (NPs), such as silicon NPs, were considered as effective carriers in their response to a magnetic field at optical frequencies. Such NPs play an important role in many state-of-the-art technologies in nano-optics. Although the resonance properties of these NPs when varying their structural parameters have been studied intensely in the past few years, their interaction with the underlying substrate has seldom been discussed, in particular, when the substrate is a waveguide structure that significantly modulates the optical responses of the NPs. We proposed and studied a selective magnetic coupling system comprising a Si-NP on a metal-dielectric waveguide (MDW). The MDW structure supports either a transverse electric (TE) or a transverse magnetic (TM) mode that induces a large polarization dependence in the magnetic resonance. A new manifestation of the optical spin Hall effect was demonstrated in which a vertical rotating magnetic dipole excites a TE-type waveguide mode with a specific unidirectional emission. Making use of this polarization response, we developed a scanning imaging system that can selectively map the transverse or longitudinal magnetic field component of a focused beam depending on the type of MDW used in the system. This selective magnetic resonance coupling system is expected to be valuable for studying the fundamental interactions between the magnetic field and matter and for developing related nano-applications.

Azimuthal Imaginary Poynting Momentum Density

The momentum of light beams can possess azimuthal densities, circulating around the beam axis and inducing intriguing mechanical effects in local light-matter interaction. Belinfante's spin momentum loops in circularly polarized beams, while the canonical momentum spirals in helically phased beams. However, a similar behavior of their imaginary counterpart, the so-called imaginary Poynting momentum (IPM), has not yet emerged. The foremost purpose of the present work is to put forward the discovery of this IPM vortex. We show that a simple superposition of radially and azimuthally polarized beams can form an IPM of completely azimuthal density. Additionally, the azimuthal IPM density can exist with a donut beam-intensity distribution and even with a vanishing azimuthal component of all other momenta. This uncovers the existence of a new mechanical effect which broadens the area of optical micromanipulation by achieving optical rotation of isotropic spheres, in the absence of both spin and orbital angular momenta. Our findings enrich the local dynamic properties of electromagnetic fields, highlighting the rotational action of their IPM, and thus its mechanical effect on microparticles and nanoparticles.

Linear control of light scattering with multiple coherent waves excitation

With the wave interferometric approach, we study how extrinsically coherent waves excitation can dramatically alter the overall scattering properties, resulting in tailoring the energy assignment between radiation and dissipation, as well as filtering multipolar resonances. As an illustration, we consider cylindrical passive systems encountered by arbitrary configurations of incident waves with various illuminating directions, phases, and intensities. With formulas for dissipation and radiation powers, we demonstrate that a coherent superposition of incident waves extrinsically interferes with the targeted channels in a desirable way. Moreover, the interferometric results can be irrespective of inherent system properties such as size, material, and structure. Our approach paves a non-invasive solution to manipulate wave-obstacle interaction at will.