Deep Subwavelength Resolution Research Articles

Multiple and spectrally robust photonic magic angles in reconfigurable α-MoO3 trilayers.

The emergence of a topological transition of the polaritonic dispersion in twisted bilayers of anisotropic van der Waals materials at a given twist angle-the photonic magic angle-results in the diffractionless propagation of polaritons with deep-subwavelength resolution. This type of propagation, generally referred to as canalization, holds promise for the control of light at the nanoscale. However, the existence of a single photonic magic angle hinders such control since the canalization direction in twisted bilayers is unique and fixed for each incident frequency. Here we overcome this limitation by demonstrating multiple spectrally robust photonic magic angles in reconfigurable twisted α-phase molybdenum trioxide (α-MoO3) trilayers. We show that canalization of polaritons can be programmed at will along any desired in-plane direction in a single device with broad spectral ranges. These findings open the door for nanophotonics applications where on-demand control is crucial, such as thermal management, nanoimaging or entanglement of quantum emitters.

Imaging the field inside nanophotonic accelerators

Controlling optical fields on the subwavelength scale is at the core of nanophotonics. Laser-driven nanophotonic particle accelerators promise a compact alternative to conventional radiofrequency-based accelerators. Efficient electron acceleration in nanophotonic devices critically depends on achieving nanometer control of the internal optical nearfield. However, these nearfields have so far been inaccessible due to the complexity of the devices and their geometrical constraints, hampering the design of future nanophotonic accelerators. Here we image the field distribution inside a nanophotonic accelerator, for which we developed a technique for frequency-tunable deep-subwavelength resolution of nearfields based on photon-induced nearfield electron-microscopy. Our experiments, complemented by 3D simulations, unveil surprising deviations in two leading nanophotonic accelerator designs, showing complex field distributions related to intricate 3D features in the device and its fabrication tolerances. We envision an extension of our method for full 3D field tomography, which is key for the future design of highly efficient nanophotonic devices.

Nanoscale Electric Field Probing in a Single Nanowire with Raman Spectroscopy and Elastic Strain.

In this work we investigate the Raman response of extremely strained gallium phosphide nanowires. We analyze new strain-induced spectral phenomena such as 2-fold and 3-fold phonon peak splitting which arise due to nontrivial internal electric field distribution coupled with inhomogeneous strain. We show that high bending strain acts as a probe allowing us to define the electric field distribution with deep subwavelength resolution using the corresponding changes of the Raman spectra. We investigate the nature of the localization with respect to nanowire diameter, excitation spot position, and light polarization, supporting the experiment with 3D numerical modeling. Based on our findings we propose a research tool allowing to precisely localize the electric field in a certain subwavelength region of the nanophotonic resonator.

High-Refractive-Index Chip with Periodically Fine-Tuning Gratings for Tunable Virtual-Wavevector Spatial Frequency Shift Universal Super-Resolution Imaging.

Continued research in fields such as materials science and biomedicine requires the development of a super‐resolution imaging technique with a large field of view (FOV) and deep subwavelength resolution that is compatible with both fluorescent and nonfluorescent samples. Existing on‐chip super‐resolution methods exclusively focus on either fluorescent or nonfluorescent imaging, and, as such, there is an urgent requirement for a more general technique that is capable of both modes of imaging. In this study, to realize labeled and label‐free super‐resolution imaging on a single scalable photonic chip, a universal super‐resolution imaging method based on the tunable virtual‐wavevector spatial frequency shift (TVSFS) principle is introduced. Using this principle, imaging resolution can be improved more than threefold over the diffraction limit of a linear optical system. Here, diffractive units are fabricated on the chip's surface to provide wavevector‐variable evanescent wave illumination, enabling tunable spatial frequency shifts in the Fourier space. A large FOV and resolutions of λ/4.7 and λ/7.1 were achieved for label‐free and fluorescently labeled samples using a gallium phosphide (GaP) chip. With its large FOV, compatibility with different imaging modes, and monolithic integration, the proposed TVSFS chip may advance fields such as cell engineering, precision industry inspection, and chemical research.

A topological lattice of plasmonic merons

Topology is an intrinsic property of the orbital symmetry and elemental spin–orbit interaction, but also, intriguingly, designed vectorial optical fields can break existing symmetries, to impose (dress) topology through coherent interactions with trivial materials. Through photonic spin–orbit interaction, light can transiently turn on topological interactions, such as chiral chemistry, or induce non-Abelian physics in matter. Employing electromagnetic simulations and ultrafast, time-resolved photoemission electron microscopy, we describe the geometric transformation of a normally incident plane wave circularly polarized light carrying a defined spin into surface plasmon polariton field carrying orbital angular momentum which converges into an array of plasmonic vortices with defined spin textures. Numerical simulations show how within each vortex domain, the photonic spin–orbit interaction molds the plasmonic orbital angular momentum into quantum chiral spin angular momentum textures resembling those of a magnetic meron quasiparticles. We experimentally examine the dynamics of such meron plasmonic spin texture lattice by recording the ultrafast nanofemto plasmonic field evolution with deep subwavelength resolution and sub-optical cycle time accuracy from which we extract the linear polarization, L-line singularity distribution, that defines the periodic lattice boundaries. Our results reveal how vectorial optical fields can impress their topologically nontrivial spin textures by coherent dressing or chiral excitations of matter.

Experimental demonstration of a three-dimensional acoustic hyperlens for super-resolution imaging

Acoustic hyperlenses have recently attracted much attention for promising applications in various fields. Yet the experimental realization of an acoustic hyperlens working in a real three-dimensional (3D) world is still lacking. Here, we theoretically propose and experimentally demonstrate a 3D acoustic hyperlens capable of producing super-resolution imaging for broadband airborne sound. A simple nonresonant metamaterial is designed as a practical implementation that simultaneously ensures tessellation of the curved surface and deep-subwavelength resolution. We analyze the dispersion relationship of the designed metamaterial that converts the evanescent waves into radially propagating modes based on positive extreme anisotropy. The effectiveness of our mechanism is demonstrated both numerically and experimentally via the production of 3D magnifying super-resolution imaging of small objects containing subwavelength patterns within a broad frequency range. We envision the realization of a 3D acoustic hyperlens to offer possibilities for the design of acoustic super-resolution imaging devices and their application in diverse scenarios ranging from medical ultrasound imaging to noninvasive evaluation.

Resolving deep sub-wavelength scattering of nanoscale sidewalls using parametric microscopy

The quantitative optical measurement of deep sub-wavelength features with sub-nanometer sensitivity addresses the measurement challenge in the semiconductor fabrication process. Optical scatterings from the sidewalls of patterned devices reveal abundant structural and material information. We demonstrated a parametric indirect microscopic imaging (PIMI) technique that enables recovery of the profile of wavelength-scale objects with deep sub-wavelength resolution, based on measuring and filtering the variations of far-field scattering intensities when the illumination was modulated. The finite-difference time-domain (FDTD) numerical simulation was performed, and the experimental results were compared with atomic force microscopic (AFM) images to verify the resolution improvement achieved with PIMI. This work may provide a new approach to exploring the detailed structure and material properties of sidewalls and edges in semiconductor-patterned devices with enhanced contrast and resolution, compared with using the conventional optical microscopy, while retaining its advantage of a wide field of view and relatively low cost.

Far-field transient absorption nanoscopy with sub-50 nm optical super-resolution

Nanoscopic imaging or characterizing is the mainstay of the development of advanced materials. Despite great progress in electronic and atomic force microscopies, label-free and far-field characterization of materials with deep sub-wavelength spatial resolution has long been highly desired. Herein, we demonstrate far-field super-resolution transient absorption (TA) imaging of two-dimensional material with a spatial resolution of sub-50 nm. By introducing a donut-shaped blue saturation laser, we effectively suppress the TA transition driven by near-infrared (NIR) pump–probe photons, and push the NIR-TA microscopy to sub-diffraction-limited resolution. Specifically, we demonstrate that our method can image the individual nano-grains in graphene with lateral resolution down to 36 nm. Further, we perform super-resolution TA imaging of nano-wrinkles in monolayer graphene, and the measured results are very consistent with the characterization by an atomic force microscope. This direct far-field optical nanoscopy holds great promise to achieve sub-20 nm spatial resolution and a few tens of femtoseconds temporal resolution upon further improvement and represents a paradigm shift in a broad range of hard and soft nanomaterial characterization.

Three-dimensional-subwavelength field localization, time reversal of sources, and infinitely asymptotic degeneracy in spherical structures

High-resolution field localization in three dimensions is one of the main challenges in optics and has immense importance in fields such as chemistry, biology, and medicine. Time-reversal symmetry of waves has been a fertile ground for applications such as generating a subwavelength focal spot and coherent-perfect absorption. However, in order to generate the time-reversed signal of a monochromatic source, discrete sources that are modulated according to the wave amplitude on a spherical envelope are required, rendering it applicable only in acoustics. Here we approach these challenges by introducing a spherical layer with a resonant permittivity, which naturally generates the spatially continuous time-reversed signal of an atomic and molecular multipole transition at the origin. We start by utilizing a spherical layer with a resonant TM $l=1$ permittivity situated in a uniform medium to generate a free-space-subwavelength focal spot at the origin. We remove the degeneracy of the eigenfunctions of the composite medium by situating a point current source (or polarization) directed parallel to the spherical layer, which generates a focal spot at the origin independently of its location. The free-space focal spot has a FWHM of $0.4\ensuremath{\lambda}$ in the lateral axes and $0.58\ensuremath{\lambda}$ in the axial axis, which is tighter by a factor of $\sqrt{2}$ in each dimension in excitation-collection mode, overcoming the $\ensuremath{\lambda}/2$ far-field resolution limit in three dimensions. We then explore two directions to localize electric field with deep-subwavelength resolution in three dimensions using this setup. Since the imaginary part of the eigenvalue is also realized in the physical parameter and the setup can be in an exact resonance, it can also open avenues in fields such as cavity QED, entanglement, and quantum information. In addition, we show that spherical structures exhibit a unique type of degeneracy in which an infinite number of eigenvalues asymptotically coalesce. This high degeneracy results in a variety of optical phenomena, such as strong scattering and enhancement of absorption and emission from an atom or molecule by orders of magnitude compared with a standard resonance.

Structured channel metamaterials for deep sub-wavelength resolution in guided ultrasonics

Experimental results on deep subwavelength resolution of defects are presented for the first time in the context of guided ultrasonic wave inspection of defects, using novel “structured channel” metamaterials. An Aluminum bar with side-drilled holes is used as a test sample, interrogated by the fundamental bar-guided symmetric mode. Simulations were conducted to optimize dimensional parameters of the metamaterial structure. Experiments using metamaterials fabricated accordingly demonstrate a resolution down to 1/72 of the operating wavelength, potentially bringing the resolution of guided wave inspection to the same range as that of bulk ultrasonics. This work has much promise for remote inspection in industry and biomedicine.

Ultrafast vector imaging of plasmonic skyrmion dynamics with deep subwavelength resolution.

Plasmonic skyrmions are an optical manifestation of topological defects in a continuous vector field. Identifying them requires characterization of the vector structure of the electromagnetic near field on thin metal films. Here we introduce time-resolved vector microscopy that creates movies of the electric field vectors of surface plasmons with subfemtosecond time steps and a 10-nanometer spatial scale. We image complete time sequences of propagating surface plasmons as well as plasmonic skyrmions, resolving all vector components of the electric field and their time dynamics, thus demonstrating dynamic spin-momentum coupling as well as the time-varying skyrmion number. The ability to image linear optical effects in the spin and phase structures of light in the single-nanometer range will allow for entirely novel microscopy and metrology applications.

Highly Uniform Plasmonic Interference Lithography Assisted by Hyperbolic Multilayer Graphene

Plasmonic interference lithography (PIL) is able to break the diffraction limit, realizing deep-subwavelength resolution in a large area. However, because the energy of surface plasmon polariton (SPP) wave is mainly confined on the surface of the metallic layer, the interference patterns in the photoresist layer often suffer from shallowness and nonuniformity. In this work, a PIL structure with highly uniform patterns is proposed, working in the wavelength of 193 nm. The structure consists of a silicon substrate and a photoresist sandwiched by two multilayer graphene films. Due to the hyperbolic property of the multilayer graphene, a slot SPP mode which is weakly dependent on the photoresist thickness is stimulated through volume plasmon polaritons in the hyperbolic multilayer graphene. A uniform periodic pattern with a half-pitch resolution of 29 nm (λ/6.7) maintains even the photoresist thickness that is larger than 80 nm. Our findings may provide a new method to realize robust PIL patterns.

Characterization of Deep Sub-wavelength Sized Horizontal Cracks Using Holey-Structured Metamaterials

Image resolution in classical wave applications is limited by the diffraction limit which corresponds to half the operating wavelength (λ). To achieve higher resolution, ways of overcoming the natural diffraction limits are of interest. In this paper, an experimental demonstration of deep sub-wavelength resolution in the ultrasonic regime using a metamaterial lens is presented. Metamaterial lenses effectively transfer the evanescent waves to the imaging plane, which carry the details of the sub-wavelength features. The successful transmission of the decaying evanescent waves which contain much larger wave vectors than the propagating waves enables to overcome the diffraction limit set by the operating wavelength. We report an imaging technique using optimized holey-structured metamaterial lens to characterize a horizontal crack of size λ/25 in a layered aluminium sample.

Plasmonic roller lithography

Photo roller lithography systems can generate patterns continuously over large areas by employing flexible photomasks on rotating quartz cylinders. In comparison, plasmonic lithography systems can reach deep sub-wavelength resolution utilizing evanescent waves carrying high spatial frequency components. In this work, we demonstrate a plasmonic roller system by integrating a quartz mechanical roller with a specially designed photomask based on plasmonic waveguide lithography. Deep sub-wavelength uniform patterns with high aspect ratios were printed continuously over a moving substrate. The plasmonic roller system may find practical applications in the large-scale production of electronic and photonic devices in a cost-effective way.

\u201cPlasmonics\u201d in free space: observation of giant wavevectors, vortices, and energy backflow in superoscillatory optical fields

Evanescent light can be localized at the nanoscale by resonant absorption in a plasmonic nanoparticle or taper or by transmission through a nanohole. However, a conventional lens cannot focus free-space light beyond half of the wavelength λ. Nevertheless, precisely tailored interference of multiple waves can form a hotspot in free space of an arbitrarily small size, which is known as superoscillation. Here, we report a new type of integrated metasurface interferometry that allows for the first time mapping of fields with a deep subwavelength resolution ~λ/100. The findings reveal that an electromagnetic field near the superoscillatory hotspot has many features similar to those found near resonant plasmonic nanoparticles or nanoholes: the hotspots are surrounded by nanoscale phase singularities and zones where the phase of the superoscillatory field changes more than tenfold faster than a free-propagating plane wave. Areas with high local wavevectors are pinned to phase vortices and zones of energy backflow (~λ/20 in size) that contribute to tightening of the main focal spot size beyond the Abbe–Rayleigh limit. Our observations reveal some analogy between plasmonic nanofocusing of evanescent waves and superoscillatory nanofocusing of free-space waves and prove the fundamental link between superoscillations and superfocusing, offering new opportunities for nanoscale metrology and imaging.

Near-field probing the magnetic field vector of visible light with a silicon nanoparticle probe and nanopolarimetry.

Magnetic light-matter interaction plays a crucial role in nanophysics, such as in photonic topological insulators and metamaterials. Recent advances in all-dielectric nanophotonics especially demand vectorial mapping of magnetic light at visible wavelengths. Here, we report that a novel functional nanoprobe decorated with a silicon nanoparticle predominantly senses both the vertical and lateral magnetic field, that is, the magnetic field vector, complementary to a metal nanoparticle probe detecting the local electric field vector. As a proof-of-principle experiment, we demonstrate the mapping of magnetic field vectors in a transverse electric (TE) evanescent standing wave by this probe in a scanning near-field optical microscope (SNOM) with nanopolarimetry. It is for the first time that the full magnetic field vector of visible light, whose frequency exceeds 550 THz, can be directly detected with deep subwavelength resolution. Such functional probe and nanopolarimetry may pave the way toward complete vectorial near-field characterization over the whole visible band for nano-optics and subwavelength optics.

Multifrequency excitation and detection scheme in apertureless scattering near-field scanning optical microscopy.

Near-field scanning optical microscopy has revolutionized the study of fundamental physics, as it is one of very few label-free optical noninvasive nanoscale-resolved imaging techniques. However, its resolution remains strongly limited by the poor discrimination of weak near-field optical signals from a far-field background. Here, we theoretically and experimentally demonstrate a multifrequency excitation and detection scheme in apertureless near-field optical microscopy that exceeds current state-of-the-art sensitivity and background suppression. We achieved a twofold enhancement in sensitivity and deep subwavelength resolution in optical measurements. This method offers rich control over experimental degrees of freedom, breaking the ground for noninterferometric complete retrieval of the near-field signal.

Self-Assembled, Nanostructured, Tunable Metamaterials via Spinodal Decomposition.

Self-assembly via nanoscale phase separation offers an elegant route to fabricate nanocomposites with physical properties unattainable in single-component systems. One important class of nanocomposites are optical metamaterials which exhibit exotic properties and lead to opportunities for agile control of light propagation. Such metamaterials are typically fabricated via expensive and hard-to-scale top-down processes requiring precise integration of dissimilar materials. In turn, there is a need for alternative, more efficient routes to fabricate large-scale metamaterials for practical applications with deep-subwavelength resolution. Here, we demonstrate a bottom-up approach to fabricate scalable nanostructured metamaterials via spinodal decomposition. To demonstrate the potential of such an approach, we leverage the innate spinodal decomposition of the VO2-TiO2 system, the metal-to-insulator transition in VO2, and thin-film epitaxy, to produce self-organized nanostructures with coherent interfaces and a structural unit cell down to 15 nm (tunable between horizontally and vertically aligned lamellae) wherein the iso-frequency surface is temperature-tunable from elliptic to hyperbolic dispersion producing metamaterial behavior. These results provide an efficient route for the fabrication of nanostructured metamaterials and other nanocomposites for desired functionalities.

Three-dimensional collimated self-accelerating beam through acoustic metascreen.

We report the generation of three-dimensional acoustic collimated self-accelerating beam in non-paraxial region with sourceless metascreen. Acoustic metascreen with deep subwavelength spatial resolution, composed of hybrid structures combining four Helmholtz resonators and a straight pipe, transmitting sound efficiently and shifting fully the local phase is evidenced. With an extra phase profile provided by the metascreen, the transmitted sound can be tuned to propagate along arbitrary caustic curvatures to form a focused spot. Due to the caustic nature, the formed beam possesses the capacities of bypassing obstacles and holding the self-healing feature, paving then a new way for wave manipulations and indicating various potential applications, especially in the fields of ultrasonic imaging, diagnosis and treatment.

Cathodoluminescence microscopy: Optical imaging and spectroscopy with deep-subwavelength resolution

Abstract