Subwavelength Resolution Research Articles

Phase-assisted angular-multiplexing nanoprinting based on the Jacobi-Anger expansion.

Featuring with ultracompactness and subwavelength resolution, metasurface-assisted nanoprinting has been widely researched as an optical device for image display. It also provides a platform for information multiplexing, and a series of multiplexed works based on incident polarizations, operating wavelengths and observation angles have emerged. However, the angular-multiplexing nanoprinting is realized at the cost of image resolution reduction or the increase of fabrication difficulty, hindering its practical applications. Here, inspired by the Jacobi-Anger expansion, a phase-assisted design paradigm, called Bessel metasurface, was proposed for angular multiplexing nanoprinting. By elaborately designing the phase distribution of the Bessel metasurface, the target images can be encoded into the desired observation angles, reaching angular multiplexing. With the merits of ultracompactness and easy fabrication, we believe that our design strategy would be attractive in the real-world applications, including optical information storage, encryption/concealment, multifunctional switchable optical devices, and 3D stereoscopic displays, etc.

Terahertz non-destructive testing of power generator bars with a dielectric waveguide antenna

AbstractTerahertz technologies for non-destructive testing (NDT) are continuing to find their way into the industrial sector in the context of very specific inspection tasks. Part of this development is the capability to adapt terahertz systems in such a way that they can meet the sometimes harsh challenges and requirements of real-world industrial scenarios. One such scenario is the inspection of components with limited available measurement space. In particular, we show here the terahertz NDT inspection of the mica insulation of generator bars of turbogenerators at power plants, where an early on-site detection of defects and cracks in the insulation can be crucial, but where only few centimeters of space between adjacent bars are available. To address this problem, we have developed a measurement system combining a 100 GHz all-electronic terahertz transceiver with a low-loss dielectric waveguide antenna with 90 degree tip. We achieve sub-wavelength image resolution by scanning the waveguide antenna's tip over the surface of the generator bars in a near-field measurement setup. Employing a frequency-modulated continuous wave technique, we obtain depth-resolved, volumetric terahertz images of the objects under test. We discuss here the implementation and performance of the implemented measurement system for terahertz NDT inspection. keywords: terahertz, non-destructive testing, dielectric waveguides, frequency-modulated continuous wave, millimeter waves, power generators

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.

Digital and Gradient Refractive Index Planar Optics by Nanoimprinting Mesoporous Silicon

AbstractWe report the realization of digital and gradient index flat‐optics and planar waveguides using the ‘nanoimprinting refractive index’ (NIRI) technique applied to mesoporous silicon. This technique combines the distinct optical and mechanical metamaterial qualities of mesoporous silicon, including its widely tunable effective refractive index and ability to undergo plastic deformation with a near zero Poisson ratio. Nanoimprinting with premastered and reusable stamps containing analog or digital features enables the continuous or discontinuous patterning of refractive index with high contrast Δn ≥ 0.8 and subwavelength resolution. Using NIRI we experimentally demonstrate a wavefront shaping flat microlens array operating in the visible (405–635 nm) and mesoporous silicon and silica waveguides operating near 1310 nm. This study demonstrates the viability of patterning arbitrary refractive index distributions, n(x,y), on the surface of a chip while circumventing the challenges and limitations of top‐down lithographic techniques – thus opening a low‐cost and scalable approach for the realization of advanced planar optical technologies.

A Cost-Effective Approach for Achieving Subwavelength THz Imaging Using Photoinduced Coded-Apertures on Mesa-Array Structures

A novel and cost-effective approach for high-resolution terahertz (THz) imaging using photoinduced (PI) coded-apertures on micromachined mesa-array structures is reported. The mesa-array structures are designed to have subwavelength electrically isolated mesas for generation of high-fidelity photopatterns for THz imaging. An Si mesa-array prototype structure was successfully fabricated and tested. Measurements show that a modulation depth of ∼22 dB was obtained in the frequency range of 740–750 GHz under a light intensity of 11.7 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which agrees well with theoretical analysis and full-wave simulation. Initial imaging experiments were first performed using 64 pixels (over different areas) for validation and performance evaluation. More advanced imaging using quadrupled pixel number (256 pixels) was conducted in order to resolve subwavelength features. As a result, a wavelength-scale spatial resolution of 400 μm has been successfully demonstrated, and a subwavelength resolution as small as ∼250 μm (0.625 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> at 740 GHz) has been shown to be potentially achievable. The proposed PI coded-aperture imaging using mesa arrays is promising for developing real-time high-resolution THz imaging with simple system and low cost.

Subwavelength full-field terahertz ptychography via longitudinal shifts

We demonstrate subwavelength resolution, full-field, and lensless terahertz ptychography via longitudinal shifts of the sample. An extended illumination is adopted to realize full-field recording. Compared to a typical scheme, this implementation requires fewer diffraction patterns, which improves the imaging throughput. To suppress the incoherent infrared background, an optical chopper is introduced and a preprocessing algorithm based on digital phase-locked is proposed. By combining the ideas of multi-plane phase retrieval and ptychographic reconstruction, a reconstruction algorithm is proposed to simultaneously retrieve the object and the probe. Due to the lensless single-beam mode and full-field illumination, the sample is placed as close to the detector as possible, and the aperture of the whole sensor is utilized to achieve the highest numerical aperture. Note that the proposed approach could be applied to other wavebands as well. We apply this method to a biological material, a polymer sample, and a resolution target. The uneven illumination, as well as the diffraction effects associated with detector edges, is separated from the object reconstruction. The secondary veins of the dragonfly forewing, as small as ∼20 µm, are observed, and the lateral resolution achieves 88 µm (0.74λ).

Sub-wavelength patterned pulse laser lithography for efficient fabrication of large-area metasurfaces

Rigorously designed sub-micrometer structure arrays are widely used in metasurfaces for light modulation. One of the glaring restrictions is the unavailability of easily accessible fabrication methods to efficiently produce large-area and freely designed structure arrays with nanoscale resolution. We develop a patterned pulse laser lithography (PPLL) approach to create structure arrays with sub-wavelength feature resolution and periods from less than 1 μm to over 15 μm on large-area thin films with substrates under ambient conditions. Separated ultrafast laser pulses with patterned wavefront by quasi-binary phase masks rapidly create periodic ablated/modified structures by high-speed scanning. The gradient intensity boundary and circular polarization of the wavefront weaken diffraction and polarization-dependent asymmetricity effects during light propagation for high uniformity. Structural units of metasurfaces are obtained on metal and inorganic photoresist films, such as antennas, catenaries, and nanogratings. We demonstrate a large-area metasurface (10 × 10 mm2) revealing excellent infrared absorption (3–7 μm), which comprises 250,000 concentric rings and takes only 5 minutes to produce.

Superresolution Imaging Using a Tapered Bundle of High-Refractive-Index Optical Fibers

The spatial resolution of an optical fiber bundle is limited by the size of a mode propagating in an individual fiber, which can be as small as a single wavelength approximately $\ensuremath{\lambda}$ for conventional low-to-medium refractive-index optical fibers. High-refractive-index optical fiber bundles have a potential for the subwavelength-resolution imaging due to strong mode confinement in a fiber core, but they suffer from inconvenient image readout from the bundle output end, which is done by energy-inefficient near-field probes. To address this issue, this work introduces a tapered high-refractive-index terahertz (THz) optical fiber bundle. It comprises the subwavelength-diameter sapphire optical fibers, that are stacked tightly in the object plane to sample the near field with a subwavelength resolution. The fibers diverge from the object plane and, thus, stretch the captured near field for its further read out from the output bundle end using conventional diffraction-limited lens. Such a tapered fiber bundle was investigated numerically and then implemented experimentally at $0.33$ THz. The subwavelength resolution of this bundle was confirmed, and varies over its aperture and can be as high as $0.35\ensuremath{\lambda}$. The developed imaging principle allows the $\ensuremath{\simeq}0.5\ensuremath{\lambda}$ Abbe resolution limit of a free-space focusing to be overcome and almost any common diffraction-limited optics for the near-field applications to be adapted.

Inverse design in flat optics

Due to the sub-wavelength resolution, compact size, and ease of integration, flat optics has emerged as a pivotal player for applications in diffractive metalens, structured-light generation, holograms, nonlocal metasurfaces, etc. In the design of flat optics devices, the traditional forward design method demonstrates insurmountable limitations while inverse design has developed into an indispensable approach to focus designers only on high-level optical functionalities while handing over the task of finding conforming structures to machines. Here, we outline a cross-section of main advances of inverse design in flat optics: moving from basic theories and algorithms to further developments and diversified applications.

High resolution in non-destructive testing: A review

Since the beginning of the applications of non-destructive testing/evaluation (NDT/NDE) techniques, efforts have been made consistently to improve their detection sensitivity and resolution. In the present paper, factors governing lateral resolution in three major NDT techniques, viz., ultrasonic testing (UT), x-ray radiographic testing (XRT), and eddy current testing (ECT) are presented. Furthermore, a review of recent advances in these NDT techniques to reach the theoretically achievable resolution limit or even surpassing the same using alternate approaches is also discussed. For example, resolution in UT is theoretically limited to half the wavelength by the Rayleigh limit; however, subwavelength resolutions have been achieved through the applications of near field methods by capturing the evanescent field. On the other hand, the resolution achieved in XRT is primarily limited to half the source/focal spot size, which is many orders of magnitude larger than the wavelength. Over the years, the reduction in the focal spot from macro-focus to micro-focus and now to nano-focus has led to improvement in the resolution to a few nanometers, of course, in combination with suitable magnification required due to detectors with limited pixel size (a few μm to a few 10 s of μm). Similarly, innovations in electromagnetic/magnetic sensors have significantly improved the resolution achieved in ECT. Atomic force microscopy, metamaterials, and artificial neural network-based methods have been employed for obtaining high-resolution NDE images. At the end, authors' perspective toward possible directions for high-resolution NDT is presented.

Large-scale vivid metasurface color printing using advanced 12-in. immersion photolithography

Nanostructures exhibiting optical resonances (so-called nanoantennas) have strong potential for applications in color printing and filtering with sub-wavelength resolution. While small scale demonstrations of these systems are interesting as a proof-of-concept, their large scale and volume fabrication requires deeper analysis and further development for industrial adoption. Here, we evaluate the color quality produced by large size nanoantenna arrays fabricated on a 12-in. wafer using deep UV immersion photolithography and dry etching processes. The color reproduction and quality are analyzed in context of the CIE color diagram, showing that a vivid and vibrant color palette, almost fully covering the sRGB color space, can be obtained with this mass-manufacturing-ready fabrication process. The obtained results, thus, provide a solid foundation for the potential industrial adoption of this emerging technology and expose the limits and challenges of the process.

Spatio-temporal ultrafast pulse shaping at the femtosecond-nanometer scale.

Optical pulse shaping is a fundamental tool for coherent control of the light-matter interaction. While such control enables the measurement of ultrafast temporal dynamics, simultaneous spatiotemporal control is required for studying non-local ultrafast charge dynamics at the nanoscale. However, obtaining accurate spatial control at a sub-wavelength resolution with conventional optical elements poses significant difficulty. Here, we use the spatiotemporal coupling naturally arising in a spatial light modulator based pulse shaping apparatus to achieve accurate control with femto-nano spatiotemporal resolution. We experimentally demonstrate spatial steering at the sub-micron scale of second harmonic generation from nanostructures. In addition, we apply an absolute-value spectral phase to achieve controlled double pulses for nanoscale excitation. We introduce a novel, to the best of our knowledge, scheme for accurate tunable spatiotemporal pump-probe experiments. This method offers rich insight into materials with ultrafast transport phenomena at the femtosecond-nanometer regimes.

Nanoscale reaction monitoring using localized surface plasmon resonance scatterometry

Heterogeneous reactions are highly dependent upon the local structure and environment of the catalyst surface within a nanoscale. Among numerous techniques for monitoring heterogeneous reactions, dark-field microscopy offers reliable data regardless of specific reaction conditions. In addition, plasmonic nanoprobes provide high sensitivity in a sub-wavelength resolution due to localized surface plasmon resonances susceptible to the dielectric change of objects and surroundings. By clever reaction cell design and data analysis, nanoparticle signals can be parallelly analyzed under variable reaction conditions in a controlled manner. This technique effectively measures the heterogeneity of individual nanoparticles for reaction monitoring. A wide range of chemical and electrochemical reactions have been monitored in situ and in operando at a single-particle level in this way. The advancement of localized surface plasmon scatterometry with simulation techniques approaches sub-particle accuracy in a high temporal resolution up to microseconds. Combining other in situ spectroscopic methods would make dark-field scatterometry a versatile tool for various reaction monitoring and sensing applications.

Forward and Backward Multibeam Scanning Controlled by a Holographic Acoustic Metasurface

Acoustic metasurfaces have great potential in the field of beam-forming acoustic antennas due to their thin, two-dimensional, and miniature artificial materials that can freely control sound waves. Here, we propose an acoustic metasurface, with multibeam acoustic radiation serving as leaky-wave antennas used for acoustic beam steering. Based on the holographic principle, we pattern a sinusoidal modulated admittance surface, which is designed as a periodic arrangement of cylindrical holes with varying depth profiles. An omnidirectional sound generated from a tiny hole located at the center of the patterned plate creates acoustic surface waves on the modulated surface while simultaneously generating leaky-wave radiation. For multibeam steering, the holographic admittance surfaces are designed with multiple subregions of the metasurface, with each subregion emitting single-beam radiation in the desired direction. Furthermore, the holographic metasurfaces are programmed separately to have forward and backward leaky-wave radiation. In this study, we use three-dimensional additive fabrication to print acoustic holograms to craft the surface-admittance variation at subwavelength resolution. Experimental results on these printed holograms show multidirectional acoustic beam steering in close agreement with the theoretical analysis and numerical results. Forward and backward multibeam frequency scanning is also demonstrated by frequency variation. Thus, we expect that this planar surface can be used in applications such as acoustic communications, levitation, and imaging.

Seismic surface wave focal spot imaging: numerical resolution experiments

SUMMARY Numerical experiments of seismic wave propagation in a laterally homogeneous layered medium explore subsurface imaging at subwavelength distances for dense seismic arrays. We choose a time-reversal approach to simulate fundamental mode Rayleigh surface wavefields that are equivalent to the cross-correlation results of three-component ambient seismic field records. We demonstrate that the synthesized 2-D spatial autocorrelation fields in the time domain support local or so-called focal spot imaging. Systematic tests involving clean isotropic surface wavefields but also interfering body wave components and anisotropic incidence assess the accuracy of the phase velocity and dispersion estimates obtained from focal spot properties. The results suggest that data collected within half a wavelength around the origin is usually sufficient to constrain the used Bessel functions models. Generally, the cleaner the surface wavefield the smaller the fitting distances that can be used to accurately estimate the local Rayleigh wave speed. Using models based on isotropic surface wave propagation we find that phase velocity estimates from vertical–radial component data are less biased by P-wave energy compared to estimates obtained from vertical–vertical component data, that even strong anisotropic surface wave incidence yields phase velocity estimates with an accuracy of 1 per cent or better, and that dispersion can be studied in the presence of noise. Estimates using a model to resolve potential medium anisotropy are significantly biased by anisotropic surface wave incidence. The overall accurate results obtained from near-field measurements using isotropic medium assumptions imply that dense array seismic Rayleigh wave focal spot imaging can increase the depth sensitivity compared to ambient noise surface wave tomography. The analogy to elastography focal spot medical imaging implies that a high station density and clean surface wavefields support subwavelength resolution of lateral medium variations.

Photoacoustic microscopy with subwavelength resolution and enhanced spatial isotropy by using an aspheric acoustic mirror group

Photoacoustic microscopy (PAM) is an emerging biomedical imaging modality, combining good acoustic resolution in deep tissue and rich functional biomedical information. Spatial resolution and spatial isotropy are two of the most important indexes of a PAM. In this study, we propose a PAM scheme based on an aspheric acoustic mirror group to achieve the subwavelength resolution and enhance the spatial isotropy. The mirror group consists of an ellipsoid acoustic mirror and a parabolic acoustic mirror. The ellipsoid acoustic mirror expands the cone angle of signal collection, and the parabolic acoustic mirror reduces the cone angle of the PA beam. Their combination equivalently expands the numerical aperture of acoustic detection. Phantom experiments demonstrate that the proposed scheme can achieve the subwavelength resolution. Simultaneously, its spatial isotropy is improved from 48.2% to 96.7%. An in vivo mouse brain imaging experiment examines the biomedical practicability of the method. This method may be found wide potential applications in biomedicine.

Nanoscale Manipulating Silver Adatoms for Aqueous Plasmonic Electrochromic Devices

AbstractPlasmonic colors are attractive building blocks for flat panel displays due to their broad color gamut and unprecedented subwavelength resolution. The exploration of reversible silver (Ag) electrodeposition for switchable plasmonic colors is considered as a promising strategy toward dynamically reconfigurable color displays. To date, the current reversible Ag electrodeposition‐based electrochromic devices are energy‐inefficient as the platforms are realized in nonaqueous electrolytes that require high working voltage. However, the high working voltage in an aqueous electrolyte leads to uncontrolled nucleation growth of Ag particles, thus rendering its application for color displays impractical. Herein, the first demonstration of the manipulation of Ag adatoms for aqueous plasmonic electrochromic devices through underpotential deposition is presented. It is shown that the Ag nanoparticles underpotential deposition not only enables a reversible voltage‐activated wide range of dynamic plasmonic color change of 100 nm, but also facilitates size control of the grown Ag nanoparticles. These findings provide a favorable and novel platform for low energy‐consumption tunable photonic and nanoplasmonic devices, as well as a simple and reliable process for rapid, scalable, and green preparation of tunable plasmonic Ag nanoparticles.

Far-Field Acoustic Subwavelength Imaging with Blind Structured Illumination and Joint-Sparsity Reconstruction

The resolution of far-field imaging systems suffers from the diffraction limit. In this work, a blind structured illumination microscopy (blind-SIM) system that achieves subwavelength resolution in acoustic far-field imaging is developed. In blind-SIM, precise knowledge of the structured illumination (SI) patterns is not required. Instead, the acoustic SIs are generated by a randomly located acoustic diffraction grating with a subwavelength grating period. The spatial frequency mixing between the object and the SIs converts evanescent waves to propagating waves that can reach sensors in the far field. The image of the object is then reconstructed from multiple far-field measurements utilizing a joint-sparsity compressive sensing algorithm. Numerical simulations and experiments in kilohertz airborne sound are performed to validate the proposed system. A subwavelength imaging resolution of around 1/3 wavelength is achieved in experiments, and around 1/5 wavelength in simulations. The blind-SIM system largely reduces the imaging hardware complexity compared with existing acoustic subwavelength technologies that mostly rely on metamaterial lenses, time-reversal mirrors, or the precise localization of time-varying contrast agents. The combination of blind-SIM and the joint-sparsity compressive sensing algorithm for image reconstruction reduces the requirements for an object's sparsity level and enhances the system's robustness against noise. Our work has potential in improving the practicality of acoustic far-field subwavelength imaging in medical diagnoses, nondestructive testing, and underwater acoustic imaging and communications.

End-to-end design of metasurface-based complex-amplitude holograms by physics-driven deep neural networks

Abstract Holograms which reconstruct the transverse profile of light with complex-amplitude information have demonstrated more excellent performances with an improved signal-to-noise ratio compared with those containing amplitude-only and phase-only information. Metasurfaces have been widely utilized for complex-amplitude holograms owing to its capability of arbitrary light modulation at a subwavelength scale which conventional holographic devices cannot achieve. However, existing methods for metasurface-based complex-amplitude hologram design employ single back-diffraction propagation and rely on the artificial blocks which are able to independently and completely control both amplitude and phase. Here, we propose an unsupervised physics-driven deep neural network for the design of metasurface-based complex-amplitude holograms using artificial blocks with incomplete light modulation. This method integrates a neural network module with a forward physical propagation module and directly maps geometric parameters of the blocks to holographic images for end-to-end design. The perfect reconstruction of holographic images verified by numerical simulations has demonstrated that compared with the complete blocks, an efficient utilization, association and cooperation of the limited artificial blocks can achieve reconstruction performance as well. Furthermore, more restricted controls of the incident light are adopted for robustness test. The proposed method offers a real-time and robust way towards large-scale ideal holographic displays with subwavelength resolution.

Terahertz near-field microscopy based on an air-plasma dynamic aperture

Terahertz (THz) near-field microscopy retains the advantages of THz radiation and realizes sub-wavelength imaging, which enables applications in fundamental research and industrial fields. In most THz near-field microscopies, the sample surface must be approached by a THz detector or source, which restricts the sample choice. Here, a technique was developed based on an air-plasma dynamic aperture, where two mutually perpendicular air-plasmas overlapped to form a cross-filament above a sample surface that modulated an incident THz beam. THz imaging with quasi sub-wavelength resolution (approximately λ/2, where λ is the wavelength of the THz beam) was thus observed without approaching the sample with any devices. Damage to the sample by the air-plasmas was avoided. Near-field imaging of four different materials was achieved, including metallic, semiconductor, plastic, and greasy samples. The resolution characteristics of the near-field system were investigated with experiment and theory. The advantages of the technique are expected to accelerate the advancement of THz microscopy.