Topological Structures of Energy Flow: Poynting Vector Skyrmions.

Topological properties of energy flow of light are fundamentally interesting and may introduce novel physical phenomena associated with directional light scattering and optical trapping. In this Letter, skyrmionlike structures formed by Poynting vectors are unveiled in the focal region of two pairs of counterpropagating cylindrical vector vortex beams in free space. The appearance of local phase singularities, and the distinct traveling and standing wave modes of different field components passing through the focal spot lead to a Néel-Bloch-Néel transition of Poynting vector skyrmion textures along the light propagating direction. By shaping the wave front of the incident beams with patterned amplitudes, the topological invariant of the Poynting vector skyrmions can be further tuned in a prospective area. This work expands the family of optical skyrmions and holds great potential in energy flow associated applications.

Ultrahigh precision laser nanoprinting based on defect-compensated digital holography for fast-fabricating optical metalenses.

The 3D structured light field manipulated by a digital-micromirror-device (DMD)-based digital hologram has demonstrated its superiority in fast-fabricating stereo nanostructures. However, this technique intrinsically suffers from defects of light intensity in generating modulated focal spots, which prevents from achieving high-precision micro/nanodevices. In this Letter, we have demonstrated a compensation approach based on adapting spatial voxel density for fabricating optical metalenses with ultrahigh precision. The modulated focal spot experiences intensity fluctuations of up to 3% by changing the spatial position, leading to a 20% variation of the structural dimension in fabrication. By altering the voxel density to improve the uniformity of the laser cumulative exposure dosage over the fabrication region, we achieved an increased dimensional uniformity from 94.4% to 97.6% in fabricated pillars. This approach enables fast fabrication of metalenses capable of sub-diffraction focusing of 0.44λ/NA with the increased mainlobe-sidelobe ratio from 1:0.34 to 1:0.14. A 6 × 5 supercritical lens array is fabricated within 2 min, paving a way for the fast fabrication of large-scale photonic devices.

Multifocal meta-fiber based on the fractional Talbot effect.

Multi-focusing of light is a crucial capability for photonic devices that can be effectively achieved by precisely modulating the phase delay on the incident wavefront. However, integrating functional structures into optical fibers for remote light focusing remains challenging due to the complex device design and limited fabrication approaches. Here, we present the design and fabrication of metalens array on the end-face of a tailored single-mode step-index fiber for focusing light field into closely packed focal spot array. The metalenses are configured based on the fractional Talbot effect and benefit a modular design capability. Light passing through the optical fiber can be focused into different focal planes. With a synergistic 3D laser nanoprinting technique based on two-photon polymerization, high-quality meta-fibers are demonstrated for focusing light parallelly with a uniform numerical aperture (NA) as high as approximately 0.77. This may facilitate various applications such as optical trapping, generation of sophisticated beam profiles, and boosting light coupling efficiencies.

Ultrasensitive Photothermal Switching with Resonant Silicon Metasurfaces at Visible Bands.

Dynamic access to quasi-bound states in the continuum (q-BICs) offers a highly desired platform for silicon-based active nanophotonic applications, while the prevailing tuning approaches by free carrier injections via an all-optical stimulus are yet limited to THz and infrared ranges and are less effective in visible bands. In this work, we present the realization of active manipulations on q-BICs for nanoscale optical switching in the visible by introducing a local index perturbation through a photothermal mechanism. The sharp q-BIC resonance exhibits an ultrasensitive susceptibility to the complex index perturbation, which can be flexibly fulfilled by optical heating of silicon. Consequently, a mild pump intensity of 1 MW/cm2 can yield a modification of the imaginary part of the refractive index of less than 0.05, which effectively suppresses the sharp q-BIC resonances and renders an active modulation depth of reflectance exceeding 80%. Our research might open up an enabling platform for ultrasensitive dynamic nanophotonic devices.

A Digitally-Controlled Integrated Circuit Solution for Tinnitus Treatment with Charge Balancing.

An integrated circuit specified for tinnitus treatment is described. This chip, realized using a 0.18um BCD high-voltage CMOS process, is capable of generating current stimulus with any wave shape directly into the inner-ear tissue without the need for off-chip control circuitry. Used as part of a multi-chip module that can be implanted into the inner ear, this core chip contains an 8-bit digital-to-analog converter, an amplitude control block, a novel high-voltage drive and charge balance circuit, a high-voltage level shifter, an SRAM, a ROM, and an on-chip central control unit. The chip can achieve ±0.1 mV charge-balance precision.

An optically fabricated gradient nanochannel array to access the translocation dynamics of T4-phage DNA through nanoconfinement.

It has been widely recognized that nanostructures in natural biological materials play important roles in regulating life machinery. Even though nanofabrication techniques such as two-photon polymerization (TPP) provide sub-100 nm fabrication resolution, it remains technologically challenging to produce 3D nanoscale features modeling the complexity in vivo. We herein demonstrate that a nanochannel array carrying different sizes and nanostructures with gradually transitioning dimensions can be easily produced on a slightly tilted nano-stage. Using the gradient nanochannel array, we systematically investigate the factors affecting the dynamics of DNA translocation through nanoconfinement, including the size of biomolecules and geometrical features of the physical environment, which resembles the selectivity of nanopores in the cell membrane. It is observed that T4-phage DNA shows distinctive conformational transition dynamics during translocation through nanochannels driven by electric field or flow, and the deformation energy required for DNA to enter the nanochannels depends on both chemical environmental conditions, i.e., the ionic strength regulating DNA persistence length and nanochannel dimension. In the electric field, DNA repeatedly gets stretched and compressed during its migration through the nanochannel, reflected by elevated fluctuation in extension, which is substantially greater than the thermal fluctuation. However, driven by flow, DNA remains stretched during translocation through nanochannels, and shows variances in extension of merely a few hundred nanometers. These results indicate that the optically fabricated gradient nanochannel array is a suitable platform for optimizing the experimental conditions for biomedical applications such as gene mapping, and verify that production of complex three dimensional (3D) nanostructures can be greatly simplified by including slight inclination during TPP fabrication.

Aberration-compensated supercritical lens for sub-diffractive focusing within 20° field of view.

The supercritical lens has shown a remarkable capability of achieving far-field sub-diffraction limited focusing through elaborating a modulated interference effect. Benefiting from the relative high energy utilization efficiency and weak sidelobe properties, the supercritical lens holds significant advantage in a series of application scenarios. However, all of the demonstrated supercritical lenses mainly work in the on-axis illumination condition, so the off-axis aberration effect will severely deteriorate its sub-diffraction limit focusing capability for the illuminating beam with an oblique angle. In this work, an aberration-compensated supercritical lens with single-layer configuration is proposed and experimentally demonstrated. Such a single-layer supercritical lens consists of multilevel phase configurations patterned with the two-photon polymerization lithography technique. The simulation and experimental recorded results show that the aberration-compensated supercritical lens with a numerical aperture value of 0.63 could achieve a far-field sub-diffraction limited focusing property within 20° field of view at a wavelength of λ = 633 nm. This monochromatic aberration-compensated supercritical lens with single-layer configuration indicates excellent potential in the development of laser scanning ultrahigh optical storage and label free super-resolution imaging.

Active Droplet Transport Induced by Moving Meniscus on a Slippery Magnetic Responsive Micropillar Array.

Intelligent droplet manipulation plays a crucial role in both scientific research and industrial technology. Inspired by nature, meniscus driving is an ingenious way to spontaneously transport droplets. However, the shortages of short-range transport and droplet coalescence limit its application. Here, an active droplet manipulation strategy based on the slippery magnetic responsive micropillar array (SMRMA) is reported. With the aid of a magnetic field, the micropillar array bends and induces the infusing oil to form a moving meniscus, which can attract nearby droplets and transport them for a long range. Significantly, clustered droplets on SMRMA can be isolated by micropillars, avoiding droplet coalescence. Moreover, through adjusting the arrangement of the micropillars of SMRMA, multi-functional droplet manipulation such as unidirectional droplet transport, multi-droplet transport, droplet mixing, and droplet screening can be achieved. This work provides a promising approach for intelligent droplet manipulation and unfolds broad application prospects in microfluidics, microchemical reaction, biomedical engineering, and other fields.

Gradient Nanoconfinement Facilitates Binding of Transcriptional Factor NF-κB to Histone- and Protamine-DNA Complexes.

Mechanically induced chromosome reorganization plays important roles in transcriptional regulation. However, the interplay between chromosome reorganization and transcription activities is complicated, such that it is difficult to decipher the regulatory effects of intranuclear geometrical cues. Here, we simplify the system by introducing DNA, packaging proteins (i.e., histone and protamine), and transcription factor NF-κB into a well-defined fluidic chip with changing spatical confinement ranging from 100 to 500 nm. It is uncovered that strong nanoconfinement suppresses higher-order folding of histone- and protamine-DNA complexes, the fracture of which exposes buried DNA segments and causes increased quantities of NF-κB binding to the DNA chain. Overall, these results reveal a pathway of how intranuclear geometrical cues alter the open/closed state of a DNA-protein complex and therefore affect transcription activities: i.e., NF-κB binding.

Encoding Coacervate Droplets with Paramagnetism for Dynamical Reconfigurability and Spatial Addressability.

It is an ongoing endeavor in chemistry and materials science to regulate coacervate droplets on a physiologically relevant spatiotemporal scale to ultimately match or even surpass living cells' precision, complexity, and functionality. Herein, we develop a magnetic strategy orthogonal to the thermal, pH, light, or chemical counterparts that are commonly employed by biotic or artificial systems; its successful implementation thus adds a missing piece to the current arsenal of manipulative methodologies. Specifically, we paramagnetize the otherwise diamagnetic coacervate droplets by cooperatively combining paramagnetic ingredients (including organic radicals, metal ions, and Fe3O4 nanoparticles) and coacervate ingredients to obtain "MagCoa" droplets. A simple model is derived theoretically to account for migration and division of MagCoa droplets in an uneven magnetic field. Experimentally, we produce an array of compartmentalized and monodispersed droplets using microfluidics and magnetically steer them with uniformity and synchronicity. We design and fabricate spatial magnetic modulators to engineer the landscape of a magnetic field that, in turn, directs the MagCoa droplets into predesigned patterns in a reconfigurable fashion. These programmable liquid patterns can be potentially extended to dynamic assembly and information encryption. We envision that the toolbox established here is of generality and multitudes to serve as a practical guide to control droplets magnetically.

Sub-100†nm pixel pitch via STED photolithography with a nanoprinting-at-expansion/employments-at-recovery strategy.

Featured with its extraordinary super-resolution capability, the advent of stimulated emission depletion (STED) lithography has allowed for vastly reduced minimum feature size of a single pixel down to the deep sub-diffraction scale so as to produce unprecedented nanofeatures. However, the anticipated sub-diffraction pixel pitch down below 100 nm remains out of reach due to redundant polymerization of adjacent exposures at a short distance, so called memory effect. In this work, a nanoprinting-at-expansion/employments-at-recovery strategy is applied in the dual-beam STED lithography technique to surmount the memory effect and break adjacent-exposure limit imposed on minimizing the pixel pitch. The implementation of a femtosecond laser at a wavelength of 532 nm, the same as the inhibition laser beam, working as the initiation laser beam, can drastically reduce the saturated inhibition laser intensity by 74% for abating redundant polymerization subjected to multiple exposures in realizing nanoscale pixel pitch. The adjacent-exposure zone can be separated by isotropically expanding an elastic PDMS substrate for further diminishing redundant polymerization. Applying stretching ratio of 30%, a minimum super-resolved nanodots pixel pitch of 96 nm was achieved with single-dot size of 34 nm on both planar and hierarchical substrate, which offers a record-close distance for printing adjacent pixels. With its nanometer discernibility, this method holds great promise for future versatile utilization in advanced nanoimprinting, high density data storage, etc.

Elliptical Supercritical Lens for Shaping Sub-Diffractive Transverse Optical Needle.

Supercritical lens can create a sub-diffraction-limited focal spot in the far field, providing a promising route for the realization of label-free super-resolution imaging through the point scanning mechanism. However, all of the reported supercritical lenses have circular shape configurations, and produce isotropic sub-diffraction-limited focal spots in the focal plane. Here, we propose and experientially demonstrate a sub-diffraction transverse optical needle by using an elliptical supercritical lens. Through breaking the circular symmetry and introducing ellipticity to the lens, a uniform sub-diffractive transverse optical needle with lateral length and width of 6λ/NA and 0.45λ/NA, respectively, was successfully created in the focal plane. Further, elliptical sector-shape cutting with an optimized apex angle of 60 degrees can lead to suppressed subsidiary focusing for improved uniformity and condensed field intensity of the transverse optical needle. The demonstration of sub-diffractive transverse optical needle with a high aspect ratio (length to width ratio) of 13:1 may find potential applications in line-scanning microscopy for video-rate label-free super-resolution imaging, and also enable advances in the fields from laser manufacturing to optical manipulation.

Meniscus-Induced Directional Self-Transport of Submerged Bubbles on a Slippery Oil-Infused Pillar Array with Height-Gradient.

Directional manipulation of submerged bubbles is fundamental for both theoretical research and industrial production. However, most current strategies are limited to the upward motion direction, complex surface topography, and additional apparatuses. Here, we report a meniscus-induced self-transport platform, namely, a slippery oil-infused pillar array with height-gradient (SOPAH) by combining femtosecond laser drilling and replica mold technology. Owing to the unbalanced capillary force and Laplace pressure difference, bubbles on SOPAH tend to spontaneously transport along the meniscus gradient toward a higher elevation. The self-transport performances of bubbles near the pillars depend on the complex meniscus shape. Significantly, to understand the underlying transport mechanism, the 3D meniscus profile is simulated by solving the Young-Laplace equation. It is found that the concave valleys formed between the adjacent pillars can change the gradient direction of the meniscus and lead to the varied transport performances. Finally, by taking advantage of a water electrolysis system, the assembled SOPAH serving as a bubble-collecting device is successfully deployed. This work should not only bring new insights into the meniscus-induced self-transport dynamics but also benefit potential applications in the field of intelligent bubble manipulation.

Subwavelength generation of orientation-unlimited energy flow in 4π microscopy.

Manipulation of light energy flow within the tight focus not only is important to the fundamental study of light-matter interactions but also underpins significant practical applications. However, the coupling between the electric and the magnetic fields of a focused light beam sets a fundamental barrier for independent control of these field components, restricting the focal energy flow primarily in the axial direction. In this paper, a 4π microscopic configuration is theoretically proposed to untangle the tight relation between the electric field and the magnetic field in a subwavelength-scale focal voxel. By independently altering the amplitudes of different field components in the focal region, energy flow with three-dimensionally unlimited orientation and ultra-high orientation purity (more than 90%) can be generated. This result expands the flexibility of energy flow manipulations and holds great potential in nanophotonics such as light scattering and optical force at subwavelength dimensions.

Laser nanoprinting of floating three-dimensional plasmonic color in pH-responsive hydrogel.

Recent demonstrations of metasurfaces present their great potential to implement flat and multifunctional optical elements, which are accomplished with the designs of planar optics and micro-/nano- fabrications. Integrating metasurfaces in three dimensions has manifested drastically increasing advantages in manipulating light fields by extending design freedom. However, fabricating three-dimensional metasurfaces remain a tough challenge due to the lack of stereo printing protocols. Herein, we demonstrate laser nanoprinting of floated silver nanoparticle array in transparent hydrogel films for 3D metasurface to achieve color patterning. It is found that spatially resolved nanoparticles can be produced through laser induced photoreduction of silver ions and robustly anchored to the gel backbones by a focused femtosecond laser beam within a pH-responsive smart hydrogel matrix. With the aid of expansion properties of the pH-responsive hydrogel, repetitive coloration of the patterned plasmonic nanoparticle array over a wide spectrum range is achieved via reversible regulation of nanoparticle spacing from 550 to 350 nm and vice versa. This approach allows broadband 3D color-regulation in nanoscale for applications in active spectral filtering, information encryption, security tagging and biological colorimetric sensing, etc.

Security-Enhanced 3D Data Encryption Using a Degradable pH-Responsive Hydrogel.

Based on degradable pH-responsive hydrogel, we report on an enhanced three-dimensional data encryption security technique in which a pH value is used for information manipulation. Featuring three types of states upon the pH value variation, namely, shrinkage, expansion and degradation, the hydrogel renders a limited pH value window as the "key" for information decryption. The pH-dependent shrinkage-to-expansion conversion of the hydrogel leads to a threshold pH value for retrieving the recorded data, whilst the degradability of the hydrogel, which can be tuned by adjusting the composition ratio of PEGDA/AAc, gives rise to a second threshold pH value for irreversibly sabotaging the retrieved data. Pre-doping silver ions in the hydrogel facilitates explicit recording and reading of binary data in forms of three-dimensional silver patterns through photoreduction and scattering, respectively, with a femtosecond laser. By accurately matching the vertical spacing of the encoded silver nanopatterns with the diffraction-limited focal depth of the decryption microscope, we can tune the pH value to encrypt and retrieve information recorded in layers and set a critical pH value to smash encoded information, which proves a highly secured 3D data encoding protocol. This strategy can effectively enrich data encryption techniques, vastly enhancing data security within unattained chemical dimensions.

Environmentally robust immersion supercritical lens with an invariable sub-diffraction-limited focal spot.

Planar metalenses provide an effective way to break the diffraction barrier in the far field. Their physical mechanism and applications have been intensively studied in the past decade. These investigations on sub-diffraction-limited light modulations have only been applied to specified single immersion environments; however, changing immersion environments can severely degrade their focusing performance, limiting their application potential. In this work, we propose and experimentally demonstrate an environmentally robust immersion supercritical lens (SCL) that can work in various immersion environments. The design of such a lens is based on the vectorial Rayleigh-Sommerfeld diffraction theory combined with a multi-objective optimization algorithm. The sub-diffraction-limited focusing effect has been experimentally demonstrated in commonly used media, including air, water, and oil, with refractive indices of 1.0, 1.33, and 1.51, respectively. Moreover, such a lens can maintain its effective numerical aperture at a fixed value, bringing a unique advantage in that the lateral size of the focal spots exhibits a similar value of ${{317}}\;{{\pm}}\;{{7}}\;{\rm{nm}}$ in all three media. Our demonstration provides the feasibility of SCLs in various application scenarios with multi-immersion environments, such as bioimaging, light trapping, and optical storage.

Ultra-sensitive nanometric flat laser prints for binocular stereoscopic image.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with tantalizing layer-dependent electronic and optical properties have emerged as a paradigm for integrated flat opto-electronic devices, but their widespread applications are hampered by challenges in deterministic fabrication with demanded shapes and thicknesses, as well as light field manipulation in such atomic-thick layers with negligible thicknesses compared to the wavelength. Here we demonstrate ultra-sensitive light field manipulation in full visible ranges based on MoS2 laser prints exfoliated with nanometric precisions. The nontrivial interfacial phase shifts stemming from the unique dispersion of MoS2 layers integrated on the metallic substrate empower an ultra-sensitive resonance manipulation up to 13.95 nm per MoS2 layer across the entire visible bands, which is up to one-order-of-magnitude larger than their counterparts. The interlayer van der Waals interactions and the anisotropic thermal conductivity of layered MoS2 films endow a laser exfoliation method for on-demand patterning MoS2 with atomic thickness precision and subwavelength feature sizes. With this, nanometric flat color prints and further amplitude-modulated diffractive components for binocular stereoscopic images can be realized in a facile and lithography-free fashion. Our results with demonstrated practicality unlock the potentials of, and pave the way for, widespread applications of emerging 2D flat optics.

Arbitrary polarization conversion dichroism metasurfaces for all-in-one full Poincaré sphere polarizers.

The control of polarization, an essential property of light, is of broad scientific and technological interest. Polarizers are indispensable optical elements for direct polarization generation. However, arbitrary polarization generation, except that of common linear and circular polarization, relies heavily on bulky optical components such as cascading linear polarizers and waveplates. Here, we present an effective strategy for designing all-in-one full Poincaré sphere polarizers based on perfect arbitrary polarization conversion dichroism and implement it in a monolayer all-dielectric metasurface. This strategy allows preferential transmission and conversion of one polarization state located at an arbitrary position on the Poincaré sphere to its handedness-flipped state while completely blocking its orthogonal state. In contrast to previous methods that were limited to only linear or circular polarization, our method manifests perfect dichroism of nearly 100% in theory and greater than 90% experimentally for arbitrary polarization states. By leveraging this attractive dichroism, our demonstration of the generation of polarization beams located at an arbitrary position on a Poincaré sphere directly from unpolarized light can substantially extend the scope of meta-optics and dramatically promote state-of-the-art nanophotonic devices.

π-phase modulated monolayer supercritical lens.

The emerging monolayer transition metal dichalcogenides have provided an unprecedented material platform for miniaturized opto-electronic devices with integrated functionalities. Although excitonic light-matter interactions associated with their direct bandgaps have received tremendous research efforts, wavefront engineering is less appreciated due to the suppressed phase accumulation effects resulting from the vanishingly small thicknesses. By introducing loss-assisted singular phase behaviour near the critical coupling point, we demonstrate that integration of monolayer MoS2 on a planar ZnO/Si substrate, approaching the physical thickness limit of the material, enables a π phase jump. Moreover, highly dispersive extinctions of MoS2 further empowers broadband phase regulation and enables binary phase-modulated supercritical lenses manifesting constant sub-diffraction-limited focal spots of 0.7 Airy units (AU) from the blue to yellow wavelength range. Our demonstrations downscaling optical elements to atomic thicknesses open new routes for ultra-compact opto-electronic systems harnessing two-dimensional semiconductor platforms with integrated functionalities.