Toward Practical, Subwavelength, Visible-Light Photolithography with Hyperlens

Three-in-One: Dye-Volatile Cocrystals Exhibiting Intensity-Dependent Photochromic, Photomechanical, and Photocarving Response

Cocrystallization of a cis-azobenzene dye with volatile molecules, such as pyrazine and dioxane, leads to materials that exhibit at least three different light-intensity-dependent responses upon irradiation with low-power visible light. The halogen-bond-driven assembly of the dye cis-(p-iodoperfluorophenyl)azobenzene with volatile halogen bond acceptors produces cocrystals whose light-induced behavior varies significantly depending on the intensity of the light applied. Low-intensity (<1 mW·cm-2) light irradiation leads to a color change associated with low levels of cis → trans isomerization. Irradiation at higher intensities (150 mW·mm-2) produces photomechanical bending, caused by more extensive isomerization of the dye. At still higher irradiation intensities (2.25 W·mm-2) the cocrystals undergo cold photocarving; i.e., they can be cut and written on with micrometer precision using laser light without a major thermal effect. Real-time Raman spectroscopy shows that this novel photochemical behavior differs from what would be expected from thermal energy input alone. Overall, this work introduces a rational blueprint, based on supramolecular chemistry in the solid state, for new types of crystalline light-responsive materials, which not only respond to being exposed to light but also change their response based on the light intensity.

Ultrasonic-Assisted Electrochemical Nanoimprint Lithography: Forcing Mass Transfer to Enhance the Localized Etching Rate of GaAs

Electrochemical nanoimprint lithography (ECNL) has emerged as a promising technique for fabricating three-dimensional micro/nano-structures (3D-MNSs) directly on semiconductor wafers. This technique is based on a localized corrosion reaction induced by the contact potential across the metal/semiconductor boundaries. The anodic etching of semiconductor and the cathodic reduction of electron acceptors occur at the metal/semiconductor/electrolyte interface and the Pt mold surface, respectively. However, the etching rate is limited by the mass transfer of species in the ultrathin electrolyte layer between the mold and the workpiece. To overcome this challenge, we introduce the ultrasonics effect into the ECNL process to facilitate the mass exchange between the ultrathin electrolyte layer and the bulk solution, thereby improving the imprinting efficiency. Experimental investigations demonstrate a positive linear relationship between the reciprocal of the area duty ratio of the mold and the imprinting efficiency. Furthermore, the introduction of ultrasonics improves the imprinting efficiency by approximately 80 %, irrespective of the area duty ratio. The enhanced imprinting efficiency enables the fabrication of 3D-MNSs with higher aspect ratios, resulting in a stronger light trapping effect. These results indicate the prospective applications of ECNL in semiconductor functional devices, such as photoelectric detection and photovoltaics.

Sub-10 nm radial resolution achieved by cascading a graded structure outside a spherical hyperlens

Due to the excellent ability to break the diffraction limit in the subwavelength range, metamaterial-based hyperlens has received extensive attention. Unfortunately, radial resolution of most current hyperlens is not high enough, which is a huge obstacle to the application in 3D super-resolution imaging. In this paper, we propose a theoretical solution to this issue by cascading a graded structure outside the conventional Ag-TiO₂ spherical hyperlens. The product of the thickness and the refractive index (RI) of the dielectric layer in the graded structure is fixed to 19.8 while RI increases linearly from 1.38 to 3.54 along the radial direction. By reducing the asymptote slope of the dispersion curve, the coupling of the wave vectors to the hyperlens is enhanced and thus radial resolution is significantly improved to 5 nm while ensuring that the focus is still detectable in the far-field. This design paves the way to high-performance hyperlens for 3D imaging and biosensing in the future.

Cold photo-carving of halogen-bonded co-crystals of a dye and a volatile co-former using visible light

The formation of co-crystals by the assembly of molecules with complementary molecular recognition functionalities is a popular strategy to design or improve a range of solid-state properties, including those relevant for pharmaceuticals, photo- or thermoresponsive materials and organic electronics. Here, we report halogen-bonded co-crystals of a fluorinated azobenzene derivative with a volatile component-either dioxane or pyrazine-that can be cut, carved or engraved with low-power visible light. This cold photo-carving process is enabled by the co-crystallization of a light-absorbing azo dye with a volatile component, which gives rise to materials that can be selectively disassembled with micrometre precision using low-power, non-burning laser irradiation or a commercial confocal microscope. The ability to shape co-crystals in three dimensions using laser powers of 0.5-20 mW-substantially lower than those used for metals, ceramics or polymers-is rationalized by photo-carving that targets the disruption of weak supramolecular interactions, rather than the covalent bonds or ionic structures targeted by conventional laser beam or focused ion beam machining processes.

Hipster microcarriers: exploring geometrical and topographical cues of non-spherical microcarriers in biomedical applications

Structure and organisation are key aspects of the native tissue environment, which ultimately condition cell fate via a myriad of processes, including the activation of mechanotransduction pathways. By modulating the formation of integrin-mediated adhesions and consequently impacting cell contractility, engineered geometrical and topographical cues may be introduced to activate downstream signalling and ultimately control cell morphology, proliferation, and differentiation. Microcarriers appear as attractive vehicles for cell-based tissue engineering strategies aiming to modulate this 3D environment, but also as vehicles for cell-free applications, given the ease in tuning their chemical and physical properties. In this review, geometry and topography are highlighted as two preponderant features in actively regulating interactions between cells and the extracellular matrix. While most studies focus on the 2D environment, we focus on how the incorporation of these strategies in 3D systems could be beneficial. The techniques applied to design 3D microcarriers with unique geometries and surface topographical cues are covered, as well as specific tissue engineering approaches employing these microcarriers. In fact, successfully achieving a functional histoarchitecture may depend on a combination of fine-tuned geometrically shaped microcarriers presenting intricately tailored topographical cues. Lastly, we pinpoint microcarrier geometry as a key player in cell-free biomaterial-based strategies, and its impact on drug release kinetics, the production of steerable microcarriers to target tumour cells, and as protein or antibody biosensors.

Ultrahigh-Resolution, Label-Free Hyperlens Imaging in the Mid-IR

The hyperbolic phonon polaritons supported in hexagonal boron nitride (hBN) with long scattering lifetimes are advantageous for applications such as super-resolution imaging via hyperlensing. Yet, hyperlens imaging is challenging for distinguishing individual and closely spaced objects and for correlating the complicated hyperlens fields with the structure of an unknown object underneath. Here, we make significant strides to overcome each of these challenges. First, we demonstrate that monoisotopic h¹¹BN provides significant improvements in spatial resolution, experimentally resolving structures as small as 44 nm and those with sub 25 nm spacings at 6.76 μm free-space wavelength. We also present an image reconstruction algorithm that provides a structurally accurate, visual representation of the embedded objects from the complex hyperlens field. Further, we offer additional insights into optimizing hyperlens performance on the basis of material properties, with an eye toward realizing far-field imaging modalities. Thus, our results significantly advance label-free, high-resolution, spectrally selective hyperlens imaging and image reconstruction methodologies.

Reconfigurable meta-radiator based on flexible mechanically controlled current distribution in three-dimensional space

In this paper, we provide an experimental proof-of-concept of this dynamic three-dimensional (3D) current manipulation through a 3D-printed reconfigurable meta-radiator with periodically slotted current elements. By utilizing the working frequency and the mechanical configuration comprehensively, the radiation pattern can be switched among 12 states. Inspired by maximum likelihood method in digital communications, a robustness-analysis method is proposed to evaluate the potential error ratio between ideal cases and practice. Our work provides a previously unidentified model for next-generation information distribution and terahertz-infrared wireless communications.

Conformal Self-Assembly of Nanospheres for Light-Enhanced Airtightness Monitoring and Room-Temperature Gas Sensing

The fabrication of conformal nanostructures on microarchitectures is of great significance for diverse applications. Here a facile and universal method was developed for conformal self-assembly of nanospheres on various substrates including convex bumps and concave holes. Hydrophobic microarchitectures could be transferred into superhydrophilic ones using plasma treatment due to the formation of numerous hydroxyl groups. Because of superhydrophilicity, the nanosphere suspension spread on the microarchitectures quickly and conformal self-assembly of nanospheres can be realized. Besides, the feature size of the conformal nanospheres on the substrates could be further regulated by plasma treatment. After transferring two-dimensional tungsten disulfide sheets onto the conformal nanospheres, the periodic nanosphere array was demonstrated to be able to enhance the light harvesting of WS2. Based on this, a light-enhanced room-temperature gas sensor with a fast recovery speed (<35 s) and low detecting limit (500 ppb) was achieved. Moreover, the WS2-covered nanospheres on the microarchitectures were very sensitive to the changes in air pressure due to the formation of suspended sheets on the convex bumps and concave holes. A sensitive photoelectronic pressure sensor that was capable of detecting the airtightness of vacuum devices was developed using the WS2-decorated hierarchical architectures. This work provides a simple method for the fabrication of conformal nanospheres on arbitrary substrates, which is promising for three-dimensional microfabrication of multifunctional hierarchical microarchitectures for diverse applications, such as biomimetic compound eyes, smart wetting surfaces and photonic crystals.

Influence of Nonlocality on Transmittance and Reflectance of Hyperbolic Metamaterials

In this paper we investigate transmittance and reflectance spectra of multilayer hyperbolic metamaterials in the presence of strong spatial dispersion. Our analysis revealed a number of intriguing optical phenomena, which cannot be predicted with the local response approximation, such as total reflectance for small angles of incidence or multiple transmittance peaks of resonant character (instead of the respective local counterparts, where almost complete transparency is predicted for small angles of incidence and the broad-angle transparency can be observed within a range of larger angles of incidence). We believe that the observed effects may serve as a working principle in a number of new potential applications, such as spatial filtering, biosensing, or beam steering.

Designing with Light: Advanced 2D, 3D, and 4D Materials

Recent achievements and future opportunities for the design of 2D, 3D, and 4D materials using photochemical reactions are summarized. Light is an attractive stimulus for material design due to its outstanding spatiotemporal control, and its ability to mediate rapid polymerization under moderate reaction temperatures. These features have been significantly enhanced by major advances in light generation/manipulation with light-emitting diodes and optical fiber technologies which now allows for a broad range of cost-effective fabrication protocols. This combination is driving the preparation of sophisticated 2D, 3D, and 4D materials at the nano-, micro-, and macrosize scales. Looking ahead, future challenges and opportunities that will significantly impact the field and help shape the future of light as a versatile and tunable design tool are highlighted.

Planar cascaded triangular hyperlens structures

Planar-ports hyperlens structures made of two or four cascaded triangular cuts of planar periodic structures are presented. The hyperlenses are capable of converting electromagnetic evanescent fields to propagating waves, featuring subwavelength resolution and image magnification. One of the two proposed structures features parallel input and output ports. The proposed structures improve the phase and amplitude unbalances, magnification, and resolution of previous planar hyperlenses. Compared to several previous cylindrical hyperlens structures, the proposed structures show competitive or better features. One of the best designs of the proposed structures offers a magnification of 3.86, a resolution of 45 nm, $\lambda /{8}$λ/8 at the free space wavelength of 365 nm, optical modulation, a measure of image contrast of 0.286, and amplitude unbalance, a measure of image quality of 0.08, the smallest among all previous structures. Detailed comparative data of performance are provided. Although the magnification of the proposed planar structures is somewhat smaller than some of the previous cylindrical ones, the performance of the proposed hyperlenses is better or competitive with respect to resolution and image quality.

Investigation of effective media applicability for ultrathin multilayer structures

Multilayer hyperbolic metamaterials (HMMs) are highly anisotropic media consisting of alternating metal and dielectric layers with their electromagnetic properties defined by the effective medium approximation (EMA). EMA is generally applied for a large number of subwavelength unit cells or periods of a multilayer HMM. However, in practice, the number of periods is limited. To the best of our knowledge, a comparison between rigorous theory, EMA and experiments to investigate the minimum number of layers that allow for the low error of EMA results has not yet been investigated. In this article, we compared the reflectance response of the effective anisotropic HMMs predicted by the scattering matrix method (SMM) and EMA with optical characterization data, having the unit cell twenty times smaller than the vacuum wavelength in the visible range. The fabricated HMM structures consist of up to ten periods of alternating 10 nm thick Au and Al₂O₃ layers deposited by sputtering and atomic layer deposition, respectively. The two deposition techniques enable us to achieve a high quality HMM with low roughness: the root mean square (RMS) is less than 1 nm. We showed that the multilayer structure behaves like an effective medium from the fourth period onwards as the EMA calculation and experimental results agree well having below 4% mean square standard deviation of reflectance (MSDR) for the wavelength range from 500 to 1750 nm with a wide incident angle range. These results could have an impact on the design and development of active metamaterials and their applications ranging from imaging to nonlinear optics and sensing.

Metamaterial Lensing Devices

In recent years, the development of metamaterials and metasurfaces has drawn great attention, enabling many important practical applications. Focusing and lensing components are of extreme importance because of their significant potential practical applications in biological imaging, display, and nanolithography fabrication. Metafocusing devices using ultrathin structures (also known as metasurfaces) with superlensing performance are key building blocks for developing integrated optical components with ultrasmall dimensions. In this article, we review the metamaterial superlensing devices working in transmission mode from the perfect lens to two-dimensional metasurfaces and present their working principles. Then we summarize important practical applications of metasurfaces, such as plasmonic lithography, holography, and imaging. Different typical designs and their focusing performance are also discussed in detail.

Ultrabroadband Three-Dimensional Printed Radial Perfectly Symmetric Gradient Honeycomb All-Dielectric Dual-Directional Lightweight Planar Luneburg Lens

An ultrabroadband all-dielectric planar Luneburg lens has been designed and fabricated in this study, which is in the form of a radial gradient lightweight honeycomb column. Because of the novel design of a radial symmetric honeycomb-like microstructure in the subwavelength dimension and the radial gradient configuration according to the refractive index distribution of Luneburg lens, the present lens can focus incident plane waves on the opposite side with high convergence, and its operating frequency range is rather broadband, spanning from 6 to 16 GHz. Besides, the all-dielectric honeycomb-like lens is lightweight with a mass density of 0.23 g/cm³, and its broadband transmittance is higher than the reported cases consisting of metallic metamaterial or gradient photonic crystal structures. A prototype of the lens is fabricated by using 3D printing techniques, on which the electric near-field distribution and far-field radiation pattern measurements have been carried out, and the aforementioned performances were demonstrated experimentally. It was also observed that for two point sources placed at the edge of the lens whose intersection angle with the center of the lens is 90°, the far-field radiation pattern was still kept highly directional, which means that the lens can generate two highly directional beams simultaneously, and is an efficient double input-double output device.

Nanoscale chiral surface relief of azo-polymers with nearfield OAM light

An optical vortex with orbital angular momentum (OAM) can be used to induce microscale chiral structures in various materials. Such chiral structures enable the generation of a nearfield vortex, i.e. nearfield OAM light on a sub-wavelength scale, thereby leading to further nanoscale mass-transport. We report on the formation of a nanoscale chiral surface relief in azo-polymers due to nearfield OAM light. The resulting nanoscale chiral relief exhibits a diameter of ca. 400 nm, which corresponds to less than 1/5-1/6th of the original chiral structure (ca. 2.1 µm). Such a nanoscale chiral surface relief is established by the simple irradiation of uniform visible plane-wave light with an intensity of <500 mW/cm².