Fabricating 3D freestanding metamaterials on elastic substrates via the shadow metal-sputtering and plastic deformation.

3D metamaterials have gained considerable attention owing to their extraordinary optical properties and promising applications beyond natural materials. However, fabricating 3D metamaterials with high resolution and reliable controllability is still a significant challenge. Here, a novel approach to manufacturing various 3D freestanding plasmonic nanostructures on elastic substrates is demonstrated using the shadow metal-sputtering and plastic deformations. A critical step is constructing a freestanding "Γ-shape" gold structural array in poly(methyl methacrylate) (PMMA) hole array via the shadow metal-sputtering and following a multifilm transfer procedure. This "Γ-shape" structural array undergoes plastic deformation to form 3D freestanding metamaterials for removing the PMMA resist by the oxygen plasma. The approach allows accurate manipulations of the morphology, size, curvature, and bend orientation of 3D nanostructures. The spectral response of the 3D cylinder array was experimentally confirmed and understood by the simulations based on the finite element method (FEM). Moreover, the bulk refractive index (RI) sensitivity of up to 858 nm RIU-1 was achieved theoretically for this cylinder array. The proposed approach provides a new possibility to realize the fabrication of 3D freestanding plasmonic metamaterials with high resolution and compatible planar lithography procedures.

Tunable Fabry-Pérot Resonator with Dynamic Structural Color: A Visual and Ultrasensitive Hydrogen Sensor.

Hydrogen detection is crucial for the forthcoming hydrogen economy. Here, we present a visual, ultrasensitive, optical hydrogen sensor based on a tunable Fabry-Pérot (FP) resonator, which can fully release the volume expansion of palladium during hydrogenation and transfer this volume expansion into an optical signal. The FP resonator consists of a suspended polymethylmethacrylate/palladium (PMMA/Pd) bilayer on a gold (Au) square-hole array. The bottom of the gold square hole and hydrogen-sensitive PMMA/Pd bilayer form a dynamically tunable FP resonator. When hydrogen gas (H2) is loaded, the hydrogen-induced lateral expanding stress concavely deforms the suspended bilayer downward to the substrate, narrowing the metal-air-metal gap at the center of the hole, and finally leading to a spectral blue shift. Our experimental results show a giant spectral shift of 279 nm with a reflectance variation of 57% on exposure to 0.6% H2 mixed with air. Such an ultrahigh optical response results in a significant color change, enabling visual hydrogen detection. In addition, the sensor has a H2 detection limit down to 0.1% and good recyclability. These advantages indicate that the sensor has excellent potential for hydrogen sensing applications.

Integrated plasmonic biosensor on a vertical cavity surface emitting laser platform.

Plasmonic devices can modulate light beyond the diffraction limit and thus have unique advantages in realizing an ultracompact feature size. However, in most cases, external light coupling systems are needed, resulting in a prohibitively bulky footprint. In this paper, we propose an integrated plasmonic biosensor on a vertical cavity surface emitting laser (VCSEL) platform. The plasmonic resonant wavelength of the nanohole array was designed to match (detune) with the emission peak wavelength of the VCSEL before (after) binding the molecules, thus the refractive index that represents the concentration of the molecule could be measured by monitoring the light output intensity. It shows that high contrast with relative intensity difference of 98.8% can be achieved for molecular detection at conventional concentrations. The size of the device chip could be the same as a VCSEL chip with regular specification of hundreds of micrometers in length and width. These results suggest that the proposed integrated sensor device offers great potential in realistic applications.

Polarization-insensitive graphene photodetectors enhanced by a broadband metamaterial absorber.

Graphene, combined with plasmonic nanostructures, shows great promise for achieving desirable photodetection properties and functionalities. Here, we theoretically proposed and experimentally demonstrated a graphene photodetector based on the metamaterial absorber in the visible and near-infrared wavebands. The experimental results show that the metamaterial-based graphene photodetector (MGPD) has achieved up to 3751% of photocurrent enhancement relative to an antennasless graphene device at zero external bias. Furthermore, the polarization-independent of photoresponse has resulted from the polarization-insensitive absorption of symmetric square-ring antennas. Moreover, the spectral-dependent photocurrent enhancement, originated from the enhanced light-trapping effect, was experimentally confirmed and understood by the simulated electric field profiles. The design contributes to the development of high-performance graphene photodetectors and optoelectronic devices.

Highly Sensitive and Durable Sea-Urchin-Shaped Silver Nanoparticles Strain Sensors for Human-Activity Monitoring.

High-performance strain sensors, composed of various artificial sensing materials on/in stretchable substrates, show great promise for applications in flexible electronic devices. Here, we demonstrated a highly sensitive and durable strain sensor consisting of a ribbon of close-packed sea-urchin-shaped silver nanoparticles (SUSNs) sandwiched between two layers of poly(dimethylsiloxane) (PDMS). Each of SUSNs possesses high-density and spherically distributed sharp spines over the body, which promotes electron transduction and further improves signal detection. This SUSN-based sensor possesses a desirable integration of high sensitivity (a gauge factor of 60) and large stretchability (up to 25%) at tensile sensing, broadening its application in wearable devices. Moreover, it also shows fast response (48 ms), good reproducibility, and long-term stability (>2500 cycles at 20% strain). It can also be used to detect compressing (sensitivity up to 31.5) and folding-type bending deformations. The sensing mechanism, the resistance of the sensors varying as the deformation load, results from the inter-spine contacts change and the microcracks evolution caused by variation in the gap between SUSNs. The sensor's sensitivity at different degrees of strain was also achieved by controlling the width of the close-packed SUSNs ribbon. For practical demonstration, the SUSN-based sensors could be used as wearable devices for monitoring human activities ranging from subtle deformations to substantial movements.

High-Contrast Dynamic Reflecting System Based on Pneumatic Micro/Nanoscale Surface Morphing.

Cephalopods offer a fascinating dynamic reflecting system to create desired colors and patterns through contracting and releasing their soft skins in response to environmental stimuli. Inspired by this natural display strategy, we designed a novel dynamic reflecting system based on pneumatic micro/nanoscale surface morphing. This system consists of a thin metal skin/elastomer bilayer modulated by a microfluidic-based gas injector. Benefited from the "wrinkled-specular" transition of the metal's surface under a small pneumatic actuation (4 kPa), an unprecedented reflectance contrast of 93 for broad-band (500-750 nm) modulation is achieved. This remarkable response also has excellent cycle stability (>2500 times) and fast response time (∼0.2 s). These advantages enable a robust and ultrasensitive optical gas pressure sensor with a sensitivity of 178 kPa-1, which is 3-4 orders of magnitude higher than those of conventional optical gas pressure sensors based on either a Fabry-Pérot interferometer or a Mach-Zehnder interferometer. Moreover, as proof-of-concept applications, we also experimentally demonstrated a curvature-variable convex mirror and noniridescent dynamic display, suggesting that our pneumatically dynamic reflecting system will potentially broaden the applications in adaptive optical devices, sensors, and displays.

Flexible plasmonic modulators induced by the thermomechanical effect.

Reconfigurable plasmon-based flexible devices, composed of artificial plasmonic nanostructures on stretchable substrates, show great promise for dynamic functionalities such as tunability, switching and modulation of electromagnetic waves. Here, we theoretically proposed and experimentally demonstrated a simple and efficient flexible plasmonic modulator based on an array of gold nanostructures on a poly(dimethylsiloxane) (PDMS) substrate. Arising from the current-induced local Joule heat, the local expansion of the PDMS substrate widens the gap distances between the neighboring gold wires, which results in a spectral shift of the plasmon resonance. The experimental results show that the plasmon resonance has a blue-shift of 39 nm under a total power consumption of only 10.5 mW, which results in a high modulation depth of up to 30.5% for the modulator. Such a low power consumption can be ascribed to the small active area and excellent thermal isolation of the PDMS. The optical and thermomechanical responses were confirmed and understood by the electromagnetic and thermomechanical co-simulations based on the finite-difference time-domain and finite-element methods. This novel mechanism to manipulate light provides new opportunities for active optical components and integrated circuits.

Metal-Assisted Transfer Strategy for Construction of 2D and 3D Nanostructures on an Elastic Substrate.

Compared with conventional rigid devices, the elastic substrates integrated with functional components offer various advantages, such as flexibility, dynamic tunability, and biocompatibility. However, the reliable formations of 2D nanoparticles, nanogaps, and 3D nanostructures on elastic substrates are still challenging. The conventional transfer method plays an important role in the fabrication of microstructures on elastic substrates; however, it could not fabricate structures with feature size less than a few micrometers. In this article, we have developed a flexible technique based on the "metal-assisted transfer" strategy. The key concept is to introduce a metal film as an assistant layer between nanostructures and silicon substrates to help the fabrication of nanostructures which cannot be successfully transferred in the conventional transfer method. Various 2D nanostructures, which are difficult to achieve on elastic substrates, could be reliably defined using this approach. The desired gap distances and even sub-10 nm metal gaps between adjacent nanoparticles can be controllably achieved. Moreover, 3D nanostructures can be directly assembled from the prestrained 2D precursors based on the developed technique. Comparing with the previous reports, our fabrication method contains only a one-step transfer process without selective bonding or a second transfer process. Significantly, the 3D nanostructures presented here are 2 orders of magnitude smaller than the state-of-the-art mechanically assembled 3D structures in unit cell size. The proposed method may become a mainstream technology for the nano-optics and ultracompact optoelectronic devices due to its multifunctionalities and superior advantages in achieving tunable nanoparticles as well as 3D nanostructures.

Low-loss integrated electrical surface plasmon source with ultra-smooth metal film fabricated by polymethyl methacrylate 'bond and peel' method.

External light sources are mostly employed to functionalize the plasmonic components, resulting in a bulky footprint. Electrically driven integrated plasmonic devices, combining ultra-compact critical feature sizes with extremely high transmission speeds and low power consumption, can link plasmonics with the present-day electronic world. In an effort to achieve this prospect, suppressing the losses in the plasmonic devices becomes a pressing issue. In this work, we developed a novel polymethyl methacrylate 'bond and peel' method to fabricate metal films with sub-nanometer smooth surfaces on semiconductor wafers. Based on this method, we further fabricated a compact plasmonic source containing a metal-insulator-metal (MIM) waveguide with an ultra-smooth metal surface on a GaAs-based light-emitting diode wafer. An increase in propagation length of the SPP mode by a factor of 2.95 was achieved as compared with the conventional device containing a relatively rough metal surface. Numerical calculations further confirmed that the propagation length is comparable to the theoretical prediction on the MIM waveguide with perfectly smooth metal surfaces. This method facilitates low-loss and high-integration of electrically driven plasmonic devices, thus provides an immediate opportunity for the practical application of on-chip integrated plasmonic circuits.