Flexible plasmonic modulators induced by the thermomechanical effect

Bottom-Up Metasurfaces for Biotechnological Applications

Metasurfaces are the 2D counterparts of metamaterials, and their development is accelerating rapidly in the past years. This progress enables the creation of devices capable of uniquely manipulating light, with applications ranging from optical communications to remote biosensing. Metasurfaces are engineered by rational assembly of subwavelength elements, defined as meta-atoms, giving rise to unique physical properties arising from the collective behavior of meta-atoms. These meta-atoms are typically organized using effective, reproducible, and precise nanofabrication methods that require a lot of effort to achieve scalable and cost-effective metasurfaces. In contrast, bottom-up methods based on colloidal nanoparticles (NPs) have developed in the last decade as a fascinating alternative for accelerating the technological spread of metasurfaces. The present review takes stock of recent advances in the fabrication and applications of hybrid metasurfaces prepared by bottom-up methods, resulting in disordered metasurfaces. In particular, metasurfaces prepared with plasmonic NPs are emphasized for their multifold applications, which are discussed from a biotechnology perspective. However, some examples of organized metasurfaces prepared by merging bottom-up and top-down approaches are also described. Finally, leveraging the historical disordered metasurface evolution, the review draws new perspectives for random metasurface design and applications.

Thermally and Electrically Regulated Plasmonic Devices Based on VO2-Covered Gold Nanoplate Arrays with SiO2 Interface Layer for Large Plasmon Shifts

Integrating metal nanoparticles with vanadium dioxide (VO2) is an effective means to realize active plasmonic regulation which has great application potential in optical devices that respond in real-time to external stimuli. However, the high temperature necessary for VO2 growth severely reshapes the metal nanoparticles, causing reduced refractive index (RI) sensitivity and degraded modulation performance. Herein, we construct a large-area dynamically tunable plasmonic system composed of a VO2-covered array of hexagonal gold nanoplates (AuNPLs). By introducing a SiO2 interface layer, the thermal tolerance of the AuNPLs is effectively improved, making the high RI sensitivity (∼368.3 nm/RIU at 855 nm) survive the subsequent VO2 deposition. Through tuning the localized surface plasmon resonance (LSPR) of the AuNPL array and the thickness of the VO2 film, the LSPR-related transmission dip can be tailored to the near-infrared region where VO2 shows a large two-phase RI contrast, a dip shift up to 272 nm is therefore realized upon VO2 phase transition. Furthermore, electro-optic modulation is demonstrated through electrically triggered VO2 partial phase transition which is accompanied by a gradually changed effective dielectric permittivity, and a continuous shift of the transmission dip from 1070 to 860 nm is achieved by varying the applied electrical current flowing through the film. This work provides a feasible route for controllably constructing stimuli-response optical devices with large wavelength modulation amplitude.

Tailor-Made Gold Nanomaterials for Applications in Soft Bioelectronics and Optoelectronics

In modern nanoscience and nanotechnology, gold nanomaterials are indispensable building blocks that have demonstrated a plethora of applications in catalysis, biology, bioelectronics, and optoelectronics. Gold nanomaterials possess many appealing material properties, such as facile control over their size/shape and surface functionality, intrinsic chemical inertness yet with high biocompatibility, adjustable localized surface plasmon resonances, tunable conductivity, wide electrochemical window, etc. Such material attributes have been recently utilized for designing and fabricating soft bioelectronics and optoelectronics. This motivates to give a comprehensive overview of this burgeoning field. The discussion of representative tailor-made gold nanomaterials, including gold nanocrystals, ultrathin gold nanowires, vertically aligned gold nanowires, hard template-assisted gold nanowires/gold nanotubes, bimetallic/trimetallic gold nanowires, gold nanomeshes, and gold nanosheets, is begun. This is followed by the description of various fabrication methodologies for state-of-the-art applications such as strain sensors, pressure sensors, electrochemical sensors, electrophysiological devices, energy-storage devices, energy-harvesting devices, optoelectronics, and others. Finally, the remaining challenges and opportunities are discussed.

Optical Printing of Silicon Nanoparticles as Strain-Driven Nanopixels

Silicon nanoparticles (Si NPs) supporting Mie resonances exhibit vivid structural colors on the subwavelength scale. For future wearable devices, next generation Si-based optical units need to be dynamic and stretchable for display, sensing, or signal processing required by human-computer interaction. Here, by utilizing the distance-sensitive electromagnetic coupling of Mie resonances, we maximize the active tuning effect of Si NP-based structures including dimers, oligomers, and NPs on WS2, which we called Si nanopixels. Through the optical tweezers-assisted printing of Si nanopixels, patterns can be formed on arbitrary flexible substrates. The strain-sensitive tuning of scattering spectra indicates their promising application on strain sensing of various stretchable substrates via a simple "spray and test" process. In the case of Si nanopixels on polydimethylsiloxane (PDMS), local strains around 1% can be detected by a scattering measurement. Moreover, we demonstrate that the scattering intensity variation of Si nanopixels printed on wrinkled tungsten disulfide (WS2) is pixel-dependent and wavelength-dependent. This property facilitates the application of information encryption, and we demonstrate that three barcodes can be independently encoded into the R, G, and B scattering channels through ternary logic represented by the strain-tuning effects of scattering.

High-performance plasmonic polymer modulators through mode hybridization and electro-thermomechanical effects

In this work, an electro-optical polymer modulator with double-layered gold nanostrips, a polymer nanograting, and a metal substrate is proposed and designed. Interestingly, mode hybridization between the Fabry-Pérot (F-P) and anti-bonding modes is formed, and strongly depends on the nanograting size, which can be controllably modulated by an injection current. The simulation and calculation results show that the temperature sensitivity and large structural sensitivity for the polymer modulator could remain constant during the current-tuning process, and a near-zero reflectance and a low linewidth of 13.8 nm in the red region corresponding to a high quality (Q) factor of 51 is achieved. In addition, a large redshift of 60.7 nm and a super-high modulation depth of 424 are obtained at only 8 µA.

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.

Photoinduced Tunable and Reconfigurable Electronic and Photonic Devices Using a Silk-Based Diffractive Optics Platform

A remarkable feature of modern electronic and photonic devices is the ability to maintain their geometric and physical properties in various circumstances for practical applications. However, there is an increasing demand for reconfigurable devices and systems that can be triggered or switched by external stimuli to change geometric, physical, and/or biochemical properties to meet specific requirements such as compact, lightweight, energy-efficient, and tunable features. Here, a set of phototunable and photoreconfigurable electronic and photonic devices composed of reconfigurable arithmetic circuits and programmable coding metamaterials at terahertz frequencies, empowered by a diffractive optics platform using naturally extracted silk proteins, is reported. These protein-based diffract optics are precisely manufactured into special microstructures for phase modulation of incident light and can be programmed to degrade at controlled rates. This allows spatial and temporal transformation of the incident light into desired intensity profiles to modulate the electrical properties of multiple photosensitive elements/components within the device simultaneously or discretely. Thus, the optoelectronic functionality of fabricated devices can be tailored to specific applications. Therefore, the approach makes it possible to efficiently fabricate tunable, reconfigurable transient electronic and photonic devices and systems.

Soft optical metamaterials

Optical metamaterials consist of artificially engineered structures exhibiting unprecedented optical properties beyond natural materials. Optical metamaterials offer many novel functionalities, such as super-resolution imaging, negative refraction and invisibility cloaking. However, most optical metamaterials are comprised of rigid materials that lack tunability and flexibility, which hinder their practical applications. This limitation can be overcome by integrating soft matters within the metamaterials or designing responsive metamaterial structures. In addition, soft metamaterials can be reconfigured via optical, electrical, thermal and mechanical stimuli, thus enabling new optical properties and functionalities. This paper reviews different types of soft and reconfigurable optical metamaterials and their fabrication methods, highlighting their exotic properties. Future directions to employ soft optical metamaterials in next-generation metamaterial devices are identified.

Deformable molecularly imprinted nanogels permit sensitivity-gain in plasmonic sensing

Soft molecularly imprinted nanogels (nanoMIPs), selective for human transferrin (HTR), were prepared via a template assisted synthesis. Owing to their soft matter, the nanoMIPs were observed to deform at binding to HTR: while no relevant changes were observed in the hydrodynamic sizes of HTR-free compared to HTR-loaded nanoMIPs, the HTR binding resulted in a significant increment of the nanoMIP stiffness, with the mean Young's modulus measured by AFM passing from 17 ± 6 kPa to 56 ± 18 kPa. When coupled to a plastic optical fibre (POF) plasmonic platform, the analyte-induced nanoMIP-deformations amplified the resonance shift, enabling to attain ultra-low sensitivities (LOD = 1.2 fM; linear dynamic range of concentrations from 1.2 fM to 1.8 pM). Therefore, soft molecularly imprinted nanogels that obey to analyte-induced deformation stand as a novel class of sensitivity-gain structures for plasmonic sensing.