Strain mapping with optically coupled plasmonic particles embedded in a flexible substrate

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.

Angstrom-Scale Active Width Control of Nano Slits for Variable Plasmonic Cavity

Nanogap slits can operate as a plasmonic Fabry–Perot cavity in the visible and infrared ranges due to the gap plasmon with an increased wavenumber. Although the properties of gap plasmon are highly dependent on the gap width, active width tuning of the plasmonic cavity over the wafer length scale was barely realized. Recently, the fabrication of nanogap slits on a flexible substrate was demonstrated to show that the width can be adjusted by bending the flexible substrate. In this work, by conducting finite element method (FEM) simulation, we investigated the structural deformation of nanogap slit arrays on an outer bent polydimethylsiloxane (PDMS) substrate and the change of the optical properties. We found that the tensile deformation is concentrated in the vicinity of the gap bottom to widen the gap width proportionally to the substrate curvature. The width widening leads to resonance blueshift and field enhancement decrease. Displacement ratio ((width change)/(supporting stage translation)), which was identified to be proportional to the substrate thickness and slit period, is on the order of 10⁻⁵ enabling angstrom-scale width control. This low displacement ratio comparable to a mechanically controllable break junction highlights the great potential of nanogap slit structures on a flexible substrate, particularly in quantum plasmonics.

Mechanochromism in Structurally Colored Polymeric Materials

Mechanochromic effects in structurally colored materials are the result of deformation-induced changes to their ordered nanostructures. Polymeric materials which respond in this way to deformation offer an attractive combination of characteristics, including continuous strain sensing, high strain resolution, and a wide strain-sensing range. Such materials are potentially useful for a wide range of applications, which extend from pressure-sensing bandages to anti-counterfeiting devices. Focusing on the materials design aspects, recent developments in this field are summarized. The article starts with an overview of different approaches to achieve mechanochromic effects in structurally colored materials, before the physical principles governing the interaction of light with each of these materials types are summarized. Diverse methodologies to prepare these polymers are then discussed in detail, and where applicable, naturally occurring materials that inspired the design of artificial systems are discussed. The capabilities and limitations of structurally colored materials in reporting and visualizing mechanical deformation are examined from a general standpoint and also in more specific technological contexts. To conclude, current trends in the field are highlighted and possible future opportunities are identified.

Continuous reversible tuning of the gap size and plasmonic coupling of bow tie nanoantennas on flexible substrates

As a multifunctional device for sensing experiments and fundamental research, tailor-made plasmonic nanostructures with continuously tunable resonances are created by preparing bow tie-shaped nanostructures on a flexible substrate. The bow ties are fabricated by electron beam lithography on a chromium sacrificial layer and transferred to a polydimethylsiloxane (PDMS) substrate. The structures on PDMS are analyzed by reflection dark-field spectroscopy and scanning electron microscopy. Dark-field spectra of individual nano-antennas are obtained while the substrate is relaxed, and while strain is applied and the substrate is elastically stretched. Depending on the alignment of the bow ties relative to the direction of the strain, the deformation of the substrates leads to an increase or decrease of the nanostructure gaps, and therefore to a fully reversible decrease or increase of the antenna coupling, respectively. The continuous change in coupling is visible as a blue-shift in the resonance of the coupling mode for increasing gap widths, and a red-shift for decreasing gap widths. This configuration offers interesting perspectives for molecular transport and sensing investigations under variable coupling conditions as well as for tunable SERS substrates and optical strain sensor applications. In particular, very narrow gaps are within reach in the transversal configuration.

Monolayer Graphene Coupled to a Flexible Plasmonic Nanograting for Ultrasensitive Strain Monitoring

Plasmonically coupled graphene structures have shown great promise for sensing applications. Their complex and cumbersome fabrication, however, has prohibited their widespread application and limited their use to rigid, planar surfaces. Here, a plasmonic sensor based on gold nanowire arrays on an elastomer with an added graphene monolayer is introduced. The stretchable plasmonic nanostructures not only significantly enhance the Raman signal from graphene, but can also be used by themselves as a sensor platform for 2D strain sensing. These nanowire arrays on an elastomer are fabricated by template-stripping based nanotransfer printing, which enables a simple and fast production of stable nanogratings. The ultrasmooth surfaces of such transferred structures facilitate reliable large-area transfers of graphene monolayers. The resulting coupled graphene-nanograting construct exhibits ultrahigh sensitivity to applied strain, which can be detected by shifts in the plasmonic-enhanced Raman spectrum. Furthermore, this sensor enables the detection of adsorbed molecules on nonplanar surfaces through graphene-assisted surface enhanced Raman spectroscopy (SERS). The simple fabrication of the plasmonic nanowire array platform and the graphene-coupled devices have the potential to trigger widespread SERS applications and open up new opportunities for high-sensitivity strain sensing applications.

A flexible pressure responsive device based on the interaction between silver nanoparticles and an aluminum reflector

The unique and tunable optical properties of metal nanoparticles have attracted intense and sustained academic attention in recent years. In tandem with the demand for low-cost responsive materials, one particular topic of interest is the development of mechanically responsive device structures. This work describes the design, fabrication, and testing of a mechanically responsive plasmonic device structure that has been integrated onto a standard commercial plastic substrate. With a low actuation force and a visually perceivable color shift, this device would be attractive for applications requiring responsive features that can be activated by the human hand.

A flexible pressure sensitive colour changing device using plasmonic nanoparticles

We report on the fabrication of a flexible pressure sensitive device based on near-field coupling between silver nanoparticles and an underlying conductor. Visually apparent colour changes can be realized with minimal change in separation owing to the high fields localized to the particle's surface. The use of soft and compliant materials enables actuation of the device at low strain.

Single step synthesis and organization of gold colloids assisted by copolymer templates

We report here an original single-step process for the synthesis and self-organization of gold colloids by simply incorporating gold salts into a solution prepared using polystyrene (PS)-polymethylmethacrylate copolymer and thiolated PS with propylene glycol methyl ether acetate as a solvent. The spin-coating and annealing of this solution then allows the formation of PS domains. Depending on the polymer concentration of the as-prepared solution, there can be either one or several gold nanoparticles (Au NPs) per PS domain. For high concentrations of Au NPs in PS domains, the coupling between plasmonic NPs leads to the observation of a second peak in the optical extinction spectrum. Such a collective effect could be relevant for the development of optical strain sensors in the near future.

Controlled in situ nanoscale enhancement of gold nanowire arrays with plasmonics

The controlled in situ growth of ordered gold nanoparticles and nanowire arrays has been studied by optically tracking changes in the local surface plasmon resonance (LSPR) spectrum. A spectrometer and custom-programmed analysis software track changes in the LSPR spectrum. The peak position, peak height (i.e. extinction intensity) and peak width (e.g. radius of curvature) were tracked over time to quantify the dynamic growth of gold as soon as the system was exposed to a commercial gold enhancement solution. This enables the controlled dynamic growth of nano-objects without the necessity of characterizing the growth and aggregation kinetics of the gold enhancement solution. The result was the successful enhancement of their electrically conductive and plasmonic properties, as well as the controlled growth and transformation of line-patterned nanoparticles into conductive particle-based nanowires.

Strain effects on the SERS enhancements for spherical silver nanoparticles

We demonstrate in the present work through the utilization of classical Mie scattering theory in conjunction with a radiation damping and dynamic depolarization-corrected electrostatic approximation the significant effect that mechanical strain has on the optical properties of spherical silver nanoparticles. Through appropriate modifications of the bulk dielectric functions, we find that the application of tensile strain generates significant enhancements in the local electric field for the silver nanoparticles, leading to large SERS enhancements of more than 300% compared to bulk, unstrained nanoparticles when a 5% tensile strain is applied. While the strain-induced SERS enhancements are found to be strongest for nanoparticle diameters where radiation damping effects are minimized, we find that the surface plasmon resonance wavelengths are relatively unchanged by mechanical strain, and that the various measures of the far field optical efficiencies (absorption, scattering, extinction) can be enhanced by up to 150% through the application of tensile strain. The present findings indicate the opportunity to actively engineer and enhance the optical properties of silver nanoparticles through the application of mechanical deformation.

Electrochemistry on a localized surface plasmon resonance sensor

The optical signal of a localized surface plasmon resonance (LSPR)-based sensor combined with electrochemistry was investigated. Gold nanoparticles were immobilized on an indium tin oxide (ITO) substrate, which functioned as working electrode. Using cyclic voltammetry synchronized with LSPR sensing, surface reactions on gold were detected both electrically and optically. In the capacitive charging regime, optical signals linear to the applied potential were detected. Gold was found to be dissolved above the oxidation potential and partially redeposited during the reduction, which changed size and conformation of the gold nanoparticles. In kinetic measurements, slower potential establishment was observed at lower salt concentrations. Simulations by multiple multipole program (MMP) suggested the formation of a lossy layer by combination of charge depletion of gold and negative ion adsorption even below the reaction potential. We consider the results presented here of importance for any future sensors based on combined plasmonics and electrochemistry.

Optical sensing and determination of complex reflection coefficients of plasmonic structures using transmission interferometric plasmonic sensor

The combination of interferometry and plasmonic structure, which consists of gold nanoparticle layer, sputter coated silicon oxide spacer layer, and aluminum mirror layer, was studied in transmission mode for biosensing and refractive index sensing applications. Because of the interferometric nature of the system, the information of the reflection amplitude and phase of the plasmonic layer can be deduced from one spectrum. The modulation amplitude in the transmission spectrum, caused by the interference between the plasmonic particle layer and the mirror layer, increases upon the refractive index increase around the plasmonic particles due to their coherent backscattering property. Our proposed evaluation method requires only two light sources with different wavelengths for a stable self-referenced signal, which can be easily and precisely tuned by a transparent spacer layer thickness. Unlike the standard localized surface plasmon sensors, where a sharp resonance peak is essential, a broad band plasmon resonance is accepted in this method. This leads to large fabrication tolerance of the plasmonic structures. We investigated bulk and adsorption layer sensitivities both experimentally and by simulation. The highest sensitivity wavelength corresponded to the resonance of the plasmonic particles, but useful signals are produced in a much broader spectral range. Analysis of a single transmission spectrum allowed us to access the wavelength-dependent complex reflection coefficient of the plasmonic particle layer, which confirmed the reflection amplitude increase in the plasmonic particle layer upon molecular adsorption.