Hybrid Metrology for Nanostructured Optical Metasurfaces.

Metasurfaces have garnered increasing research interest in recent years due to their remarkable advantages, such as efficient miniaturization and novel functionalities compared to traditional optical elements such as lenses and filters. These advantages have facilitated their rapid commercial deployment. Recent advancements in nanofabrication have enabled the reduction of optical metasurface dimensions to the nanometer scale, expanding their capabilities to cover visible wavelengths. However, the pursuit of large-scale manufacturing of metasurfaces with customizable functions presents challenges in controlling the dimensions and composition of the constituent dielectric materials. To address these challenges, the combination of block copolymer (BCP) self-assembly and sequential infiltration synthesis (SIS), offers an alternative for fabrication of high-resolution dielectric nanostructures with tailored composition and optical functionalities. However, the absence of metrological techniques capable of providing precise and reliable characterization of the refractive index of dielectric nanostructures persists. This study introduces a hybrid metrology strategy that integrates complementary synchrotron-based traceable X-ray techniques to achieve comprehensive material characterization for the determination of the refractive index on the nanoscale. To establish correlations between material functionality and their underlying chemical, compositional and dimensional properties, TiO2 nanostructures model systems were fabricated by SIS of BCPs. The results from synchrotron-based analyses were integrated into physical models, serving as a validation scheme for laboratory-scale measurements to determine effective refractive indices of the nanoscale dielectric materials.

Developing Quantitative Nondestructive Characterization of Nanomaterials: A Case Study on Sequential Infiltration Synthesis of Block Copolymers.

The sequential infiltration synthesis (SIS) of inorganic materials in nanostructured block copolymer templates has rapidly progressed in the last few years to develop functional nanomaterials with controllable properties. To assist this rapid evolution, expanding the capabilities of nondestructive methods for quantitative characterization of the materials properties is required. In this paper, we characterize the SIS process on three model polymers with different infiltration profiles through ex situ quantification by reference-free grazing incidence X-ray fluorescence. More qualitative depth distribution results were validated by means of X-ray photoelectron spectroscopy and scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy.

Role of probe design and bioassay configuration in surface enhanced Raman scattering based biosensors for miRNA detection.

The accurate design of labelled oligo probes for the detection of miRNA biomarkers by Surface Enhanced Raman Scattering (SERS) may improve the exploitation of the plasmonic enhancement. This work, thus, critically investigates the role of probe labelling configuration on the performance of SERS-based bioassays for miRNA quantitation. To this aim, highly efficient SERS substrates based on Ag-decorated porous silicon/PDMS membranes are functionalized according to bioassays relying on a one-step or two-step hybridization of the target miRNA with DNA probes. Then, the detection configuration is varied to evaluate the impact of different Raman reporters and their labelling position along the oligo sequence on bioassay sensitivity. At high miRNA concentration (100-10 nM), a significantly increased SERS intensity is detected when the reporters are located closer to the plasmonic surface compared to farther probe labelling positions. Counterintuitively, a levelling-off of the SERS intensity from the different configurations is recorded at low miRNA concentration. Such effect is attributed to the increased relative contribution of Raman hot-spots to the whole SERS signal, in line with the electric near field distribution simulated for a simplified model of the Ag nanostructures. However, the beneficial effect of reducing the reporter-to-surface distance is partially retained for a two-step hybridization assay thanks to the less sterically hindered environment in which the second hybridization occurs. The study thus demonstrates an improvement of the detection limit of the two-step assay by tuning the probe labelling position, but sheds at the same time light on the multiple factors affecting the sensitivity of SERS-based bioassays.

Materials Testing for the Development of Biocompatible Devices through Vat-Polymerization 3D Printing.

Light-based 3D printing techniques could be a valuable instrument in the development of customized and affordable biomedical devices, basically for high precision and high flexibility in terms of materials of these technologies. However, more studies related to the biocompatibility of the printed objects are required to expand the use of these techniques in the health sector. In this work, 3D printed polymeric parts are produced in lab conditions using a commercial Digital Light Processing (DLP) 3D printer and then successfully tested to fabricate components suitable for biological studies. For this purpose, different 3D printable formulations based on commercially available resins are compared. The biocompatibility of the 3D printed objects toward A549 cell line is investigated by adjusting the composition of the resins and optimizing post-printing protocols; those include washing in common solvents and UV post-curing treatments for removing unreacted and cytotoxic products. It is noteworthy that not only the selection of suitable materials but also the development of an adequate post-printing protocol is necessary for the development of biocompatible devices.

Laser-Triggered Writing and Biofunctionalization of Thiol-Ene Networks.

The light responsivity of ortho-nitrobenzyl esters (o-NBE) is exploited to inscribe µ-scale 2.5D patterns in thiol-ene networks by direct laser writing. For this purpose, a multifunctional thiol and a photosensitive alkene with an o-NBE chromophore are cured upon visible light exposure without inducing a premature photocleavage of the o-NBE links. Once the network is formed, a laser beam source with a wavelength of 375 nm is used for selectively inducing the photocleavage reaction of the o-NBE groups. Positive tone patterns are directly inscribed onto the sample surface without the requirement of a subsequent development step (removing soluble species in an appropriate organic solvent). Along with the realization of dry-developable micropatterns, the chemical surface composition of the exposed areas can be conveniently adjusted since different domains with a tailored content of carboxylic groups are obtained simply by modulating the laser energy dose. In a following step, those are activated and exploited as anchor points for attaching an Alexa-546 conjugated Protein A. Thus, the laser writable thiol-ene networks do not only provide a convenient method for the fabrication of positive tone patterns but also open future prospectives for a wide range of biosensing applications.

Real-time and reversible light-actuated microfluidic channel squeezing in dye-doped PDMS.

The azobenzene chromophore is used as a functional dye for the development of smart microfluidic devices. A single layer microfluidic channel is produced, exploiting the potential of a dye doped PDMS formulation. The key advantage of this approach is the possibility to control the fluid flow by means of a simple light stimulus. Furthermore, the deformation can be controlled in time, space and intensity, giving rise to several degrees of freedom in the actuation of the channel squeezing. A future perspective will be the implementation of the microfluidic platform with structured light, to have the possibility to control the flow in a parallel and reversible manner at several points, modifying the pattern in real time.

Core-shell silica-rhodamine B nanosphere for synthetic opals: from fluorescence spectral redistribution to sensing.

Photonic crystals are a unique tool to modify the photoluminescence of light-emitting materials. A variety of optical effects have been demonstrated by infiltrating opaline structures with photoactive media. On the other hand, the fabrication of such structures includes complex infiltration steps, that often affect the opal lattice and decrease the efficiency of light emission control. In this work, silica nanospheres were directly functionalized with rhodamine B to create an emitting shell around the dielectric core. Simple tuning of the microsphere preparation conditions allows selecting the appropriate sphere diameter and polydispersity index approaching 5%. These characteristics allow facile self-assembling of the nanospheres into three-dimensional photonic crystals whose peculiar density of photonic states at the band-gap edges induces spectral redistribution of the rhodamine B photoluminescence. The possibility to employ the new stable structure as sensor is also investigated. As a proof of principle, we report the variation of light emission obtained by exposure of the opal to vapor of chlorobenzene.

Three-Dimensional Printed Photoluminescent Polymeric Waveguides.

In this work, we propose an innovative strategy for obtaining functional objects employing a light-activated three-dimensional (3D) printing process without affecting the materials' printability. In particular, a dye is a necessary ingredient in a formulation for a digital light processing 3D printing method to obtain precise and complex structures. Here, we use a photoluminescent dye specifically synthesized for this purpose that enables the production of 3D printed waveguides and splitters able to guide the luminescence. Moreover, copolymerizing the dye with the polymeric network during the printing process, we are able to maintain the solvatochromic properties of the dye toward different solvents in the printed structures, enabling the development of solvents' polarity sensors.

Reconfigurable elastomeric graded-index optical elements controlled by light.

In many optical applications, there is an increasing need for dynamically tunable optical elements that are able to shape the wavefront of light 'on demand'. In this work, an elastomeric easy-to-fabricate optical element whose transmission functions can be reversibly phase configured by visible light is demonstrated. The light responsivity of proper azopolymers incorporated within an elastomeric matrix is exploited to induce a light-controlled graded refractive index (GRIN) distribution within the bulk compound. The induced refractive index distribution is continuous and conformal to the intensity profile of the illumination at moderate power. A 100 mW doubled-frequency Nd:YAG Gaussian beam focused to a 650 µm waist is shown to induce a maximum relative refractive index change of ~0.4% in the elastomeric matrix, with an approximately parabolic profile. The restoring characteristics of the elastomeric matrix enable full recovery of the initial homogeneous refractive index distribution within a few seconds when the incident laser is switched off. As an exemplary application, the configurable GRIN element is used in a microscope-based imaging system for light control of the effective focal length.

Light-Driven Reversible Shaping of Individual Azopolymeric Micro-Pillars.

Azopolymers are known to exhibit a strong light responsivity known as athermal photofluidization. Although the underlying physics is still under debate, athermal photofluidization has been demonstrated to trigger mass-migration according to the polarization of a proper illumination light. Here, a polymer blend is proposed wherein a commercial azo-polyelectrolyte is mixed with a passive polymer. The blend is patterned as an array of micro-pillars that are individually exposed to visible laser illumination. Thanks to the interplay between the two blend components, a reversible and controlled deformation of the micro-pillars by periodically tuning the laser polarization in time is demonstrated. A reduced mobility of the azo-compound allows to repeatibly elongate and rotate micro-pillars along specific directions, with no significant material flow outisde the initial volume and no significant degradation of the structure morphology over several cycles. The proposed work suggests new degrees of freedom in controlling the mechanical features of micro-patterned light-responsive materials that can be usefully exploited in many application fields.

Bloch Surface Wave-Coupled Emission at Ultra-Violet Wavelengths.

The interaction of fluorophores with nearby metallic structures is now an active area of research. Dielectric photonic structures offer some advantages over plasmonic structures, namely small energy losses and less quenching. We describe a dielectric one-dimensional photonic crystal (1DPC), which supports Bloch surface waves (BSWs) from 280 to 440 nm. This BSW structure is a quartz slide coated with alternating layers of SiO2 and Si3N4. We show that this structure displays BSWs and that the near-UV fluorophore, 2-aminopurine (2-AP), on the top surface of the structure couples with the BSWs. Fluorophores do not have to be inside the structure for coupling and show a narrow angular distribution, with an angular separation of wavelengths. The Bloch wave-coupled emission (BWCE) radiates through the dielectric layer. These BSW structures, with useful wavelength range for detection of intrinsic protein and cofactor fluorescence, provide opportunities for novel optical configurations for bioassays with surface-localized biomolecules and for optical imaging using the coupled emission.

Resonant evanescent complex fields on dielectric multilayers.

Complex light fields, including evanescent Bessel beams, can be generated at dielectric interfaces by means of oil-immersion optics operating in total internal reflection conditions. Here we report on the observation of evanescent complex fields produced on a dielectric multilayer through the interference of surface modes resonantly sustained by the multilayer itself. The coupling to surface modes is attained by modifying the wavefront of an incident laser beam in such a way that the resulting intensity distribution in k-space matches the dispersion of the surface mode. The phase of surface modes can be further controlled, and two-dimensional vortex beams can also be produced according to the same working principle.

Easy Tuning of Surface and Optical Properties of PDMS Decorated by Ag Nanoparticles.

Herein, we report a systematic study on the wetting and optical properties of a PDMS surface coated by silver nanoparticles. A uniform Ag nanoparticles distribution onto PDMS membrane was obtained through dc room-temperature sputtering. The effect of sputtering current and PDMS mixing ratio between oligomer and curing agent was investigated by means of UV-vis spectroscopy and contact angle measurements. The results clearly show that the wettability and optical properties of the silver-coated elastomeric substrate were strongly affected by the sputtering current and by the PDMS composition with a marked decrease of the water contact angle and the spectral shift of well-defined plasmonic dips in the transmittance spectra related to the nanoparticles morphology. The finite element method was employed to model the optical experimental results. The observed tunable properties can find huge applications in several technological fields in which PDMS was usually employed as the structural and/or plasmonic active element.

One-dimensional photonic crystals with cylindrical geometry.

A one-dimensional photonic crystal (1DPC) consisting of a stack of alternate TiO(2) and Al(2)O(3) layers is deposited on the side wall of a glass rod by Atomic Layer Deposition. The stack is designed to sustain TE-polarized Bloch Surface Waves (BSW) in the visible spectrum at wavelengths shorter than 650 nm. Experimental evidence of light coupling and guiding capabilities of the 1DPC is provided together with a possible application for fluorescence-based remote sensors.

Focusing and extraction of light mediated by Bloch surface waves.

The control of emission from localized light sources is an objective of outstanding relevance in nanophotonics. In a recent past, a large number of metallic nanostructures has been proposed to this end, wherein plasmonic modes are exploited as energy carriers on a subwavelength scale. As an interesting alternative, we present here the use of surface modes on patterned dielectric multilayers to deliver electromagnetic power from free-space to localized volumes and vice versa. Thanks to this low-loss energy transfer, proper periodic ring structures are shown to provide a subwavelength focusing of an external radiation onto the multilayer surface. By reciprocity, the radiated power from emitters within the ring center is shown to be efficiently beamed in the free-space, with a well-controlled angular divergence. This mechanism overcomes some important limitations involved in the all-plasmonic approach, while opening new opportunities for hybrid devices in photon management applications such as optical sensing and lighting.

Leakage radiation interference microscopy.

We present a proof of principle for a new imaging technique combining leakage radiation microscopy with high-resolution interference microscopy. By using oil immersion optics it is demonstrated that amplitude and phase can be retrieved from optical fields, which are evanescent in air. This technique is illustratively applied for mapping a surface mode propagating onto a planar dielectric multilayer on a thin glass substrate. The surface mode propagation constant estimated after Fourier transformation of the measured complex field is well matched with an independent measurement based on back focal plane imaging.