Focused Ion Beam Processing for 3D Chiral Photonics Nanostructures

Magnetic assembly of plasmonic chiral superstructures with dynamic chiroptical responses

Plasmonic nanostructures exhibiting dynamically tunable chiroptical responses hold great promise for broad applications such as sensing, catalysis, and enantioselective analysis. Despite the successful fabrication of chiral structures through diverse templates, creating dynamic chiroptical materials with fast and reversible responses to external stimuli is still challenging. This work showcases reversible magnetic assembly and active tuning of plasmonic chiral superstructures by introducing a chiral magnetic field from a cubic permanent magnet. Manipulating the strength and direction of the magnetic field controls both the chiral arrangement and plasmonic coupling of the nanoparticle assembly, enabling fast and reversible tunability in not only the handedness of the superstructures but also the spectral characteristics of their chiroptical properties. The dynamic tunability further enables the fabrication of color-changing optical devices based on the optical rotatory dispersion effect, showcasing their potential for application in anti-counterfeiting and stress sensors.

Assessing the Quality of Oxygen Plasma Focused Ion Beam (O-PFIB) Etching on Polypropylene Surfaces Using Secondary Electron Hyperspectral Imaging

The development of Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) systems has provided significant advances in the processing and characterization of polymers. A fundamental understanding of ion-sample interactions is still missing despite FIB-SEM being routinely applied in microstructural analyses of polymers. This study applies Secondary Electron Hyperspectral Imaging to reveal oxygen and xenon plasma FIB interactions on the surface of a polymer (in this instance, polypropylene). Secondary Electron Hyperspectral Imaging (SEHI) is a technique housed within the SEM chamber that exhibits multiscale surface sensitivity with a high spatial resolution and the ability to identify carbon bonding present using low beam energies without requiring an Ultra High Vacuum (UHV). SEHI is made possible through the use of through-the-lens detectors (TLDs) to provide a low-pass SE collection of low primary electron beam energies and currents. SE images acquired over the same region of interest from different energy ranges are plotted to produce an SE spectrum. The data provided in this study provide evidence of SEHI's ability to be a valuable tool in the characterization of polymer surfaces post-PFIB etching, allowing for insights into both tailoring polymer processing FIB parameters and SEHI's ability to be used to monitor serial FIB polymer surfaces in situ.

Laser-Induced Chirality of Plasmonic Nanoparticles Embedded in Porous Matrix

Chiral plasmonic nanostructures have emerged as promising objects for numerous applications in nanophotonics, optoelectronics, biosensing, chemistry, and pharmacy. Here, we propose a novel method to induce strong chirality in achiral ensembles of gold nanoparticles via irradiation with circularly-polarized light of a picosecond Nd:YAG laser. Embedding of gold nanoparticles into a nanoporous silicate matrix leads to the formation of a racemic mixture of metal nanoparticles of different chirality that is enhanced by highly asymmetric dielectric environment of the nanoporous matrix. Then, illumination with intense circularly-polarized light selectively modifies the particles with the chirality defined by the handedness of the laser light, while their "enantiomers" survive the laser action almost unaffected. This novel modification of the spectral hole burning technique leads to the formation of an ensemble of plasmonic metal nanoparticles that demonstrates circular dichroism up to 100 mdeg. An unforeseen peculiarity of the chiral nanostructures obtained in this way is that 2D and 3D nanostructures contribute almost equally to the observed circular dichroism signals. Thus, the circular dichroism is neither even nor odd under reversal of direction of light propagation. These findings will help guide the development of a passive optical modulator and nanoplatform for enhanced chiral sensing and catalysis.

Surface Lattice Resonances in 3D Chiral Metacrystals for Plasmonic Sensing

Chiral lattice modes are hybrid states arising from the chiral plasmonic particles assembled in ordered arrays with opportune periodicity. These resonances exhibit dependence on excitation handedness, and their observation in plasmonic lattices is strictly related to the chiroptical features of the fundamental plasmonic unit. Here, the emergence of chiral surface lattice resonances (c-SLRs) is shown in properly engineered arrays of nanohelices (NHs), fully three dimensional (3D) chiral nano-objects fabricated by focused ion beam processing. By tuning the relative weight of plasmonic and photonic components in the hybrid mode, the physical mechanism of strong diffractive coupling leading to the emergence of the lattice modes is analyzed, opening the way to the engineering of chiral plasmonic systems for sensing applications. In particular, a coupling regime is identified where the combination of a large intrinsic circular dichroism (CD) of the plasmonic resonance with a well-defined balance between the photonic quality factor (Q factor) and the plasmonic field enhancement (M) maximizes the capability of the system to discriminate refractive index (RI) changes in the surrounding medium. The results lay the foundation for exploiting CD in plasmonic lattices to high performance refractometric sensing.

Molecular Plasmonics with Metamaterials

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

Recent Advances in Silver Nanostructured Substrates for Plasmonic Sensors

Noble metal nanostructures are known to confine photon energies to their dimensions with resonant oscillations of their conduction electrons, leading to the ultrahigh enhancement of electromagnetic fields in numerous spectroscopic methods. Of all the possible plasmonic nanomaterials, silver offers the most intriguing properties, such as best field enhancements and tunable resonances in visible-to-near infrared regions. This review highlights the recent developments in silver nanostructured substrates for plasmonic sensing with the main emphasis on surface plasmon resonance (SPR) and surface-enhanced Raman spectroscopy (SERS) over the past decade. The main focus is on the synthesis of silver nanostructured substrates via physical vapor deposition and chemical synthesis routes and their applications in each sensing regime. A comprehensive review of recent literature on various possible silver nanostructures prepared through these methodologies is discussed and critically reviewed for various planar and optical fiber-based substrates.

Template-assisted self-assembly of achiral plasmonic nanoparticles into chiral structures

The acquisition of strong chiroptical activity has revolutionized the field of plasmonics, granting access to novel light-matter interactions and revitalizing research on both the synthesis and application of nanostructures. Among the different mechanisms for the origin of chiroptical properties in colloidal plasmonic systems, the self-assembly of achiral nanoparticles into optically active materials offers a versatile route to control the structure-optical activity relationships of nanostructures, while simplifying the engineering of their chiral geometries. Such unconventional materials include helical structures with a precisely defined morphology, as well as large scale, deformable substrates that can leverage the potential of periodic patterns. Some promising templates with helical structural motifs like liquid crystal phases or confined block co-polymers still need efficient strategies to direct preferential handedness, whereas other templates such as silica nanohelices can be grown in an enantiomeric form. Both types of chiral structures are reviewed herein as platforms for chiral sensing: patterned substrates can readily incorporate analytes, while helical assemblies can form around structures of interest, like amyloid protein aggregates. Looking ahead, current knowledge and precedents point toward the incorporation of semiconductor emitters into plasmonic systems with chiral effects, which can lead to plasmonic-excitonic effects and the generation of circularly polarized photoluminescence.

Magnetically induced transparency in helically structured periodic crystals

We investigated the specific properties of magnetically induced transparency (MIT) and magnetically induced absorption (MIA) in helically structured periodic crystals (HSPCs). We showed that for the wavelength of MIT we have an ideal optical diode: The forward signal passes fully, while the backward signal is completely absorbed and not reflected. A formula for the wavelength λ_{t} of MIT and MIA resonance based on the numerical simulations was analytically obtained. The influence of HSPC parameters on the wavelength λ_{t} and on Δλ_{t}, the transparency line half width, was investigated by numerical simulations. The specific properties of light energy density, the ellipticity e_{in}, and azimuth φ_{in} of the total wave excited in the HSPC layer for MIT and MIA modes were investigated, too.

Chiral Plasmonic Nanowaves by Tilted Assembly of Unidirectionally Aligned Block Copolymers with Buckling-Induced Microwrinkles

Chiral-structured nanoscale materials exhibit chiroptical properties with preferential absorptions of circularly polarized light. The distinctive optical responses of chiral materials have great potential for advanced optical and biomedical applications. However, the fabrication of three-dimensional structures with mirrored nanoscale geometry is still challenging. This study introduces chiral plasmonic nanopatterns in wavy shapes based on the unidirectional alignment of block copolymer thin films and their tilted transfer, combined with buckling processes. The cylindrical nanodomains of polystyrene-block-poly(2-vinylpyridine) thin films were unidirectionally aligned over a large area by the shear-rolling process. The aligned block copolymer thin films were transferred onto uniaxially prestrained polydimethylsiloxane films at certain angles relative to the stretching directions. The strain was then released to induce buckling. The aligned nanopatterns across the axis of the formed microwrinkles were selectively infiltrated with gold ions. After reduction by plasma treatment, chiral plasmonic nanowave patterns were fabricated with the presence of mirror-reflected circular dichroism spectra. This fabrication method does not require any lithography processing or innately chiral biomaterials, which can be advantageous over other conventional fabrication methods for artificial nanoscale chiral materials.

High Resolution Patterning of an Organic-Inorganic Photoresin for the Fabrication of Platinum Microstructures

Platinum (Pt) is an interesting material for many applications due to its high chemical resilience, outstanding catalytic activity, high electrical conductivity, and high melting point. However, microstructuring and especially 3D microstructuring of platinum is a complex process, based on expensive and specialized equipment often suffering from very slow processing speeds. In this work, organic-inorganic photoresins, which can be structured using direct optical lithography as well as two-photon lithography (TPL) with submicrometer resolution and high-throughput is presented. The printed structures are subsequently converted to high-purity platinum using thermal debinding of the binder and reduction of the salt. With this technique, complex 3D structures with a 3D resolution of 300 nm were fabricated. At a layer thickness of 35 nm, the patterns reach a high conductivity of 67% compared to bulk platinum. Microheaters, thermocouple sensors as well as a Lab-on-a-Chip system are presented as exemplary applications. This technology will enable a broad range of application from electronics, sensing and heating elements to 3D photonics and metamaterials.

Editorial for the Special Issue on Nanofabrication with Focused Electron/Ion Beam Induced Processing

Focused electron beam (FEB) and focused ion beam (FIB) technologies have opened novel paths for material science research and technology at the micro and nano scales in recent decades [...].

Femtomolar Biodetection by a Compact Core-Shell 3D Chiral Metamaterial

Advanced sensing tools, detecting extremely low concentrations of circulating biomarkers, can open unexplored routes toward early diagnostics and diseases progression monitoring. Here, we demonstrate the sensing capabilities of a chip-based metamaterial, combining 3D chiral geometry with a functional core-shell nanoarchitecture. The chiral metamaterial provides a circular polarization-dependent optical response, allowing analysis in a complex environment without significant background interferences. The functional nanoarchitecture, based on the conformal coating with a polymer shell, modifies the chiral metamaterial near- and far-field optical response because of the energy transfer between dielectric shell polarization charges and plasmonic core free electrons, leading to efficient interaction with biomolecules. The system sensitivity slope is 27 nm/pM, in the detection of TAR DNA-binding protein 43, clinically relevant for neurodegenerative diseases. Measurements were performed in spiked solution and in human serum with concentrations from 1 pM down to 10 fM, which is a range not accessible with common immunological assays, opening new perspectives for next-generation biomedical systems.

Monitoring Carbon in Electron and Ion Beam Deposition within FIB-SEM

It is well known that carbon present in scanning electron microscopes (SEM), Focused ion beam (FIB) systems and FIB-SEMs, causes imaging artefacts and influences the quality of TEM lamellae or structures fabricated in FIB-SEMs. The severity of such effects depends not only on the quantity of carbon present but also on its bonding state. Despite this, the presence of carbon and its bonding state is not regularly monitored in FIB-SEMs. Here we demonstrated that Secondary Electron Hyperspectral Imaging (SEHI) can be implemented in different FIB-SEMs (ThermoFisher Helios G4-CXe PFIB and Helios Nanolab G3 UC) and used to observe carbon built up/removal and bonding changes resulting from electron/ion beam exposure. As well as the ability to monitor, this study also showed the capability of Plasma FIB Xe exposure to remove carbon contamination from the surface of a Ti6246 alloy without the requirement of chemical surface treatments.