Gold triple-helix mid-infrared metamaterial by STED-inspired laser lithography

Printing 3D Metallic Structures in Porous Matrix

The fabrication of metallic micro/nanostructures has great potential for advancing optoelectronic microdevices. Over the past decade, femtosecond laser direct writing (FsLDW) technology has played a crucial role in driving progress in this field. In this study, silica gel glass is used as a supporting medium, and FsLDW is employed to reduce gold and palladium ions using 7-Diethylamino-3-thenoylcoumarin (DETC) as a two-photon sensitizer, enabling the printing of conductive multilayered and 3D metallic structures. How the pore size of the silica gel glass affects the electrical conductivity of printed metal wires is systematically examined. This 3D printing method is versatile and offers expanded opportunities for applying metallic micro/nanostructures in optoelectronic devices.

Direct-write 3D printing of plasmonic nanohelicoids by circularly polarized light

Chiral plasmonic surfaces with 3D "forests" from nanohelicoids should provide strong optical rotation due to alignment of helical axis with propagation vector of photons. However, such three-dimensional nanostructures also demand multi-step nanofabrication, which is incompatible with many substrates. Large-scale photonic patterns on polymeric and flexible substrates remain unattainable. Here, we demonstrate the substrate-tolerant direct-write printing and patterning of silver nanohelicoids with out-of-plane 3D orientation using circularly polarized light. Centimeter-scale chiral plasmonic surfaces can be produced within minutes using inexpensive medium-power lasers. The growth of nanohelicoids is driven by the symmetry-broken site-selective deposition and self-assembly of the silver nanoparticles (NPs). The ellipticity and wavelength of the incident photons control the local handedness and size of the printed nanohelicoids, which enables on-the-fly modulation of nanohelicoid chirality during direct writing and simple pathways to complex multifunctional metasurfaces. Processing simplicity, high polarization rotation, and fine spatial resolution of the light-driven printing of stand-up helicoids provide a rapid pathway to chiral plasmonic surfaces, accelerating the development of chiral photonics for health and information technologies.

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.

Hierarchically manufactured chiral plasmonic nanostructures with gigantic chirality for polarized emission and information encryption

Chiral metamaterials have received significant attention due to their strong chiroptical interactions with electromagnetic waves of incident light. However, the fabrication of large-area, hierarchically manufactured chiral plasmonic structures with high dissymmetry factors (g-factors) over a wide spectral range remains the key barrier to practical applications. Here we report a facile yet efficient method to fabricate hierarchical chiral nanostructures over a large area (>11.7 × 11.7 cm2) and with high g-factors (up to 0.07 in the visible region) by imparting extrinsic chirality to nanostructured polymer substrates through the simple exertion of mechanical force. We also demonstrate the application of our approach in the polarized emission of quantum dots and information encryption, including chiral quick response codes and anti-counterfeiting. This study thus paves the way for the rational design and fabrication of large-area chiral nanostructures and for their application in quantum communications and security-enhanced optical communications.

Meta-Atom Coupling Induced Chiral Hotspot in Silicon Nitride Staggered Nanorods Meta-Surface

Dielectric meta-surfaces have emerged as an effective way for fabricating chiral optical devices, and the chiral meta-surfaces are usually constituted by periodic chiral meta-atom structures. Here, we report a chiral meta-surface consisting of nonchiral silicon nitride rectangular nanorods. The chiral hotspots are generated between the staggered nanorods due to the coupling between the two nearest neighbor nanorod units. 14.6% macroscopic circular dichroism (CD) is achieved experimentally with larger area staggered nanorods. Meanwhile, we demonstrate that the wavelength tuning capability of this design from 696 to 820 nm by simply modulating the overlap length of nanorods. Our work highlights the mechanisms for CD hotspot generation without complex chiral units, which paves a novel way for future on-chip photon-spin selective devices.

Photonically active bowtie nanoassemblies with chirality continuum

Chirality is a geometrical property described by continuous mathematical functions^1-5. However, in chemical disciplines, chirality is often treated as a binary left or right characteristic of molecules rather than a continuity of chiral shapes. Although they are theoretically possible, a family of stable chemical structures with similar shapes and progressively tuneable chirality is yet unknown. Here we show that nanostructured microparticles with an anisotropic bowtie shape display chirality continuum and can be made with widely tuneable twist angle, pitch, width, thickness and length. The self-limited assembly of the bowties enables high synthetic reproducibility, size monodispersity and computational predictability of their geometries for different assembly conditions⁶. The bowtie nanoassemblies show several strong circular dichroism peaks originating from absorptive and scattering phenomena. Unlike classical chiral molecules, these particles show a continuum of chirality measures² that correlate exponentially with the spectral positions of the circular dichroism peaks. Bowtie particles with variable polarization rotation were used to print photonically active metasurfaces with spectrally tuneable positive or negative polarization signatures for light detection and ranging (LIDAR) devices.

Recent Advances in Ultrathin Chiral Metasurfaces by Twisted Stacking

Artificial chiral nanostructures have been subjected to extensive research for their unique chiroptical activities. Planarized chiral films of ultrathin thicknesses are in particular demand for easy on-chip integration and improved energy efficiency as polarization-sensitive metadevices. Recently, controlled twisted stacking of two or more layers of nanomaterials, such as 2D van der Waals materials, ultrathin films, or traditional metasurfaces, at an angle has emerged as a general strategy to introduce optical chirality into achiral solid-state systems. This method endows new degrees of freedom, e.g., the interlayer twist angle, to flexibly engineer and tune the chiroptical responses without having to change the material or the design, thus greatly facilitating the development of multifunctional metamaterials. In this review, recent exciting progress in planar chiral metasurfaces are summarized and discussed from the viewpoints of building blocks, fabrication methods, as well as circular dichroism and modulation thereof in twisted stacked nanostructures. The review further highlights the ever-growing portfolio of applications of these chiral metasurfaces, including polarization conversion, information encryption, chiral sensing, and as an engineering platform for hybrid metadevices. Finally, forward-looking prospects are provided.

Ultra-broadband and high-efficiency planar chiral metamaterial

To date, the helix-like assemblies are known for delivering the most broadband chiroptic response; however, as their dimensions shrink to the nanoscale, it becomes increasingly difficult to realize three-dimensional (3D) building blocks and accurate alignments. In addition, a continuous optical channel requirement hinders the downsizing for integrated photonics. Here, we introduce an alternative approach based on two assembled layers of dielectric-metal nanowires to demonstrate that chiroptic effects similar to helix-like metamaterials can be realized with an ultracompact planar structure by creating dissymmetry using orientation and making use of interference phenomena. We constructed two polarization filters for the near-(NIR) and the mid-infrared (MIR) spectrums that exhibit a broadband (0.835-2.11 µm and 3.84-10.64 µm) chiroptic response with maximum transmission and circular dichroism (CD) of approximately 0.965 and extinction ratio > 600. The structure is easy to fabricate, independent of alignments, and scalable from the visible to MIR range for applications including imaging, medical diagnostics, polarization conversion, and optical communication.

Angle-selective chiral absorption induced by diffractive coupling in metasurfaces

Here we report that a simple chiral metasurface with twisted metallic cut-wire arrays enables highly efficient and continuously tunable chiral absorption over a broad spectral range by scanning the incidence angle over a few degrees. The angle-selective chiral absorption results from the surface plasmon resonance (SPR) excited by diffractive effects of the metasurface. The diffraction-assisted chiral metasurface provides a straightforward strategy for achieving dynamically tunable chiral devices and offers intriguing possibilities for various applications in on-chip chiral detectors/emitters, chiral spectrometers, chiral lasers, and so on.

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.

DNA-Helix Inspired Wire Routing in Cylindrical Structures and Its Application to Flexible Surgical Devices

In general wire-driven continuum robot mechanisms, the wires are used to control the motion of the devices attached at the distal end. The slack and taut wire is one of the challenging issues to solve in flexible mechanism. This phenomenon becomes worse when the continuum robot is inserted into the natural orifices of the human body, which inherently have uncertain curvilinear geometries consisting of multiple curvatures. Inspired by the unique characteristic of DNA-helix structure that the length of the helix remains almost constant regardless of the deflection of the DNA structure, this article proposes a new idea to design useful flexible mechanism to resolve slack of wires. Using modern Lie-group screw theory, the analytic model for length of helix wire wrapped around a single flexible backbone is proposed and then extended to a general model with multiple flexible backbones and different curvatures. Taking advantage of this helix type wire mechanism, we designed and implemented a flexible surgical device suitable for laryngopharyngeal surgery. The effectiveness of the proposed flexible mechanism is demonstrated through both simulation and phantom experiment.

Non-iterative multifold strip segmentation phase method for six-dimensional optical field modulation

In this Letter, we propose a non-iterative multifold strip segmentation phase method for a spatial light modulator (SLM) to generate multifocal spots of diverse beams (Airy, spiral, perfect vortex, and Bessel-Gaussian beams) in a high-numerical-aperture system, with up to 6D controllability. The method is further validated by an inverted fluorescence microscope. By adjusting the bright and dark voltage parameters of the SLM, zero-order light caused by the pixelation effect of the SLM has been successfully eliminated. We hope this research provides a more flexible and powerful approach for the rapid modulation of multi-focus light fields in the development of biomedicine and lithography.

Three-dimensional artificial chirality towards low-cost and ultra-sensitive enantioselective sensing

Artificial chiral structures have potential applications in the field of enantioselective signal sensing. Advanced nanofabrication methods enable a large diversity in geometric structures and broad selectivity of materials, which can be exploited to manufacture artificial three-dimensional chiral structures. Various chiroptical phenomena exploiting spin and orbital angular momentum at the nanoscale have been continuously exploited as a way to effectively detect enantiomers. This review introduces precisely controlled bottom-up and large-area top-down metamaterial fabrication methods to solve the limitations of high manufacturing cost and low production speed. Particle synthesis, self-assembly, glanced angled vapor deposition, and three-dimensional plasmonic nanostructure printing are introduced. Furthermore, emerging sensitive chiral sensing methods such as cavity-enhanced chirality, photothermal circular dichroism, and helical dichroism of single particles are discussed. The continuous progress of nanofabrication technology presents the strong potential for developing artificial chiral structures for applications in biomedical, pharmaceutical, nanophotonic systems.

Nanophotonic Chiral Sensing: How Does It Actually Work?

Nanophotonic chiral sensing has recently attracted a lot of attention. The idea is to exploit the strong light-matter interaction in nanophotonic resonators to determine the concentration of chiral molecules at ultralow thresholds, which is highly attractive for numerous applications in life science and chemistry. However, a thorough understanding of the underlying interactions is still missing. The theoretical description relies on either simple approximations or on purely numerical approaches. We close this gap and present a general theory of chiral light-matter interactions in arbitrary resonators. Our theory describes the chiral interaction as a perturbation of the resonator modes, also known as resonant states or quasi-normal modes. We observe two dominant contributions: A chirality-induced resonance shift and changes in the modes' excitation and emission efficiencies. Our theory brings deep insights for tailoring and enhancing chiral light-matter interactions. Furthermore, it allows us to predict spectra much more efficiently in comparison to conventional approaches. This is particularly true, as chiral interactions are inherently weak and therefore perturbation theory fits extremely well for this problem.

Fabrication of silver helix microstructures in a large area by a two-photon absorption DLW method

Large-area helix microstructures intended for metamaterials were fabricated using a negative photoresist, SU-8 using a two photon absorption direct laser writing (TPA-DLW). Two types of helix structures were fabricated. One type is those with no neighboring distance. In this case, compact helix structures with radius of 2.5 and 1.0 μm were fabricated. Another type is those with enough neighboring distance. The helix structures with shorter neighboring distance below 6.0 μm were collapsed, whereas those with longer neighboring distance more than 6.5 μm, free-standing helix structures could successfully be built. To stabilize the fabricated free-standing helix microstructures with a 1 μm radius, circular foundations with a radius of 1.3 μm and elevation angle of 10, 12, or 14° were built in advance. The foundation is useful to avoid collapsing the helix microstructures. Due to the useful foundation, over 18,000 helical structures were fabricated in a large area. The fabricated helical structures were coated with silver using an electroless plating method to produce 3D metallic helix structures. Silver coating was measured using a EDX measurement. The obtained helical structures have the potential for metamaterials to control the handedness of a circularly polarized infrared beam.

Controllable double-helical microstructures by photonic orbital angular momentum for chiroptical response

Three-dimensional helical microstructures are abundant in nature and can be applied as chiral metamaterials for advanced nanophotonics. Here we report a flexible method to fabricate double-helical microstructures with single exposure by recording the chirality of incident optical vortices. Two coaxial optical vortices can interfere to generate a helical optical field, confirmed by the numerical simulation. The diameters of double-helical microstructures can be tailored by the magnitude of topological charges. This fast manufacturing strategy provides the opportunity to efficiently yield helical microstructures. Finally, the chirality of double-helical microstructures can be reversibly read by optical vortices, demonstrating a strong chiroptical response.

Focused Ion Beam Processing for 3D Chiral Photonics Nanostructures

The focused ion beam (FIB) is a powerful piece of technology which has enabled scientific and technological advances in the realization and study of micro- and nano-systems in many research areas, such as nanotechnology, material science, and the microelectronic industry. Recently, its applications have been extended to the photonics field, owing to the possibility of developing systems with complex shapes, including 3D chiral shapes. Indeed, micro-/nano-structured elements with precise geometrical features at the nanoscale can be realized by FIB processing, with sizes that can be tailored in order to tune optical responses over a broad spectral region. In this review, we give an overview of recent efforts in this field which have involved FIB processing as a nanofabrication tool for photonics applications. In particular, we focus on FIB-induced deposition and FIB milling, employed to build 3D nanostructures and metasurfaces exhibiting intrinsic chirality. We describe the fabrication strategies present in the literature and the chiro-optical behavior of the developed structures. The achieved results pave the way for the creation of novel and advanced nanophotonic devices for many fields of application, ranging from polarization control to integration in photonic circuits to subwavelength imaging.

3D micro/nanoshaping of metal strip arrays by direct imprinting for chiral metasurfaces

An original hybrid method for three-dimensional (3D) shaping of micro- and nanostructures by direct imprinting of lithographically patterned metal 2D-elements on a polymer substrate is proposed. The technological features of the method are demonstrated by transforming Au micro/nanostrip arrays into 3D half-turn helices. The metasurface formed by an array of 3D half-turn Au microhelices in the surface layer of a polymer substrate rotates the polarization plane of transmitted 3 THz radiation through an angle in excess of 50°. Both experimentally and numerically, it is established that the metasurface permits to control the state of polarization and the intensity of transmitted linearly polarized THz radiation using half-wavelength and guided-mode resonances as well as diffraction effects. The proposed approach enables the fabrication of double-layer arrays of half-helices, designed for infrared metasurfaces, from 80 nm wide Au nanostrips. These studies not only pave the way towards the fabrication of large-area chiral metamaterials but, also, open a new avenue in the development of 3D micro/nanostructures for other applications.

Linear and nonlinear chiroptical response from individual 3D printed plasmonic and dielectric micro-helices

Sub-wavelength chiral resonators formed from artificial structures exhibit exceedingly large chiroptical responses compared to those observed in natural media. Owing to resonant excitation, chiral near fields can be significantly enhanced for these resonators, holding great promise for developing enantioselective photonic components such as biochemical sensors based on circular dichroism (CD) and spin-dependent nonlinear imaging. In the present work, strong linear and nonlinear chiroptical responses (scattering CD > 0.15 and nonlinear differential CDs > 0.4) at visible and near infrared frequencies are reported for the first time for individual micrometer-scale plasmonic and dielectric helical structures. By leveraging dark-field spectroscopy and nonlinear optical microscopy, the circular-polarization-selective scattering behavior and nonlinear optical responses (e.g., second harmonic generation and two-photon photoluminescence) of 3D printed micro-helices with feature sizes comparable to the wavelength (total length is ∼5λ) are demonstrated. These micro-helices provide potential for readily accessible photonic platforms, facilitating an enantiomeric analysis of chiral materials. One such example is the opportunity to explore ultracompact photonic devices based on single, complex meta-atoms enabled by state-of-the-art 3D fabrication techniques.

Dynamical generation of multiple focal spot pairs with controllable position and polarization

We report a flexible method to dynamically generate multiple sub-wavelength focal spot pairs with adjustable polarization, position and number at the focal plane of a high numerical aperture (NA) objective lens. The desired incident field on the pupil plane can be analytically derived by employing the time-reversed method combined with the dipole antenna radiation theory. The numerical simulations of the corresponding tightly focused field are conducted using the Richards-Wolf vectorial diffraction theory. The validity of the presented method is demonstrated through experimental generation of several designed pupil fields with a versatile vectorial optical field generator and characterization of the produced focused fields with Stokes parameter measurements.

Increasing the circular dichroism of the planar chiral nanostructure by inducing coupling between the coverage layer and the planar nanostructure

Circular dichroism (CD) has been widely studied in recent decades because of its wide application in biomedical detection. Nanostructures with different heights (NDH) usually increase the transmission CD effect. To achieve such nanostructures, one needs to repeatedly perform the electron-beam lithography (EBL) method twice or more, layer-by-layer, which is a very complicated process. Here, we propose a method to prepare NDH by combining the EBL and oblique angle deposition (OAD) techniques. L-shaped planar silver nanostructures are prepared using EBL and normal electron beam deposition, and the OAD method is then used to partially cover one arm of the L-shaped nanostructure. Numerical simulations reveal that the height difference in the two arms of the L-shaped NDH (LSNDH) causes a difference in the polarization directions of the left- (LCP) and right-circularly polarized (RCP) incident light, thereby, generating CD effects. A 2D material is used to cover the LSNDH to further increase the charge polarization direction differences, which considerably increases the CD effect. These results are useful in simplifying and increasing the convenience of the preparation method of 3D chiral nanostructures. Furthermore, the proposed nanostructure may have potential application in biosensor, such as chiral enantiomer sensors.

Chiral Ligand-Free, Optically Active Nanoparticles Inherently Composed of Chiral Lattices at the Atomic Scale

Bulk metals lack chirality. Recently, metals have been sculptured with metastable chirality varying from the micro- to nano-scale. The manipulation of molecular chirality could be novelly performed using metals composed of chiral lattices at atomic scales (i.e., chiral nanoparticles or CNPs) if one could fundamentally understand the interactions between molecules and the chiral metal lattices. The incorporation of chiral ligands has been generally adapted to form metal CNPs. However, post-fabrication removal of chiral ligands usually causes relaxation of the metastable chiral lattices to thermodynamically stable achiral structures, and thus the coexisting chiral ligands will unavoidably disturb or screen the interactions of interest. Herein, a concept of metal CNPs that are free of chiral ligands and consist of atomically chiral lattices is introduced. Without chiral ligands, shear forces applied by substrate rotation along with the translation of incident atoms lead to imposing the metastable chiral lattices onto metals. Metal CNPs show not only the chiroptical effect but the enantiospecific interactions of chiral lattices and molecules. These two unique chiral effects have resulted in the applications of enantiodifferentiation and asymmetric synthesis. Prospectively, the extension in composition space and constituent engineering will apply alloy CNPs to enantiodiscrimination, enantioseperation, bio-imaging, bio-sensing, and asymmetric catalysis.

Controlling the spin-selective absorption with two-dimensional chiral plasmonic gratings

Spin-selective absorption in a two-dimensional (2D) chiral plasmonic grating is observed by excitation of chiral-dependent plasmonic cavity resonance. For the proposed structures, the incident right-handed circularly polarized light is absorbed with nearly 100% efficiency, whereas the incident left-handed circularly polarized light is reflected with same handedness. Moreover, we show that the location of spin-selective absorption can be controlled flexibly by tuning the plasmonic cavity dimension. The intensity of spin-selective absorption can be enhanced as well as suppressed based on Fabry-Perot interference phase relation. Such 2D chiral plasmonic gratings could find many potential applications in novel photon-spin selective devices, such as circularly polarized light detectors/emitters, chiral sensors, chiral cavities, and spin lasers.

High-Performance All-Organic DFB and DBR Waveguide Laser with Various Grating Height Fabricated by a Two-Photon Absorption DLW Method

Organic solid-state lasers (OSSLs) with distributed feedback (DFB) structures or distributed Bragg reflectors (DBRs) are promising for potential application in bio-sensing and hazardous materials detection. Here, the laser performances of the all-organic DFB waveguide lasers with various grating heights ranging from 0.4 to 4.7 μm were investigated. The grating structures used as the lasing cavity were fabricated using a two-photon absorption (TPA) direct laser writing (DLW) method with an SU-8 negative photoresist. The laser active layer consisted of a rhodamine 6G (R6G) laser dye and a cellulose acetate (CA) matrix. The R6G/CA solution was spin-coated onto the quartz substrate with the cavity (grating) structures to fabricate the DFB waveguide laser devices. The diffraction order of lasing ranged from m = 4 to 7. As the grating height was increased to 1.9 μm, the slope efficiency increased for all diffraction orders and the threshold decreases for each diffraction order. The dependence of the cavity (grating) length on the laser performances was investigated. The slope efficiency increased as the cavity length increased to 300 μm. The effect of the cavity (grating) position on the slope efficiency and the threshold position of the cavity (grating) was also studied. A maximum slope efficiency of 10.2% was achieved for the DFB waveguide laser device with a cavity (grating) length of 300 μm, a cavity position at 6 mm from the emission edge of the waveguide, and an aspect ratio ≈3 between the grating height of 1.74 μm and the grating width of 0.6 μm for the diffraction order m = 6 for lasing.

Synthesis and Characterization of the Conducting Polymer Micro-Helix Based on the Spirulina Template

As one of the most interesting naturally-occurring geometries, micro-helical structures have attracted attention due to their potential applications in fabricating biomedical and microelectronic devices. Conventional processing techniques for manufacturing micro-helices are likely to be limited in cost and mass-productivity, while Spirulina, which shows natural fine micro-helical forms, can be easily mass-reproduced at an extremely low cost. Furthermore, considering the extensive utility of conducting polymers, it is intriguing to synthesize conducting polymer micro-helices. In this study, PPy (polypyrrole), PANI (polyaniline), and PEDOT (poly(3,4-ethylenedioxythiophene)) micro-helices were fabricated using Spirulinaplatensis as a bio-template. The successful formations of the conducting polymer micro-helix were confirmed using scanning electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) and Raman and X-ray diffraction (XRD) were employed to characterize the molecular structures of the conducting polymer in micro-helical forms. In the electrochemical characterization, the optimized specific capacitances for the PPy micro-helix, the PANI micro-helix, and the PEDOT micro-helix were found to be 234 F/g, 238 F/g at the scan rate of 5 mV/s, and 106.4 F/g at the scan rate of 10 mV/s, respectively. Therefore, it could be expected that other conducting polymer micro-helices with Spirulina as a bio-template could be also easily synthesized for various applications.

Three dimensional chiral plasmon rulers based on silver nanorod trimers

The symmetry dependences of plasmon excitation modes are studied in 3D silver nanorod trimers. The degenerate plasmon modes split into chiral modes by breaking the inversion and mirror symmetry of the nanorod trimer through translation and/or rotation of the middle rod. With a translation operation, successive evolution of the circular dichroism (CD) spectrum can be achieved through gradual breaking of the inversion symmetry. An additional rotation operation produces even dramatic spectral changes due to breaking a quasi-mirror symmetry resulted from the same angular distance of the middle rod to the top and bottom rods. Especially, pairs of new chiral modes can be excited due to the contact of the middle rod with the top-bottom rod pair. The spectral changes in the simulations, which are also demonstrated experimentally, envision the 3D chiral nanorod trimer system as plasmon ruler for spatial configuration retrieval and dynamic bio-process analysis at the single molecule level.

Towards three-dimensional optical metamaterials

Metamaterials have opened up the possibility of unprecedented and fascinating concepts and applications in optics and photonics. Examples include negative refraction, perfect lenses, cloaking, perfect absorbers, and so on. Since these metamaterials are man-made materials composed of sub-wavelength structures, their development strongly depends on the advancement of micro- and nano-fabrication technologies. In particular, the realization of three-dimensional metamaterials is one of the big challenges in this research field. In this review, we describe recent progress in the fabrication technologies for three-dimensional metamaterials, as well as proposed applications.

Direct laser writing for micro-optical devices using a negative photoresist

Direct laser writing (DLW) via two-photon absorption (TPA) has attracted much attention as a new microfabrication technique because it can be applied to fabricate complex, three-dimensional (3D) microstructures. In this study, 3D microstructures and micro-optical devices of micro-lens array on the micrometer scale are fabricated using the negative photoresist SU-8 through TPA with a femtosecond laser pulse under a microscope. The effects of the irradiation conditions on linewidths, such as laser power, writing speed, and writing cycles (a number of times a line is overwritten), are investigated before the fabrication of the 3D microstructures. Various microstructures such as woodpiles, hemisphere and microstructures, 3D micro-lens and micro-lens array for micro-optical devices are fabricated. The shape of the micro-lens is evaluated using the shape analysis mode of a laser microscope to calculate the working distance of the fabricated micro-lenses. The calculated working distance corresponds well to the experimentally measured value. The focusing performance of the fabricated micro-lens is confirmed by the TPA fluorescence of an isopropyl thioxanthone (ITX) ethanol solution excited by a Ti:sapphire femtosecond laser at 800 nm. Micro-lens array (assembled 9 micro-lenses) are fabricated. Nine independent woodpile structures are simultaneously manufactured by DLW via TPA to confirm the multi-focusing ability using the fabricated micro-lens array.

3D Laser Micro- and Nanoprinting: Challenges for Chemistry

3D printing is a powerful emerging technology for the tailored fabrication of advanced functional materials. This Review summarizes the state-of-the art with regard to 3D laser micro- and nanoprinting and explores the chemical challenges limiting its full exploitation: from the development of advanced functional materials for applications in cell biology and electronics to the chemical barriers that need to be overcome to enable fast writing velocities with resolution below the diffraction limit. We further explore chemical means to enable direct laser writing of multiple materials in one resist by highly wavelength selective (λ-orthogonal) photochemical processes. Finally, chemical processes to construct adaptive 3D written structures that are able to respond to external stimuli, such as light, heat, pH value, or specific molecules, are highlighted, and advanced concepts for degradable scaffolds are explored.

Chiroptically Active Metallic Nanohelices with Helical Anisotropy

Manipulation of circular polarization states of light in the ultraviolet-visible-near infrared range, which cannot be operated using natural materials, is of fundamental interest derived from a wide range of chirality-associated optical, chemical and biological applications. To achieve this objective, chiral metamaterials made of metallic nanohelices (NHs) have been artificially designed and generated. The circular polarization manipulation is as a result of helical anisotropy and localized surface plasmon resonance of metallic NHs, and engineering helical structures and metallic composition gives rise to flexible tailoring of the circular polarization manipulation. Herein, we review the latest development of metallic NHs in terms of fabrication, fundamentally optical and magnetic properties, and chiroptical applications, followed by envisioning prospective applications closely related to molecular chirality. We strongly believe that the helicity-induced anisotropy of metallic NHs will provide us a wide range of opportunities to tackle some prominent problems and challenges intrinsically associated with the natural homochirality.

Nanolattices: An Emerging Class of Mechanical Metamaterials

In 1903, Alexander Graham Bell developed a design principle to generate lightweight, mechanically robust lattice structures based on triangular cells; this has since found broad application in lightweight design. Over one hundred years later, the same principle is being used in the fabrication of nanolattice materials, namely lattice structures composed of nanoscale constituents. Taking advantage of the size-dependent properties typical of nanoparticles, nanowires, and thin films, nanolattices redefine the limits of the accessible material-property space throughout different disciplines. Herein, the exceptional mechanical performance of nanolattices, including their ultrahigh strength, damage tolerance, and stiffness, are reviewed, and their potential for multifunctional applications beyond mechanics is examined. The efficient integration of architecture and size-affected properties is key to further develop nanolattices. The introduction of a hierarchical architecture is an effective tool in enhancing mechanical properties, and the eventual goal of nanolattice design may be to replicate the intricate hierarchies and functionalities observed in biological materials. Additive manufacturing and self-assembly techniques enable lattice design at the nanoscale; the scaling-up of nanolattice fabrication is currently the major challenge to their widespread use in technological applications.

Precise detection of circular dichroism in a cluster of nano-helices by photoacoustic measurements

Compact samples of nano-helices built by means of a focused ion beam technology with large bandwidth and high dichroism for circular polarization are promising for the construction of built-in-chip sensors, where the ideal transducer must be sufficiently confined without compromising its filtering ability. Direct all-optical measurements revealed the sample’s dichroic character with insufficient details because of scattering and diffraction interference. On the other hand, photoacoustic measurements resulted to be a possible alternative investigation, since they directly deal with absorbed power and allow to get clear evidences of the differential selection for the two opposite polarization states. Multi-level numerical simulations confirmed the experimental results, proving once again the reliability of photoacoustic technique and the versatility of this class of dichroic artificial materials.

Sub-diffraction-limited fluorescent patterns by tightly focusing polarized femtosecond vortex beams in a silver-containing glass

We report that the shape and size of fluorescent patterns can be controlled by the focused laser intensity distribution, which depends on irradiation conditions as well as on the spin and orbital angular momenta being carried by light, inducing the formation of silver cluster patterns in a silver-containing zinc phosphate glass. In particular, we demonstrate that sub-diffraction-limited inner structures of fluorescent patterns can be generated by direct laser writing (DLW) with tightly focused femtosecond laser vortex beams as Laguerre-Gauss modes (LG0l) with linear and left-handed circular polarizations. We believe this technique, further combined with dual-color DLW, can be useful and powerful for developing structured light enabled nanostructures.

Chiral plasmonics

We present a comprehensive overview of chirality and its optical manifestation in plasmonic nanosystems and nanostructures. We discuss top-down fabricated structures that range from solid metallic nanostructures to groupings of metallic nanoparticles arranged in three dimensions. We also present the large variety of bottom-up synthesized structures. Using DNA, peptides, or other scaffolds, complex nanoparticle arrangements of up to hundreds of individual nanoparticles have been realized. Beyond this static picture, we also give an overview of recent demonstrations of active chiral plasmonic systems, where the chiral optical response can be controlled by an external stimulus. We discuss the prospect of using the unique properties of complex chiral plasmonic systems for enantiomeric sensing schemes.

Additive Manufacturing of Metal Structures at the Micrometer Scale

Currently, the focus of additive manufacturing (AM) is shifting from simple prototyping to actual production. One driving factor of this process is the ability of AM to build geometries that are not accessible by subtractive fabrication techniques. While these techniques often call for a geometry that is easiest to manufacture, AM enables the geometry required for best performance to be built by freeing the design process from restrictions imposed by traditional machining. At the micrometer scale, the design limitations of standard fabrication techniques are even more severe. Microscale AM thus holds great potential, as confirmed by the rapid success of commercial micro-stereolithography tools as an enabling technology for a broad range of scientific applications. For metals, however, there is still no established AM solution at small scales. To tackle the limited resolution of standard metal AM methods (a few tens of micrometers at best), various new techniques aimed at the micrometer scale and below are presently under development. Here, we review these recent efforts. Specifically, we feature the techniques of direct ink writing, electrohydrodynamic printing, laser-assisted electrophoretic deposition, laser-induced forward transfer, local electroplating methods, laser-induced photoreduction and focused electron or ion beam induced deposition. Although these methods have proven to facilitate the AM of metals with feature sizes in the range of 0.1-10 µm, they are still in a prototype stage and their potential is not fully explored yet. For instance, comprehensive studies of material availability and material properties are often lacking, yet compulsory for actual applications. We address these items while critically discussing and comparing the potential of current microscale metal AM techniques.

Strong Rotational Anisotropies Affect Nonlinear Chiral Metamaterials

Masked by rotational anisotropies, xthe nonlinear chiroptical response of a metamaterial is initially completely inaccessible. Upon rotating the sample the chiral information emerges. These results highlight the need for a general method to extract the true chiral contributions to the nonlinear optical signal, which would be hugely valuable in the present context of increasingly complex chiral meta/nanomaterials.

Spin-orbit interactions in optically active materials

We investigate the inherent influence of light polarization on the intensity distribution in anisotropic media undergoing a local inhomogeneous rotation of the principal axes. Whereas, in general, such a configuration implies a complicated interaction between the geometric and the dynamic phase, we show that, in a medium showing an inhomogeneous circular birefringence, the geometric phase vanishes. Due to the spin-orbit interaction, the two circular polarizations perceive reversed spatial distribution of the dynamic phase. Based on this effect, polarization-selective lenses, waveguides, and beam deflectors are proposed.

All-dielectric metasurface circular dichroism waveplate

We propose and experimentally demonstrate a high efficient circularly polarizing dichroism waveplate (CPDW) using a Si-based all-dielectric 2Dchiral metasurface. We demonstrate that the CPDW exhibits a unique dichroism in that it functions as a transmissive quarter waveplate for one of either left-or right-handed circularly polarized incident lightand a reflective mirror for the opposite polarization. The circular polarization dichroism (CPD = I_RCP − I_LCP) in transmission at wavelength ~1.5 μm reaches 97% and the extinction ratio (ER = I_RCP/I_LCP) is as high as 345:1. Experimental fabrications and measurements of the proposed all-dielectric metasurface are implemented and found to be in excellent agreement with the simulations. The proposed all-dielectric chiral metasurface is of advantages of high-dichroism, easy-fabrication and standard semiconductor fabrication techniques compatible, which could lead to enhanced security in fiber and free-space communications, as well as imaging and sensing applications for circularly polarized light with a highly integrated photonic platform.

Bragg-mirror-like circular dichroism in bio-inspired quadruple-gyroid 4srs nanostructures

The smooth and tailorable spectral response of Bragg mirrors has driven their pervasive use in optical systems requiring customizable spectral control of beam propagation. However, the simple nature of Bragg mirror reflection prevents their application to the control of important polarization states such as circular polarization. While helical and gyroid-based nanostructures exhibiting circular dichroism have been developed extensively to address this limitation, they are often restricted by the spectral inconsistency of their optical response. Here we present the fabrication and characterization of quadruple-gyroid 4srs nanostructures exhibiting bio-inspired Bragg-mirror-like circular dichroism: a smooth and uniform band of circular dichroism reminiscent of the spectrum of a simple multilayer Bragg-mirror. Furthermore, we demonstrate that the circular dichroism produced by 4srs nanostructures are robust to changes in incident angle and beam collimation, providing a new platform to create and engineer circular dichroism for functional circular polarization manipulation.

Programmable Extreme Chirality in the Visible by Helix-Shaped Metamaterial Platform

The capability to fully control the chiro-optical properties of metamaterials in the visible range enables a number of applications from integrated photonics to life science. To achieve this goal, a simultaneous control over complex spatial and localized structuring as well as material composition at the nanoscale is required. Here, we demonstrate how circular dichroic bands and optical rotation can be effectively and independently tailored throughout the visible regime as a function of the fundamental meta-atoms properties and of their three dimensional architecture in a the helix-shaped metamaterials. The record chiro-optical effects obtained in the visible range are accompanied by an additional control over optical efficiency, even in the plasmonic context. These achievements pave the way toward fully integrated chiral photonic devices.

Manipulation of wavefront using helical metamaterials

Helical metamaterials, a kind of 3-dimensional structure, has relatively strong coupling effect among the helical nano-wires. Therefore, it is expected to be a good candidate for generating phase shift and controlling wavefront with high efficiency. In this paper, using the finite-difference time-domain (FDTD) method, we studied the phase shift properties in the helical metamaterials. It is found that the phase shift occurs for both transmitted and reflected light waves. And the maximum of reflection coefficients can reach over 60%. In addition, the phase shift (φ) is dispersionless in the range of 600 nm to 860 nm, that is, it is only dominated by the initial angle (θ) of the helix. The relationship between them is φ = ± 2θ. Using Jones calculus we give a further explanation for these properties. Finally, by arranging the helixes in an array with a constant phase gradient, the phenomenon of anomalous refraction was also observed in a broad wavelength range.

Broadband circularly polarizing dichroism with high efficient plasmonic helical surface

We propose and experimentally demonstrate a broadband and high efficient circularly polarizing dichroism using a simple single-cycle and single-helical plasmonic surface array arranged in square lattice. Two types of helical surface structures (partially or completely covered with a gold film) are investigated. It is shown that the circular polarization dichroism in the mid-IR range (3µm - 5µm) can reach 80% (when the surface is partially covered with gold) or 65% (when the surface is completely covered with gold) with a single-cycle and single-helical surface. Experimental fabrications of the proposed helical plasmonic surface are implemented with direct 3D laser writing followed by electron beam evaporation deposition of gold. The experimental evaluations of the circular polarization dichroism are in excellent agreement with the simulation. The proposed helical surface structure is of advantages of easy-fabrication, high-dichroism and scalable to other frequencies as a high efficient broadband circular polarizer.

Formation of Enhanced Uniform Chiral Fields in Symmetric Dimer Nanostructures

Chiral fields with large optical chirality are very important in chiral molecules analysis, sensing and other measurements. Plasmonic nanostructures have been proposed to realize such super chiral fields for enhancing weak chiral signals. However, most of them cannot provide uniform chiral near-fields close to the structures, which makes these nanostructures not so efficient for applications. Plasmonic helical nanostructures and blocked squares have been proved to provide uniform chiral near-fields, but structure fabrication is a challenge. In this paper, we show that very simple plasmonic dimer structures can provide uniform chiral fields in the gaps with large enhancement of both near electric fields and chiral fields under linearly polarized light illumination with polarization off the dimer axis at dipole resonance. An analytical dipole model is utilized to explain this behavior theoretically. 30 times of volume averaged chiral field enhancement is gotten in the whole gap. Chiral fields with opposite handedness can be obtained simply by changing the polarization to the other side of the dimer axis. It is especially useful in Raman optical activity measurement and chiral sensing of small quantity of chiral molecule.