Spiral Nanostructures Research Articles

Controlled generation of cylindrical vector vortex beams using an optically active material.

An efficient method for the generation of tunable cylindrical vector vortex beams is proposed and demonstrated experimentally using an optically active (OA) material. The uniqueness of the proposed methodology lies in the control over the tunability of the cylindrical vector vortex beams using different concentrations of the optically active material. The efficiency of the generated beams is enhanced by using a single low oblique incidence angle of the input beam on a spatial light modulator with a single-phase profile. Two global parameters are measured experimentally to show the quantitative tunability and efficiency of the generated beams. The proposed method can find applications in the fabrication of various kinds of spiral nanostructures.

Simultaneous Circular Dichroism and Wavefront Manipulation with Generalized Pancharatnam-Berry Phase Metasurfaces.

Simultaneous circular dichroism and wavefront manipulation have gained considerable attention in various applications, such as chiroptical spectroscopy, chiral imaging, sorting and detection of enantiomers, and quantum optics, which can improve the miniaturization and integration of the optical system. Typically, structures with n-fold rotational symmetry (n ≥ 3) are used to improve circular dichroism, as they induce stronger interactions between the electric and magnetic fields. However, manipulating the wavefront with these structures remains challenging because they are commonly considered isotropic and lack a geometric phase response in linear optics. Here, we propose and experimentally demonstrate an approach to achieve simultaneous circular dichroism (with a maximum value of ∼0.62) and wavefront manipulation using a plasmonic metasurface made up of C3 Archimedes spiral nanostructures. The circular dichroism arises from the magnetic dipole-dipole resonance and strong interactions between adjacent meta-atoms. As a proof of concept, two metadevices are fabricated and characterized in the near-infrared regime. This configuration possesses the potential for future applications in photodetection, chiroptical spectroscopy, and the customization of linear and nonlinear optical responses.

Improvement in optical nonlinearity of spiral square Mn-based nanostructures with increasing the number of square sides

Spiral square Mn-based nanostructures with different numbers of square sides were grown on glass substrate using the glancing angle deposition method. The crystallinity of the products was determined by X-ray diffraction. The surface properties of the synthesized nanostructures were analyzed by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). 2D AFM images have been used for more detail on grain growth evolution and height distribution. The band gaps of the nanostructures were determined using the Tauc formula in the range of 2.72–3.06 eV. It was observed a decrease in band gap energy of spiral square Mn-based nanostructures with increasing the number of square sides. Nonlinear optical measurements of synthesized samples were performed using a Z-scan setup under a CW laser irradiated at 532 nm. Values of the order of 10−4 cm W−1 and 10−7 cm2 W−1 were calculated for nonlinear absorption coefficients and refraction indices of spiral square nanostructures, respectively. This investigation revealed an increase of about 1.7 times for nonlinear optical parameters of Mn-based nanostructures with increasing the number of square sides from 8 up to 10 that is significant increment. These results suggest spiral square Mn-based nanostructures as a potential material for application in optics.

Photofabrication of chiral plasmonic nanospiroids

Nanofabrication of three-dimensional chiral plasmonic structures has been a challenging research topic. In the present study, we shaped dielectric caps on plasmonic gold nanocubes (AuNCs) into three-dimensional nanospiroids by circularly polarized light (CPL) as the chirality source, without using lithographic methods or chiral molecules. AuNCs adsorbed on a TiO2 substrate were irradiated with right or left CPL in the presence of Pb2+ for the deposition of PbO2 on AuNCs. The Au–PbO2 nanocomposites, thus, obtained are the first spiral plasmonic nanostructures prepared by CPL. They exhibit strong and sharp signals of circular dichroism, and the signs of the signals are reversed by changing the rotation direction of the CPL used. Their g-factor values are highest among the chiral plasmonic nanostructures fabricated by CPL.

Polarization-directed growth of spiral nanostructures by laser direct writing with vector beams

Chirality is pivotal in nature which attracts wide research interests from all disciplines and creating chiral matter is one of the central themes for chemists and material scientists. Despite of significant efforts, a simple, cost-effective and general method that can produce different kinds of chiral metamaterials with high regularity and tailorability is still demanding but greatly missing. Here, we introduce polarization-directed growth of spiral nanostructures via vector beams, which is simple, tailorable and generally applicable to both plasmonic and dielectric materials. The self-aligned near field enhances the photochemical growth along the polarization, which is crucial for the oriented growth. The obtained plasmonic chiral nanostructures present prominent optical activity with a g-factor up to 0.4, which can be tuned by adjusting the spirality of the vector beams. These spiral plasmonic nanostructures can be used for the sensing of different chiral enantiomers. The dielectric chiral metasurfaces can also be formed in arrays of sub-mm scale, which exhibit a g-factor over 0.1. However, photoluminescence of chiral cadmium sulfide presents a very weak luminescence g-factor with the excitation of linearly polarized light. A number of applications can be envisioned with these chiral nanostructures such as chiral sensing, chiral separation and chiral information storage.

Fabrication of zigzag and square spiral Cu nanostructures: Influence of substrate rotation on the structural, optical and electrical properties

Copper (Cu) thin films were deposited by using glancing angle deposition in the form of zigzag and square spiral nanostructures. Thin films were fabricated under different azimuthal rotations (φ = 180°, 90°, 45°, 22.5° and 11°), to the thickness of approximately 300 nm. Morphological, chemical and structural properties were investigated by field-emission scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. For the optical characterization of the samples spectroscopic ellipsometry was used, while the electrical properties were analyzed by four-point probe method. It was found that as the azimuthal rotation decreases, deposited nanostructures become more porous with larger number of grown arms. Both, optical and electrical analyses revealed that the properties of the obtained thin films are significantly influenced by growth mechanism. By decreasing the φ parameter, the surface plasmon resonance peak was shifted from 1038.9 nm to 1676.6 nm, which can be correlated with the porosity of Cu structures. Besides, it was found that the optical and electrical properties of the thin films are also strongly dependent on the concentration of defects.

(Invited) Photoelectrochemical Fabrication of Chiral Plasmonic Nanostructures By Circularly Polarized Light

Chiral plasmonic nanostructures attracts attention because they are potentially applicable to optical materials such as enantioselective sensors and metamaterials, as well as photoelectrochemical devices. Chiral nanostructures are often prepared by electron beam lithography or synthesis based on DNA templates. We have recently developed a photoelectrochemical method, in which handedness of the chiral nanostructure can be controlled by right- or left- circularly polarized light.The photoelectrochemical method is based on plasmon-induced charge separation (PICS),1,2 in which electrons are injected from a plasmonic metal nanoparticle to a semiconductor such as titania in direct contact. In PICS, anodic reactions often occur at the resonance sites of the plasmonic nanoparticle, at which electron oscillation is localized.3,4 Energetic electron-hole pairs generate at the resonance site, and holes are used for the local anodic reaction, probably via trap sites. On the basis of the mechanisms, we have demonstrated site-selective etching of silver nanoparticles and site-selective deposition of lead oxide on gold nanoparticles.Under right-circularly polarized light (CPL), distribution of the resonance sites could be the mirror image of that under left-CPL.5 Therefore, we performed site-selective deposition of lead oxide on gold nanocuboids on titania under right- or left-CPL.6 As a result, lead oxide was deposited on the gold nanocuboids in a chiral geometry. The nanostructures thus obtained exhibited circular dichroism (CD), and the CD spectrum obtained for the structure prepared under right-CPL was opposite to that obtained for the structure prepared under left-CPL. Reversible switching of the handedness of the chiral plasmonic nanostructures can also be possible.7 This method also allows us to fabricate spiral nanostructures. 1. Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005). 2. T. Tatsuma, H. Nishi, and T. Ishida, Chem. Sci., 8, 3325 (2017) [review]. 3. I. Tanabe and T. Tatsuma, Nano Lett., 12, 5418 (2012). 4. T. Tatsuma and H. Nishi, Nanoscale Horiz., 5, 597 (2020) [review]. 5. S. Hashiyada, T. Narushima, and H. Okamoto, J. Phys. Chem. C, 118, 22229 (2014). 6. K. Saito and T. Tatsuma, Nano Lett., 18, 3209 (2018). 7. K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano, 14, 3603 (2020).

Electrically Tunable and Dramatically Enhanced Valley-Polarized Emission of Monolayer WS2 at Room Temperature with Plasmonic Archimedes Spiral Nanostructures.

Monolayer transition metal dichalcogenides (TMDs) have intrinsic valley degrees of freedom, making them appealing for exploiting valleytronic applications in information storage and processing. WS2 monolayer possesses two inequivalent valleys in the Brillouin zone, each valley coupling selectively with a circular polarization of light. The degree of valley polarization (DVP) under the excitation of circularly polarized light (CPL) is a parameter that determines the purity of valley polarized photoluminescence (PL) of monolayer WS2 . Here efficient tailoring of valley-polarized PL from monolayer WS2 at room temperature (RT) through surface plasmon-exciton interactions with plasmonic Archimedes spiral (PAS) nanostructures is reported. The DVP of WS2 at RT can be enhanced from <5% to 40% and 50% by using 2 turns (2T) and 4 turns (4T) of PAS, respectively. Further enhancement and control of excitonic valley polarization is demonstrated by electrostatically doping monolayer WS2 . For CPL on WS2 -2TPAS heterostructures, the 40% valley polarization is enhanced to 70% by modulating the carrier doping via a backgate, which may be attributed to the screening of momentum-dependent long-range electron-hole exchange interactions. The manifestation of electrically tunable valley-polarized emission from WS2 -PAS heterostructures presents a new strategy toward harnessing valley excitons for application in ultrathin valleytronic devices.

Ultra-sensitive amplitude engineering and sign reversal of circular dichroism in quasi-3D chiral nanostructures.

Circular dichroism (CD), as one of the most representative chiroptical effects, provides a simple strategy for the detection and characterization of the molecular chirality. The enhancement and sign reversal of CD are of great importance for its practical applications in chiral bio-sensing, chirality switching and optical filtering, etc. Here, we realize considerable adjustments and the sign reversal of CD in quasi-three-dimensional (quasi-3D) combined Archimedean spiral nanostructures. With special local and lattice configurations, the nanostructures have both right-handed and left-handed geometric chirality, which are designed based on the proximity effect of stencil lithography. We find that the CD response of the nanostructures becomes obvious once its height exceeds 200 nm and can be adjusted by the further increase of the height or the change of the blade spacing of the nanostructures. The CD reversal is achieved by utilizing the competition of two chiral centers when the height or blade spacing exceeds a critical value. Further analysis of the scattering power of multipole moments reveals that the CD modulation is determined by both magnetic dipole moment and electric quadrupole moment. Benefiting from the highly sensitive CD response to the height, the extreme sign reversal of CD is achieved when a sub-10-nm ultrathin medium layer is anchored on the surface of the nanostructures, which provides a promising strategy for ultra-sensitive chiral bio-sensing.

Chiral graphene plasmonic Archimedes' spiral nanostructure with tunable circular dichroism and enhanced sensing performance.

Circular dichroism spectroscopy is frequently used to characterize the chiral biomolecules by measuring the absorption spectra contrast between the left-handed circularly polarized light and the right-handed circularly polarized light. Compared with biomolecules, chiral metal plasmonic nanostructures also produce a strong circular dichroism response in the range of near-infrared. However, due to the large damping rate, the non-adjustable resonant frequency of the conventional metals, the applications of chiral metal plasmonic nanostructures in the fields of photoelectric detection and chemical and biochemical sensing are restricted. Here, we present a chiral graphene plasmonic Archimedes' spiral nanostructure that displays a significant circular dichroism response under the excitation of two polarizations of circularly polarized light. By manipulating the material and geometric parameters of the Archimedes' spiral, the stronger circular dichroism responses and modulation of the resonant wavelength are achieved. The optimized plasmonic nanostructure has outstanding refractive index sensing performance, where the sensitivity and figure of merit reach 7000nm/RIU and 68.75, respectively. Our proposed chiral graphene plasmonic Archimedes' spiral nanostructure might find potential applications in the fields of optical detection and high performance of index sensing.

How does PEDOT grow like nanoropes? A theoretical investigation in pace with experimental insights

Poly 3–4 ethylene dioxythiophene (PEDOT) is a highly conducting polymer insoluble and intractable when synthesized under normal synthesis conditions. This polymer was found to assume spiral or ropelike nanostructures with some modification in physical properties when grown in presence of a liquid crystal 5CB. The present work focuses on finding out the underlying reason behind the unique morphology of PEDOT theoretically, using density functional theory (DFT) as implemented in the Gaussian 03 suite of programs. From the energy minimized structures of 5CB clusters as well as PEDOT-5CB combinations it was observed that in the clusters of 6 or 10 molecules, 5CB is self-assembled in spiral orientation which might have guided the similar growth of PEDOT. It was also established from molecular orbital studies that the neighboring 5CB environment undergoes some sort of nonbonding interaction with the growing PEDOT chains and has resulted in the modification of solubility and few other properties of PEDOT. Therefore the PEDOT nanostructure obtained under the present synthesis condition is supposed to be a hybrid material having optical and electrical properties dominated by the PEDOT counterpart.

α spiral nanoslit and the higher order plasmonic vortex generation

In view of the conciseness of a spiral nanoslit and the limited order of the generated vortex, a kind of nanometer spiral, named α spirals, is proposed to generate a higher order plasmonic vortex. Theoretical analysis provides the basis for the advancement of an α spiral. The proposed spiral can generate the plasmonic vortex and the extreme order of the generated vortex depends on parameter α. The numerical simulations express the valid region of the plasmonic vortex generated by the α spiral. Discussions about the validity range of the α spiral nanoslit and the influence of the film material are beneficial to generate a high order vortex. This work builds a platform for the generation of the higher order plasmonic vortex using the simple spiral nanostructure and it can extend the potential applications of higher order plasmonic vortices.

Mechanism of Extreme Optical Nonlinearities in Spiral WS2 above the Bandgap.

Layered two-dimensional transition-metal dichalcogenides (2D-TMDs) are promising building blocks for ultracompact optoelectronic applications. Recently, a strong second harmonic generation (SHG) was observed in spiral stacked TMD nanostructures which was explained by its low crystal symmetry. However, the relationship between the efficiency of SHG signals and the electronic band structure remains unclear. Here, we show that the SHG signal in spiral WS2 nanostructures is strongly enhanced (∼100 fold increase) not only when the second harmonic signal is in resonance with the exciton states but also when the excitation energy is slightly above the electronic band gap, which we attribute to a large interband Berry connection associated with certain optical transitions in spiral WS2. The giant SHG enhancement observed and explained in this study could promote the understanding and utility of TMDs as highly efficient nonlinear optical materials and potentially lead to a new pathway to fabricate more efficient optical energy conversion devices.

Triple sum frequency pump-probe spectroscopy of transition metal dichalcogenides

Triple sum-frequency (TSF) spectroscopy measures multidimensional spectra by resonantly exciting multiple quantum coherences of vibrational and electronic states. In this work we demonstrate pump-TSF-probe spectroscopy in which a pump excites a sample and some time later three additional electric fields generate a probe field which is measured. We demonstrate pump-TSF-probe spectroscopy on polycrystalline, smooth, thin films and spiral nanostructures of both MoS2 and WS2. The pump-TSF-probe spectra are qualitatively similar to the more conventional transient-reflectance spectra. While transient-reflectance sensitivity suffers under low surface coverage, pump-TSF-probe sensitivity is independent of the sample coverage and nanostructure morphologies. Our results demonstrate that pump-TSF-probe is a valuable methodology for studying microscopic material systems.

Anisotropic nature of thermal conductivity in graphene spirals revealed by molecular dynamics simulations

Spiral nanostructures with many potential applications in next-generation high-tech industries can show outstanding mechanical, electrical and thermal properties thanks to their special topologies. In this study, we report a detailed analysis of heat transport in graphene spiral nanostructures. The non-equilibrium molecular dynamics simulation is used to investigate the thermal conductivity of the spirals in both axial (symmetric) and radial (asymmetric) directions for different geometries. The results reveal that with increasing widths of the graphene spirals, effective thermal conductivity (kA) in the axial direction and the thermal conductivity (k) in the radial direction increase. But with increasing the lengths of the graphene spirals (number of turns), only the axial thermal conductivity increases. Furthermore, the axial tensile strain and the addition of an extra layer to form a double-layer spiral structure have positive effects on increasing the thermal conductivity in the axial direction. Moreover, the results show that the thermal conductivity in radially inward and outward directions differ from each other arising from asymmetricity between the two radial directions where the heat flux prefers the inward direction.

Role of Chemical Doping in Large Deformation Behavior of Spiral Carbon-Based Nanostructures: Unraveling Geometry-Dependent Chemical Doping Effects

The ever-increasing need for miniaturization of electromechanical devices has led us to exploit the properties of nanomaterials as well as controlling them. Chemical doping is one of the most commonly used techniques for controlling the properties of nanomaterials. Spiral carbon-based nanostructures possess excellent electrical properties, which are highly improved with chemical doping; however, the effect of chemical doping on their mechanical properties is still unknown. In this study, molecular dynamics simulation is conducted to study the effect of random/patterned boron and nitrogen doping in different percentages on the mechanical properties of spiral carbon-based nanostructures. The results show a significant impact of the geometry on the mechanical response of doped spiral nanostructures. Furthermore, increasing the percentage of the chemical doping influences the mechanical behavior of these nanoparticles, which can reduce their extensive stretchability even up to 50%. Chemical doping at the posi...

Magnetization reversal in ferromagnetic Fibonacci nano-spirals

Abstract Nanostructured ferromagnetic materials are applicable in a broad range of technologies, especially in the areas of data storage and spintronics. In addition, special structures are of high interest in basic research, aiming at understanding magnetization reversal processes on the nano-scale and thus possibly creating new applications. The strong influence of the shape anisotropy in magnetic nanoparticles enables tailoring magnetization reversal processes by the shape of such a particle. Typically, symmetrical structures are investigated, such as nano-dots, doughnuts, squares, rectangles, etc. Here we report on spiral nano-structures with constantly varying bending radii, modelled according to the Fibonacci spiral which is well-known from diverse plants and biological processes and also related to many mathematical problems. The magnetic Fibonacci nano-spirals with a thicker and a thinner lateral structure size as well as a mirrored form, building a heart-like shape, show large numbers of steps in the hysteresis loops, corresponding to stable intermediate states, due to nucleation and annihilation of domains which do not propagate along the spirals. The numbers of these stable intermediate states which could be utilized for data storage as well as the coercive fields depend strongly on the spiral dimensions and the orientation of the external magnetic field. Simulating minor loops showed the high stability of the intermediate states and thus underlined the possibility to use such structures for data storage.

Spatially and spectrally resolved orbital angular momentum interactions in plasmonic vortex generators

Understanding the near-field electromagnetic interactions that produce optical orbital angular momentum (OAM) is crucial for integrating twisted light into nanotechnology. Here, we examine the cathodoluminescence (CL) of plasmonic vortices carrying OAM generated in spiral nanostructures. The nanospiral geometry defines a photonic local density of states that is sampled by the electron probe in a scanning transmission electron microscope (STEM), thus accessing the optical response of the plasmonic vortex with high spatial and spectral resolution. We map the full spectral dispersion of the plasmonic vortex in spiral structures designed to yield increasing topological charge. Additionally, we fabricate nested nanospirals and demonstrate that OAM from one nanospiral can be coupled to the nested nanospiral, resulting in enhanced luminescence in concentric spirals of like handedness with respect to concentric spirals of opposite handedness. The results illustrate the potential for generating and coupling plasmonic vortices in chiral nanostructures for sensitive detection and manipulation of optical OAM.

High degree of circular polarization in WS2 spiral nanostructures induced by broken symmetry

We present helicity resolved photoluminescence (PL) measurements of WS2 spiral (SPI) nanostructures. We show that very high degree of circular polarization (DCP) (~94 ± 4%) is obtained from multilayer SPI samples at room temperature upon excitation with a circularly polarized laser at a wavelength near-resonant with the A-exciton (633 nm). TEM analysis showed that these SPI nanostructures have AB stacking in which the inversion symmetry is broken, and hence this leads to very high DCP. Comparison with PL from monolayer and bi-layer WS2 samples, along with polarization resolved PL studies provide evidence for suppression of interlayer/intravalley scattering in the multilayer SPI samples.

Insight into Geometry-Controlled Mechanical Properties of Spiral Carbon-Based Nanostructures

The spiral structures of carbon-based materials such as coiled carbon nanotube (CCNT) and graphene helicoid have attracted great attention for use in electrical and mechanical nanodevices. There are a couple of main reasons for this attitude such as striking properties and behavioral diversity with regard to the ever-increasing need for miniaturization of devices. In this research, using atomistic simulations, the effects of geometric parameters (e.g., cross-sectional shape, pitch angle, inner diameter, and outer diameter) on the mechanical properties of CCNT are studied. Interestingly, the results show that the mechanical properties (e.g., Young’s modulus, stretchability, etc.) have a heavy reliance on CCNTs’ geometric parameters. The stretching of the CCNT increases with the raising inner radius. Geometric changes affect the various stages that the CCNTs encounter during tensile and compression tests. The different mechanical behavior of various types of CCNTs leads to their diverse applications. Thus, ...