Miniaturized fractal optical nanoantennas defined by focused helium ion beam milling

Nano-antennas with decoupled transparent leads for optoelectronic studies

Performing electrical measurements on single plasmonic nanostructures presents a challenging task due to the limitations in contacting the structure without disturbing its optical properties. In this work, we show two ways to overcome this problem by fabricating bow-tie nano-antennas with indium tin oxide leads. Indium tin oxide is transparent in the visible range and electrically conducting, but non-conducting at optical frequencies. The structures are prepared by electron beam lithography. Further definition, such as introducing small gaps, is achieved by focused helium ion beam milling. Dark-field reflection spectroscopy characterization of the dimer antennas shows typical unperturbed plasmonic spectra with multiple resonance peaks from mode hybridization.

Bio-Inspired Nanomembranes as Building Blocks for Nanophotonics, Plasmonics and Metamaterials

Nanomembranes are the most widespread building block of life, as they encompass cell and organelle walls. Their synthetic counterparts can be described as freestanding or free-floating structures thinner than 100 nm, down to monatomic/monomolecular thickness and with giant lateral aspect ratios. The structural confinement to quasi-2D sheets causes a multitude of unexpected and often counterintuitive properties. This has resulted in synthetic nanomembranes transiting from a mere scientific curiosity to a position where novel applications are emerging at an ever-accelerating pace. Among wide fields where their use has proven itself most fruitful are nano-optics and nanophotonics. However, the authors are unaware of a review covering the nanomembrane use in these important fields. Here, we present an attempt to survey the state of the art of nanomembranes in nanophotonics, including photonic crystals, plasmonics, metasurfaces, and nanoantennas, with an accent on some advancements that appeared within the last few years. Unlimited by the Nature toolbox, we can utilize a practically infinite number of available materials and methods and reach numerous properties not met in biological membranes. Thus, nanomembranes in nano-optics can be described as real metastructures, exceeding the known materials and opening pathways to a wide variety of novel functionalities.

A Parabola-like Gold Nanobowtie on a Sapphire Substrate as a Nano-Cavity

Plasmonic, metallic nanostructures have attracted much interest for their ability to manipulate light on a subwavelength scale and for their related applications in various fields. In this work, a parabola-like gold nanobowtie (PGNB) on a sapphire substrate was designed as a nano-cavity for confining light waves in a nanoscale gap region. The near-field optical properties of the innovative PGNB structure were studied comprehensively, taking advantage of the time-resolved field calculation based on a finite-difference time-domain algorithm (FDTD). The calculation result showed that the resonance wavelength of the nano-cavity was quite sensitive to the geometry of the PGNB. The values that related to the scattering and absorption properties of the PGNB, such as the scattering cross section, absorption cross section, extinction cross section, scattering ratio, and also the absorption ratio, were strongly dependent on the geometrical parameters which affected the surface area of the nanobowtie. Increased sharpness of the gold tips on the parabola-like nano-wings benefited the concentration of high-density charges with opposite electric properties in the narrow gold tips with limited volume, thus, resulting in a highly enhanced electric field in the nano-cavity under illumination of the light wave. Reduction of the gap size between the two gold nano-tips, namely, the size of the nano-cavity, decreased the distance that the electric potential produced by the highly concentrated charges on the surface of each gold nano-tip had to jump across, therefore, causing a significantly enhanced field in the nano-cavity. Further, alignment of the linearly polarized electric field of the incident light wave with the symmetric axis of the PGNB efficiently enabled the free electrons in the PGNB to concentrate on the surface of the sharp gold tips with a high density, thus, strongly improving the field across the nano-cavity. The research provides a new insight for future design, nanofabrication, and characterization of PGNBs for applications in devices that relate to enhancing photons emission, improving efficiency for energy harvesting, and improving sensitivity for infrared detection.

Self-Organized Fractal Structures on Plasma-Exposed Silver Surface

Planar fractal microstructure is observed on the silver film treated by positive corona discharge for the first time. Due to the abundant positive ions driven by the electrical field of positive polarity, surface modification is mainly induced by the plasma oxidation effect, resulting in a large scale of dendritic pattern with self-similarity and hierarchy. In contrast, negative ions dominate the plasma-film interaction under negative corona discharge condition, leading to a different surface morphology without fractal characteristics. A growth model based on the modified diffusion-limited aggregation (DLA) theory is proposed to describe the formation of the dendritic fractal structure, whilst the physics behind is attributed to the electric field directed diffusion of the positive ions around the surface roughness. Numerical simulation verifies the high density of the hot spot in the dendritic pattern, which may enable potential applications in fractal photonic metamaterials.

Electro-active properties of nanostructured films of cytosine and guanine nucleobases

The discovery of multifunctional properties related to electro-activity of organic systems of biomolecules is important for a variety of applications, especially for devices in the realm of biocompatible sensors and/or bioactuators. A further step towards such applications is to prepare thin films with the required properties. Here, the investigation is focused on the characterization of films of guanine and cytosine nucleobases, prepared by thermal evaporation-an industrial accessible deposition technique. The cytosine films have an orthorhombic non-centrosymmetric structure and grow in two interconnected nanostructured fractal patterns, of nearly equal proportion. Piezoresponse force microscopy images acquired at room temperature on the cytosine films display large zones with antiparallel alignment of the vertical components of the polarization vector. Guanine films have a dense nano-grained morphology. Our studies reveal electrical polarization switching effects which can be related to ferroelectricity in the films of guanine molecules. Characteristic ferroelectric polarization-electric-field hysteresis loops showing large electrical polarization are observed at low temperatures up to 200 K. Above this temperature, the guanine films have a preponderant paraelectric phase containing residual or locally induced nano-scopic ferroelectric domains, as observed by piezoresponse force microscopy at room temperature.

A review of defect engineering, ion implantation, and nanofabrication using the helium ion microscope

The helium ion microscope has emerged as a multifaceted instrument enabling a broad range of applications beyond imaging in which the finely focused helium ion beam is used for a variety of defect engineering, ion implantation, and nanofabrication tasks. Operation of the ion source with neon has extended the reach of this technology even further. This paper reviews the materials modification research that has been enabled by the helium ion microscope since its commercialization in 2007, ranging from fundamental studies of beam-sample effects, to the prototyping of new devices with features in the sub-10 nm domain.