Small footprint optoelectrodes using ring resonators for passive light localization.

The combination of electrophysiology and optogenetics enables the exploration of how the brain operates down to a single neuron and its network activity. Neural probes are in vivo invasive devices that integrate sensors and stimulation sites to record and manipulate neuronal activity with high spatiotemporal resolution. State-of-the-art probes are limited by tradeoffs involving their lateral dimension, number of sensors, and ability to access independent stimulation sites. Here, we realize a highly scalable probe that features three-dimensional integration of small-footprint arrays of sensors and nanophotonic circuits to scale the density of sensors per cross-section by one order of magnitude with respect to state-of-the-art devices. For the first time, we overcome the spatial limit of the nanophotonic circuit by coupling only one waveguide to numerous optical ring resonators as passive nanophotonic switches. With this strategy, we achieve accurate on-demand light localization while avoiding spatially demanding bundles of waveguides and demonstrate the feasibility with a proof-of-concept device and its scalability towards high-resolution and low-damage neural optoelectrodes.

Nanoimprinted High-Refractive Index Active Photonic Nanostructures Based on Quantum Dots for Visible Light.

A novel method to realizing printed active photonic devices was developed using nanoimprint lithography (NIL), combining a printable high-refractive index material and colloidal CdSe/CdS quantum dots (QDs) for applications in the visible region. Active media QDs were applied in two different ways: embedded inside a printable high-refractive index matrix to form an active printable hybrid nanocomposite, and used as a uniform coating on top of printed photonic devices. As a proof-of-demonstration for printed active photonic devices, two-dimensional (2-D) photonic crystals as well as 1D and 2D photonic nanocavities were successfully fabricated following a simple reverse-nanoimprint process. We observed enhanced photoluminescence from the 2D photonic crystal and the 1D nanocavities. Outstandingly, the process presented in this study is fully compatible with large-scale manufacturing where the patterning areas are only limited by the size of the corresponding mold. This work shows that the integration of active media and functional materials is a promising approach to the realization of integrated photonics for visible light using high throughput technologies. We believe that this work represents a powerful and cost-effective route for the development of numerous nanophotonic structures and devices that will lead to the emergence of new applications.

Campanile Near-Field Probes Fabricated by Nanoimprint Lithography on the Facet of an Optical Fiber.

One of the major challenges to the widespread adoption of plasmonic and nano-optical devices in real-life applications is the difficulty to mass-fabricate nano-optical antennas in parallel and reproducible fashion, and the capability to precisely place nanoantennas into devices with nanometer-scale precision. In this study, we present a solution to this challenge using the state-of-the-art ultraviolet nanoimprint lithography (UV-NIL) to fabricate functional optical transformers onto the core of an optical fiber in a single step, mimicking the 'campanile' near-field probes. Imprinted probes were fabricated using a custom-built imprinter tool with co-axial alignment capability with sub <100 nm position accuracy, followed by a metallization step. Scanning electron micrographs confirm high imprint fidelity and precision with a thin residual layer to facilitate efficient optical coupling between the fiber and the imprinted optical transformer. The imprinted optical transformer probe was used in an actual NSOM measurement performing hyperspectral photoluminescence mapping of standard fluorescent beads. The calibration scans confirmed that imprinted probes enable sub-diffraction limited imaging with a spatial resolution consistent with the gap size. This novel nano-fabrication approach promises a low-cost, high-throughput, and reproducible manufacturing of advanced nano-optical devices.

Low temperature dry etching of chromium towards control at sub-5 nm dimensions.

Patterned chromium and its compounds are crucial materials for nanoscale patterning and chromium based devices. Here we investigate how temperature can be used to control chromium etching using chlorine/oxygen gas mixtures. Oxygen/chlorine ratios between 0% and 100% and temperatures between -100 °C and +40 °C are studied. Spectroscopic ellipsometry is used to precisely measure rates, chlorination, and the thickness dependence of n and k. Working in the extremes of oxygen content (very high or very low) and lower temperatures, we find rates can be controlled to nanometers per minute. Activation energies are measured and show that etch mechanisms are both temperature and oxygen level dependent. Furthermore, we find that etching temperature can manipulate the surface chemistry. One surprising consequence is that at low oxygen levels, Etching rates increase with decreasing temperature. Preliminary feature-profile studies show the extremes of temperature and oxygen provide advantages over commonly used room temperature processing conditions. One example is with higher ion energies at -100 °C, where etching products deposit.

Nanoimprint of a 3D structure on an optical fiber for light wavefront manipulation.

Integration of complex photonic structures onto optical fiber facets enables powerful platforms with unprecedented optical functionalities. Conventional nanofabrication technologies, however, do not permit viable integration of complex photonic devices onto optical fibers owing to their low throughput and high cost. In this paper we report the fabrication of a three-dimensional structure achieved by direct nanoimprint lithography on the facet of an optical fiber. Nanoimprint processes and tools were specifically developed to enable a high lithographic accuracy and coaxial alignment of the optical device with respect to the fiber core. To demonstrate the capability of this new approach, a 3D beam splitter has been designed, imprinted and optically characterized. Scanning electron microscopy and optical measurements confirmed the good lithographic capabilities of the proposed approach as well as the desired optical performance of the imprinted structure. The inexpensive solution presented here should enable advancements in areas such as integrated optics and sensing, achieving enhanced portability and versatility of fiber optic components.

High refractive index Fresnel lens on a fiber fabricated by nanoimprint lithography for immersion applications.

In this Letter, we present a Fresnel lens fabricated on the end of an optical fiber. The lens is fabricated using nanoimprint lithography of a functional high refractive index material, which is suitable for mass production. The main advantage of the presented Fresnel lens compared to a conventional fiber lens is its high refractive index (n=1.68), which enables efficient light focusing even inside other media, such as water or an adhesive. Measurement of the lens performance in an immersion liquid (n=1.51) shows a near diffraction limited focal spot of 810 nm in diameter at the 1/e2 intensity level for a wavelength of 660 nm. Applications of such fiber lenses include integrated optics, optical trapping, and fiber probes.

Printable photonic crystals with high refractive index for applications in visible light.

Nanoimprint lithography (NIL) of functional high-refractive index materials has proved to be a powerful candidate for the inexpensive manufacturing of high-resolution photonic devices. In this paper, we demonstrate the fabrication of printable photonic crystals (PhCs) with high refractive index working in the visible wavelengths. The PhCs are replicated on a titanium dioxide-based high-refractive index hybrid material by reverse NIL with almost zero shrinkage and high-fidelity reproducibility between mold and printed devices. The optical responses of the imprinted PhCs compare very well with those fabricated by conventional nanofabrication methods. This study opens the road for a low-cost manufacturing of PhCs and other nanophotonic devices for applications in visible light.

Low-temperature plasma etching of high aspect-ratio densely packed 15 to sub-10 nm silicon features derived from PS-PDMS block copolymer patterns.

The combination of block copolymer (BCP) lithography and plasma etching offers a gateway to densely packed sub-10 nm features for advanced nanotechnology. Despite the advances in BCP lithography, plasma pattern transfer remains a major challenge. We use controlled and low substrate temperatures during plasma etching of a chromium hard mask and then the underlying substrate as a route to high aspect ratio sub-10 nm silicon features derived from BCP lithography. Siloxane masks were fabricated using poly(styrene-b-siloxane) (PS-PDMS) BCP to create either line-type masks or, with the addition of low molecular weight PS-OH homopolymer, dot-type masks. Temperature control was essential for preventing mask migration and controlling the etched feature's shape. Vertical silicon wire features (15 nm with feature-to-feature spacing of 26 nm) were etched with aspect ratios up to 17 : 1; higher aspect ratios were limited by the collapse of nanoscale silicon structures. Sub-10 nm fin structures were etched with aspect ratios greater than 10 : 1. Transmission electron microscopy images of the wires reveal a crystalline silicon core with an amorphous surface layer, just slightly thicker than a native oxide.

Minimizing electrostatic charging of an aperture used to produce in-focus phase contrast in the TEM.

Microfabricated devices designed to provide phase contrast in the transmission electron microscope must be free of phase distortions caused by unexpected electrostatic effects. We find that such phase distortions occur even when a device is heated to 300 °C during use in order to avoid the formation of polymerized, carbonaceous contamination. Remaining factors that could cause unwanted phase distortions include patchy variations in the work function of a clean metal surface, radiation-induced formation of a localized oxide layer, and creation of a contact potential between an irradiated area and the surround due to radiation-induced structural changes. We show that coating a microfabricated device with evaporated carbon apparently eliminates the problem of patchy variation in the work function. Furthermore, we show that a carbon-coated titanium device is superior to a carbon-coated gold device, with respect to radiation-induced electrostatic effects. A carbon-coated, hybrid double-sideband/single-sideband aperture is used to record in-focus, cryo-EM images of monolayer crystals of streptavidin. Images showing no systematic phase error due to charging are achievable under conditions of low-dose data collection. The contrast in such in-focus images is sufficient that one can readily see individual streptavidin tetramer molecules. Nevertheless, these carbon-coated devices perform well for only a limited length of time, and the cause of failure is not yet understood.