Jong G. Ok

Structural Colors: From Plasmonic to Carbon Nanostructures

In addition to colorant-based pigmentation, structure is a major contributor to a materials color. In nature, structural color is often caused by the interaction of light with dielectric structures whose dimensions are on the order of visible-light wavelengths. Different optical interactions including multilayer interference, light scattering, the photonic crystal effect, and combinations thereof give rise to selective transmission or reflection of particular light wavelengths, which leads to the generation of structural color. Recent developments in nanofabrication of plasmonic and carbon nanostructures have opened another efficient way to control light properties at the subwavelength scale, including visible-light wavelength selection, which can produce structural color. In this Concept, the most relevant and representative achievements demonstrated over the last several years are presented and analyzed. These plasmonic and carbon nanostructures are believed to offer great potential for high-resolution color displays and spectral filtering applications.

Carbon-Nanotube Optoacoustic Lens for Focused Ultrasound Generation and High-Precision Targeted Therapy

We demonstrate a new optical approach to generate high-frequency (>15 MHz) and high-amplitude focused ultrasound, which can be used for non-invasive ultrasound therapy. A nano-composite film of carbon nanotubes (CNTs) and elastomeric polymer is formed on concave lenses, and used as an efficient optoacoustic source due to the high optical absorption of the CNTs and rapid heat transfer to the polymer upon excitation by pulsed laser irradiation. The CNT-coated lenses can generate unprecedented optoacoustic pressures of >50 MPa in peak positive on a tight focal spot of 75 μm in lateral and 400 μm in axial widths. This pressure amplitude is remarkably high in this frequency regime, producing pronounced shock effects and non-thermal pulsed cavitation at the focal zone. We demonstrate that the optoacoustic lens can be used for micro-scale ultrasonic fragmentation of solid materials and a single-cell surgery in terms of removing the cells from substrates and neighboring cells.

Carbon nanotube composite optoacoustic transmitters for strong and high frequency ultrasound generation.

We demonstrate carbon nanotube (CNT) composite-based optoacoustic transmitters that generate strong and high frequency ultrasound. The composite consists of CNTs grown on a substrate, which are embedded in elastomeric polymer used as an acoustic transfer medium. Under pulsed laser excitation, the composite generates very strong optoacoustic pressure: 18 times stronger than a Cr film reference and five times stronger than a gold nanoparticle composite with the same polymer. This enhancement persists over a broadband frequency range of up to 120 MHz and is confirmed by calculation. We suggest the CNT-polymer composites as highly efficient optoacoustic transmitters for high resolution ultrasound imaging.

Low density carbon nanotube forest as an index-matched and near perfect absorption coating

We demonstrate broadband, near perfect absorption with a conformal coating of a multi-walled carbon nanotube (CNT) forest on an arbitrarily shaped surface. The complex refractive index of such a CNT forest is retrieved from the measured transmission and reflection spectra using Kramers-Kronig constrained variational analysis, which gives a typical value of neff = 1.04 + 0.01i at visible wavelengths. Therefore, when used as a conformal coating on an object, a thick layer of the CNT forest can provide an excellent impedance match to air and near perfect absorption, preventing any detectable light reflection and scattering from the object.

Continuous and scalable fabrication of flexible metamaterial films via roll-to-roll nanoimprint process for broadband plasmonic infrared filters

We demonstrate the continuous fabrication of large-area flexible metamaterial films via roll-to-roll (R2R) nanoimprint lithography (NIL) technique that can be conducted in an ambient environment at high speed. The plasmonic metal-insulator-metal structure is successfully fabricated by R2R NIL to continuously pattern the sub-wavelength scale metal disk array on flexible substrates. The patterned metal disks having varying diameters and sub-micron spacing with few defects lead to the desired broadband IR filtering performance at the designed dual-band, which correlates well with simulation analysis. Our method realizes a simple and high-throughput fabrication of plasmonic metamaterials for scalable and flexible optoelectronic and photonic applications.

Photo-roll lithography (PRL) for continuous and scalable patterning with application in flexible electronics.

A novel nanofabrication methodology for continuous, scalable, and geometry-tunable lithography is developed, named photo-roll lithography (PRL), by integrating photolithography with rollable processing. As a flexible mask attached to a quartz cylinder containing a UV source rolls over a photoresistcoated substrate, PRL realizes continuous photolithographic fabrication of various micro/nanoscale patterns with geometry that is tunable by controlling mask-substrate motions.

Facile route of flexible wire grid polarizer fabrication by angled-evaporations of aluminum on two sidewalls of an imprinted nanograting

In this study, we report a new method to fabricate a wire grid polarizer (WGP) that greatly relaxes the requirement on patterning and etching, and can be easily applied to produce flexible WGPs. The technique is to pattern a high aspect ratio and narrow linewidth grating by nanoimprint lithography followed by two angled aluminum depositions in opposite directions to produce the narrow spacing between the aluminum lines required for a visible band WGP. Anisotropic reactive ion etching is used to remove the aluminum deposited at the top of the grating but leave the aluminum layer on the grating sidewalls, thereby forming a metal wire grid with much smaller spacings than a lithographically defined grating. As a result, the fabricated WGP showed good performance in a wide range of visible wavelength.

Continuous phase-shift lithography with a roll-type mask and application to transparent conductor fabrication

We report the development of a near-field optical nanolithography method using a roll-type phase-shift mask. Sub-wavelength resolution is achieved using near-field exposure of photoresist through a cylindrical phase mask, allowing dynamic and high throughput continuous patterning. As an application, we present the fabrication of a transparent electrode in the form of a metallic wire grid by using the roller-based optical lithography method. To fabricate a mesh-type metal pattern, a specific phase-shift mask was designed and critical experimental parameters were also studied. As a result, a transparent conductor with suitable properties was achieved with a recently built cylindrical phase-shift lithography prototype designed to pattern on 100 mm(2) of substrate area.

Rapid anisotropic photoconductive response of ZnO-coated aligned carbon nanotube sheets.

We investigate the rapid and anisotropic UV-induced photoconductive response of hybrid thin films comprising zinc oxide (ZnO) nanowires (NWs) directly grown on horizontally aligned (HA-) carbon nanotube (CNT) sheets. The films exhibit anisotropic photoconductivity; along the CNTs, conductivity is dominated by the CNTs and the photoconductive gain is lower, whereas perpendicular to the CNTs the photoconductive gain is higher because transport is influenced by ZnO nanoclusters bridging CNT-CNT contacts. Because of the distributed electrical contact provided by the large number of ZnO NWs on top of the HACNT film, this hybrid nanoarchitecture has a significantly greater photocurrent than reported for single ZnO NW-based devices at comparable UV illumination intensity. Moreover, the hybrid architecture where a thin basal film of ZnO ohmically contacts metallic CNTs enables rapid transport of photogenerated electrons from ZnO to CNTs, resulting in sub-second photoresponse upon pulsed illumination. The built-in potential generated across ZnO-CNT heterojunctions competes with the externally applied bias to control the photocurrent amplitude and direction. By tuning the anisotropic conductivity of the CNT network and the morphology of the ZnO or potentially other nanostructured coatings, this material architecture may be engineered in the future to realize high-performance optical and chemical sensors.

Continuous patterning of nanogratings by nanochannel-guided lithography on liquid resists.

Department of Mechanical Engineering University of Michigan Ann Arbor, MI 48109, USA H. J. Park , Dr. C. A. Pina-Hernandez , Prof. L. J. Guo Macromolecular Science and Engineering University of Michigan Ann Arbor, MI 48109, USAE-mail: guo@umich.edu Dr. M. K. Kwak , Prof. L. J. Guo Department of Electrical Engineering and Computer Science University of Michigan Ann Arbor, MI 48109, USA [ +] Present address: Molecular Imprints, Inc., Austin, TX 78758, USA