Yifang Chen

Nanofabrication and coloration study of artificial Morpho butterfly wings with aligned lamellae layers

The bright and iridescent blue color from Morpho butterfly wings has attracted worldwide attentions to explore its mysterious nature for long time. Although the physics of structural color by the nanophotonic structures built on the wing scales has been well established, replications of the wing structure by standard top-down lithography still remains a challenge. This paper reports a technical breakthrough to mimic the blue color of Morpho butterfly wings, by developing a novel nanofabrication process, based on electron beam lithography combined with alternate PMMA/LOR development/dissolution, for photonic structures with aligned lamellae multilayers in colorless polymers. The relationship between the coloration and geometric dimensions as well as shapes is systematically analyzed by solving Maxwell’s Equations with a finite domain time difference simulator. Careful characterization of the mimicked blue by spectral measurements under both normal and oblique angles are carried out. Structural color in blue reflected by the fabricated wing scales, is demonstrated and further extended to green as an application exercise of the new technique. The effects of the regularity in the replicas on coloration are analyzed. In principle, this approach establishes a starting point for mimicking structural colors beyond the blue in Morpho butterfly wings.

High-resolution plasmonic structural colors from nanohole arrays with bottom metal disks.

We present transmissive plasmonic structural colors from subwavelength nanohole arrays with bottom metal disks for scaled-up manufacturing by nanoimprint lithography (NIL). Comprehensive theoretical and experimental studies are carried out to understand the specific extraordinary optical transmission behavior of the structures with such bottom metal disks. Distinctive colors covering the entire visible spectrum can be generated by changing the structural dimensions of hole arrays in Ag covered by the metal disks. The plasmonic energy hybridization theory is applied to explain the unstable color output with shallow holes so that a large processing window during NIL could be achieved for mass production. A high-resolution of 127,000 dots per inch is demonstrated with potential applications, including color filters and displays, high-resolution color printing, CMOS color imaging, and anti-counterfeiting.

Characterizations of nanoembossed Pb(Zr0.3,Ti0.7)O3 ferroelectric films

Ferroelectric thin films may find potential applications in a broad range of ferroelectronic devices such as mass-storage memories. In this article, arrays of Pb(Zr0.3,Ti0.7)O3 (PZT) ferroelectric cells with minimum lateral size down to 500 nm were fabricated by nanoembossing technique. Structural characterizations of embossed PZT film were carried out by Raman spectroscopy and x-ray diffraction. Ferroelectronic properties of embossed PZT film were investigated by using piezoresponse force microscopy and Radiant Technologies precision material analyzer. Excellent ferroelectric and piezoelectric characteristics observed in the embossed PZT films suggest that the nanoembossing process proposed in this article is promising to become a new manufacturing approach for high density PZT based memory devices at significantly lower cost than the existing technique.

Gold nanopillar arrays as biosensors fabricated by electron beam lithography combined with electroplating

We report our work on the development of subwavelength gold pillar arrays as local surface plasmonic (LSP) resonators for sensor applications. These arrays are fabricated by electron beam lithography combined with electroplating. The conical shape, instead of flat one, on the top of Au pillars, induced by uneven current density in the plating, may affect the LSP resonance (LSPR). This paper aims to carry out a systematic study of LSPR behavior in nanopillar arrays with both flat and conical shapes on the top, trying to prove the feasibility of the developed nanoprocess. Both numerical simulations by the finite-difference time-domain (FDTD) method and experimental characterization on fabricated LSP resonators for reflectance spectra were carried out. Our experiments indicate that the fabricated nanopillar arrays in Au demonstrate the promising capability of refractive index sensing with sensitivity of 270xa0nm/refractive index unit. FDTD simulation of electric field density in the gap between pillars reveals the correlation between the resonant absorption of the incident light and the standing waves of localized surface plasmon polaritons in the gaps of the pillar array, despite the conical shape of the pillars. Moreover, it was discovered that the resonant absorption becomes stronger when the light incident angle is increased. The proposed nanoprocess for pillar arrays should possess great prospects for manufacturing Au pillars with high aspect ratio for achieving higher sensitivity at an economical cost.

Subwavelength Gold Grating as Polarizers Integrated with InP-Based InGaAs Sensors

There are currently growing needs for polarimetric imaging in infrared wavelengths for broad applications in bioscience, communications and agriculture, etc. Subwavelength metallic gratings are capable of separating transverse magnetic (TM) mode from transverse electric (TE) mode to form polarized light, offering a reliable approach for the detection in polarization way. This work aims to design and fabricate subwavelength gold gratings as polarizers for InP-based InGaAs sensors in 1.0-1.6 μm. The polarization capability of gold gratings on InP substrate with pitches in the range of 200-1200 nm (fixed duty cycle of 0.5) has been systematically studied by both theoretical modeling with a finite-difference time-domain (FDTD) simulator and spectral measurements. Gratings with 200 nm lines/space in 100-nm-thick gold have been fabricated by electron beam lithography (EBL). It was found that subwavelength gold gratings directly integrated on InP cannot be applied as good polarizers, because of the existence of SPP modes in the detection wavelengths. An effective solution has been found by sandwiching the Au/InP bilayer using a 200 nm SiO2 layer, leading to significant improvement in both TM transmission and extinction ratio. At 1.35 μm, the improvement factors are 8 and 10, respectively. Therefore, it is concluded that the Au/SiO2/InP trilayer should be a promising candidate of near-infrared polarizers for the InP-based InGaAs sensors.

Photon nanojet lens: Design, fabrication and characterization

In this paper, a novel nanolens with super resolution, based on the photon nanojet effect through dielectric nanostructures in visible wavelengths, is proposed. The nanolens is made from plastic SU-8, consisting of parallel semi-cylinders in an array. This paper focuses on the lens designed by numerical simulation with the finite-difference time domain method and nanofabrication of the lens by grayscale electron beam lithography combined with a casting/bonding/lift-off transfer process. Monte Carlo simulation for injected charge distribution and development modeling was applied to define the resultant 3D profile in PMMA as the template for the lens shape. After the casting/bonding/lift-off process, the fabricated nanolens in SU-8 has the desired lens shape, very close to that of PMMA, indicating that the pattern transfer process developed in this work can be reliably applied not only for the fabrication of the lens but also for other 3D nanopatterns in general. The light distribution through the lens near its surface was initially characterized by a scanning near-field optical microscope, showing a well defined focusing image of designed grating lines. Such focusing function supports the great prospects of developing a novel nanolithography based on the photon nanojet effect.

APPLICATIONS OF NANOIMPRINT LITHOGRAPHY FOR BIOCHEMICAL AND NANOPHOTONIC STRUCTURES USING SU-8

Nanoimprint lithography (NIL) technology has aroused great interests in both academia and industry due to its high resolution, low-cost, and high-volume nanopatterning capability. And as an expoxy resin-based negative amplified photoresist, SU-8 is an ideal candidate for NIL because of its low-glass-transition temperature, low-volume shrinkage coefficient, and good optical properties. In this reviewing paper, we highlight the major technical achievements in NIL on epoxy resin and its applications for bio- and nanophotonic structures. NIL was also applied for the duplication of imprint templates, originally fabricated by e-beam lithography (EBL) followed by reactive ion etch (RIE), using a SU-8/SiO2/PMMA tri-layer technique. And nanoimprint properties were systematically investigated for optimization. The developed nanoimprint process for different applications indicates promising industrial potentials in the next generation lithography resolution.

Low-voltage-exposure-enabled hydrogen silsesquioxane bilayer-like process for three-dimensional nanofabrication.

We report a bilayer-like electron-beam lithographic process to obtain three-dimensional (3D) nanostructures by using only a single hydrogen silsesquioxane (HSQ) resist layer. The process utilizes the short penetration depth of low-energy (1.5 keV) electron irradiation to first obtain a partially cross-linked HSQ top layer and then uses a high-voltage electron beam (30 keV) to obtain self-aligned undercut (e.g. mushroom-shaped) and freestanding HSQ nanostructures. Based on the well-defined 3D resist patterns, 3D metallic nanostructures were directly fabricated with high fidelity by just depositing a metallic layer. As an example, Ag-coated mushroom-shaped nanostructures were fabricated, which showed lower plasmon resonance damping compared to their planar counterparts. In addition, the undercut 3D nanostructures also enable more reliable lift-off in comparison with the planar nanostructures, with which high-quality silver nanohole arrays were fabricated which show distinct and extraordinary optical transmission in the visible range.

Simulation and experimental study of aspect ratio limitation in Fresnel zone plates for hard-x-ray optics.

For acquiring high-contrast and high-brightness images in hard-x-ray optics, Fresnel zone plates with high aspect ratios (zone height/zone width) have been constantly pursued. However, knowledge of aspect ratio limits remains limited. This work explores the achievable aspect ratio limit in polymethyl methacrylate (PMMA) by electron-beam lithography (EBL) under 100 keV, and investigates the lithographic factors for this limitation. Both Monte Carlo simulation and EBL on thick PMMA are applied to investigate the profile evolution with exposure doses over 100 nm wide dense zones. A high-resolution scanning electron microscope at low acceleration mode for charging free is applied to characterize the resultant zone profiles. It was discovered for what we believe is the first time that the primary electron-beam spreading in PMMA and the proximity effect due to extra exposure from neighboring areas could be the major causes of limiting the aspect ratio. Using the optimized lithography condition, a 100 nm zone plate with aspect ratio of 15/1 was fabricated and its focusing property was characterized at the Shanghai Synchrotron Radiation Facility. The aspect ratio limit found in this work should be extremely useful for guiding further technical development in nanofabrication of high-quality Fresnel zone plates.

Multistep Aztec profiles by grayscale electron beam lithography for angle-resolved microspectrometer applications

In this paper, grayscale electron beam lithography is applied to generate multistep Aztec profiles (MAPs) for angle-resolved spectral applications such as microspectrometers. Monte Carlo simulations taking into consideration the proximity effect are carried out to calculate the spatial dose distributions for desired profiles, using actual dissolution rates measured on the same resist. The MAPs in PMMA resist with step heights from 50 to 200u2009nm and step widths from 0.1 to 5u2009μm are achieved by high-resolution electron beam lithography, and high-resolution scanning electron microscopy and atomic force microscopy are used to characterize the quality of the MAPs. Angle-resolved spectra of the reflectance are obtained using a finite-difference time-domain simulator and by experimental measurements. A distinct angle selection of the wavelengths is demonstrated, though the high surface roughness measured on the deeper steps may cause broadening of the spectral peaks. Initial investigations into the origin of the ...