We present a visible light interference lithography technique that utilizes a 2x2 cm metasurface mask to enable fabrication of 8x8 cm continuous and homogenous nano-architected materials. Patterns are resolved both in commercial 20-60 um films of SU-8 and >20 um films of custom glycidyl methacrylate-derived negative-tone photoresists. The combination of our metasurface-enabled large-scale 3D patterning technique with customizable photoresist chemistry provides a new pathway for scalable production of architected materials with nanometer feature resolution and advanced functional properties. Impact experiments using Laser-Induced Particle Impact Testing (LIPIT) were conducted to probe mechanical response and material homogeneity.
Nano-architected materials have the potential to be adopted in several areas including photonic devices and structural materials. We present a 3D interference lithography technique with dielectric metasurfaces at visible wavelengths that allows patterning of thick epoxide films over areas on the order of 10 cm^2 with 100 nm resolution. By leveraging the ability of the metasurface to control the amplitude and phase of a wavefront, complex near-field 3D interference patterns can be designed. Pyrolysis of 3D patterned SU-8 produces a carbon-based material with sub-100 nm features and enhanced mechanical properties.
Fabrication of 3D dielectric photonic crystals in the visible and in the infrared range typically requires sub-micron structural features and high-refractive index materials. We developed a template-free additive manufacturing (AM) process based on direct laser writing (DLW) that can create complex 3D architectures out of titania (TiO2) with ~100 nm resolution. In this process, we synthesize hybrid organic-inorganic materials that contain titanium clusters coordinated with acrylic ligands to prepare a photoresist that is amenable to two-photon lithography (TPL). We sculpt a pre-ceramic architecture using TPL and then pyrolyze in air at 900°C to remove the organic constituents to produce a replica of the original structure with ~70% reduced linear dimensions. Energy-Dispersive Spectroscopy (EDS) and Raman spectroscopy confirm the constituent solid to consist predominantly out of rutile titania.
We demonstrate this process by fabricating titania woodpile structures with lateral dimensions of 70 × 70 μm and lateral periodicities between 1.0 and 1.3 μm. Fourier Transform Infrared (FTIR) spectroscopy reveals passive tuning of the reflectance peak between 1.7 and 2.3 μm, which agrees with Plane Wave Expansion simulations. This titania AM process offers a promising pathway to efficiently fabricate complex 3D nano-architectures out of a high-index material for 3D dielectric photonic crystals in the visible and the infrared.
Interest in negative refraction has been motivated by the possibility of creating a “superlens” as proposed by Pendry (Phys. Rev. Lett. 85, 3966 (2000)). This theoretical work showed that a material capable of negative refraction amplifies evanescent waves and allows this material to act as a lens with a resolution not limited by working wavelength. Although theory and some experiments have shown that certain metamaterials and photonic crystals (PhCs) can act as superlenses, realistic demonstration of negative refraction in the optical and infrared range remains a challenge. This is because most metamaterials employ lossy metal elements and most PhC structures found to exhibit negative refraction are made of positive index dielectric materials and are two-dimensional. Subwavelength imaging of a 3D object requires a 3D PhC capable of negative refraction.
Inspired by the numerical simulations of Luo, et. al. (Appl. Phys. Lett. 81, 2352 (2002)), we demonstrate the fabrication and characterization of a 500nm-diameter polymer core, 250nm-thick Germanium shell 3D photonic crystal lattice that exhibits negative refraction in the mid-infrared, centered around 8µm. This 3D photonic crystal resembles a BCC lattice of air cubes in dielectric media and was fabricated using two-photon lithography direct laser writing of an acrylic polymer resin scaffold followed by RF sputtering of Ge. The band structure of the lattice was mapped using FTIR spectroscopy reflectance measurements, and negative refraction was observed using far-field IR transmission imaging.
Sub-wavelength arrays have garnered significant interest for many potential optoelectronics applications. We fabricated sub-wavelength silicon nanopillar arrays with a ratio of radius, r and a center-to-center distance, a, of r/a ≈ 0.2 that were fully embedded in SiO2 for narrow stopband filters that are compact and straightforward to fabricate compared to conventional Bragg stack reflectors. These arrays are well-suited for hyperspectral filtering applications in the infrared. They are ultra-thin (<0.1λ), polarization-independent, and attain greater efficiencies enabled by low loss compared to plasmonic-based designs. The choice of Si as the nanopillar material stems from its low cost, high index of refraction, and a band gap of 1.1 eV near the edge of the visible.
These arrays exhibit narrow near-unity reflectivity resonances that arise from coupling of an incident wave into a leaky waveguide mode via a grating vector that is subsequently reradiated, also known as guided mode resonances (GMRs). Simulations reveal reflectivities of >99% with full width at half maxima (FWHM) of ≈0.01 μm. We demonstrate a fabrication route for obtaining nanopillar arrays that exhibit these GMRs. We experimentally observed a GMR with an amplitude of ~0.8 for filter arrays fabricated on silicon on insulator (SOI) substrates, combined with Fabry-Perot interference that stems from the underlying silicon layer.
We use a plane wave expansion method to define parameters for the fabrication of 3-dimensional (3D) core-shell photonic crystals (PhCs) with lattice geometries that are capable of all-angle negative refraction (AANR) in the midinfrared centered around 8.0 μm. We discuss the dependence of the AANR frequency range on the volume fraction of solid within the lattice and on the ratio of the low index core material to the high index shell material. Following the constraints set by simulations, we fabricate two types of nanolattice PhCs: (1) polymer core-germanium shell and (2) amorphous carbon core-germanium shell to enable experimental observation of 3D negative refraction and related dispersion phenomena at infrared and eventually optical frequencies.
Three-dimensional (3D) photonic crystals have potential in solid state lighting applications due to their advantages over conventional planar thin film devices. Periodicity in a photonic crystal structure enables engineering of the density of states to improve spontaneous light emission according to Fermi’s golden rule. Unlike planar thin films, which suffer significantly from total internal reflection, a 3D architectured structure is distributed in space with many non-flat interfaces, which facilitates a substantial enhancement in light extraction. We demonstrate the fabrication of 3D nano-architectures with octahedron geometry that utilize luminescing silicon nanocrystals as active media with an aluminum cathode and indium tin oxide anode towards the realization of a 3D light emitting device. The developed fabrication procedure allows charge to pass through the nanolattice between two contacts for electroluminescence. These initial fabrication efforts suggest that 3D nano-architected devices are realizable and can reach greater efficiencies than planar devices.
We provide an overview of our work where carbon-based nanostructures have been applied to twodimensional
(2D) planar and three-dimensional (3D) vertically-oriented nano-electro-mechanical (NEM)
switches. In the first configuration, laterally oriented single-walled nanotubes (SWNTs) synthesized using
thermal chemical vapor deposition (CVD) were implemented for forming bridge-type 2D NEMS switches,
where switching voltages were on the order of a few volts. In the second configuration, vertically oriented
carbon nanofibers (CNFs) synthesized using plasma-enhanced (PE) CVD have been explored for their
potential application in 3D NEMS. We have performed nanomechanical measurements on such vertically
oriented tubes using nanoindentation to determine the mechanical properties of the CNFs. Electrostatic
switching was demonstrated in the CNFs synthesized on refractory metallic nitride substrates, where a
nanoprobe was used as the actuating electrode inside a scanning-electron-microscope. The switching voltages
were determined to be in the tens of volts range and van der Waals interactions at these length scales appeared
significant, suggesting such structures are promising for nonvolatile memory applications. A finite element
model was also developed to determine a theoretical pull-in voltage which was compared to experimental
results.
We have developed manufacturable approaches to form single, vertically aligned carbon nanotubes,
where the tubes are centered precisely, and placed within a few hundred nm of 1-1.5 μm deep trenches.
These wafer-scale approaches were enabled by chemically amplified resists and inductively coupled
Cryo-etchers to form the 3D nanoscale architectures. The tube growth was performed using dc plasmaenhanced
chemical vapor deposition (PECVD), and the materials used for the pre-fabricated 3D
architectures were chemically and structurally compatible with the high temperature (700 °C) PECVD
synthesis of our tubes, in an ammonia and acetylene ambient. The TEM analysis of our tubes revealed
graphitic basal planes inclined to the central or fiber axis, with cone angles up to 30° for the particular
growth conditions used. In addition, bending tests performed using a custom nanoindentor, suggest that
the tubes are well adhered to the Si substrate. Tube characteristics were also engineered to some extent,
by adjusting growth parameters, such as Ni catalyst thickness, pressure and plasma power during growth.
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