Pure edge-contact devices on single-layer-CVD-graphene integrated into a single chip.

We present a simple and cost-effective fabrication technique for on-chip integration of pure edge contact two-terminal (2T) and Graphene field effect transistor (GFET) devices with low contact resistance and nonlinear characteristics based on single-layer chemical-vapor-deposited (CVD) graphene. We use a smart print-based mask projection technique with a 10X magnification objective lens for maskless lithography followed by thermal evaporation of the contact material Cr-Pd-Au through three different angles (90° and ± 45°) using a customized inclined-angle sample-holder to control the angle during normal incidence evaporation for edge-contact to graphene. Our fabrication technique, graphene quality, and contact geometry enable pure metal contact to 2D single-layer graphene allowing electron transport through the 1D atomic edge of graphene. Our devices show some signatures of edge contact to graphene in terms of very low contact resistance of 23.5 Ω, the sheet resistance of 11.5 Ω, and sharp nonlinear voltage-current characteristics (VCC) which are highly sensitive to the bias voltage. This study may find application in future graphene-integrated chip-scale passive or active low-power electronic devices.

Solar Steam Generation and Desalination Using Ultra-Broadband Absorption in Plasmonic Alumina Nanowire Haze Structure-Graphene Oxide-Gold Nanoparticle Composite.

Solar steam generation is a promising solar energy harvesting technology to address the global clean energy and water deficiency cost-effectively. In this work, we present a compact plasmonic nanostructure-nanoparticle composite based on haze/GO-rGO/Au enabling multiple purposes such as broadband solar absorption, solar steam generation, and solar desalination. The graphene oxide-reduced GO (GO-rGO) combination allows broadband optical absorption, and the presence of 5 nm Au nanoparticles creates high-density localized hotspots for enhanced photothermal effect as evidenced through Raman signal enhancement studies. Anodized aluminum oxide-based haze nanostructures provide maximum light interaction volume through multiple nanogaps and porosity through vertically aligned nanowires of 20-26 nm width and 5-10 µm depth for water channelization. The haze sample coated with GO-rGO/Au shows high solar absorbance of 92.5% over the 300-2500 nm wavelength range covering the whole solar spectrum (ultraviolet-visible-near-infrared). We have achieved 50 and 71.1% of solar-to-thermal conversion efficiencies in a single-layer microscale sample for saline and freshwater, respectively, with a maximum surface temperature of 95.7 °C. The efficiencies increase to 64 and 77% for two layers of the sample at 5 suns (5 kW m-2) illumination.

Broadband, wide-angle antireflection in GaAs through surface nano-structuring for solar cell applications.

We demonstrate broadband and wide-angle antireflective surface nanostructuring in GaAs semiconductors using variable dose electron-beam lithography (EBL). Various designed structures are written with EBL on a positive EB-resist coated GaAs and developed followed by shallow inductively coupled plasma etching. An optimized nanostructured surface shows a reduced surface reflectivity down to less than 2.5% in the visible range of 450-700 nm and an average reflectance of less than 4% over a broad near-infrared wavelength range from 900-1400 nm. The results are obtained over a wide incidence angle of 33.3°. This study shows the potential for anti-reflective structures using a simpler reverse EBL process which can provide optical absorption or extraction efficiency enhancement in semiconductors relevant to improved performance in solar photovoltaics or light-emitting diodes.

Design and studies on gradient index metasurfaces for broadband polarization-independent, subwavelength, and dichroic focusing.

This paper presents designs for and simulation studies on planar gradient index metasurfaces for polarization-independent and dichroic subwavelength focusing for broadband applications. Polarization-independent lenses are designed based on dielectric (Si3N4 and TiO2) gradient nanopillars. Dichroic metalenses are designed based on gradient aluminum nanohelices for helicity-dependent focusing of circularly polarized light. The helical shape is considered due to its sensitivity to circularly polarized light of a specific handedness depending upon the orientation of the helices in the lattice; this may help in 3D imaging. In the designed metalenses, the variation in the spatial dimension (fill factor) is in a gradient manner, which leads to directional bending of electromagnetic waves, and strong coupling between the bent electromagnetic waves leads to subwavelength focusing over the high numerical aperture. The designed metasurface can be materialized through multibeam interference using a combination of n plane beams and a nondiffracting Bessel beam of either zeroth or first order presented through the simulated irradiance profile and a proposed single-step experimental setup. The designed TiO2-based metalens focuses the incident arbitrary or plane polarized light to a spot sized 0.314λ, at a wavelength of 635 nm, that is based on Si3N4, enabling polarization-independent subwavelength focusing over a broad (436-810 nm) wavelength range. Realization of these lenses will enable polarization-independent high-numerical-aperture focusing and super-resolution real-time imaging of biological samples.

Metamaterial structures of variable and gradient basis orientations embedded with periodic linear defects: phase engineered design, single step optical realization, and applications.

Metamaterial structures of different basis shapes and orientations and with gradient refractive index variations are applicable in integrated photonics, miniaturized optoelectronics, diffraction limited focusing, and super-resolution imaging. We present design and experimental realizations of gradient metamaterial structures embedded with linear periodic defects and propose its applications in on-substrate color filtering through a simulation-based study. A combination of phase engineered plane beams in double cone geometry and an axial plane beam are interfered to obtain different gradient basis metamaterial structures with linear defects in two dimensions and three dimensions, respectively. The defect size and spatial gradient amplitude modulations can be controlled computationally through shifts in the interference angle for some of the plane beams in double cone geometry without changing any optical components in the experiment. The designed and realized metamaterial structures upon transferring to certain materials will find application in optical circuits and metalenses for enhanced light matter interactions.

Fabrication of helical photonic structures with submicrometer axial and spatial periodicities following "inverted umbrella" geometry through phase-controlled interference lithography.

In this Letter we report for the first time, to the best of our knowledge, a phase spatial light modulator (SLM)-based interference lithography (IL) approach for the realization of hexagonally packed helical photonic structures with a submicrometer scale spatial, as well as axial, periodicity over a large area. A phase-only SLM is used to electronically generate six phase-controlled plane beams. These six beams from the front side and a direct central backside beam are used together in an "inverted umbrella" geometry setup to realize the desired submicrometer axial periodic chiral photonic structures through IL. The realized structures with 650 nm spatial and 353 nm axial periodicities on negative photoresist can be used as an optical filter and refractive index sensor, as evidenced from the FDTD-based simulation study on its optical properties. Further, the fabricated templates can be transferred to metals such as silver or aluminum for the realization of a metamaterial-based broadband circular polarizer ranging from 1 to 3.5 µm of near-infrared spectra.

Design and fabrication of woodpile photonic structures through phase SLM-based interference lithography for omnidirectional optical filters.

In this Letter, we report a large-area and single-step optical fabrication technique based on phase engineering interference lithography that is scalable and reconfigurable for the realization of submicrometer scale periodic face-centered cubic inverse woodpile photonic structures. The realized inverse woodpile structure on positive having four number axial layers with 740 nm spatial and 1046 nm axial periodicities shows 10% reflectance and 90% transmittance at 776 nm wavelength that can further be improved for the addition of axial layers. The realized structure can be transferred to crystalline silicon for realizing a bandpass/rejection near-infrared filter in a reflection/transmission mode. Further, woodpile structures based on low-contrast silicon nitride (Si3N4) are designed as selective narrow frequency filters at 1310 and 1550 nm wavelengths for telecommunication applications and omnidirectional red-green-blue filters for display devices by tuning the design parameters.

Single-step optical realization of bio-inspired dual-periodic motheye and gradient-index-array photonic structures.

This Letter demonstrates a single-step optical realization method for hexagonal and square lattice-based dual periodic motheye and gradient-index-array photonic structures over large areas. Computed phase mask of gradient interference patterns are used as inputs to a phase-only spatial light modulator (SLM), and the first-order diffracting beams are coherently superposed with the help of a 2f-2f Fourier filtering setup to avoid complex optical geometry for generation and control of individual beams. The simulated interference patterns are verified experimentally through a CMOS camera. The fabricated micro-structures on a positive photoresist are shown to have a major periodicity of 638 µm and minor periodicity of 25.2 µm, with the air hole diameter varying from 22.7 to 6.9 µm along the X and Y axes. The depth of the fabricated structure gradually varies from 4.203 µm at the center to 1.818 µm at the corner. These structures may be scaled down to submicron features that can show improved anti-reflection properties for solar energy harvesting and GRIN lens for optical wavelength region.

Submicrometer photonic structure fabrication by phase spatial-light-modulator-based interference lithography.

We present a large-area and single-step fabrication approach based on phase spatial light modulator (SLM)-assisted interference lithography for the realization of submicrometer photonic structures on photoresist. A multimirror beam steering unit is used to reflect the SLM-generated phase-engineered beams leading to a large angle between interfering beams while also preserving the large area of the interfering plane beams. Both translational and rotational periodic submicrometer structures are experimentally realized. This approach increases the flexibility of interference lithography to fabricate more complex submicrometer photonic structures and photonic metamaterial structures for future applications.

N-single-helix photonic-metamaterial based broadband optical range circular polarizer by induced phase lags between helices.

In this work, we have designed a photonic-metamaterial based broadband circular polarizer using N=4 phase-lagged aluminum single helices arranged in a square array as a unit cell. The effect of phase differences between the helices in an array on the optical performance of the structure is studied, and a comparative study is done with that of multi-intertwined helices. It is observed that the proposed metamaterial structure shows circular polarization sensitivity over a broad optical wavelength range (≈450-900 nm), with improved optical performance in average extinction ratio and broad positive circular dichroism in comparison to multiple intertwined helices. The induced phase lag between the helices in a square-array based unit cell reduces the linear birefringence and leads to the recovery of circular space symmetry in the structure.