Abhijeet Bagal

Fabrication of subwavelength periodic nanostructures using liquid immersion Lloyd's mirror interference lithography.

We have developed a liquid immersion Lloyds mirror interference lithography system to fabricate subwavelength periodic nanostructures. In this approach, we construct the Lloyds mirror interferometer within a liquid medium to increase the ambient index. The light wavelength is scaled by the refractive index of the immersion fluid, reducing the minimum interference pattern period and increasing the spatial resolution. The all-liquid system ensures continuous fluid contact with the sample without an external mechanism, allows rapid adjustment of pattern period with subwavelength resolution, and retains the passive vibration-correction capability of Lloyds mirror interferometers. Using this approach, we have successfully fabricated a grating structure with 112 nm period using a laser with 325 nm wavelength, attaining a numerical aperture of 1.45. The proposed immersion strategy can be adapted to improve pattern resolution of more complex interference lithography systems.

Antireflection effects at nanostructured material interfaces and the suppression of thin-film interference

Thin-film interference is a well-known effect, and it is commonly observed in the colored appearance of many natural phenomena. Caused by the interference of light reflected from the interfaces of thin material layers, such interference effects can lead to wavelength and angle-selective behavior in thin-film devices. In this work, we describe the use of interfacial nanostructures to eliminate interference effects in thin films. Using the same principle inspired by moth-eye structures, this approach creates an effective medium where the index is gradually varying between the neighboring materials. We present the fabrication process for such nanostructures at a polymer-silicon interface, and experimentally demonstrate its effectiveness in suppressing thin-film interference. The principle demonstrated in this work can lead to enhanced efficiency and reduce wavelength/angle sensitivity in multilayer optoelectronic devices.

Multifunctional nano-accordion structures for stretchable transparent conductors

Multifunctional nano-accordion structures exhibiting a unique combination of conductivity, stretchability, and transparency are fabricated through a combination of nanolithography and atomic layer deposition. The nanostructured material demonstrated two orders-of-magnitude improvement in stretchability, repeatable electrical performance for cyclic stretching and bending, and broadband optical transmission up to 70%. The proposed experimental techniques and analytical models enable the deterministic design of nano-accordion geometry to control material stretchability. The proposed nanostructures are promising for applications in transparent flexible electronics, stretchable displays, and wearable sensors.

Fabrication and structural properties of AlN submicron periodic lateral polar structures and waveguides for UV-C applications

Periodically poled AlN thin films with submicron domain widths were fabricated for nonlinear applications in the UV-VIS region. A procedure utilizing metalorganic chemical vapor deposition growth of AlN in combination with laser interference lithography was developed for making a nanoscale lateral polarity structure (LPS) with domain size down to 600 nm. The Al-polar and N-polar domains were identified by wet etching the periodic LPS in a potassium hydroxide solution and subsequent scanning electron microscopy (SEM) characterization. Fully coalesced and well-defined vertical interfaces between the adjacent domains were established by cross-sectional SEM. AlN LPSs were mechanically polished and surface roughness with a root mean square value of ∼10 nm over a 90 μm × 90 μm area was achieved. 3.8 μm wide and 650 nm thick AlN LPS waveguides were fabricated. The achieved domain sizes, surface roughness, and waveguides are suitable for second harmonic generation in the UVC spectrum.

Wicking Enhancement in Three-Dimensional Hierarchical Nanostructures

Wicking, the absorption of liquid into narrow spaces without the assistance of external forces, has drawn much attention due to its potential applications in many engineering fields. Increasing surface roughness using micro/nanostructures can improve capillary action to enhance wicking. However, reducing the structure length scale can also result in significant viscous forces to impede wicking. In this work, we demonstrate enhanced wicking dynamics by using nanostructures with three-dimensional (3D) hierarchical features to increase the surface area while mitigating the obstruction of liquid flow. The proposed structures were engineered using a combination of interference lithography and hydrothermal synthesis of ZnO nanowires, where structures at two length scales were independently designed to control wicking behavior. The fabricated hierarchical 3D structures were tested for water and ethanol wicking properties, demonstrating improved wicking dynamics with intermediate nanowire lengths. The experimental data agree with the derived fluid model based on the balance of capillary and vicious forces. The hierarchical wicking structures can be potentially used in applications in water harvesting surfaces, microfluidics, and integrated heat exchangers.

Study on dielectric and piezoelectric properties of 0.7 Pb(Mg1/3Nb2/3)O3-0.3 PbTiO3 single crystal with nano-patterned composite electrode

Effect of nano-patterned composite electrode and backswitching poling technique on dielectric and piezoelectric properties of 0.7 Pb(Mg1/3Nb2/3)O3-0.3 PbTiO3 was studied in this paper. Composite electrode consists of Mn nano-patterns with pitch size of 200 nm, and a blanket layer of Ti/Au was fabricated using a nanolithography based lift-off process, heat treatment, and metal film sputtering. Composite electrode and backswitching poling resulted in 27% increase of d33 and 25% increase of dielectric constant, and we believe that this is attributed to regularly defined nano-domains and irreversible rhombohedral to monoclinic phase transition in crystal. The results indicate that nano-patterned composite electrode and backswitching poling has a great potential in domain engineering of relaxor single crystals for advanced devices.

Large-Area Nanolattice Film with Enhanced Modulus, Hardness, and Energy Dissipation

We present an engineered nanolattice material with enhanced mechanical properties that can be broadly applied as a thin film over large areas. The nanolattice films consist of ordered, three-dimensional architecture with thin-shell tubular elements, resulting in favorable modulus-density scaling (n ~ 1.1), enhanced energy dissipation, and extremely large material recoverability for strains up to 20% under normal compressive loading. At 95.6% porosity, the nanolattice film has demonstrated modulus of 1.19 GPa and specific energy dissipation of 325.5 kJ/kg, surpassing previously reported values at similar densities. The largest length scale in the reported nanolattice is the 500 nm unit-cell lattice constant, allowing the film to behave more like a continuum material and be visually unobservable. Fabricated using three-dimensional colloidal nanolithography and atomic layer deposition, the process can be scaled for large-area patterning. The proposed nanolattice film can find applications as a robust multifunctional insulating film that can be applied in integrated photonic elements, optoelectronic devices, and microcircuit chips.

Nanostructured Antireflective In-Plane Solar Harvester

In this work, we demonstrate a two-dimensional nano-hole array that can reduce reflection losses while passively trapping and harvesting incident light. The surface structure is designed to scavenge a small portion of incident light that would typically be lost due to Fresnel reflection, while the majority of light transmits unobstructed like a regular window. The trapping mechanism is dependent on angle and wavelength, and can be designed to selectively trap narrow wavelength bands using the constructed theoretical models. We demonstrate that structures with periods of 275 nm and 325 nm can trap different wavelength range within the visible spectrum, while simultaneously suppressing reflection losses. The trapping effect can be observed visually, and can be converted to a current output using a photovoltaic (PV) cell on the glass edge. The fabrication of such materials employs a simple replication process, and can be readily scaled up for large-scale manufacturing. The demonstrated solar harvester can be potentially be widely deployed in residential and commercial buildings as multifunctional windows for solar energy harvesting, scavenging, spectra splitting, and anti-glare properties.

Nanostructured In-Plane Solar Concentrator

We present a solar concentrator utilizing two-dimensional (2D) periodic nanostructure on a glass window to redirect a portion of the incoming light to the edge pane to be harnessed by a small efficient photovoltaic cell.