Chaitanya K. Ullal

Here we demonstrate the volumetric patterning of photochromic gels by super-resolution interference lithography. Judicious choice of solvent, which impacts the photophysics of the photochromic switch, and ensuring the swollen nature of the gel, facilitate the highly parallelized patterning of thick gels via two-color interference lithography using cost-effective UV LEDs and visible-light laser diodes. Based on the predictions of a simple photokinetic model of the reversibly saturable writing process for contrast and feature size widths on the solvent-dependent thermal back reaction constant, ethylene glycol is chosen as the solvent. Polyacrylamide gels designed to maximize swelling in ethylene glycol are postfunctionalized with spirothiopyran photoswitches and simultaneously exposed to a featureless illumination of UV light and a standing wave of green light, resulting in volumetric patterning of films that are tens of microns in thickness. Features with super-resolved critical dimensions ∼ 90 nm are achieved. Resolutions of ∼ 220 nm are achieved by phase-shifted double exposures. Reducing exposure times from 80 s to 100 ms while keeping dose constant comes at the cost of reduced contrast.

Inspired by the ability of super-resolved fluorescence microscopy to circumvent the diffraction barrier, two-color super-resolution interference lithography exploits non-equilibrium kinetics in materials to achieve large area nanopatterning while using visible light. Periodic patterns with super-resolved features down to tens of nanometers have been demonstrated in thin films and monolayers. Extending these advances to the bulk nanopatterning of thick films requires a quantitative understanding of the time-dependent interactions of optical dynamics, including absorption, diffraction and intensity modulation at two wavelengths, with the photo-activated and inhibited reaction kinetics. Here, we develop an efficient electromagnetic (EM) perturbation theory approach that facilitates for the first time fully-coupled simulations of EM and chemical kinetics in two-color interference lithography. Applied to a spirothiopyran-functionalized photoresist system, these simulations show that diffraction and absorption effects are negligible (< 0.1%) for depths up to 10 µm, and that tuning exposure time and intensities can lead to concentration contrasts up to 80%. We investigate multiple exposure strategies to reduce pitch of the line pattern including sequential exposures with different times to achieve uniform lines, and multiplexed exposures with equal periods. This capability to rapidly and accurately predict the coupled optical and chemical dynamics facilitates computational design of high-precision patterns in two-color interference lithography.

Super resolution microscopy (SRM) brings the advantages of optical microscopy to the imaging of nanostructured soft matter, and in colloidal microgels, promises to quantify variations of crosslink densities at unprecedented length scales. However, the distribution of all crosslinks does not coincide with that of dye-tagged crosslinks, and density quantification in SRM is not guaranteed due to over/under-counting dye molecules. Here we demonstrate that SRM images of microgels encode reaction rate constants of functional cross linkers, which hold the key to correlating these distributions. Combined with evolution of microgel particle radii, the functional cross linker distributions predict consumption versus time with high fidelity. Using a Bayesian regression approach, we extract reaction rate constants for homo and cross propagation of the functional crosslinker, which should be widely useful for predicting spatial variations in crosslink density of gels.

The non-equilibrium kinetics of spirothiopyran monolayers are studied to enable large area interference lithography with feature dimensions that circumvent the diffraction barrier.

The Dirac point and linear band structure in graphene bestow it with remarkable electronic and optical properties, a subject of intense ongoing research. Explanations of high electronic mobility in graphene often invoke the masslessness of electrons based on the effective relativistic Dirac-equation behavior, which are inaccessible to most undergraduate students and are not intuitive for non-physics researchers unfamiliar with relativity. Here, we show how to use only basic concepts from semiconductor theory and the linear band structure of graphene to explain its unusual effective mass and mobility, and compare them with conventional metals and semiconductors. We discuss the more intuitive concept of transverse effective mass, which emerges naturally from these basic derivations, and which approaches zero in the limit of undoped graphene at low temperature and is responsible for its extremely high mobility.