Chaitanya K. Ullal

Resolution scaling in STED microscopy

We undertake a comprehensive study of the inverse square root dependence of spatial resolution on the saturation factor in stimulated emission depletion (STED) microscopy and generalize it to account for various focal depletion patterns. We used an experimental platform featuring a high quality depletion pattern which results in operation close to the optimal optical performance. Its superior image brightness and uniform effective resolution <25 nm are evidenced by imaging both isolated and self-organized convectively assembled fluorescent beads. For relevant saturation values, the generalized square-root law is shown to predict the practical resolution with high accuracy.

Three-dimensional nanoscopy of colloidal crystals.

We demonstrate the direct three-dimensional imaging of densely packed colloidal nanostructures using stimulated emission depletion microscopy. A combination of two de-excitation patterns yields a resolution of 43 nm in the lateral and 125 nm in the axial direction and an effective focal volume that is by 126-fold smaller than that of a corresponding confocal microscope. The mapping of a model system of spheres organized by confined convective assembly unambiguously identified face-centered cubic, hexagonal close-packed, random hexagonal close-packed, and body-centered cubic structures.

Block copolymer nanostructures mapped by far-field optics.

We demonstrate stimulated emission depletion microscopy using opposing objective lenses to noninvasively reveal the nanoscale morphology of block copolymers in three dimensions with focused light. This is exemplified in a poly(styrene-block-2-vinylpyridine) model system in which contrast is achieved by specifically staining the vinylpyridine phase with a fluorescent dye. We image swelling induced mesopores and other convoluted structures within the bulk of samples, at scales that have so far required electron and scanning probe microscopes.

Dynamic imaging of colloidal-crystal nanostructures at 200 frames per second

The dynamic noninvasive imaging of colloidal nanostructures has been precluded by the diffraction-limited resolution of (confocal) light microscopy. Using Fast Stimulated Emission Depletion (STED) microscopy, we demonstrate the ability to resolve the formation of a colloidal crystal (monolayer) from particles of 200 nm size, where the voids in the crystal are as small as 30 nm. With a temporal resolution of 5 ms, we exemplify the technique by visualizing the annealing of potential point defects during the formation of the colloidal crystal.

Bimodal “matrix-free” polymer nanocomposites

Maximum performance enhancement in polymer nanocomposites is predicated on the simultaneous realization of maximum filler loading, controlled filler dispersion and structural integrity. Through the example of high refractive index ZrO2/polydimethylsiloxane encapsulants for LEDs for enhanced light extraction efficiency, this paper demonstrates that all three properties can be achieved by eliminating the matrix in a single component polymer nanocomposite. Surface bound polymer brushes serve as both the matrix and stabilizing agent, to ensure uniform filler dispersion. The use of multimodal brush configurations that are at least bimodal is the key enabler. This provides sufficient crowding near the particle surface to screen core–core attraction, as well as entanglement between sparsely grafted long brushes to prevent premature cracking. A further widening of the applicability and processing windows are achieved by introducing crosslinkable moieties into the brushes.

Wetting Regimes for Residual-Layer-Free Transfer Molding at Micro- and Nanoscales

Transfer molding offers a low-cost approach to large-area fabrication of isolated structures in a variety of materials when recessed features of the open-faced mold are filled without leaving a residual layer on the plateaus of the mold. Considering both macroscale dewetting and microscale capillary flow, a proposed map of wetting regimes for blade meniscus coating provides a guide for achieving discontinuous dewetting at maximum throughput. Dependence of meniscus morphology on the azimuthal orientation of the stamp provides insight into the dominant mechanisms for discontinuous dewetting of one-dimensional (1-D) patterns. Critical meniscus velocity is measured and residual-layer-free filling is demonstrated for 1-D patterned soft molds (stamps) with periods ranging from 140 nm to 6 μm. Transfer of isolated lines, and multilayer woodpile structures were achieved through plasma bonding. These results are relevant to other roll-to-roll compatible processes for scalable production of high-resolution structures across large areas.

Hypersonic phononic crystals

Phononic crystals are structures with spatially periodic variations in density and sound velocities. The most fascinating feature of phononic crystals is the existence of stop bands or band gaps in their dispersion relation, where no phonons are allowed to propagate. As a result, phononic crystals have great promise for achieving precise control over propagation of sound and other mechanical waves. In this talk our recent investigation of 2‐D and 3‐D hypersonic phononic crystals with band gaps in GHz frequency range will be presented and their potential applications as acoustic mirrors, lenses, waveguides, etc. will be discussed. High‐quality single‐crystalline structures were fabricated using interference lithography. Their phonon dispersion relation was directly measured with Brillouin light scattering. Finite‐element analysis was employed to compute theoretical dispersion relation and provide interpretation for the experimentally observed propagation modes [T. Gorishnyy et al., Phys. Rev. Let. 94, 1155...

Exploring Space Groups for Three Dimensional Photonic Band Gap Structures Via Level Set Equations: The Face Centered Cubic Lattice

A level set approach was used to study photonic band gaps for dielectric composites with symmetries of the eleven face centered cubic lattices. Candidate structures were modeled for each group by a 3D surface given by f(x,y,z)-t=0 obtained by equating f to an appropriate sum of structure factor terms. This approach allows us to easily map different structures and gives us an insight into the effects of symmetry, connectivity and genus on photonic band gaps. It is seen that a basic set of symmetries defines the essential band gap and connectivity. The remaining symmetry elements modify the band gap. The eleven lattices are classified into four fundamental topologies on the basis of the occupancy of high symmetry Wyckoff sites. Of the fundamental topologies studied, three display band gaps--- including two: the (F-RD) and a group 216 structure that have not been reported previously.