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Research

My work comprises theory and experiment on optical and electromagnetic materials with complex morphology. Links to various publications are listed, along with descriptions of several research projects.

Publications

The results are described in several publications, including the following:

Projects

I have conducted both theoretical and experimental research on various optical systems which span length scales. This work has included studies on optical properties of molecules and coherent control, optical materials with microscale and nanoscale structure, and classical optical design. All the projects have in common a focus on complex optical and electromagnetic materials, and their applications to optical devices and energy systems. Here, a complex material is defined as one which breaks many symmetries on a given length or energy scale. For example photonic crystals, which are materials with morphology endowing them with periodic dielectric contrast, break translational and rotational symmetries on the scale of an electromagnetic wavelength.

I have worked on several projects, including the following:

Rolling mask lithography

Rolling mask lithography (RML) is an industrial-scale nanopatterning technique for making nanostructured thin films. Applications of the method include moth-eye antireflection coatings and transparent metal electrodes. RML is being developed and commercialized by the company Rolith.

Patterning method

RML works as follows. A flexible transparent phase mask mounted on a quartz cylinder rolls conformally across the surface of a photoresist-coated substrate. A lamp inside the rolling mask shines ultraviolet light through it, generating an interference pattern which exposes the photoresist. Subsequent development steps convert this latent image into a topography variation in the photoresist. The patterned photoresist can then serve as a mask to deposit material onto or etch permanent features into the substrate.

Moth-eye antireflection coatings

Moth-eye coatings comprise a close-packed forest of nanopillars whose tapered shape creates a smoother gradient in the refractive index between one material and another, thus reducing Fresnel reflections. Such antireflection coatings have applications for more effective photovoltaics, high-performance windows, better solid-state lighting, and more efficient displays.

Transparent metal electrodes

Transparent metal electrodes comprise a mesh of submicron-width metal wires (typically silver or aluminum) arranged in a regular grid. The wires' slender profiles renders them invisible to the human eye, while the metal's high electrical conductivity endows the film with low sheet resistance. The electrodes can be made on both rigid (glass) and flexible polymer (PET) substrates. These properties give transparent metal electrodes advantages in both transparency and conductivity compared to competitor technologies such as indium tin oxide and random metal nanowire coatings. The films can be used in solid state lighting, photovoltaics, and electrochromic windows.

Transfer-printed optoelectronics

Transfer printing is a materials fabrication technique which allows for heterogeneous integration of materials, i.e. combining materials of many different types or functions. The transferred materials, or inks as they are often called in analogy with the usual techniques for printing on paper, are patterned in such a way that they can be moved from the substrate on which they are patterned (donor material or substrate) to another material substrate (acceptor material or substrate). A stamp is used to pick up the substrate and move it from the donor to the acceptor material.

The acceptor substrate can have very different properties than the donor, e.g. flexibility of silicone. Transfer printing allows rigid, planar semiconductor devices with high quality electrical and optical properties to be integrated with polymers whose mechanical properties can yield composite systems with high quality electrical and optical properties as well as flexibility or curved geometries.

The transferred inks can be passive, including complex optical materials like photonic or plasmonic crystals or active, like photodetectors, photovoltaic cells, or light emitting diodes. The transfer printing process allows disparate materials to be combined to make novel optical systems. One theme in the following projects is the novel optical effects, advantages, and expansions of the design space afforded by the heterogeneous integration by transfer printing.

Transfer printing technology has been advanced by the the Rogers research group and others at the University of Illinois at Urbana-Champaign (UIUC).

Overlapping scales

The aim of one project is to develop technology for transfer printing overlapping scales of complex optical materials from the donor substrate on which they are made (e.g. silicon) onto different acceptor substrates (e.g. flexible polymer). The scales are small tiles or platelets of material possessing optical functionality. For example, they can exhibit functional photonic or plasmonic properties. My collaborators and I are are working both to design novel optical devices that utilize the pleated geometry of overlapping scales, and to create active substrates on which the scales can be printed to allow their movement in response to stimuli such as temperature or illumination changes, or electrical signals.

Curved imagers

The aberration of field curvature can be reduced or eliminated by having an optical system form an image on a curved surface of prescribed geometry. The ability to create cameras with electronic detectors on such curved surfaces would greatly simplify their optics in many cases. However, typical semiconductor manufacturing technologies are incompatible with the small radii of curvature needed by these cameras. Several collaborators succeeded in developing a method of transferring a silicon photodetector array to a hemispherical surface with ~1 cm radius of curvature. I developed an optical ray tracing model to design and analyze the optics for a camera incorporating the curved detector, and helped design experiments to show its advantages over a camera with a planar detector. I predicted the optimal curved focal surface for comparison with experiments and helped show that a simple hemispherical detector and lens, which is about the size and shape of the human eye, can create sharper images than a planar detector across a larger region of the detector surface.

Photovoltaic modules

Several collaborators developed a new kind of flexible solar cell array using silicon transfer technology they developed. The modules consist of an array of thin silicon microcells connected by electrodes. In one incarnation, the solar microcells are transferred to a flexible substrate. In another incarnation, the microcells are printed on a flat substrate and an array of cylindrical lenses is placed on top of them. I used optical ray tracing models to analyze the performance one of their array designs that incorporate lenses.

Thin silicon microcells are desired for several reasons. First, lower quality and hence cheaper silicon can be used. Second, if a method can be found to allow the thin silicon microcells to be more absorptive in a thin form factor, good efficiencies and with the flexible designs can be combined. Third, after the microcells have been transferred from the donor wafer to the device, the wafer can be polished and a new set of microcells can be prepared. In this way, the wafer material can be more efficiently utilized.

Holographic photonic crystals

Holographic photonic crystals are optical materials with periodic structure created by the interference of several laser beams in a photosensitive polymer. The void regions in the developed photoresist structures can be filled with inorganic materials via atomic layer deposition or electrodeposition, followed by the removal of the polymer. The periodic microstructure endows holographic photonic crystals with reflectance peaks in their optical and infrared spectra. Members of the Braun and Wiltzius research groups developed a photoresist formulation that yields holographic photonic crystals with higher reflectance than were achieved previously. I developed algorithms to calculate reflection and transmission spectra of holographic photonic crystals when their dielectric contrast is low.

Nonlinear optical composites

Nonlinear optical materials are important for such applications as wavelength conversion, generation of entangled photons, and switching. Unfortunately for optical engineers, most materials have weak nonlinear properties, so development of materials with enhanced response is desired. Using ideas and techniques from homogenization theory, I predicted that certain layered metal-dielectric composites could exhibit very large optical nonlinearities under certain conditions. These composites consist of alternating layers of metal and dielectric with thicknesses much less than those of the optical wavelengths at which they are designed to operate. When the appropriate volume fraction of metal is chosen, the composites can exhibit optical nonlinearities far greater than the already large intrinsic optical nonlinearities of their metal components along the direction perpendicular to the layers. Moreover, in some cases the absorption in these materials could be limited or even lower than that of the bulk metal component.

Coherent control for Raman spectroscopy

Optical imaging, like other types of imaging, requires a mechanism to provide contrast. Contrast is the ability to distinguish between different physical structures as recorded in the image. Typically for optical imaging, contrast requires differences in optical properties that affect either the phase, intensity, or both of the light passing through the object to be imaged. To image chemical information in addition to the structural information usually imaged, exogenous contrast agents (e.g. fluorophores that bind selectively to certain chemical species) must be added to the sample (e.g. biological tissue). Endogenous chemical contrast can arise from the properties of the molecules in the sample themselves. One such source of contrast is the Raman spectra of the molecules, which depends on their vibrational properties. Incoherent Raman scattering typically yields a very weak signal, but coherent Raman processes such as coherent anti-Stokes Raman scattering (CARS) can yield much stronger signals.

For the past several years, the Biophotonics Imaging Laboratory has been developing a variant of optical coherence tomography called NIVI to image both the structural and chemical content of biological tissues, with the goals of detecting early-stage cancers and aiding drug development. I helped develop an optical pulse shaping algorithm to coherently excite the vibrational normal modes of a chemical species of interest. The concept is to design an optical pulse whose intensity approximates the matched filter of the third-order nonlinear susceptibility component responsible for the Raman effect. The algorithm was originally developed for use in biological imaging, but I intend to extend its range of applicability to more general problems of coherent control. I also worked with colleagues on ultrafast optics experiments.

Clarifying the refractive index concept

The refractive index is a crucial parameter to understand for any optical material, if it is to be used in practical devices. The refractive index of an isotropic material is defined as the square root of the product of its relative permittivity and relative permeability, which contains an ambiguity: which root is the physical one? The proper choice of refractive index is challenging for a certain class of dielectrics comprising passive and active components. I performed time-domain calculations that unambiguously determined the refractive index one must choose for such media. In related work, my collaborators (Akhlesh Lakhtakia and Tom Mackay) and I showed that the sign of the real part of the refractive index is immaterial to the solution of many electromagnetic problems of engineering interest.

Sculptured thin films

Sculptured thin films (STFs) are a class of nanoengineered materials comprised of parallel nanowires whose shapes are twisted and bent during their deposition. STFs are made via deposition of material by evaporation under vacuum at oblique angles onto a relatively cool substrate that is tilted and/or rotated during fabrication. When the individual nanowires are helical, the resulting films are called chiral STFs. Sufficiently thick chiral STFs exhibit a circular-polarized-dependent photonic bandgap in one dimension. These films have many potential applications, some of which have been realized in experiments. I calculated the effects of propagation through chiral STFs on ultrashort optical pulses. This work gave me a strong background in electromagnetic theory and optics, particularly nanophotonics.

These calculations were done via a numerical finite-difference method that solves the Maxwell equations in the time domain subject to various initial and boundary conditions. These calculations revealed effects of the carrier phase on ultrashort optical pulses, as well as effects on optical pulse shapes and durations in one- and two-dimensional calculations. They also highlighted the trinity of phase, length and time in nanophotonics.


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