AVS2016 Session EM-MoM: Advances in Photonics

The mid-infrared (mid-IR) spectral range (3-15µm) has become a burgeoning and dynamic field of research both for fundamental exploration as well as for more applied research in health and the environment, security and defense, communication, and sensing. New optoelectronic devices, in particular sources and detectors, have allowed for the rapid growth of the mid-IR, and the development of a range of optical systems for a variety of applications. At the same time, the areas of plasmonics and metamaterials have experienced explosive growth over the past decade, fueled in part by rapid developments in fabrication, characterization, computational science, and theory. These have become increasing important in the mid-IR, where sub-wavelength confinement and manipulation of light offer the potential for new types of optical materials and structures for integration with the ever-improving mid-IR optoelectronic devices. Yet all of the above developments are, in some ways, spectrally limited to wavelengths <20μm, due largely to the strong absorption of light in semiconductors at, or near, to optical phonon energies. The strong absorption of optical phonons results in a far-IR wavelength band, which for simplicity sake we refer to as the Reststrahlen region, from 20-60μm where little to no optical and optoelectronic infrastructure exists.

In this talk, I will discuss our group’s recent work developing novel optoelectronic, plasmonic, and phononic devices and structures for far-IR applications. I will present far-IR perfect absorber structures and discuss the potential and limitations of selective thermal emitters in the far-IR. In addition, I will demonstrate how phononic materials can behave similarly to plasmonic materials in a limited spectral band near the longitudinal optical (LO) phonon, showing coupling to both propagating and localized surface phonon modes. I will also show how careful control of doping in semiconductor epilayers offers some wavelength flexibility in designing materials capable of supporting hybrid plasmonic/phononic modes. Finally, I will discuss the opportunities for development of far-IR optoelectronic devices capable of serving as light sources at far-IR wavelengths. In all, I hope to present a picture of the far-IR as a new optical frontier where we can not only apply the lessons from shorter wavelength photonic structures, but also explore exciting new approaches to the development of Reststrahlen band optical infrastructure.

The field of nanophotonics is based on the ability to confine light to sub-diffractional dimensions. Up until recently, research in this field has been primarily focused on the use of plasmonic metals. However, the high optical losses inherent in such metal-based surface plasmon materials has led to an ever-expanding effort to identify, low-loss alternative materials capable of supporting sub-diffractional confinement. Beyond this, the limited availability of high efficiency optical sources, refractive and compact optics in the mid-infrared to THz spectral regions make nanophotonic advancements imperative. One highly promising alternative are polar dielectric crystals whereby sub-diffraction confinement of light can be achieved through the stimulation of surface phonon polaritons within an all-dielectric, and thus low loss material system. Due to the wide array of high quality crystalline species and varied crystal structures, a wealth of unanticipated optical properties have recently been reported. However, these materials also have some limitations, primarily in the limited spectral bandwidth of operation for any given material. This talk will discuss recent advancements to improve the material lifetime and to induce additional functionality through isotopic enrichment and hybridization of polaritonic modes for realizing low-loss, actively tunable/modulated nanophotonic materials.

While various materials have been investigated for photovoltaics, solar cells based on crystalline silicon (c-Si) dominate the current photovoltaics market. To reduce the cost of c-Si cells, wafer manufacturing companies have produced competitively priced thin c-Si films, ranging from a few microns to tens of microns, using a kerfless process. In such thin-film c-Si cells, light absorption becomes poorer than in thick films and light trapping is crucial to increase the photovoltaic efficiency. Han et al. have demonstrated that, among various light-trapping schemes, symmetry breaking in photonic nanostructures can approach the Lambertian light-trapping limit very closely. However, fabricating symmetry-breaking nanostructures in a scalable, cost-effective, manufacturable manner remains elusive. Here, we introduce a new approach to systematically break the symmetry in photonic nanostructures on c-Si surface. Using our approach, we fabricate low-symmetry inverted nanopyramid structures. Our method makes use of low-cost, manufacturable wet etching steps on c-Si(100) wafers without relying on expensive off-cut wafers. Our experiment and computational modeling demonstrate that the symmetry breaking can increase the Shockley-Queisser efficiency from 27.0 to 27.9% for a 10-micron-thick c-Si film. Further, our computation reveals that this improvement would increase from 28.1 to 30.0% with over-etching for a 20-micron-thick c-Si film.

The synthesis of beta-phase silicon carbide nanoparticles exhibiting a hollow core-shell morphology is demonstrated by means of a two-step non-thermal plasma method.[1] Crystalline silicon nanoparticles are nucleated from silane precursor gas in a non-thermal plasma reactor similar to the one described in [2] and injected into a secondary methane-containing plasma reactor, where they are carbonized to form beta-phase silicon carbide nanoshells. Furthermore, at sufficiently high input power in the second plasma, a single-layer graphene coating forms around the silicon carbide particles. These findings are consistent with XRD and Raman spectra. An analytical solution of the 1D diffusion equation in spherical coordinates, as well as the lattice volume expansion from silicon to silicon carbide are used to explain the formation of the interior void. This provides an alternative to previously reported results, which invoke the nanoscale Kirkendall effect to explain the void formation by means of fast out-diffusion of the core element.[3] Further consideration of the system kinetics indicates interactions with the ionized gas leads to particle superheating, allowing for the diffusion of carbon into the silicon matrix and nucleation of beta-phase silicon carbide to occur during the short residence time of the particles in the system. This work expands upon the materials achievable by non-thermal plasma synthesis and suggests that such systems offer the capability to engineer particle morphology, as well as grow conformal 2D materials on freestanding nanoparticles.

References:

[1] Hollow silicon carbide nanoparticles from a non-thermal plasma process. D. Coleman, T. Lopez, O. Yasar-Inceoglu, and L. Mangolini. J. Appl. Phys. In press (2015).

[2] Silicon nanocrystal production through non-thermal plasma synthesis: a comparative study between silicon tetrachloride and silane precursors. O. Yasar-Inceoglu, T. Lopez, E. Farshihagro, and L. Mangolini, Nanotechnology 23, 255604 (2012).

[3] Formation of Hollow Nanocrystals Through the Nanoscale Kirkendall Effect. Y. Yin, R. M. Rioux, C. K. Erdonmez, S. Hughes, G. A. Somorjai, and A. P. Alivisatos, Science 304, 711 (2004).

It is an accepted truism that the behavior of surfaces and interfaces is dictated by the interactions of atoms. Consequently, understanding atomic interactions at surfaces and interfaces is the foundational basis for predicting, controlling, and designing devices and processes. This is particularly relevant to devices for memory storage, sensing, and photonics in which electrical contacts must be made at very small scales. We use an ideal system to examine the size and structure dependence of interfaces.

We report that the size dependence of electronic properties at nanosized metal−semiconducting oxide (Au nanoparticle/SrTiO3 ) interfaces is significantly affected by the interface atomic structure. The properties of interfaces with two orientations are compared over size range of 20−200 nm. Three different mechanisms of size dependence occur at various size regimes. The difference in interface atomic structure leads to electronic structure differences that alter electron transfer paths. Specifically, interfaces with a higher concentration of undercoordinated Ti result in enhanced tunneling due to the presence of defect states or locally reduced tunnel barrier widths.

In the case of materials which exhibit resistive switching the observed “eight-wise” bipolar resistive hysteresis loop is modulated by trap/detrap process. The size-dependent high resistance state is consistent with changes in both the interfacial area and Schottky properties. The low resistance state exhibits size independent resistance through the dominant fast conductive path. Detrapping requires more work for smaller interfaces due to the associated larger built-in electric field.