Browsing by Author "Varnavides, Georgios"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Mid- to Far-Infrared Anisotropic Dielectric Function of HfS2 and HfSe2(Advanced Optical Materials, 2022-09-06) Kowalski, Ryan A.; Nolen, Joshua Ryan; Varnavides, Georgios; Silva, Sebastian Mika; Allen, Jack E.; Ciccarino, Christopher J.; Juraschek, Dominik M.; Law, Stephanie; Narang, Prineha; Caldwell, Joshua D.The far-infrared (far-IR) remains a relatively underexplored region of the electromagnetic spectrum extending roughly from 20 to 100 µm in free-space wavelength. Research within this range has been restricted due to a lack of optical materials that can be optimized to reduce losses and increase sensitivity, as well as by the long free-space wavelengths associated with this spectral region. Here the exceptionally broad Reststrahlen bands of two Hf-based transition metal dichalcogenides (TMDs) that can support surface phonon polaritons (SPhPs) within the mid-infrared (mid-IR) into the terahertz (THz) are reported. In this vein, the IR transmission and reflectance spectra of hafnium disulfide (HfS2) and hafnium diselenide (HfSe2) flakes are measured and their corresponding dielectric functions are extracted. These exceptionally broad Reststrahlen bands (HfS2: 165 cm−1; HfSe2: 95 cm−1) dramatically exceed that of the more commonly explored molybdenum- (Mo) and tungsten- (W) based TMDs (≈5–10 cm−1), which results from the over sevenfold increase in the Born effective charge of the Hf-containing compounds. This work therefore identifies a class of materials for nanophotonic and sensing applications in the mid- to far-IR, such as deeply sub-diffractional hyperbolic and polaritonic optical antennas, as is predicted via electromagnetic simulations using the extracted dielectric function.Item The Role of Optical Phonon Confinement in The Infrared Dielectric Response of III-V Superlattices(Advanced Materials, 2023-12-01) Matson, Joseph R.; Alam, Md Nazmul; Varnavides, Georgios; Sohr, Patrick; Knight, Sean; Darakchieva, Vanya; Stokey, Megan; Schubert, Mathias; Said, Ayman; Beechem, Thomas; Narang, Prineha; Law, Stephanie; Caldwell, Joshua D.Polar dielectrics are key materials of interest for infrared (IR) nanophotonic applications due to their ability to host phonon-polaritons (PhPs) that allow for low-loss, subdiffractional control of light. The properties of phonon-polaritons are limited by the characteristics of optical phonons, which are nominally fixed for most “bulk” materials. Superlattices composed of alternating atomically-thin materials offer control over crystal anisotropy through changes in composition, optical phonon confinement, and the emergence of new modes. In particular, the modified optical phonons in superlattices offer the potential for so-called crystalline hybrids whose IR properties cannot be described as a simple mixture of the bulk constituents. To date, however, studies have primarily focused on identifying the presence of new or modified optical phonon modes rather than assessing their impact on the IR response. This study focuses on assessing the impact of confined optical phonon modes on the hybrid IR dielectric function in superlattices of GaSb and AlSb. Using a combination of first principles theory, Raman, FTIR, and spectroscopic ellipsometry, the hybrid dielectric function is found to track with the confinement of optical phonons, leading to optical phonon spectral shifts of up to 20 cm−1. These results provide an alternative pathway towards designer IR optical materials.