Joshua D. Caldwell

We experimentally observe a significant improvements in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics, with resolution greater than \lambda/125 observed in a preliminary hyperlens design. Our results provide the foundation for a materials-growth-directed approach aimed at realizing the loss control necessary for the development of next generation PhP-based nanophotonic devices.

Metasurfaces control light propagation at the nanoscale for applications in both free-space and surface-confined geometries. However, all recent designs have exhibited concepts using geometrically fixed structures, or used materials with excessive propagation losses, thereby limiting potential applications. Here we show how to overcome these limitations using a reconfigurable hyperbolic metasurface comprising a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with a phase-change material (PCM), single crystal vanadium dioxide (VO2). Metallic and dielectric domains in VO2 provide spatially localized changes in the local dielectric environment to tune the wavelength of hyperbolic phonon polaritons (HPhPs) supported in hBN by a factor of 1.6. This contrasts with earlier work using surface phonon polaritons, where propagation could only be observed above a low-loss dielectric phase. We demonstrate the first realization of in-plane HPhP refraction, which obeys Snell’s law and the means for launching, reflecting and transmitting HPhPs at the PCM domain boundaries. To demonstrate practical applications of this platform, we show how hBN could be combined with either VO2 or GeSbTe glasses to make refractive nanophotonic waveguides and lenses. This approach offers control of in-plane HPhP propagation at the nanoscale and exemplifies a reconfigurable framework combining hyperbolic media and PCMs to design new optical functionalities including resonant cavities, beam steering and waveguiding.

It is two decades since the first reports that the insulator-to-metal transition (IMT) in vanadium dioxide (VO2) occurred on an ultrafast time scale, followed by growing interest in the potential use of this strongly correlated oxide in a variety of switching schemes. At first glance, VO2 would seem to be ideally suited to a variety of applications in electro-optics and all optical switching: The IMT occurs on a sub-picosecond time scale; it is fully reversible and has a large dielectric contrast at wavelengths in the near- to mid-infrared; and the material itself is fully compatible with many optical and electronic materials of interest. However, there are also well-known difficulties, chief among them the fact that the IMT, if fully completed, is accompanied by a structural phase transition (SPT) that requires nanoseconds to return from the rutile, metallic state to the monoclinic insulating ground state – thus essentially limiting switching speeds to time scales similar to those in amorphous-to-crystalline transitions in chalcogenide glasses. Here we discuss the ways in which the very considerable advantages of VO2 as a modulating or threshold switch can be amplified by deploying it appropriately in silicon photonic modulators, switchable metasurfaces, plasmonic heterostructures, and two-dimensional materials that can support phonon polariton optics. We focus particularly on ways of tailoring the physical properties of the VO2 component of a system to meet the requirements of operating in particular wavelength regions, meeting specific threshold requirements and choosing electrical or optical initiation of the IMT.

The current state-of-the-art in materials used for IR optical components (e.g. focusing elements, waveplates or prisms) is far from ideal. This problem is exacerbated by the long free-space wavelengths associated with the mid-wave IR (MWIR) to terahertz (THz) spectral domains. Through the use of polaritons, one can surpass the diffraction limit and thus the limitations of these long free-space wavelengths can be circumvented. The two most prevalent varieties are the surface plasmon (SPP) and surface phonon polariton (SPhP), resulting from the coupling of light with electronic charges in a metal or ionic charges on a polar lattice, respectively. Each exhibits significant limitations, for instance the narrow, material specific operational "Reststrahlen band" of SPhPs and the relatively high optical losses in SPPs. Thus, it would be ideal to identify a method to dictate the IR/THz response, while retaining the positive attributes of both.

Conference Title: 2018 Conference on Lasers and Electro-Optics (CLEO) Conference Start Date: 2018, May 13 Conference End Date: 2018, May 18 Conference Location: San Jose, CA, USA We demonstrate high-contrast infrared absorption spectroscopy on ultrathin protein films enabled by coaxial nanogap resonators, where light can be efficiently coupled with vibrational modes of proteins at the zeroth-order Fabry-Perot resonance condition.

Analogous to surface plasmon polaritons in metals, polar dielectrics support surface phonon polaritons (SPhP) in the mid infrared (MIR) spectral region. In contrast to their plasmonic counterpart, however, SPhPs exhibit much longer lifetimes, and hence constitute a promising alternative in the development of nanophotonics. Employing an MIR free-electron laser, we here experimentally reveal resonant second harmonic generation from SPhPs, excited in the Otto geometry for prism coupling. In a second system featuring an ultrathin layer on top of the SPhP active material, we observe strong coupling of the bulk SPhP to the epsilon near zero mode supported by the thin layer. Our experimental findings are corroborated using a specifically developed matrix formalism for anisotropic multilayer wave propagation.