Refractive Index-Based Control of Hyperbolic Phonon-Polariton Propagation.

Hyperbolic phonon polaritons (HPhPs) are generated when infrared photons couple to polar optic phonons in anisotropic media, confining long-wavelength light to nanoscale volumes. However, to realize the full potential of HPhPs for infrared optics, it is crucial to understand propagation and loss mechanisms on substrates suitable for applications from waveguiding to infrared sensing. We employ scattering-type scanning near-field optical microscopy (s-SNOM) and nano-Fourier transform infrared (FTIR) spectroscopy, in concert with analytical and numerical calculations, to elucidate HPhP characteristics as a function of the complex substrate dielectric function. We consider propagation on suspended, dielectric and metallic substrates to demonstrate that the thickness-normalized wavevector can be reduced by a factor of 25 simply by changing the substrate from dielectric to metallic behavior. Moreover, by incorporating the imaginary contribution to the dielectric function in lossy materials, the wavevector can be dynamically controlled by small local variations in loss or carrier density. Counterintuitively, higher-order HPhP modes are shown to exhibit the same change in the polariton wavevector as the fundamental mode, despite the drastic differences in the evanescent ranges of these polaritons. However, because polariton refraction is dictated by the fractional change in the wavevector, this still results in significant differences in polariton refraction and reduced sensitivity to substrate-induced losses for the higher-order HPhPs. Such effects may therefore be used to spatially separate hyperbolic modes of different orders and for index-based sensing schemes. Our results advance our understanding of fundamental hyperbolic polariton excitations and their potential for on-chip photonics and planar metasurface optics.

Reconfigurable Multistate Optical Systems Enabled by VO2 Phase Transitions.

Reconfigurable optical systems are the object of continuing, intensive research activities, as they hold great promise for realizing a new generation of compact, miniaturized, and flexible optical devices. However, current reconfigurable systems often tune only a single state variable triggered by an external stimulus, thus, leaving out many potential applications. Here we demonstrate a reconfigurable multistate optical system enabled by phase transitions in vanadium dioxide (VO2). By controlling the phase-transition characteristics of VO2 with simultaneous stimuli, the responses of the optical system can be reconfigured among multiple states. In particular, we show a quadruple-state dynamic plasmonic display that responds to both temperature tuning and hydrogen-doping. Furthermore, we introduce an electron-doping scheme to locally control the phase-transition behavior of VO2, enabling an optical encryption device encoded by multiple keys. Our work points the way toward advanced multistate reconfigurable optical systems, which substantially outperform current optical devices in both breadth of capabilities and functionalities.