Polariton Design and Modulation via Van der Waals / Doped Semiconductor Heterostructures

Due to the long free-space wavelength of mid- to far-infrared (IR) light, the realization of deeply subdiffractional photon confinement via the stimulation of polaritons is critical for IR nanophotonic applications, such as miniaturized optical components, on-chip photonics, polariton waveguides, and nanolasers. Specifically, hyperbolic polaritons supported in extremely anisotropic media, i.e., those featuring permittivity tensor components with opposite signs along different optical axes, can offer significant promise for many nanophotonic applications where stronger confinement and improved control over propagation are beneficial. Applications of these properties include hyperlensing, metasurface-based optical components, quantum optics, and probes of nanoscale defects. While hyperbolicity was first demonstrated with artificial dielectric/metal stacks, it was later discovered that a list of natural materials, including hexagonal boron nitride (hBN), MoO₃, V₂O₅, support hyperbolic phonon polaritons (HPhPs). Such HPhPs in natural crystals feature exceptionally low optical losses, as the polaritons are derived from optic phonons instead of scattering from free carriers.<br/>Compared to surface polaritons, HPhPs offer further confinement of long-wavelength light to deeply subdiffractional scales. Furthermore, the evanescent field of volume propagation could interact with the local environment, e.g., substrate, allowing for tuning of wavevectors without modifying the hyperbolic media itself. Therefore, it is possible to realize polaritonic resonators and near-field polariton propagation without deleterious etching of hyperbolic materials. While it is promising to utilize the substrate-induced effect in engineering polaritonic devices, conventionally used noble metal and dielectric substrates restrict the tunability of this approach, leaving most of the wavevectors in the dispersion inaccessible.<br/>Here, we present our recent work on exploiting substrate induced HPhP tuning. We demonstrate that using doped semiconductors, e.g., InAs and CdO, can enable near-continuous tuning and access to both the maximum and minimum wavevectors (~8.3 times experimentally demonstrated). We further elucidate HPhP tuning with the plasma frequency of an InAs substrate, which features a significant wavevector discontinuity and modal order transition when the substrate permittivity crosses -1 in the Reststrahlen band. Around the transition point, the HPhP system is sensitive to perturbations, e.g., the working frequency, InAs plasma frequency and superstrate, thus it is suitable for sensing and modulation applications. Although the loss of HPhPs peaks around the transition point, where the largest tunability is observed, we highlight that the increased loss can be neglected in some proposed applications/devices. We also illustrate that the hBN/InAs platform allows for active modulation at picosecond timescales by photo-injecting carriers into the InAs substrate, demonstrating a dynamic wavevector change of ~20%. Overall, the demonstrated hBN/doped semiconductor platform offers significant improvements towards manipulating HPhPs, and enormous potential for engineered and modulated polaritonic systems for applications in on-chip photonics and planar metasurface optics.