Polariton design and modulation via van der Waals / doped semiconductor heterostructures

Hyperbolic phonon polaritons (HPhPs) can be supported in highly anisotropic materials, where the real parts of their permittivities along different directions are opposite in sign as a result of spectrally offset optical phonons. Compared to surface polaritons, HPhPs offer further confinement of long-wavelength light to deeply subdiffractional scales, and volume propagation that enables control of the polariton wavevector by changing the underlying medium. This allows for greater control of polaritonic resonators and near-field polariton propagation without deleterious etching of hyperbolic materials. Yet, conventionally used noble metal and dielectric substrates restrict the tunability of this approach, leaving most of the wavevector inaccessible. To overcome this challenge, 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. 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.