Opportunities rooted in programmable quantum matter are abundant. Nano-optical exploration of quantum materials can leverage polaritons: where strong dipole active resonances hybridize with photons. Infrared nano-imaging can be used to detect these polaritonic waves, yielding insights into the properties of host materials that support polaritons. The naturally layered class of van der Waals (vdW) materials are particularly intriguing in this regard due, in-part, to their often strongly anisotropic behavior. In materials hosting both strong anisotropy and strong dipole active resonances non-intuitive optical properties can emerge. Chief among them are sub-diffractional polaritonic wave packets that can travel as conical rays with hyperbolic dispersion throughout the materials’ bulk. We utilized femtosecond photoexcitation to agitate the vdW semiconductor WSe₂ and interrogated these crystals in the transient state [1,2]. In the mid-infrared range of the electromagnetic spectrum, we detect on-demand hyperbolic exciton-polaritons, which propagate throughout the bulk of WSe₂. Our time-resolved nano-imaging data reveals key signatures of optical hyperbolicity, appearing on the sub-picosecond timescale. By varying the photo-excitation power programmable trajectories of hyperbolic polaritons, propagating within the crystal, are observed [1].

[1] A. J. Sternbach et al., Science 371, 617 (2021)
[2] A. J. Sternbach et al., Nature Communications 11, 3567 (2020)

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