Shanhui Fan, Postdoctoral Faculty Sponsor
Hyperbolic shear polaritons in low-symmetry crystals.
2022; 602 (7898): 595-600
The lattice symmetry of a crystal is one of the most important factors in determining its physical properties. Particularly, low-symmetry crystals offer powerful opportunities to control light propagation, polarization and phase1-4. Materials featuring extreme optical anisotropy can support a hyperbolic response, enabling coupled light-matter interactions, also known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales5. Here we show that monoclinic crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid-infrared to far-infrared due to shear phenomena in the dielectric response. This feature emerges in materials in which the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals in which several oscillators with non-orthogonal relative orientations contribute to the optical response6,7. Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic1,3,4 and hexagonal8,9 crystal systems, unveiling new features, such as the continuous evolution of their propagation direction with frequency, tilted wavefronts and asymmetric responses. The interplay between diagonal loss and off-diagonal shear phenomena in the dielectric response of these materials has implications for new forms of non-Hermitian and topological photonic states. We anticipate that our results will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals10, many common oxides11 and organic crystals12, greatly expanding the material base and extending design opportunities for compact photonic devices.
View details for DOI 10.1038/s41586-021-04328-y
View details for PubMedID 35197618
Interface nano-optics with van der Waals polaritons
2021; 597 (7875): 187-195
Polaritons are hybrid excitations of matter and photons. In recent years, polaritons in van der Waals nanomaterials-known as van der Waals polaritons-have shown great promise to guide the flow of light at the nanoscale over spectral regions ranging from the visible to the terahertz. A vibrant research field based on manipulating strong light-matter interactions in the form of polaritons, supported by these atomically thin van der Waals nanomaterials, is emerging for advanced nanophotonic and opto-electronic applications. Here we provide an overview of the state of the art of exploiting interface optics-such as refractive optics, meta-optics and moiré engineering-for the control of van der Waals polaritons. This enhanced control over van der Waals polaritons at the nanoscale has not only unveiled many new phenomena, but has also inspired valuable applications-including new avenues for nano-imaging, sensing, on-chip optical circuitry, and potentially many others in the years to come.
View details for DOI 10.1038/s41586-021-03581-5
View details for Web of Science ID 000695818300008
View details for PubMedID 34497390
Ghost hyperbolic surface polaritons in bulk anisotropic crystals
2021; 596 (7872): 362-+
Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale1-10. So far these polaritons have been limited to two classes: bulk polaritons, which propagate inside a material, and surface polaritons, which decay exponentially away from an interface. Here we report a near-field observation of ghost phonon polaritons, which propagate with in-plane hyperbolic dispersion on the surface of a polar uniaxial crystal and, at the same time, exhibit oblique wavefronts in the bulk. Ghost polaritons are an atypical non-uniform surface wave solution of Maxwell's equations, arising at the surface of uniaxial materials in which the optic axis is slanted with respect to the interface. They exhibit an unusual bi-state nature, being both propagating (phase-progressing) and evanescent (decaying) within the crystal bulk, in contrast to conventional surface waves that are purely evanescent away from the interface. Our real-space near-field imaging experiments reveal long-distance (over 20 micrometres), ray-like propagation of deeply subwavelength ghost polaritons across the surface, verifying long-range, directional and diffraction-less polariton propagation. At the same time, we show that control of the out-of-plane angle of the optic axis enables hyperbolic-to-elliptic topological transitions at fixed frequency, providing a route to tailor the band diagram topology of surface polariton waves. Our results demonstrate a polaritonic wave phenomenon with unique opportunities to tailor nanoscale light in natural anisotropic crystals.
View details for DOI 10.1038/s41586-021-03755-1
View details for Web of Science ID 000686681500012
View details for PubMedID 34408329
Topological polaritons and photonic magic angles in twisted alpha-MoO(3)bilayers
2020; 582 (7811): 209-+
Twisted two-dimensional bilayer materials exhibit many exotic electronic phenomena. Manipulating the 'twist angle' between the two layers enables fine control of the electronic band structure, resulting in magic-angle flat-band superconductivity1,2, the formation of moiré excitons3-8 and interlayer magnetism9. However, there are limited demonstrations of such concepts for photons. Here we show how analogous principles, combined with extreme anisotropy, enable control and manipulation of the photonic dispersion of phonon polaritons in van der Waals bilayers. We experimentally observe tunable topological transitions from open (hyperbolic) to closed (elliptical) dispersion contours in bilayers of α-phase molybdenum trioxide (α-MoO3), arising when the rotation between the layers is at a photonic magic twist angle. These transitions are induced by polariton hybridization and are controlled by a topological quantity. At the transitions the bilayer dispersion flattens, exhibiting low-loss tunable polariton canalization and diffractionless propagation with a resolution of less than λ0/40, where λ0 is the free-space wavelength. Our findings extend twistronics10 and moiré physics to nanophotonics and polaritonics, with potential applications in nanoimaging, nanoscale light propagation, energy transfer and quantum physics.
View details for DOI 10.1038/s41586-020-2359-9
View details for Web of Science ID 000542696500004
View details for PubMedID 32528096
- Coherent steering of nonlinear chiral valley photons with a synthetic Au-WS2 metasurface NATURE PHOTONICS 2019; 13 (7): 467-+