Dissertations, Theses, and Capstone Projects
Date of Degree
9-2026
Document Type
Doctoral Dissertation
Degree Name
Doctor of Philosophy
Program
Physics
Advisor
Andrea Alù
Committee Members
Eitan Bachmat
Li Ge
Vinod Menon
Ali Miri
Subject Categories
Condensed Matter Physics | Engineering Physics | Optics | Other Physics
Keywords
Hyperbolic phonon polaritons, Symmetry breaking, Mid-infrared nanophotonics, Metasurfaces, Floquet engineering, Dispersion engineering
Abstract
The confinement and directional propagation of light in conventional optical media are fundamentally restricted by the diffraction limit. Polaritons, hybrid excitations of light and a material resonance, overcome this constraint by confining light to deeply subwavelength volumes, and in hyperbolic media they additionally support highly directional, ray-like propagation. Natural polar dielectrics such as hexagonal boron nitride (hBN) and $\alpha$-MoO$_3$ realize such hyperbolic media at mid-infrared frequencies through their phonon polaritons, low-loss surface modes that are anisotropic by nature. In these materials, the lower the crystal symmetry, the more directional the phonon resonances and the more exotic the supported polaritons, so that broken symmetry directly produces directional wave phenomena. In natural crystals, however, this response is inherited rather than engineered: the operating frequency is locked to the lattice Reststrahlen bands, and the anisotropy is fixed once the crystal has grown. In this work, we show that these constraints are not fundamental, and that symmetry breaking, whether intrinsic to the material or imposed by design, provides a general tool to engineer hyperbolicity and the directional phenomena it produces. The treatment is developed for phonon polaritons in the mid-infrared but applies to hyperbolic media at any frequency.
First, we study shear hyperbolic polaritons in natural monoclinic crystals, in which non-orthogonal, detuned phonon resonances rotate the optical axis with frequency and redistribute loss asymmetrically between opposite directions. This response is fixed by the lattice and limited to a few crystals. We reproduce and surpass it in an engineered metasurface of two detuned resonator families twisted away from orthogonality, obtaining programmable axial dispersion and loss asymmetry stronger than in any natural monoclinic crystal, in both electromagnetic and elastic platforms. Next, we address how to access these modes. The hyperbolic dispersion is open and rotationally asymmetric, and its polaritons are so strongly confined that free-space light cannot couple to them along more than one direction at a time. We solve this with a bowtie grating, designed by Fourier-space matching to the hyperbolic contour, which couples far-field radiation to the confined polaritons along every direction of the open branch and enhances the local light--matter interaction. We then break symmetry in time. An intense optical pump that periodically modulates a polar crystal couples free-space light to surface phonon polaritons all-optically, without any grating, and drives their dispersion through topological transitions purely by tuning the pump. Because these polaritons are tightly confined, manipulating them requires dedicated nanoscale optics, whose design is complicated by the non-orthogonality of phase and energy flow in hyperbolic media. We show that a Minkowski, or Lorentzian, geometric description restores conventional ray optics and yields a simple, closed-form route to design such devices, and we use it to realize nanolenses that focus phonon polaritons far below the diffraction limit. Finally, we combine this strong confinement with cavity-induced symmetry breaking. A hyperbolic medium enclosed in a resonator with a single tilted wall supports a new class of cavity modes, the hyperbolic wave attractors: stable, chiral field patterns absent from conventional resonators, whose physics we unveil and exploit for broadband frequency sorting.
Together, these results establish symmetry breaking, intrinsic or engineered, as a deliberate design principle for directional wave phenomena in hyperbolic media. Although demonstrated for phonon polaritons in the mid-infrared, the approach is general and applies at any frequency, laying the groundwork for reconfigurable mid-infrared photonics, label-free sensing, nanoscale thermal management, and magnet-free nonreciprocal devices.
Recommended Citation
Renzi, Enrico Maria, "Engineering Wave Phenomena in Hyperbolic Materials" (2026). CUNY Academic Works.
https://academicworks.cuny.edu/gc_etds/6821
Included in
Condensed Matter Physics Commons, Engineering Physics Commons, Optics Commons, Other Physics Commons
