Joshua D. Caldwell

Hyperbolic phonon polaritons ‐ hybridized modes arising from the ultrastrong coupling of infrared light to strongly anisotropic lattice vibrations in uniaxial or biaxial polar crystals ‐ enable to confine light to the nanoscale with low losses and high directionality. In even lower symmetry materials, such as monoclinic ‐Ga 2 O 3 (bGO), hyperbolic shear polaritons (HShPs) further enhance the directionality. Yet, HShPs are intrinsically supported only within narrow frequency ranges defined by the phonon frequencies of the host material. Here, we report spectral tuning of HShPs in bGO by isotopic substitution. Employing near‐field optical microscopy to image HShPs in 18 O bGO films homoepitaxially grown on a 16 O bGO substrate, we demonstrate a spectral redshift of 40 cm −1 for the 18 O bGO, compared to 16 O bGO. The technique allows for direct observation and a model‐free estimation of the spectral shift driven by isotopic substitution without the need for knowledge of the dielectric tensor. Complementary far‐field measurements and ab initio calculations ‐ in good agreement with the near‐field data ‐ confirm the effectiveness of this estimation. This multifaceted study demonstrates a significant isotopic substitution induced spectral tuning of HShPs into a previously inaccessible frequency range, creating new avenues for technological applications of such highly directional polaritons.

Controlling the spatial distribution and field localization of plasmonic elements is crucial to designing systems which rely on modal overlap. Such overlaps can lead to strong coupling, as has previously been demonstrated for mid-infrared surface plasmon polariton (SPP) and epsilon-near-zero (ENZ) modes in cadmium oxide (CdO) thin films. Insight into how local cavity confinement alters modal frequency, spatial field distribution, and coupling behavior is desirable for further control of CdO-supported polaritons. Here, we utilize monochromated electron energy loss spectroscopy in the scanning transmission electron microscope to locally probe and map out the plasmonic response of CdO pillars comprised of SPP- and ENZ-sustaining layers with simultaneously high spatial and energy resolution. We demonstrate that multiple localized surface plasmons (LSP) appear strongly asymmetric due to convoluting effects of uneven carbon coating and crosstalk between microresonators. Several of these modes are nonradiative in nature, and the diameter-independent modes convolve with diameter-dependent modes. Effects of Ga+ irradiation and hydration are additionally investigated as factors impacting the plasmonic response. These results provide fundamental insights into how radiant and subradiant modes manifest within propagating infrared plasmonic materials that have been shaped into individual resonators, offering relevant insights into the photonic density of states of hybrid resonators.

Vibrational ultrastrong coupling (USC) opens new opportunities for controlling chemical reactivity and advancing mid-infrared nanophotonics. Here, we demonstrate USC in an ultrathin polar dielectric using acoustic graphene plasmons (AGPs). Unlike conventional graphene plasmons, whose coupling weakens with a reduced dielectric thickness, AGPs maintain strong coupling through enhanced field confinement in narrow dielectric gaps. This enables strong coupling in a polar film as thin as λ/8,000 and USC at λ/2,000. While USC is usually attributed to transverse optical phonons, we identify interface phonon polaritons at polar-nonpolar boundaries as an additional contributor. By tuning the AGP coupling to both phonon types, we observe a transition from a single anticrossing to two distinct anticrossings. This platform, sustaining USC in an extremely small volume, provides a promising basis for mid-infrared nanophotonics and quantum optics.

Distinct optical properties of nanostructures have enabled infrared light sources with high spatial and/or spectral coherence and offered IR absorption/emission cross sections well beyond the physical dimensions. Yet, the implementation of such structures for heat dissipation through radiation or enhanced conduction via polaritons is still in the nascent stage. While thermal emissivity could reveal information critical to radiation and light-matter interactions, direct experimental measurements of individual nanostructures' emissivity have long been impeded due to the extremely small signal with respect to the thermal background. In this study, we introduce a platform to quantify the far-field thermal emission from nanoscale samples, which allows for the detection of small signals through differential calorimetry employing a Wheatstone bridge scheme. Experimental validation through measurements of the emissivity of SiO2 nanoflakes demonstrates good agreement with simulation results, highlighting size-dependent emissivity trends and distinct resonant behavior in the Reststrahlen band. This platform provides a route for characterizing small thermal emission signals, which can be used to probe the thermal radiation properties of various nanostructures, thereby aiding in the fundamental understanding of nanostructures' thermal emissivity for the design of novel IR emitters and effective strategies for microdevice thermal management.

Hexagonal boron nitride (hBN) is an important 2D material for van der Waals heterostructures, single photon emitters, and infrared nanophotonics. The optical characterization of mono- and few-layer samples of hBN however, remains a challenge as the material is almost invisible optically. Here, phase-resolved sum-frequency microscopy is introduced as a technique for imaging monolayers of hBN grown by chemical vapor deposition (CVD) and visualizing their crystal orientation. Femtosecond mid-infrared (IR) and visible laser pulses are used for sum-frequency generation (SFG), which is imaged in a wide-field optical microscope. The IR laser resonantly excites a phonon of hBN that leads to an ≈800-fold enhancement of the SFG intensity, making it possible to image large 100 × 100 µm sample areas in less than 1 s. Heterodyne detection combined with azimuthal sample rotation further provides full crystallographic information. Combined knowledge of topography and crystal orientation reveals that triangular domains of CVD-grown monolayer hBN have nitrogen-terminated zigzag edges. Overall, SFG microscopy is an ultra-sensitive tool with the potential to image crystal structure, strain, stacking sequences, and twist angles in a wide range of van der Waals structures, where locating and identifying monolayer regions and interfaces with broken inversion symmetry is of paramount importance.

Metal‐based thermal metasurfaces exhibit stable spectral characteristics under temperature fluctuations, in contrast to more traditional gray‐ and near black‐bodies, as well as some dielectric metasurfaces, whose emission spectra shift with changing temperatures. However, they often suffer from limited quality (Q) factors due to significant non‐radiative ohmic losses. This study addresses the challenge of achieving high emissivity and Q‐factors in metal‐based thermal emitters. By leveraging the coupling of three surface lattice resonances that support bound states in the continuum and electromagnetically induced absorption (EIA), this work designs a metal‐based thermal metasurface with a near‐unity emissivity (0.96) and a Q factor as high as 320 through simulations. Experimental validation yields an emissivity of 0.82 and a Q factor of 202, representing an approximately fivefold improvement in the experimentally measured Q factor compared to the state‐of‐the‐art metal‐based thermal metasurfaces. This work offers a promising approach for developing efficient, narrow‐band, directional thermal emitters with stable emission spectra across a wide temperature range.

Low-symmetry crystals have emerged as a platform for exploring unique light-matter interactions in the form of hyperbolic shear polaritons. These excitations exhibit unique properties such as frequency-dispersive optical axes and asymmetric light propagation and energy dissipation. However, only non-vdW materials have been demonstrated to support hyperbolic shear polaritons, limiting their exotic properties and potential applications. Here, we introduce shear phenomena in low symmetry crystals by demonstrating elliptical and canalized shear phonon polaritons in gypsum, an exfoliable monoclinic sulphate mineral. Our results unveil a topological transition from hyperbolic shear to elliptical shear polaritons, passing through a canalization regime with strong field confinement. We observe a notable slowdown of group velocity, reaching values as low as 0.0005 . These findings expand the application scope of low-symmetry crystals with the benefits that an exfoliable material provides, such as stronger field confinement, tunability, and versatility for its incorporation in complex photonic devices.