Phonon Polaritons Research Articles

Optical Imaging of Ultrafast Phonon-Polariton Propagation through an Excitonic Sensor.

Hexagonal boron nitride (hBN) hosts phonon polaritons (PhP), hybrid light-matter states that facilitate electromagnetic field confinement and exhibit long-range ballistic transport. Extracting the spatiotemporal dynamics of PhPs usually requires "tour de force" experimental methods such as ultrafast near-field infrared microscopy. Here, we leverage the remarkable environmental sensitivity of excitons in two-dimensional transition metal dichalcogenides to image PhP propagation in adjacent hBN slabs. Using ultrafast optical microscopy on monolayer WSe2/hBN heterostructures, we image propagating PhPs from 3.5 K to room temperature with subpicosecond and few-nanometer precision. Excitons in WSe2 act as transducers between visible light pulses and infrared PhPs, enabling visible-light imaging of PhP transport with far-field microscopy. We also report evidence of excitons in WSe2 copropagating with hBN PhPs over several micrometers. Our results provide new avenues for imaging polar excitations over a large frequency range with extreme spatiotemporal precision and new mechanisms to realize ballistic exciton transport at room temperature.

Ultra-broadband mid-infrared absorber based on hyperbolic α-MoO3

Owing to their broad operational bandwidth, broadband absorbers are extensively used in various applications, such as solar cells, microbolometers, and light modulators. To enhance energy harvesting, it is essential to achieve insensitivity to wide incident angles and polarization-independent absorption. α-MoO3 is a natural polar van der Waals crystal that hosts phonon polaritons in the mid-infrared spectral region. These polaritons can manipulate the infrared light at the nanoscale and break the diffraction limit, offering a great opportunity for the design of broadband absorbers. In this study, we propose a strategy based on an array of crossed the permittivity of α-MoO3 in [100] and [001] crystallographic axes to expand its broadband absorption. Triangular one-dimensional grating pattern is initially considered, which shows an ultra-broadband and wide-angle insensitive absorption within the range of 10.23–18.07 μm. Subsequently, square pyramidal two-dimensional arrays are investigated, which not only show ultra-broadband absorption and wide-angle insensitivity, but also exhibit polarization independence within the range of 10.46–18.32 μm. This remarkable broadband absorption is elucidated by the effective medium theory and the local power dissipation density. These results demonstrate an effective strategy for constructing ultra-broadband absorbers based on α-MoO3 for applications in solar thermal and photovoltaic systems.

Lithography-free high sensitivity perfect absorption based on Graphene/α-MoO3/SiC and Tamm plasmonic structure

In this paper, we propose a tunable perfect-absorption lithography-free refractive index sensor structure, which couples the Tamm phonon polariton (TPhP) mode and Graphene/α-MoO3/SiC (GMS) mode. By aligning the modal frequencies of TPhP and GMS, the coupling of the two modes is realized with perfect absorption. We utilize the transfer matrix method (TMM), resonance equation and coupled mode theory (CMT) to theoretically analyze the absorption of different modes and apply the structure to refractive index sensor. By modulating the structure parameter, the sensitivity of the sensor can be as high as 6 μm/RIU. This work could contribute to several fields such as refractive index sensors, photodetectors, absorbers, and energy harvesting devices.

Near- and Far-Field Observation of Phonon Polaritons in Wafer-Scale Multilayer Hexagonal Boron Nitride Prepared by Chemical Vapor Deposition.

Polaritons in layered materials (LMs) are a promising platform to manipulate and control light at the nanometer scale. Thus, the observation of polaritons in wafer-scale LMs is critically important for the development of industrially relevant nanophotonics and optoelectronics applications. In this work, phonon polaritons (PhPs) in wafer-scale multilayer hexagonal boron nitride (hBN) grown by chemical vapor deposition are reported. By infrared nanoimaging, the PhPs are visualized, and PhP lifetimes of ≈0.6ps are measured, comparable to that of micromechanically exfoliated multilayer hBN. Further, PhP nanoresonators are demonstrated. Their quality factors of ≈50 are about 0.7 times that of state-of-the-art devices based on exfoliated hBN. These results can enable PhP-based surface-enhanced infrared spectroscopy (e.g., for gas sensing) and infrared photodetector applications.

Longitudinal-optical-phonon resonant thermal emission efficiency and spectrum control of metal-GaAs surface micro-stripe structures

Phonon-based tera-Hertz or mid-infrared emission has been investigated because it can be applied to high-efficiency light sources for optical communication and material diagnostics, where thermal energy can be utilized as an energy source of emission. Particularly, nano- and micro-surface structures have been investigated for versatile emission controllability. We present the polarized longitudinal optical (LO) phonon resonant thermal emission of radiation (LORE) peaked at approximately 8.5 THz in a range of 450–650 K from simple surface Au–GaAs–Au micro-stripe structures on undoped (u-) GaAs wafers, where the metal component, which has been avoided for surface phonon polariton (SPhP) emission, is introduced. This LORE is based on electric dipoles formed by coherently oscillating polarization charges on pairs of interfaces and has a feature of structure size or emission direction-independent frequency, which is quite different from the properties of SPhPs. The dependence of emission properties on structural factors and temperature reveals the approximately evenly matched radiative and nonradiative LO-phonon annihilation rates in the local surface field and shows the controllability of emission and absorption balance based on the surface structures. Higher emission efficiency in the higher temperature region is found for structures with narrow emission windows of less than 2 μm. The decrease in the volume occupied by electric dipoles reduces the reabsorption probability of LORE, which shortens the effective radiative lifetime and, resultantly, increases the fraction of the radiative rate in the total annihilation rate. By comparing the emission from a pure graphite wafer, a short LORE mean time interval of approximately 2 ps is found. The background emission subject to Planck’s law is significantly reduced particularly in the region of 250–300 cm−1 using emission window widths narrower than 2 μm because of the sub-diffraction phenomenon.

Terahertz Twistoptics-Engineering Canalized Phonon Polaritons.

The terahertz (THz) frequency range is key to studying collective excitations in many crystals and organic molecules. However, due to the large wavelength of THz radiation, the local probing of these excitations in smaller crystalline structures or few-molecule arrangements requires sophisticated methods to confine THz light down to the nanometer length scale, as well as to manipulate such a confined radiation. For this purpose, in recent years, taking advantage of hyperbolic phonon polaritons (HPhPs) in highly anisotropic van der Waals (vdW) materials has emerged as a promising approach, offering a multitude of manipulation options, such as control over the wavefront shape and propagation direction. Here, we demonstrate the THz application of twist-angle-induced HPhP manipulation, designing the propagation of confined THz radiation between 8.39 and 8.98 THz in the vdW material α-molybdenum trioxide (α-MoO3), hence extending twistoptics to this intriguing frequency range. Our images, recorded by near-field optical microscopy, show the frequency- and twist-angle-dependent changes between hyperbolic and elliptic polariton propagation, revealing a polaritonic transition at THz frequencies. As a result, we are able to allocate canalization (highly collimated propagation) of confined THz radiation by carefully adjusting these two parameters, i.e. frequency and twist angle. Specifically, we report polariton canalization in α-MoO3 at 8.67 THz for a twist angle of 50°. Our results demonstrate the precise control and manipulation of confined collective excitations at THz frequencies, particularly offering possibilities for nanophotonic applications.

Colossal Optical Anisotropy from Atomic-Scale Modulations.

Materials with large birefringence (Δn, where n is the refractive index) are sought after for polarization control (e.g., in wave plates, polarizing beam splitters, etc.), nonlinear optics, micromanipulation, and as a platform for unconventional light-matter coupling, such as hyperbolic phonon polaritons. Layered 2Dmaterials can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optical axis is out of the plane of the layers and the layers are weakly attached. This work demonstrates that a bulk crystal with subtle periodic modulations in its structure-Sr9/8 TiS3 -is transparent and positive-uniaxial, with extraordinary index ne = 4.5 and ordinary index no = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3 , results in the formation of TiS6 trigonal-prismatic units that break the chains of face-sharing TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr form highly oriented electron clouds, which selectively boost the extraordinary index ne and result in record birefringence (Δn > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation.

Hybridized Phonon Polaritons Assisted Broad Range SEIRA-Based Multimolecular Sensing

Hybridized Phonon Polaritons Assisted Broad Range SEIRA-Based Multimolecular Sensing

Directive and coherent thermal emission of hybrid surface plasmon-phonon polaritons in n-GaN gratings of linear and radial shapes

Beaming and coherent thermal emission of the hybrid surface plasmon phonon polaritons (SPPhPs) was numerically and experimentally investigated employing the n-GaN surface relief gratings (SRGs) shaped in a linear and radial geometry. The polariton propagation losses were minimized numerically with the help of a rigorous coupled wave analysis model, while the spatial and temporal quality of selected mode radiation in a normal direction was maximized by fixing the grating period value at 17.5 µm and varying the grating filling factor from 75% to 25%. A set of optimal design linear and radial geometry SRG samples were fabricated in order to validate the emission characteristics of hybrid SPPhPs found by numerical modeling. We demonstrated that both efficient emission and beaming are possible to achieve through the excitation and interference of the same number but opposite sign hybrid polariton modes in n-GaN SRG.

Controlling the propagation asymmetry of hyperbolic shear polaritons in beta-gallium oxide

Structural anisotropy in crystals is crucial for controlling light propagation, particularly in the infrared spectral regime where optical frequencies overlap with crystalline lattice resonances, enabling light-matter coupled quasiparticles called phonon polaritons (PhPs). Exploring PhPs in anisotropic materials like hBN and MoO3 has led to advancements in light confinement and manipulation. In a recent study, PhPs in the monoclinic crystal β-Ga2O3 (bGO) were shown to exhibit strongly asymmetric propagation with a frequency dispersive optical axis. Here, using scanning near-field optical microscopy (s-SNOM), we directly image the symmetry-broken propagation of hyperbolic shear polaritons in bGO. Further, we demonstrate the control and enhancement of shear-induced propagation asymmetry by varying the incident laser orientation and polariton momentum using different sizes of nano-antennas. Finally, we observe significant rotation of the hyperbola axis by changing the frequency of incident light. Our findings lay the groundwork for the widespread utilization and implementation of polaritons in low-symmetry crystals.

Addressing the Impact of Surface Roughness on Epsilon-Near-Zero Silicon Carbide Substrates.

Epsilon-near-zero (ENZ) media have been very actively investigated due to their unconventional wave phenomena and strengthened nonlinear response. However, the technological impact of ENZ media will be determined by the quality of realistic ENZ materials, including material loss and surface roughness. Here, we provide a comprehensive experimental study of the impact of surface roughness on ENZ substrates. Using silicon carbide (SiC) substrates with artificially induced roughness, we analyze samples whose roughness ranges from a few to hundreds of nanometer size scales. It is concluded that ENZ substrates with roughness in the few nanometer scale are negatively affected by coupling to longitudinal phonons and strong ENZ fields normal to the surface. On the other hand, when the roughness is in the hundreds of nanometers scale, the ENZ band is found to be more robust than dielectric and surface phonon polariton (SPhP) bands.

Near-field radiative modulator based on α-MoO3 films

Near-field radiative heat transfer (NFRHT) has great potential in thermal management and energy harvesting due to the giant radiative heat flux. Modulation of NFRHT is another important research direction, where relative rotation between the emitter and receiver is a typical method. However, the enhancement of modulation contrast (MC) is mainly concerned in previous works, while large heat flux is not taken into account. In this work, we investigate the heat transfer coefficient (HTC) between α-MoO3 films along different crystal directions of α-MoO3. We take account of the modulator with high heat flux and high MC. The results show that the HTC and MC can reach 223.2 W/(m2·K) (36 times of blackbody limit) and 6.33 at a gap distance of 100 nm when NFRHT is along [010] crystal direction of α-MoO3, respectively. These are attributed to the excitation and mismatch of hyperbolic phonon polaritons (HPhPs). Furthermore, when NFRHT is along [100] and [001] crystal directions of α-MoO3, MCs reach 5.84 and 2.38, respectively. The results are strongly related to the crystal direction. In addition, the impact of film thickness on the MC for different crystal directions is investigated. This work is conducive to the applications of hyperbolic materials in non-contact thermal modulators.

Tunable optical topological transitions of plasmon polaritons in WTe2 van der Waals films

Naturally existing in-plane hyperbolic polaritons and the associated optical topological transitions, which avoid the nano-structuring to achieve hyperbolicity, can outperform their counterparts in artificial metasurfaces. Such plasmon polaritons are rare, but experimentally revealed recently in WTe2 van der Waals thin films. Different from phonon polaritons, hyperbolic plasmon polaritons originate from the interplay of free carrier Drude response and interband transitions, which promise good intrinsic tunability. However, tunable in-plane hyperbolic plasmon polariton and its optical topological transition of the isofrequency contours to the elliptic topology in a natural material have not been realized. Here we demonstrate the tuning of the optical topological transition through Mo doping and temperature. The optical topological transition energy is tuned over a wide range, with frequencies ranging from 429 cm−1 (23.3 microns) for pure WTe2 to 270 cm−1 (37.0 microns) at the 50% Mo-doping level at 10 K. Moreover, the temperature-induced blueshift of the optical topological transition energy is also revealed, enabling active and reversible tuning. Surprisingly, the localized surface plasmon resonance in skew ribbons shows unusual polarization dependence, accurately manifesting its topology, which renders a reliable means to track the topology with far-field techniques. Our results open an avenue for reconfigurable photonic devices capable of plasmon polariton steering, such as canaling, focusing, and routing, and pave the way for low-symmetry plasmonic nanophotonics based on anisotropic natural materials.

Optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3.

Phonon polaritons (PhPs), collective modes hybridizing photons with lattice vibrations in polar insulators, enable nanoscale control of light. In recent years, the exploration of in-plane anisotropic PhPs has yielded new levels of confinement and directional manipulation of nano-light. However, the investigation of in-plane anisotropic PhPs at the atomic layer limit is still elusive. Here, we report the optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3 (5 atomic layers). We show that narrow α-MoO3 nanoribbons as thin as a few atomic layers can support anisotropic PhPs modes with a high confinement ratio (∼133 times smaller wavelength than that of light). The anisotropic PhPs interference fringe patterns in atomic layers are tunable depending on the PhP wavelength via changing the illumination frequency. Moreover, spatial control over the PhPs interference patterns is also achieved by varying the nanostructures' shape or nanoribbon width of atomically-thin α-MoO3. Our work may serve as an empirical reference point for other anisotropic PhPs that approach the thickness limit and pave the way for applications such as atomically integrated nano-photonics and sensing.

Van der Waals isotope heterostructures for engineering phonon polariton dispersions

Element isotopes are characterized by distinct atomic masses and nuclear spins, which can significantly influence material properties. Notably, however, isotopes in natural materials are homogenously distributed in space. Here, we propose a method to configure material properties by repositioning isotopes in engineered van der Waals (vdW) isotopic heterostructures. We showcase the properties of hexagonal boron nitride (hBN) isotopic heterostructures in engineering confined photon-lattice waves—hyperbolic phonon polaritons. By varying the composition, stacking order, and thicknesses of h10BN and h11BN building blocks, hyperbolic phonon polaritons can be engineered into a variety of energy-momentum dispersions. These confined and tailored polaritons are promising for various nanophotonic and thermal functionalities. Due to the universality and importance of isotopes, our vdW isotope heterostructuring method can be applied to engineer the properties of a broad range of materials.

Near-field thermal rectification via an InSb/graphene/3C–SiC-nanowire heterostructure

Thermal rectifiers allow asymmetric heat flux when the temperature bias between two terminals is reversed. Based on the near-field radiative heat transfer, the performance of thermal rectifiers can be significantly enhanced. Here, we propose a near-field thermal rectifier based on the InSb/graphene/3C–SiC-nanowire heterostructure. The rectifier consists of two terminals, one terminal is an InSb slab with a graphene-coated bottom, and the other terminal is a 3C–SiC nanowire array with a graphene cover on its top. By using fluctuational electrodynamics, we calculate the radiation heat flux and the corresponding thermal rectification efficiency (TRE). The results show that the strong asymmetric surface plasmon polaritons (SPPs) and hyperbolic phonon polaritons (HPPs) in the forward and reverse heat transfer scenarios guarantee a robust TRE. Besides, the TRE of the proposed heterostructure can be dynamically controlled and optimized via the chemical potential of graphene, the volumetric filling ratio of the nanowire array, and the terminal temperature. Specifically, the thermal rectification efficiency (1 – min (Qf, Qr)/max (Qf, Qr) with Qf and Qr denoting the forward and reverse heat flux, respectively) can exceed 80% at a wide-ranged terminal gap and reaches a maximum of 92.96%.

Long-range polaritonic heat conduction in asymmetric surrounding media

Surface Phonon Polaritons (SPhPs) as an evanescent electromagnetic surface wave supports long propagation which readily surpasses the mean free path of classical heat carriers, e.g., phonons and electrons in solids. SPhPs have emerged as a promising candidate to dominate heat transfer in thin films. Polaritonic heat transfer has two distinct advantages: superior thermal conduction and a wide range of manipulation. Here, we study the upper limit of the thermal conductivity mediated by long-range polaritons in asymmetric surrounding media, where its surface effect overwhelms the volumetric one. The thin film structure strengthens the interactions of two surface waves at the top and bottom surfaces, and the asymmetric surrounding media makes an evanescent surface wave to further penetrate the free space with a higher refractive index, but it requires a fine tuning of asymmetric permittivity of the surrounding media to reach the upper limit of energy transmission efficiency near to the modal cut-off, where the transverse wavevector becomes zero. Both analytical and numerical simulations were introduced to investigate dispersion in asymmetric surrounding media and to model the thermal conductivity of glass thin films. Anomalously high thermal conductivity of 248 W/m-K was achieved with a 50 nm thick SiO2 film in asymmetric surrounding media, yet subtly dissimilar.

Phonon Polaritons: A New Paradigm for Light‐Controllable Heat Sources

AbstractPhotothermal applications, such as therapy, imaging, and catalysis, necessitate heat sources capable of generating high temperatures under mid‐infrared (mid‐IR) illumination. However, commonly used metal nanostructures suffer from low efficiency due to their high carrier concentrations, resulting in shallow surface heating and optical range restrictions. To overcome these limitations, this work proposes a novel approach employing materials that support phonon polaritons (PhPs) as a promising paradigm for light‐controllable heat sources. The theoretical demonstration reveals that hexagonal boron nitride (hBN) nanorods can produce up to 46 times more heat compared to plasmonic gold counterparts under resonant monochromatic light. This superior heating capability stems from the unique properties of PhPs, which enable stronger field confinement and deeper penetration within the nanostructure, leading to higher efficiency by circumventing the electrostatic shielding effect associated with plasmonic heating. Furthermore, this work demonstrates that the heating performance of hBN antennas can be optimized by manipulating their size, geometry, and material loss. Notably, the use of isotopically pure hBN can triple the heat power. These findings highlight the tremendous potential of hBN antennas as light‐controllable heat sources, opening up new possibilities for IR photothermal applications by harnessing materials that support PhPs.

Polaritonic Probe of an Emergent 2D Dipole Interface.

The use of work-function-mediated charge transfer has recently emerged as a reliable route toward nanoscale electrostatic control of individual atomic layers. Using α-RuCl3 as a 2D electron acceptor, we are able to induce emergent nano-optical behavior in hexagonal boron nitride (hBN) that arises due to interlayer charge polarization. Using scattering-type scanning near-field optical microscopy (s-SNOM), we find that a thin layer of α-RuCl3 adjacent to an hBN slab reduces the propagation length of hBN phonon polaritons (PhPs) in significant excess of what can be attributed to intrinsic optical losses. Concomitant nano-optical spectroscopy experiments reveal a novel resonance that aligns energetically with the region of excess PhP losses. These experimental observations are elucidated by first-principles density-functional theory and near-field model calculations, which show that the formation of a large interfacial dipole suppresses out-of-plane PhP propagation. Our results demonstrate the potential utility of charge-transfer heterostructures for tailoring optoelectronic properties of 2D insulators.

Flatband polaritonic router in twisted bilayer van der Waals materials.

In recent years, van der Waals (vdW) polaritons excited by the hybrid of matter and photons have shown great promise for applications in nanoimaging, biosensing, and on-chip light guiding. In particular, polaritons with a flatband dispersion allow for mode canalization and diffractionless propagation, which showcase advantages for on-chip technologies requiring long-range transportation of optical information. Here, we propose a flatband polaritonic router based on twisted α-MoO3 bilayers, which allows for on-chip routing of highly confined and low-loss phonon polaritons (PhPs) along multichannel propagating paths under different circular polarized dipole excitations. Our work combines flatband physics and optical spin- orbit coupling, with potential applications in nanoscale light propagation, on-chip optical switching, and communication.