Hyperbolic Phonon Polaritons Research Articles

On-chip phonon-enhanced IR near-field detection of molecular vibrations

Phonon polaritons – quasiparticles formed by strong coupling of infrared (IR) light with lattice vibrations in polar materials – can be utilized for surface-enhanced infrared absorption (SEIRA) spectroscopy and even for vibrational strong coupling with nanoscale amounts of molecules. Here, we introduce and demonstrate a compact on-chip phononic SEIRA spectroscopy platform, which is based on an h-BN/graphene/h-BN heterostructure on top of a metal split-gate creating a p-n junction in graphene. The metal split-gate concentrates the incident light and launches hyperbolic phonon polaritons (HPhPs) in the heterostructure, which serves simultaneously as SEIRA substrate and room-temperature infrared detector. When thin organic layers are deposited directly on top of the heterostructure, we observe a photocurrent encoding the layer’s molecular vibrational fingerprint, which is strongly enhanced compared to that observed in standard far-field absorption spectroscopy. A detailed theoretical analysis supports our results, further predicting an additional sensitivity enhancement as the molecular layers approach deep subwavelength scales. Future on-chip integration of infrared light sources such as quantum cascade lasers or even electrical generation of the HPhPs could lead to fully on-chip phononic SEIRA sensors for molecular and gas sensing.

Deep subwavelength topological edge state in a hyperbolic medium.

Topological photonics offers the opportunity to control light propagation in a way that is robust from fabrication disorders and imperfections. However, experimental demonstrations have remained on the order of the vacuum wavelength. Theoretical proposals have shown topological edge states that can propagate robustly while embracing deep subwavelength confinement that defies diffraction limits. Here we show the experimental proof of these deep subwavelength topological edge states by implementing periodic modulation of hyperbolic phonon polaritons within a van der Waals heterostructure composed of isotopically pure hexagonal boron nitride flakes on patterned gold films. The topological edge state is confined in a subdiffraction volume of 0.021 µm3, which is four orders of magnitude smaller than the free-space excitation wavelength volume used to probe the system, while maintaining the resonance quality factor above 100. This finding can be directly extended to and hybridized with other van der Waals materials to broadened operational frequency ranges, streamline integration of diverse polaritonic materials, and compatibility with electronic and excitonic systems.

Landau-phonon polaritons in Dirac heterostructures.

Polaritons are light-matter quasiparticles that govern the optical response of quantum materials at the nanoscale, enabling on-chip communication and local sensing. Here, we report Landau-phonon polaritons (LPPs) in magnetized charge-neutral graphene encapsulated in hexagonal boron nitride (hBN). These quasiparticles emerge from the interaction of Dirac magnetoexciton modes in graphene with the hyperbolic phonon polariton modes in hBN. Using infrared magneto-nanoscopy, we reveal the ability to completely halt the LPP propagation in real space at quantized magnetic fields, defying the conventional optical selection rules. The LPP-based nanoscopy also tells apart two fundamental many-body phenomena: the Fermi velocity renormalization and field-dependent magnetoexciton binding energies. Our results highlight the potential of magnetically tuned Dirac heterostructures for precise nanoscale control and sensing of light-matter interaction.

Enhanced and tunable near-field thermophotovoltaics driven by hybrid polaritons

Near-field thermophotovoltaics (NF-TPV) offers the potential for achieving elevated power density and conversion efficiency by leveraging the amplification of thermal radiation within a nanoscale gap. Here, we propose an NF-TPV device with a sandwich emitter composed of calcite film and graphene layer. The results show that this sandwich configuration can significantly enhance output power, outperforming monolayer-graphene-covered heterostructures and the single calcite film. These are because the sandwich configuration can enhance hybrid polaritons, which are formed by the coupling between surface plasmon polaritons in graphene and hyperbolic phonon polaritons in calcite. In addition, the effects of graphene chemical potentials on the performance of NF-TPV devices are also studied. The tunable power density range of the sandwich structure can be up to 3.26 times that of other structures by altering the chemical potential of graphene. The findings presented here may unpack a promising path for enhancing and manipulating the performance of NF-TPV at the nano- and microscale.

Configurable lateral optical forces from twisted mixed-dimensional MoO3 homostructures

Abstract In recent years, the concept of hyperbolic phonon polaritons (HPPs) has revolutionized the field of nanophotonic, enabling unprecedented control over light-matter interactions at the nanoscale. Here, we theoretically propose and study the lateral optical forces in twisted mixed-dimensional MoO3 homostructures. Assisted with the low-symmetry HPPs, we realize a lateral optical force exerted on the Au nanoparticles near the surface of mixed-dimensional MoO3 homostructures with a linear polarized incident light. By controlling the polarization state, incident angle of light and the twisted angle of MoO3, the amplitude and direction of the lateral optical forces can be tailored in the mid-infrared range. Our findings provide a new platform for engineering lateral optical forces to manipulate diverse objects in a flexible and efficient manner.

Low Dielectric Medium for Hyperbolic Phonon Polariton Waveguide in van der Waals Heterostructures.

Polar van der Waals (vdW) crystals, composed of atomic layers held together by vdW forces, can host phonon polaritons-quasiparticles arising from the interaction between photons in free-space light and lattice vibrations in polar materials. These crystals offer advantages such as easy fabrication, low Ohmic loss, and optical confinement. Recently, hexagonal boron nitride (hBN), known for having hyperbolicity in the mid-infrared range, has been used to explore multiple modes with high optical confinement. This opens possibilities for practical polaritonic nanodevices with subdiffractional resolution. However, polariton waves still face exposure to the surrounding environment, leading to significant energy losses. In this work, we propose a simple approach to inducing a hyperbolic phonon polariton (HPhP) waveguide in hBN by incorporating a low dielectric medium, ZrS2. The low dielectric medium serves a dual purpose-it acts as a pathway for polariton propagation, while inducing high optical confinement. We establish the criteria for the HPhP waveguide in vdW heterostructures with various thicknesses of ZrS2 through scattering-type scanning near-field optical microscopy (s-SNOM) and by conducting numerical electromagnetic simulations. Our work presents a feasible and straightforward method for developing practical nanophotonic devices with low optical loss and high confinement, with potential applications such as energy transfer, nano-optical integrated circuits, light trapping, etc.

Unidirectional hyperbolic whispering-gallery phonon-polariton excitation in boron nitride nanotubes.

In two-dimensional (2D) hyperbolic materials, energy is directed into their deep subwavelength polaritonic modes through four narrow beams. Hyperbolic whispering-gallery mode nanocavity-confined phonon polaritons (PhPs) display a strongly enhanced light-matter interaction in the infrared regime. Particularly, the unidirectional phonon-polarization excitation in nanocavities has a potential application value in an on-chip integrated optical circuit design, efficient optical sensors, and enhanced spectral technology. Here, we explore the hyperbolic whispering-gallery mode PhPs on the cross section of a hexagonal BN nanotube (BNNT) and demonstrate that efficient unidirectional excitation can be achieved using a circularly polarized electric dipole, combining with optical spin-orbit coupling. Our results demonstrated that the undirectionality of the hyperbolic polariton propagation in a nanocavity can be conveniently achieved, independent of the structure symmetry of the nanocavity, providing potential applications in nanoscale light propagation, on-chip optical devices, and communication.

Atomic-force-microscopy-based time-domain two-dimensional infrared nanospectroscopy.

For decades, infrared (IR) spectroscopy has advanced on two distinct frontiers: enhancing spatial resolution and broadening spectroscopic information. Although atomic force microscopy (AFM)-based IR microscopy overcomes Abbe's diffraction limit and reaches sub-10 nm spatial resolutions, time-domain two-dimensional IR spectroscopy (2DIR) provides insights into molecular structures, mode coupling and energy transfers. Here we bridge the boundary between these two techniques and develop AFM-2DIR nanospectroscopy. Our method offers the spatial precision of AFM in combination with the rich spectroscopic information provided by 2DIR. This approach mechanically detects the sample's photothermal responses to a tip-enhanced femtosecond IR pulse sequence and extracts spatially resolved spectroscopic information via FFTs. In a proof-of-principle experiment, we elucidate the anharmonicity of a carbonyl vibrational mode. Further, leveraging the near-field photons' high momenta from the tip enhancement for phase matching, we photothermally probe hyperbolic phonon polaritons in isotope-enriched h10BN. Our measurements unveil an energy transfer between phonon polaritons and phonons, as well as among different polariton modes, possibly aided by scattering at interfaces. The AFM-2DIR nanospectroscopy enables the in situ investigations of vibrational anharmonicity, coupling and energy transfers in heterogeneous materials and nanostructures, especially suitable for unravelling the relaxation process in two-dimensional materials at IR frequencies.

Greatly Enhanced Radiative Transfer Enabled by Hyperbolic Phonon Polaritons in α‐MoO3

AbstractOrthorhombic molybdenum trioxide (α‐MoO3) is a highly anisotropic hyperbolic material in nature. Within its wide Reststrahlen bands, α‐MoO3 has hyperboloidal dispersion that supports bulk propagation of high‐k phonon polariton modes. These modes can serve as energy transport channels to greatly enhance radiative heat transfer inside the material. In this work, large radiative transfer enabled by phonon polaritons in α‐MoO3 is demonstrated. The study first determines the temperature‐dependent permittivity of α‐MoO3 from polarized Fourier‐Transform Infrared (FTIR) spectroscopy measurements and then uses a many‐body radiative heat transfer model to predict the equivalent radiative thermal conductivity of hyperbolic phonon polariton. Contribution of radiative transfer to the total thermal transport is experimentally determined from the Time‐Domain Thermoreflectance (TDTR) measurements in a temperature range from −100 to 300 °C. It is found that radiative transfer can account for ≈60% of the total thermal transport at a temperature of 300 °C. That is, conductive thermal transport is enhanced by >100% by radiative transfer, or radiation inside α‐MoO3 is greater than that of conduction. These additional energy pathways will have important implications in thermal management in new materials and devices.

Active control of nonreciprocal near-field radiative heat transfer with a drift-current biased graphene/α-MoO3 heterostructure.

The interaction between the drift-current biased graphene plasmonics and the hyperbolic phonon polaritons of α-MoO3 provides a promising way to manipulate near-field radiation heat transfer (NFRHT). Through examination of the drift biased graphene/α-MoO3 heterostructure, it has been discovered that drift-current applied to the graphene effectively enhances photon tunneling. Consequently, they dynamically modulate the coupling effect of the two excitations, thereby offering a reliable pathway for the modulation of NFRHT. Furthermore, the influencing mechanism of vacuum gaps on nonreciprocal NFRHT with different drift-current rates is revealed, and it is discovered that the vacuum gaps can filter the nonreciprocal surface plasmon polaritons with high nonreciprocity. Our findings make it possible to manipulate nanoscale thermal rectification and noncontact thermal modulation.

Modulations of superradiance and quantum entanglement of quantum emitters in terahertz frequency

We study the superradiance and entanglement phenomena between two quantum emitters (QEs) operating at terahertz frequencies, facilitated by the presence of a topological insulator bismuth selenide (Bi2Se3). Controlled by the hyperbolic phonon polaritons and electron surface state in the terahertz frequency range, the interaction between QEs is significantly enhanced both within and outside the hyperbolic bands. Moreover, the entanglement between two QEs through Bi2Se3 is studied in detail. Certain initial entanglement states undergo sudden death of entanglement when the system is in non-interaction. However, in the cases of moderate or strong interaction, the entanglement can be revive after experiencing the sudden death phenomenon. These findings hold significant implications for controlling the interaction of QEs and their potential applications in the fields of quantum metrology and quantum imaging in the terahertz frequency region.

Tailoring intrinsic chiroptical responses via twisted bilayer α-MoO3 separated by a VO2 film

Flexible control of intrinsic chiroptical responses within compact nanostructures is crucial for flat optics, topological photonics, and chiroptics. However, previous approaches require complicated patterns with both in-plane and out-of-plane mirror symmetry breaking to achieve intrinsic chirality, and their chiroptical responses cannot be dynamically controlled as well. Herein, we demonstrated that near-perfect intrinsic circular dichroism (CD) can be achieved within a lithography-free structure consisting of the twisted bilayer α-MoO3 separated by a vanadium dioxide (VO2) film. By twisting the bilayer α-MoO3, dual-band intrinsic chiroptical responses can be realized due to the excitations of the hyperbolic phonon polaritons modes in the mid-infrared. It is the spin-selected average electric-field enhancement instead of the chiral absorption that is responsible for the intrinsic CD of the device. In addition, the chiroptical responses are insensitive to the variation of the thickness of the structure as well as the incident angle, and high contrast CD can be dynamically tuned by varying the volume fraction of VO2.

Hyperbolic phonon polaritons-induced photonic spin Hall effect in an α -MoO3 thin film

Photonic spin Hall effect (PSHE) can be achieved by using the patterned structures or metal/dielectric multilayers in the nanophotonic systems; however, the complicated structures of these devices hinder their further applications. Herein, we demonstrate that highly directional PSHE can be realized through the excitation of hyperbolic phonon polaritons (HPPs) in a comparably simple architecture based on an anisotropic α-MoO3 thin film. It is shown that the propagation of the HPP modes of α-MoO3 in the reststrahlen (RS) bands exhibits topological transitions between open hyperbola and closed ellipse in both real space and momentum space (k-space) due to the extreme in-plane anisotropy. Specifically, larger dispersion angle possesses larger figure of merit (FoM), and high k mode of HPPs exhibits robust propagation properties at the maximum dispersion angle. Spin-selected propagation with asymmetric ratio of intensity equal to ±0.94 can be realized by changing the handedness of the dipole emitters. By changing the incident wavelengths of the RS bands, the propagation angle of the HPP modes can be dynamically tuned in wide angular and wavelength ranges.

Airy‐Like Hyperbolic Shear Polaritons in High Symmetry van der Waals Crystals

AbstractControlling light at the nanoscale by exploiting ultra‐confined polaritons—hybrid light and matter waves—in various van der Waals (vdW) materials empowers unique opportunities for many nanophotonic on‐chip technologies. So far, mainstream approaches have relied on interfacial techniques (e.g., refractive optics, meta‐optics, and moire engineering) to manipulate the polariton wavefront. Here, it is proposed that orbital angular momentum (OAM) of incident light can offer a new degree of freedom to structure vdW polaritons. With vortex excitations, a new class of accelerating polariton waves is observed—Airy‐like hyperbolic phonon polaritons (PhPs) in high‐symmetry orthorhombic vdW crystal α‐MoO3. In analogous to the well‐known Airy beams in free space, such Airy‐like PhPs also exhibit self‐accelerating, nonspreading, and self‐healing characteristics. Interestingly, the helical phase gradient of the vortex beam leads to asymmetry excitation of polaritons, as a result, the Airy‐like PhPs possess asymmetric propagation features even with a symmetric mode, analogous to the asymmetry hyperbolic shear polaritons in low‐symmetry crystals. The finding highlights the potential of OAM to manipulate polaritons in vdW materials, which can be further extended into a variety of applications such as active structured polaritonic devices.

A high figure of merit of phonon-polariton waveguide modes with hbn/SiO2/graphene /hBN ribs waveguide in mid-infrared range

Natural hyperbolic materials can confine electromagnetic waves at the nanoscale. In this study, we propose a waveguide design that combines a high quality factor (FOM) with low loss, utilizing hexagonal boron nitride and graphene and gold substrate. The waveguide consists of a dielectric rib with a graphene layer sandwiched between two hBN ribs. Numerical simulations demonstrate the existence of two guided modes in the proposed waveguide within the second reststrahlen band (1360.0 cm−1<ω < 1609.8 cm−1) of hBN. These modes are formed by coupling the hyperbolic phonon polariton (HPhP) of two hBN rib in the middle dielectric rib and are subsequently modulated by a graphene layer. Interestingly, we observe variations in four transmission parameters, namely effective length, figure of merit, device length, and propagation loss of the guided modes, with respect to the operation frequency and gate voltage. By optimizing the waveguide's geometry parameters and dielectric permittivity, the modal properties were analyzed. Simulation results indicate that optimizing the waveguide size parameters enables us to achieve a high FOM of 4.0 × 107. The proposed waveguide design offers a promising approach for designing tunable mid infrared range waveguides on photonic chips, and this concept can be extended to other 2D materials and hyperbolic materials.

Coupling of Surface Plasmon Polaritons and Hyperbolic Phonon Polaritons on the Near-Field Radiative Heat Transfer Between Multilayer Graphene/hBN Structures

ABSTRACT Near-field radiative heat transfer (NFRHT) between multilayer graphene/hBN heterostructures has been demonstrated to exceed the blackbody limit due to the coupling mechanism of surface plasmon polaritons and hyperbolic phonon polaritons, opening the door to applications in thermal management, thermophotovoltaics, and nanoscale metrology. Recent studies have shown that adding vacuum layers within multilayer structures can effectively promote surface modes and thus enhance NFRHT. However, the influence of vacuum layers on NFRHT between multilayer graphene/hBN heterostructures has not been investigated. Moreover, the influence of vacuum layers on coupled resonance modes excited in multilayer structures is worth discussing. In this work, we study the NFRHT based on multilayer graphene/vacuum/hBN/vacuum structures. The results show that as the gap distance increases from 20 nm to 100 nm, the NFRHT of three-cell and six-cell configurations is enhanced, while that of unit-cell configuration is suppressed. The potential mechanism can be identified as the excitation of surface plasmon-phonon polaritons (SPPPs) and hyperbolic plasmon-phonon polaritons (HPPPs) in multilayer structures. The enhancement factor of the six-cell configurations is up to 4.82 when the gap distance is 80 nm. Moreover, the influences of the chemical potential of graphene and the layer thickness on the NFRHT are discussed. The interesting results in this work indicate the perspectives for future near-field research involving coupling of SPPPs and HPPPs, and shed new light on high-performance devices introducing vacuum layers based on near-field radiative heat transfer.

Influence of substrate effect on near-field radiative modulator based on biaxial hyperbolic materials

Relative rotation between the emitter and receiver could effectively modulate the near-field radiative heat transfer (NFRHT) in anisotropic media. Due to the strong in-plane anisotropy, natural hyperbolic materials can be used to construct near-field radiative modulators with excellent modulation effects. However, in practical applications, natural hyperbolic materials need to be deposited on the substrate, and the influence of substrate on modulation effect has not been studied yet. In this work, we investigate the influence of substrate effect on near-field radiative modulator based on α-MoO3. The results show that compared to the situation without a substrate, the presence of both lossless and lossy substrate will reduce the modulation contrast (MC) for different film thicknesses. When the real or imaginary component of the substrate permittivity increases, the mismatch of hyperbolic phonon polaritons (HPPs) weakens, resulting in a reduction in MC. By reducing the real and imaginary components of substrate permittivity, the MC can be significantly improved, reaching 4.64 for ε s = 3 at t = 10 nm. This work indicates that choosing a substrate with a smaller permittivity helps to achieve a better modulation effect, and provides guidance for the application of natural hyperbolic materials in the near-field radiative modulator.

Tunable hyperbolic polaritons with plasmonic phase-change material In3SbTe2

Abstract Hyperbolic polaritons that originate from the extreme optical anisotropy in van der Waals (vdW) crystals have gained much attention for their potential in controlling nanolight. For practical use, there has been a strong interest to develop various manipulation strategies to customize the propagation of hyperbolic polaritons on a deeply sub-diffractional scale. In this regard, phase-change materials (PCMs) that possess two phases with different refractive indices offer suitably a tunable dielectric environment. Here, we report on the tuning of hyperbolic phonon polaritons in natural vdW crystals, hexagonal boron nitride (hBN), and alpha-phase molybdenum trioxide (α-MoO3), using the plasmonic phase-change material In3SbTe2 (IST). Unlike conventional PCMs whose both phases are dielectric, IST features a metallic crystalline phase that is stable at room temperature. The coupling between polaritons with their mirror charges in the underneath crystalline IST triggers an even stronger field confinement for polaritons. Moreover, benefited from the metallicity of laser-writable crystalline IST, we show an all-optical material platform in which crystalline IST boundaries efficiently excite and focus hyperbolic phonon polaritons in α-MoO3. Our experiments highlight the possibility to obtain new degrees of freedom in polariton engineering with plasmonic PCMs, thereby expanding the toolkit of tunable nanophotonics with flexible, on-demand fabrication and reconfiguration capabilities.

High-quality nanocavities through multimodal confinement of hyperbolic polaritons in hexagonal boron nitride.

Compressing light into nanocavities substantially enhances light-matter interactions, which has been a major driver for nanostructured materials research. However, extreme confinement generally comes at the cost of absorption and low resonator quality factors. Here we suggest an alternative optical multimodal confinement mechanism, unlocking the potential of hyperbolic phonon polaritons in isotopically pure hexagonal boron nitride. We produce deep-subwavelength cavities and demonstrate several orders of magnitude improvement in confinement, with estimated Purcell factors exceeding 108 and quality factors in the 50-480 range, values approaching the intrinsic quality factor of hexagonal boron nitride polaritons. Intriguingly, the quality factors we obtain exceed the maximum predicted by impedance-mismatch considerations, indicating that confinement is boosted by higher-order modes. We expect that our multimodal approach to nanoscale polariton manipulation will have far-reaching implications for ultrastrong light-matter interactions, mid-infrared nonlinear optics and nanoscale sensors.

Actively tuning anisotropic light–matter interaction in biaxial hyperbolic material α-MoO3 using phase change material VO2 and graphene

Anisotropic hyperbolic phonon polaritons (PhPs) in natural biaxial hyperbolic material α-MoO3 has opened up new avenues for mid-infrared nanophotonics, while active tunability of α-MoO3 PhPs is still an urgent problem necessarily to be solved. In this study, we present a theoretical demonstration of actively tuning α-MoO3 PhPs using phase change material VO2 and graphene. It is observed that α-MoO3 PhPs are greatly dependent on the propagation plane angle of PhPs. The insulator-to-metal phase transition of VO2 has a significant effect on the hybridization PhPs of the α-MoO3/VO2 structure and allows to obtain actively tunable α-MoO3 PhPs, which is especially obvious when the propagation plane angle of PhPs is 90°. Moreover, when graphene surface plasmon sources are placed at the top or bottom of α-MoO3 in α-MoO3/VO2 structure, tunable coupled hyperbolic plasmon–phonon polaritons inside its Reststrahlen bands (RBs) and surface plasmon–phonon polaritons outside its RBs can be achieved. In addition, the above-mentioned α-MoO3-based structures also lead to actively tunable anisotropic spontaneous emission (SE) enhancement. This study may be beneficial for realization of active tunability of both PhPs and SE of α-MoO3, and facilitate a deeper understanding of the mechanisms of anisotropic light–matter interaction in α-MoO3 using functional materials.